Influence of ambient temperature on the thermoregulatory responses to 5‐hydroxytryptamine, noradrenaline and acetylcholine injected into the lateral cerebral ventricles of sheep, goats and rabbits

Abstract

SUMMARY1. The influences of ambient temperature (Ta) on the thermoregulatory effector activities and the body temperature (Tb) of intraventricular injections into the sheep, goat and rabbit of 5-hydroxytryptamine (5-HT), noradrenaline (NA), acetylcholine (ACh), carbachol and eserine, have been interpreted in terms of a simple neuronal model of the pathways between thermosensors and thermoregulatory effectors.2. In all three species 5-HT in minimal doses caused a rise in respiratory frequency (RF) and a fall in Tb at high Ta, and a reduction in EMG activity and a fall in Tb at low Ta. These effects could be interpreted as those of an excitatory transmitter acting on the warm receptor-heat loss pathway. 3. In all three species NA caused a reduction in RF and a rise in Tb at high Ta, and a reduction in EMG activity and a fall in Tb at low Ta. These effects are interpreted as those of an inhibitory transmitter acting both on the warm sensor-heat loss pathways and on the cold sensor-heat production pathway.4. The effects of ACh and the cholinomimetic substances carbachol and eserine are complex and more difficult to interpret. In small doses the effects on the sheep and goat are those of an excitatory transmitter on the cold sensor-heat production pathway. There was an increase in EMG activity and a rise in Tb at low Ta, and a reduction in RF and a rise in Tb at high Ta. At higher dose levels in the goat and at all dose levels in the

The effect of noradrenaline, injected into the hypothalamus, on thermoregulation in the cat.

“…In contrast, the direction of temperature change to intraventricular injection of NA varies in sheep, goat and rabbits depending on ambient temperature conditions (Bligh et al 1971). Thus our data supports the concept that, even when differential responses due to environmental temperature are considered, there does appear to be a species difference in the temperature response to substances injected directly into the brain (Bligh et al 1971;Veale & Cooper, 1973).…”

“…During PGE, induced fever in rabbits (Stitt, 1973) and sheep (Bligh & Milton, 1973), the temperature rise in the cold is brought about largely by an increase in heat production, whereas in a hot environment, inhibition of heat loss mechanisms causes a similar elevation. The use of different mechanisms to change body temperature at different environmental temperatures is also commonly observed following injection of various amines into the cerebral ventricles of sheep, goats and rabbits (Bligh et al 1971 (Myers, 1974) envisages NA acting only in an inhibitory manner on the pathway leading to heat conservation and production. Our data suggests that this model requires modification and we propose that NA be given an additional role as acting in an excitatory manner within a heat loss pathway in the AH/POA.…”

The effect of noradrenaline and 5‐hydroxytryptamine injected into a lateral cerebral ventricle, on thermoregulation in the new‐born lamb.

“…It is interesting, however, that the NA did not appear to affect heat conservation mechanisms such as reduced respiratory rate and ear vasomotor tone. The lack of effect of NA on heat conservation in the cold had been previously observed in adult sheep (Bligh et al 1971). There is a possibility, however, that the intraventricular injection of NA did decrease the neural drive for vasomotor tone but was of insufficient magnitude to reduce vasoconstriction enough for the effect to be manifest as an increase in ear temperature.…”

“…It is unlikely that heat production was activated by the NA, since there is evidence that heat production pathways within the c.w.s. of the adult sheep are cholinergically mediated (Bligh & Maskrey, 1969;Bligh et al 1971). Furthermore, the lambs did not shiver, which is an important source of heat in new-born lambs exposed to cold (Alexander & Williams, 1968).…”

Cholinergic Mechanisms in Central Thermoregulation in Pigeons

“…Since then abundant evidence has been obtained in support of this suggestion in a number of mammals by studying the effects of cholinomimetic substances on body temperature following their injection into the cerebral ventricles or into different regions of the hypothalamus. Experiments of this kind were done in mice (Friedman & Jaffe, 1969), rats (Hulst & de Wied, 1967;Meeter & Wolthuis, 1968;Myers & Yaksh, 1968;Lomax, Foster & Kirkpatrick, 1969;Meeter, 1969Meeter, , 1971Avery, 1972;Baird & Lang, 1973), sheep, goats and rabbits (Bligh, Cottle & Maskrey, 1971), cats (Baird & Lang, 1973) and monkeys (Myers & Yaksh, 1969). The nature of the thermoregulatory response to cholinomimetic substances was found to differ in different species and to be dependent on the dose of the substance injected, the ambient temperature, and when injected into the hypothalamus on the actual site of injection.…”