Snout contact fixation, climbing and gnawing during apomorphine stereotypy in rats from two substrains

Apomorphine, at doses greater than or equal to 10 mg/kg (intraperitoneally), produced two patterns of stereotypy. In rats from one supplier it induced predominantly gnawing while in those from another predominantly climbing, suggesting that the response to the drug is influenced by genetic and/or experimental factors. At lower doses, apomorphine induced climbing in both groups (ED50 = 1.4 mg/kg in each group) but oral behavior in only one of them (ED50 = 1.3 mg/kg in one, and 8 mg/kg in the second group). Thus, at a given dose of apomorphine, different patterns of stereotypy may result from an interaction between two phenomena: the relative setting of the thresholds to mouth and to climb, and an inverse relation between oral activity and climbing. Analysis of climbing suggests that this response is comprised of two (previously unidentified) fundamental effects of apomorphine: snout contact fixation and bodywise forward progression.