Differential effects of clonidine on pain, arterial blood pressure, and heart rate in the cat: Lack of interactions with naloxone

In cats anesthetized with α-chloralose and urethane, intravertebral administration of clonidine (4 and 10 μg/kg) dose-dependently suppressed the jaw-opening reflex, arterial blood pressure, and heart rate. For a given dose, there was a differential degree of inhibition in the order of analgesia ≫ hypotension > bradycardia. Naloxone injections (0.4 and 1.0 mg/kg, i.vert.) essentially failed to antagonize these effects, suggesting the lack of involvement of the opiate receptors or endogenous opioids in these processes. Furthermore, pain suppression by clonidine appeared to be independent of the vasodepression and cardioinhibition it promoted. It is possible that neural mechanisms responsible for clonidine-induced antinociception, hypotension, and bradycardia are likely to have differential sensitivities to the imidazoline compound, regardless of whether they exist in separate central sites or in subpopulations of neurons within common neural substrates.