Comparative immunocytochemical study of the catecholaminergic and peptidergic afferent innervation to the dorsal vagal complex in rat and guinea pig

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Brainstem Neuronal Circuitries Controlling Gastric Tonic and Phasic Contractions: A Review

This review is on how current knowledge of brainstem control of gastric mechanical function unfolded over nearly four decades from the perspective of our research group. It describes data from a multitude of different types of studies involving retrograde neuronal tracing, microinjection of drugs, whole-cell recordings from rodent brain slices, receptive relaxation reflex, accommodation reflex, c-Fos experiments, immunohistochemical methods, electron microscopy, transgenic mice, optogenetics, and GABAergic signaling. Data obtained indicate the following: (1) nucleus tractus solitarius (NTS)-dorsal motor nucleus of the vagus (DMV) noradrenergic connection is required for reflex control of the fundus; (2) second-order nitrergic neurons in the NTS are also required for reflex control of the fundus; (3) a NTS GABAergic connection is required for reflex control of the antrum; (4) a single DMV efferent pathway is involved in brainstem control of gastric mechanical function under most experimental conditions excluding the accommodation reflex. Dual-vagal effectors controlling cholinergic and non-adrenergic and non-cholinergic (NANC) input to the stomach may be part of the circuitry of this reflex. (5) GABAergic signaling within the NTS via Sst-GABA interneurons determine the basal (resting) state of gastric tone and phasic contractions. (6) For the vagal-vagal reflex to become operational, an endogenous opioid in the NTS is released and the activity of Sst-GABA interneurons is suppressed. From the data, we suggest that the CNS has the capacity to provide region-specific control over the proximal (fundus) and distal (antrum) stomach through engaging phenotypically different efferent inputs to the DMV.

Insulin-responsive autonomic neurons in rat medulla oblongata

Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin-responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 U/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase-stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N-methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9 ± 0.3 mmol/L to 1.7 ± 0.2 mmol/L (mean ± SEM; n = 6); saline injection had no effect. In insulin-treated rats, many PNMT-immunoreactive C1 neurons had Fos-immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3% ± 1.4% (n = 8) of C1 neurons were Fos-positive. Insulin also induced Fos in 47.2% ± 2.0% (n = 5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT-positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2, or C3 neurons and no DMV neurons were Fos-positive. The high numbers of PNMT-immunoreactive and ChAT-immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.

Oxytocin-immunoreactive innervation of identified neurons in the rat dorsal vagal complex

BACKGROUND

Oxytocin (OXT) has been implicated in reproduction and social interactions and in the control of digestion and blood pressure. OXT-immunoreactive axons occur in the dorsal vagal complex (DVC; nucleus tractus solitarius, NTS, dorsal motor nucleus of the vagus, DMV, and area postrema, AP), which contains neurons that regulate autonomic homeostasis. The aim of the present work is to provide a systematic investigation of the OXT-immunoreactive innervation of dorsal motor nucleus of the vagus (DMV) neurons involved in the control of gastrointestinal (GI) function.

METHODS

We studied DMV neurons identified by (i) prior injection of retrograde tracers in the stomach, ileum, or cervical vagus or (ii) induction of c-fos expression by glucoprivation with 2-deoxyglucose. Another subgroup of DMV neurons was identified electrophysiologically by stimulation of the cervical vagus and then juxtacellularly labeled with biotinamide. We used two- or three-color immunoperoxidase labeling for studies at the light microscopic level.

KEY RESULTS

Close appositions from OXT-immunoreactive varicosities were found on the cell bodies, dendrites, and axons of DMV neurons that projected to the GI tract and that responded to 2-deoxyglucose and juxtacellularly labeled DMV neurons. Double staining for OXT and choline acetyltransferase revealed that OXT innervation was heavier in the caudal and lateral DMV than in other regions. OXT-immunoreactive varicosities also closely apposed a small subset of tyrosine hydroxylase-immunoreactive NTS and DMV neurons.

CONCLUSIONS & INFERENCES

Our results provide the first anatomical evidence for direct OXT-immunoreactive innervation of GI-related neurons in the DMV.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Receptors and neural effects of oxytocin in the rodent hypothalamus and preoptic region

Vasopressin and oxytocin are produced in and secreted from not only hypothalamo-hypophysial neurons which shed their products into the circulation to act as hormones or releasing factors, but also from neurons whose axons form tracts which remain within the central nervous system. Using tritiated or radioiodinated ligands, binding sites for vasopressin and for oxytocin have been detected by in vitro autoradiography. In the rat hypothalamus binding sites for vasopressin are present in the suprachiasmatic, sigmoid and arcuate nuclei, and oxytocin receptors in the area of the ventromedial nucleus. Electrophysiological evidence obtained using single cell recordings in slices suggests that oxytocin-binding sites present in the ventromedial hypothalamus and in the bed nucleus of the stria terminalis mostly represent functional, neuronal receptors. The expression of these receptors (but not of the vasopressin receptors) depends on gonadal steroid hormones, as does that of uterine and mammary gland oxytocin receptors. Modifications of the hormonal status associated with, for example, puberty and lactation cause 'up-regulation' of central and peripheral oxytocin receptors. The central administration of oxytocin facilitates (and the administration of oxytocin agonists inhibits) maternal behaviour and the milk ejection reflex, therefore the hormonal and neural actions of oxytocin appear to be complementary in ensuring the birth and development of the offspring.

Hindbrain chemical mediators of reflex-induced inhibition of gastric tone produced by esophageal distension and intravenous nicotine

The purpose of this study was to activate a vagovagal reflex by using esophageal distension and nicotine and test whether hindbrain nitric oxide and norepinephrine are involved in this reflex function. We used double-labeling immunocytochemical methods to determine whether esophageal distension (and nicotine) activates c-Fos expression in nitrergic and noradrenergic neurons in the nucleus tractus solitarii (NTS). We also studied c-Fos expression in the dorsal motor nucleus of the vagus (DMV) neurons projecting to the periphery. Esophageal distension caused 19.7 +/- 2.3% of the noradrenergic NTS neurons located 0.60 mm rostral to the calamus scriptorius (CS) to be activated but had little effect on c-Fos in DMV neurons. Intravenous administration of nicotine caused 19.7 +/- 4.2% of the noradrenergic NTS neurons 0.90 mm rostral to CS to be activated and, as reported previously, had no effect on c-Fos expression in DMV neurons. To determine whether norepinephrine and nitric oxide were central mediators of esophageal distension-induced decrease in intragastric pressure (balloon recording), N(G)-nitro-L-arginine methyl ester microinjected into the NTS (n = 5), but not into the DMV, blocked the vagovagal reflex. Conversely, alpha2-adrenergic blockers microinjected into the DMV (n = 7), but not into the NTS, blocked the vagovagal reflex. These data, in combination with our earlier pharmacological microinjection data with nicotine, indicate that both esophageal distension and nicotine produce nitric oxide in the NTS, which then activates noradrenergic neurons that terminate on and inhibit DMV neurons.

Monoaminergic and peptidergic axonal projections to the vagal motor cell column of a teleost, the filefish Stephanolepis cirrhifer

In an immunohistochemical study, the vagal motor nucleus of a teleost, the filefish Stephanolepis cirrhifer, could be divided into a rostral part and a caudal part, and the former into a dorsolateral group and a ventromedial group. The dorsolateral group consisted of neurons immunoreactive for calcitonin gene-related peptide, whereas the ventrolateral-caudal group was negative for calcitonin gene-related peptide. The latter group was retrogradely labeled after dextran amine injection to the visceral ramus of the vagus nerve, suggesting that it is a general visceral efferent column, made up of parasympathetic preganglionic neurons, whereas the dorsolateral rostral group is a special visceral efferent column. In the general visceral efferent column, a dense concentration of nerve fibers immunoreactive for serotonin, tyrosine hydroxylase, cholecystokinin-8, and substance P, and a small number of fibers immunoreactive for neuropeptide Y was observed. Perikarya in contact with varicose terminals immunoreactive for these substances were frequently seen. In contrast, in the special visceral efferent column, only a moderate concentration of neuropeptide Y-immunoreactive nerve fibers and a sparse distribution of fibers immunoreactive for tyrosine hydroxylase were observed. Perikarya in contact with varicose terminals immunoreactive for these substances were rare. These results suggest that the vagal parasympathetic preganglionic neurons might receive multiple inputs of monoaminergic and peptidergic fibers involved in the regulation of the visceral organs. On the other hand, monoaminergic and peptidergic afferent fibers might be of much less significance in the activity of the special visceral efferent component of the vagus nerve.

CNS site of action and brainstem circuitry responsible for the intravenous effects of nicotine on gastric tone

The purposes of our study were to determine (1) the effects of intravenous (i.v.) nicotine on gastric mechanical function of anesthetized rats, (2) the CNS site of action of nicotine to produce these effects, (3) the CNS nicotinic acetylcholine receptor (nAChR) subtype(s) responsible for mediating the i.v. effects of nicotine, and (4) the brainstem neurocircuitry engaged by i.v. nicotine for eliciting its gastric effects. This was accomplished by monitoring intragastric pressure (gastric tone) and contractility of the fundus and antrum while administering five doses of i.v. nicotine and microinjecting nicotine into specific brainstem nuclei. Additionally, c-Fos expression in the brainstem after i.v. nicotine and pharmacological agents were used as tools to identify the CNS site and circuitry and reveal the nAChR subtype(s) mediating the gastric effects of nicotine. Using these experimental approaches, we found the following. (1) When given intravenously in doses of 56.5, 113, 226, 452, and 904 nmol/kg, nicotine elicited only inhibitory effects on gastric mechanical function. The most sensitive area of the stomach to nicotine was the fundus, and this effect was mediated by the vagus nerve at doses of 56.5, 113, and 226 nmol/kg. (2) The CNS site of action and nAChR subtype responsible were glutamatergic vagal afferent nerve terminals in the medial subnucleus of the tractus solitarious (mNTS) and alpha4beta2, respectively. (3) The brainstem neurocircuitry that was involved appeared to consist of a mNTS noradrenergic pathway projecting to the dorsal motor nucleus of the vagus (DMV). This pathway seems to be activated via nitriergic interneurons engaged by vagally released glutamate in the mNTS and results in alpha2 adrenergic receptor-mediated inhibition of DMV neurons projecting to the fundus and controlling gastric tone.

The dorsal vagal complex of the ferret: anatomical and immunohistochemical studies

To further the understanding of gastrointestinal function in this species, and in particular to advance our own work concerning central emetic pathways, the cytoarchitecture and the distribution of eight neurochemicals were studied in the ferret dorsal vagal complex (DVC; area postrema, nucleus of the solitary tract [nTS] and dorsal motor nucleus of the vagus). The cytoarchitectural features of this region in the ferret were similar to those seen in other species; however, the ferret possesses a particularly large and distinct subnucleus gelatinosus of the nTS. Dense calcitonin gene-related peptide-immunoreactivity was found in the gelatinous, interstitial and commissural subnuclei of the nTS, with lesser amounts in other regions of the DVC. Enkephalin-immunoreactivity of varying densities was found throughout the DVC. Moderate to dense galanin-immunoreactivity was observed throughout the DVC, with the exception of the subnucleus gelatinosus of the nTS, from which it was virtually absent. Dense neuropeptide Y-immunoreactivity was observed in the subnucleus gelatinosus and interstitial subnucleus, with moderate staining in other regions of the DVC. Neurotensin immunoreactivity was very sparse or absent. Immunoreactivity for serotonin was sparsely distributed throughout the DVC. Moderate somatostatin-immunoreactivity was observed over a large portion of the DVC, but was virtually absent from the gelatinosus and interstitial subnuclei. Substance P immunoreactivity was observed throughout the DVC and was particularly dense in the dorsal/dorsolateral subnucleus and the dorsal aspects of the medial and commissural subnuclei. In terms of its cytoarchitecture the DVC of the ferret is more similar to the cat than the rat, especially with regard to the area postrema and the subnucleus gelatinosus of the nTS. The distribution of neuroactive substances was largely similar to other species; however, differences were present particularly in patterns of immunoreactivity for enkephalin, serotonin, neuropeptide Y and somatostatin.

Ontogeny of oxytocin-like immunoreactivity in the Brazilian opossum brain

The neuropeptide oxytocin (OT) has been shown to function as a neurotransmitter and/or neuromodulator in addition to its hormonal function in the periphery in the adult central nervous system (CNS). Previously, we have studied the postnatal neurogenesis of the paraventricular and supraoptic nuclei and ontogeny of arginine vasopressin-like immunoreactivity in the Brazilian opossum brain, Monodelphis domestica. In this study, we have described the ontogeny of oxytocin-like immunoreactivity (OT-IR) in the opossum brain. As a marsupial, opossum pups are in an extremely immature state, with neurogenesis and morphogenesis continuing into the second week of postnatal life. Thus, opossum pups are a good model for developmental studies. In the adult opossum brain, OT-IR was localized in regions as reported for the adult rat and other species, except for a few differences. These findings suggest similar functional roles for OT in the adult opossum brain as in other mammals. Unlike the prenatal expression of arginine vasopressin, OT-IR was first detected in the forming median eminence on day 1 of postnatal life (1 PN). Between 3 and 5 PN, OT-IR was present in the hypothalamic supraoptic and paraventricular nuclei and posterior pituitary. At this time, neurogenesis of these nuclei is not completed. By 10 to 15 PN, OT-IR was seen in several brain areas, and begins to resemble that of the adult between 45 and 60 PN. These results indicate that the time course of appearance of the OTnergic system does not directly parallel the early expression of the vasopressinergic system. However, the expression of OT-IR in the opossum brain before neurogenesis and morphogenesis is completed suggests a potential role for OT in developmental events. Similar to arginine vasopressin, oxytocin may also be involved in the regulation of autonomic functions that are essential for the opossum's adaptation to an ex utero environment. Future studies utilizing experimental manipulations of the OTnergic system will help determine the significance of this peptide in the neonatal opossum.

A rise in the intracellular Ca2+ concentration of isolated rat supraoptic cells in response to oxytocin

1. Intracellular Ca2+ concentration ([Ca2+]i) was monitored in single cells isolated from adult rat supraoptic (SO) nuclei. The great majority of cells (85%) were neurones and most were immunoreactive to oxytocin or to vasopressin (AVP). 2. The resting [Ca2+]i of the majority (80%) of the neurones remained stable while 20% of the neurones displayed spontaneous [Ca2+]i oscillations which disappeared in low-Ca2+ (100 nM) EGTA buffer. 3. Addition of 100 nM oxytocin increased the [Ca2+]i in both stable and oscillating cells. Two types of responses were observed: (i) a sustained response with [Ca2+]i being maintained at an elevated level and (ii) a brief response with [Ca2+]i quickly returning to a near-resting level. Responses were reproducible, dose dependent and blocked with a specific oxytocin antagonist. 4. Removal of extracellular Ca2+ did not block the oxytocin response. In EGTA buffer, application of thapsigargin (200 nM) onto oxytocin-sensitive cells induced an increase in [Ca2+]i and inhibited the oxytocin response. These effects were not induced by other intracellular Ca2+ mobilizers such as tBuBHQ (see Methods) or caffeine. 5. In conclusion, half of the SO cells respond to oxytocin with a rise in [Ca2+]i. The effect is mediated by oxytocin receptors and results from release of Ca2+ from thapsigargin-sensitive stores.

Electrophysiological actions of oxytocin in the dorsal vagal complex of the female rat in vitro: changing responsiveness during the oestrous cycle and after steroid treatment

The effect of the oxytocin-specific agonist Thr4,Gly7-oxytocin (TGOT) was tested on neurones in tissue slices of the dorsal vagal complex, obtained from virgin female rats at different stages of the oestrous cycle. The proportion of neurones excited by TGOT (0.1 microM) was independent of the day of the cycle, but both the basal activity and magnitude of response induced by TGOT were significantly reduced on the day of oestrus by comparison with dioestrus. This was due to a small but significant shift in the dose-response relationship. The magnitude of the excitation of neurones obtained from animals at proestrus did not differ significantly from either oestrus or dioestrus, but lay between the two. Ovariectomy 6 days prior to recording reduced the proportion of responsive neurones (35% vs. 69% at dioestrus), but had only a small effect on the amplitude of the averaged responses. Daily injection of 10 micrograms oestradiol benzoate had no additional effect on the proportion of responsive neurones (40%), but caused a marked suppression of the amplitude of the response at all doses (change in firing rate caused by 0.1 microM: 1.68 +/- 0.27 Hz vs. 2.69 +/- 0.39 Hz). In contrast, injections of 5 mg progesterone caused a small increase in the amplitude of the response. The data show that ovarian steroids have a marked effect on oxytocin-sensitive neurones of the dorsal vagal complex, causing dynamic changes in responsiveness over the oestrous cycle. This is discussed with respect to the effects of ovarian steroids on central oxytocin receptors and the possible involvement in regulating autonomic functions.

Differences in electrophysiological properties between neurones of the dorsal motor nucleus of the vagus in rat and guinea pig

We have examined the electrophysiological properties of neurones in the dorsal motor nucleus of the vagus (DMV) in rats and guinea pigs in transverse medullary slices maintained in vitro. There were only minor differences in the morphology of the neurones between the species, and their passive electrical properties were very similar. However, action potentials in guinea pig neurones had larger amplitudes and longer half-widths than did those in rat neurones. In both species, action potentials were followed by prolonged afterhyperpolarisations (AHPs). In the majority of guinea pig neurones, two calcium-activated potassium currents underlying the AHP could be separated into an early apamin-sensitive component and a late apamin-insensitive component. In rat neurones, the current underlying the AHP was briefer and entirely apamin-sensitive. In response to a step of depolarising current, neurones in the guinea pig only discharged once or twice and then ceased firing. In rat neurones, this manoeuvre produced repetitive firing. An inward rectifier was larger in neurones of the guinea pig than in those in the rat. The effects of 5-hydroxytryptamine and noradrenaline also differed between neurones of each species. We conclude that, despite many similarities of size and electrical properties, DMV neurones in the two species differ in terms of several voltage- and calcium-dependent conductances which determine their active electrical behaviour.

Mechanism of action of oxytocin in rat vagal neurones: induction of a sustained sodium-dependent current

1. The mechanism of action of oxytocin on vagal neurones of the rat was studied using single-electrode voltage-clamp recordings from brainstem slices. The ionic basis of the oxytocin-induced current was examined by changing the composition of the perfusion solution and by making use of channel blockers. 2. In neurones clamped at or near their resting potential, oxytocin generated a sustained, TTX-insensitive inward current whose peak amplitude was concentration related. This current was detectable at 10 nM, was half-maximal at about 100 nM and was maximal at micromolar concentrations of peptide. 3. The oxytocin current was inward over membrane potentials ranging from -110 to -20 mV and was voltage dependent, since it increased in magnitude as the membrane was depolarized from the resting potential toward less negative potentials. 4. Partial replacement of extracellular sodium by equimolar N-methyl-D-glucamine reversibly attenuated or suppressed the oxytocin current. By contrast, substituting part of extracellular chloride or blocking calcium currents did not modify it. Increasing the transmembrane potassium gradient was also without effect and none of the potassium channel blockers TEA, 4-amino pyridine (4-AP), apamin, caesium or barium affected the oxytocin current. This current is thus at least in part carried by sodium. 5. The activation of the oxytocin current as a function of the membrane potential could be quantitatively simulated using a Boltzmann equation, suggesting that oxytocin acts by inducing the opening of a voltage-dependent channel which can exist in either of two states, open or closed. 6. Lowering the extracellular calcium concentration from 2 to 0.1 mM, while keeping the magnesium concentration constant at 1 mM, enhanced the response to oxytocin. This low calcium-induced potentiation of the oxytocin current was 1.4-3-fold and was reversible. 7. We conclude that oxytocin increases the excitability of vagal neurones by generating a persistent, voltage-gated current which is sodium dependent, is insensitive to TTX and is modulated by divalent cations.

Electrophysiology of oxytocin actions on central neurons

The action of oxytocin on neurons located in the dorsal motor nucleus of the vagus nerve was studied in brain slices in vitro. It acted postsynaptically and caused a reversible, concentration-dependent excitation of vagal motoneurons in rats. This effect is specific, since it could be mimicked by a selective agonist and suppressed by an oxytocin antagonist. Single-electrode voltage-clamp recordings from rat vagal motoneurons indicated that oxytocin generates a noninactivating inward current, whose amplitude increased as the membrane was depolarized. This current was insensitive to TTX, to a reduction of membrane calcium currents, and to a reversal in the transmembrane chloride gradient; and it was unaffected by several potassium channel blockers. By contrast, it was reversibly reduced by partially substituting extracellular sodium with equimolar N-methyl-D-glucamine. These results suggest that oxytocin exerts its neuronal action in the rat brainstem by generating a sustained voltage-dependent sodium current. Vasopressin activates a similar current when acting on motoneurons located in the facial nucleus of newborn rats. These fast, neurotransmitter-like actions of oxytocin and of vasopressin may provide an explanation--though not necessarily the sole explanation--for their central effects on maternal, sexual, and social behaviors.

Morphological and electrophysiological evidence for postsynaptic localization of functional oxytocin receptors in the rat dorsal motor nucleus of the vagus nerve

The vagal complex is innervated by oxytocin immunoreactive axons of hypothalamic origin. The presence of oxytocin binding sites in the dorsal motor nucleus of the vagus nerve of the rat was evidenced by autoradiography with a radioiodinated oxytocin antagonist as ligand. Two weeks following a unilateral vagotomy, distal to the nodose ganglion, binding sites were reduced below the level of detection in the ipsilateral dorsal motor nucleus of the vagus nerve. Choline acetyltransferase immunoreactivity was also markedly reduced in the vagal motoneurons whose axons had been transected. Electrophysiological studies were performed in vitro in brainstem slices from control rats. In antidromically identified vagal motoneurones, oxytocin applied at 0.1-1.0 microM either caused a reversible depolarization or generated, under voltage-clamp conditions, a transient inward current. These responses persisted under the condition of synaptic uncoupling. Taken together these observations favour the notion that oxytocin of hypothalamic origin acts directly on rat vagal motoneurones.

Impact of antral mechanoreceptor activation on the vago-vagal reflex in the rat: functional zonation of responses

1. Activation of gastric sensory afferents alters gastric motor and secretory function via the gastric vago-vagal reflex. In this report, we investigated in the rat the impact of gastric mechanoreceptor activation on the brain stem components of the reflex, which are located in the dorsal vagal complex (DVC), i.e. the nucleus of the solitary tract (NTS) and the subjacent dorsal motor nucleus (DMN). 2. In our extracellular recordings of single-cell activity in the DVC, we observed a relation between the response to antral distention and the location of the cell in the DVC. Specifically, cells that were excited by antral distention (ON cells) were located dorsal to those that were inhibited (OFF cells) by the same stimulus (mean depth = 536 +/- 15 and 627 +/- 14 microns for ON and OFF cells, respectively). 3. For a subset of DVC cells, the location was marked by ionophoretic ejection of Pontamine Blue from the recording barrel. Histological analysis indicated that ON cells were located in the NTS, and OFF cells were located in the ventral NTS or within the boundaries of the DMN. Together, these data led to the hypothesis that ON and OFF cells are functionally different groups of neurones, i.e. ON cells may be NTS neurones, and OFF cells may be DMN neurones. We tested this directly by employing both an intragastric balloon and a non-traumatic vagal stimulating electrode to determine whether inflation-related cells were NTS or DMN cells via orthodromic and antidromic activation, respectively. 4. Almost all ON cells (12/13) were orthodromically activated by vagal stimulation, i.e. they were NTS neurones. One ON cell was antidromically activated, and therefore was a DMN neurone. Of the twenty-eight OFF cells that were encountered, ten were classified as NTS neurones because they were orthodromically inhibited by vagal stimulation. The remaining eighteen OFF cells were orthodromically inhibited and antidromically activated (i.e. DMN neurones). Thus, our results support the hypothesis that ON and OFF cells can be functionally distinct populations of neurones, in that almost all ON cells are NTS cells and approximately 2/3 of the OFF cells are DMN neurones. 5. The response to mechanoreceptor activation was different for NTS and DMN neurones. NTS cells were activated (55%) or inhibited (45%) by balloon distention of the stomach, whereas DMN cells were almost exclusively inhibited (95%) by this stimulus. This information provides insight into the organization of excitatory and inhibitory connections of the brain stem components that mediate gastric vago-vagal reflexes.

Light and electron microscopical localisation of 5-HT-immunoreactive boutons in the rat trigeminal motor nucleus

We have used pre-embedding EM immunohistochemical methods to obtain quantitative data on the frequency and post-synaptic targets of 5-hydroxytryptamine-immunoreactive (5-HT-IR) boutons within the rat V motor nucleus. Thirteen percent (69/531) of all synaptic contacts in the motor nucleus involved 5-HT-IR boutons. Seventy-four percent of 5-HT-IR boutons made axo-dendritic contacts, 20% axo-somatic contacts, and 6% axo-axonic contacts. We conclude that a significant fraction of boutons in the motor nucleus are 5-HT-IR and most contribute to postsynaptic rather than presynaptic effects on trigeminal motoneurones.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Microinjection of oxytocin into the dorsal vagal complex decreases pancreatic insulin secretion

Microinjections of oxytocin and of an oxytocin antagonist into the dorsal vagal complex of the medulla oblongata were performed in order to study the possible role of the oxytocin containing axons that innervate this region in the regulation of pancreatic insulin secretion. No significant effect was produced by the intramedullary injection of the oxytocin vehicle alone or of 0.04 pM oxytocin. Injections of 4 and 20 pM oxytocin produced a reversible decrease of plasmatic insulin levels which fall to 59% of basal levels 15 min after the injection. Such an effect was abolished when 4 pM oxytocin was injected to animals which have been previously bilaterally vagotomized. In contrast to oxytocin, intramedullary injection of a specific antagonist of oxytocin to intact animals induced a marked increase of plasmatic insulin levels which raised 131% of basal levels 15 min after the injection. In animals receiving such an injection of oxytocin antagonist, a secondary injection of 4 pM oxytocin produced a slight but not significant decrease of plasmatic insulin levels. These data strongly suggest that the hypothalamic neurons producing oxytocin that densely project to the dorsal vagal complex may be involved in an inhibitory control of the vagal preganglionic neurons that innervate the pancreas.

Thyrotropin-releasing hormone-immunoreactive nerve terminals synapse on the dendrites of gastric vagal motoneurons in the rat

Thyrotropin-releasing hormone stimulates vagally mediated gastric acid secretion and motility by an undefined central mechanism in the rat. The present study sought to determine the anatomical basis for this stimulatory effect by examining the ultrastructural relationship of nerve terminals immunoreactive for thyrotropin-releasing hormone with the dendrites of gastric vagal motoneurons. A light and electron microscopic double immunostaining technique was employed using the beta subunit of unconjugated cholera toxin as a neural tracer. Cholera toxin (50 microliters, 0.25%) was injected into the ventral stomach musculature in five rats. After 72 hours' survival, animals were sacrificed by transcardiac perfusion fixation. Retrogradely transported cholera toxin was immunocytochemically localized in vagal gastric motoneurons and their dendrites in the dorsal motor nucleus of the vagus and nucleus of the solitary tract, alone or in combination with the immunocytochemical localization of thyrotropin-releasing hormone. Ultrastructural analysis of double-labeled material revealed thyrotropin-releasing hormone-immunoreactive nerve terminals making asymmetric synaptic contacts on the retrogradely labeled dendrites of vagal gastric motoneurons. Nerve terminals immunoreactive for thyrotropin-releasing hormone also made asymmetric and symmetric synaptic contacts with unlabeled dendrites of undetermined perikaryal origin. In addition, nonsynaptic varicosities immunoreactive for thyrotropin-releasing hormone were frequently observed in the vagal nuclei. The synaptic contacts between thyrotropin-releasing hormone-immunoreactive nerve terminals and vagal gastric motoneuronal dendrites provide one possible basis for the profound stimulatory effect of central thyrotropin-releasing hormone on gastric vagal motor activity.