Inhibition of the spontaneous climbing behavior elicited in mice by opiates

The spontaneous climbing behavior (SCB) developed by naive mice when they are introduced into small individual cylindrical cages with walls of vertical bars is inhibited in a dose-dependent manner by all the tested opiates. Only nalorphine administered over a large scale of doses never completely inhibits the SCB; its maximal effect is only about 50% of total inhibition. The efficacy of opiates on this special component of the locomotor activity has been compared with that on the horizontal displacements measured with an actometer equipped with photoelectric cells. Whereas the drugs known as specific kappa agonists depress both the horizontal locomotor activity and the SCB, most of the other tested opiates inhibit the SCB at doses which do not modify the horizontal locomotion. The low efficacy of loperamide, an opiate agent crossing the blood-brain barrier with difficulty, as well as the strong efficacy of a morphine low dose injected i.c.v. suggest that the opiate-induced SCB inhibition is centrally mediated. Finally, the involvement of mu and kappa receptors is discussed from the observed interactions with various opiate agonists, partial agonist (nalorphine) and antagonists (naloxone and diprenorphine).