Neurons of the vagal division of the solitary nucleus activated by the paraventricular nucleus of the hypothalamus

Noninvasive Neuromodulation of Peripheral Nerve Pathways Using Ultrasound and Its Current Therapeutic Implications

This review describes work from several research groups in which ultrasound is being used to target the peripheral nervous system and perform neuromodulation noninvasively. Although these techniques are in their infancy compared to implant-based and electrical nerve stimulation, if successful this new noninvasive method for neuromodulation could solve many of the challenges facing the field of bioelectronic medicine. The work outlined herein shows results in which two different (potentially therapeutic) targets are stimulated, a neuroimmune pathway within the spleen and a nutrient/sensory pathway within the liver. Both data and discussion are provided that compare this new noninvasive technique to implant-based nerve stimulation.

Overactivity of Liver-Related Neurons in the Paraventricular Nucleus of the Hypothalamus: Electrophysiological Findings in db/db Mice

Preautonomic neurons in the paraventricular nucleus (PVN) of the hypothalamus play a large role in the regulation of hepatic functions via the autonomic nervous system. Activation of hepatic sympathetic nerves increases glucose and lipid metabolism and contributes to the elevated hepatic glucose production observed in the type 2 diabetic condition. This augmented sympathetic output could originate from altered activity of liver-related PVN neurons. Remarkably, despite the importance of the brain-liver pathway, the cellular properties of liver-related neurons are not known. In this study, we provide the first evidence of overall activity of liver-related PVN neurons. Liver-related PVN neurons were identified with a retrograde, trans-synaptic, viral tracer in male lean and db/db mice and whole-cell patch-clamp recordings were conducted. In db/db mice, the majority of liver-related PVN neurons fired spontaneously; whereas, in lean mice the majority of liver-related PVN neurons were silent, indicating that liver-related PVN neurons are more active in db/db mice. Persistent, tonic inhibition was identified in liver-related PVN neurons; although, the magnitude of tonic inhibitory control was not different between lean and db/db mice. In addition, our study revealed that the transient receptor potential vanilloid type 1-dependent increase of excitatory neurotransmission was reduced in liver-related PVN neurons of db/db mice. These findings demonstrate plasticity of liver-related PVN neurons and a shift toward excitation in a diabetic mouse model. Our study suggests altered autonomic circuits at the level of the PVN, which can contribute to autonomic dysfunction and dysregulation of neural control of hepatic functions including glucose metabolism.SIGNIFICANCE STATEMENT A growing body of evidence suggests the importance of the autonomic control in the regulation of hepatic metabolism, which plays a major role in the development and progression of type 2 diabetes mellitus. Despite the importance of the brain-liver pathway, the overall activity of liver-related neurons in control and diabetic conditions is not known. This is a significant gap in knowledge, which prevents developing strategies to improve glucose homeostasis via altering the brain-liver pathway. One of the key findings of our study is the overall shift toward excitation in liver-related hypothalamic neurons in the diabetic condition. This overactivity may be one of the underlying mechanisms of elevated sympathetic activity known in metabolically compromised patients and animal models.

Lipid Processing in the Brain: A Key Regulator of Systemic Metabolism

Metabolic disorders, particularly aberrations in lipid homeostasis, such as obesity, type 2 diabetes mellitus, and hypertriglyceridemia often manifest together as the metabolic syndrome (MetS). Despite major advances in our understanding of the pathogenesis of these disorders, the prevalence of the MetS continues to rise. It is becoming increasingly apparent that intermediary metabolism within the central nervous system is a major contributor to the regulation of systemic metabolism. In particular, lipid metabolism within the brain is tightly regulated to maintain neuronal structure and function and may signal nutrient status to modulate metabolism in key peripheral tissues such as the liver. There is now a growing body of evidence to suggest that fatty acid (FA) sensing in hypothalamic neurons via accumulation of FAs or FA metabolites may signal nutritional sufficiency and may decrease hepatic glucose production, lipogenesis, and VLDL-TG secretion. In addition, recent studies have highlighted the existence of liver-related neurons that have the potential to direct such signals through parasympathetic and sympathetic nervous system activity. However, to date whether these liver-related neurons are FA sensitive remain to be determined. The findings discussed in this review underscore the importance of the autonomic nervous system in the regulation of systemic metabolism and highlight the need for further research to determine the key features of FA neurons, which may serve as novel therapeutic targets for the treatment of metabolic disorders.

Brain-liver connections: role of the preautonomic PVN neurons

Diabetes mellitus and the coexisting conditions and complications, including hypo- and hyperglycemic events, obesity, high cholesterol levels, and many more, are devastating problems. Undoubtedly, there is a huge demand for treatment and prevention of these conditions that justifies the search for new approaches and concepts for better management of whole body metabolism. Emerging evidence demonstrates that the autonomic nervous system is largely involved in the regulation of glucose homeostasis; however, the underlying mechanisms are still under investigation. Within the hypothalamus, the paraventricular nucleus (PVN) is in a unique position to integrate neural and hormonal signals to command both the autonomic and neuroendocrine outflow. This minireview will provide a brief overview on the role of preautonomic PVN neurons and the importance of the PVN-liver pathway in the regulation of glucose homeostasis.

Neuroregulative mechanism of hypothalamic paraventricular nucleus on gastric ischemia-reperfusion injury in rats

A rat model of gastric ischemia-reperfusion injury (GI-RI) was established by clamping the celiac artery for 30 min and allowing reperfusion for 1 h, on which the regulatory effect of the paraventricular nucleus (PVN) and its neural mechanisms were investigated. The results were: 1. Electrical stimulation of the PVN obviously attenuated the GI-RI. Microinjection of L-glutamic acid into PVN produced an effect similar to that of PVN stimulation. 2. Electrolytic ablation of the PVN aggravated the GI-RI. 3. Nucleus tractus solitarius (NTS) ablation could eliminate the protective effect of electrical stimulation of PVN on GI-RI. 4. Hypophysectomy did not alter the effect of electrical stimulation of PVN. 5. Vagotomy or sympathectomy both could increase the effect of PVN stimulation on GI-RI. These results indicate that the PVN participates in the development of GI-RI as a specific area in the CNS, exerting protective effects on the GI-RI. The NTS and vagus and sympathetic nerve may be involved in the regulative mechanism of PVN on GI-RI, but the PVN mechanism here is independent of the PVN-hypophyseal pathway.

Subthreshold aortic nerve inputs to neurons in nucleus of the solitary tract

Subthreshold aortic nerve (AN) inputs to neurons receiving a monosynaptic AN-evoked input (MSNs: respond to each of two AN stimuli separated by 5 ms) and neurons receiving a polysynaptic AN input (PSNs) in the nucleus of the solitary tract (NTS) were identified in anesthetized rats. In extracellular recordings from 24 MSNs and 49 PSNs, 12% of MSNs and 29% of PSNs only responded to AN stimulation during the application of excitatory amino acids. In intracellular recordings from 24 MSNs and 22 PSNs, 12% of MSNs and 14% of PSNs responded to AN stimulation with excitatory postsynaptic potentials that did not evoke action potential discharge. Reductions in arterial pressure produced minimal changes in the spontaneous discharge of suprathreshold AN-evoked neurons, suggesting that these neurons receive excitatory inputs from nonbaroreceptor sources. The results suggest that some baroreflex-related NTS neurons exist in a "reserve state and can be changed to an active state or vice versa. This will change the number of neurons involved in baroreflex circuits and provides a novel mechanism for regulating baroreflex function independently of alterations in peripheral afferent input.

The role of paraventricular nucleus of hypothalamus in stress-ulcer formation in rats

The rat stress model of restraint and cold water immersion was used to investigate the effect of stimulating the paraventricular nucleus (PVN) of hypothalamus on the development of stress-induced gastric ulceration. The results were (1) electric stimulation of the PVN increased the stress ulceration, while electrolytic lesion of the PVN decreased it; (2) intracerebroventricular injection (i.c.v.) of acetylcholine (Ach) enhanced the effect of PVN stimulation on stress ulcers, and the M-receptor was involved; (3) i.c.v. norepinephrine (NE) attenuated the effect of PVN stimulation on stress ulcers in a dose-dependent manner, and the beta-receptor was involved; (4) i.c.v. 5-hydroxytryptamine (5-HT) enhanced the effect of PVN stimulation on stress ulcers; (5) electrolytic lesions of dorsal raphe nucleus (DR) attenuated the effect of PVN stimulation on stress ulcers, while electrolytic lesions of the locus ceruleus (LC) aggravated the effect; (6) thyroidectomy, adrenalectomy, ovariectomy, vagotomy and sympathectomy all attenuated the effect of PVN stimulation on stress ulcers; (7) electric stimulation of the PVN produced no effect on gastric juice volume, acidity, total acid output, pepsin activity or the gastric barrier mucus; but greatly reduced gastric mucosal blood flow. These results indicate that the PVN is an important brain site regulating the development of stress-induced gastric ulcers, that the classical neurotransmitters Ach, NE and 5-HT are involved, and that in the periphery, both the parasympathetic and sympathetic nervous systems and the three endocrine glands (thyroid, adrenal and gonad) take part in the effect.

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.

c-fos expression in specific rat brain nuclei after intestinal anaphylaxis: involvement of 5-HT3 receptors and vagal afferent fibers

The c-fos immediate-early gene is acutely induced in brain after various stimuli from the digestive tract. 5-HT3 receptors and vagal afferents have been found involved in intestinal motor disturbances induced by intestinal anaphylaxis. Our aim was to determine whether intestinal anaphylaxis activates brain structures, using c-fos expression, and to evaluate the modulation of c-fos induction by 5-HT3 receptors and vagal afferents. The effects of antigen challenge on intestinal motility were evaluated in ovalbumin-sensitized Hooded Lister rats chronically fitted with NiCr electrodes in the jejunal wall. Intestinal motility was assessed in conscious rats pretreated or not by perivagal capsaicin or a 5-HT3 antagonist (ondansetron). In sensitized rats, ovalbumin disrupted for 62.4 +/- 9.5 min the jejunal migrating motor complexes (MMC) and an important c-fos expression was detected in the nucleus tractus solitarius (NTS), lateral parabrachial nucleus (LPB) and paraventricular nucleus of the hypothalamus (PVN). Intraperitoneal administration of ondansetron or perivagal capsaicin treatment significantly reduced the duration of MMC disruption and attenuated markedly c-fos staining in the 3 brain sites. In contrast, intracerebroventricular administration of ondansetron significantly reduced jejunal motor alterations but did not diminish the c-fos expression, suggesting a role of central 5-HT3 receptors in the efferent control of the intestinal disturbances. Blockade of both c-fos expression and MMC disruption by systemic ondansetron and by perivagal capsaicin indicates that some brainstem nuclei are involved in digestive disturbances after intestinal anaphylaxis, and reflects an involvement of peripheral 5-HT3 receptors on vagal afferents. The reduction of c-fos staining in NTS as well as in LPB and PVN after perivagal capsaicin suggests that the NTS is the primary relay in the activation of the central nervous system during intestinal allergic challenge.

Microinjection of thyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus stimulates gastric contractility

Changes in gastric contractility following microinjection of thyrotropin-releasing hormone (TRH) into the paraventricular nucleus of the hypothalamus (PVN) were examined in fasted, urethane-anesthetized rats. Gastric contractility was measured with extraluminal force transducers and analysed by computer. Unilateral and bilateral PVN microinjections of TRH (0.5 and 1.0 microgram) significantly increased the force index of gastric contractions from 0 to 60 min postinjection, when compared with animals microinjected with 0.1 microgram TRH, 0.1% BSA or TRH (0.5 and 1.0 microgram TRH) in sites adjacent to the PVN. The gastric force index was also significantly elevated from 61 to 120 min postinjection in rats receiving bilateral PVN microinjections of TRH (0.5 and 1.0 microgram). Peak gastric responses occurred within 10-20 min postinjection and represented an approximately eight-fold increase over basal values. In the remaining groups, the force index was not significantly altered from preinjection values. The excitatory action of TRH (1.0 microgram) on gastric contractility was completely abolished by subdiaphragmatic vagotomy. These results suggest that TRH acts within the PVN to stimulate gastric contractility via vagal-dependent pathways.

CRF in the paraventricular nucleus of the hypothalamus induces dose-related behavioral profile in rats

Corticotropin-releasing factor (CRF) administered into the cerebrospinal fluid (CSF) alters grooming and locomotion in rats. The present study was designed to investigate if CRF microinjected into the paraventricular nucleus of the hypothalamus (PVN) influences grooming and spontaneous locomotor behavior in fasted rats maintained in a familiar environment. Unilateral microinfusion of CRF (0.06, 0.2 and 0.6 nmol) into the PVN induced a dose-related increase in grooming, whereas locomotion was dually affected. At lower doses of CRF (0.06 and 0.2 nmol) spontaneous locomotion was significantly increased. At the highest dose, locomotor activity was markedly reduced and, in about 30% of animals, freezing behavior occurred intermittently. The behavioral effects of CRF were maintained throughout the 60 min post injection period. Microinjection of CRF (0.2 nmol) into the lateral hypothalamus, or outside of PVN boundaries had no effect on these behavioral parameters. These results demonstrate that the PVN is a selective and potent site of action for CRF to induce a dose-dependent range of alterations in grooming and locomotion that mimics those observed after CSF injection in a familiar environment. These data also suggest that CRF in the PVN may be involved in mediating behavioral activation and the anxiogenic effect.

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.

Localization of central prostaglandin E2 antisecretory effects

Intracerebroventricular prostaglandin E2 (PGE2) inhibits stimulated gastric acid secretion; however, the central site of action is unknown. Specific PGE2 binding sites have been localized to the ventromedial hypothalamic nucleus and central amygdala (A). The nuclear accumbens has been shown to play a role in central neurotensin-induced antisecretory effects. These studies tested the hypothesis that microinjections of PGE2 into the ventromedial hypothalamic nucleus, central amygdala, and nuclear accumbens inhibit stimulated gastric acid secretion. The hippocampus served as a cerebral control region. Two days before the experiments, metal cannulas were stereotaxically positioned bilaterally into specific areas of the brain, and metal gastric cannulas were operatively implanted, under nembutal anesthesia, in male 250-g Sprague-Dawley rats. On the experimental day, the rats, fasted for 14 hours, were given saline or PGE2 (0.1-1.0 micrograms in 0.2 microL/side) through the central cannulas 10 minutes before administering pentagastrin (40 micrograms/kg SC). Gastric secretion was measured at 30-minute intervals and expressed as acid output, micromoles per hour. Acid output (mean +/- SE) in control animals was 161 +/- 14 mumol/h. Prostaglandin E2 administration at doses of 0.10, 0.50, and 1.0 micrograms/side (a) into ventromedial hypothalamic nucleus reduced acid output to 53 +/- 11,* 36 +/- 10,* and 27 +/- 11* mumol/h regularly; (b) into NACB reduced acid output to 157 +/- 36, 60 +/- 12,* and 38 +/- 12* mumol/h; and (c) into A reduced acid output to 144 +/- 31, 141 +/- 26, and 90 +/- 19* mumol/h, respectively (*P less than 0.05 by Neuman-Keuls test). Prostaglandin E2 (0.50 micrograms/side) administration into hippocampus had no significant effect on acid output (134 +/- 28 mumol/h). Although central PGE2 administration was associated with hyperthermia, this occurred at lower doses than those required to inhibit acid secretion. Prostaglandin E2 administration into specific brain areas known to have PGE2 receptors, the central amygdala and ventromedial hypothalamic nucleus, and into nuclear accumbens inhibits stimulated gastric acid secretion. These observations suggest that PGE2 may have a physiological role in the central control of gastric acid secretion.

Locus coeruleus projections to the dorsal motor vagus nucleus in the rat

The origin of the noradrenergic innervation of the preganglionic autonomic nuclei in the medulla oblongata and spinal cord is still controversial. In this investigation descending connections of the locus coeruleus to the dorsal motor vagus nucleus in the rat are studied with Phaseolus vulgaris leucoagglutinin and horseradish peroxidase as neuroanatomical tracers. Locus coeruleus projections in the motor vagus nucleus are found in the medial part at rostral levels and in the lateral part at intermediate levels of this nucleus. The terminal labeling in the lateral intermediate part of the vagus nucleus appears in an area where possibly preganglionic parasympathetic cardiac neurons are located, suggesting that the locus coeruleus might be involved in regulation of cardiovascular functions. After small iontophoretic injections of horseradish peroxidase in the motor vagus nucleus, retrogradely labeled cells are found in the ventral part of the locus coeruleus and occasionally in the dorsal part of the nucleus. The results show that the locus coeruleus-dorsal motor vagus nucleus pathway may participate in the inhibition of the cardiac preganglionic neurons in the dorsal motor vagus nucleus by the hypothalamic paraventricular nucleus.

Oxytocin excites gastric-related neurones in rat dorsal vagal complex

1. Dorsal medullary injections of oxytocin (OT) influence gastric motor and secretory function via a vagally mediated mechanism. Thus, it was hypothesized that OT altered the firing rate of brain stem vagal neurones that were specifically related to gastric function. 2. To study this, glass microelectrode/injection pipette arrays were used to record the activity of gastric-related neurones in the dorsal vagal complex (DVC), which includes vagal sensory neurones in the nucleus tractus solitarius (NTS) and motor neurones in the dorsal motor nucleus (DMN). After identifying such a neurone, spontaneous activity was monitored before and after micropressure injection of OT and vehicle solutions from the pipettes. 3. Two methods were used to identify neurones that were related to gastric function. One method employed a gastric balloon to identify DVC neurones that were responsive to gastric inflation. The second method employed a gastric vagal stimulating electrode, which permitted the identification of gastric-related NTS and DMN cells via orthodromic or antidromic activation, respectively. 4. Twenty-four of forty-two gastric-inflation-related neurones responded to administration of OT (100-400 fmol in 100-400 pl). The majority of those responding to OT were activated by this peptide (21/24). All the cells tested (n = 13) remained sensitive to gastric inflation after administration of OT. Also, OT was found to excite the majority of cells that were identified as gastric-related NTS (nine excited; one no effect) or DMN cells (eleven excited; two no effect). 5. These studies support the hypothesis that central oxytocinergic neurones influence gastric motility and secretion by increasing the excitability of central vagal neurones in the NTS and DMN that are related to gastric function.

Regional changes in brain 14C-2-deoxyglucose uptake after feeding-inducing intrahypothalamic norepinephrine injections

Although norepinephrine (NE) injections into the paraventricular hypothalamus (PVN) have been extensively documented to induce feeding in satiated rats, there have been few systematic attempts to elucidate the neural circuitry subserving this response. In this study quantitative 14C-2-deoxyglucose (14C-2DG) autoradiography was used to map regional brain changes induced by PVN NE injections. Male Wistar rats, bearing PVN cannulae and previously shown to be positive responders for NE-induced feeding, were given 125 microCi/kg 14C-2DG IV immediately following a PVN injection of either 40 nmol NE or vehicle, then killed 45 min later. 14C-2DG uptake was examined in 97 brain structures using computerized densitometry. PVN NE injections resulted in small, localized changes in brain 14C-2DG uptake. Forebrain structures affected included the somatosensory parietal cortex (+15%), the CA3 hippocampal field (-8%), and the reticular thalamic nucleus (+14%). Midbrain changes involved the anterior pretectal area (+8%) and the central gray area (-11%). At the hindbrain level, the lateral reticular nucleus showed the most pronounced changes of all brain regions examined (-24%), followed by the nucleus of the solitary tract (-16%) and the laterodorsal tegmental nucleus (+16%). No changes were seen in the median eminence or in other hypothalamic areas. This pattern of results largely agrees with recent proposals for the circuitry of a PVN-hindbrain system subserving NE-induced as well as hypothalamic lesion-induced feeding effects. In addition, however, they suggest the possibility that altered activity in some forebrain structures may also be involved in the NE response.

Somatic and vagal afferent convergence on solitary tract neurons in cat: electrophysiological characteristics

Electrophysiological characteristics are described for 67 neurones localized to subnuclei of the solitary tract or the area of the dorsal motor nucleus of the vagus in alpha-chloralose-anesthetized, paralysed cats which received vagal and hindlimb sural or peroneal nerve excitation. The peroneal and sural nerves were stimulated in an exposed hindlimb preparation; the ipsilateral vagus was stimulated at the cervical level. Compound action potentials were recorded from all three nerves. Neurons were recorded with extracellular microelectrodes from the brain stem solitary area contralateral to the stimulated somatic nerves. Ninety-one percent of the recorded neurons were spontaneously active. Eighteen percent and 5% of the neurons received only peroneal or sural excitation, respectively, while 59% of the neurons received convergent peroneal and sural excitation. Thirty-nine of the 67 neurons were also tested for vagal input of which 41% responded with excitation. All of the neurons tested for vagal input also received converging excitation from one or both of the somatic nerves. Thirty-one percent of the vagal-excited neurons received converging input from both the peroneal and sural nerves. The combined mean minimal conduction velocity for peroneal and sural input was 31 +/- 1 m/s (mean +/- 1 S.E., range 9-54 m/s). Thirty-six percent of the peroneal and 31% of the sural afferents were Group II fibers. Significant periods of inhibition of spontaneous neuronal spike activity followed peroneal and sural excitation in 43 and 39% of the neurons, respectively. In many neurons, both excitation and inhibition of spike activity could be elicited at stimulus intensities as low as 1.2 times threshold for the lowest threshold fibers in each nerve. Somatic nerve-induced inhibition of spontaneous neuron activity without prior excitation was also observed. These results suggest that neurons of the solitary tract nuclei receive Group II and Group III somatic afferents which converge on neurons also receiving excitatory vagal input. Consequently, somesthetic and kinesthetic as well as visceral receptor activation may directly modulate solitary tract neurons. A possible conclusion is that the nucleus tractus solitarius is the initial central site of mediation of somatosympathetic reflexes. Modulation of the nucleus tractus solitarius by somatic afferents may then adjust sympathetic tone, via modulation of other medullary centers, in visceral and somatic tissues to match somatic metabolic needs.

Injection of neuropeptide Y into the paraventricular nucleus of the hypothalamus inhibits gastric acid secretion in the rat

Neuropeptide Y (NPY) was injected into the paraventricular nucleus (PVN) of the hypothalamus of anesthetized rats in order to assess its effect on gastric acid secretion. NPY evoked a dose-dependent decrease of interdigestive gastric acid output when injected directly into the PVN or immediately ventral to it. Intracerebroventricular NPY and saline injections did not alter acid output. Injection of NPY into adjacent non-PVN hypothalamic areas resulted in either an elevated acid output or had no effect depending on the site of injection. Mean arterial blood pressure and heart rate were not consistently affected by NPY. These results show that injection of NPY into the PVN of anesthetized rats inhibits interdigestive gastric acid output in a dose-dependent manner.

PVN-hindbrain pathway involved in the hypothalamic hyperphagia-obesity syndrome

This study examined the involvement of caudal brainstem projections of the hypothalamic paraventricular nucleus (PVN) in the medial hypothalamic (MH) hyperphagia-obesity syndrome. Experiment 1 demonstrated that a unilateral parasagittal knife cut in the MH combined with a contralateral coronal knife cut in either the ventrolateral pons (vP) or ventrolateral medulla (vM) significantly increased food intake and body weight in adult female rats. Overeating and overweight were also produced by a unilateral MH knife cut combined with a contralateral oblique cut under the nucleus of the solitary tract and dorsal motor nucleus of the vagus complex (NST/DX). In contrast, an MH cut x dorsolateral medullary cut combination did not increase food intake or body weight compared to a MH cut alone or sham surgery. Experiment 2 demonstrated that the hyperphagia/obesity effect of MH x vP knife cuts was comparable to that obtained with bilateral PVN lesions, but less than that produced by bilateral MH knife cuts. Bilateral vP cuts also increased body weight but the effect was less than that obtained with the other experimental treatments. Feeding the rats a high-fat diet rather than chow potentiated the hyperphagia and obesity syndromes produced by the various lesion conditions. Taken together, these findings suggest that the medial hypothalamic hyperphagia and obesity syndrome is due, in part, to damage to PVN projections to the caudal brainstem, the NST/DX complex in particular. The functional significance of this PVN-hindbrain "feeding" pathway and the identity of extra-PVN components of the hyperphagia-obesity syndrome remain to be established.

Histochemical identification of a PVN-hindbrain feeding pathway

Unilateral coronal knife cuts through the ventrolateral pontine reticular formation produce overeating and overweight when combined with contralateral parasagittal knife cuts in the medial hypothalamus (MH). The knife cuts were in a position to sever fiber projections from the paraventricular nucleus to the hindbrain. The present study used histochemical techniques to confirm that hyperphagia-producing knife cuts transect PVN-hindbrain fiber connections. In Experiment 1, adult female rats received a unilateral coronal knife cut in the ventrolateral pontine reticular formation. Horseradish peroxidase (HRP) was applied to the knife cut region and two to three days later brains were processed for the localization of neurons labeled with HRP. HRP-labeled neurons were found in the PVN, particularly in the caudal parvocellular region. Additional HRP-labeled neurons were observed in other medial hypothalamic areas but none were found in the ventromedial nucleus. HRP-filled cells were also found in the lateral hypothalamus, central nucleus of the amygdala, and in the nucleus of the solitary tract (NST). Many of the PVN projections to the hindbrain contain oxytocin and Experiment 2 determined if hyperphagia-inducing knife cuts sever PVN oxytocinergic fibers. Adult female rats received unilateral MH cuts, unilateral pontine cuts, or a contralateral combination of both cuts. One to eight days later the brains were processed for immunocytochemistry. The MH cuts and pontine cuts were found to interrupt descending oxytocinergic fibers. Taken together, these results support the hypothesis that interruption of a direct PVN-hindbrain oxytocinergic projection is responsible for the hypothalamic hyperphagia-obesity syndrome. However, the results do not rule out the involvement of a multisynaptic pathway or additional neurochemical systems.

Hypothalamic paraventricular nucleus stimulation-induced gastric acid secretion and bradycardia suppressed by oxytocin antagonist

Unilateral microstimulation of the medial parvocellular division of the hypothalamic paraventricular nucleus (PVNmp) elicits significant increases in gastric acid secretion and bradycardia. An injection of 25 picomoles of the oxytocin antagonist dET2Tyr(Et)Orn8 Vasotocin (ETOV), suspended in 5 nanoliters of artificial of cerebrospinal fluid (CSF), into the dorsal motor nucleus of the vagus (DMN) immediately preceding microstimulation of the PVNmp suppresses this change in gastric acid secretion and heart rate. The injection of an equal volume (5 nanoliters) of artificial CSF vehicle solution into this region of the DMN, prior to PVNmp microstimulation, has no effect on either the subsequent stimulation-evoked changes in acid secretion or cardiac activity. This suppression of PVNmp stimulation-evoked changes in gastric acid levels and heart rate by the presence of the oxytocin antagonist, ETOV, within the DMN supports the hypothesis that oxytocin may be a neurotransmitter used for descending communication from the PVNmp to neurons within the DMN that regulates these two functions.

Gastric-vagal solitary neurons excited by paraventricular nucleus microstimulation

Electrophysiological studies were performed to determine whether neurons of the nucleus of the solitary tract (NST) which receive sensory input from the stomach via vagal afferents are activated by microstimulation of the medial parvocellular division of the paraventricular nucleus of the hypothalamus (PVHmp). We found that 37% of the NST neurons orthodromically activated by gastric vagal nerve stimulation are also orthodromically excited by PVHmp microstimulation. Not only do these NST neurons receive convergent input from the PVHmp as well as gastric afferent input (vagal), but this input from the PVHmp may also bias the responsiveness of these NST neurons to incoming afferent information. This PVHmp influence on NST excitability is probably mediated by a direct, monosynaptic projection between these two nuclei. These data support the hypothesis that neurons in the PVHmp can control gastric function by altering the sensitivity of neurons which form the sensory limb of gastric vago-vagal reflexes. This ability of the PVHmp to bias the responsiveness of NST neurons to incoming vagal afferent information is probably mediated by a direct, monosynaptic projection between these two nuclei.

The action of centrally administered arginine vasopressin on blood pressure in the conscious rabbit

In addition to its peripheral endocrine actions, arginine vasopressin (AVP) has been implicated in the central control of blood pressure. Intracerebroventricular (i.c.v.) injections (0.01-1.0 nmol) of AVP or arginine vasotocin (AVT), but not oxytocin (OXY), into unanesthetized rabbits caused a rapid, dose related rise in blood pressure as well as increases in heart rate. The lowest centrally administered dose of AVP and AVT (0.01 nmol) had no effect on blood pressure when given intravenously. In search of tissue locus for the pressor effect of AVP microinjection of AVP and OXY into the posterior hypothalamus and septum of conscious rabbits was without effect. However, microinjection (0.01-0.04 nmol) of AVP into the nucleus tractus solitarius of anesthetized rabbits caused a rise in blood pressure similar to the response seen after i.c.v. injection. Comparable volumes of the vehicle into the ventricle or the tissue sites had no effect on resting blood pressure. The pressor response after AVP given i.c.v. was significantly reduced up to 3 h after administration of the ganglionic blocker, chlorisondamine HCl. The central antagonist, d(CH2)5Tyr (Me) vasopressin, eliminated the usual increase in blood pressure after administration of AVP in half the animals tested. The results indicate that AVP acts centrally to mediate cardiovascular responses in unanesthetized as well as anesthetized rabbits.

Dorsal medullary oxytocin, vasopressin, oxytocin antagonist, and TRH effects on gastric acid secretion and heart rate

Injections of oxytocin and TRH (11 picomoles), centered on the dorsal motor nucleus of the vagus, substantially increased gastric acid secretion. Additionally, oxytocin, but not TRH, simultaneously produced a consistent reduction in heart rate. Vasopressin injected into the same locus, at doses of 11 and 110 picomoles, had no effect on either function. Both the gastric and cardiac effects of oxytocin were eliminated by the central injections of oxytocin antagonist dEt2Tyr(Et)Orn8Vasotocin (ETOV; 6 picomoles) or peripheral administration of atropine (300 micrograms/kg, IP). Application of oxytocin or TRH to the area postrema, at double the dosage (22 picomoles) yielded no consistent effects on either gastric secretion or heart rate. These findings indicate that oxytocin in the dorsal motor nucleus of the vagus may act as a regulator of vagally-mediated gastric and cardiovascular functions while TRH effects, in this medullary area, seem limited to the regulation of gastric function.

Response of rat paraventricular neurones with central projections to suckling, haemorrhage or osmotic stimuli

In lactating, urethane-anaesthetized female rats extracellular recordings were made from paraventricular nucleus (PVN) neurones that were antidromically activated following electrical stimulation of the neurohypophysis, amygdala or nucleus tractus solitarius/vagal complex (NTS/VC). Of the PVN units, 98 projected to the neurohypophysis but none of these neurosecretory neurones were found to simultaneously project to extrahypothalamic areas. From the firing patterns and the response of these neurons to suckling, haemorrhage or osmotic stimuli both 'vasopressinergic' and 'oxytocinergic' neurones were identified. We found 43 PVN units to project to the NTS/VC and 22% of tested neurones were activated by osmotic or haemorrhage stimuli; no phasic activity was associated with this activation. The suckling stimulus failed to elicit any response from these units. Upon testing the PVN units that projected to the amygdala (n = 35), it was found that haemorrhage and suckling stimuli were without effect, while the osmotic stimulus activated one of 6 units tested. Thus, the extrahypothalamic PVN projections examined in this study were not associated with the suckling reflex response, although there is evidence for their limited involvement in neural response to osmotic or haemorrhage stimuli.

Vagal afferent stimulation-evoked gastric secretion suppressed by paraventricular nucleus lesion

Studies were performed to evaluate the possibility that the paraventricular nucleus (PVN) of the hypothalamus modulates gastric acid secretion by changing the sensitivity of the gastric secretory control mechanism to vagal afferent input. Under pentobarbital anesthesia, 17 rats were prepared with esophageal and pyloric catheters such that the stomach could be perfused continuously on a flow-through basis. Thus, acid secretion could be monitored throughout the experiment. Stimulating electrodes were attached to the central cut end of the cervical vagus nerve. Unilateral stimulation of cervical vagal afferents resulted in a substantial increase in gastric acid secretion. This vagal afferent-mediated increase in acid outflow was suppressed following a single PVN lesion ipsilateral to the side of afferent stimulated output. Given the nature of PVN connections with brainstem regions responsible for the elaboration of vago-vagal reflexes, our results suggest that the PVN may control gastric acid outflow by changing the gain of gastric vago-vagal reflexes.

Interaction between descending paraventricular neurons and vagal motor neurons

The hypothalamic paraventricular nucleus is known to send projections to the dorsal medullary area where it appears to innervate the nucleus tractus solitarius and the dorsal motor nucleus of the vagus. Experiments were carried out in urethane anesthetized rats to identify single vagal motor neurons identified by antidromic invasion and to determine their response to stimulation of the paraventricular nucleus (PVN) of the hypothalamus. From the ipsilateral vagus, 26 single units were antidromically activated. Of these, the majority were unresponsive to PVN stimulation; however, 3 of these neurons demonstrated orthodromic excitation following paraventricular stimulation. These studies provide electrophysiological evidence in support of an interaction between descending paraventricular neurons and vagal motor neurons, but indicate that the majority of such vagal neurons are not influenced directly by the PVN.