Intraventricular self-administration of morphine in naive laboratory rats

Male Wistar rats implanted with cannulae aimed at the left lateral cerebral ventricle were individually maintained in Skinner boxes for 11 consecutive days. Animals were neither predependent on morphine nor shaped to press the operant lever. Experimental animals (n = 7) obtained intraventricular infusions of a 1% morphine HCl solution (2 mul per 5-s infusion) for each lever press while control animals (n = 7) received only the vehicle. Four animals were yoked to experimental animals and received equivalent but non-contingent morphine HCl infusions. The mean number of lever presses per day for the experimental group was significantly higher than for the vehicle control or yoked control groups suggesting that naive rats will work for the positive reinforcing properties of morphine when it is infused centrally.