Dopamine microinjected into the nucleus ambiguus elicits vagal bradycardia in spinal rats

To investigate the effects of dopamine (DA) on vagal efferent activity, DA was microinjected into the right nucleus ambiguus (NA) in rats. Experiments were done in 19 urethane anaesthetized, artificially ventilated spinal (C1) rats. Sites in the right NA containing cardioinhibitory neurons were identified by observing a marked and reproducible decrease in heart rate (HR; 64.9 + 2.8 bpm; n = 36) elicited by microinjecting L-glutamate (GLU; 1.5. nmol in 10 nl). No decreases in arterial pressure (AP) were obtained at these sites. Microinjection of DA (1-15 nmol in 10 nl) into 24 of these 36 sites caused a dose-dependent decrease in HR. The responses to 1 nmol and 3 nmol DA were blocked by (+/-)-sulpiride, a specific D2 receptor antagonist (0.1 nmol in 10 nl). A higher dose of (+/-)-sulpiride (1 nmol in 10 nl) was required to block the responses to 15 nmol of DA. Bradycardia elicited by even the lowest amount of DA (1 nmol) was not blocked by SCH-23390, a specific D1 receptor antagonist. These experiments demonstrate that the bradycardia caused by microinjection of DA into the NA is due to the excitation of dopamine D2 receptors present on vagal preganglionic cardioinhibitory neurons controlling HR.