Excitatory and inhibitory actions of intrathecally administered 5-hydroxytryptamine on sympathetic nerve activity in the rat

Effects of intracerebroventricular injections of 5-HT on systemic vascular resistances of conscious rats

The aims of this study were to determine (i) the effects of intracerebroventricular (i.c.v.) injections of 5-hydroxytryptamine (5-HT, 10μg) on mean arterial blood pressure (MAP), heart rate (HR) and mesenteric (MR), renal (RR) and hindquarter (HQR) vascular resistances of conscious rats, (ii) the central 5-HT receptor subtype which mediates these effects, and (iii) the role of nitric oxide (NO) in the expression of these responses. The i.c.v. injection of 5-HT had minor effects on MAP but produced a decrease in HR (-18±4%), which lasted for 20min. The i.c.v. injection of 5-HT elicited marked increases in MR (+50±7%) and reductions in HQR (-31±3%). These responses occurred promptly and lasted for 25-35min. 5-HT also produced a transient decrease in RR (-26±8% at 10min). All of these responses were prevented by the prior i.c.v. injection of the 5-HT1/5-HT2-receptor antagonist, methysergide (10μg). The intravenous injection of the NO synthesis inhibitor, L-NAME (25μmol/kg), produced a sustained pressor response, bradycardia and increases in MR, RR and HQR. Subsequent i.c.v. injection of 5-HT produced a minor pressor response (+7±2%), bradycardia (-18±3%), an increase in MR (+52±8%) but no decreases in RR or HQR. This study demonstrates that i.c.v. 5-HT differentially affects peripheral vascular resistances by activation of central 5-HT1/5-HT2-receptors. It appears that L-NAME did not interfere with the central actions of 5-HT as it did not prevent the 5-HT-induced bradycardia or mesenteric vasoconstriction. Since the 5-HT-induced falls in RR and HQR were abolished by L-NAME, it is possible that these responses are mediated by an active neurogenic process involving the release of NO within the vasculature.

Cardiovascular responses produced by 5-hydroxytriptamine:a pharmacological update on the receptors/mechanisms involved and therapeutic implications

The complexity of cardiovascular responses produced by 5-hydroxytryptamine (5-HT, serotonin), including bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, has been explained by the capability of this monoamine to interact with different receptors in the central nervous system (CNS), on the autonomic ganglia and postganglionic nerve endings, on vascular smooth muscle and endothelium, and on the cardiac tissue. Depending, among other factors, on the species, the vascular bed under study, and the experimental conditions, these responses are mainly mediated by 5-HT(1), 5-HT(2), 5-HT(3), 5-HT(4), 5-ht(5A/5B), and 5-HT(7) receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT(6) receptors do not seem to be involved in the cardiovascular responses to 5-HT. Regarding heart rate, intravenous (i.v.) administration of 5-HT usually lowers this variable by eliciting a von Bezold-Jarisch-like reflex via 5-HT(3) receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT(1B/1D) receptors and, in the case of the rat, an additional 5-ht(5A/5B) receptor component. Moreover, i.v. 5-HT can increase heart rate in different species (after vagotomy) by a variety of mechanisms/receptors including activation of: (1) myocardial 5-HT(2A) (rat), 5-HT(3) (dog), 5-HT(4) (pig, human), and 5-HT(7) (cat) receptors; (2) adrenomedullary 5-HT(2) (dog) and prejunctional sympatho-excitatory 5-HT(3) (rabbit) receptors associated with a release of catecholamines; (3) a tyramine-like action mechanism (guinea pig); and (4) unidentified mechanisms (certain lamellibranch and gastropod species). Furthermore, central administration of 5-HT can cause, in general, bradycardia and/or tachycardia mediated by activation of, respectively, 5-HT(1A) and 5-HT(2) receptors. On the other hand, the blood pressure response to i.v. administration of 5-HT is usually triphasic and consists of an initial short-lasting vasodepressor response due to a reflex bradycardia (mediated by 5-HT(3) receptors located on vagal afferents, via the von Bezold-Jarisch-like reflex), a middle vasopressor phase, and a late, longer-lasting, vasodepressor response. The vasopressor response is a consequence of vasoconstriction mainly mediated by 5-HT(2A) receptors; however, vasoconstriction in the canine saphenous vein and external carotid bed as well as in the porcine cephalic arteries and arteriovenous anastomoses is due to activation of 5-HT(1B) receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT(7) receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT(1A/1B/1D) receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT(2B) and/or 5-HT(1B/1D) receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT(1A) receptors) and hypertension (mainly mediated by 5-HT(2) receptors). The increasing availability of new compounds with high affinity and selectivity for the different 5-HT receptor subtypes makes it possible to develop drugs with potential therapeutic usefulness in the treatment of some cardiovascular illnesses including hypertension, migraine, some peripheral vascular diseases, and heart failure.

Generation of a physiological sympathetic motor rhythm in the rat following spinal application of 5-HT

When applied in vitro to various CNS structures 5-HT and/or NMDA have been observed to generate rhythmic nervous activity. In contrast, reports of similar in vivo actions are relatively rare. Here we describe a physiological sympathetic motor rhythm regulating the thermoregulatory circulation of the rat tail (T-rhythm; 0.40-1.20 Hz) that can be elicited following intrathecal (i.t.) application of 5-HT to an in situ'isolated' spinal cord preparation (anaesthetized rats spinalized at T10-T11 and cauda equina cut). i.t. injections were delivered to L1 as sympathetic neuronal activity to the tail (SNAT) arises from preganglionic neurones at T11-L2. SNAT was abolished after spinal transection (n = 18) and it did not return spontaneously. The administration of 5-HT (250 nmol) generated rhythmic sympathetic discharges (n = 6). The mean frequency of the T-like rhythm during the highest level of activity was 0.88 +/- 0.04 Hz which was not significantly different from the T-rhythm frequency observed in intact animals (0.77 +/- 0.02 Hz; P > 0.05 n = 16). In contrast, NMDA (1 micromol) generated an irregular tonic activity, but it failed to generate a T-like rhythm (n = 9), even though the mean levels of activity were not significantly different to those produced by 5-HT. However, 5-HT (250 nmol) applied after NMDA generated a T-like rhythm (0.95 +/- 0.11 Hz, n = 6). Our observations support the idea that 5-HT released from rostral ventromedial medullary neurones, known to innervate sympathetic preganglionic neurones, can induce sympathetic rhythmic activity.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Activation of bulbospinal serotonergic neurons during cold exposure

In a four-part study, we expand on our previous report that bulbospinal serotonin (5HT) neuronal activation occurs with 24 h of cold exposure. To characterize temporal aspects, rats were exposed to 3°C or were maintained at 22°C for 2, 8, 48, or 96 h (experiment 1) or for 15, 30, or 60 min (experiment 2). To ensure that cold-induced changes in 5HT activity were not due to disturbances in diurnal pattern, rats in experiment 3 were exposed to cold (8 h) during the dark cycle. To explore the hypothesis that cold-induced 5HT activation is part of a broad metabolic response that includes activation of the sympathetic nervous system, metabolically impaired (hypothyroid) rats were exposed to 8°C in experiment 4. Significant increments in 5-hydroxyindoleacetic acid (5HIAA) concentration were evident by 60 min of cold exposure and existed at all later time points measured. These findings were most robust in spinal cord and rostral brainstem. Activation in spinal cord was also found when rats were exposed to 8 h of cold during the dark cycle, the active period for rats. In experiment 4, hypothyroid rats exhibited significantly greater norepinephrine excretion compared with control rats exposed to the same cold stimulus; this finding was accompanied by significantly greater increments in 5HIAA concentration in rostral brainstem and spinal cord of hypothyroid rats. In addition, significant elevations in tryptophan concentration were noted throughout the brainstem and spinal cord of cold-exposed, hypothyroid rats relative to room temperature, hypothyroid rats. This finding suggested that elevations in 5HIAA concentration in these rats were due to increases in precursor availability. The implications of these findings relative to autonomic and metabolic control are discussed.Key words: serotonin, spinal cord, raphe, cold, sympathetic nervous system.

Cuneiform nucleus stimulation-induced sympathoexcitation: role of adrenoceptors, excitatory amino acid and serotonin receptors in rat spinal cord

Stimulation of the midbrain cuneiform nucleus has previously been shown to produce increases in arterial blood pressure and lumbar sympathetic nerve activity. While this sympathoexcitatory effect is, in part, due to excitation of premotor sympathoexcitatory neurons in the rostral ventrolateral medulla, the specific spinal neurotransmitter systems recruited by cuneiform nucleus stimulation remains to be elucidated. In this study, mean arterial pressure, resting and cuneiform nucleus stimulation-evoked lumbar sympathetic nerve activity were analysed following intrathecal injections of an excitatory amino acid antagonist (kynurenic acid), alpha1-adrenoceptor antagonist (prazosin) and a serotonin receptor antagonist (methiothepin) in anesthetized, paralysed male Sprague-Dawley rats. Mean arterial pressure and resting sympathetic nerve discharge were decreased by all treatments (n = 6/group) compared to the vehicle control group. Intermittent electrical stimulation of the cuneiform nucleus produced a bimodal sympathoexcitatory response, of which the short latency peak was significantly attenuated (43% reduction) by intrathecal kynurenate whereas the long latency peak was reduced by intrathecal prazosin (decrease of 21%) and methiothepin (38% attenuation). These results are consistent with the significant roles of excitatory amino acid, alpha1-adrenergic and serotonin receptors in modulating the activity of sympathetic vasomotor preganglionic neurons supplying the lumbar sympathetic nerve trunk, and suggest the existence of at least three neuronal groups and/or pathways associated with the sympathoexcitatory response to cuneiform nucleus stimulation.

Serotonin inputs to rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion or adrenal medulla

The input from serotonin-containing nerve fibres to rabbit sympathetic preganglionic neurons projecting to either the superior cervical ganglion or the adrenal medulla was investigated by combining retrograde tracing with the B subunit of cholera toxin and immunocytochemistry for serotonin. There were pronounced rostrocaudal variations in the density of serotonin fibres in the rabbit intermediolateral cell column from T1 to L4; maximum numbers of fibres were found in T3-6 and L3-4 and minimum numbers in T1 and T10-12. By light microscopy, retrogradely labelled sympathetic preganglionic neurons projecting to the superior cervical ganglion or the adrenal medulla received variable densities of close appositions from serotonin-immunoreactive fibres. Some neurons from each population received many close appositions, whereas others received moderate numbers or few appositions. Appositions occurred on the cell bodies, dendrites, and occasionally axons of sympathetic preganglionic neurons. Rare neurons in both groups of retrogradely labelled cells received no appositions from serotonin-containing nerve fibres. At the ultrastructural level, synapses were found between serotonin-positive boutons and sympathetic preganglionic neurons projecting either to the superior cervical ganglion or to the adrenal medulla. These results indicate that, through direct synaptic contacts, serotonin-immunoreactive, presumably bulbospinal, nerve fibres affect the activity of the vast majority of sympathetic preganglionic neurons that send axons either to the superior cervical ganglion or to the adrenal medulla. This serotonin input may be sympathoexcitatory and could mediate increases in sympathetic nerve activity and in the release of catecholamines from the adrenal medulla.

Mediation of baroreceptor inhibition of sympathetic nerve activity via both a brainstem and spinal site in rats

1. The possible involvement of a spinal site of sympatho-inhibitory action in the baroreceptor reflex pathways was investigated by determining the effect of phenylephrine-induced, baroreceptor-mediated inhibition on a spinally evoked excitatory response of renal nerve activity before, during and after removal of tonic descending excitatory drive from the rostral ventrolateral medulla (RVLM). 2. Stimulation of descending excitatory axons at the C4 vertebral level evoked a highly reproducible excitatory response (mean variance, 2.2 +/- 0.6%) in a renal sympathetic nerve with a latency of 72 +/- 5.7 ms and duration of 211 +/- 32 ms. 3. Following baroreceptor activation, the magnitude of this spinally evoked response was reduced by 33.5 +/- 4.2% compared with control spinal response. 4. To remove tonic descending excitatory drive, glycine was microinjected bilaterally into the RVLM (RVLM block); this briefly and reversibly abolished spontaneous sympathetic nerve activity. 5. Following RVLM block, the spinally evoked response in a renal sympathetic nerve was inhibited by 16.9 +/- 4.4% during baroreceptor activation. 6. Intrathecal administration of the glycine antagonist strychnine to the lower thoracic segments of the spinal cord virtually abolished this inhibition. 7. It was concluded that baroreceptor inhibition of sympathetic activity occurs at a spinal site as well as a supraspinal one. Glycine is a likely mediator of the inhibition at the spinal site.

Distribution and co-localization of 5-hydroxytryptamine, thyrotropin-releasing hormone and substance P in the cat medulla

This study demonstrates the co-existence of three neurochemicals in ventral medullary neurons of the cat utilizing fluorescence immunohistochemistry. Neurons containing 5-hydroxytryptamine, thyrotropin-releasing hormone and substance P were identified within the rostrocaudal extent of the medulla, specifically within the raphe pallidus and raphe magnus and in the reticular formation of the ventrolateral medulla in the nucleus paragigantocellularis lateralis. Within the raphe pallidus the majority of 5-hydroxytryptamine-containing neurons were co-localized with thyrotropin-releasing hormone and substance P. However, in the raphe magnus the majority of stained neurons contained 5-hydroxytryptamine and thyrotropin-releasing hormone but were devoid of substance P. In the ventrolateral medulla two major populations of neurons were identified rostral to the inferior olivary nuclei, one containing 5-hydroxytryptamine and thyrotropin-releasing hormone, while a more lateral group contained substance P alone. More caudally, at the level of the inferior olives, the majority of 5-hydroxytryptamine-containing cells also displayed immunoreactivity for thyrotropin-releasing hormone and substance P. A consistent finding in both the ventromedial and ventrolateral regions of the medulla was a population of 5-hydroxytryptamine-containing cells which did not stain for either thyrotropin-releasing hormone or substance P. The functional role of co-localized neurochemicals remains unknown but co-existence of neurotransmitter substances in medullary neurons may allow for specific and multiple actions in the spinal cord.

Cardiovascular effects of intrathecal administration of agents active at 5-hydroxytryptamine1-receptors in the rat: modulation by substance P and a substance P antagonist

The intrathecal (i.th., T8-10) administration in conscious rats of the 5-hydroxytryptamine (5-HT)1A agonist 8-OH-DPAT (10 nmol), and to a lesser extent the 5-HT1B agonist CGS 12066B (6 nmol), resulted in a blood pressure reduction and a bradycardia. These responses were prevented by the i.th. pretreatment with substance P (SP) (6.5 nmol) and enhanced following pretreatment with the non-peptide SP antagonist CP-96,345 (10 nmol). The partial 5-HT1A agonist 8-MeO-CLEPAT (10 nmol) had by itself small cardiovascular effects. Following the pretreatment with SP, 8-MeO-CLEPAT caused a pressor response and a tachycardia whereas the opposite effects were observed following pretreatment with the SP antagonist. These results support the notion of a functional interaction between serotonergic and SP mechanisms at the level of the preganglionic sympathetic nerves in the spinal cord.

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.

A role for serotonin in the elaboration of a differential pattern of activity in sympathetic nerves to kidney and skeletal muscle vasculature

The role of serotonin (5-HT) in the elaboration of patterns of regional sympathetic discharge evoked from the ventral medulla was examined in cats under Saffan anesthesia. A differential response pattern consisting of an increase in renal sympathetic nerve activity simultaneous with no change in activity to skeletal muscle vasculature was evoked from a group of ventral medullary neurons by microinjection of D,L-homocysteic acid. Subsequent intrathecal administration of the 5-HT receptor antagonist methysergide (200 micrograms) attenuated or abolished the excitatory response to the kidney on restimulation of the medulla, without affecting sympathetic activity to muscle. Methysergide did not affect an excitatory response evoked in the outflow to both the kidney and skeletal muscle vasculature from an adjacent region of the ventral medulla. These results indicate a role for 5-HT in the elaboration of a specific differential pattern of sympathetic response evoked from medullary neurons.

Excitatory and indirect inhibitory actions of 5-hydroxytryptamine on sympathetic preganglionic neurones in the neonate rat spinal cord in vitro

The action of 5-hydroxytryptamine (5-HT) on sympathetic preganglionic neurones (SPN) was studied by intracellular recordings in thin slices of neonatal rat spinal cord in vitro. Superfusion of 5-HT (1-270 microM) to SPN caused a concentration dependent slow depolarization or inward current and an increase in synaptic activity consisting of both EPSPs and IPSPs. The slow depolarization was still present after superfusion with TTX. Similar effects were seen during superfusion with 5-carboxamidotryptamine (5-CT) or alpha-methyl-5-hydroxytryptamine (alpha-me-5-HT). A comparison with the potency of 5-HT was made for 5-CT or alpha-me-5-HT on the same neurone by determining the magnitude of the slow depolarization to different concentrations of agonist. This showed that the apparent potency of the agonists was 5-CT > 5-HT > alpha-me-5-HT even in the presence of fluoxetine, a 5-HT uptake inhibitor. The 5-HT-induced slow depolarization was partially blocked by ketanserin but full recovery was not observed. The results suggest that the excitatory action of 5-HT on SPN is mediated via an atypical 5-HT2 receptor or a 5-HT1C-like receptor. The 5-HT-induced IPSPs were reversibly blocked by superfusion with strychnine, suggesting they were mediated by glycine.

Effects of neonatal adrenalectomy on 5-HT and TRH-like immunoreactivity in the rat spinal cord

This study investigated the effects of neonatal unilateral adrenalectomy on the serotonin- (5-HT) and thyrotropin-releasing hormone-like immunoreactivities (TRH-LI) in the intermediolateral cell column (IML) of the rat spinal cord where the sympathetic preganglionic neurons innervating the adrenal medulla are located. The density of the innervation was measured by a computer-assisted image analysis. Two weeks after the lesion, only one rat (1/3) showed a 30% decrease of 5-HT-LI in the ipsilateral IML, although no modifications were observed for TRH-LI. At 1 month, we observed a variable decrease of 19% to 30% for either 5-HT- or TRH-LI in the ipsilateral IML. However, one animal (1/3) showed nonsignificant modifications. At 3 months, we showed a 17% mean loss of 5-HT-LI and TRH-LI in the ipsilateral IML. However, the decreases of 5-HT-LI and TRH-LI did not always appear similar. These results appear different from those obtained after neonatal removal of the superior cervical ganglion.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Cardiovascular effects of intrathecal administration of substance P in the rat: interactions with serotonergic mechanisms

Intrathecal (i.th.) administration of substance P (SP, 6.5 nmol) at the Th 8-10 level in conscious rats increased blood pressure (carotid artery), heart rate and plasma catecholamine concentrations. The responses were antagonized by the intravenous (i.v.) but not i.th. pretreatment with the 5-HT2-receptor antagonists ketanserin and ritanserin and intrathecally administered serotonin (5-HT, 10 micrograms). The pressor response and the increase in plasma noradrenaline concentrations were also antagonized by i.v. or i.th. pretreatment with the 5-HT1A-agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). In contrast the pressor response to SP was facilitated by the 5-HT1A-antagonist 1-pindolol (i.v. or i.th). Pretreatment with SP (i.th) reduced the hypotensive response to i.v. 8-OH-DPAT. These results demonstrate functional interactions between SP and serotonergic mechanisms in the central system, but the precise location and nature were not elucidated.

Serotoninergic denervation suppresses the sympathetic outflow induced by thyrotropin-releasing hormone in conscious rats

Intrathecal administration of thyrotropin-releasing hormone (TRH) resulted in an increase in plasma epinephrine (E) and glucose levels in conscious rats. To elucidate the mechanisms by which TRH stimulates the release of E, the animals were pretreated with 5,7-dihydroxytryptamine, parachlorophenylalanine, methysergide or ketanserin. Serotoninergic denervation suppressed the TRH-induced increase in plasma E and glucose levels, whereas inhibition of serotonin (5-HT) synthesis or blockade of 5-HT receptors did not suppress the responses. These findings suggest that the serotoninergic neurons, but not 5-HT itself, are involved in stimulating the sympathetic outflow by TRH at the spinal level.

Characterization of the 5-HT receptor subtypes involved in the motor behaviours produced by intrathecal administration of 5-HT agonists in rats

1. The motor behavioural effects of intrathecal injections of 5-hydroxytryptamine (5-HT) and a variety of 5-HT receptor agonists were examined in adult Wistar rats to establish; (a) which 5-HT receptor subtype/s elicit each behaviour and (b) whether these receptors are located within the spinal cord. 2. Intrathecal injection of 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) or 2,5-dimethoxy-alpha,4-dimethylbenzene ethamine hydrochloride (DOM) produced dose-related back muscle contractions (BMC) and wet dog shakes (WDS) which were both markedly attenuated by intraperitoneal pretreatment with either ritanserin (1 mg kg-1), ketanserin (0.16 mg kg-1) or mianserin (0.6 mg kg-1) indicating the involvement of 5-HT2 receptors in both these motor behaviours. Both fluoxetine (1-20 mg kg-1, i.p.) and high doses of 5-HT (50 micrograms) following fluoxetine (5 mg kg-1, i.p.) also elicited BMC, further confirming the involvement of 5-HT in this behaviour. 3. Intrathecal 5-carboxamidotryptamine (5-CT) evoked a marked wet-dog shake response without producing any BMC. Intrathecal pretreatment with 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) enhanced, while in contrast 2-methyl-5-HT pretreatment attenuated, 5-HT agonist-induced BMC without affecting WDS. These data suggest that the spinal 5-HT2 receptors mediating BMC are positively modulated by 5-HT1A but negatively influenced by 5-HT3 receptor activation and may be of a different subtype to the supra-spinal 5-HT2 receptors which elicit WDS. 4. A contrast, reciprocal forepaw treading, lateral head weaving, flat body posture and Straub-tail were evoked by 5-MeODMT, 8-OH-DPAT or 5-CT but not by DOI or DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. Ritanserin (1 mg kg- 1, i.p.) or ketanserin (0.16mgkg-1, i.p.) pretreatment reduced the reciprocal forepaw treading induced by high intrathecal doses of either 5-MeODMT (25.pg) or 5-CT (50,ug) suggesting that this behaviour may be facilitated by 5-HT2 receptor activation. 5. Intrathecal injection of 5-HT (0.05-50pg, after systemic fluoxetine, 5mg kg 1, i.p.), or 1-(3-chlorophenyl) piperazine (mCPP) produced dose-related forepaw-licking and grooming, neither of which were attenuated by ketanserin (0.16 mgkg-1, i.p.) pretreatment suggesting these behaviours may be mediated by 5-HT1c receptors. In contrast, 2-methyl-5-HT (50 and 100pg) produced sideward tail-flicks, not evoked by any other 5-HT agonist and could therefore be mediated by spinal 5-HT3 receptor activation. 6. These data provide behavioural evidence for the existence of spinal 5-HT2 receptors which produce a novel motor behaviour, BMC. Ligand binding studies and dose-response studies with a range of selective 5-HT antagonists are required to establish whether BMC and WDS are mediated by different subtypes of 5-HT2 receptors.

Studies on the site and mechanism of the sympatholytic action of 8-OH DPAT

Studies in our laboratory indicate that the 5-HT1A agonist 8-OH DPAT acts in the central nervous system at postsynaptic receptor sites to inhibit sympathetic nerve activity and lower arterial blood pressure. The present study was designed to investigate possible postsynaptic sites on central sympathetic neurons where 8-OH DPAT might produce its sympatholytic action in anesthetized cats. The sympatholytic effect of 8-OH DPAT was compared in midcollicular transected and sham operated control animals. Administration of 8-OH DPAT (0.01-1.0 mg/kg, i.v.) inhibited sympathetic activity and decreased blood pressure in both the transected and sham animals to a similar degree. The effects of microiontophoretically applied 8-OH DPAT and 5-HT on antidromically identified sympathetic preganglionic neurons were determined. Microiontophoretically applied 5-HT consistently increased the firing rate of sympathetic preganglionic neurons. Iontophoretic 8-OH DPAT failed to affect the firing of sympathetic preganglionic neurons but blocked the excitatory effects of 5-HT. The effects of 8-OH DPAT and 5-HT on the firing of sympathoexcitatory neurons located in the rostral ventrolateral medulla were also determined. Sympathoexcitatory neurons were identified using spike triggered averaging techniques and by their response to baroreceptor activation. Intravenous administration of 8-OH DPAT inhibited the firing of sympathoexcitatory neurons in the rostral ventrolateral medulla. The inhibition of unit firing produced by 8-OH DPAT was exactly paralleled by the shutoff of inferior cardiac nerve activity. Microiontophoretic application of 8-OH DPAT and 5-HT onto sympathoexcitatory neurons in the rostral ventrolateral medulla failed to affect the firing rate of these neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the site and mechanism of the sympathoexcitatory action of 5-HT2 agonists

Intravenous administration of the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), produced an increase in sympathetic activity recorded from the inferior cardiac nerve in chloralose-anesthetized cats. Microiontophoretically applied 5-HT increased the firing rate of antidromically identified sympathetic preganglionic neurons. Microiontophoretic DOI failed to affect the firing of sympathetic preganglionic neurons. The effect of DOI was also studied on medullospinal sympathoexcitatory neurons located in the rostral ventrolateral medulla. Intravenous DOI increased the firing of sympathoexcitatory neurons and sympathetic nerve discharge to a similar extent. Microiontophoretic application of DOI failed to affect the firing of sympathoexcitatory neurons. The data are discussed in relation to the site and mechanism of the sympathoexcitatory action of 5-HT2 agonists.

The influence of 5-hydroxytryptamine agonists and antagonists on identified sympathetic preganglionic neurones in the rat, in vivo

1. 5-Hydroxytryptamine (5-HT) was applied by microiontophoresis in the vicinity of identified sympathetic preganglionic neurones in the upper thoracic spinal cord of the rat, in vivo. 2. Sympathetic preganglionic neurones responded in one of three ways to 5-HT: by (a) excitation (76%), (b) inhibition (4%) or (c) in a biphasic manner (5%). 3. The excitatory responses evoked by 5-HT were mimicked by 5-carboxamidotryptamine (5-CT) and alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT). The inhibitory and biphasic responses evoked by 5-HT were mimicked by 2-methyl-5-hydroxytryptamine (2-Me-5-HT). The observed responses evoked by 5-HT and selective agonists may be different on the same cell. In several instances a single neurone excited by one agonist was inhibited by another agonist. 4. The 5-HT2-receptor antagonists, ketanserin and LY 53857, failed to abolish selectively the excitatory responses evoked by 5-HT and alpha-Me-5-HT, when applied by microiontophoresis. The antagonists non-selectively reduced the excitatory responses evoked by 5-HT, 5-CT, alpha-Me-5-HT, D,L-homocysteic acid (DLH) and noradrenaline (NA). A reduction in synaptically evoked activity was also observed. 5. The 5-HT3-receptor antagonist, ICS 205-930, failed to abolish the inhibitory responses evoked by 5-HT. 6. It was concluded that the excitatory responses evoked by 5-HT are mediated by a receptor that is neither 5-HT2 or 5-HT3, but shows similarities to the 5-HT1-like receptor profile. The inhibitory actions of 5-HT are mimicked by 2-Me-5-HT, but the receptor is not 5-HT3, or 5-HT1-like or 5-HT2.

Examination of the effects of some 5-HT2 receptor antagonists on central sympathetic outflow and blood pressure in anaesthetised cats

Thoracic preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose response curves were constructed for the 5-HT2 antagonists cinanserin, ritanserin, cyproheptadine, methiothepin, metergoline and ICI 169,369. These antagonists showed differing effects on the above parameters. Methiothepin and cyproheptadine caused hypotension and sympathoinhibition at low and high doses, whilst metergoline and ritanserin caused these effects at high doses, above 1 mg kg-1. Cinanserin and ICI 169,369 did cause large transient changes in heart rate, blood pressure and sympathetic outflow. However, all the antagonists except ICI 169,369 and methiothepin caused an increase in femoral arterial conductance which was not associated with hypotension. It was therefore concluded that using the above antagonists it was difficult to attribute changes in blood pressure and central sympathetic outflow to blockade of 5-HT2 receptors. However, it is suggested that 5-HT2 receptors may be involved in the control of skeletal muscle and/or skin vascular beds.

A comparison of the effects of intrathecally administered 5-hydroxytryptamine and thyrotropin-releasing hormone on renal and muscle sympathetic nerve activity

Thyrotropin-releasing hormone (TRH) has been shown to coexist with 5-hydroxytryptamine (5-HT) in spinally projecting raphe neurones, some of which terminate in the sympathetic nuclei of the spinal cord. In an attempt to mimic the actions of these neurones, the effects of intrathecal administration of 5-HT was compared to that of TRH on activity in two sympathetic postganglionic nerves, the renal nerve and sympathetic fibres to skeletal muscle of the hind limb. In chloralose-urethane anaesthetised rats intrathecal infusion of TRH at the T9 level in doses of 5-30 micrograms increased activity in renal nerve as did low doses (20-100 micrograms) of 5-HT. In contrast, intrathecal infusion of TRH in doses of 5-30 micrograms and 5-HT in doses of 20-800 micrograms at L4 level decreased activity in sympathetic fibres to muscle. The results show that TRH, unlike 5-HT, has an excitatory but not an inhibitory action on renal sympathetic activity whereas both substances have only an inhibitory action on muscle sympathetic activity.