Pre-weaning non-handling of rats disrupts latent inhibition in males, and results in persisting sex- and area-dependent increases in dopamine and serotonin turnover

Pre-weaning "handling" of rat pups (1-21 days) is reported to result in a number of behavioural differences from "nonhandled" pups, persisting into adult life. In general, these are associated with altered emotional reactivity. We now report the results of a replication of a previous finding that handling also affects performance on a latent inhibition (LI) task, which involves learning in the absence of motivation. The effect of 30 pre-exposures to a tone stimulus on the formation of an association from two pairings between that stimulus and footshock was determined. The association was indexed by the suppression of licking resulting from tone presentation during licking for water reward. In adult female rats, pre-exposure prevented the formation of this association (i.e. LI was present) whether they had been handled or nonhandled pre-weaning. However in adult males, pre-exposure was effective only in handled rats, and not in nonhandled. This confirms the striking pattern of results reported previously by another group (Weiner et al., 1987). The turnover of dopamine (DA) and serotonin was subsequently determined post mortem from the ratios of metabolites to amines in DA-innervated brain areas of the rats used in the present study. Pre-weaning nonhandling and female sex, were independently associated with increased dopamine turnover, and to a lesser extent with increased serotonin turnover. While these increases were not sex-specific, nonhandled males did show a pattern of increased DA turnover relative to serotonin turnover in limbic areas; previous pharmacological and physiological studies support the idea that this pattern may be associated with impairment of LI. It is concluded that preweaning nonhandling is indeed associated with an impairment of LI restricted to males, and is associated with enduring changes in DA and 5HT turnover in both sexes. Further studies are indicated to determine the precise role of these changes in the behavioural effects of pre-weaning nonhandling.