The effects of GABAA receptor blockade in the dorsomedial hypothalamic nucleus on corticotrophin (ACTH) and corticosterone secretion in male rats

doi:10.1016/S0006-8993(96)00810-4

Brain Research

Volume 739, Issues 1–2, 11 November 1996, Pages 46-51

https://doi.org/10.1016/S0006-8993(96)00810-4 Get rights and content

Abstract

Experiments were conducted to test if blockade of GABA_A receptors in the dorsomedial hypothalamic nucleus (DMH) of rats, which is known to elicit cardiovascular and anxiety responses, would also elicit changes in the plasma levels of adrenocorticotrophic hormone (ACTH) and corticosterone. Male Sprague-Dawley rats were anesthetized with pentobarbital, fitted with femoral arterial catheters and implanted with microinjection cannulae into the DMH or the sites anterior to the DMH (i.e., closer to the paraventricular nucleus (PVN) of the hypothalamus). The rats were then injected with either artificial cerebrospinal fluid (aCSF; 100 nl) or the GABA_A antagonist, bicuculline methiodide (BMI; 50 pmol in 100 nl) and their plasma samples obtained at 5, 30, 60, and 120 min after microinjection. Plasma ACTH and corticosterone were measured by using a radioimmunoassay. Rats injected with BMI, but not aCSF, into the DMH showed significant increases in heart rate (HR, 110 ± 16 beats/min), blood pressure (BP; 30 ± 4 mmHg), and plasma levels of both ACTH (64 ± 10 pg/ml) and corticosterone (170 ± 25 ng/ml) from baseline. BMI injections into the anterior sites closer to the PVN did not elicit significant increases in HR, BP, or plasma levels of ACTH and corticosterone. These results suggest that a tonic GABA_A receptor-mediated inhibition system regulates a coordinated physiological and neuroendocrine response in the DMH and that this neuroendocrine response is not due to diffusion of BMI to the PVN of rats.

Introduction

Blocking GABA_A neurotransmission in the dorsomedial hypothalamus (DMH) of rats elicits increases in heart rate (HR), blood pressure (BP), respiration rate (RR), plasma catecholamines and peripheral blood flow patterns similar to a defense reaction 7, 8, 29. GABA blockade in the cardiostimulatory region of the DMH of conscious rats elicits ‘escape’ oriented locomotion [19], a selective enhancement of ‘fear’ responses [20], and an increase in experimental anxiety as measured in a ‘conflict’ [21], the elevated plus maze [22], and social interaction [24]tests. All of these data suggest that GABAergic neurons in the DMH tonically inhibit a coordinated pattern of physiological and behavioral responses associated with stress and anxiety. Therefore, it could be predicted that one possible outcome of a stress response elicited by blocking this GABAergic inhibition in the DMH would be activation of the pituitary-adrenal axis.

The paraventricular nucleus (PVN) of the hypothalamus is another region where blockade of GABA_A receptor function is known to elicit cardiovascular (particularly pressor) responses [12]. The PVN is known to be the primary site for corticotrophin-releasing hormone (CRF)-containing neurons that regulate adrenocorticotrophic hormone (ACTH) release 6, 25. Anatomical studies have demonstrated that the DMH has efferent projections to a number of areas that regulate emotional and physiological responses [28], including the PVN. The DMH appears to be a major relay center to the PVN for the circadian rhythm influences from the suprachiasmatic nucleus of the hypothalamus [4]and lesioning the DMH disrupts the diurnal pattern of corticosterone secretion [3]. Thus, it appears that the PVN and DMH are part of a hypothalamic circuit that coordinates the physiological, behavioral, and neuroendocrine responses associated with stress and emotion. At least some aspects of this circuit appear to be regulated by GABAergic inhibition in both the DMH and PVN.

Therefore, the aim of the present study was to determine if GABA_A receptor blockade in the DMH would activate the release of ACTH and corticosterone in addition to the cardiorespiratory responses associated with stress. Furthermore, we tested the effects of GABA_A receptor blockade in the area anterior to the DMH (i.e., closer to the PVN) to assess the possible diffusion of drugs to this region and eliciting release of ACTH and corticosterone.

Section snippets

Surgical procedures

Male Sprague-Dawley (275–300 g) rats were individually housed under 12-h light-dark conditions and allowed free access to food and water. The rats were anesthetized with pentobarbital (50 mg/kg, i.p.) and arterial catheters were inserted into the femoral artery, by methods previously described [17]. The catheters were filled with heparinized saline (25 U/ml) and routed subcutaneously to the dorsal midline of the neck where they were fixed using a leather jacket. The animal was then placed into

Results

Histological examinations confirmed that a total of 6 rats had cannulae in the DMH and 5 rats had implantations in areas anterior to the DMH. Representative injection sites in the DMH (A) or the anterior sites (B) marked with India ink are shown in Fig. 1 (see arrows).

There were significant increases observed in HR and BP following an acute microinjection of 50 pmol of BMI into the DMH of anesthetized rats (Fig. 2A). When BMI was injected into the sites anterior to the DMH, there were no

Discussion

The data presented above clearly indicate that blocking GABA_A receptor-mediated inhibition in the DMH of rats results in increases in ACTH and corticosterone secretion in addition to the increases in HR and BP (Fig. 2 and Fig. 3). This activation of the hypothalamic-pituitary-adrenal (HPA) axis by GABA_A receptor blockade in the DMH appears consistent with previous reports which suggest that blocking GABA_A receptors in the DMH elicit fear [20]and anxiety responses 22, 23, while enhancing GABA

Acknowledgements

This study was supported by PHS Grant MH 45362 and research funds from the Association for the Advancement of Mental Health Research and Education and the Indiana State Division of Mental Health. The authors wish to thank Dawn Davis and Dr. Debomoy Lahiri for their assistance in conducting the ACTH and corticosterone assays.

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Research reportThe effects of GABAA receptor blockade in the dorsomedial hypothalamic nucleus on corticotrophin (ACTH) and corticosterone secretion in male rats

Abstract

Introduction

Section snippets

Surgical procedures

Results

Discussion

Acknowledgements

Brain Res. Rev.

B. Rec. Prog. Brain Res.

Brain Res.

Brain Res.

Brain Res.

Brain Res.

Neuropharmacology

Brain Res.

Brain Res.

Brain Res.

Pharmacol. Biochem. Behav.

Effects of destruction of the suprachiasmatic nuclei on the circadian rhythms in plasma corticosterone, body temperature, feeding and plasma thyrotropin

Neuroendocrinology

Effect of ventromedial and dorsomedial hypothalamic lesions on circadian corticosterone rhythms

Neuroendocrinology

Projections of the suprachiasmatic nucleus to stress-related areas in the rat hypothalamus: a light and electron microscopic study

J. Comp. Neurol.

Research report
The effects of GABA_A receptor blockade in the dorsomedial hypothalamic nucleus on corticotrophin (ACTH) and corticosterone secretion in male rats