Biomedical subjects

Michael T Shipley

Publications and source records attributed to Michael T Shipley.

10 recordsLinked to original sources

PubMed2006

Coding and synaptic processing of sensory information in the glomerular layer of the olfactory bulb.

Input from olfactory receptor neurons is first organized and processed in the glomerular layer of the olfactory bulb. Olfactory glomeruli serve as functional units in coding olfactory information and contain a complex network of synaptic connections. Odor information has long been thought to be represented by spatial patterns of glomerular activation; recent work has, additionally, shown that these patterns are temporally dynamic. At the same time, recent advances in our understanding of the glomerular network suggest that glomerular processing serves to temporally sharpen these dynamics and to modulate spatial patterns of glomerular activity. We speculate that odor representations and their postsynaptic processing are tuned to and shaped by the sniffing behavior of the animal.

Animals↗

PubMed2005

Olfactory bulb external tufted cells are synchronized by multiple intraglomerular mechanisms.

In rat olfactory bulb slices, external tufted (ET) cells spontaneously generate spike bursts. Only ET cells affiliated with the same glomerulus exhibit significant synchronous activity, suggesting that synchrony results mainly from intraglomerular interactions. The intraglomerular mechanisms underlying their synchrony are unknown. Using dual extracellular and patch-clamp recordings from ET cell pairs of the same glomerulus, we found that the bursting of ET cells is synchronized by several mechanisms. First, ET cell pairs of the same glomerulus receive spontaneous synchronous fast excitatory synaptic input that can also be evoked by olfactory nerve stimulation. Second, they exhibit correlated spontaneous slow excitatory synaptic currents that can also be evoked by stimulation of the external plexiform layer. These slow currents may reflect the repetitive release of glutamate via spillover from the dendritic tufts of other ET or mitral/tufted cells affiliated with the same glomerulus. Third, ET cells exhibit correlated bursts of inhibitory synaptic activity immediately after the synchronous fast excitatory input. These bursts of IPSCs were eliminated by CNQX and may therefore reflect correlated feedback inhibition from periglomerular cells that are driven by ET cell spike bursts. Fourth, in the presence of fast synaptic blockers, ET cell pairs exhibit synchronous slow membrane current oscillations associated with rhythmic spikelets, which were sensitive to the gap junction blocker carbenoxolone. These findings suggest that coordinated synaptic transmission and gap junction coupling synchronize the spontaneous bursting of ET cells of the same glomerulus.

Animals↗

PubMed2005

Inhibition [corrected] of olfactory receptor neuron input to olfactory bulb glomeruli mediated by suppression of presynaptic calcium influx.

We investigated the cellular mechanism underlying presynaptic regulation of olfactory receptor neuron (ORN) input to the mouse olfactory bulb using optical-imaging techniques that selectively report activity in the ORN presynaptic terminal. First, we loaded ORNs with calcium-sensitive dye and imaged stimulus-evoked calcium influx in a slice preparation. Single olfactory nerve shocks evoked rapid fluorescence increases that were largely blocked by the N-type calcium channel blocker omega-conotoxin GVIA. Paired shocks revealed a long-lasting suppression of calcium influx with approximately 40% suppression at 400-ms interstimulus intervals and a recovery time constant of approximately 450 ms. Blocking activation of postsynaptic olfactory bulb neurons with APV/CNQX reduced this suppression. The GABA(B) receptor agonist baclofen inhibited calcium influx, whereas GABA(B) antagonists reduced paired-pulse suppression without affecting the response to the conditioning pulse. We also imaged transmitter release directly using a mouse line that expresses synaptopHluorin selectively in ORNs. We found that the relationship between calcium influx and transmitter release was superlinear and that paired-pulse suppression of transmitter release was reduced, but not eliminated, by APV/CNQX and GABA(B) antagonists. These results demonstrate that primary olfactory input to the CNS can be presynaptically regulated by GABAergic interneurons and show that one major intracellular pathway for this regulation is via the suppression of calcium influx through N-type calcium channels in the presynaptic terminal. This mechanism is unique among primary sensory afferents.

2-Amino-5-phosphonovalerate↗

PubMed2004

Transmembrane dye labeling and immunohistochemical staining of electrophysiologically characterized single neurons.

Numerous studies have used whole-cell patch recording to characterize the electrophysiology of neurons and, via intracellular dye filling, the detailed morphology of the same cells. However, it has been difficult to demonstrate the presence of small soluble molecules within such cells, because washout of the soluble contents of the cell into the patch pipette precludes their later detection by immunohistochemistry. This leaves a major gap in our understanding of circuits made up of neurochemically heterogeneous neurons. To bridge this gap we have developed a transmembrane labeling approach, employing membrane-permeant dye in conjunction with perforated patch electrophysiology. Using this method we have successfully recorded from juxtaglomerular cells in the olfactory bulb, reconstructed the morphology of the cells, and demonstrated expression of soluble neurochemical markers within the same cells. This new technique provides a reliable means to link the physiology, morphology, and neurochemistry of single identified neurons studied using patch-clamp recording.

Animals↗

PubMed2004

External tufted cells: a major excitatory element that coordinates glomerular activity.

The glomeruli of the olfactory bulb are the first site of synaptic processing in the olfactory system. The glomeruli contain three types of neurons that are referred to collectively as juxtaglomerular (JG) cells: external tufted (ET), periglomerular (PG), and short axon (SA) cells. JG cells are thought to interact synaptically, but little is known about the circuitry linking these neurons or their functional roles in olfactory processing. Single and paired whole-cell recordings were performed to investigate these questions. ET cells spontaneously fired rhythmic spike bursts in the theta frequency range and received monosynaptic olfactory nerve (ON) input. In contrast, all SA and most PG cells lacked monosynaptic ON input. PG and SA cells exhibited spontaneous, intermittent bursts of EPSCs that were highly correlated with spike bursts of ET cells in the same but not in different glomeruli. Paired recording experiments demonstrated that ET cells provide monosynaptic excitatory input to PG/SA cells; the ET to PG/SA cell synapse is mediated by glutamate. ET cells thus are a major excitatory linkage between ON input and other JG cells. Spontaneous bursting is highly correlated among ET cells of the same glomerulus, and ET cell activity remains correlated when all fast synaptic activity is blocked. The findings suggest that multiple, synchronously active ET cells synaptically converge onto single PG/SA cells. Synchronous ET cell bursting may function to amplify transient sensory input and coordinate glomerular output.

Action Potentials↗

PubMed2004

Olfactory bulb glomeruli: external tufted cells intrinsically burst at theta frequency and are entrained by patterned olfactory input.

Glomeruli, the initial sites of synaptic processing in the olfactory system, contain at least three types of neurons collectively referred to as juxtaglomerular (JG) neurons. The role of JG neurons in odor processing is poorly understood. We investigated the morphology, spontaneous, and sensory-evoked activity of one class of JG neurons, external tufted (ET) cells, using whole-cell patch-clamp and extracellular recordings in rat olfactory bulb slices. ET cells have extensive dendrites that ramify within a single glomerulus or, rarely, in two adjacent glomeruli. All ET neurons exhibit spontaneous rhythmic bursts of action potentials (approximately 1-8 bursts/sec). Bursting is intrinsically generated; bursting persisted and became more regular in the presence of ionotropic glutamate and GABA receptor antagonists. Burst frequency is voltage dependent; frequency increased at membrane potentials depolarized relative to rest and decreased during membrane potential hyperpolarization. Spontaneous bursting persisted in blockers of calcium channels that eliminated low-threshold calcium spikes (LTS) in ET cells. ET cells have a persistent sodium current available at membrane potentials that generate spontaneous bursting. Internal perfusion with a fast sodium channel blocker eliminated spontaneous bursting but did not block the LTS. These results suggest that persistent sodium channels are essential for spontaneous burst generation in ET cells. ET cell bursts were entrained to ON stimuli delivered over the range of theta frequencies. Thus, ET cells appear to be tuned to the frequency of sniffing.

Action Potentials↗

PubMed2003

Afferent connections of the parvocellular subparafascicular thalamic nucleus in the rat: evidence for functional subdivisions.

The parvocellular subparafascicular nucleus of the thalamus (SPFp) consists of separate subdivisions, i.e., a medial portion containing galanin-immunoreactive (-IR) axons and a lateral portion containing calcitonin gene related peptide (CGRP)-IR neurons and fibers. These subdivisions appear to have distinct functional roles. In particular, ejaculation-induced Fos expression is expressed in the medial SPFp. Hence, it was hypothesized that medial SPFp is involved in relay of copulation-related information. In contrast, lateral SPFp is involved in the processing of auditory and visual signals involved in fear-conditioned responses. Here we tested the hypothesis that medial and lateral subdivisions of SPFp receive different sets of afferents and that these differences contribute to the separate functional roles of the two subdivisions. Inputs to medial and lateral SPFp were identified following injections of FG restricted to either division in male rats. The medial SPFp receives unique inputs from lumbar spinothalamic cells and brain regions involved in processing of visceral stimuli, supporting the hypothesis that the medial SPFp is involved in the relay of genitosensory information critical for the expression of male sexual behavior. The afferents of the lateral SPFp include brain regions involved in processing of visual and auditory signals and support a role for this subdivision in relay of visual and auditory information. Thus, the two subdivisions of SPFp are anatomically and functionally distinctive.

Afferent Pathways↗

PubMed2003

Parvocellular subparafascicular thalamic nucleus in the rat: anatomical and functional compartmentalization.

The parvocellular subparafascicular thalamic nucleus (SPFp) is located in the posterior thalamus, consists of horizontally oriented cells, and extends from rostromedial to caudolateral, fusing with the posterior intralaminar nucleus and the peripeduncular nucleus. The present study demonstrates a chemoarchitechtonic and functional parcellation of the rat SPFp. Analysis of the distributions of the neuropeptides galanin, calcitonin gene related peptide (CGRP), substance P, and calbindin revealed the existence of a medial and lateral subdivision within SPFp, and a possible intermediate subdivision. The medial subdivision contains a dense population of galanin-immunoreactive fibers, originating from galanin neurons in the lumbosacral spinal cord. In contrast, the lateral subdivision contains CGRP-positive fibers and neurons. The presence of substance P and calbindin immunoreactivity throughout the entire nucleus suggests that these are separate subdivisions of SPFp, rather than different subnuclei. The present study also investigated the functional association of the separate subdivisions of SPFp for male and female rat sexual behavior. In the medial subdivision, Fos-positive neurons were activated in males by display of ejaculation and in females by vaginocervical stimulation. Thus, Fos induction in medial SPFp appears to reflect processing of inputs related to those events. In contrast, sexual behavior did not induce Fos in the lateral SPFp. Taken together, the present results indicate the existence of separate subdivisions in SPFp that are involved in different behavioral functions. The medial SPFp may process inputs important for sexual behavior, whereas the lateral SPFp may be involved in convergence of auditory and nociceptive inputs important for conditioned fear responses.

Animals↗

PubMed2003

A device for automated control of pipette internal pressure for patch-clamp recording.

Formation of a high-resistance seal between the tip of a glass recording pipette and the membrane of the recorded cell is the crucial step in patch clamping, or whole cell recording with patch pipettes. Formation of the seal, and subsequent rupture of the membrane for whole cell recording, requires a specific sequence of changes in pipette internal hydrostatic pressure. Generating this sequence of pressure changes adds to the complexity of setting up, gaining proficiency, and performing experiments. Automation of routine pipette pressure manipulations would simplify seal formation, and benefit productivity. Here we describe a device that automates control of patch pipette internal pressure. Solenoid valves sequentially operated by manual switching, or external electronic control, automatically provide the necessary sequence of connections to the pipette interior. This greatly simplifies the operations performed to obtain membrane seals and whole cell recordings and improves standardization and reproducibility in patch recording.

Electronics↗

PubMed2003

Activation of a subset of lumbar spinothalamic neurons after copulatory behavior in male but not female rats.

The precise pathways that convey copulation-related information to forebrain regions activated during male and female sexual behavior are poorly understood. Previous work from our laboratory and others has demonstrated the existence of a spinothalamic pathway that is a candidate to relay information to these areas. This pathway originates from a population of spinothalamic neurons in the lumbar spinal cord containing several neuropeptides including galanin, located in laminas 7 and 10 of the lumbar segments 3 and 4. To investigate the involvement of these lumbar spinothalamic neurons in conveying copulation-related information, we tested the hypothesis that these cells are activated after ejaculation in male rats and vaginocervical stimulation in female rats. This was assessed using galanin or cholecystokinin as a marker for this subset of spinothalamic neurons and Fos-immunoreactivity as a marker for neuronal activation. The results demonstrated that activation of these spinothalamic neurons is triggered by stimuli associated with ejaculation. Fos induction was specifically associated with ejaculation, because mounts or intromissions did not trigger expression. Moreover, these spinothalamic neurons were not activated by vaginocervical stimulation in female rats. Spinothalamic neurons have generally been associated with signaling pain and temperature information. The present findings demonstrate that a specific subpopulation of spinothalamic neurons signals information associated with ejaculation.

8-Hydroxy-2-(di-n-propylamino)tetralin↗