Cardiovascular influences on rat parabrachial an electrophysiological study JACK

H. JHAMANDAS,

SVEN

E. AIPPERSBACH,

AND

KIM

nucleus:

H. HARRIS

Division of Neurology, Department of Medicine, Faculty of Medicine, University of Alberta, Edmonton, Alberta T6G 2B7, Canada

H., SVEN E. AIPPERSBACH, AND KIM H. influences on rat parabrachial nucleus: an electrophysioZogicaZ study. Am. J. Physiol. 260 (Regulatory Integrative Comp. Physiol. 29): R225-R231, 1991.-The rat pontine parabrachial nucleus is a prominent recipient of autonomic-related information from the more caudal levels of the neuraxis. The present experiments examined the responsiveness of neurons within the parabrachial nucleus to the following three specific cardiovascular stimuli: the activation of peripheral arterial baroreceptors, right atria1 stretch receptors, and the administration of systemic angiotensin (ANG) II. Extracellular recordings in urethan-anesthetized animals indicate the presence of cells, mostly within the lateral parabrachial nucleus, that increase (17.5%, 28 of 160 cells) and decrease (4&l%, 77 of 160 cells) their excitability consequent to baroreceptor activation. A similar profile of alteration in cellular firing rates was observed with intravenous ANG II (increase in l&8%, 16 of 101 cells; decrease in 28.7%, 29 of 101 cells). In contrast, fewer neurons located within the medial parabrachial and the Kolliker-Fuse nuclei were activated by these stimuli. A majority of cells (80%, n = 15) displayed a lack of response to right atria1 stretch receptor activation. Of ANG II-sensitive lateral parabrachial cells, 23% (n = 43) revealed an alteration in excitability that could not be explained on the basis of a response to elevation in blood pressure. It is possible that this group of cells may be activated by the actions of systemic ANG II on neurons of the area postrema, a circumventricular structure, whose central projections are directed toward the parabrachial nucleus. These findings also support the notion of a diversity of cardiovascular inputs to topographically segregated regions within the parabrachial nucleus. JHAMANDAS, JACK HARRIS. Cardiovascular

nucleus of the tractus solitarius (NTS) (23, 26, 32). In particular, the pathways from the NTS appear to be topographically organized within the PBN, so that the rostra1 portion of the NTS that receives gustatory input projects to the caudomedial PBN (11, 17). In contrast, the lateral PBN and the adjacent Kolliker-Fuse nucleus are recipients of cardiovascular-related information from mediocaudal subdivision of the NTS, which is the primary medullary site for the termination of baro- and chemoreceptor afferents from the periphery (8,14). More recently, another cardiovascular input from the brain stem to the lateral PBN has also been described to originate in the area postrema, a circumventricular organ devoid of the blood-brain barrier (33, 35). The area postrema is viewed as a window whereby circulating substances, notably angiotensin (ANG) II, can influence central autonomic nervous system outflow. In view of these observations, neurons within the PBN are in a unique position to receive cardiovascular-related signals from a number of different sources. Although these neurons have been shown to alter their excitability consequent to electrical stimulation of buffer nerves (3), data concerning the response of PBN neurons to activation of specific cardiovascular inputs, through their receptors in the periphery, are at present lacking. The present study was thus aimed at examining the response of identified single neurons within the PBN to baroreceptor and right atria1 stretch receptor activation and systemic ANG II.

brain stem; blood pressure MATERIALS

(PBN), consisting of the dorsal and ventral subdivisions, lies in the dorsolateral pons and flanks the brachium conjunctivium. There is increasing evidence to implicate this structure as a major participant in the control of a wide array of visceral and autonomic functions, including taste (11)) respiration (2)) fluid balance (27), central cardiovascular regulation (3, 25,38), and neuroendocrine responses (34). With respect to the latter of these, electrical and chemical stimulation within the PBN region evoke potent pressor responses (25, 37) and influence release of the neurohypophysial hormone vasopressin (28, 34). A significant number of afferent projections to the PBN from caudal levels of the neuraxis originate in the spinal cord (4), the ventrolateral medulla (2l), and the THE

PARABRACHIAL

NUCLEUS

0363-6119/91

$1.50 Copyright

AND METHODS

Male Sprague-Dawley rats (ZOO-350 g; n = 25) were anesthetized with urethan (1.2-1.5 g/kg). The femoral artery and vein were catheterized (PE-50, Intramedic) to monitor arterial blood pressure continuously and permit the administration of drugs, respectively. Heart rate was monitored throughout the experiment, and body temperature was maintained at 37°C using a feedback-controlled heating blanket. Animals were placed in a stereotaxic frame, and burr holes were drilled in the skull for subsequent placement of recording electrodes. Recording of parabrachial neurons. Extracellular single-unit recordings were obtained from neurons in the region of the parabrachial nucleus using glass micropipettes filled with 2% Pontamine sky blue in 0.5 M sodium acetate and with a tip impedance of 2-5 MQ. The electrodes were stereotaxically positioned within the para-

0 1991 the American

Physiological

Society

R225

Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (130.070.008.131) on November 7, 2018. Copyright © 1991 American Physiological Society. All rights reserved.

R226

CARDIOVASCULAR

INPUTS

TO

brachial area (coordinates 6.8-7.4 mm posterior to bregma and 2.0-2.4 mm lateral to midline; 31). Neural signals were amplified conventionally, filtered (band pass 300 Hz-10 kHz), and displayed on an oscilloscope. Action potentials were isolated from background activity with a window discriminator and led to an IBM/PS-2 personal computer programmed for on-line spike train analysis. Impulse activity was also monitored using an audio amplifier. Cardiovascular stimuli and responses of parabrachial neurons. In cases where stable, baseline recording was

obtained for >3 min, the responsiveness of these neurons to baroreceptor and right atria1 stretch receptor activation and the administration of systemic ANG II was assessed. Activation of peripheral arterial baroreceptors was achieved by a brief intravenous administeration of 2-10 pg of the a-adrenergic agonist metaraminol, which was sufficient to elevate mean arterial pressure by at least 40-60 mmHg. The doses of systemically administered ANG II (Bachem) ranged from 50 to 500 ng and were based on previous studies demonstrating the ability of intravenous ANG II, at these doses, to alter the excitability of forebrain neurons (9) and also to release the neurohypophysial hormones vasopressin and oxytotin (18). The effects of two or more different doses of ANG II were assessed where possible. In three animals, right atria1 balloons were implanted 1 wk before electroph .ysiological experiments by methods previously described (15). At th .e time of ne ura l recordings, right atria1 stretch receptors were activated by atria1 distension, consequent to the inflation of a balloon with 0.05-0.10 ml of physiological saline. In the rat, the threshold volume of balloon inflation required to evoke a reproducible alteration in the excitability of locus coeruleus neurons has been shown to be on the average 0.05 ml (13). Balloon placement at the right atrial-superior vena cava junction was visually identified at the end of each experiment. Patterns of response (an increase or decrease in firing rate) consequent to cardiovascular stimuli were derived from ratemeter records; a 30% stimulus-induced alteration of baseline excitability was required to classify a response as being facilitatory or depressant. The baseline firing rate (spikes/s) was determined first by calculating the mean bin count (bin resolution 1 s) for a duration of 100 s before the cardiovascular stimulus. In a similar fashion, a positive response, i.e., one that revealed an alteration of 30% from baseline firing rate, was derived from a 100-s period after application of the stimulus. In all instances, the return of a cell to its baseline firing rate was confirmed by determining its firing frequency over a 100 s duration before the delivery of the next stimulus. Histology. For each experiment, the site of final electrode penetration was marked by iontophoretically depositing Pontamine sky blue dye from the electrode tip using ~-PA cathodal current for 10 min. The brains were fixed during intracardiac perfusion of deeply anesthetized animals with 10% formaldehyde and 50-pm serial coronal sections cut using a vibratome. The sections were mounted and stained with safranin. Recording sites cor-

RAT

PARABRACHIAL

NUCLEUS

responding to serial electrode penetrations were reconstructed by extrapolation from one marked reference point (Pontamine sky blue deposit) in all 25 animals. Experiments where the dye spot could not be reliably detected were excluded from subsequent analysis. RESULTS

The spontaneous firing frequency of 114 PBN neurons, with stable baseline activity of >5-min duration, was assessed and found to range from 0 to 29 Hz (Fig. 1). A majority of these (75.4%) were determined to fire at frequencies