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D J Uhlrich

Publications and source records attributed to D J Uhlrich.

At least 19 recordsLinked to original sources
PubMed2005

Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure.

It is well established that patterns of sensory input can affect neuroplastic changes during early development. The scope and consequences of experience-dependent plasticity in the adult are less well understood. We studied the possibility that repeated exposure to trains of stroboscopic stimuli could induce a sensitized and potentially aberrant response in ordinary individuals. Chronic electrocorticographic recording electrodes enabled measurement of responses in awake, freely moving animals. Normal adult rats, primarily Sprague-Dawley, were exposed to 20-40 strobe trains per day after a strobe-free adaptation period. The common response to strobe trains changed in 34/36 rats with development of a high-amplitude spike-wave response that emerged fully by the third day of photic exposure. Onset of this sensitized response was marked by short-term augmentation of response to successive strobe flashes. The waveform generalized across the brain, reflected characteristics of the visual stimulus, as well as an inherent 6- to 8-Hz pacing, and was suppressed with ethosuximide administration. Spike-wave episodes were self-limiting but could persist beyond the strobe period. Sensitization lasted 2-4 wk after last strobe exposure. The results indicate visual stimulation, by itself, can induce in adult rats an enduring sensitization of visual response with epileptiform characteristics. The results raise the question of the effects of such neuroplastic change on sensation and epileptiform events.

Adaptation, Physiological↗
PubMed1999

Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey.

The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. Brain tissue from macaques was immunoreacted by means of antibodies to choline acetyltransferase (ChAT) or to histamine and processed for light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3-6 and relatively sparse in the interlaminar zones. Varicosities associated with the cholinergic axons had an average of three conventional asymmetric synapses per varicosity, and these appeared to contact dendrites of both thalamocortical cells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelming majority of varicosities were found en passant and rarely displayed conventional synapses, despite the abundance of synaptic vesicles, and were not associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergic systems is consistent with their proposed role in state dependent modulation of thalamic activity. The dense and highly synaptic innervation by cholinergic axons supports the proposal of additional involvement of these axons in functions related to eye movements.

Animals↗
PubMed1999

Muscarinic receptor subtypes in the lateral geniculate nucleus: a light and electron microscopic analysis.

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, ml-m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3-positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2-positive synaptic contacts were classified as F2-type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype-specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic-mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons.

Animals↗
PubMed1999

Burst firing in identified rat geniculate interneurons.

We used whole-cell patch recording to study 102 local interneurons in the rat dorsal lateral geniculate nucleus in vitro. Input impedance with this technique (607.0+/-222.4 MOhm) was far larger than that measured with sharp electrode techniques, suggesting that interneurons may be more electrotonically compact than previously believed. Consistent and robust burst firing was observed in all interneurons when a slight depolarizing boost was given from a potential at, or slightly hyperpolarized from, resting membrane potential. These bursts had some similarities to the low-threshold spike described previously in other thalamic neuron types. The bursting responses were blocked by Ni+, suggesting that the low-threshold calcium current I(T), responsible for the low-threshold spike, was also involved in interneuron burst firing. Compared to the low-threshold spike of thalamocortical cells, however, the interneuron bursts were of relatively long duration and low intraburst frequency. The requirement for a depolarizing boost to elicit the burst is consistent with previous reports of a depolarizing shift of the I(T) activation curve of interneurons relative to thalamocortical cells, a finding we confirmed using voltage-clamp. Voltage-clamp study also revealed an additional long-lasting current that could be tentatively identified as the calcium activated non-selective cation current, I(CAN), based on reversal potential and on pharmacological characteristics. Computer simulation of the interneuron burst demonstrated that its particular morphology is likely due to the interaction of I(T) and I(CAN). In the slice, bursts could also be elicited by stimulation of the optic tract, suggesting that they may occur in response to natural stimulation. Synaptically triggered bursts were only partially blocked by Ni+, but could then be completely blocked by further addition of (+/-)-2-amino-5-phosphonopentanoic acid. The existence of robust bursts in this cell type suggests an additional role for interneurons in sculpting sensory responses by feedforward inhibition of thalamocortical cells. The low-threshold spike is a mechanism whereby activity in a neuron is dependent on a prior lack of activity in that same neuron. Understanding of the low-threshold spike in the other major neuron types of the thalamus has brought many new insights into how thalamic oscillations might be involved in sleep and epilepsy. Our description of this phenomenon in the interneurons of the thalamus suggests that these network oscillations might be even more complicated than previously believed.

Animals↗
PubMed1999

Properties of a hyperpolarization-activated cation current in interneurons in the rat lateral geniculate nucleus.

A hyperpolarization-activated cation conductance contributes to the membrane properties of a variety of cell types. In the thalamus, a prominent hyperpolarization-activated cation conductance exists in thalamocortical cells, and this current is implicated in the neuromodulation of complex firing behaviors. In contrast, the GABAergic cells in the reticular nucleus in the thalamus appear to lack this conductance. The presence and role of this cation conductance in the other type of thalamic GABAergic cells, local interneurons, is still unclear. To resolve this issue, we studied 54 physiologically and morphologically identified local interneurons in the rat dorsal lateral geniculate nucleus using an in vitro whole-cell patch recording technique. We found that hyperpolarizing current injections induced depolarizing voltage sags in these geniculate interneurons. The I-V relationship revealed an inward rectification. Voltage-clamp study indicated that a slow, hyperpolarization-activated cation conductance was responsible for the inward rectification. We then confirmed that this slow conductance had properties of the hyperpolarization-activated cation conductance described in other cell types. The slow conductance was insensitive to 10 mM tetraethylammonium and 0.5 mM 4-aminopyridine, but was largely blocked by 1-1.5 mM Cs+. It was permeable to both K+ and Na+ ions and had a reversal potential of -44 mV. The voltage dependence of the hyperpolarization-activated cation conductance in interneurons was also studied: the activation threshold was about -55 mV, half-activation potential was about -80 mV and maximal conductance was about 1 nS. The activation and deactivation time constants of the conductance ranged from 100 to 1000 ms, depending on membrane potential. The depolarizing voltage sags and I-V relationship were further simulated in a model interneuron, using the parameters of the hyperpolarization-activated cation conductance obtained from the voltage-clamp study. The time-course and voltage dependence of the depolarizing voltage sags and I-V relationship in the model cell were very similar to those found in geniculate interneurons in current clamp. Taken together, the results of the present study suggest that thalamic local interneurons possess a prominent hyperpolarization-activated cation conductance, which may play important roles in determining basic membrane properties and in modulating firing patterns.

Animals↗
PubMed1999

An intrinsic oscillation in interneurons of the rat lateral geniculate nucleus.

By using the whole cell patch recording technique in vitro, we examined the voltage-dependent firing patterns of 69 interneurons in the rat dorsal lateral geniculate nucleus (LGN). When held at a hyperpolarized membrane potential, all interneurons responded with a burst of action potentials. In 48 interneurons, larger current pulses produced a bursting oscillation. When relatively depolarized, some interneurons produced a tonic train of action potentials in response to a depolarizing current pulse. However, most interneurons produced only oscillations, regardless of polarization level. The oscillation was insensitive to the bath application of a combination of blockers to excitatory and inhibitory synaptic transmission, including 30 microM 6,7-dinitroquinoxaline-2,3-dione, 100 microM (+/-)-2-amino-5-phosphonopentanoic acid, 20 microM bicuculline, and 2 mM saclofen, suggesting an intrinsic event. The frequency of the oscillation in interneurons was dependent on the intensity of the injection current. Increasing current intensity increased the oscillation frequency. The maximal frequency of the oscillation was 5-15 Hz for most cells, with some ambiguity caused by the difficulty of precisely defining a transition from oscillatory to regular firing behavior. In contrast, the interneuron oscillation was little affected by preceding depolarizing and hyperpolarizing pulses. In addition to being elicited by depolarizing current injections, the oscillation could also be initiated by electrical stimulation of the optic tract when the interneurons were held at a depolarized membrane potential. This suggests that interneurons may be recruited into thalamic oscillations by synaptic inputs. These results indicate that interneurons may play a larger role in thalamic oscillations than was previously thought.

Action Potentials↗
PubMed1998

Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus.

We used the whole-cell recording technique in an in vitro preparation to examine the electrophysiological actions of the muscarinic receptors on relay cells in the rat lateral geniculate nucleus. Drop application of the muscarinic agonist acetyl-beta-methylcholine resulted in a slow depolarization that persisted for several minutes. The response was insensitive to the nicotinic antagonist hexamethonium, but was blocked by atropine, a muscarinic antagonist. The response was also insensitive to blockade of synaptic transmission by tetrodotoxin, indicating a direct muscarinic effect. The muscarinic depolarization consisted of two components that were somewhat separated in time. The early portion of the muscarinic response was mediated by a large inward current with little change in input resistance, while the later portion was mediated by a small inward current associated with a large increase in input resistance. Pharmacological agents were used to distinguish the two components. Drop application of McN-A-343, an ml receptor agonist, could only mimic the later component of the muscarinic response. This was supported by the result that the later component was blocked by low concentrations of pirenzepine. These data suggest that the ml receptor only mediates the late component of the muscarinic response, while the early component is mainly mediated by the m3 receptor. The idea that both ml and m3 receptors were involved in the muscarinic depolarization was further supported by voltage-clamp analysis. This revealed that activation of the ml receptor was associated with a decrease in an inward potassium current, IKleak, while activation of the m3 receptor was likely associated with both a decrease in IKleak and an increase in the hyperpolarization-activated cation current Ih. In summary, our data suggest that muscarinic responses in geniculate relay cells result from the activation of two receptors, which modulate IKleak and Ih. Given the fact that the ascending aminergic systems also depolarize geniculate relay cells via two receptors acting on IKleak and Ih, we concluded that ascending activating systems use common mechanisms to enact the depolarizing form of arousal in relay neurons.

(4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethy↗
PubMed1997

Nicotinic receptor-mediated responses in relay cells and interneurons in the rat lateral geniculate nucleus.

We used the in vitro whole-cell recording technique to study the nicotinic responses of relay cells and interneurons in the adult rat dorsal lateral geniculate nucleus, the thalamic nucleus that conveys visual signals from the retina to the cortex. These geniculate relay cells and interneurons were identified by their physiological and morphological properties. We found that, in the presence of a muscarinic antagonist, atropine, acetylcholine induced a depolarization in relay cells. A similar depolarization was induced by application of nicotine. These depolarizations were completely blocked by a nicotinic antagonist, hexamethonium, but were little affected by bath solution that contained tetrodotoxin and/or low calcium concentration to block synaptic transmission. This suggests that the depolarization is mediated directly by nicotinic receptors in relay cells. Application of nicotine also induced a depolarization in geniculate interneurons. The interneurons continued to exhibit a response to nicotine in the presence of synaptic blockade, although the time-course of the response was altered. The nicotinic responses in relay cells and interneurons shared many similar properties. Both exhibited desensitization, although this characteristic was much more pronounced in the interneurons. In both cell types, the nicotinic response activated a relatively linear conductance with a slight inward rectification. The reversal potential for the conductance was about - 33 mV, which is consistent with a permeability to sodium and potassium ions. The reversal potential shifted negatively by 5-6 mV when the bath solution contained low calcium, which further suggests a permeability to calcium ions. Our results indicate that nicotinic receptors are present in both geniculate relay cells and interneurons. The nicotinic depolarization in relay cells may serve to enhance transmission of visual signals through the lateral geniculate nucleus as well as to contribute to a voltage-dependent shift in the response mode of geniculate relay cells from burst to tonic (single-spike) firing. The nicotinic depolarization in interneurons may provide an explanation for reports that activation of the cholinergic system can enhance inhibitory tuning in the lateral geniculate nucleus.

Acetylcholine↗
PubMed1996

Histamine-immunoreactive neurons and their innervation of visual regions in the cortex, tectum, and thalamus in the primate Macaca mulatta.

The histaminergic system is involved in the control of arousal in the brain and may impact significantly on visual processing. However, little is known about the histaminergic innervation of visual areas, or the histamine system in the primate brain, in general. We examined in Macaca mulatta the location of histamine-immunoreactive neurons and the innervation of important cortical and subcortical visual areas by histamine-immunoreactive axons. Brain sections were treated with an antibody to histamine and processed with standard immunohistological procedures. Histamine-immunoreactive neurons (20-45 microns in diameter) were localized bilaterally in the hypothalamus, particularly in ventral, lateral, posterior, and perimammillary hypothalamic areas. These hypothalamic cells appear to provide the sole neural source of histamine in the macaque brain. A plexus of varicose histamine-immunoreactive axons was present throughout the superior colliculus, the dorsal and ventral lateral geniculate nuclei of the thalamus, the reticular nucleus of the thalamus, the lateral posterior/pulvinar complex, and the visual cortex, including areas 17, 18, and the nearby extrastriate cortex. The axons nearly homogeneously innervated every region and layer in these structures, except for an increase in density in layer 1 of the visual cortex and in the superficial-most layers of the superior colliculus. Histaminergic axons broadly innervated every visual region examined. In comparison with the other aminergic and the cholinergic projection systems, which show considerable projection specificity, the histaminergic projection exhibited great homogeneity. The breadth of the distribution of histaminergic axons ensures that virtually all levels of visual processing in the primate can be influenced, either directly or indirectly, by the neuromodulatory effects of histamine.

Animals↗
PubMed1995

Projection of individual axons from the pretectum to the dorsal lateral geniculate complex in the cat.

The dorsal lateral geniculate nucleus of the thalamus transmits visual signals from the retina to the cortex. Within the lateral geniculate nucleus, the ascending visual signals are modified by the actions of a number of afferent pathways. One such projection originates in the pretectum and appears to be active in association with oculomotor activity. Much remains unknown about the pretectal-geniculate projection. Our purpose was to examine for the first time individual axon arbors from the pretectum that project to the lateral geniculate nucleus, describing their topography and nuclear and laminar targets. We made injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the cat pretectum, targeting the nucleus of the optic tract. Serial 40 microns coronal sections were processed by using immunohistochemistry to reveal labeled axons that were then serially reconstructed using light microscopy. Pretectal-geniculate axons appeared morphologically heterogeneous in terms of swelling size, branching patterns, and laminar target. Most axons innervated the geniculate A laminae. A separate, smaller population innervated the C laminae. All axons exhibited substantially greater spread medial-laterally than rostral-caudally in the lateral geniculate nucleus, displaying a topographical organization for visual field elevation, but not azimuth. Many pretectal axons that projected to the LGN also innervated adjacent structures, including the medial interlaminar nucleus, the perigeniculate nucleus, and/or the pulvinar. These results indicate that the projection from the pretectum to the dorsal lateral geniculate nucleus is heterogeneous, is semitopographical, and may coordinate neural activity in the lateral geniculate nucleus and in neighboring visual thalamic structures in association with oculomotor events.

Animals↗
PubMed1995

Morphology of physiologically identified retinal X and Y axons in the cat's thalamus and midbrain as revealed by intraaxonal injection of biocytin.

Prior morphological studies of individual retinal X and Y axon arbors based on intraaxonal labeling with horseradish peroxidase have been limited by restricted diffusion or transport of the label. We used biocytin instead as the intraaxonal label, and this completely delineated each of our six X and 14 Y axons, including both thalamic and midbrain arbors. Arbors in the lateral geniculate nucleus appeared generally as has been well documented previously. Interestingly, all of the labeled axons projected a branch beyond thalamus to the midbrain. Each X axon formed a terminal arbor in the pretectum, but none continued to the superior colliculus. In contrast, 11 of 14 Y axons innervated both the pretectum and the superior colliculus, one innervated only the pretectum, and two innervated only the superior colliculus. Two of the Y axons were quite unusual in that their receptive fields were located well into the hemifield ipsilateral with respect to the hemisphere into which they were injected. These axons exhibited remarkable arbors in the lateral geniculate nucleus, diffusely innervating the C-laminae and medial interlaminar nucleus, but, unlike all other X and Y arbors, they did not innervate the A-laminae at all. In addition to these qualitative observations, we analyzed a number of quantitative features of these axons in terms of numbers and distributions of terminal boutons. We found that Y arbors contained more boutons than did X arbors in both thalamus and midbrain. Also, for axons with receptive fields in the contralateral hemifield (all X and all but two Y axons), 90-95% of their boutons terminated in the lateral geniculate nucleus; the other two Y axons had more of their arbors located in midbrain.

Animals↗
PubMed1995

Effects of brain stem parabrachial activation on receptive field properties of cells in the cat's lateral geniculate nucleus.

1. The lateral geniculate nucleus is the primary thalamic relay for the transfer of retinal signals to the visual cortex. Geniculate cells are heavily innervated from nonretinal sources, and these modify retinogeniculate transmission. A major ascending projection to the lateral geniculate nucleus arises from cholinergic cells in the parabrachial region of the brain stem. This is an important pathway in the ascending control of arousal. In an in vivo preparation, we used extracellular recordings to study the effects of electrical activation of the parabrachial region on the spontaneous activity and visual responses of X and Y cells in the lateral geniculate nucleus of the cat. 2. We studied the effects of two patterns of parabrachial activation on the spontaneous activity of geniculate cells. Burst stimulation consisted of a short pulse at high frequency (16 ms at 250 Hz). Train stimulation was of longer duration at lower frequency (e.g., 1 s at 50 Hz). The firing rate of almost all geniculate cells was enhanced by either pattern of stimulation. However, the burst pattern of stimulation elicited a short, modulated response with excitatory and inhibitory epochs. We found that the different epochs could differentially modulate the visual responses to drifting gratings. Thus the temporal alignment of the brain stem and visual stimuli was critical with burst stimulation, and varied alignments could dramatically confound the results. In comparison, the train pattern of stimulation consistently produced a relatively flat plateau of increased firing, after a short initial period of more variable effects. We used the less confounding pattern of train stimuli to study the effects of parabrachial activation on visual responses. 3. Our main emphasis was to examine the parabrachial effects on the visual responses of geniculate cells. For most visual stimuli, we used drifting sine wave gratings that varied in spatial frequency; these evoked modulated responses from the geniculate cells. Parabrachial activation enhanced the visual responses of almost all geniculate cells, and this enhancement included both increased depth of modulation and greater response rates. 4. Our results were incorporated quantitatively into a difference-of-Gaussians model of visual receptive fields in order to study the parabrachial effects on the spatial structure of the receptive field. This model fit our data well and provided measures of the response amplitude and radius of the receptive field center (Kc and Rc, respectively) and the response amplitude and radius of the receptive field surround (Ks and Rs, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Histaminergic and non-histamine-immunoreactive mast cells within the cat lateral geniculate complex examined with light and electron microscopy.

Mast cells and their location in the cat lateral geniculate complex of the thalamus were examined by means of histamine immunohistochemistry and the mast cell stain pinacyanol erythrosinate. Brain sections from seven normal adult pigmented cats were processed for light or electron microscopy. Histamine-containing and pinacyanol erythrosinate-stained mast cells were widespread throughout the dorsal and ventral lateral geniculate nuclei and the surrounding regions. Mast cells were especially numerous rostrally in the complex and in the geniculate C laminae. The cells were found consistently in association with blood vessels, ranging from capillary size to vessels c. 150 microns diameter, and twice as often with arterioles as with venules. Large clusters of many mast cells associated with single blood vessels were seen. Individual mast cells were typically 8 microns in diameter and somewhat oval, although multipolar and crescent-shaped cells were also seen, up to twice as long. The amount of histamine labeling varied across cells. When histamine-labeled material was secondarily stained with pinacyanol erythrosinate, many mast cells were double labeled. In addition, there was a small population of mast cells that stained only with pinacyanol erythrosinate, but was otherwise identical to the histamine-immunoreactive mast cells. Electron microscopic examination showed that the mast cells lie on the brain side of the blood-brain barrier. Mast cells were found in close proximity to the thalamic neuropil, primarily apposed to the processes of astrocytes, but also apposed to neural elements. The distinctive electron-dense cytoplasmic granules in the fully granulated, mature state were largely amorphous in appearance and as large as 700 nm in diameter. Histamine was dispersed throughout some granules and contained within restricted areas of other granules. In degranulated mast cells, large, irregularly shaped, electron-lucent granules were seen fused with the cell membrane on the neuropil side, as well as the lumen side of the mast cell. More mast cells were observed at the electron microscopic level than were expected from the light level observations, which suggests that, despite the numbers of mast cells labeled, these results may still underestimate the total mast cell population present in this region of the thalamus. Mast cells, by their numbers, their distribution and the potent chemical substances they contain, may significantly influence vascular and neural function, directly and indirectly, in the cat lateral geniculate complex.

Animals↗
PubMed1993

Ultrastructure of synapses from the pretectum in the A-laminae of the cat's lateral geniculate nucleus.

We have recently shown in cats that many neurons projecting to the lateral geniculate nucleus from the pretectum use gamma-amino butyric acid (GABA) as their neurotransmitter. We sought to determine the morphology of synaptic terminals and synapses formed by these pretectal axons and the extent to which they resemble other GABAergic terminals found in the geniculate neuropil (i.e., from geniculate interneurons and cells of the nearby perigeniculate nucleus). To do this, we labeled a population of pretectal axons with the anterograde tracer Phaseolus vulgaris leucoagglutinin and analyzed the morphology and synaptology of labeled pretectal terminals in the A-laminae of the cat's lateral geniculate nucleus. The pretectal projection, which arises primarily from the nucleus of the optic tract (NOT), provides synaptic innervation to elements in the geniculate neuropil. The labeled NOT terminals are densely packed with vesicles, contain dark mitochondria, and form symmetrical synaptic contacts. These are characteristics of the F1 type of terminal, and we know from other studies that GABAergic axon terminals from interneurons and perigeniculate cells also give rise to F1 terminals. We compared our population of NOT terminals with labeled perigeniculate and unlabeled F1 terminals selected from the geniculate neuropil and found that all three populations share many morphological characteristics. Both qualitative and quantitative assessments of the pretectal terminals suggest that these are a type of F1 terminal. Most pretectal terminals selectively form synapses onto geniculate profiles that contain irregularly distributed vesicles and dark mitochondria and that are postsynaptic to other types of terminals. These postsynaptic targets thus exhibit features of another class of inhibitory, GABAergic terminal known as F2 terminals, which are the specialized appendages of geniculate interneurons. Pretectal inputs, being GABAergic, may thus serve to inhibit local interneuronal outputs. Pretectal axons also innervate the perigeniculate nucleus, in which the only targets are the other main type of inhibitory, GABAergic neurons. These results suggest that the pretectum may facilitate retinal transmission through the lateral geniculate nucleus by providing inhibition to the local inhibitory cells: the interneurons and probably perigeniculate cells. This would serve to release geniculate relay cells from inhibition.

Animals↗
PubMed1993

The distribution of histaminergic axons in the superior colliculus of the cat.

The histaminergic projection from the hypothalamus to the superior colliculus was examined immunohistochemically in the cat brain using an antibody to histamine. The source of histaminergic fibers in the brain is a group of neurons in the posterior hypothalamus, located primarily in ventrolateral and periventricular regions and collectively referred to as the tuberomammillary nucleus. All laminae of the superior colliculus--including the superficial, intermediate, and deep layers, as well as the central gray--were blanketed with histamine-immunoreactive axonal fibers. Overall, labeling in the superior colliculus was moderately dense compared to other locations in the cat brain, with some variation in fiber density. Individual labeled fibers resembled histaminergic fibers described previously in the brain. Labeled axonal fibers showed infrequent branching and were beaded with numerous en passant varicosities that were typically 1 micron or smaller, but as large as 2.5 micron in diameter. Varicosity size differed significantly at different depths in the colliculus. The histaminergic projection appears to be separate from a previously reported, apparently non-histaminergic projection from neurons in the dorsal hypothalamic area to discrete regions of intermediate and deep colliculus. These results indicate that the histaminergic projection from the tuberomammillary nucleus of the hypothalamus projects extensively throughout the superior colliculus. Histamine, which is believed to act as a neuromodulator in the brain, is in a position to influence sensory and motor-related processes in every layer of the cat superior colliculus.

Afferent Pathways↗
PubMed1991

Electron-microscopic analysis of synaptic input from the perigeniculate nucleus to the A-laminae of the lateral geniculate nucleus in cats.

The perigeniculate nucleus of carnivores is thought to be a part of the thalamic reticular nucleus related to visual centers of the thalamus. Physiological studies show that perigeniculate neurons, which are primarily GABAergic, provide feedback inhibition onto neurons in the lateral geniculate nucleus. However, little is known about the anatomical organization of this feedback pathway. To address this, we used two complementary tracing methods to label perigeniculate axons for electron microscopic study in the geniculate A-laminae: intracellular injection of horseradish peroxidase (HRP) to fill an individual perigeniculate cell and its axon; and anterograde transport of Phaseolus vulgaris leucoagglutinin to label a population of perigeniculate axons. Labeled perigeniculate terminals display features of F1 terminals in the geniculate neuropil: they are small, contain dark mitochondria, and form symmetric synaptic contacts. We found that most of the perigeniculate terminals (greater than 90%) contact geniculate cell dendrites in regions that also receive a rich innervation from terminals deriving from visual cortex (e.g., "cortico-recipient" dendrites). The remainder of the perigeniculate synapses (10%) contacted dendrites in regions that also received direct retinal input (e.g., "retino-recipient" dendrites). Serial reconstruction of segments of dendrites postsynaptic to perigeniculate terminals suggests that these terminals contact both classes of relay cell in the A-laminae (X and Y), although our preliminary conclusion is that an individual perigeniculate cell contacts only one class. Finally, our quantitative comparison between labeled perigeniculate terminals and unlabeled F1 terminals indicates that these perigeniculate terminals form a distinct subset of F1 terminals. We quantitatively compared the labeled perigeniculate terminals to unlabeled F1 terminals. Although the parameters of the perigeniculate terminals fell entirely within the range of those for the unlabeled F1 terminals, as populations, we found consistent differences between these two groups. We thus conclude that, as populations, other sources of F1 terminals are morphologically distinct from perigeniculate terminals and innervate different targets.

Animals↗
PubMed1991

Morphology and axonal projection patterns of individual neurons in the cat perigeniculate nucleus.

1. The lateral geniculate nucleus is the primary thalamic relay through which retinal signals pass en route to cortex. This relay is gated and can be suppressed by activity among local inhibitory neurons that use gamma-aminobutyric acid (GABA) as a neurotransmitter. In the cat, a major source of this GABAergic inhibition seems to arise from cells of the perigeniculate nucleus, which lies just dorsal to the A-laminae of the lateral geniculate nucleus. However, the morphological characteristics of perigeniculate cells, and particularly the projection patterns of their axons, have never been fully characterized. We thus examined the morphology of these cells: individually by intracellular injection of horseradish peroxidase (HRP) and en masse with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL). 2. We recorded from 12 perigeniculate cells that we impaled and successfully labeled with HRP. These cells exhibited response properties generally consistent with those described previously. They had long response latencies to stimulation of the optic chiasm and relatively large, often diffuse, receptive fields. The visually evoked responses of most of the cells were dominated by one eye. Compared with cells of the lateral geniculate nucleus, perigeniculate cells had large somata (517 +/- 136 microns 2 in cross-sectional area, mean +/- SD), which were fusiform or multipolar in shape, and dendritic arbors that extended a considerable distance (1,095 +/- 167 microns) parallel to the border between the perigeniculate and lateral geniculate nuclei. Terminal arbors of some dendrites were quite complex and beaded. 3. The axons of six perigeniculate cells were labeled sufficiently well to trace and reconstruct over a considerable distance. Each of these axons formed branches that descended to innervate the lateral geniculate nucleus, and this geniculate innervation was exclusively limited to the A-laminae. Terminal boutons within the A-laminae were nearly all en passant, which gave the axons a beaded appearance. Furthermore, branches of five of these six axons provided local innervation of the perigeniculate nucleus, generally within each labeled cell's own dendritic arbor. Three of the cells also exhibited an axon branch that extended medially and caudally away from the soma, but we were unable to trace these axon branches to their targets. 4. Within the lateral geniculate nucleus, each arbor of perigeniculate axons derived from two main components. One was a narrow, sparse medial component that innervated laminae A and A1.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗