The catecholamine precursor L-3,4-dihydroxyphenylalanine inhibits both heat production and heat loss mechanisms in the rabbit.

The effects of catecholamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA) on the thermoregulatory responses of conscious rabbits to different ambient temperatures (Ta) (2, 22, and 30 degrees C) were assessed. Intravenous administration of L-DOPA alone, intravenous administration of L-DOPA plus R04-4602 (a peripheral decarboxylase inhibitor), and intraventricular administration of L-DOPA or norepinephrine all produced a hypothermia at Ta 2 degrees C. The hypothermia was due to a decrease in metabolic heat production (M). On the other hand, L-DOPA or norepinephrine produced both behavioral excitation and hyperthermia at both Ta 22 and 32 degrees C. At Ta 22 degrees C, the hyperthermia was due to decreased ear skin blood flow (EBF) and slightly increased M (due to behavioral excitation) whereas at Ta 32 degrees C the hyperthermia was due to EBF, decreased respiratory evaporative heat loss, and slightly increased M (due to behavioral excitation). Further, the temperature effects induced by L-DOPA were antagonized by pretreatment with 6-hydroxydopamine (a relative depletor of catecholaminergic nerve fibers) but not with haloperidol (a relative blocker of dopaminergic receptors). The data indicate that activation of central adrenergic receptors via the endogeneous release of norepinephrine with L-DOPA inhibits both heat production and heat loss mechanisms in the rabbit.