Drug discrimination following morphine injection into rat para ventricular nucleus (PVN)

Five male Sprague-Dawley rats were trained to press both levers in a two-lever opérant apparatus. Following lever press training each subject was surgically implanted with a permanently indwelling cannula aimed at the PVN. Stereotaxic co-ordinates with the incisor bar set at 3.0 mm below the interaural line were 0.75 mm lateral and 1.8 mm posterior to the bregma, to 8.0 mm below the surface of the skull. Subjects were allowed to recover for seven days following surgery. During discrimination training subjects were injected with either 0.3 ng/0.5 p.1 morphine or 0.5 |il saline thirty minutes prior to each experimental session. Sessions were conducted daily and morphine or saline administration alternated randomly, with the restriction that no more than two consecutive sessions of morphine or saline occurred. Responses on the right lever were reinforced1 following morphine injection and responses on the left lever were reinforced following saline injection. The number of responses required for each reinforcer or time-out was increased to 20-responses for all animals. The criterion for aquisition of discrimination was selected as X5% of responses prior to the first consequence occuring at the appropriate lever in 8 out of 10 consecutive sessions. The mean percentage of responses on the morphine lever was 85% (SE=6.7) following morphine injection and 15% (SE=6.3) following saline injection (mean=37.4 sessions), indicating aquisition of discrimination.