PRESSOR EFFECTS ELICITED BY STIMULATION WITHIN THE MEDULLARY RAPHE NUCLEI OF THE GOLDEN-HAMSTER (MESOCRICETUS-AURATUS)

The medullary raphe nuclei are involved in central autonomic regulatio n. In all species investigated, electrical stimulation of the raphe nu clei causes cardiovascular responses, although these changes vary betw een species. The present study was designed to investigate the partici pation of these nuclei in cardiovascular regulation in the hamster. We studied the effect on arterial blood pressure (BP) and heart rate (HR ) of electrical stimulation (isolated cathodal square wave pulses for 10 sec at 100 Hz, 40-100 mu A and 1-msec pulse duration) within the me dullary raphe nuclei in urethane-anesthetized (1.2 g/kg, iv, after eth er induction) golden hamsters (Mesocricetus auratus, 130 to 150 g, eit her sex), Electrical stimulation of the same sites was performed on a group of paralyzed (Flaxedil(R), 1 mg/kg, iv) and artificially ventila ted animals. Stimulation sites were histologically defined and maps of the stimuli were obtained for the effect of electrical stimulation on arterial blood pressure, In another series of experiments L-glutamate (0.18 M) was microinjected (75 to 150 nl) into the nucleus raphe obsc urus. Electrical stimulation of the raphe nuclei produced predominantl y presser responses (Delta = +12 to +100 mmHg; 49.3% of the stimulated sites), Hypotension (Delta = -5 to -20 mmHg; 14.6% of the stimulated sites) and no change in BP (35.3%) were evoked from fewer stimulation sites. Presser responses were also predominant in paralyzed animals (D elta = +15 to +95 mmHg; 62.5% of the stimulated sites), and after micr oinjection of L-glutamate into the raphe obscurus (Delta = +35 to +135 mmHg). The present results demonstrate that in the hamster the stimul ation of these nuclei evokes mainly presser responses, These responses are similar to those obtained in the rat and guinea pig but opposite to those observed in the cat and rabbit.