Nociceptive reactivity and precipitated abstinence in hypophysectomized rats.

Nociceptive reactions were determined using the hot plate technique (55 degrees C) in sham-operated and hypophysectomized rats. No significant differences were noticed in the latencies to lick, jump, or escape between the two groups. These experiments indicate that pituitary endorphins are not the sole endogenous substances involved in the regulation of thermonociception. In hypophysectomized rats rendered acutely dependent on morphine, following naloxone, some signs of precipitated abstinence (chewing and teeth chattering) were significantly diminished, some others (abnormal posture, body tremor and pilo-erection) were significantly enhanced, and many others (urination, paw shakes, body shakes, diarrhoea, jump, rearing, eye blinking, head shakes and grooming) were not modified. These observations indicate that the pituitary has a complex role in the expression of signs of abstinence. However, concomitant lesions of the adjacent areas such as the hypothalamus might have also contributed to the observed modifications of the abstinence signs. Hence, it may be advisable always to interpret the results obtained with hypophysectomized rats with some caution.