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PubMed1979

Evidence of a neurotransmitter role for aspartate and gamma-aminobutyric acid in the rat olfactory cortex.

1. Using a cortical cup technique, the release of seven endogenous amino acids from the isolated rat olfactory cortex slice has been monitored.2. Electrical stimulation of the lateral olfactory tract at a frequency of 4 min(-1) was accompanied by a significant increase in the release of aspartate, GABA and taurine; the release of GABA and aspartate but not that of taurine was Ca(2+)-dependent.3. Chronic unilateral bulbectomy was accompanied by a specific, significant fall in the aspartate content of the olfactory cortex which reached a maximum 5 days after surgery and persisted for at least a further 5 days. Electrical stimulation of the lateral olfactory tract of such preparations did not release any of the amino acids under investigation.4. When slices from the unoperated side were exposed to solutions containing 25 mM-K+ or stimulated with electrodes placed directly on the cortex enclosed by the cup there was a Ca(2+)-dependent release of aspartate and GABA accompanied by a Ca(2+)-independent release of taurine. Following chronic bulbectomy, these procedures failed to evoke significant release of aspartate whereas the characteristics of GABA and taurine release were unaltered.5. It is concluded that aspartate may be the excitatory transmitter of some of the terminals of the lateral olfactory tract fibres and that GABA may be a transmitter at some inhibitory synapses of the rat olfactory cortex.

Amino Acids↗

PubMed1992

Further study of the aberrant optic nerve projection to olfactory cortex.

When implanted into the cerebral hemisphere, the regenerating optic nerve of the adult frog (Rana pipiens) forms a well-defined terminal field in the pars ventralis of the lateral (olfactory) cortex, and sometimes expands medially into the postolfactory eminence. These adjacent areas receive their normal input from the main olfactory bulb. The aberrant projection extends caudally toward the core neuropil of the medial amygdaloid nucleus, which receives its normal input from the accessory olfactory bulb, but does not enter this vomeronasal sector of the amygdala. The present study tests whether: 1) optic fibers would innervate the vomeronasal amygdala after surgical ablation of the accessory olfactory bulb, 2) the projection would transpose into adjacent cortex after olfactory cortex lesions, and 3) the projection would overflow into adjacent areas after being amplified by hemisection at the di-telencephalic junction (to minimize escape of fibers into the diencephalon). The retinal projection always terminated in the olfactory cortex when this area was intact, or in spared fragments of it after radical cortical lesions, but never entered the vomeronasal amygdala in any specimen, as studied by autoradiographic and horseradish peroxidase tracing techniques. With forebrain hemisection, the cortical terminal field increased in thickness but remained confined to the olfactory area. However, the interruption of the lateral forebrain bundle induced a new projection to the striatum in a region neighboring but separate from the olfactory cortical field. These findings support the hypothesis that retinal fibers have a specific affinity for primary olfactory cortex that is not normally allowed expression in development. Retinal fibers may also have a latent affinity for the striatum that is unmasked after deafferentation.

Animals↗

PubMed1972

Potentiation and depression of synaptic transmission in the olfactory cortex of the guinea-pig.

1. The extracellular field potentials of the olfactory cortex evoked by stimulation of the lateral olfactory tract (l.o.t.) were studied in in vitro preparations from the olfactory cortex. The field potentials comprised an initial diphasic wave - the l.o.t. compound action potential - followed by a negative wave of about 10 msec duration which in turn was followed by a low amplitude positive wave of long duration (100 msec or more). In this paper, the size of the negative field potential (extracellularly recorded EPSP) has been studied during and after periods of repetitive stimulation of the l.o.t.2. If two identical volleys were delivered to the l.o.t. the second evoked EPSP was not the same size as the conditioning EPSP. At brief conditioning intervals (up to 10 msec) the second (test) EPSP was smaller than the control. For conditioning intervals between 10 and 200 msec, the test EPSP was potentiated over the control. For long conditioning intervals (300 msec up to 5 sec) the test EPSP was again slightly smaller than the control EPSP. After a brief conditioning train, the depression of a test EPSP (elicited 300 msec or more after the conditioning train) was more pronounced and lasted longer. These changes of test EPSP size were attributed to the presence of two opposing processes: an initial potentiation superimposed on a more prolonged but less pronounced depression.3. During prolonged repetitive stimulation the final steady amplitude of an EPSP varied with the frequency of stimulation. At low frequencies (0.5-2/sec) the steady EPSP amplitude was 90-95% of the initial control amplitude. At moderate frequencies (5-20/sec) the steady EPSP amplitude was greater than the initial control. At high frequencies (above 20/sec) the steady amplitude of the EPSPs declined with increasing frequency of stimulation. Potentiation of EPSPs was observed early in a train of impulses when the stimulation frequency was 5-70/sec.4. After a large number of stimuli at frequencies from 20 to 100/sec the amplitude of individual, infrequently evoked, EPSPs passed through a phase of depression that lasted about 30 sec. This depression was followed by a phase of potentiation (post-tetanic potentiation). The amplitude and duration of post-tetanic potentiation appeared to depend on the characteristics of the conditioning train.5. The discussion compares the results obtained with those obtained for other mammalian synapses. It is suggested that the transmitter in the presynaptic terminals could be in three parts, (a) immediately available transmitter (b) conditionally available transmitter requiring a single nerve impulse for its availability and (c) main depot transmitter which replenishes the other two stores. Potentiation and depression of evoked EPSPs were interpreted in terms of changes in the amount of transmitter released by the test volley. According to this analysis, a fixed proportion (about 10%) of the immediately available transmitter is released by each nerve impulse.

Action Potentials↗

PubMed1986

Binding characteristics of the selective alpha 2-adrenoceptor antagonist [3H]idazoxan to rat olfactory cortex membranes.

[3H]Idazoxan binding to membranes prepared from rat olfactory cortex obeyed saturation kinetics and was to a single population of sites. Although the density of sites was dependent on the incubation medium, binding was of high affinity (KD approximately 5.5 nM) with a Hill coefficient close to unity. Competition studies with a range of adrenoceptor agonists and antagonists confirmed that [3H]idazoxan binding was to alpha 2-adrenoceptors. Neither chemical lesions with the neurotoxin kainic acid nor chronic unilateral bulbectomy significantly altered any of the [3H]idazoxan binding parameters. These findings suggest that alpha 2-adrenoceptors are not located on the lateral olfactory tract terminals or pyramidal cells of the olfactory cortex.

Adrenergic alpha-Antagonists↗

PubMed1977

Early cortical histogenesis in the primary olfactory cortex of the mouse.

The primary olfactory cortex of mouse embryos varying in age from the 13th to the 15th day of gestation is investigated with the aid of several techniques: Nissl and Golgi preparations, autoradiography and electron microscopy. It is observed that the earliest cells to complete their migrations are diffusely scattered at all levels of the emerging cortical plate. The dendrites of these cells are predominantly tangentially aligned. Subsequently these early formed cells are subdivided into two groups. One group remains superficial in close relation to the lateral olfactory tract while the second is displaced to the deepest level of the cortex. The dendrites of many cells which remain at superficial cortical levels become realigned in a radially upward direction. "Maculae adherens diminutae" are abundant at points of apposition of membranes of cells of all classes at superficial cortical levels. It is suggested that these serve to stabilize cell position and that the upward dendritic realignment is an expression of radially directed stresses. Synapses appear relatively late and probably play no role as determinants of cell position.

Animals↗

PubMed1984

The development of lamination of afferent fibers to the olfactory cortex in rats, with additional observations in the adult.

The complementary distribution of the fibers from the olfactory bulb and the intracortical associational fibers to layers Ia and Ib, respectively, of the olfactory cortex has been examined in both adult and neonatal rats, using horseradish peroxidase (HRP) and 3H-leucine as double tracers in the same animal. The observations presented here confirm and extend the previous demonstration (Price, '73) that in the adult the two projections are essentially nonoverlapping throughout the olfactory cortex. Indeed, when the distribution of axons from the olfactory bulb (labeled by HRP inserted into a cut in the LOT) is compared on the same section with that of associational fibers (labeled by 3H-leucine injected into the cortex), the overlap between the two projections is limited to a zone only 5-10 micron in width in both the piriform cortex and olfactory tubercle. In contrast, at P1 the two projections overlap throughout layer I, although the bulbar and associational fibers are slightly concentrated superficially and deeply in layer I, respectively. This overlap is especially prominent in the part of the anterior piriform cortex deep to the LOT. During the remainder of the first postnatal week, this overlap resolves and by P7 the segregation of the two sets of afferent fibers is nearly equivalent to that seen in the adult. However, there are several instances in adults where the segregation of these afferents does not develop. First, a relatively small population of aberrant axons derived from the LOT may be traced from layer Ia into layer Ib and then back to layer Ia. Most of these axons are large in diameter and lack the boutonlike varicosities found on smaller axons in layer Ia. They are most prominent in areas where the cortex is highly curved. Second, in layer I of the nucleus of the lateral olfactory tract, bulbar and associational fibers are extensively intermingled. In this case also, the bulbar fibers are large in diameter with only a few boutonlike varicosities. The developmental emergence of afferent segregation and its breakdown in cases where the fibers from the olfactory bulb do not form boutons suggest that an interaction between the two distinct sets of fibers and the dendritic field is responsible for the normal development of this segregation and that this interaction depends on the process of synaptogenesis.

Afferent Pathways↗

PubMed2000

A neural mechanism of hierarchical discrimination of odors in the olfactory cortex based on spatiotemporal encoding of odor information.

We propose a neural mechanism for discrimination of different complex odors in the olfactory cortex based on the dynamical encoding scheme. Both constituent molecules of the odor and their mixing ratios are encoded simultaneously into a spatiotemporal activity pattern (limit cycle attractor) in the olfactory bulb [Hoshino O, Kashimori Y, Kambara T (1998) Biol Cybern 79:109-120]. We present a functional model of the olfactory cortex consisting of some dynamical mapping modules. Each dynamical map is represented by itinerancy among the limit cycle attractors. When a temporal sequence of spatial activity patterns corresponding to a complex odor is injected from the bulb to the network of the olfactory cortex, the neural activity state of each mapping module is fixed to a relevant spatial pattern injected. Recognition of an odor is accomplished by a combination of firing patterns fixed in all the mapping modules. The stronger the response strength of the component, the earlier the component is recognized. The hierarchical discrimination of an odor is made by recognizing the components in order of decreasing response strengths.

Discrimination Learning↗

PubMed1984

Tangential organization of olfactory, association, and commissural projections to olfactory cortex in a species of reptile (Trionyx spiniferus), bird (Aix sponsa), and mammal (Tupaia glis).

Small amounts of tritiated leucine were injected into the olfactory bulb or anterior olfactory cortex of softshell turtles, wood ducks, and tree shrews in order to compare quantitatively the laminar distribution of olfactory bulb, association, and commissural projections to olfactory cortex. In all three species, a similar colaminar distribution of olfactory and association projections was found: the olfactory projections are restricted to the superficial cortical layer Ia, while the association projections are distributed into the deeper cortical layers Ib, II, and III. Differences among these three species were found in the origin and distribution of commissural projections. Whereas in tree shrews these fibers originate from third-order neurons and project into the deeper layers of the contralateral cortex (with the homolateral olfactory bulb projections), in softshell turtles and wood ducks, they originate from second-order neurons and project into the superficial layer of the contralateral cortex (with the homolateral olfactory bulb projections). These results, in conjunction with those obtained previously in other species, indicate that the basic tangential organization of mammalian olfactory cortex is retained, albeit with some modification, from a remote, reptilian ancestor.

Animals↗

PubMed1987

Synapse formation in the olfactory cortex by regenerating optic axons: ultrastructural evidence for polyspecific chemoaffinity.

The optic nerve was severed at its entry into the optic chiasma and implanted into the striatal region of the ipsilateral cerebral hemisphere in adult frogs (Rana pipiens). Regenerating retinal ganglion cell axons, observed by the autoradiographic tracing method and by horseradish peroxidase (HRP) fiber filling, grew anteriorly along the olfactory tracts and posteriorly along the ipsilateral lateral forebrain bundle and stria medullaris. Many of the regenerating axons ultimately joined the ipsilateral optic tract. The optic axons formed terminal plexuses in the olfactory cortex, lateral geniculate complex, pretectum, tectum, and basal optical nucleus but not in the amygdala or other cerebral territories not postsynaptic to the olfactory bulb, nor in the cell groups associated with the lateral forebrain bundle or stria medullaris. Optic axon terminals labeled with HRP were observed by electron microscopy in the ipsilateral olfactory cortex and in the normal projection areas of the optic nerve, although they were misplaced to the ipsilateral side. They contained clear, spherical synaptic vesicles and pale mitochondria and made Gray type I, asymmetric contacts on dendrites. The retinal projection to the olfactory cortex was formed early in regeneration and was maintained to some degree for periods up to 39 weeks. It was absent in a specimen surviving 50 weeks. Retinal innervation appeared earlier in the lateral geniculate complex and pretectum than in the tectum. These observations suggest that regenerating retinal ganglion cell axons have an affinity for neurons in the olfactory cortex, as well as for the neurons in the optic pathway to which they are normally postsynaptic. Unless the apparent selectivity of the aberrant projection is regulated by principles other than those that bring about the reinnervation of the normal optic centers, the data further suggest that the nature of the molecular mechanisms conveying synaptic specificity must be broad enough to permit the formation of limited sets of alternative synaptic connections. The ability to innervate selectively targets other than those normally specified is termed, here, polyspecificity. Since polyspecificity refers to the the affinity of retinal ganglion axons, as a class, for target structures considered as unit aggregates, it is conceptually different from the graded affinity of ganglion cells in different regions of the retina for target neurons in different regions of the tectum.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed2004

Learning-induced long-term synaptic modifications in the olfactory cortex.

The idea that memory is manifested at the cellular level by enhancement of synaptic connections between simultaneously activated neurons has been suggested half a century ago by Hebb, and is widely accepted since. Much effort is done to describe such enhancement and reveal the underlying mechanisms. Learning-induced synaptic modifications were studied in the last decade with in-vitro brain slices preparations. Several forms of long-term enhancement of synaptic connections between layer II pyramidal neurons in the piriform cortex accompany olfactory learning. Such modifications were described also in other brain areas, following other training paradigms. Post-synaptic enhancement of synaptic transmission is indicated by reduced rise time of (post synaptic potentials) PSPs and formation of new synaptic connections is indicated by increased spine density along dendrites of these neurons. Enhanced synaptic release is indicated by reduced paired-pulse facilitation. In slices from trained rats predisposition for long-term potentiation is decreased and predisposition for long-term depression is increased. These modifications are attributed to olfactory-discrimination rule learning, rather than to memories for specific odors, and may be subsequent to intrinsic modifications in pyramidal neurons that create favorable conditions for activity-dependent synaptic enhancement.

Animals↗

PubMed1982

Antagonism of gamma-aminobutyric acid and muscimol by picrotoxin, bicuculline, strychnine, bemegride, leptazol, D-tubocurarine and theophylline in the isolated olfactory cortex.

gamma-Aminobutyric acid (GABA) applied to neurones in the olfactory cortex slice in vitro, increases input conductance and produces a small depolarization, which mimics the action of the inhibitory transmitter. In previous experiments it was shown that this inhibition could be blocked by picrotoxin, bicuculline, strychnine, leptazol, bemegride, theophylline and d-tubocurarine. In the present study the effects of the above blockers on the action of bath-applied GABA were assessed. These blockers all antagonised the action of GABA at concentrations similar to those required to block synaptic inhibition. However, the amount of antagonism of GABA action was variable and this variability was attributed to the cellular uptake of GABA. The variability was circumvented by using muscimol, a GABA agonist not subjected to uptake. This GABA antagonism explains the convulsant action of many of the agents studied and reinforces the idea that GABA mediated inhibitory transmission in the olfactory cortex.

Animals↗

PubMedDate not supplied

Posttetanic and frequency potentiation in slices of rat olfactory cortex.

The electrical tetanization of the lateral olfactory tract at a frequency of 30/sec for 15 sec elicited the development of posttetanic potentiation of populational EPSP and IPSP in surviving slices of rat olfactory cortex. The stimulation of the lateral olfactory tract by series of stimuli at a constant frequency of 10/sec and with intervals of 4-5 sec between series facilitates the emergence of the phenomenon of frequency potentiation. The data obtained indicate that such forms of functional plasticity as posttetanic and frequency potentiation are manifested in the pyriform cortex.

Action Potentials↗

PubMed1988

Current generators and properties of late components evoked in rat olfactory cortex.

Following main olfactory bulb (MOB) stimulation at frequencies of 0.1-0.3 Hz, in addition to early field potentials, a frequency-sensitive, surface negative late N2 wave (latency range: 63-96 msec) followed occasionally by a late N3 transient, was evoked in the piriform cortex and endopiriform nucleus of the rat. The N2 wave inverted polarity at the Ib-II cortical layer interface (P2 wave) and was associated with late unit discharges 200 to 1200 microns deep to the turnover point. Response probability, peak latency, recovery curve and frequency-sensitivity of the P2 wave were not significantly different in animals under urethane or pentobarbital. Current-source-density (CSD) analysis revealed that the N2 wave generators were localized to the Ib-II layer interface. Since inhibitory activity does not contribute substantially to the second derivative curve, CSD analysis strengthens the assumption that late components (LCs) are excitatory events (compound EPSPs) presumably generated on the proximal apical dendritic segments of pyramidal cells by association axons. The early "b" wave in a test response was facilitated, rather than occluded, when a LC was present in the conditioning response, or when the priming volley was delivered to the mediodorsal thalamic nucleus. Clustering of unit and field activity in two distinct periods of the evoked response separated by a prolonged interval of cell silence suggests that cortical coding of olfactory cues might be more efficiently achieved by temporal modulation of the neuronal response rather than by spatial distribution of firing patterns.

Animals↗

PubMed1986

Ca-channel blockers and the electrophysiology of synaptic transmission of the guinea-pig olfactory cortex.

Slices of guinea-pig olfactory cortex have been used to compare the potency of various Ca-blockers on the electrophysiology of synaptic transmission. Listed in the order of potency, the divalent cations Cd2+, Ni2+, Mn2+, Co2+, La3+ and Mg2+ depressed synaptic transmission. The organic Ca-blockers, nifedipine or nimodipine or verapamil and diltiazem were ineffective up to 0.01 mmol/l. Verapamil, D600 or diltiazem (0.1-0.3 mmol/l) depressed both synaptic transmission and the sodium-mediated presynaptic action potential. These results reaffirm the idea that 'organic Ca-antagonist' do not block all Ca-channels in brain and the high Cd2+ sensitivity suggests the Ca-channels in post- and presynaptic membranes have dissimilar pharmacological profiles.

Animals↗

PubMed2000

Time course of odorant-induced activation in the human primary olfactory cortex.

Paradoxically, attempts to visualize odorant-induced functional magnetic resonance imaging (fMRI) activation in the human have yielded activations in secondary olfactory regions but not in the primary olfactory cortex-piriform cortex. We show that odorant-induced activation in primary olfactory cortex was not previously made evident with fMRI because of the unique time course of activity in this region: in primary olfactory cortex, odorants induced a strong early transient increase in signal amplitude that then habituated within 30-40 s of odorant presence. This time course of activation seen here in the primary olfactory cortex of the human is almost identical to that recorded electrophysiologically in the piriform cortex of the rat. Mapping activation with analyses that are sensitive to both this transient increase in signal amplitude, and temporal-variance, enabled us to use fMRI to consistently visualize odorant-induced activation in the human primary olfactory cortex. The combination of continued accurate odorant detection at the behavioral level despite primary olfactory cortex habituation at the physiological level suggests that the functional neuroanatomy of the olfactory response may change throughout prolonged olfactory stimulation.

Adult↗

PubMed1985

Spontaneous inhibitory postsynaptic potentials in guinea pig neocortex and olfactory cortex neurones.

The membrane potential of olfactory cortex and neocortex neurones in vitro was recorded using conventional microelectrode techniques. During recordings with KCl- or CsCl-filled microelectrodes, spontaneous, subthreshold, transient membrane depolarizations were observed. These were abolished by the GABAA-receptor antagonist, bicuculline methiodide, and were prolonged by the barbiturate pentobarbitone. In most cells they were abolished by tetrodotoxin. It is concluded that these spontaneous depolarizations are inhibitory postsynaptic potentials arising from spontaneous activity in inhibitory interneurones.

Animals↗

PubMed1986

An analysis of the action of pentobarbitone on the excitatory postsynaptic potentials and membrane properties of neurones in the guinea-pig olfactory cortex.

Intracellular recordings were made from neurones in slices of guinea-pig olfactory cortex maintained in vitro at 37 degrees C. The average membrane potential was 63 +/- 12 mV and the input resistance of these cells was 42 +/- 20 M omega (mean +/- s.d.). Stimulation of the lateral olfactory tract (l.o.t.) generated a transient depolarization in these cells which had the characteristics of an excitatory postsynaptic potential (e.p.s.p.). If the e.p.s.p. was of sufficient amplitude it culminated in an action potential. The e.p.s.p. was potentiated by repetitive stimulation at 10-50 Hz and showed post-tetanic potentiation after a prolonged period of high frequency stimulation (50-100 Hz for 30-60 s). Pentobarbitone (0.1-0.5 mM) depressed the e.p.s.p. reversibly but was without effect on the resting membrane potential, input resistance or time constant of the neurones. Pentobarbitone did not inhibit potentiation of the e.p.s.p. by a preceding conditioning shock. It is concluded that pentobarbitone does not affect the passive membrane properties of neurones in the olfactory cortex. The depressant action of pentobarbitone on synaptic transmission results from a decrease in the amount of transmitter released in response to a nerve impulse, or a decrease in the sensitivity of the postsynaptic membrane to the transmitter or a combination of both effects.

Animals↗

PubMed1986

Sites and mechanisms of action of catechol (1,2-dihydroxybenzene) in the rat olfactory cortex slice.

Synaptic transmission in the isolated olfactory cortex slice from the rat was monitored by recording the surface field potentials evoked on lateral olfactory tract (LOT) stimulation. Catechol (approximately 0.05 to 2 mM) caused a concentration-dependent, partially reversible increase in the amplitudes of all field potentials. In a series of conditioning experiments, catechol (1 mM) potentiated postsynaptic inhibition by a mechanism which was at least partially picrotoxin-insensitive. When the relationship between the stimulus input and evoked output was investigated in picrotoxin-treated slices, for a given tract action potential amplitude, catechol (0.25 and 0.5 mM) increased the amplitude of the field potential known as the N-wave; in contrast, for a given N-wave amplitude, the latency of the population spike was increased. Catechol (1 mM) increased the K+-evoked release of endogenous aspartate by a tetrodotoxin-insensitive mechanism whereas the release of glutamate and gamma-aminobutyric acid (GABA) was unaffected. Catechol (1 mM) had no effect on submaximal depolarizations evoked by L-aspartate, L-glutamate or GABA. It is concluded that catechol potentiates excitatory transmission at the LOT-superficial pyramidal cell synapse, possibly by increasing evoked transmitter release. Other synaptic actions of catechol may be consequent upon this increased excitatory input but the results do not exclude the possibility of separate and distinct actions on polysynaptic transmission.

Action Potentials↗