Deficits in the control of food intake after hypothalamic paraventricular nucleus lesions

Noradrenergic mechanisms of the hypothalamic paraventricular nucleus (PVN) have been shown to play an important role in the stimulation of feeding To determine the influence of this nucleus in monitoring and controlling responses to physiological and pharmacological challenges, PVN electrolytic lesion rats were tested for their behavioral responsiveness to agents known to affect the alpha-2 noradrenergic system as well as release of corticosterone, and to short- and long-term periods of food deprivation. Discrete lesions of the PVN produced enhanced feeding, particularly of carbohydrate, in freely-feeding rats maintained on a macronutrient self-selection paradigm. Lesion rats demonstrated a behavioral deficit in food intake regulation (a decrease in carbohydrate ingestion) in response to 5-hr and 24-hr fasts, showed a disturbance in circadian feeding, and exhibited a dramatic decrease in circulating corticosterone. However, feeding in response to 2-deoxy-D-glucose and insulin remained intact, suggesting that noradrenergic receptors within the PVN are not involved in the mediation of glucoprivic-induced feeding.     

Feeding induced by GABA(A) receptor stimulation within the nucleus accumbens shell: regional mapping and characterization of macronutrient and taste preference.

This study investigated the areas of the nucleus accumbens shell involved in the modulation of feeding behavior by GABAergic stimulation and characterized this response using macronutrient diets as well as saline, sucrose, and saccharin solutions. The GABA agonist muscimol induced a pronounced feeding response when infused in the medial nucleus accumbens shell but not in the ventral or lateral accumbens shell. In the macronutrient preference study, muscimol increased the intake of both high fat and high carbohydrate diets when presented separately. When both diets were available simultaneously, muscimol stimulated feeding of both diets to the same degree. Muscimol elicited a robust increase in the consumption of sucrose solution. However, no effect of muscimol was demonstrated for water, saline, or saccharin intake. These findings provide evidence for a selective role for GABA-sensitive neurons in the medial accumbens shell in the regulation of ingestive behavior and further suggest that GABA(A) receptors in this region do not modulate palatability, macronutrient selection, or rewarding properties of food.     

Antinociceptive effects of galanin in the nucleus accumbens of rats

It has been demonstrated that galanin plays important roles in the modulation of nociceptive information in rats. The present study is performed to investigate the regulating role of galanin in nociception in the nucleus accumbens (NAc) of rats. Intra-NAc administration of galanin induces dose-dependent increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. Furthermore, the galanin-induced antinociceptive effects are blocked by following intra-NAc injection of the galanin receptor antagonist galantide. The results demonstrate that galanin induces antinociceptive effects in the NAc of rats, and galanin receptors are involved in the galanin-induced antinociception effects.     

Galanin in the PVN increases nutrient intake and changes peripheral hormone levels in the rat

In self-selection feeding paradigms, rats display differential patterns of nutrient (protein, carbohydrate or fat) intake. Factors known to influence this selection include brain peptides as well as circadian parameters. In this series of experiments we investigated the role of PVN galanin in nutrient intake during the early and late dark periods in the rat. Rats were allowed to select between three isocaloric diets enriched in protein, carbohydrate or fat. Following a 2-week adaptation period, the animals' 24-h intake was monitored for 4 weeks. Galanin was injected into the PVN and food intake was measured 1, 2 and 24 h post-injection. Galanin significantly increased the 1 h total food intake but it failed to increase the intake of any particular nutrient. Galanin had no effect 2 or 24 h post-injection. Analysis of the data grouped by preference based on the rats 24 h baseline selection patterns over the 4-week period revealed that galanin seem to increase the preferred nutrient. That is, galanin preferentially increased the intake of the carbohydrate- or fat-rich diet in animals with high (over 40% of the total food intake) 24-h baselines in this particular nutrient. Finally, analysis of the plasma hormone levels after paraventricular galanin administration revealed a significant increase in noradrenaline levels, a small reduction in plasma insulin with no effects on adrenaline, glucose or corticosterone. The data revealed that galanin in the PVN influences both food intake and metabolic functioning. PVN galanin significantly increases sympathetic outflow and seems to stimulate the intake of the individual rat's preferred macronutrient.     

Dietary sodium deprivation reduces gustatory neural responses of the parabrachial nucleus in rats

Acute sodium depletion induced by furosemide reduces gustatory responses of parabrachial nucleus (PBN) neurons to 0.3-0.5M NaCl in rats. However, in the rat nucleus of the solitary tract (NST), where taste-responsive cells project to the PBN, acute sodium depletion and dietary sodium deprivation elicit different response profiles to lingual NaCl stimulation. To examine the effect of dietary sodium deprivation on the responses of PBN gustatory neurons, we observed the taste responses of the PBN neurons to the four taste qualities and serial concentrations of NaCl in 15-day dietary sodium-deprived and control rats. The results showed that sodium deprivation reduced the responses of PBN taste neurons to 0.1-1.0M NaCl, but not to other tastants. Based on the analyses classified by best-stimulus categories, the number of NaCl-best neurons decreased from 68% to 45% following dietary sodium deprivation, and the responses of the NaCl-best neurons to 0.03-1.0M NaCl were significantly inhibited. Multidimensional scaling illustrated that sodium deprivation increased the similarity of the response profiles of the NaCl-best neurons. These findings suggest that dietary sodium deprivation might modulate sodium intake via increasing aversive threshold for salt rather enhancing salt discrimination.     

Long-lasting inhibition of the cardiovascular responses to glutamate and the baroreceptor reflex elicited by neuropeptide Y injected into the nucleus tractus solitarius of the rat.

Neuropeptide Y (NPY) microinjected unilaterally into the nucleus tractus solitarii (NTS) of anesthetized paralyzed rats elicits a gradual dose-dependent and reversible fall in arterial pressure (AP) and heart rate (HR) lasting 20 min. It also abolished the brief (less than 1 min) dose-dependent and reversible fall of AP and HR elicited by L-glutamate (L-Glu) injected into the nucleus. The blockade of L-Glu by NPY appeared gradually and was prolonged, lasting over 2 h, and recovering by 24 h. It was not replicated by desamido-NPY or galanin. Unlike 2% lidocaine it did not block the hypotension elicited by focal electrical stimulation at the injection site indicating the response was not that of a local anesthetic. Bilateral injection of NPY into the NTS resulted, after an initial fall, in an elevation of AP (+48 +/- 10.6 mmHg). At this time the reflex bradycardia evoked by elevating AP with phenylephrine was markedly reduced. We conclude that in the NTS, NPY antagonizes the actions of L-Glu and may attenuate baroreceptor reflexes. Since the NTS is richly innervated by NPY neurons and contains many NPY binding sites and since primary baroreceptor afferents appear to be glutamatergic the results suggested that NPY may serve in NTS as a long-term regulator of baroreceptor reflex activity.     

Feeding and macronutrient selection patterns in rats: adrenalectomy and chronic corticosterone replacement

To analyze further the role of corticosterone (CORT) in the control of feeding behavior, we examined the impact of adrenalectomy (ADX) and chronic CORT implants on the food intake and macronutrient self-selection patterns of adult male rats at different periods of the diurnal cycle. Consistent with a separate study of acute CORT injection in ADX rats (Kumar and Leibowitz, 1988), the present findings indicate that ADX significantly attenuated the rats' daily (24 hr) ingestion of all three macronutrients, namely, protein, carbohydrate and fat. However, food intake in the dark cycle, specifically during the first few hours after dark onset, was significantly more affected (-70%) than feeding in the later dark and light periods (-25%). Moreover, during this early dark time when circulating CORT level normally peaks, ADX appeared to have its strongest suppressive effect on carbohydrate ingestion. Chronic subcutaneous CORT implants in the ADX animals reversed these effects of surgery and generally restored the rats' eating patterns to that of the cholesterol-implanted SHAM animals. These findings suggest that CORT exerts a decisive influence on caloric intake, on the diurnal pattern of feeding, and on appetite for specific macronutrients. The impact of CORT on carbohydrate intake is apparent specifically during the active eating period, particularly at dark onset when endogenous CORT levels normally peak and carbohydrate is exhibited as the preferred macronutrient.     

Antinociceptive effects induced by injection of the galanin receptor 1 agonist M617 into central nucleus of amygdala in rats

The present study was performed to explore the antinociceptive effects of M617, a selective galanin receptor 1 agonist, in the central nucleus of amygdala (CeA) of rats. Intra-CeA injection of 0.1 nmol, 0.5 nmol and 1 nmol of M617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, rats received intra-CeA administration of M617 and galanin. The HWL to noxious thermal and mechanical stimulations increased markedly, and there were no significant differences in HWLs of rats received intra-CeA administration of M617 and galanin. The results demonstrated that intra-CeA injection of M617 induced significant antinociceptive effects in CeA of rats, indicating that galanin receptor 1 may be involved in M617-induced antinociception in the CeA of rats.     

Bilateral lesions of the SCN abolish lipolytic and hyperphagic responses to 2DG

The effects of intracranial injection of 2-deoxy-D-glucose (2DG) on the plasma free fatty acid (FFA) concentration and food intake in the light (11:00 hr) and dark (23:00 hr) period were examined in male Wistar rats under a 12-hr light (08:00–20:00) and 12-hr dark (20:00–08:00) cycle. In sham-operated rats 2DG injection induced increase in plasma FFA concentration and in food intake in the light period, but not in the dark period. Bilateral lesions of the suprachiasmatic nucleus (SCN) resulted in complete loss of the lipolytic and feeding responses to 2DG in the light period. From these findings it is concluded that the lipolytic and feeding responses to 2DG are photoperiodic and that the SCN is involved in these phenomena.     

Glutamatergic vestibular neurons express Fos after vestibular stimulation and project to the NTS and the PBN in rats

In this study, retrograde tracing method combined with phosphate-activated glutaminase (PAG) and Fos immunofluorescence histochemistry was used to identify glutamatergic vestibular nucleus (VN) neurons receiving vestibular inputs and projecting to the nucleus of the solitary tract (NTS) and the parabrachial nucleus (PBN). Conscious animals were subjected to 120 min Ferris-wheel like rotation stimulation. Neuronal activation was assessed by Fos expression in the nucleus of VN neurons. After Fluoro-gold (FG) injection into the caudal NTS, approximately 48% FG-labeled VN neurons were immunoreactive for PAG, and about 14% PAG/FG double-labeled neurons co-existed with Fos. Following FG injection into the PBN, approximately 56% FG-labeled VN neurons were double-labeled with PAG, and about 12% of the PAG/FG double-labeled neurons also expressed Fos. Careful examination of the typology and distribution pattern of these PAG-immunoreactive neurons indicated that the vast majority of these neurons were glutamatergic rather than GABAergic. These results suggest that PAG-immunoreactive VN neurons might constitute excitatory glutamatergic VN-NTS and VN-PBN transmission pathways and these pathways might be involved in vestibulo-autonomic reflexes during vestibular stimulation.     

Effects of adrenal steroid agonists on food intake and macronutrient selection.

These experiments tested the effects of subcutaneous (SC) and paraventricular nucleus (PVN) administration of the steroid receptor agonists, corticosterone (CORT), aldosterone (ALDO), RU28362, and dexamethasone (DEX), on food intake and macronutrient selection during the first h of the dark feeding period in the rat. Results indicate that CORT and the selective type II receptor agonist RU28362 specifically stimulate carbohydrate ingestion after SC or PVN administration, while DEX has no effect on feeding. This selective effect of SC CORT on carbohydrate ingestion is dose dependent, seen at doses ranging from 0.125 to 2.0 mg/kg. Moreover, the stimulatory effects of CORT and RU28362 on carbohydrate intake are observed in ADX rats but not in sham rats. This is in contrast to SC and PVN administration of the type I receptor agonist ALDO, which specifically enhances fat ingestion in both sham and ADX rats. These results, with both peripheral and central steroid administration, reveal selective effects of type I and type II receptor stimulation on fat and carbohydrate intake, respectively.     

Pro-opiomelanocortin gene transfer to the nucleus of the solitary track but not arcuate nucleus ameliorates chronic diet-induced obesity

Short-term pharmacological melanocortin activation deters diet-induced obesity (DIO) effectively in rodents. However, whether central pro-opiomelanocortin (POMC) gene transfer targeted to the hypothalamus or hindbrain nucleus of the solitary track (NTS) can combat chronic dietary obesity has not been investigated. Four-weeks-old Sprague–Dawley rats were fed a high fat diet for 5 months, and then injected with either the POMC or control vector into the hypothalamus or NTS, and body weight and food intake recorded for 68 days. Insulin sensitivity, glucose metabolism and adrenal indicators of central sympathetic activation were measured, and voluntary wheel running (WR) assessed. Whereas the NTS POMC-treatment decreased cumulative food consumption and caused a sustained weight reduction over 68 days, the hypothalamic POMC-treatment did not alter cumulative food intake and produced weight loss only in the first 25 days. At death, only the NTS-POMC rats had a significant decrease in fat mass. They also displayed enhanced glucose tolerance, lowered fasting insulin and increased QUICK value, and elevated adrenal indicators of central sympathetic activation. Moreover, the NTS-POMC animals exhibited a near 20% increase in distance ran relative to the respective controls, but the ARC-POMC rats did not. In conclusion, POMC gene transfer to the NTS caused modest anorexia, persistent weight loss, improved insulin sensitivity, and increased propensity for WR in DIO rats. These metabolic improvements may involve stimulation of energy expenditure via centrally regulated sympathetic outflow. The similar POMC treatment in the hypothalamus had minimal long-term physiological or metabolic impact. Thus, melanocortin activation in the brainstem NTS region effectively ameliorates chronic dietary obesity whilst that in the hypothalamus fails to do so.     

The pattern of c-Fos immunoreactivity in the hindbrain of the rat following stomach distension

It has been previously shown that the walls of the stomach contain vagal and splanchnic afferents, connected to low and high threshold (LT and HT) gastric receptors, that convey physiological and noxious information to areas of the hindbrain involved mainly in the control of gastrointestinal function. Because distension of the stomach also reflexly increases the sympathetic drive to the cardiovascular system, the present study was planned to examine the pattern of activation of all nuclei encountered throughout the hindbrain in response to gastric distension. In anaesthetized rats, the stimulus was controlled by employing different transmural pressures and frequencies of distension, and c-Fos immunohistochemistry was used to characterize neuronal activation. Low intensity stimulation induced c-Fos expression in the cranial part of nucleus of solitary tract (NTS), the nucleus ambiguus (NA), the lateral reticular area (LRt) and the ventrolateral medulla (RVL/CVL). At low frequency of stimulation c-Fos positive nuclei (p.n.) were found in NTS only. At high frequency of stimulation an increase in c-Fos immunoreactivity was found. High intensity stimulation induced c-Fos expression in area postrema (AP), the lateral vestibular nucleus (LVe) and the caudal part of the NTS. At low frequency, only the number of c-Fos p.n. was increased. Increasing the frequency of stimulation induced c-Fos expression in further nuclei such as the parabrachial nucleus (PBN), the inferior olive subnuclei (IOn), the oral part of spinal trigeminal nucleus (Sp5O) and locus coeruleus (LC). At higher frequencies c-Fos immunoreactivity decreased in NTS and LRt, disappeared in VLM and increased in NA. Thus stomach distension activated several neuronal excitatory and inhibitory circuits that are involved in the control of gastrointestinal function as well as in cardiovascular, respiratory and pain regulation. The differences in c-Fos immunoreactivity induced by changing the distension patterns suggested interactions between groups of vagal and splanchnic afferents.     

Galanin stimulation of feeding is blocked by the addition of a response requirement

Administration of the neuropeptide galanin increases food intake in laboratory rats and mice, however this increase has only been observed under conditions of free-feeding. As there is a growing distinction between consummatory and instrumental behavior, we assessed whether galanin would differentially affect food consumption when food was freely available or when the same food was response-contingent. We also tested whether food restriction would interact with galanin's effect on food consumption in either condition. As in previous studies, galanin significantly increased food consumption under free access conditions. However, when food was contingent upon lever-pressing, galanin had no such stimulatory effect. Food-restriction increased basal feeding levels in both tasks but there was no interaction between food restriction and free or response-contingent food intake. These results demonstrate that galanin-induced feeding stimulation is limited to conditions of free access and support the theoretical distinction between consummatory and instrumental behavior.     

Development of taste responses in the rat parabrachial nucleus

Extracellular responses from neurons in the parabrachial nuclei (PBN) were studied in rats 4 days old to adulthood during chemical stimulation of the tongue with monochloride salts, citric and hydrochloric acids, sucrose, sodium saccharin, and quinine hydrochloride. Multiunit taste responses were recorded in rats at 4-7 days of age and single-unit responses were recorded from 121 neurons in four other age groups of 14-20 days, 25-35 days, 50-60 days, and adults. PBN neurons in rats 4-7 days old consistently responded to 0.1 M solutions of NH4Cl and NaCl, to 0.5 M solutions of NH4Cl, NaCl, and KCl, and to 1.0 M sucrose, 0.1 M sodium saccharin, 0.1 M citric acid, and 0.1 N HCl. They often did not respond, however, to 0.1 M KCl and 0.01 M quinine hydrochloride. Single PBN neurons in rats 14 days old and older characteristically responded to all stimuli, which consisted of 0.1 and 0.5 M salts, acids, sucrose, sodium saccharin, and quinine hydrochloride. Thus no developmental differences occurred in the number of stimuli to which neurons responded after rats were 14 days old. With the exception of responses to hydrochloric acid, there were significant increases in response frequencies to all stimuli after 14 days of age. Average response frequencies to NH4Cl and citric acid increased after 20 days of age and those to NaCl, LiCl, KCl, sucrose, sodium saccharin, and quinine hydrochloride increased after 35 days of age. Average response frequencies for hydrochloric acid did not alter after 14 days of age. The proportion of single PBN neurons that responded maximally to specific monochloride salts did not change during development. Most single neurons in all age groups responded equally well to NH4Cl, NaCl, and LiCl. No PBN neuron responded maximally to KCl. Developmental differences in response frequencies of third-order gustatory neurons in the PBN generally reflect developmental response changes in first-order neurons of the chorda tympani nerve and second-order neurons of the solitary nucleus. However, unique developmental changes are evident in the PBN. Thus the ontogenetic changes that occur in PBN responses likely relate to modifications of lower-order peripheral and central nervous system afferents and peripheral receptor sensitivities.     

The orexigenic effect of peripheral ghrelin differs between rats of different age and with different baseline food intake, and it may in part be mediated by the area postrema

Ghrelin is mainly secreted during fasting. While an orexigenic effect of peripherally injected ghrelin has been reported, reproducing this effect has often proven difficult. Here, we hypothesized that ghrelin's effect to increase food intake may depend on the experimental conditions (e.g., age of animals). We therefore investigated the effect of an IP ghrelin injection (100 microg/kg) on food intake in rats of different age and at different times during the light-dark cycle, i.e. with different levels of baseline food intake. Ghrelin injected at dark onset in ad libitum fed young rats (body weight [BW] 92 g) slightly increased feeding while no such effect was observed in 12 h food deprived rats (BW 150 g). In the middle of the light phase, ghrelin significantly increased feeding up to 2 h after injection in ad libitum fed rats (BW 130 g; food intake 1 h after injection: NaCl 0.4 +/- 0.2 g versus ghrelin 1.2 +/- 0.3 g [p < 0.05]). In various subsequent experiments, older rats (BW 300-490 g) tested under the same conditions did not respond to a single ghrelin injection. However repeated ghrelin injection (15 microg/kg/day once daily at light onset) over 10 days significantly increased food intake in rats (BW 400-460 g) starting from day 4 of the experiment (24 h food intake: NaCl approx. 19.5 g, ghrelin 22.5 g). Interestingly, the latter effect was completely abolished in rats lesioned in the area postrema (AP). Cumulative food intake was also increased in SHAM but not in AP-X animals (e.g., after 7 days: SHAM/NaCl 135.1 +/- 5.3 g versus SHAM/ghrelin 149.7 +/- 3.5 g [p < 0.05], AP-X/NaCl 127.2 +/- 16.4 versus AP-X/ghrelin 127.9 +/- 5.3). We conclude that ghrelin's effect to increase food intake can best be demonstrated when basal food intake is low. Ghrelin increases feeding mainly in young, fast growing animals. Ghrelin may therefore link the high energy needs to body growth in young individuals. In older animals, peripheral ghrelin increased feeding when injected repeatedly over several days. At least under these conditions, ghrelin's effect was mediated by the AP/NTS region. Using repeated administration, ghrelin might be an interesting tool to increase feeding in patients suffering from wasting diseases such as cancer anorexia.     

Rapid inhibition of neural excitability in the nucleus tractus solitarii by leptin: implications for ingestive behaviour

The fat-derived peptide leptin regulates cellular activity in areas of the CNS related to feeding, and application of leptin to the fourth ventricle or the nucleus tractus solitarii (NTS) inhibits food intake and weight gain. The hypothesis that leptin modulates cellular activity in the NTS was tested using whole-cell patch-clamp recordings in brainstem slices. Leptin caused a rapid membrane hyperpolarization in 58% of rat NTS neurones, including neurones receiving tractus solitarius input (i.e. viscerosensory) and those involved in regulating output to the stomach, identified after gastric inoculation with a transneuronal retrograde viral label. The hyperpolarization was accompanied by a decrease in input resistance and cellular responsiveness, reversed near the K⁺ equilibrium potential, and was prevented by intracellular Cs⁺. Perfusion of tolbutamide (200 μ m ) or wortmannin (100–200 n m ) prevented the hyperpolarization, indicating activation of an ATP-sensitive K⁺ channel via a PI3 kinase-dependent mechanism. Constant latency tractus solitarius-evoked EPSCs were decreased in amplitude by leptin, and the paired-pulse ratio was increased, suggesting effects on evoked EPSCs involved activation of receptors on vagal afferent terminals. Leptin reduced the frequency of spontaneous and miniature EPSCs, whereas IPSCs were largely unaffected. Leptin's effects were observed in neurones from lean, but not obese, Zucker rats. Neurones that expressed enhanced green fluorescent protein (EGFP) in a subpopulation of putative GABAergic neurones in transgenic mice did not respond to leptin, whereas unlabelled murine neurones responded similarly to rat neurones. Leptin therefore directly and rapidly suppresses activity of excitatory NTS neurones likely to be involved in viscerosensory integration and/or premotor control of the stomach.     

DOPA cyclohexyl ester, a DOPA antagonist, blocks the depressor responses elicited by microinjections of nicotine into the nucleus tractus solitarii of rats

Nicotinic cholinergic receptors play a role in cardiovascular regulation in the lower brain stem. Herein, we present evidence that l-3,4-dihydroxyphenylalanine (DOPA), a putative neurotransmitter in the central nervous system, is involved in the depressor response to microinjection of nicotine into the nucleus tractus solitarii (NTS). Microinjection of nicotine into the medial area of the NTS led to decreases in arterial blood pressure and heart rate in anesthetized rats. Mecamylamine, a nicotinic receptor antagonist, microinjected into NTS, blocked the depressor and bradycardic responses to nicotine. Nicotine-induced depressor and bradycardic responses were blocked by DOPA cyclohexyl ester (DOPA CHE), an antagonist for DOPA. DOPA CHE did not modify the action of carbachol on excitatory postsynaptic potential in rat cortical slices. These results suggest that endogenous DOPA is involved in nicotine-induced depressor responses in the NTS of anesthetized rats.     

Central gustatory lesions: II. Effects on sodium appetite, taste aversion learning, and feeding behaviors.

Intake and taste reactivity tests were used to determine the effects of bilateral lesions of the gustatory portions of the nucleus of the solitary tract (NST), the parabrachial nucleus (PBN), and the ventral posteromedial nucleus of the thalamus (VPMpc) on several complex ingestive behaviors. In the 1st experiment, lesions of the PBN and the NST blocked, and VPMpc lesions impaired, the behavioral expression of salt appetite. In the 2nd experiment, alanine was paired with injections of LiCl. Control rats as well as rats with NST and VPMpc lesions acquired the taste aversion, but rats with PBN lesions did not. In the 3rd experiment, all animals increased their food intake after injections of 2 U/kg insulin and 250 mg/kg 2-deoxy-D-glucose, and their food intake was suppressed after nutritive stomach loads.     

Role of nucleus tractus solitarius 5-HT3 receptors in the defense reaction-induced inhibition of the aortic baroreflex in rats

Different stressful conditions elicit a typical behavior called the defense reaction. Our aim was to determine whether 5-HT3 receptors in the nucleus tractus solitarius (NTS) are involved in 1) the inhibition of the baroreflex bradycardia and 2) the rise in blood pressure, which are known to occur during the defense reaction. In urethane-anesthetized rats, the defense reaction was elicited by electrical stimulation of the dorsomedial nucleus of the hypothalamus (DMH) or the dorsal part of the periaqueductal gray (dPAG). Direct electrical stimulation of the aortic depressor nerve was used to trigger the typical baroreflex responses. Aortic stimulation at high (100-150 microA) and low (50-90 microA) intensity produced a decrease in heart rate of -39 to -44% (relative to baseline, Group 1 responses, n = 113) and -19 to -24% (Group 2 responses, n = 43), respectively. In spontaneously breathing rats, Group 1 and Group 2 bradycardiac responses were inhibited during DMH (-75 +/- 4% and -96 +/- 4%, n = 38 and n = 11, respectively), as well as dPAG (-81 +/- 3% and -95 +/- 4%, n = 36 and n = 10, respectively) stimulation. The aortic baroreflex bradycardia was hardly affected by DMH or dPAG stimulation when bicuculline (5 pmol), a specific GABAA receptor antagonist, had previously been microinjected into the NTS. Likewise, NTS microinjections of granisetron, a specific 5-HT3 receptor antagonist, prevented, in a dose-dependent manner, the baroreflex bradycardia inhibition. In addition, intra-NTS granisetron did not affect the rise in blood pressure induced by either site stimulation. These data show that 5-HT3 receptors in the NTS are involved in the GABAergic inhibition of the aortic baroreflex bradycardia, but not in the rise in blood pressure, occurring during the defense reaction elicited by DMH or dPAG stimulation.