264 Brain Research, 535 (1990) 264-270 Elsevier BRES 16121 Mechanism of the cardiovascular effects of GABA B receptor activation in the nucleus tractus solitarii of the rat Alberto Florentino, Karoly Varga and George Kunos Laboratory of Physiological and Pharmacological Studies, National Institute on Alcohol Abuse and Alcoholism, Bethesda, MD 20892 (U.S.A.) (Accepted 26 June 1990) Key words: Baclofen; 2-hydroxy-Saclofen; Phaclofen; Glutamate; Baroreflex The effects of baclofen microinjected into the nucleus tractus solitarii (NTS) on blood pressure, heart rate and baroreflex bradycardia were studied in urethane-anesthetized rats, Baclofen caused dose-dependent pressor and tachycardic effects and inhibited the reflex bradycardia elicited by i.v. phenylephrine. The effects of baclofen were inhibited by similarly administered GABA B receptor antagonists, phaclofen and 2-OH-saclofen, or the non-NMDA glutamate receptor antagonist, DNQX, or by pretreatment of rats with intracisternally administered pertussis toxin. DNQX and pertussis toxin, but not the NMDA antagonist, MK-801, also inhibited baroreflex bradycardia. Intra-NTS injections of glutamate caused hypotension and bradycardia, which were potentiated by baclofen, and were not affected by either DNQX or MK-801, or by pretreatment with pertussis toxin. These findings indicate that the cardiovascular effects of stimulation of GABAa receptors in the NTS are due, at least in part, to inhibition of the depressor baroreflex response. Inhibition of the release and/or postsynaptic action of an excitatory amino acid transmitter other than glutamate is the most likely mechanism. INTRODUCTION The medullary nucleus tractus solitarii (NTS) is the site of the first synapse of the baroreflex arc and thus it plays an important role in the reflex regulation of blood pressure and heart rate. While the chemical nature of the primary neurotransmitter in this synapse has not been definitively determined, various neurotransmitter sub- stances have been shown to modulate the sensitivity of the baroreflex at the level of the NTS. GABA is the main inhibitory neurotransmitter in the brain, and GABAergic mechanisms in the hypothalamus ~-3, at various medullary sites 4-6, as well as in the spinal cord 7 have been shown to be involved in cardiovascular regulation. The receptor mediating the effects of GABA at most of these sites is the bicuculline-sensitive GABA A receptor. The NTS region is rich in GABA 8'9 and stimulation of GABAA receptors in the NTS by muscimol causes a bicuculline- sensitive pressor response 4. However, the GABAB re- ceptor agonist, baclofen, causes a bicuculline-insensitive pressor response when injected into the NTS 1°-~4, and there is evidence that the pressor response to endogenous GABA in the NTS is mediated by bicuculline-insensitive GABA B receptors similar to those involved in the actions of baclofen 13. Recent studies indicate the existence of two subtypes of GABA B receptors, distinguished by their differential sensitivity to the GABAB receptor antago- nist, phaclofen, or to inactivation by pertussis toxin 15. The purpose of the present experiments was to analyze the mechanism of the pressor response to GABA 8 receptor stimulation in the NTS of urethane-anesthetized rats, and to pharmacologically characterize the receptors involved. MATERIALS AND METHODS Male Sprague-Dawley rats weighing 250-350 g were anesthetized by urethane, 400 mg/kg i.p. +800 mg/kg i.v. Arterial blood pressure was measured through a cannula inserted into the femoral artery and connected to a pressure transducer and polygraph. Heart rate was monitored through a tachograph preamplifier driven by the pressure wave. Intravenous injections were given through a cannula in the femoral vein. For microinjection of various agents into the NTS, the anesthe- tized animal was mounted in a stereotaxie apparatus, the head was tilted 45 ° downward and the brainstem was exposed by a midline incision and cutting through the atlanto-occipital membrane. The obex was located at the caudal tip of the area postrema and the NTS was determined from this reference as 0.0 mm (A/P), +0.5 mm (M/L), -0.4 mm (D/V). The tip of a single barrelled, drug filled glass microcannula was stereotaxically inserted into the NTS. Drugs were dissolved in normal saline or, in the case of DNQX (see below), in 10% dimethylsulfoxide (DMSO) in normal saline. Drugs were injected over a period of 5-10 s, in volumes of 50-100 hi/side. Microinjection of saline or 10% DMSO in the same volume caused no change in blood pressure, heart rate, or in the reflex bradycardic response to phenylephrine. Using these low injection volumes is Correspondence: G. Kunos, LPPS, NIAAA, 12501 Washington Avenue, Rockville, MD 20852, U.S.A.