Elsevier

Brain Research

Volume 900, Issue 2, 11 May 2001, Pages 306-313
Brain Research

Short communication
The discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input

https://doi.org/10.1016/S0006-8993(01)02294-6Get rights and content

Abstract

In order to determine whether serotonergic cells in the medullary raphe magnus (RM) receive baroreceptor input, cells were tested for their responses to descending aortic occlusion, aortic nerve stimulation, or systemic phenylephrine administration in the lightly anesthetized rat. Serotonergic cells were identified physiologically by a quantitative analysis of their slow and steady discharge. Greater than 40% of the serotonergic RM cells tested responded to brief occlusion of the descending aorta at the level of the coeliac arteries, a stimulus that elevated blood pressure by about 30 mmHg. Similarly, about 40% of the serotonergic RM cells responded to stimulation of the aortic nerve, a nerve that contains primarily baroreceptor afferents from the aortic arch. Greater than 70% of RM serotonergic cells responded to phenylephrine administration which elevated blood pressure by an average of 50 mmHg. Serotonergic cell responses to all methods of baroreceptor activation were small in magnitude and were largely restricted in time to the stimulus duration. The results indicate that a subset of serotonergic cells in RM are influenced by baroreceptor activity.

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.

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    Present address: Dept. of Psychiatry, MetroHelath Medical Center, 2500 MetroHealth Drive, Cleveland, OH 44109-1998, USA.

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