Brain stem regions mediating the cardiovascular responses elicited from the posterior cerebellar cortex in the rabbit.

1. In this study we have examined the roles of the lateral parabrachial nucleus (lateral PBN) and nucleus tractus solitarius (NTS) as sites mediating the circulatory responses evoked from lobule IX b of the posterior cerebellar vermis in the decerebrate and anaesthetized decerebrate rabbit. Microinjection of either kainic acid or bicuculline into the lateral PBN and NTS was undertaken to assess the importance of neurones and GABAergic mechanisms, respectively, in expressing the cardiovascular responses evoked from lobule IX b. 2. In both the decerebrate and anaesthetized decerebrate rabbit activation of neurones within the lateral PBN with a microinjection of glutamate elicited tachycardia and a pressor response together with an increase in renal sympathetic nerve discharge and vasoconstriction in the hindlimb. A microinjection of GABA into the lateral PBN evoked an opposite pattern of response. Chemical lesioning of neurones within the rostral region of the lateral PBN or pharmacological blockade of GABAA receptor, abolished or significantly attenuated the cardiovascular response (bradycardia/depressor effect) evoked from lobule IX b in the anaesthetized decerebrate rabbit. 3. In the decerebrate animal the tachycardia/pressor response elicited from lobule IX b was suppressed when either kainic acid or bicuculline was microinjected into the caudal division of the lateral PBN. 4. A bradycardia, depressor response, an abolition of on-going renal nerve activity and vasodilatation in the femoral bed were evoked by activating neurones within a restricted region of the NTS with glutamate in the decerebrate and anaesthetized decerebrate rabbit. A microinjection of GABA into the NTS produced a qualitatively opposite pattern of response. The bradycardia/depressor response evoked from lobule IX b in the anaesthetized decerebrate rabbit was not altered significantly following a microinjection of either kainic acid or bicuculline into the ipsilateral NTS. However, in the decerebrate preparation following a microinjection of bicuculline into the ipsilateral NTS the tachycardia/pressor response evoked from IX b was either abolished or in many cases reversed to the pattern of response seen in the anaesthetized decerebrate rabbit. 5. The present experiments provide evidence for two functionally distinct pathways from the cardiovascular region of lobule IX b to the lateral PBN.(ABSTRACT TRUNCATED AT 400 WORDS)