Additive effects of dopamine and 8-OH-DPAT microinjected into the nucleus ambiguus in eliciting vagal bradycardia in rats.

The effects of combined microinjection into the nucleus ambiguus (NA) of dopamine (DA) and the 5-hydroxytryptamine-1A (5HT-1A) receptor agonist 8-hydroxy-2-[di-n-propylamino]tetralin (8-OH-DPAT) on arterial pressure (AP) and heart rate (HR) were studied in 24 urethane-anaesthetized, artificially ventilated spinal (C1) rats. Sites from which bradycardia was elicited by microinjection of L-glutamate (GLU) were selected for microinjection of DA and 8-OH-DPAT into the NA. Microinjections of 8-OH-DPAT (60-300 pmol in 10 nl) elicited a dose-dependent bradycardia; the HR responses elicited by microinjection of 300 pmol of 8-OH-DPAT were significantly larger (-34.0 +/- 3.0) than responses elicited by the threshold dose of 60 pmol (-3.1 +/- 0.1 bpm). The onset latency of the HR responses elicited by the 300 pmol of 8-OH-DPAT was 6.5 +/- 0.5 s and the peak was reached in 40.0 +/- 8.0 s. The duration of these responses was 615 +/- 3.5 s. Microinjection of DA (1 nmol in 10 nl) within 3-5 s from the time of 8-OH-DPAT microinjection (60 pmol in 2 nl) into the NA at sites previously shown to elicit decreases in HR following microinjection of GLU, produced significant additive effects in eliciting bradycardia (20.4 +/- 2.9 bpm) when compared with decreases in HR elicited by microinjection of DA (11.1 +/- 1.8 bpm) or of 8-OH-DPAT (3.1 +/- 0.1 bpm) alone. There were no changes in AP after microinjections of DA, 8-OH-DPAT or of these two substances combined.(ABSTRACT TRUNCATED AT 250 WORDS)