In the present study we analyzed effects of bilateral microinjections of muscimol (a GABA A agonist) and baclofen (a GABA B agonist) into the nucleus tractus solitarius (NTS) on bradycardic and pressor responses to chemoreflex activation (potassium cyanide, 40 µg/rat iv) in awake rats. Bilateral microinjections of muscimol (25 and 50 pmol/50 nl) into the NTS increased baseline mean arterial pressure (MAP): 119 Ϯ 8 vs. 107 Ϯ 2 mmHg (n ϭ 6) and 121 Ϯ 8 vs. 103 Ϯ 3 mmHg (n ϭ 6), respectively.

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... mol at 25 pmol/50 nl reduced the bradycardic response to chemoreflex activation 5 min after microinjection; with 50 pmol/50 nl the bradycardic response to chemoreflex activation was reduced 5, 15, 30, and 60 min after microinjection. Neither muscimol dose produced an effect on the pressor response of the chemoreflex. Effects of muscimol (50 pmol/50 nl) on basal MAP and on the bradycardic response of the chemoreflex were prevented by prior microinjection of bicuculline (a GABA A antagonist, 40 pmol/50 nl) into the NTS. Bilateral microinjections of baclofen (12.5 and 25 pmol/50 nl) into the NTS produced an increase in baseline MAP [137 Ϯ 9 vs. 108 Ϯ 4 (n ϭ 7) and 145 Ϯ 5 vs. 105 Ϯ 2 mmHg (n ϭ 7), respectively], no changes in basal heart rate, and no effects on the bradycardic response; 25 pmol/50 nl only attenuated the pressor response to chemoreflex activation. The data show that activation of GABA A receptors in the NTS produces a significant reduction in the bradycardic response, whereas activation of GABA B receptors produces a significant reduction in the pressor response of the chemoreflex. We conclude that 1) GABA A but not GABA B plays an inhibitory role in neurons of the lateral commissural NTS involved in the parasympathetic component of the chemoreflex and 2) attenuation of the pressor response of the chemoreflex by activation of GABA B receptors may be due to inhibition of sympathoexcitatory neurons in the NTS or may be secondary to the large increase in baseline MAP produced by baclofen. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.