Evidence for two distinct sympathoinhibitory bulbo-spinal systems.

Yohimbine hydrochloride (0.5 mg/kg, i.v.) caused a long-lasting potentiation of electrodermal (sympathetic-cholinergic) reflexes in intact anaesthetized and decerebrate unanaesthetized cats. Transection of the cervical spinal cord also resulted in an increased amplitude of the sudomotor reflex in unanaesthetized decerebrate preparations. Depletion of monoamines in the CNS by pretreatment with reserpine (5 mg/kg, i.p.) and alpha-methyl-p-tyrosine (2 X 300 mg/kg, i.p.) reduced the concentrations of norepinephrine, dopamine and serotonin to less than 93% of control levels in the thoracic spinal cord. In monoamine-depleted preparations, yohimbine no longer facilitated the reflex amplitude whereas the effect of spinal transection was not altered. These results suggest that there are two distinct sympathoinhibitory systems in the lower brain stem that converge on spinal sympathetic neurons, one of which is monoaminergic and one of which is not. Evidence for the baroreceptor-independent nature of these descending inhibitory systems is discussed.