EFFECTS OF PEDUNCULOPONTINE TEGMENTAL NUCLEUS LESIONS ON MORPHINE-INDUCED CONDITIONED PLACE PREFERENCE AND ANALGESIA IN THE FORMALIN TEST

The development of a conditioned place preference to morphine (2 mg/kg ; three pairings) and the analgesic effect of morphine (0, 4 or 8 mg/k g) in the formalin test were studied in rats with sham or neurotoxin l esions of the pedunculopontine tegmental nucleus. Lesions were induced by bilateral infusions of N-methyl-D-aspartate (0.5 mul of 0. 1 M sol ution) or vehicle over 10 min. No anti-seizure medication was administ ered in the first experiment, whereas animals in the second experiment were injected with diazepam (I mg/kg) immediately after surgery. In E xperiment 1, behaviour in the conditioned place preference and formali n tests was assessed in separate groups of lesioned and control rats. In Experiment 2, the same animals received both sets of tests. In both experiments lesions of the pedunculopontine tegmental nucleus blocked the development of a conditioned place preference to morphine, but ha d no effect on the behavioural response to formalin, or on its inhibit ion by morphine. Examination of cholinergic-stained cells found no cor relation between the magnitude of behavioural effects and the number o f acetylcholine cells destroyed by the lesions. These results confirm that the pedunculopontine tegmental nucleus mediates the development o f a morphine-induced conditioned place preference, but not the analges ic effect of morphine.