Raphe pallidus and parapyramidal neurons regulate ear pinna vascular conductance in the rabbit

We have determined whether alteration of neuronal function in raphe pallidus and the parapyramidal region alters ear blood flow, measured by an implanted Doppler ultrasonic probe, in anesthetized rabbits. Injection of GABA (5 nmol in 50 nl) increased ear flow from 6.0 +/- 1.0 to 31 +/- 10 kHz, without changing arterial pressure or heart rate. Focal electrical stimulation of raphe pallidus at low current amplitude caused ear pinna blood flow to fall from 41 +/- 6 to 9 +/- 3 kHz, again with little or no change in arterial pressure. These excitatory and inhibitory stimuli did not affect superior mesenteric blood flow. The fall in ear flow in response to electrical stimulation of raphe pallidus was not prevented by tetrodotoxin-mediated inhibition of the rostral ventrolateral medulla. Thus raphe pallidus and parapyramidal region may regulate ear pinna vascular conductance via a direct spinal projection.