Research Report Microinjection of muscimol into caudal periaqueductal gray lowers body temperature and attenuates increases in temperature and activity evoked from the dorsomedial hypothalamus Rodrigo C.A. de Menezes b , Dmitry V. Zaretsky a , Marco A.P. Fontes b , Joseph A. DiMicco a, ⁎ a Department of Pharmacology and Toxicology, Indiana University School of Medicine, Indianapolis, IN 46202, USA b Laboratório de Hipertensão, Department de Fisiol. e Biofísica, ICB–UFMG, Brazil ARTICLE INFO ABSTRACT Article history: Accepted 23 March 2006 Available online 4 May 2006 Microinjection of the neuronal inhibitor muscimol into the midbrain lateral/dorsolateral periaqueductal gray (l/dlPAG) suppresses increases in heart rate (HR) and mean arterial pressure (MAP) evoked by microinjection of the GABA A receptor antagonist bicuculline methiodide (BMI) into the dorsomedial hypothalamus (DMH) in rats. Injection of BMI into the DMH also increases body temperature (Tco) and motor activity. Here, our goal was to extend previous findings by examining the effect of microinjection of muscimol into the PAG on these thermogenic and behavioral responses in conscious freely moving rats. Microinjection of muscimol (300 pmol and 1 nmol) alone into the l/dlPAG reduced baseline Tco without affecting activity, HR, or MAP. Similar injection of a dose that failed to alter baseline Tco (100 pmol) suppressed the increases in Tco evoked from the DMH and significantly attenuated DMH-induced increases in locomotor activity. Whereas microinjection of 1 nmol muscimol into the ldlPAG abolished the increases in Tco evoked from the DMH and in fact lowered body temperature to a degree similar to that seen after this dose of muscimol alone, 1 nmol muscimol at adjacent sites outside the targeted region of the PAG had no significant effect on DMH-induced increases in Tco or any other parameter. These results indicate a role for neuronal activity in the l/dlPAG in (1) the temperature and behavioral responses to disinhibition of neurons in the DMH, and (2) the maintenance of basal body temperature in conscious freely moving rats. © 2006 Elsevier B.V. All rights reserved. Keywords: Bicuculline GABA Body temperature Locomotor activity 1. Introduction Recent studies indicate that neurons in the region of dorsomedial hypothalamus (DMH) are essential components in the integration of the physiological and behavioral responses to emotional stress in rats (for review, see DiMicco et al., 2002). Accordingly, microinjection of the GABA A receptor antagonist bicuculline methiodide (BMI) into the DMH evokes a pattern of physiological changes similar to those seen in emotional stress, including increases in heart rate (HR) and mean arterial pressure (MAP; De Novellis et al., 1995; Fontes et al., 2001; Samuels et al., 2002, 2004; da Silva et al., 2003; Horiuchi et al., 2004; Soltis et al., 1991), plasma ACTH (Bailey and DiMicco, 2001) and sympathetic nerve activity (Wible et BRAIN RESEARCH 1092 (2006) 129 – 137 ⁎ Corresponding author. Fax: +1 317 274 7714. E-mail address: jdimicco@iupui.edu (J.A. DiMicco). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.03.080 available at www.sciencedirect.com www.elsevier.com/locate/brainres