Effects of adrenoceptor blocking agents on body temperature

Area of Science:

• Neuroscience
• Pharmacology
• Physiology

Background:

• Adrenoceptors play a role in regulating physiological functions, including body temperature.
• The central nervous system, particularly the hypothalamus, is crucial for thermoregulation.
• Noradrenaline is a key neurotransmitter involved in various central nervous system functions.

Purpose of the Study:

• To investigate the effects of alpha- and beta-adrenoceptor blocking agents on rectal temperature in unanaesthetized rabbits, cats, and rats.
• To explore the role of central noradrenergic pathways in thermoregulation by examining the impact of adrenoceptor blockade on temperature.
• To determine if intraventricular administration of these agents has different effects compared to systemic administration.

Main Methods:

• Chronic intraventricular cannulation was performed in rabbits, cats, and rats.
• Adrenoceptor blocking agents (phenoxybenzamine, phentolamine, ergotamine, propranolol, pronethalol, Trasicor) were administered intraventricularly and intravenously/intraperitoneally.
• Rectal temperature was monitored in unanaesthetized animals maintained at room temperature.
• Noradrenaline stores in the hypothalamus were depleted using reserpine in some rabbits.

Main Results:

• Intraventricular phenoxybenzamine induced significant hypothermia in rabbits and rats, but not intravenously.
• The hypothermic effect of phenoxybenzamine in rabbits was abolished after hypothalamic noradrenaline depletion.
• Intraventricular phenoxybenzamine caused hyperthermia in some cats, while other agents showed varied effects.
• Beta-adrenoceptor blockers had no significant effect on temperature in any species studied.

Conclusions:

• Central alpha-adrenoceptor blockade, particularly with phenoxybenzamine, plays a significant role in inducing hypothermia in rabbits and rats.
• These findings suggest that continuous release of noradrenaline from adrenergic neurons innervating the anterior hypothalamus influences body temperature.
• The differential effects observed in cats may indicate species-specific or dose-dependent mechanisms in central thermoregulation.