5-HT2 receptors in the nucleus tractus solitarius: characterisation and role in cardiovascular regulation in the rat

Key role of 5-HT3 receptors in the nucleus tractus solitarii in cardiovagal stress reactivity

Serotonin plays a modulatory role in central control of the autonomic nervous system (ANS). The nucleus tractus solitarii (NTS) in the medulla is an area of viscerosomatic integration innervated by both central and peripheral serotonergic fibers. Influences from different origins therefore trigger the release of serotonin into the NTS and exert multiple influences on the ANS. This major influence on the ANS is also mediated by activation of several receptors in the NTS. In particular, the NTS is the central zone with the highest density of serotonin₃ (5-HT₃) receptors. In this review, we present evidence that 5-HT₃ receptors in the NTS play a key role in one of the crucial homeostatic responses to acute and chronic stress: inhibitory modulation of the parasympathetic component of the ANS. The possible functional interactions of 5-HT₃ receptors with GABA_A and NK₁ receptors in the NTS are also discussed.

5-hydroxytryptamine 2C receptors tonically augment synaptic currents in the nucleus tractus solitarii

The nucleus tractus solitarii (nTS) is the primary termination and integration point for visceral afferents in the brain stem. Afferent glutamate release and its efficacy on postsynaptic activity within this nucleus are modulated by additional neuromodulators and transmitters, including serotonin (5-HT) acting through its receptors. The 5-HT(2) receptors in the medulla modulate the cardiorespiratory system and autonomic reflexes, but the distribution of the 5-HT(2C) receptor and the role of these receptors during synaptic transmission in the nTS remain largely unknown. In the present study, we examined the distribution of 5-HT(2C) receptors in the nTS and their role in modulating excitatory postsynaptic currents (EPSCs) in monosynaptic nTS neurons in the horizontal brain stem slice. Real-time RT-PCR and immunohistochemistry identified 5-HT(2C) receptor message and protein in the nTS and suggested postsynaptic localization. In nTS neurons innervated by general visceral afferents, 5-HT(2C) receptor activation increased solitary tract (TS)-EPSC amplitude and input resistance and depolarized membrane potential. Conversely, 5-HT(2C) receptor blockade reduced TS-EPSC and miniature EPSC amplitude, as well as input resistance, and hyperpolarized membrane potential. Synaptic parameters in nTS neurons that receive sensory input from carotid body chemoafferents were also attenuated by 5-HT(2C) receptor blockade. Taken together, these data suggest that 5-HT(2C) receptors in the nTS are located postsynaptically and augment excitatory neurotransmission.

Dorsomedial medullary 5-HT2 receptors mediate immediate onset of initial hyperventilation, airway dilation, and ventilatory decline during hypoxia in mice

The dorsomedial medulla oblongata (DMM) includes the solitary tract nucleus and the hypoglossal nucleus, to which 5-HT neurons project. Effects of 5-HT in the DMM on ventilatory augmentation and airway dilation are mediated via 5-HT2 receptors, which interact with the CO(2) drive. The interaction may elicit cycles between hyperventilation with airway dilation and hypoventilation with airway narrowing. In the present study, effects of 5-HT2 receptors in the DMM on hypoxic ventilatory and airway responses were investigated, while 5-HT release in the DMM was monitored. Adult male mice were anesthetized, and then a microdialysis probe was inserted into the DMM. The mice were placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to hypoxic gas (7% O(2) in N(2)) for 5 min with or without a 5-HT2 receptor antagonist (LY-53857) perfused in the DMM. 5-HT release in the DMM was increased by hypoxia regardless of the presence of LY-53857. Immediate onset and the peak of initial hypoxic hyperventilatory responses were delayed. Subsequent ventilatory decline and airway dilation during initial hypoxic hyperventilation were suppressed with LY-53857. These results suggest that 5-HT release increased by hypoxia acts on 5-HT2 receptors in the DMM, which contributes to the immediate onset of initial hypoxic hyperventilation, airway dilation, and subsequent ventilatory decline. Hypoxic ventilatory and airway responses mediated via 5-HT2 receptors in the DMM may play roles in immediate rescue and defensive adaptation for hypoxia and may be included in periodic breathing and the pathogenesis of obstructive sleep apnea.

Activation of nucleus tractus solitarius 5-HT2A but not other 5-HT2 receptor subtypes inhibits the sympathetic activity in rats

Our first aim was to elucidate the mechanisms underlying the hypotensive response elicited by 5-HT(2) receptor activation in the nucleus tractus solitarius (NTS). In pentobarbitone-anaesthetized rats, intra-NTS administration of 2,5-dimethoxy-4-iodoamphetamine (DOI), a wide spectrum 5-HT(2) receptor agonist, but not an antagonist of selective 5-HT(2B) and 5-HT(2C) receptors, produced a decrease in blood pressure and heart rate. The maximal cardiovascular changes obtained by DOI (0.5 pmol) could be almost completely abolished by prior intra-NTS microinjection (10 pmol) of MDL-100907, a selective 5-HT(2A) receptor antagonist, but not by 5-HT(2B) or 5-HT(2C) receptor antagonists. In addition, using extracellular recordings we found that the large majority of identified cardiovascular rostroventrolateral medulla (RVLM) neurons were almost totally inhibited by NTS 5-HT(2A) receptor stimulation. We then investigated whether intra-NTS administration of a subthreshold dose (0.05 pmol) of DOI, known to facilitate the cardiovagal component of the baroreflex, could also modulate the sympathoinhibitory component of this reflex. These experiments showed that neither the decrease in the activity of the cardiovascular RVLM neurons and lumbar sympathetic nerve activities produced by aortic occlusion (gain of the baroreflex), nor the hypotensive response elicited by aortic nerve stimulation, were potentiated by the microinjection of DOI under such conditions. These data show that activation of 5-HT(2A), but not 5-HT(2B) or 5-HT(2C), receptors, located on NTS neurons, elicits depressor and bradycardic responses, and that this 5-HT(2A)-mediated hypotension is produced via the inhibition of RVLM cardiovascular neurons. In addition, NTS 5-HT(2A) receptor activation facilitates the cardiac but not the sympathetic baroreflex response.

Cardiac baroreflex facilitation evoked by hypothalamus and prefrontal cortex stimulation: role of the nucleus tractus solitarius 5-HT2Areceptors

We previously showed that serotonin (5-HT2) receptor activation in the nucleus of the tractus solitarius (NTS) produced hypotension, bradycardia, and facilitation of the baroreflex bradycardia. Activation of the preoptic area (POA) of the hypothalamus, which is involved in shock-evoked passive behaviors, induces similar modifications. In addition, previous studies showed that blockade of the infralimbic (IL) part of the medial prefrontal cortex, which sends projections to POA, produced an inhibitory influence on the baroreflex cardiac response. Thus, to assess the possible implication of NTS 5-HT2receptors in passive cardiovascular responses, we analyzed in anesthetized rats the effects of NTS inhibition and NTS 5-HT2receptor blockade on the cardiovascular modifications induced by chemical (0.3 M d,l-homocysteic acid) and electrical (50 Hz, 150–200 μA) stimulation of IL or POA. Intra-NTS microinjections of muscimol, a GABAAreceptor agonist, prevented the decreases in blood pressure and heart rate normally evoked by IL or POA activation. In addition, we found that intra-NTS microinjection of R(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol, a specific 5-HT2Areceptor antagonist, did not affect the decreases in cardiovascular baseline parameters induced by IL or POA stimulation but prevented the facilitation of the aortic baroreflex bradycardia normally observed during IL (+65 and +60%) or POA (+70 and +69%) electrical and chemical stimulation, respectively. These results show that NTS 5-HT2Areceptors play a key role in the enhancement of the cardiac response of the baroreflex but not in the changes in basal heart rate and blood pressure induced by IL or POA stimulation.

Contribution of 5-HT2 receptor subtypes to sleep-wakefulness and respiratory control, and functional adaptations in knock-out mice lacking 5-HT2A receptors

Serotonin (5-hydroxytryptamine; 5-HT) plays key roles in sleep-wakefulness regulation. Evidence indicates that 5-HT2 receptors are involved mainly in non-rapid eye movement sleep (NREMS) regulation and respiratory control. Here, we investigated the relative contribution of 5-HT(2A), 5-HT(2B), and 5-HT(2C) receptor subtypes to NREMS and breathing during sleep, using 5-HT2 subtype-selective ligands in wild-type (5-HT(2A)+/+) and knock-out (5-HT(2A)-/-) mice that do not express 5-HT(2A) receptors. Acute blockade of 5-HT(2A) receptors induced an increase in NREMS in 5-HT(2A)+/+ mice, but not 5-HT(2A)-/- mutants, which spontaneously expressed less NREMS than wild-type animals. In 5-HT(2A)+/+ mice, 5-HT(2B) receptor blockade produced a reduction of NREMS, whereas receptor activation induced an increase in this sleep stage. These effects were less pronounced in 5-HT(2A)-/- mice, indicating a lower sensitivity of 5-HT(2B) receptors in mutants, with no change in 5-HT(2B) mRNA. Blockade of 5-HT(2C) receptors had no effect on NREMS in both strains. In addition, an increase in EEG power density after sleep deprivation was observed in 5-HT(2A)+/+ mice but not in 5-HT(2A)-/- mice. Whole-body plethysmographic recordings indicated that 5-HT(2A) receptor blockade in 5-HT(2A)+/+ mice reduced NREMS apneas and bradypneas that occurred after sighs. In contrast, in 5-HT(2A)-/- mutants, NREMS apneas were not modified, and bradypnea after sighs were more pronounced. Our results demonstrate that 5-HT exerts a 5-HT(2B)-mediated facilitation of NREMS, and an influence respectively inhibitory on NREMS and facilitatory on sleep apnea generation, via 5-HT(2A) receptors. Moreover, 5-HT(2A) gene knock-out leads to functional compensations yielding adaptive changes opposite to those caused by pharmacological blockade of 5-HT(2A) receptors in 5-HT(2A)+/+ mice.

Action site of ketanserin enhancing baroreflex function is within the rostral ventrolateral medulla in anesthetized rats

In our previous studies, it was found that ketanserin enhanced baroreflex sensitivity (BRS) in rats and this effect was not blood pressure dependent. The present work was designed to investigate the effects of ketanserin on BRS within the nucleus tractus solitarus (NTS) and the rostral ventrolateral medulla (RVLM). In anesthetized rats, BRS were evaluated by the changes in depressor action of aortic nerve stimulation and bradycardiac response to rapid pressor action of intravenous phenylephrine. It was found that bilateral injection of ketanserin (250 pmol for each side) into the NTS not only significantly increased blood pressure, but also attenuated baroreflex function. Interestingly, when ketanserin was bilaterally injected into RVLM, a significant enhancement of BRS was found and there were no modifications in basal blood pressure and heart period. The present study demonstrates that ketanserin-induced enhancement of BRS mainly occurred within the RVLM, which might contribute to the systemic effects of ketanserin on BRS.

Carotid sinus nerve section abolishes NMDA evoked respiratory effects in anaesthetised rats

Respiratory effects of NMDA injection into the right atrium were investigated in 11 urethane-chloralose anaesthetised and spontaneously breathing rats. The animals were initially vagotomised and six of them were subdued to the subsequent carotid sinus nerve section, and the other five were treated by NMDA antagonist. Bolus injection of NMDA (27 micromol/kg) induced the depression of ventilation in all rats, due to the decrease in tidal volume from a baseline of 2.98 +/- 0.4 to 2.63 +/- 0.3 ml (P < 0.01), and slowing down of the respiratory rate from a baseline of 56 +/- 2.6 to 27 +/- 2.0 breaths min(-1) (P < 0.0001). Section of the carotid sinus nerves (CSNs) precluded the respiratory depression. Prolongation of the expiratory time was reduced by this neurotomy from 5.07 +/- 2.6 to 1.04 +/- 0.03 (P < 0.05). In five rats the blockade of NMDA receptors with the selective antagonist (AP-7) was likewise efficient in eliminating the post-NMDA respiratory response. NMDA increased mean arterial blood pressure and this rise occurred beyond the afferentation from the carotid bodies and the blockade of NMDA receptors. Results of this study indicate that inhibition of the respiratory drive evoked by NMDA administered via the peripheral circulation requires intact carotid bodies and activation of NMDA receptors.

Serotonin(3) receptor stimulation in the nucleus tractus solitarii activates non-catecholaminergic neurons in the rat ventrolateral medulla

The present study was performed to determine whether or not the increased arterial pressure triggered by 5-HT(3) receptor stimulation in the nucleus tractus solitarii and underlain by a sympathoexcitation is associated with the activation of ventromedullary cells known to be involved in vascular regulation, i.e. the C1 and A1 catecholaminergic cells. For this purpose, double immunohistochemical labeling for tyrosine hydroxylase and c-fos protein was performed all along the ventrolateral medulla after microinjection of 1-(m-chlorophenyl)-biguanide, a selective and potent 5-HT(3) receptor agonist, into the nucleus tractus solitarii of alpha-chloralose/urethane-anaesthetized rats. This treatment produced a significant elevation of arterial pressure ( approximately +35 mm Hg). Concomitantly, a significant increase in the number of c-fos expressing neurons was observed in the rostral ventrolateral medulla (+63%), in particular in its most anterior part (+78%), and in the medullary region surrounding the caudal part of the facial nucleus (+91%). Retrograde labeling with gold-horseradish peroxidase complex showed that at least some of these activated c-fos expressing cells project to the spinal cord. However, the number of double-stained neurons, i.e. c-fos and tyrosine hydroxylase positive neurons, did not increase at any level of the ventrolateral medulla. In contrast, under the same alpha-chloralose/urethane anesthesia, systemic infusion of sodium nitroprusside appeared to produce a hypotension and a marked increase in the density of such double c-fos and tyrosine hydroxylase expressing cells in the rostral ventrolateral medulla and the caudal medullary region surrounding the caudal part of the facial nucleus. These data indicate that medullary catecholaminergic C1 and A1 neurons are not involved in the pressor effect elicited by 5-HT(3) receptor stimulation in the nucleus tractus solitarii. However, this 5-HT(3) receptor-mediated effect is clearly associated with the excitation of (non-catecholaminergic) neurons within the pressor region of the ventral medulla.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Stimulation of 5-HT2 receptors in the nucleus tractus solitarius enhances NMDA receptor-mediated reflex-evoked bradycardiac responses in the rat

The modulation by 5-HT2 receptors in the nucleus tractus solitarius of the reflex bradycardia evoked by stimulation of peripheral baroreceptors and cardiopulmonary chemoreceptors, and their possible functional interactions with local NMDA receptors, were investigated in pentobarbital- and urethane-anaesthetized rats, respectively. Microinjection of the 5-HT2 receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (0.1-0.5 pmol), into the nucleus tractus solitarius elicited a dose-dependent hypotension and bradycardia. Bilateral microinjections at the same site of a subthreshold dose of 2,5-dimethoxy-4-iodoamphetamine (0.05 pmol) significantly enhanced the aforementioned reflex-evoked bradycardiac responses. In contrast, local bilateral microinjections of the NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (500 and 1000 pmol), reduced, in a dose-dependent manner, both reflex-evoked responses. The facilitatory effect of 2,5-dimethoxy-4-iodoamphetamine upon these reflex-evoked bradycardiac responses was prevented by prior local microinjection of low doses of either the selective 5-HT2 receptor antagonist, ketanserin (10 pmol), or 2-amino-5-phosphonopentanoic acid (100 pmol), which, on their own, did not affect the reflex-associated bradycardia. These data suggest that 5-HT2 receptors within the nucleus tractus solitarius participate in a facilitatory modulation of the reflex control of heart rate, probably through functional interactions with local NMDA receptors.

In vivo modulation of vagal-identified dorsal medullary neurones by activation of different 5-Hydroxytryptamine(2) receptors in rats

1. In in vivo experiments, DOI (a 5-HT(2) receptor agonist), MK-212 (a 5-HT(2C) receptor agonist), and BW-723C86 (a 5-HT(2B) receptor agonist) were applied by ionophoresis to neurones in the rat nucleus tractus solitarius (NTS) receiving vagal afferent input. 2. The majority of the putative 'monosynaptically' vagal activated cells were inhibited by both MK-212 (4/6) and DOI (2/4), but unaffected by BW-723C86 (12/14). In contrast, 'polysynaptically' activated NTS cells were excited by both BW-723C86 (13/19) and DOI (9/10). Inactive 'intermediate' cells were inhibited by BW-723C86 (9/12), MK-212 (5/6) and DOI (3/4), whilst active cells of this group were excited by BW-723C86 (7/13) and DOI (5/5). 3. The selective 5-HT(2B) receptor antagonist LY-202715 significantly reduced the excitatory actions of BW-723C86 on 'intermediate' and 'polysynaptic' cells (13/13), but not the inhibitory effects observed on inactive Group 2 cells (n=5) whereas the selective 5-HT(2C) receptor antagonist RS-102221 reversed the inhibitory effects of MK-212 and DOI on 'monosynaptic and 'intermediate' neurones. 4. Cardio-pulmonary afferent stimulation inhibited two of four putative 'monosynaptically' activated calls and all four inactive intermediate cells. These were also inhibited by DOI and MK-212. In contrast, cardio-pulmonary afferents excited all five active intermediate cells and all six putative 'polysynaptically' activated NTS cells, while all were also previously excited by BW-723C86 and/or DOI. 5. In conclusion, these data demonstrate that neurones in the NTS are affected differently by 5-HT(2) receptor ligands, in regard of their vagal postsynaptic location, the type of cardio-pulmonary afferent they receive and the different 5-HT(2) receptors activated.

In vivo modulation of nucleus tractus solitarius (NTS) neurones by activation of 5-hydroxytryptamine(2) receptors in rats

In in vivo experiments, 5-hydroxytryptamine (5-HT) and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI), a 5-HT(2) receptor agonist, were applied by ionophoresis to rat nucleus tractus solitarius (NTS) neurones identified by their vagal and cardiopulmonary afferent inputs to test whether the response of NTS cells to 5-HT(2) receptor activation was related to whether they received mono- or polysynaptic vagal inputs and their presumed function as defined by their afferent input. Cells were classified on the basis of the variability of the latency of the vagal-evoked spikes: this varied by less than 3 ms for Group 1, from 3 to 5 ms for Group 2, and more than 5 ms for Group 3. Both 5-HT and DOI inhibited most Group 1 cells (16/18) and inactive (without ongoing activity) cells (8/13) in Group 2. Cells inhibited by DOI were also inhibited by cardiopulmonary afferent stimulation, evoked by atrial phenylbiguanide administration. By contrast, application of 5-HT and DOI excited the majority of Group 3 cells (14/19) and Group 2 with ongoing activity (7/9). Cells excited by DOI were also activated by cardiopulmonary stimulation. Both actions of DOI were reversed by application of ketanserin (n=15). In conclusion, these data demonstrate that activation of 5-HT(2) receptors in the NTS produces different effects dependent on whether the neurones received mono- or polysynaptic vagal input and their response to cardiopulmonary afferent stimulation.

Glutamate stimulation of raphe pallidus attenuates the cardiopulmonary reflex in anaesthetised rats

Serotonin has been implicated as having a modulatory action on NTS neurones mediating vagal inputs controlling the cardiorespiratory system. Since raphe pallidus and raphe obscurus both send serotonergic projections to the NTS, we have investigated a neuromodulatory role for these structures on the cardiopulmonary reflex. A multibarrelled microelectrode positioned around the level of the area postrema was placed at varying depths into mid-line brainstem structures and the effect of glutamate stimulation on the cardiopulmonary reflex tested. Excitatory chemical stimulation in the region of raphe pallidus, but not raphe obscurus, attenuated significantly the respiratory and bradycardic components of the cardiopulmonary reflex. This attenuation was reversed by an NTS microinjection of RS-39604, a selective 5-HT, receptor antagonist. We propose that neurones in raphe pallidus that project to the NTS can release serotonin which acts via 5-HT4 receptors to attenuate the reflex phrenic nerve activity and heart rate components of the cardiopulmonary reflex.

Inhibition of rat nucleus tractus solitarius neurones by activation of 5-HT2C receptors

In vivo, nucleus tractus solitarius (NTS) neurones receiving monosynaptic vagal input and inactive intermediate neurones were inhibited by both DOI and a selective 5-HT2C receptor agonist, MK-212. Cells receiving a more polysynaptic input were excited by DOI and although MK-212 also excited a few of these cells, the majority of cells in these groups were unaffected by MK-212. The inhibitory, but not the excitatory actions of both MK-212 and DOI were prevented by a selective 5-HT2C receptor antagonist, RS-102221. In contrast, most dorsal vagal preganglionic neurones were unaffected by application of either DOI or MK-212, the few remaining cells being excited by both agonists. These data demonstrate that DOI-evoked inhibition of NTS cells activated by vagal afferent input and DOI-evoked excitation of vagal preganglionic neurones is mediated by 5-HT2C receptors.

Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats

Previous studies have demonstrated that injection of serotonin into the nucleus tractus solitarius (NTS) elicits hypotension and bradycardia in rats. The present study sought to further characterize this response and to examine the role of serotonergic mechanisms in the NTS in cardiovascular regulation in spontaneously hypertensive (SHR) rats. Injections of picomole amounts of serotonin into the NTS of chloralose-anesthetized normotensive Sprague-Dawley (S-D) or Wistar-Kyoto (WKY) rats produced hypotension and bradycardia that were eliminated by prior injection into the NTS of the selective 5HT(2) antagonist sarpogrelate. Bilateral injection of sarpogrelate did not alter blood pressure or reflex changes in heart rate in response to phenylephrine-induced increases in blood pressure or nitroprusside-induced decreases in blood pressure. In SHR rats, the depressor response produced by injection of serotonin into the NTS was markedly larger than in WKY rats, and was larger than depressor responses previously reported for other excitatory substances injected into the NTS. In SHR rats bilateral injection of sarpogrelate produced an increase in blood pressure, although it did not alter baroreceptor-evoked changes in heart rate. These results provide further support for the hypothesis that stimulation of 5HT(2) receptors in the NTS contributes to cardiovascular regulation independent of the baroreceptor reflex. Furthermore, this serotonergic system is altered in SHR rats, apparently acting tonically to reduce blood pressure.

Dorsal medullary 5-HT3 receptors and sympathetic premotor neurones in the rat

1. Our aim was to determine whether the cardiovascular neurones in the rostro-ventrolateral medulla (CV-RVLM neurones) were involved in the sympathoexcitation induced by stimulation of 5-HT3 receptors in the region of the nucleus tractus solitarii (NTS). Experiments were performed in pentobarbitone-anaesthetized rats, artificially ventilated and paralysed with pancuronium bromide. 2. Using extracellular recordings, different types of RVLM neurones were characterized: cardiovascular (CV), ventilation-related and baroreflex-insensitive (unidentified) neurones. The CV-RVLM cells were further subdivided into three populations according to their axonal conduction velocities: A (1.2 +/- 0.1 m s-1), B (2.5 +/- 0.2 m s-1) and C (6.8 +/- 1.1 m s-1). 3. Only the CV-RVLM neurones of the A and B categories were partially inhibited (-30 %) by a hypotensive dose (2.5 microg kg-1 i.v.) of clonidine. 4. Microinjections into the region of the commissural NTS of 1-(m-chlorophenyl)-biguanide (CPBG, 2 nmol), a selective 5-HT3 receptor agonist, elicited an increase in both lumbar sympathetic nerve discharge (SND) and arterial pressure. In addition, this treatment produced a marked excitation of CV-RVLM neurones of the A and B categories, without affecting those of the C type, as well as ventilation-related and unidentified RVLM cells. 5. The activity of the CV neurones in the caudo-ventrolateral part of the medulla oblongata (CV-CVLM) was not modified by 5-HT3 receptor stimulation in the NTS. 6. Prior intra-NTS microinjections of ondansetron (300 pmol, a selective 5-HT3 receptor antagonist) into the region of the commissural NTS prevented the excitation of A and B CV-RVLM neurones induced by CPBG. 7. Intracarotid administration of saline saturated with CO2 (chemoreceptor activation) elicited both an increase in the SND and an excitation of the clonidine-insensitive CV-RVLM neurones of the C type, without affecting A and B neurones. 8. In conclusion, the sympathoexcitation elicited following 5-HT3 receptor stimulation in the region of the commissural NTS of pentobarbitone-anaesthetized rats seems to result from the excitation of two different pools of clonidine-sensitive CV-RVLM neurones. These neurones are apparently not involved in the sympathetic component of the chemoreceptor reflex.

In vivo effects of 5-hydroxytryptamine receptor activation on rat nucleus tractus solitarius neurones excited by vagal C-fibre afferents

The effects of ionophoretically applied 5-hydroxytryptamine (5-HT) and 5-HT receptor agonists were studied on rat nucleus tractus solitarius (NTS) neurones receiving unmyelinated vagal afferent input. 5-HT excited 15 of 34 neurones (44%), inhibited 10 (29%) and had no effect on nine. 8-Hydroxy-2-(di-N-propylamino)tetralin HBr (8-OH-DPAT) excited 23 of 53 neurones (43%), inhibited 24 (45%) and had no effect on six neurones and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl activated 18 of 37 neurones (49%), inhibited nine (24%) and had no effect on 10. These results demonstrate that activation of 5-HT1A and 5-HT2 receptors can excite or inhibit populations of NTS neurones. Phenylbiguanide, however, excited 20 of 23 neurones (87%), inhibited only one (4%) and had no effect on two indicating that 5-HT3 receptor activation has an excitatory action. NTS neurones receiving cardiac vagal afferent input were more likely to be excited by 5-HT (five of five, 100%) or 8-OH-DPAT (four of five. 80%) than the population as a whole. In conclusion, the data demonstrate that 5-HT1A, 5-HT2, and 5-HT3 receptor subtypes are functionally present on NTS neurones receiving excitatory vagal afferent input. Further, the subpopulation of NTS neurones receiving input from cardiac afferents are excited by 5-HT, possibly by an action on 5-HT1A or 5-HT3 receptors.

Augmented release of serotonin by adenosine A2a receptor activation and desensitization by CGS 21680 in the rat nucleus tractus solitarius

Rat dorsomedial medullary brain segments containing primarily nucleus tractus solitarius (NTS) were employed for slice superfusion studies of electrically evoked [3H]serotonin ([3H]5-HT) release. Individual slices loaded with [3H]5-HT were stimulated two times, S1 and S2, at 3 Hz, 25 mA, 2 ms pulses for 1 min. Control NTS slices had a S2/S1 ratio of 0.94 (+/- 0.02). Superfusion of tissue slices with 0.1 nM to 100 nM 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680), a selective adenosine A2a receptor agonist, for 5 min prior to the S2 stimulus produced a significant concentration-dependent increase in the S2/S1 fractional release ratio which was maximal (37.2% increase, P < 0.01) at 1.0 nM. However, superfusion of tissue slices with CGS 21680 over the same concentration range for 20 min prior to the S2 stimulus did not significantly alter the S2/S1 ratio from control release ratios. The augmented release of [3H]5-HT mediated by 1.0 nM CGS 21680 with 5 min tissue exposure was abolished by 1.0 nM 9-chloro-2-(2-furanyl)-5, 6-dihydro-[1,2,4]-triazolo[1,5-c]quinazolin-5-imine (CGS 15943) as well as by 100 nM 8-(3-chlorostyryl)caffeine (CSC), both A2a receptor antagonists, but not by 1.0 nM 8-cyclopentyl-1,3,-dipropylxanthine (DPCPX), the A1 receptor antagonist. These results indicate that CGS 21680 augmented the evoked release of [3H]5-HT in the NTS by way of activation of presynaptic adenosine A2a receptors. It was also apparent that this population of adenosine A2a receptors in the NTS desensitized within 20 min since the augmenting action of CGS 21680 on evoked transmitter release was not evident at the longer time interval.

The effects of 5'-N-ethylcarboxamidoadenosine on evoked release of [3H]serotonin in the rat nucleus tractus solitarius

5'-N-Ethylcarboxamidoadenosine (NECA) a non-selective adenosine A1 and A2a receptor agonist was employed in stimulated (3 Hz, 25 mA, 1 min) superfused nucleus tractus solitarius (NTS) brain slices loaded with [3H]5-hydroxytryptamine ([3H]5-HT), and ligand binding with [3H]NECA on NTS membranes. Superfusion of NTS slices with 1.0 and 3.0 nM NECA for 5 min prior to S2 stimulation produced an augmented release of [3H]5-HT (35.7%) above the control S2/S1 ratio. When the duration of NECA perfusion was increased to 20 min prior to S2, the S2/S1 ratio was depressed 21% at 1.0 nM for [3H]5-HT release. The augmented release of [3H]5-HT by NECA at 5 min was blocked by the adenosine A2a antagonist 8-(3-chlorostyryl)caffeine (CSC; 100 nM), and was reduced but not blocked by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM). Saturation binding assays with [3H]NECA on NTS membranes showed two binding sites, a high affinity site with a KD 2.18 nM and low affinity site with a KD of 44.9 nM. With the selective adenosine A2a antagonist CSC the high affinity site was blocked while 10 nM DPCPX, the A1 antagonist, reduced binding of the low affinity site, but did not abolish it. NECA binds to two different adenosine receptor sites in NTS membranes with the A2a receptor being the high affinity site. The same A2a site is associated with the augmented neurotransmitter release of [3H]5-HT with 5 min tissue exposure since it is blocked by CSC. Longer tissue exposure to NECA resulted in desensitization and finally inhibition of release possibly associated with adenosine A1 receptors.

Cardiovascular effects induced by the stimulation of neuropeptide FF receptors in the dorsal vagal complex: an autoradiographic and pharmacological study in the rat

Previous studies have suggested that central administration of neuropeptide FF-related peptides may modulate cardiovascular parameters in the rat. In the present study, we investigated the role of dorsal vagal complex neuropeptide FF receptors in the central regulation of cardiovascular parameters. The fate of neuropeptide FF receptors in normal and nodose ganglionectomized rats was investigated using an autoradiographic approach with 125I-[DTyr1, (NMe)Phe3]NPFF as ligand for these receptors. We showed that neuropeptide FF binding sites are preferentially located postsynaptically with respect to the vagal afferent fibers in the nucleus tractus solitarius. Thus, ganglionectomy reduced by only 30% and 17% the density of peptide binding sites in the rostral and caudal regions of this nucleus, respectively. Bilateral microinjection of neuropeptide FF (1 nmol) into the commissural nucleus tractus solitarius produced an increase in blood pressure (+13.8 +/- 0.8 mmHg, n = 6), bradycardia (-29.0 +/- 3.2 bpm) and a significant inhibition (-47.6 +/- 3.1%) of the cardiac component of the baroreceptor reflex. Further studies with doses below 1 nmol indicate that NTS microinjections of the neuropeptide produced a dose-dependent decrease in heart rate. Similar cardiovascular effects were observed after bilateral NTS microinjections of one analog neuropeptide FF receptor agonist, [DTyr1, (NMe)Phe3]NPFF (1 nmol). Pretreatment with prazosin (100 micrograms/kg), an alpha 1-adrenoreceptor antagonist, inhibited the neuropeptide FF-evoked blood pressure effect. In addition, the neuropeptide FF-induced heart rate decrease was abolished by pretreatment with atropine (30 micrograms/kg), a muscarinic receptor antagonist. Taken together, these anatomical and pharmacological data suggest that neuropeptide FF receptors within the nucleus tractus solitarius, preferentially located on the postsynaptic component, are involved in the central reflex regulation of cardiovascular parameters.

Extracellular catechol and indole turnover in the nucleus of the solitary tract of spontaneously hypertensive and Wistar-Kyoto normotensive rats in response to drug-induced changes in arterial blood pressure

Drug-induced alterations in arterial blood pressure are reflected in the extracellular fluid neurotransmitter levels of the nucleus of the solitary tract (NTS). Urethane-anesthetized spontaneously hypertensive rats (SHRs) and Wistar-Kyoto normotensive (WKY) rats were used in this study. The extracellular neurochemical profile of the NTS was quantified using the in vivo microdialysis technique. In SHR, phenylephrine-induced hypertension produced no significant changes in the extracellular norepinephrine (NE) and dihydroxyphenylacetic acid concentrations, whereas a significant increase in the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) concentration was observed. Wistar normotensive rats, in response to phenylephrine-induced hypertension, showed a significant increase in extracellular NE and 5-HIAA concentrations. Hypotension produced by the intravenous infusion of nitroprusside failed to show significant changes in the extracellular neurotransmitters in both SHR and WKY rats. However, there was a significant increase in 5-HIAA concentration in SHRs during the rebound hypertension, which followed the nitroprusside-infused hypotension. No such change was observed in the case of the WKY rats. These results suggest the possible involvement of the serotonergic mechanisms of NTS in the regulation of normal arterial blood pressure in these two rat strains.

Effects of serotonin-1 and serotonin-2 receptor agonists on neuronal activity in the nucleus tractus solitarius

Spontaneous neuronal activity in the solitary tract nucleus was recorded extracellularly in a brain slice preparation during bath-application of 5-HT1 and 5-HT2 receptor-selective agonists and antagonists. The 5-HT1A/5-HT1B agonist 5-carboxamidotryptamine depressed activity in 20 of 25 neurons studied. The remaining five neurons were unaffected. The 5-HT1A/5-HT1B antagonist pindolol prevented the 5-carboxamidotryptamine-induced changes, whereas the 5-HT1A antagonist spiroxatrine and the 5-HT2 antagonists ketanserin and mianserin were ineffective. Application of the 5-HT1/5-HT2 agonist alpha-methylserotonin depressed activity in 16 of 19 neurons, whereas the remaining three neurons were unresponsive. Pindolol blocked alpha-methylserotonin-induced changes of activity, but spiroxatrine, ketanserin and mianserin were ineffective. Finally, the 5-HT2 agonist DOI was applied to seven neurons. Six were unresponsive to DOI, and one responded with a depression of activity. These data provide electrophysiological evidence for the presence of 5-HT1 receptors in the nTS, presumably of the 5-HT1B subclass, but cast further doubt on the contribution of 5-HT2 and 5-HT1A receptors to the actions of serotonin in the nucleus.

Presynaptic displacement of serotonin by alpha-methyl-5-hydroxytryptamine in the nucleus tractus solitarius of the rat

Rat brain slices containing the nucleus tractus solitarius loaded with [3H]5-hydroxytryptamine ([3H]5-HT) for superfusion were stimulated at (3 Hz, 25 mA, 1 min) resulting in fractional release ratios S2/S1 of 0.89 for [3H]5-HT in the presence of the serotonin uptake inhibitor 1.0 microM 6-nitroquipazine (6-NQ). alpha-Methylserotonin (alpha-Me-5-HT; 1.0 microM), a non-selective 5-HT1 and 5-HT2 receptor agonist, significantly reduced the S2/S1 ratio of [3H]5-HT without affecting the basal release ratios B2/B1 1.00 +/- 0.04. Without 6-NQ in the perfusion medium 1.0 microM alpha-Me-5-HT sharply increased the basal release B2/B1 to 2.21 (P < 0.01). In low Ca2+ medium the S2/S1 ratio was reduced to 0.06 and alpha-Me-5-HT promoted a B2/B1 release of 2.17 (P < 0.01). The 5-HT3 antagonist LY-278,584 did not block alpha-Me-5-HT induced basal release of [3H]5-HT. Both pindolol and LY-53,857 blocked the autoinhibitory effects of alpha-Me-5-HT, but only LY-53,857 and 6-NQ blocked the basal release induced by alpha-Me-5-HT. These results suggest that alpha-Me-5-HT reduces neurotransmitter release through the serotonin autoreceptor and displaces serotonin through a non-exocytotic release mechanism.

Increased sympathetic nerve discharge without alteration in the sympathetic baroreflex response by serotonin3 receptor stimulation in the nucleus tractus solitarius of the rat

Previous studies have shown that serotonin3 receptor activation in the nucleus tractus solitarius (NTS) increased mean arterial pressure (MAP) and inhibited the cardiac component of the baroreceptor reflex [9]. We have examined the effects of such stimulation upon spontaneous and evoked sympathetic nerve activity. Microinjection of serotonin (10 nmol) into the NTS of halothane-anaesthetized, paralyzed and artificially ventilated rats produced an increase in MAP and lumbar sympathetic nerve discharge which could be completely prevented by prior local microinjection of zacopride, a potent serotonin3 antagonist (200 pmol). In addition, 1-(m-chlorophenyl)-biguanide, a selective serotonin3 receptor agonist, mimicked the sympathoexcitatory effect of serotonin. Since the gain of the sympathetic component of the baroreflex was unaltered after intra-NTS microinjection of serotonin, it could be concluded that serotonin3 receptors activation in the NTS induces a sympathetic activation which is not mediated through an inhibition of the sympathetic baroreceptor reflex arc.

Cardiovascular effects of 5HT2 and 5HT3 receptor stimulation in the nucleus tractus solitarius of spontaneously hypertensive rats

The effects of local application of 5-HT2- and 5-HT3-receptor agonists in the nucleus tractus solitarius (NTS) on the cardiovascular parameters were investigated in anaesthetized spontaneously hypertensive (SH) rats and their genetically normotensive precursors (WKY). Unilateral microinjection of picomolar doses of a 5HT2 receptor agonist, 2,5-dimethoxy-3-bromo-amphetamine (DOB, 0.025-0.5 pmol), produced a dose-dependent hypotension and bradycardia in both SH and WKY rats. These effects could be prevented by prior local microinjection of a 5-HT2 receptor antagonist, ketanserin (10 pmol). However, for both cardiovascular parameters, DOB was more potent in SH than in WKY rats. Thus, the dose-related responses to DOB were shifted to the left in SH as compared to WKY rats. Bilateral microinjection of the 5-HT3 receptor agonist, phenylbiguanide (1.7-5 nmol), produced an increase in blood pressure and reduced the cardiovagal component of the baroreflex. These effects were not significantly different in SH and WKY rats. These data suggest that 5-HT2 receptors, but not 5-HT3 receptors, are supersensitive in the NTS of SH rats.

Central distribution of substance P, calcitonin gene-related peptide and 5-hydroxytryptamine in vagal sensory afferents in the rat dorsal medulla

The central distribution of vagal afferents in the medulla containing either substance P, calcitonin gene-related peptide or 5-hydroxytryptamine was examined using a double-labelling technique and laser scanning confocal microscopy. Areas of the nucleus tractus solitarii, dorsal motonucleus of the vagus nerve and area postrema were scanned for double-labelled axon profiles. Analysis of this material revealed that all three neurochemicals were contained within the central terminals of vagal nerve sensory neurons. However, the distribution of vagal nerve afferents containing each of these putative transmitters differed. Afferents containing 5-hydroxytryptamine were detected mainly in the areas postrema and the adjacent nucleus tractus solitarii, with a smaller number in the ventral subnuclei of the solitary tract. In contrast afferents containing calcitonin gene-related peptide were found primarily in the medial and commissural regions of the nucleus tractus solitarii. Afferents containing substance P-immunoreactivity were surprisingly few in number and did not appear to be associated with any particular region. These results establish the presence of 5-hydroxytryptamine, substance P and calcitonin gene-related peptide in the central axons of vagal sensory afferents. Furthermore, the differential distribution of afferents immunoreactive for these neurochemicals seen in this study, together with previous demonstrations of the viscerotopic organization of vagal sensory afferents suggests a possible "chemical coding" for individual end organs.

Nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of the pigeon: localization of peptide and 5-hydroxytryptamine immunoreactive fibers

The distribution of peptide and serotonin fibers in the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus nerve (DMNX) in the pigeon (Columba livia) was investigated immunocytochemically. This information was correlated with the viscerotopic organization of the nuclei and with central NTS circuitry to suggest the role of the neurochemical containing fibers in the regulation of organ function. The distribution of fibers containing cholecystokinin (CCK), calcitonin gene-related peptide (CGRP), enkephalin (ENK), neuropeptide Y (NPY), neurotensin (NT), substance P (SP), somatostatin (SS), vasoactive intestinal peptide (VIP), and 5-hydroxytryptamine (5-HT) was determined. Each substance had a distinct distribution within the subnuclei of NTS-DMNX, but certain generalities can be deduced. In the DMNX, fibers immunoreactive for ENK, NT, and SP were found in greatest concentration, while CGRP and 5-HT immunoreactive fibers were the least dense. This suggests that ENK, NT, and SP may have a significant modulatory effect on gastrointestinal functions. In the NTS overall, ENK, NT, SP, and VIP fibers were found in high density, CCK, NPY, SS, and 5-HT fibers were found in moderate density, and CGRP fibers were found in low density. However, some individual NTS subnuclei were found to contain moderate to high concentrations of each of the substances, including CGRP. Fibers containing CCK, ENK, NT, SP, SS, and VIP in the medial dorsal NTS subnuclei may regulate gastroesophageal functions. The caudal part of subnucleus lateralis parasolitarius did not contain most of the substances, which suggests that pulmonary function is not modulated by these neurochemicals. The boundaries of a subnucleus could sometimes be demarcated by a change in density of immunoreactive fibers between adjacent subnuclei. This was particularly evident in NTS subnuclei medialis dorsalis anterior centralis and lateralis parasolitarius, and in DMNX subnucleus posterior dorsalis magnocellularis. The selective distribution of peptide and serotonin immunoreactive fibers in various subnuclei of NTS-DMNX suggests that these substances may be differentially involved in neural circuits that mediate cardiovascular and gastrointestinal functions.