PRECOLLICULAR DECEREBRATION REDUCES THE PRESSOR RESPONSES EVOKED BY STIMULATION OF ROSTRAL PONS BUT NOT MEDULLA IN CATS

In 30 cats under chloralose (40 mg/kg) and urethane (400 mg/kg) anesth esia, the ponto-medullary region involved in cardiovascular integratio n were stimulated by rectangular pulses (0.5 ms, 80 or 5 Hz, 100 to 20 0 mu A) and/or by microinjection of sodium glutamate (Glu, 0.25-0.5 M, 70-200 nl). Changes of systemic arterial blood pressure (SAP) and ren al sympathetic nerve activity (RNA) following stimulation were compare d before and after preollicular decerebration. Precollicular decerebra tion itself resulted in an immediate but brief (5 to 15 min) hypotensi on with a decrease in SAP ranging from 40 to 100 mmHg. Stimulation of the lateral tegmental field (FTL) produced depressor responses. After precollicular decerebration, the stimulation induced depressor respons es were either abolished or converted to mild presser responses. Stimu lation of the dorsal gigantocellular tegmental field-periventricular g rey (dFTG-PVG) produced presser responses. These responses were abolis hed after precollicular decerebration without exception. On the other hand, precollicular decerebration did not reduce presser responses pro duced by stimulation of the ventrolateral medulla (VLM) and the dorsal medulla (DM). In 7 additional cats killed with an overdose of pentoba rbital, the brain stem were processed for dopamine beta-hydroxylase (D BH). The presser areas of the VLM and DM were DBH positive, indicating the presence of norepinephrine, while the dFTG-PVG and FTL were not. These findings suggest that the depressor mechanism of the FTL and the presser mechanism of the dFTG, but not of the VLM or DM depend on act ions of the brain structures rostral to superior colliculi.