Elevated spontaneous activity of phenylephrine-excited neurons in the caudal ventrolateral medulla of spontaneously hypertensive rats.

1. The caudal ventrolateral medulla (CVLM) of pentobarbital-anaesthetized spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats was identified by the depressor response with microinjection of glutamate. 2. The mean firing rate of the spontaneously active phenylephrine-excited CVLM neurons was significantly greater in SHR than in WKY. The pattern of neuronal firing in terms of coefficient of variation and skewness of the interspike interval histogram was similar in both types of rats. 3. Administration to CVLM of kainic acid, an excitotoxic agent, led to a biphasic blood pressure response--a transient reduction followed by a gradual and sustained elevation. The elevation in blood pressure was significantly greater in SHR than in WKY. 4. These findings suggest that the barosensitive CVLM neurons are more active and the tonic inhibitory influence from CVLM to rostral ventrolateral medulla (RVLM) is greater in SHR than in WKY. It is therefore not likely that the elevated spontaneous activity of the RVLM cardiovascular neurons in SHR is due to a smaller inhibitory influence from CVLM.