Brain Research 879 2000 105–114 www.elsevier.com locate bres
Research report
Autonomic and cardiovascular reflex responses to central estrogen injection in ovariectomized female rats
Monique C. Saleh, Barry J. Connell, Tarek M. Saleh
Department of Anatomy and Physiology , Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, P.E.I., Canada C1A 4P3
Accepted 25 July 2000
Abstract
The role of estrogen in central autonomic nuclei was examined in ovariectomized female Sprague–Dawley rats supplemented daily for 7 days with either estrogen 5 mg kg; sc or saline 0.9; sc. Animals were subsequently anaesthetized with sodium thiobutabarbital
Inactin; 100 mg kg; ip and instrumented to record blood pressure and heart rate. Efferent vagal parasympathetic VPNA and renal sympathetic RSNA nerve activities were recorded and used to assess baseline and reflexive changes in autonomic tone. The cardiac
baroreflex was evoked using a single bolus injection of phenylephrine 0.1 mg kg both before and following either intrathecal injection of estrogen 0.5 mM; 1 ml or bilateral injection of estrogen 0.5 mM; 100 nl side into several central autonomic nuclei. In
estrogen-replaced rats, both the baseline and PE-evoked values for mean arterial pressure and RSNA were significantly decreased following injection of estrogen into the nucleus tractus solitarius NTS, rostral ventrolateral medulla RVLM, parabrachial nucleus
PBN, central nucleus of the amygdala CNA and the intrathecal space. Baseline heart rate and VPNA were significantly decreased following injection of estrogen into NTS, nucleus ambiguus Amb, PBN and the intrathecal space. PE-evoked changes in heart rate and
VPNA were significantly enhanced following injection of estrogen into these same nuclei. Injection of estrogen into the insular cortex IC produced significant decreases in baseline and PE-evoked RSNA only. The cardiac baroreflex was significantly enhanced following
injection of estrogen into all nuclei and the intrathecal space. In saline-replaced females, injection of estrogen into NTS, RVLM, Amb and the intrathecal space had similar effects on both baseline and PE-evoked parameters although of a reduced magnitude compared to
estrogen-replaced rats. However, no significant changes in autonomic tone and baroreflex function were observed following the injection of estrogen into the PBN, CNA or IC of saline-replaced rats. These results demonstrate a role for estrogen in central autonomic nuclei in
female rats and suggest a possible alteration of estrogen receptor distribution or efficacy within the central nervous system of estrogen-deficient female rats.
2000 Elsevier Science B.V. All rights reserved.
Theme : Endocrine and autonomic regulation
Topic : Cardiovascular regulation
Keywords : Parabrachial nucleus; Central nucleus of the amygdala; Insular cortex; Nucleus ambiguus; Nucleus tractus solitarius; Rostral ventrolateral
medulla; Spinal cord
1. Introduction nucleus ambiguus and intermediolateral cell column of the
spinal cord respectively [18,19]. With the additional evi- There is increasing evidence to support a role for
dence of estrogen receptor mRNA [1,22,23] as well as the estrogen as a central modulator of autonomic tone. Experi-
estrogen-synthesizing enzyme, aromatase cytochrome P- ments in male and female rats have demonstrated that
450 within discrete regions of the central nervous system intravenous estrogen administration produces significant
[16], the contribution of locally-produced estrogen to the changes in autonomic tone via direct effects on para-
regulation of autonomic and cardiovascular reflexes be- sympathetic and sympathetic preganglionic neurons in the
comes more likely. Experiments in male rats have demonstrated that direct
injection of estrogen into cardiovascular and autonomic
Corresponding author. Tel.: 11-902-566-0819; fax: 11-902-566-
nuclei in the brain stem elicits significant changes in
0832. E-mail address
: tsalehupei.ca T.M. Saleh.
autonomic tone and baroreflex function [20]. Specifically,
0006-8993 00 – see front matter
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significant increases in parasympathetic tone and barorefl- daily subcutaneous injections of either estrogen 17b-
ex sensitivity were observed following bilateral injection estradiol-3-sulfate, water soluble form; 5 mg kg; Sigma
of estrogen into the nucleus tractus solitarius and nucleus Chemical, St. Louis, MO, USA; n555 or physiological
ambiguus. Conversely, sympathetic tone was significantly saline 0.9; n553 over an additional 7 days. This
decreased following injection of estrogen into the nucleus estrogen replacement regimen produces sustained serum
tractus solitarius and rostral ventrolateral medulla [20]. In estrogen levels similar to that in an intact rat during
general, the apparent role of estrogen in cardiovascular proestrous 40–50 pg ml [19].
nuclei is to produce a shift in sympatho-vagal balance On the day of experimentation, animals were anaesthet-
toward the parasympathetic limb resulting in an enhance- ized with sodium thiobutabarbital Inactin; RBI, Natick,
ment of baroreflex function which has been shown to MA, USA; 100 mg kg; ip and instrumented to record
provide antifibrillatory protection on the heart [5]. blood pressure, heart rate and efferent vagal and renal
Investigation into the role of estrogen in cardiovascular nerve activities as described previously [17–19]. Blood
nuclei of female rats has been limited. In estrogen-replaced pressure and heart rate data were displayed and analysed
ovariectomized female rats, the cardiovascular responses to using
POLYVIEW PRO
32 data acquisition software Grass; glutamate injection in the bed nucleus of the stria ter-
Warwick, RI, USA. The multi-unit nerve activity was minalis were significantly enhanced when preceded by
amplified by a Grass model P55 preamplifier Grass with injection of estrogen [6]. Similar injections of estrogen in
a 100-Hz to 3-kHz bandpass and 60-Hz notch filter and non-estrogen replaced ovariectomized female rats had no
then displayed using the
POLYVIEW PRO
32 data acquisition effect on the glutamate-evoked responses [6] suggesting
system Grass. A sampling rate of 2000 s was used to that the effects of estrogen within the central nervous
record the raw nerve signal. Non-biological noise was system may in fact be dependent on levels of circulating
obtained by recording from the nerve after death and was estrogen in the peripheral vasculature. The present in-
subtracted from the nerve signal recorded during the vestigation will attempt to expand our understanding of the
experiment. A venous catheter PE-10 was inserted into role of estrogen within the central nervous system as well
the right femoral vein to permit administration of drugs. as the contribution of peripheral estrogen levels to the
Respiration was facilitated via the insertion of an endotra- neuromodulatory function of estrogen. Specifically, the
cheal tube and ventilation with room air on a Harvard effects of estrogen injection into several central autonomic
rodent ventilator 65 strokes per min; 2.5-ml tidal volume. nuclei on autonomic tone and baroreflex function will be
examined in estrogen-replaced and saline-replaced ovariec- 2.2. Central injections
tomized female rats. Changes in autonomic tone will be assessed by monitoring efferent vagal and renal nerve
Animals n593 were placed in a David Kopf Tujunga, activities. Baroreflex function will be assessed using
CA, USA stereotaxic frame and small burr holes were intravenous administration of a single dose of phenylep-
drilled bilaterally through the skull to permit stereotaxic hrine hydrochloride. Finally, co-injection of a potent and
insertion of a 30-gauge stainless steel, 1-ml Hamilton selective estrogen receptor antagonist, ICI 182,780 [10,25],
micro-syringe. A bilateral injection of either estrogen with estrogen into brain nuclei will be used to verify the
17b-estradiol-3-sulphate; Sigma-Aldrich; St. Louis, MO, specificity and receptor-mediated action of estrogen on
USA; 0.5 mM; 100 nl per side or a combination of autonomic tone and baroreflex function.
estrogen 0.5 mM and ICI 182,780 Tocris; Bristol, UK; 1 pM; 100 nl per side was made into each of the following
nuclei according to coordinates obtained from a stereotaxic
2. Materials and methods atlas of the rat brain [15]: nucleus tractus solitarius NTS,
