[Alteration of the hypothalamic electrical stimulation disconnection reaction in rats during chronic morphine...

Rats with electrodes implanted in the hypothalamus were trained to switch off the central stimulation. Animals were administered 12--30 injections of morphine in increasing doses (from 20 up to 120--180 mg/kg/injection). The drug exerted a dose-proportional suppressive effect on the escape response. During repeated injections an absolute or relative tolerance to the suppressive action developed in the responses both to central and to peripheral painful stimulation. The suppressive action seems to be specific and does not correlate with the catatonic reaction and changes in general motor activity. The activation of gnawing response was noted after the morphine injections in doses of 60 mg/kg and higher. In one of the animals the transformation of the negative effect into ambivalent one as well as self-stimulation were observed. "Antiaversive" effect is considered to be one of the determinants of morphine abuse and addiction liability.