INFLUENCE OF INTRAHYPOTHALAMICALLY INJECTED CHOLINERGIC AGENTS ON TEMPERATURE REGULATION IN THE PIGEON AT AMBIENT-TEMPERATURES OF 20-DEGREES-C, 32-DEGREES-C AND 38-DEGREES-C

The effects of intrahypothalamic injections of cholinergic substances on temperature regulation of unanaesthetized pigeons were studied at ambient temperatures (Ta) of 20°C, 32°C and 38°C. At Ta 20°C, acetylcholine + physostigmine (ACh, 20 μg + Phy, 5 μg), oxotremorine (Oxo, 2, 5 and 8 μg) and carbamylcholine (CCb, 1.5 μg) induced cessation of shivering, vasodilation in the feet and a fall in body temperature. Nicotine (Nic, 2 and 20 μg) was devoid of these effects. The effect of Oxo on Tb was dose-related. Injections of ACh + Phy (5 + 5 μg and 20 + 5 μg) and Oxo (2, 5 and 8 μg) at Ta 32°C produced variable temperature responses and usually a slight elevation of heart and respiratory frequencies. Carbamylcholine (1.5 and 3 μg) caused a rise of Tb, transient acceleration of heart rate and a prolonged elevation of respiratory rate. Nicotine (2 and 20 μg) slightly lowered Tb and increased the heart rate, but lacked a detectable effect on respiratory rate. Under moderate heat load (Tb 38°C), both ACh + Phy (20 + 5 μg) and CCh (1 μg) depressed thermally-induced panting and caused a slight hyperthermia. Atropine (Atr, 2 μg) pre-injection, in most cases, prevented the depressant action of CCh on panting, while tubocurarine (TbC, 1 μg) was ineffective. In a few birds, however, TbC caused a transitory depression of panting. At a dosage of 3 μg both CCh and TbC led to lethal hyperthermia. The results indicate that cholinergic stimulation of the hypothalamus of the pigeon may inhibit opposing thermoregulatory responses (both shivering and panting) elicited by different external conditions. At Ta near thermoneutrality a slight hyperthermia is evoked. The neuronal pathways involved in these responses seem to be muscarinic. © 1979.