Central nervous system 5-hydroxytryptamine and noradrenaline specificity of ear vascular and ventilation reflexes in thermoregulating rabbits.

Contrasting hypotheses of mammalian thermoregulation were tested in unanesthetized rabbits in relation to the role of the central nervous (CNS) monoamines 5-hydroxytryptamine (5-HT) and noradrenaline (NA) in regulating lung function and ear skin blood flow (Doppler flowmeter). Normal rabbits and rabbits with CNS depletion of 5-HT and NA (caused by the neurotoxins 5,7-dihydroxytryptamine and 6-hydroxydopamine) were studied in an airconditioned chamber at ambient temperatures of 12 degrees, 22 degrees and 35 degrees C. The results suggest that CNS 5-HT plays an excitatory role in the heat conservation mechanism of cold-induced ear skin vasoconstriction, and that this effect is inhibited by CNS NA to cause heat dissipation during heat stress. Both CNS 5-HT and NA appear to exert a mild inhibitory restraint on ventilation even during heat stress. The data support the theory that CNS 5-HT is concerned with heat conservation and CNS NA with heat loss mechanisms in the cutaneous circulation, and that both monoamines moderate heat loss through panting. CNS monoamine-dependent thermoregulation in the rabbit thus resembles the model postulated for the cat, dog and monkey rather than, as previously proposed, for the sheep and goat.