Electrophysiological effects of serotonin in the solitary tract nucleus of the rat

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part I. Tissue-based evidence for serotonin receptor signaling abnormalities in cardiorespiratory- and arousal-related circuits

The sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality in the United States, is typically associated with a sleep period. Previously, we showed evidence of serotonergic abnormalities in the medulla (e.g. altered serotonin (5-HT)1A receptor binding), in SIDS cases. In rodents, 5-HT2A/C receptor signaling contributes to arousal and autoresuscitation, protecting brain oxygen status during sleep. Nonetheless, the role of 5-HT2A/C receptors in the pathophysiology of SIDS is unclear. We hypothesize that in SIDS, 5-HT2A/C receptor binding is altered in medullary nuclei that are key for arousal and autoresuscitation. Here, we report altered 5-HT2A/C binding in several key medullary nuclei in SIDS cases (n = 58) compared to controls (n = 12). In some nuclei the reduced 5-HT2A/C and 5-HT1A binding overlapped, suggesting abnormal 5-HT receptor interactions. The data presented here (Part 1) suggest that a subset of SIDS is due in part to abnormal 5-HT2A/C and 5-HT1A signaling across multiple medullary nuclei vital for arousal and autoresuscitation. In Part II to follow, we highlight 8 medullary subnetworks with altered 5-HT receptor binding in SIDS. We propose the existence of an integrative brainstem network that fails to facilitate arousal and/or autoresuscitation in SIDS cases.

Endocannabinoids blunt the augmentation of synaptic transmission by serotonin 2A receptors in the nucleus tractus solitarii (nTS)

Serotonin (5-Hydroxytryptamine, 5-HT) and the 5-HT2 receptor modulate cardiovascular and autonomic function in part through actions in the nTS, the primary termination and integration point for cardiorespiratory afferents in the brainstem. In other brain regions, 5-HT2 receptors (5-HT2R) modify synaptic transmission directly, as well as through 5-HT2AR-induced endocannabinoid release. This study examined the role of 5-HT2AR as well as their interaction with endocannabinoids on neurotransmission in the nucleus tractus solitarii (nTS). Excitatory postsynaptic currents (EPSCs) in monosynaptic nTS neurons were recorded in the horizontal brainstem slice during activation and blockade of 5-HT2ARs. 5-HT2AR activation augmented solitary tract (TS) evoked EPSC amplitude whereas 5-HT2AR blockade depressed TS-EPSC amplitude at low and high TS stimulation rates. The 5-HT2AR-induced increase in neurotransmission was reduced by endocannabinoid receptor block and increased endogenous endocannabinoids in the synaptic cleft during high frequency, but not low, TS stimulation. Endocannabinoids did not tonically modify EPSCs. These data suggest 5-HT acting through the 5-HT2AR is an excitatory neuromodulator in the nTS and its effects are modulated by the endocannabinoid system.

Ventilatory effects of muscimol microdialysis into the rostral medullary raphé region of conscious rats

We hypothesized that inhibition of the rostral medullary raphe region (MRR), a putative central chemoreceptor location, with the GABA(A) receptor agonist muscimol would decrease ventilatory responses to hypercapnia and hypoxia in conscious rats, and that its known effect at this site on body temperature might alter its effect upon these ventilatory responses. At ambient temperatures of 24.5-26.5 degrees C (Cool), microdialysis of 1mM muscimol into the MRR significantly decreased body temperature by approximately 0.5 degrees C, increased the ventilatory response to 7% CO(2) and decreased the response to 10% O(2). At ambient temperatures of 29.5-30.5 degrees C (Warm), 1 mM muscimol microdialysis no longer decreased body temperature and increased the ventilatory response to hypercapnia and to hypoxia. Muscimol did not significantly affect the VE/VO2 ratio at either temperature. Muscimol significantly increased the hypercapnic ventilatory responses in Cool and Warm conditions and the hypoxic response in Warm conditions, which indicates the presence of an inhibitory effect of rostral MRR neurons sensitive to muscimol. In the Cool condition the ventilatory response to hypoxia is inhibited but appropriately so for the lower VO2 .

Preclinical neuropharmacology of naratriptan

The basic CNS neuropharmacology of naratriptan is reviewed here. Naratriptan is a second-generation triptan antimigraine drug, developed at a time when CNS activity was thought not to be relevant to its therapeutic effect in migraine. It was, however, developed to be a more lipid-soluble, more readily absorbed and less readily metabolized variant on preexisting triptans and these variations conferred on it a higher CNS profile. Naratriptan is a 5-HT(1B/1D) receptor agonist with a highly selective action on migraine pain and nausea, without significant effect on other pain or even other trigeminal pain. Probable sites of therapeutic action of naratriptan include any or all of: the cranial vasculature; the peripheral terminations of trigeminovascular sensory nerves; the first-order synapses of the trigeminovascular sensory system; the descending pain control system; and the nuclei of the thalamus. Naratriptan may prevent painful dilatation of intracranial vessels or reverse such painful dilatation. Naratriptan can prevent the release of sensory peptides and inhibit painful neurogenic vasodilatation of intracranial blood vessels. At the first order synapse of the trigeminal sensory system, naratriptan can selectively suppress neurotransmission from sensory fibers from dural and vascular tissue, while sparing transmission from other trigeminal fibers, probably through inhibition of neuropeptide transmitter release. In the periaqueductal gray matter and in the nucleus raphe magnus, naratriptan selectively activates inhibitory neurons which project to the trigeminal nucleus and spinal cord and which exert inhibitory influences on trigeminovascular sensory input. Naratriptan has also a therapeutic effect on the nausea of migraine, possibly exerting its action at the level of the nucleus tractus solitarius via the same mechanisms by which it inhibits trigeminovascular nociceptive input. The incidence of naratriptan-induced adverse effects in the CNS is low and it is not an analgesic for pain other than that of vascular headache. In patients receiving selective serotonin uptake inhibitors (SSRIs) naratriptan may cause serotonin syndrome-like behavioral side effects. The mechanism of action involved in the production of behavioral and other CNS side effects of naratriptan is unknown.

Vagal control of the heart: central serotonergic (5-HT) mechanisms

Cardiac vagal preganglionic neurones (CVPNs) are located within the dorsal vagal nucleus (DVN) and the nucleus ambiguus (nA). In mammals, CVPNs within the nA have small myelinated axons and mediate major chronotropic effects, those in the DVN have non-myelinated axons and mediate smaller chronotropic, dromotropic and inotropic effects. Numerous studies demonstrate important influences of serotonin (5-HT) at multiple sites controlling autonomic outflows including the nucleus tractus solitarius (NTS) where cardiorespiratory afferent fibres terminate, and the CVPNs and rostral ventrolateral medulla (RVLM), the location of sympathetic premotor neurones. We have demonstrated roles for some of the numerous 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3, 5-HT4 and 5-HT7) in brainstem regions involved in cardiac control. Intracisternal application of selective ligands was used to study the effect of 5-HT receptors on heart rate and its reflex control. Further electrophysiological studies were also carried out to delineate their location and the mechanisms of action of these ligands. Blocking 5-HT1A receptors attenuated bradycardias evoked by stimulating baroreceptor and cardiopulmonary afferents but not arterial chemoreceptors, whereas antagonizing 5-HT7 receptors markedly attenuated all these reflex bradycardias. Within the DVN, nA and NTS, activation of 5-HT1A receptors could excite or inhibit neurones. In the NTS 5-HT2 receptors also had variable effects; 5-HT2B receptors excite and 5-HT2C receptors inhibit. Antagonism of 5-HT3 receptors attenuated upper airway and cardiopulmonary reflex bradycardias; this is compatible with data showing that 5-HT3 receptors excite DVN and NTS neurones by a glutamate-dependent mechanism. The origin of the glutamate (neuronal or glial) remains unresolved but glia are a possibility as baroreceptor-sensitive NTS neurones receive few direct 5-HT-containing synaptic contacts. Thus, 5-HT plays a critical role in the control of vagal outflow to the heart; however, why so many different receptors are involved, and their relative functional roles, remains unresolved.

Syncope: facts and fiction

According to the current state-of-art on the brainstem functional anatomy and reticular formation, authors believe that nucleus tractus solitarii (NTS) is the neural structure, which meets all the conditions of the hypothetical syncope generating, reflex centre. The afferent branch of this reflex arc represents information from different visceral sources including the brain itself. The efferent branch of this reflex arc is reticular activating system (RAS). The executive mechanism of syncope is deactivation of RAS done with the active engagement of NTS through solitarioreticular pathway (SRT) and parabrachial nuclear complex (PBC). The biological purpose of syncope would be resetting of the NTS in case of an unbearable vegetative input, which is code for triggering the mechanism described.

The 5-hydroxytryptamine1B/1D/1F receptor agonists eletriptan and naratriptan inhibit trigeminovascular input to the nucleus tractus solitarius in the cat

Migraine pain arises in the trigeminovascular system and is often associated with nausea and sometimes with vomiting. In this study, an in vivo cat model of trigeminovascular stimulation was used to determine first whether there is a functional connection between the trigeminovascular system and the nucleus tractus solitarius (NTS), which is involved in regulating vomiting, and second whether anti-migraine drugs have any effect on such a connection. Chloralose-anaesthetised cats (n=16) were prepared for single neuron recording. The superior sagittal sinus (SSS) was isolated and stimulated electrically. The brainstem near the obex was exposed and a metal microelectrode equipped with six glass barrels for microiontophoresis was placed in the NTS. Recordings were made from 44 NTS neurons which responded to SSS stimulation with A-delta latencies. Iontophoretic ejection (50 nA) of eletriptan or naratriptan suppressed the response in 75% (15/20) and 78% (11/14) of cells and caused an average suppression of cell firing of 42+/-5% (n=20) and 54+/-8% (n=14), respectively. This suppression could be antagonized by the concurrent ejection (20-50 nA) of the 5-HT(1B/1D) receptor antagonist GR127935. We conclude that activation of the trigeminovascular system excites cells in the NTS that can be inhibited by eletriptan and naratriptan through activation of 5-HT(1B/1D) receptors. It is possible that in patients having a migraine attack trigeminovascular activation triggers nausea and vomiting, and that the alleviation of these symptoms by anti-migraine compounds may be via an action at 5-HT(1B/1D) receptors in the NTS.

Serotonin and sleep

For 50 years, serotonin has been in the centre of the search for the mechanisms and control of sleep. Serotonergic neurotransmission is related to the behavioural state of the animal and plays an important role in modulation of the behavioural state, by interacting with other brain areas modulating circadian rhythm, sleep and waking. Serotonergic activity may be accompanied by waking or sleep depending on the brain area and receptor type involved in the response, on the current behavioural state and on the concomitant agonism/antagonism of other neurotransmitter systems.

Brain stem control of swallowing: neuronal network and cellular mechanisms

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

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.

The neurophysiology of sleep and waking: intracerebral connections, functioning and ascending influences of the medulla oblongata

This paper focuses on the successive historical papers related to medulla oblongata (M.O.) intracerebral connections, its activities and ascending influences regulating sleep waking behavior. The M.O. certainly influences the quantitative and qualitative processes of waking. However, its neurophysiological properties are often concealed by those of the upper-situated brain stem structures. The M.O., particularly the solitary tract nucleus, is involved in sleep-inducing processes. This nucleus seem to act as a deactivating system of the above situated reticular formation, but it also impacts directly on the thalamocortical slow wave and spindle-inducing processes. The M.O. is significantly involved in paradoxical sleep mechanisms. Indeed, the mesopontine executive centers are unable to induce paradoxical sleep without the M.O. Moreover, stimulation of the solitary tract nucleus afferents can induce paradoxical sleep, and the M.O. metabolic functioning is specifically disturbed by paradoxical sleep deprivation. Finally. there seems to be a paradoxical sleep Zeitgeber. Our current knowledge shows that this lowest brain stem level is crucial for sleep waking mechanisms. It will undoubtedly be further highlighted by future electrophysiologial and neurochemical studies.

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.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

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.

Evidence against a hemodynamic role for serotonin in the dorsal motor nucleus of the vagus

This study was performed to investigate the potential role of serotonin (5-HT) in the dorsal motor nucleus of the vagus (dmnX) in regulating peripheral hemodynamics. Microinjections (5 or 25 nmol in 50 nl) of the monoaminergic neurotransmitter were made into the dorsomedial medulla of the urethaneanesthetized rat during continuous recording of femoral arterial blood pressure. Heart rate was extracted electronically from the pressure waveform. Discrete injections of 5-HT placed directly in the dmnX were found to be entirely without effect on peripheral hemodynamics. In contrast, injections placed in the solitary tract nucleus, lying immediately above the dmnX, were found to have profound depressor and bradycardic effects, while the immediately subjacent hypoglossal nucleus appeared to contain both depressor and unresponsive sites. These findings cast doubt on the involvement of serotonin in the dmnX in the regulation of cardiovascular hemodynamics.