Somatostatin regulates excitatory amino acid receptor-mediated fast excitatory postsynaptic potential components in vagal motoneurons

Expression and localisation of somatostatin receptor subtypes sst1-sst5 in areas of the rat medulla oblongata involved in autonomic regulation

Somatostatin is known to modulate the activity of neurones of the medulla oblongata involved in autonomic regulation, mediated through five subtypes of G protein-coupled receptors, sst1-sst5. This study utilises reverse transcription polymerase chain reaction and immunohistochemistry to investigate the expression of sst1-sst5, including the sst2(A)/sst2(B) isoforms, in the main autonomic centres of the rat medulla oblongata: nucleus of the solitary tract (NTS), dorsal motor vagal nucleus (DVN) and ventrolateral medulla (VLM). In tissue from the cerebral cortex, hippocampus and cerebellum all subtype mRNAs were detected, but sst5 signals were weak, and the distribution of sst1-sst5 immunoreactivities was consistent with previous reports. In the medulla, all sst mRNAs gave clear amplicons and subtype-specific antibodies produced characteristic patterns of immunolabelling, frequently in areas of somatostatinergic innervation. Anti-sst1 labelled beaded fibres, sst2(A), sst2(B), sst4 and sst5 gave somatodendritic labelling and sst3 labelled presumptive neuronal cilia. In NTS tissue, sst1, sst2(A), sst4 and sst5 mRNAs were strongly expressed, while in VLM tissue sst1, sst2(A), sst2(B) and sst4 predominated. In both areas of the medulla, neurones with intense somatodendritic sst2(A) immunoreactivity were principally catecholaminergic in phenotype, being double labelled for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl-transferase (PNMT). Some TH/PNMT positive neurones were also sst2(B) and sst4 immunoreactive. Cholinergic parasympathetic neurones in the DVN were immunoreactive for the sst2(A), sst2(B), sst4 and sst5 subtypes. These observations are consistent with the proposal that multiple somatostatin receptor subtypes, possibly combining as heterodimers, are involved in mediating the modulatory effects of somatostatin on autonomic function, including cardiovascular, respiratory and gastrointestinal reflex activity.

Somatostatin immunoreactivity in axon terminals in rat nucleus tractus solitarii arising from central nucleus of amygdala: coexistence with GABA and postsynaptic expression of sst2A receptor

Axon terminals synapsing on neurones in the nucleus tractus solitarii (NTS) that originate from the central nucleus of the amygdala (CeA) have been shown to contain gamma-aminobutyric acid (GABA) immunoreactivity. Here we investigated whether such terminals also contain somatostatin (SOM), a neuropeptide found in axons distributed throughout the NTS and in somata in the CeA, and known to modulate cardiovascular reflexes when microinjected into the NTS. With fluorescence microscopy, SOM immunoreactivity was seen in the varicosities of some axons throughout the NTS that were anterogradely labelled with biotin dextran amine injected into the CeA. Such varicosities were frequently observed in close proximity to dendrites of NTS neurones that were immunoreactive for the SOM receptor sst(2A) subtype, and in many cases also for catecholamine synthesising enzymes. In the caudal, cardioregulatory zone of NTS, SOM immunoreactivity was localised by electron microscopic pre-embedding gold labelling to boutons containing dense-cored and clear pleomorphic vesicles and forming symmetrical synapses, mostly onto dendrites. Additional post-embedding gold labelling for GABA suggested that a subpopulation (29%) of GABAergic terminals sampled in this area of NTS contained SOM. Almost all boutons anterogradely labelled from the amygdala were GABA-immunoreactive (-IR) and 21% of these were SOM-IR. A similar proportion of these boutons (22%) formed synapses onto dendrites containing immunoreactivity for the SOM receptor sst(2A) subtype. These observations provide evidence that some of the GABAergic projection neurones in the CeA that inhibit baroreceptor reflex responses in the NTS in response to fear or emotional stimuli could release SOM, which might modulate the activity of NTS neurones via an action on sst(2A) receptors.

Rhombencephalic pathways and neurotransmitters controlling deglutition

Information from neuronal pathway tracing and pharmacologic microstimulation studies, in conjunction with electrophysiological data, has begun to coalesce into a coherent, if still incomplete, picture of the brain stem circuitry responsible for generating the motor patterns underlying deglutition and esophageal peristalsis in the rat. The intermediate, interstitial, and ventral subnuclei of the solitarius complex appear to play a pivotal part, as evidenced by their viscerosensory inputs and extensive projections to the parvicellular intermediate reticular formation of the medulla, to the brain stem deglutitive motor neuron pools, and to general visceral efferent preganglionic neurons controlling the upper alimentary tract striated and smooth musculature, respectively. The dense projections of the solitarial central subnucleus form a separate subcircuit controlling esophageal, and also some aspects of gastric, motility. Although not extensive, direct connections between the latter subnucleus and interneurons coordinating the buccopharyngeal stage of swallowing appear to exist. In both subcircuits, fast information transfer uses excitatory amino acidergic transmission by means of several glutamate-receptor subtypes. Release from tonic GABAergic inhibition exerted by local solitarial interneurons may provide a mechanism for triggering deglutitive premotoneuronal activity. Local or reticular cholinergic neurons are implicated in pharyngoesophageal coupling and the generation of propulsive esophagomotor output. The solitary interneurons under investigation engage in complex local dendritic and axonal projections within the solitarius complex. Further analysis of these local circuits and their transmitters should yield essential clues regarding the mechanisms underlying deglutitive motor pattern generation.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

Somatostatin-IR neurons are a major subpopulation of the cuneothalamic neurons in the rat cuneate nucleus

This study was aimed to localize and characterize the somatostatin-immunoreactive (SOM-IR) neurons in the rat cuneate nucleus (CN). By immuno-histochemistry, the SOM-IR neurons, which were widely distributed in the nucleus, were round, spindle or multiangular in shape (mean area = 226.1 +/ -3.1 microm(2), n = 1016). By electron microscopy, the neurons shared all the ultrastructural features of the cuneothalamic neurons (CTNs) which showed a slightly indented nucleus and a fairly rich cytoplasm containing well-developed Golgi apparatuses and rough endoplasmic reticulum (rER). The SOM immunoreaction product filled the cytoplasm of the neurons extending from the soma to the proximal and distal dendrites, which were postsynaptic to unlabeled boutons. In addition to soma and dendrites, SOM-IR boutons were also identified which made axodendritic synaptic contacts with SOM-IR dendrites. The SOM-IR neurons were characterized by using anti-SOM pre-embedding immunolabeling coupled with horseradish peroxidase (HRP) retrograde method, or SOM immunolabeling along with anti-glutamate, gamma-aminobutyric acid (GABA) or glycine post-embedding immunolabeling for identification of CTNs, glutamate-IR, GABA-IR and glycine-IR neurons, respectively. It was shown that more then 80% of the CTNs contained SOM and, furthermore, they contained glutamate but not GABA or glycine. On the basis of present findings, it is suggested the majority of the SOM-IR neurons in the rat CN are CTNs and that they may be involved in modulation of somatosensory synaptic transmission.

Immunohistochemical localization of the somatostatin sst2(b) receptor splice variant in the rat central nervous system

Somatostatin is a neuromodulator in the mammalian CNS. To date, genes for at least five different somatotrophin release inhibiting factor receptors, termed sst1-sst5, have been cloned. The rat sst2 receptor exists in two splice variants, sst(alpha)a) and sst2(b), which differ in their carboxy-termini. When heterologously expressed in Chinese hamster ovary-K1 cells, these splice variants show little difference in their operational characteristics. Recently, the distribution of the sst2(a) receptor was documented, yet at present no data are available about the distribution of the sst2(b) receptor in the CNS. Here, we present the characterization of a novel polyclonal anti-peptide antibody that is selective for the sst2(b) receptor splice variant. The antibody was raised against the unique intracellular carboxy-terminal portion of the receptor protein. Using this affinity-purified antibody in western blotting experiments, the sst2(b) receptor expressed in Chinese hamster ovary-K1 cells was shown to be a glycoprotein with a molecular weight centred at about 85,000. The antibody showed no cross-reactivity to any of the recombinant human sst1-5 receptors, the rat sst2(a) receptor or wild-type Chinese hamster ovary-K1 cells. Employing immunohistochemistry, we investigated the distribution of the sst2(b) receptor in the brain and spinal cord of adult rats. A distinct distribution was found throughout the rostrocaudal axis of the CNS. Somatodendritic as well as axonal staining was observed. Somatodendritic labelling was particularly obvious in the olfactory bulb, cerebral cortex, hippocampal formation, mesencephalic trigeminal nucleus and cerebellum, as well as in cranial and spinal motor areas. The results show that the distribution of the sst2(b) receptor partially overlaps with that of the sst2(b) receptor, although there were differences in a number of brain areas. The location of the sst2(b) receptor implies that it may mediate a modulatory role of somatostatin inhibitory releasing factor on sensory as well as motor functions.

Somatostatin immunoreactivity in esophageal premotor neurons of the rat

Esophageal peristalsis is coordinated by premotor neurons localized to the central subnucleus of the nucleus of the solitary tract (NTScen). These premotor neurons project directly to motoneurons within the compact formation of the nucleus ambiguus (NAc). Somatostatin immunoreactive terminals have been previously demonstrated encircling motoneurons in the (NAc) (Cunningham, E.T., Jr. and Sawchenko, P.E., J. Neurosci., 9 (1989) 1668-1682). We combined transsynaptic tracing with pseudorabies virus and immunohistochemistry to localize somatostatin to premotor neurons within the NTScen.

Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones: an electrophysiological and single cell RT-PCR study

We have previously shown that somatostatin can either enhance or decrease AMPA/kainate receptor-mediated responses to glutamate in mouse-dissociated hypothalamic neurones grown in vitro. To investigate whether this effect is due to differential activation of somatostatin (SRIF) receptor subtypes, we compared modulation of the response to glutamate by SRIF with that induced by CH-275 and octreotide, two selective agonists of sst1 and sst2/sst5 receptors, respectively. Somatostatin either significantly decreased (49%) or increased (30%) peak currents induced by glutamate, and was ineffective in the remaining cells. Only the decreased response was obtained with octreotide, whereas only increased responses were elicited by CH-275 (47 and 35% of the tested cells, respectively). Mean amplitude variations under somatostatin or octreotide on the one hand, and under somatostatin or CH-275 on the other hand, were equivalent. Pertussis toxin pretreatment significantly decreased the number of cells inhibited by somatostatin or octreotide, but had no effect on the frequency of neurones showing increased sensitivity to glutamate during somatostatin or CH-275 application. About half of the neurones tested by single cell reverse transcriptase polymerase chain reaction (RT-PCR) expressed only one sst receptor (sst1 in 26% and sst2 in 22% of studied cells). Out of the remaining neurones, 34% displayed neither sst1 nor sst2 mRNAs, whereas 18% showed a simultaneous expression of both mRNA subtypes. Expression of sst1 or sst2 mRNA subtypes matched totally with the effects of somatostatin on sensitivity to glutamate in 79% of the neurones processed for PCR after recordings. These data show that pertussis toxin-insensitive activation of the sst1 receptor subtype mediates somatostatin-induced increase in sensitivity to glutamate, whereas decrease in the response to glutamate is linked to pertussis toxin-sensitive activation of the sst2 receptor subtype.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Anti-somatostatin antisera, but neither a somatostatin agonist (octreotide) nor antagonist (CYCAM), attenuates hyperalgesia in the rat

Somatostatin has been reported to have both nociceptive and antinociceptive roles in sensory transmission in the spinal cord. In this study, antisera against SOM (alpha-SOM), a somatostatin antagonist (CYCAM) and a somatostatin agonist (octreotide), were evaluated for their role in thermal and mechanical hyperalgesia in a model of carrageenan-induced inflammatory pain in the rat. Intrathecal administration of alpha-SOM prior to hindpaw inflammation dose-dependently attenuated thermal and mechanical hyperalgesia and the increase in paw size for up to 4 h following injury. Administration of alpha-SOM 3 h following injury had no effect. Intrathecal administration of octreotide or CYCAM prior to or following injection of carrageenan had no effect on any measure. It is suggested that the lack of effect of octreotide and CYCAM resulted from low affinity for the SOM receptor subtypes in the rat spinal cord. The attenuation of hyperalgesia and paw size produced by alpha-SOM may have resulted from attenuation of somatostatin's role in producing a dorsal root reflex that modulates the increase in paw size following injury.

Ultrastructure and synaptology of the nucleus ambiguus in the rat: the compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato-somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and, occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 +/- 0.4 S.D. micrograms in diameter). Motoneuronal somatic profiles have 0-5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 x 0.7 microgram in the transverse plane and are up to 3.0 micrograms long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (> 90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injection of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis.

Modulation by somatostatin of glutamate sensitivity during development of mouse hypothalamic neurons in vitro

Glutamate sensitivity development and interactions of somatostatin (SRIF) with AMPA/Kainate receptor-mediated glutamate responses were studied in dissociated hypothalamic neurons from 16-day-old mouse embryos grown in vitro. Only 18% of functionally innervated cells could be found at 6-9 DIV whereas the percentage of innervated neurons progressively increased thereafter to reach 100% at 19-22 DIV. The glutamate sensitivity, estimated from glutamate-induced peak inward current, was very low at 6-9 DIV, sharply increased at 11-14 DIV and developed at a low increase rate thereafter. SRIF either unaffected glutamate peak current (27% of the cells), or significantly decreased (50%) or increased it (23%). Pertussis Toxin pretreatment abolished the SRIF-induced decrease of the glutamate response without affecting the excitatory effect. The number of glutamate responsive neurons inhibited by SRIF increased with time in culture whereas that of neurons responding to SRIF by an increased glutamate response was not statistically modified by functional innervation. The present data suggest that increased glutamate sensitivity coincides with the onset of functional synaptogenesis in mouse hypothalamic neurons in culture. SRIF can modulate glutamate sensitivity of hypothalamic neurons with either synergistic or antagonistic effects. Since glutamate has been shown to stimulate SRIF synthesis and secretion from hypothalamic neurons, the reverse capacity of SRIF to modulate the glutamate response suggests that both transmitters exhibit complex reciprocal interactions.

Somatostatin and leu-enkephalin in the rat auditory brainstem during fetal and postnatal development

A transient expression of the neuropeptide somatostatin has been described in several brain areas during early ontogeny and several opioid peptides, such as leu-enkephalin, have also been found in the brain at this stage in development. It is therefore believed that somatostatin and leu-enkephalin may play a role in neural maturation. The aim of the present study was to describe the spatiotemporal pattern of somatostatin and leu-enkephalin immunoreactivity in the auditory brainstem nuclei of the developing rat and to correlate it with other developmental events. In order to achieve this goal, we applied peroxidase-antiperoxidase immunocytochemistry to rat brains between embryonic day (E) 17 and adulthood. Somatostatin immunoreactivity (SIR) was found in all nuclei of the auditory brainstem, yet it was temporally restricted in most nuclei. SIR appeared prenatally and reached maximum levels around postnatal day (P) 7, when great numbers of immunoreactive neurons were present in the ventral cochlear nucleus (VCN) and in the lateral lemniscus. At that time relatively low numbers of cells were labeled in the dorsal cochlear nucleus, the lateral superior olive (LSO), and the inferior colliculus (IC). During the same period, when somata in the VCN were somatostatin-immunoreactive (SIR), a dense network of labeled fibers was also present in the LSO, the medial superior olive (MSO), and the medial nucleus of the trapezoid body (MNTB). As these nuclei receive direct input from VCN neurons, and as the distribution and morphology of the somatostatinergic fibers in the superior olivary complex (SOC) was like that of axons from VCN neurons, these findings suggest a transient somatostatinergic connection within the auditory system. Aside from the LSO, MSO, and MNTB, labeled fibers were found to a smaller extent in all other auditory brainstem nuclei. After P7, the SIR decreased and only a few immunoreactive elements were found in the adult auditory brainstem nuclei, indicating that somatostatin is transiently expressed in the rat auditory brainstem. Leu-enkephalin immunoreactivity showed a lower number and weaker intensity of labeled structures as compared to SIR, with E18 being the earliest day at which labeled fibers appeared in the SOC. At birth, immunoreactive fibers were also present in the cochlear nuclear complex and in the IC. Leu-enkephalin immunoreactive somata were found only after P12 in the CN and after P16 in the IC. Leu-enkephalin immunoreactivity was not transient, but increased progressively with age until about P21, when the adult levels were reached.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of somatostatin receptor 3 mRNA in the motorneurones of the rat spinal cord, and the sensory neurones of the spinal ganglia

At least five different types of somatostatin receptors (SSTRs): SSTR1-5 have been recently cloned. Here, we report the distribution of somatostatin receptor 3 (SSTR3) mRNA in spinal cord and spinal ganglia by in situ hybridization. We demonstrate the presence of this transcript in motorneurones of the spinal cord and other motor nuclei of the brain stem, and in the sensory neurones of the spinal ganglia. These data suggest a role for SSTR3 in motor and sensory functions.

Seizures, brain damage and brain development

Recent evidence suggests that hippocampal damage can be both the result of seizure activity and the cause of further chronic epilepsy. A review of current models of status epilepticus-induced brain damage reveals that excitotoxic mechanisms probably mediate the lesions in most brain regions. NMDA receptors appear to play a dominant role, although non-NMDA glutamate receptors are important in several specific neuronal populations. In the immature brain, a number of unique metabolic features determine a different set of vulnerabilities, resulting in a brain which is more resistant than the adult's to certain mechanisms of brain damage, but quite vulnerable to others. The inhibition of growth by severe seizure activity has implications for the developing brain that have not yet been fully explored. The mechanisms by which seizure-induced hippocampal lesions cause chronic epilepsy have been explored in several recent animal models. A rearrangement of hippocampal circuits may result from death of selected populations of inhibitory neurons, or from misdirected regeneration by excitatory neurons. It could lead to chronic epilepsy through loss of normal inhibition, through sprouting of new excitatory connections, through conservation of excitatory connections which in a healthy brain would be pruned during development, or through facilitation of kindling by one of these mechanisms. These recent results are beginning to reconcile the pathology seen in human hippocampi ablated for intractable epilepsy with that observed in experimental animals, and offer the promise of even greater advances in the future. They suggest a mechanism for Gower's dictum that "seizures beget seizures" and highlight the importance of the interneurons of the dentate gyrus in epileptogenesis.

Nicotinic cholinoceptor-mediated excitatory postsynaptic potentials in rat nucleus ambiguus

In rat brainstem slice preparations, intracellular recording from neurons (n = 39) in the compact formation of the nucleus ambiguus (AMBc) revealed spontaneous and miniature excitatory postsynaptic potentials (EPSPs; n = 11) that, along with acetylcholine-induced depolarization, were enhanced by physostigmine (10 microM; n = 2) and blocked by dihydro-beta-erythroidine 1-5 pmol (n = 4). Retrograde neuronal tracing combined with choline acetyltransferase immunocytochemistry demonstrated that the AMBc receives a projection from a subpopulation of cholinergic neurons in the zona intermedialis reticularis parvicellularis. Electrical stimulation of this region in slices evoked fast EPSPs in AMBc neurons (n = 23) that were inhibited by dihydro-beta-erythroidine 2-5 pmol (n = 8), but not by methscopolamine 1 pmol (n = 2). The present findings strongly support the existence of a cholinergic nicotinic synapse mediating fast transmission in brainstem vagal motoneurons.

Central nervous system control mechanisms of swallowing: a neuropharmacological perspective

Neuropharmacological in vivo and in vitro investigations are beginning to provide insight into chemical signaling processes within brainstem networks controlling the individual stages of swallowing. Different subtypes of excitatory amino acid (EAA) receptors operate at the level of solitarial interneurons programming the buccopharyngeal and esophageal stage, as well as motoneurons innervating esophageal striated musculature. Muscarinic cholinoceptors (MAChRs), probably activated via a propriobulbar input, are critically involved in generating output from solitarial neurons to esophageal motoneurons. Inhibition to tonically active GABAA-receptor mediated afferents to solitarial premotor neurons results in rhythmic deglutitive output, reflecting disinhibition of EAA and MACh receptor activity. Motoneuronal EAA receptors may be regulated by a somatostatinergic input arising from solitarial premotoneurons. The available evidence is consistent with a transmitter heterogeneity in esophageal premotor neurons that may operate to provide chemical coding of afferents to the motor output stage of the pattern generator for esophageal peristalsis.

The brainstem esophagomotor network pattern generator: a rodent model

The evidence reviewed in this essay supports the following working model of the central function generator for esophageal peristalsis in the rat: solitarial subnucleus centralis (NTSc) neurons operate in a dual capacity as esophagomotor reflex interneurons and as command neurons programming respective outputs from nucleus ambiguus compact formation (AMBc) motoneurons during secondary and primary peristalsis. In both conditions, there is a critical requirement for cholinergic input which enables NTSc neurons to generate the timed sequence of AMBc motoneuronal activity. In primary peristalsis, the cholinergic coupling mechanism is activated centrally, probably via projections from deglutitive premotor neurons to the parvicellular reticular formation and thence to the NTS. In reflex (or secondary) peristalsis, the cholinergic input could in part be generated by cholinergic vagal viscerosensory fibers innervating the esophagus. Postulated connections between NTS deglutitive neurons and the parvicellular cholinergic neurons of the intermediate reticular formation have yet to be demonstrated. Premotor input from NTSc to AMBc is generated by somatostatinergic and excitatory aminoacidergic neurons. Coactivation of both inputs by cholinergic afferents is necessary to generate esophagomotor output from AMBc neurons. The model under study is derived from investigations into central mechanisms governing striated muscle peristaltic activity. Whether the basic operational principles revealed thus far apply to peristaltic pattern generation in species with a smooth muscle esophagus, requires further investigation.