Cholinergic influences on wall climbing and its ontogenetic decline.

Wall climbing is an age-specific behavior that is robustly elicited in rat pups during the 2nd postnatal week, but not thereafter, by a variety of stimuli including footshock and treatment with catecholamine agonists such as clonidine. This series of experiments examined the influence of the cholinergic antagonist scopolamine on footshock- and clonidine-induced wall climbing and its ontogenetic decline in infant to adult Sprague-Dawley rats. Scopolamine was observed to partially reinstate clonidine- and footshock-precipitated wall climbing following the ontogenetic decline in this behavior pattern after the 2nd postnatal week, effects that did not appear to be related to drug-induced alterations in general activity levels or to alterations in body temperature. In contrast, wall climbing induced by both stimuli during the 2nd postnatal week was conversely reduced by scopolamine, data consistent with a number of previous reports that anticholinergic agents may produce "paradoxical" responses early in development opposite to those observed later in ontogeny. These results provide evidence that the dramatic ontogenetic decline in wall climbing may be related in part to the maturation of cholinergic subsystems with an inhibitory influence on this behavior pattern. However, the only partial reinstatement of wall climbing by cholinergic blockade suggests that the ontogenetic decline of this behavior pattern may also be related to the development of other inhibitory systems or to the emergence of competing responses elicited by formerly effective wall climbing stimuli as the organism matures.