PROCEDURAL EXAMINATION OF BEHAVIORAL SENSITIZATION TO MORPHINE - LACKOF BLOCKADE BY MK-801, OCCURRENCE OF SENSITIZED SNIFFING, AND EVIDENCE FOR CROSS-SENSITIZATION BETWEEN MORPHINE AND MK-801

Rats were tested in an open field, a ''sniffing box'' and an eight-arm maze, to examine in detail the behavioural changes induced by morphin e (10 mg/kg, i.p.) and MK-801 (0.1 mg/kg, i.p.), either alone or in co mbination, during a 10 day treatment and subsequent drug challenges. I n addition to locomotion, a number of other behaviours such as sniffin g, rearing and exploration were examined. After morphine challenge, se nsitised locomotion and rearing were found in the open held, and sensi tised sniffing and turning were observed in the sniffing box. In addit ion, in the sniffing box, saline challenge produced significant condit ioned sniffing and turning, and after a challenge with MK-801, sensiti sed sniffing and turning were seen in the group pretreated with morphi ne, suggesting a cross-sensitisation between morphine and MK-801 (but not vice verse). In the eight-arm maze, sensitised locomotion was foun d after morphine challenge. Morphine and MK-801 changed the preference for particular angles run during trials in a characteristic manner. I n none of the behavioural measures was MK-801 able to block the develo pment (and expression) of sensitisation to morphine. In several cases, rather, MK-801 enhanced the acute morphine effects. Sensitisation of sniffing suggests that sensitisation has also developed within the nig rostriatal dopamine system and not only within the mesolimbic dopamine system, as is generally discussed in the context of the most commonly assessed behaviour, locomotion. This finding argues for the additiona l use of the sniffing box in sensitisation experiments.