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Control of cardiac rate, contractility, and atrioventricular conduction by medullary raphé neurons in anesthetized rats

¹Howard Florey Institute and ²Department of Anatomy and Cell Biology, University of Melbourne, Victoria, Australia; and ³School of Biomedical Sciences, University of Newcastle, New South Wales, Australia

Submitted 29 August 2008 ; accepted in final form 2 December 2008

The sympathetic actions of medullary raphé neurons on heart rate (HR), atrioventricular conduction, ventricular contractility, and rate of relaxation were examined in nine urethane-anesthetized (1–1.5 g/kg iv), artificially ventilated rats that had been adrenalectomized and given atropine methylnitrate (1 mg/kg iv). Mean arterial pressure (MAP), ECG, and left ventricular pressure were recorded. The peak rates of rise and fall in the first derivative of left ventricular (LV) pressure (dP/dt_max and dP/dt_min, respectively) and the stimulus-R ($-R) interval were measured during brief periods of atrial pacing at 8.5 Hz before and after ventral medullary raphé neurons were activated by DL-homocysteic acid (DLH, 0.1 M) or inhibited by GABA (0.3 M) in local microinjections (90 nl). LV dP/dt_max values were corrected for the confounding effect of MAP, determined at the end of the experiments after giving propranolol (1 mg/kg iv) to block sympathetic actions on the heart. DLH microinjections into the ventral medullary raphé region increased HR by 44 ± 2 beats/min, LV dP/dt_max by 1,055 ± 156 mmHg/s, and the negative value of LV dP/dt_min by 729 ± 204 mmHg/s (all, P < 0.001) while shortening the $-R interval by 2.8 ± 0.8 ms (P < 0.01). GABA microinjections caused no significant change in HR, LV dP/dt_max, or $-R interval but reduced LV dP/dt_min from –5,974 ± 93 to –5,548 ± 171 mmHg/s and MAP from 115 ± 4 to 105 ± 5 mmHg (both, P < 0.01). Rises in tail skin temperature confirmed that GABA injections effectively inhibited raphé neurons. When activated, the neurons in the ventral medullary raphé region thus enhance atrioventricular conduction, ventricular contractility, and relaxation in parallel with HR, but they provide little or no tonic sympathetic drive to the heart.

brain stem; sympathetic; inotropic; dromotropic; lusitropic