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PubMed2000

Morphology and physiology of auditory and vibratory ascending interneurones in bushcrickets.

Auditory/vibratory interneurones of the bushcricket species Decticus albifrons and Decticus verrucivorus were studied with intracellular dye injection and electrophysiology. The morphologies of five physiologically characterised auditory/vibratory interneurones are shown in the brain, subesophageal and prothoracic ganglia. Based on their physiology, these five interneurones fall into three groups, the purely auditory or sound neurones: S-neurones, the purely vibratory V-neurones, and the bimodal vibrosensitive VS-neurones. The S1-neurones respond phasically to airborne sound whereas the S4-neurones exhibit a tonic spike pattern. Their somata are located in the prothoracic ganglion and they show an ascending axon with dendrites located in the prothoracic, subesophageal ganglia, and the brain. The VS3-neurone, responding to both auditory and vibratory stimuli in a tonic manner, has its axon traversing the brain, the suboesophageal ganglion and the prothoracic ganglion although with dendrites only in the brain. The V1- and V2-neurones respond to vibratory stimulation of the fore- and midlegs with a tonic discharge pattern, and our data show that they receive inhibitory input suppressing their spontaneous activity. Their axon transverses the prothoracic ganglion, subesophageal ganglion and terminate in the brain with dendritic branching. Thus the auditory S-neurones have dendritic arborizations in all three ganglia (prothoracic, subesophageal, and brain) compared to the vibratory (V) and vibrosensitive (VS) neurones, which have dendrites almost only in the brain. The dendrites of the S-neurones are also more extensive than those of the V-, VS-neurones. V- and VS-neurones terminate more laterally in the brain. Due to an interspecific comparison of the identified auditory interneurones the S1-neurone is found to be homologous to the TN1 of crickets and other bushcrickets, and the S4-neurone also can be called AN2. J. Exp. Zool. 286:219-230, 2000.

Acoustic Stimulation↗
PubMed1997

Neuroblast ablation in Drosophila P[GAL4] lines reveals origins of olfactory interneurons.

Hydroxyurea (HU) treatment of early first instar larvae in Drosophila was previously shown to ablate a single dividing lateral neuroblast (LNb) in the brain. Early larval HU application to P[GAL4] strains that label specific neuron types enabled us to identify the origins of the two major classes of interneurons in the olfactory system. HU treatment resulted in the loss of antennal lobe local interneurons and of a subset of relay interneurons (RI), elements usually projecting to the calyx and the lateral protocerebrum (LPR). Other RI were resistant to HU and still projected to the LPR. However, they formed no collaterals in the calyx region (which was also ablated), suggesting that their survival does not depend on targets in the calyx. Hence, the ablated interneurons were derived from the LNb, whereas the HU-resistant elements originated from neuroblasts which begin to divide later in larval life. Developmental GAL4 expression patterns suggested that differentiated RI are present at the larval stage already and may be retained through metamorphosis.

Animals↗
PubMed1996

Homosynaptic and heterosynaptic changes in driving of dentate gyrus interneurons after brief tetanic stimulation in vivo.

This study addressed changes in interneuron driving in the dentate gyrus (DG) of urethane-anaesthetized rats in response to tetanic stimulation of the perforant path (PP) or the converging dentate commissural pathway (CP). Using an extracellular tungsten electrode, we recorded from putative interneurons in the DG that fired to stimulation of both the PP and the CP. Conditioning trains (400 Hz, 17.5 ms) were delivered to each pathway individually and to the two pathways together. The primary measure of synaptic drive was the latency of interneuron discharge. High-intensity PP tetany, CP tetany, and paired tetany consistently reduced firing latency to CP driving (P < .05 for all three), indicating an LTP-like increase in synaptic activation through the CP. High-intensity PP tetany decreased latency to PP driving in only two of seven cases. Heterosynaptic changes occurred frequently in individual experiments. Activity-mediated adjustments in synaptic driving of inhibitory interneurons could play a role in normal physiological function.

Animals↗
PubMed1998

Neurocalcin-immunoreactive cells in the rat hippocampus are GABAergic interneurons.

Neurocalcin (NC) is a recently described calcium-binding protein isolated and characterized from bovine brain. NC belongs to the neural calcium-sensor proteins defined by the photoreceptor cell-specific protein recoverin that have been proposed to be involved in the regulation of calcium-dependent phosphorylation in signal transduction pathways. We analyzed the distribution and morphology of the NC-immunoreactive (IR) neurons in the rat dorsal hippocampus and the coexistence of NC with GABA and different neurochemical markers which label perisomatic inhibitory cells [parvalbumin (PV) and cholecystokinin (CCK)], mid-proximal dendritic inhibitory cells [calbindin D28k (CB)], distal dendritic inhibitory cells [somatostatin (SOM) and neuropeptide Y (NPY)], and interneurons specialized to innervate other interneurons [calretinin (CR) and vasoactive intestinal polypeptide (VIP)]. NC-IR cells were present in all layers of the dentate gyrus and hippocampal fields. In the dentate gyrus, NC-IR cells were concentrated in the granule cell layer, especially in the hilar border, whereas in the CA fields they were most frequently found in the stratum radiatum. NC-IR cells were morphologically heterogeneous and exhibited distinctive features of non-principal cells. In the dentate gyrus, pyramidal-like, multipolar and fusiform (horizontal and vertical) cells were found. In the CA3 region most NC-IR cells were multipolar, but vertical and horizontal fusiform cells also appeared. In the CA1 region, where NC-IR cells showed most frequently vertically arranged dendrites, multipolar, bitufted and fusiform (vertical and horizontal) cells could be distinguished. All the NC-IR cells were found to be GABA-IR in all hippocampal layers and regions, and they represented about 19% of the GABA-positive cells. NC/CB, NC/CR and NC/VIP double-labeled cells were found in all hippocampal regions, and represented 29%, 24% and 18% of the NC-IR cells, respectively. NC and CCK did not coexist in the dentate gyrus; however, 9% of the NC-IR cells in the CA fields also contained CCK. No coexistence of NC with PV, SOM or NPY was found in any hippocampal region. We conclude that NC is exclusively expressed by interneurons in the rat hippocampus. NC-IR cells are a morphologically and neurochemically heterogeneous subset of GABAergic non-principal cells, which, on the basis of the known termination pattern of the colocalizing markers, are also functionally heterogeneous and are mainly involved in feed-forward dendritic inhibition in the commissural-associational and Schaffer collateral termination zones (CB containing cells), in innervation of other interneurons (CR- and VIP-containing cells), and in perisomatic inhibition (CCK-containing cells). NC is never present in perisomatic inhibitory PV-containing cells, or in feed-back distal dendritic inhibitory SOM/NPY-containing cells.

Animals↗
PubMed2000

Synaptic plasticity in morphologically identified CA1 stratum radiatum interneurons and giant projection cells.

Long-term potentiation (LTP) of synaptic efficacy was examined in interneurons and giant cells in the stratum radiatum region of the hippocampal CA1 subfield. Cells were visually selected using differential interference contrast (DIC) optics and filled with biocytin while being recorded using whole-cell patch-clamp techniques. Electrophysiological criteria, including spike height, width, and degree of spike adaptation shown to sustained depolarization, proved inadequate for differentiating interneurons from giant cells. We found that cells in the stratum radiatum, however, could be reliably differentiated using DIC optics or following intracellular staining. The response of the two cell types to tetanic stimulation further dissociated them. Long-term potentiation, dependent on the activation of NMDAr (N-methyl-D-aspartate receptors), could reliably be induced in interneurons with stimuli administered at 200 Hz, but not 100 Hz. Giant cells, in contrast, exhibited NMDA receptor-dependent LTP in response to 100-Hz stimuli, but not the 200-Hz stimuli. LTP induction in interneurons also appeared temperature-dependent, being much more robust at 34 degrees C than at room temperature. The LTP in both cell types required postsynaptic calcium influx, and was not due to the passive propagation of LTP induction in neighboring pyramidal cells. These results suggest that different cell types within the hippocampal formation may preferentially alter synaptic connectivity in a frequency-specific manner.

Action Potentials↗
PubMed1992

Association interneurons of embryonic rat spinal cord transiently express the cell surface glycoprotein SNAP/TAG-1.

SNAP/TAG-1 is a 135 kDa glycoprotein of the immunoglobulin superfamily that is transiently expressed upon the surfaces of developing axons. In the embryonic rodent spinal cord, this molecule is expressed by motor neurons, dorsal root ganglion cells, and commissural neurons (Yamamoto et al.: J. Neurosci. 6:3576-3594, 1986; Dodd et al.: Neuron 1:105-116, 1988). The commissural cells are a subset of early-forming dorsal horn interneurons whose axons follow a circumferential course in the embryonic spinal cord. The axons of commissural neurons cross the developing ventral commissure to terminate on contralateral synaptic targets, whereas those of the other subset of circumferential cells, the association interneurons, remain on the same side of the spinal cord to form ipsilateral, terminal synaptic fields. The difference between the axonal trajectories of these two subsets of nerve cells raised the question of whether or not association interneurons would also express the SNAP/TAG-1 epitope and, if so, how would this expression be related to that of the commissural cells. Immunocytochemistry for SNAP/TAG-1 and choline acetyltransferase (ChAT) was used to answer these questions. The results indicated that association interneurons expressed SNAP/TAG-1 epitopes and that this expression began later and lasted longer than that of the commissural neurons. Other new findings of this study included the identification of a lateral subgroup of commissural fibers that expressed SNAP/TAG-1 later than their more medially located counterparts, and these lateral fibers were more pronounced in the thoracic spinal cord than at cervical levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1998

Striatal spiny neurons and cholinergic interneurons express differential ionotropic glutamatergic responses and vulnerability: implications for ischemia and Huntington's disease.

Striatal spiny neurons are selectively vulnerable in Huntington's disease (HD) and ischemia, whereas large aspiny (LA) cholinergic interneurons of the striatum are spared in these pathological conditions. We have investigated whether a different sensitivity to ionotropic glutamatergic agonists might account for this differential vulnerability. Intracellular recordings were obtained from morphologically identified striatal spiny neurons and LA cholinergic interneurons by using a rat brain slice preparation. The two striatal neuronal subtypes had strikingly different intrinsic membrane properties. Both subtypes responded to cortical stimulation with excitatory postsynaptic potentials: these potentials, however, had a different time course and pharmacology in the two classes of cells. Interestingly, membrane depolarizations and inward currents produced by exogenous glutamate receptor agonists (AMPA, kainate, and NMDA) were remarkably larger in spiny neurons than in LA interneurons. Moreover, concentrations of agonists producing reversible membrane changes in LA interneurons caused irreversible depolarizations in spiny cells. Our data suggest that the different physiological responses induced by the activation of ionotropic glutamate receptors may account for the cell type-specific vulnerability of striatal neurons in ischemia and HD.

Animals↗
PubMed2005

Calcium-binding proteins define interneurons in HVC of the zebra finch (Taeniopygia guttata).

Nucleus HVC of the avian song system is essential to song patterning and is a prime site for auditory-vocal integration important to vocal learning. These processes require precise, high-frequency action potential activity, which, in other systems, is often correlated with the expression of calcium-binding proteins. To characterize any such functional specializations in HVC, we retrogradely labeled projection neurons innervating HVC's known targets, namely, area X or nucleus robustus arcopallialis (RA), then stained HVC sections with antibodies to the calcium-binding proteins parvalbumin, calbindin, and calretinin. Under epifluorescent illumination, neither projection neuron type exhibited detectable levels of calcium-binding protein immunoreactivity, whereas a third cell type, made up of nonprojection neurons (interneurons), was immunopositive for one, two, or all three of the calcium-binding proteins. In fact, most of these interneurons were either doubly or triply labeled. To explore the link between the electrical and calcium-binding protein properties of individual HVC neurons, we used intracellular methods in brain slices to record from identified HVC cell types based on their intrinsic electrical properties. Intracellular neurobiotin combined with immunostaining revealed that fast-spiking interneurons, but not the slower-spiking projection neurons, were positive for one or more calcium-binding proteins. Confocal microscopy confirmed these results and also revealed that RA-projecting cells might contain very low levels of parvalbumin. These results indicate that HVC interneurons are specialized in their calcium-binding proteins and suggest how it might be possible to resolve the details of HVC microcircuits underlying song selectivity and auditory-vocal learning.

Animals↗
PubMed1976

Recovery from deafferentation by cricket interneurons after reinnervation by their peripheral field.

The physiological and morphological recovery of cricket (Acheta domesticus) abdominal giant interneurons following varying periods of deafferentation and subsequent regeneration was examined. The principal afferent input to two identified interneurons was removed by surgically ablating an abdominal sensory appendage, the cercus. Deafferentation restricts the growth of the dendrites of the medial giant interneuron. Reinnervation by the peripheral sensory field leads to recovery of the dendrite length which is dependent on the time allowed for recovery. The response properties of the reinnervated neurons never completely recovers irregardless of how short the period of deafferentation. Reinnervated neurons respond more weakly to standard tones than do control neurons. This is due in part to faciliation of an inhibitory synaptic input which is activated by the control cercus. The results suggest that the balance between the excitatory and the inhibitory synaptic inputs to these interneurons is irrevocably altered by brief periods of deafferentation early in life.

Animals↗
PubMed1977

Effects of low-level x-irradiation on cat cerebella at different postnatal intervals. III. Changes in the morphology of interneurons in the molecular layer.

The whole head of infant kittens was irradiated with fractionated doses of 150 R and 200 R at different postnatal intervals. Experimental age conditions consisted of a newborn, 1-week, 2-week, 3-week, and a 4-week age condition while the age of sacrifice remained constant at 70 days. Going analysis revealed that the interneurons found in the molecular layer of the newborn, 1-week and 2-week condition were basket cells. Stellate cells were found in increasing numbers in the 3-week and 4-week conditions. Basket calls were found to occupy the entire molecular layer in the newborn and 1-week conditions resulting in an abnormal axonal plexus. The position of the basket cell soma was found to be in the same plane as its axonal projection. The branching point of the primary dendrite of Purkinje cells was found to correlate with the amount of molecular layer occupied by the abnormal basket cell plexus. Interneurons in the molecular layer were found to show different dendritic growth patterns dependent upon where their soma was located. Interneurons in the deep molecular layer showed only a decreased dendritic field, whereas interneurons in the middle and superficial moledular layer had an abnormal growth of dendrites into the deep molecular layer. A hypothesis is presented to account for the decreased dendritic arborization and the increased length of the primary dendrite before branching, the ectopic basket cell and their abnormally directed dendritic growth, and the difference in behavioral deficits between the newborn and the 1-week conditions.

Age Factors↗
PubMed1980

Excitation and inhibition of marginal layer and interstitial interneurons in cat nucleus caudalis by mechanical stimuli.

In cats anesthetized with urethane, the caudal medulla was stabilized in preparation for intracellular recording from interstitial neurons in the descending tract of the trigeminal nerve and from neurons in lamina I of nucleus caudalis. Glass micropipets (10-50 M ohms) were advanced from the surface to a maximum depth of 350 micrometer. When DC potential shifts occurred, it was found that mechanical stimuli to the face generated bursts of positive-going spikes, followed in some cases by inhibitory postsynaptic potentials (IPSPs). Subdermal electrical stimulation of the face in each receptive field almost always enabled the same neuron to be driven electrically. Recordings were classified as from primary afferent fibers or from interneurons. Primary fibers had a purely positive spike, with a latency varying by no more than 0.05 msec, and could follow stimulation at 500 Hz. The mean latency for the fibers was 1.87 +/- 0.06 msec (n=75), and their absolute refractory period was 0.42 +/- 0.02 msec (n=36). Recordings were classed as from interneurons if there was an IPSP or the latency was at least 4 msec, with a variation of latency of at lest 0.5 msec. Responses thought to be monosynaptically driven had a mean latency of 2.09 +/0 0.07 msec (n=32) and could follow pairs of stimuli at a mean minimum interval of 0.70 +/- 0.06 msec (n=20). Responses thought to be polysynaptically driven had a mean latency of 7.9 +/- 1.08 msec (n=49) and a mean interstimulus interval of 2.96 +/- 0.84 (n=20). Most responses were generated by brushing the face (n=87), some by pressure on the face (n=25), and a few by pinching the skin of the face (n=6). Interneuron responses were most commonly recorded in the first 200 micrometer of the descending tract, and this position was confirmed by the injection of pontamine sky blue and the examination of frozen sections. The recordings were thought to be from dendrites of marginal and interstitial cells or the somas of interstitial cells. The IPSPs which followed spike potentials could only follow stimuli at 10 Hz or less. The failure appeared to be at a primary afferent synapse upon an inhibitory interneuron. Collision tests between mechanically evoked and electrically evoked responses showed long-lasting inhibition of the response to electrical stimulation after collision. Presynaptic inhibition exerted on the primary afferent excitation was suggested as the explanation.

Animals↗
PubMed1987

Segmental origins of the cricket giant interneuron system.

The segmental origins of the cricket giant interneuron system have been studied by staining these neurons with cobalt during the last half of embryonic development. The results demonstrate that the interneurons are derived from three distinct clusters of embryonic neurons that form a serially repeating pattern in each abdominal ganglion. Some of the neurons previously described in adults (Mendenhall and Murphey, '74; Murphey, '85) have been identified in embryos and are described here with respect to this pattern. These neurons include both giant interneurons and several non-giant mechanosensory interneurons that mediate several different sensory modalities. The anatomical organization of this system is compared to similar mechanosensory systems in other insects and crustacea.

Abdomen↗
PubMed1989

Fate of descending interneurons in the metamorphosing brain of an insect, the beetle Tenebrio molitor L.

The fate of descending brain-thoracic neurons in the metamorphosing supraoesophageal ganglion of the mealworm beetle, Tenebrio molitor, is described. Comparison of the descending neurons of the larval, various pupal, and adult stages outlines a high degree of topological invariance in the structure of descending interneurons; i.e., the basic organization of the imaginal set of descending neurons is anticipated by the structure of the larval neurons. Single descending neurons of analogous clusters of larval and imaginal neurons outline a virtual identical structural organization in both the larval and imaginal brain. There is a quantitative increase from approximately 70 to approximately 120 brain-thoracic interneurons during metamorphosis. This increase does not cause complex transformations in the structural organization of the descending interneurons. Experiments with the DNA-inhibitor hydroxyurea prove that the described topological invariant structure of the set of descending neurons is based on the persistence of individually descending neurons. There is evidence that the whole set of larval interneurons persists throughout the beetles' metamorphosis. The essential characteristics of the later imaginal set of descending neurons are qualitatively and quantitatively established within the first 10% of pupation. Structural invariance of the set of descending neurons is discussed with regard to the significance of cellular interaction for the mechanisms of metamorphic reorganization of nervous tissue.

Animals↗
PubMed1992

Common synaptic drive to segmentally homologous interneurons in the locust.

The aim of the present study was to examine the pattern of synaptic interactions among a set of identified homologous interneurons in the segmental nervous system of the locust. This paper presents two main findings: first, serially homologous interneurons that are the progeny of neuroblast 7-4 in the mesothoracic, metathoracic, and first abdominal neuromeres of the locust central nervous system all receive synaptic drive from one and the same presynaptic interneuron. This interneuron has its entire arborization located in these three neuromeres of the central nervous system. It synapses with cells that are siblings, bilateral homologs, and serial homologs, and is itself connected monosynaptically with auditory afferents. The neuronal network that results comprises postsynaptic cells with the same developmental lineage. The second finding is that there is an additional set of synaptic connections among the homologous neurons themselves. All these connections are excitatory, and the pattern of information flow within the network is highly directional. This may relate to the morphologies of the neurons involved and will influence the contribution of homologs from different segments to behavior.

Acoustic Stimulation↗
PubMed1993

Differential expression of neuron-glia cell adhesion molecule (Ng-CAM) on developing axons and growth cones of interneurons in the chick embryo spinal cord: an immunoelectron microscopic study.

To elucidate the role of neuron-glia cell adhesion molecule (Ng-CAM) in axonal pathway formation of avian spinal interneurons, we have examined the ultrastructural expression of Ng-CAM in the developing spinal cord, by using a preembedding immunocytochemical method. Ng-CAM immunoreactivity was punctate and was restricted to cell surfaces. In accordance with our previous light microscopic observations (Shiga et al., '90), the earliest developing spinal interneurons were Ng-CAM-positive on their cell bodies, axons, and growth cones. Axons and growth cones that were either fasciculated or in contact with each other strongly expressed Ng-CAM, thus indicating the possible involvement of Ng-CAM in fasciculation of axons and in the contact guidance of growth cones along preexisting axons. By using higher resolution immunoelectron microscopy, the present study has also revealed new information on the subcellular localization of Ng-CAM on developing spinal interneurons, neuroepithelial cells, and floor plate cells. Although Ng-CAM immunoreactivity was prominent on both axons and growth cones, these structures were Ng-CAM-negative when they contacted the basal lamina around the spinal cord. By contrast, Ng-CAM was detectable on the surface of both neuroepithelial cells and floor plate cells only when they made contact with the Ng-CAM-positive axons and growth cones of interneurons. These results suggest that the subcellular distribution of Ng-CAM is regulated differentially, depending on the apposing cell surfaces, and that such differential and developmentally regulated expression may contribute to the elongation, fasciculation, and guidance of spinal axons.

Animals↗
PubMed1993

Relay cells, not interneurons, of cat's lateral geniculate nucleus contain N-acetylaspartylglutamate.

N-acetylaspartylglutamate (NAAG) is an endogenous brain dipeptide that satisfies many of the criteria for a neurotransmitter. We have previously identified NAAG immunoreactivity in neurons of the lateral geniculate nucleus (LGN) of the cat and monkey. To determine whether all LGN neurons contain NAAG, we treated sections of cat LGN with affinity-purified antibodies to NAAG and counterstained them with thionin. The larger neurons contained NAAG, but the smaller neurons did not. We treated other sections with antiserum to glutamic acid decarboxylase (GAD), the rate-limiting enzyme in the synthesis of gamma-aminobutyric acid (GABA), in order to label interneurons of the LGN. In these sections, the smaller cells were labeled; the larger neurons were not. We hypothesized that NAAG was present in relay cells, but not interneurons. We used two double-labeling paradigms to test this hypothesis. We combined immunocytochemistry for NAAG using a fluorescent secondary antibody with either (1) fluorescent retrograde tracers (true blue, granular blue, rhodamine beads, or propidium iodide) injected into areas 17 and/or 18 or (2) immunocytochemistry for GAD using a second fluorescent secondary antibody. In the LGN, over 99% of retrogradely labeled cells contained NAAG, but few GAD-positive neurons did. In contrast, neurons of the perigeniculate nucleus contained both NAAG and GAD, demonstrating that staining by one set of antisera did not inhibit staining by the other and that perigeniculate neurons are chemically distinct from the interneurons of the LGN. We conclude that in LGN, the relay cells, which project to visual cortex, contain NAAG, whereas most of the interneurons, which contain GABA, do not.

Animals↗
PubMed1994

Distribution of nitric oxide synthase-immunoreactive interneurons in the spinal trigeminal nucleus.

The spinal trigeminal nucleus is involved in the transmission of orofacial sensory information. Neither the distribution of the neuromessenger, nitric oxide, within the trigeminal system nor the possible relationship of this simple gas with trigeminothalamic neurons has been carefully studied. Using immunocytochemical (against nitric oxide synthase) and histochemical (NADPH-diaphorase staining) techniques, we have found that nitric oxide neurons and processes are more prominent in the nucleus caudalis and the dorsomedial aspect of the nucleus oralis than in other spinal trigeminal regions. To study the relationship of nitric oxide to trigeminothalamic neurons and intertrigeminal interneurons of the spinal trigeminal nucleus, spinal trigeminal neurons were retrogradely labeled with fluorogold by thalamic injections or by injections into the junction of the nucleus interpolaris and nucleus caudalis. Medullary sections were subsequently processed with NADPH-diaphorase histochemistry. None of the diaphorase-stained neurons in the spinal trigeminal nucleus was found to contain fluorogold; however, some diaphorase-stained processes were found in close proximity to trigeminothalamic neurons. Following spinal trigeminal nucleus injections, many diaphorase-stained neurons were found to contain fluorogold, especially in the nucleus caudalis, suggesting that nitric oxide-containing neurons in the spinal trigeminal nucleus are intertrigeminal interneurons. Collectively, these data indicate that nitric oxide is most prominent in interneurons located in nucleus caudalis and that these interneurons give rise to processes that appose trigeminothalamic neurons, raising the possibility that they may indirectly influence orofacial nociceptive processing at the level of the spinal trigeminal nucleus via nitric oxide production.

Amino Acid Oxidoreductases↗
PubMed1995

Ultrastructural examination of the targets of serotonin-immunoreactive descending interneurons in the guinea pig small intestine.

Serotonin neurons are descending interneurons in the myenteric plexus of the guinea pig small intestine. Preembedding single- and double-label immunocytochemistries at the ultrastructural level were used to identify the targets of these serotonin interneurons. Serial ultrathin sections were taken through a myenteric ganglion that had been processed for serotonin immunocytochemistry. The ganglion contained the cell bodies of 69 neurons, including 2 serotonin neurons and 6 neurons with the ultrastructural features of Dogiel type II cells. For each cell body in the ganglion, the number of serotonin inputs (synapses and close contacts) was determined. About 59% of the cell bodies did not receive any serotonin inputs. The most abundant serotonin terminals were related to two targets: other serotonin descending interneurons and a population of neurons with Dogiel type I morphology, but whose neurochemistry and function is unknown. The serotonin inputs to the serotonin cell bodies were located predominantly on the lamellar dendrites. Each of the Dogiel type II neurons received 3 or fewer serotonin inputs, and none of the serotonin inputs to Dogiel type II neurons formed a synapse. Overall, about 40% of the serotonin inputs formed synapses. The serotonin inputs to neurons that received many serotonin inputs were more likely to show synaptic specializations than serotonin inputs to neurons that received few serotonin inputs. Inhibitory motor neurons contain nitric oxide synthase (NOS). At the light microscope level, serotonin nerve fibers do not form dense pericellular baskets around NOS cell bodies. To determine whether there are serotonin inputs to NOS neurons, serial ultrathin sections were taken through a myenteric ganglion that had been processed for preembedding double-label immunocytochemistry, in which the NOS neurons were labeled with peroxidase-diaminobenzidine and the serotonin neurons with silver-intensified 1 nm gold. Only 1 out of 9 NOS cells examined in serial section received more than 5 serotonin inputs. The results suggest that, in the guinea pig small intestine, the serotonin descending interneurons are not an essential element of the descending inhibitory reflex.

Animals↗