Short communicationThe discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input
Introduction
Serotonergic neurons comprise less than a quarter of the cells in the medullary raphe magnus and adjacent nucleus reticularis magnocellularis (collectively referred to as RM), but are a major source of serotonin in the spinal cord [12], [15], [46], [49]. Within the spinal cord, serotonergic RM neurons project strongly to the superficial dorsal horn, the deep dorsal horn, the intermediolateral cell column, and the central canal region [2], [8], [30]. The projection from neurons in RM and NRMC to the thoracic intermediolateral cell column includes serotonergic cells that project to preganglionic sympathetic neurons [3], [13], [24], [52]. Interestingly, the densest brainstem target of RM serotonergic cell axons is the ventrolateral medulla, a region that is important in cardiovascular control [21]. These projections suggest that serotonergic RM neurons may play a role in cardiovascular modulation. In support of this idea, RM neurons, including serotonergic cells, are consistently labeled transneuronally after virus injections into autonomic ganglia, including the stellate ganglion [28], [29]. Furthermore, unpublished observations in our laboratory provide evidence that some serotonergic RM cells discharge in relation to spontaneous changes in blood pressure. Therefore, in order to determine whether serotonergic RM cells are influenced by baroreceptor input, serotonergic RM cells were recorded during blood pressure changes evoked by descending aortic occlusion, aortic nerve stimulation, and systemic phenylephrine administration.
Section snippets
Surgical preparation
Male Sprague–Dawley rats (240–400 g; Sasco, Madison, WI) were deeply anesthetized with 1.8–2.0% halothane in oxygen via a tracheal catheter. Catheters were inserted into the brachial artery and femoral vein for recording of arterial blood pressure and administration of drugs, respectively. In some animals, the femoral artery was catheterized in order to confirm that inflation of a vascular cuff (see below) completely blocked distal blood flow. A small craniotomy was made for the introduction of
Characterization of serotonergic cells
Neurons were classified as p5 HT (n=81) by use of a previously described algorithm (see Methods and [40]). p5HT cells had a mean background discharge rate of 1.5±0.1 Hz (Fig. 1). The mean coefficient of variation of the interspike interval (CVISI) was 0.42±0.02 with a range of 0.22–0.99. All cells were located in RM or nucleus reticularis magnocellularis pars α between the levels of the facial nucleus and the superior olivary complex [45].
Responses to aortic occlusion
Inflation of a pneumatic cuff placed on the descending
Identification of serotonergic cells
All p5HT cells were identified using a quantitative algorithm that discriminates their slow and steady discharge from the fast and/or bursting discharge of non-serotonergic cells [40]. Since the development of this algorithm, it has reliably predicted the serotonin immunoreactivity of 67 of 70 RM cells tested ( [17], [18], [21] unpublished observations). It is therefore likely that all or nearly all of the RM neurons studied contained serotonin.
Baroreceptor activation
Changes in blood pressure are sensed by
Acknowledgements
This research was supported by NINDS (R01 NS33984) and the Brain Research Foundation.
References (62)
Direct evidence of an extensive GABAergic innervation of the spinal dorsal horn by fibres descending from the rostral ventromedial medulla
Neuroscience
(1996)- et al.
Thyrotropin-releasing hormone in spinal cord: coexistence with serotonin and with substance P in fibers and terminals apposing identified preganglionic sympathetic neurons
Brain Res.
(1987) - et al.
Efferent connections of the lateral hypothalamic area of the rat: an autoradiographic investigation
Brain Res. Bull.
(1982) - et al.
A laser confocal microscopic study of vagal afferent innervation of rat aortic arch: chemoreceptors as well as baroreceptors
J. Auton. Nerv. Syst.
(1997) - et al.
The response of individual sympathetic preganglionic neurones to microelectrophoretically applied endogenous monoamines
Brain Res.
(1981) Medullary neurons with projections to lamina X of the rat as demonstrated by retrograde labeling after HRP microelectrophoresis
Brain Res.
(1989)- et al.
Descending axonal projections from the medial parabrachial and Kolliker-Fuse nuclear complex to the nucleus raphe magnus in cats
Neurosci. Lett.
(1990) Anatomical and functional connections of neurons of the rostral medullary raphe of the rabbit
Brain Res.
(1988)Cardiovascular responses elicited by electrical and chemical stimulation of the rostral medullary raphe of the rabbit
Brain Res.
(1988)- et al.
Thyrotropin-releasing hormone-immunoreactive neurons project from the ventral medulla to the intermediolateral cell column: partial coexistence with serotonin
Brain Res.
(1986)
Anatomical evidence for a strong ventral parabrachial projection to nucleus raphe magnus and adjacent tegmental field
Brain Res.
Transneuronal labeling of CNS neuropeptide and monoamine neurons after pseudorabies virus injections into the stellate ganglion
Brain Res.
Antagonism of the excitatory effects of 5-hydroxytryptamine on sympathetic preganglionic neurones and neurones activated by visceral afferents
Neuropharmacology
Changes in the tail surface temperature of the rat following injection of 5-hydroxytryptamine into the ventrolateral medulla
Neuropharmacology
The actions of 5-hydroxytryptamine on the membrane of putative sympatho-excitatory neurones in the rostral ventrolateral medulla of the adult rat in vitro
Brain Res.
Cardiovascular responses to 5-HT in the ventrolateral medulla of the rat
J. Auton. Nerv. Syst.
Raphe magnus serotonergic neurons tonically modulate nociceptive transmission
Pain Forum
Evidence for a serotonergically mediated sympathoexcitatory response to stimulation of medullary raphe nuclei
Brain Res.
Identification of serotonergic and sympathetic neurons in medullary raphe nuclei
Brain Res.
Single unit activity of noradrenergic neurons in locus coeruleus and serotonergic neurons in the nucleus raphe dorsalis of freely moving cats in relation to the cardiac cycle
Brain Res.
The topographical distribution of serotoninergic terminals in the spinal cord of the cat: biochemical mapping by the combined use of microdissection and microassay procedures
Brain Res.
Interactive responses to stimulation of the amygdaloid central nucleus and baroreceptor afferent activation in the rabbit
J. Auton. Nerv. Syst.
Aortic nerve stimulation in the rat: cardiovascular and respiratory responses
Brain Res. Bull.
Evidence for co-existence of thyrotropin-releasing hormone, substance P and serotonin in ventral medullary neurons that project to the intermediolateral cell column in the rat
Neuroscience
Central neural organization and control of sympathetic nervous system in mammals
Prog. Neurobiol.
Excitatory 5-HT2-mediated effects on rostral ventrolateral medullary neurones in rats
Neurosci. Lett.
Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats
Neuroscience
The influence of the sympathetic outflow on aortic chemoreceptors of the cat during hypoxia and hypercapnia
J. Physiol. (Lond.)
The aortic arch baroreceptor response to static and dynamic stretches in an isolated aorta-depressor nerve preparation of cats in vitro
J. Physiol. (Lond.)
Electron microscopic evidence of a monosynaptic pathway between cells in the caudal raphe nuclei and sympathetic preganglionic neurons in the rat spinal cord
Exp. Brain Res.
The relation between carotid body chemoreceptor discharge, carotid sinus pressure and carotid body venous flow
J. Physiol. (Lond.)
Cited by (0)
- 1
Present address: Dept. of Psychiatry, MetroHelath Medical Center, 2500 MetroHealth Drive, Cleveland, OH 44109-1998, USA.