PAVLOVIAN CONDITIONING OF MORPHINE HYPERTHERMIA - ASSESSMENT OF INTERSTIMULUS-INTERVAL AND CS-US OVERLAP

The present study examined the effect of interstimulus interval on acq uisition of conditioned ther mal responses produced by trials in which a light/noise stimulus (CS) was repeatedly paired with infusion of mo rphine sulphate (US), Rats were implanted with a chronic intravenous c atheter for drug delivery and a biotelemetry device for remote monitor ing of core body temperature. In experiment 1, different groups receiv ed morphine either 0.5 (group P0.5) or 15 min (group P15) after onset of the 15-min CS. A third group was exposed to an identical number of CS and US presentations but in an explicitly unpaired manner (group UP ). After repeated exposure to morphine, all groups showed a more rapid rise in body temperature in response to drug infusion. Test presentat ions of CS alone revealed conditioned hyperthermic responses to CS in groups P0.5 and P15. However, the response of the P15 group was smalle r than that of the P0.5 group, suggesting weaker conditioning at the l onger interstimulus interval. The contribution of CS-US overlap to the diminished associative strength observed in the P15 group was assesse d in experiment 2. Groups P0.5/15 and P0.5/30 received infusions of mo rphine 0.5 min after onset of a 15- or 30-min CS, respectively. Group P15/30 received morphine 15 min after onset of a 30 min CS, whereas gr oup UP/30 received explicitly unpaired presentations of the US and a 3 0-min CS. Enhancement of the hyperthermic effect of morphine was obser ved in all groups after ten conditioning trials. Test presentations of the CS without drug revealed that all paired groups had acquired cond itioned hyperthermic responses. These results support the conclusion t hat drug-induced conditioning can occur at relatively long interstimul us intervals when there is sufficient temporal overlap between the CS and unconditioned response evoked by the drug US.