Opposite effects of ventral tegmental and periaqueductal gray morphine injections on lateral hypothalamic...

Eating was induced in sated animals by lateral hypothalamic electrical stimulation following central microinjections of morphine or saline. With stimulation intensity fixed at a moderate level, time to eat 3 food pellets of 45 mg decreased with increases in stimulation frequency, approaching an asymptote near 7 s at approximately 70 Hz. Ventral tegmental area morphine injections (0.8-8 nmol) reduced the minimum frequency required to produce eating of 3 pellets within 20 s and reduced the frequency at which asymptotic performance was produced; it did not substantially change the speed of eating at which performance approached asymptote. Periaqueductal gray morphine injections (1.6-16 nmol) had the opposite effect; they increased the stimulation frequency required to meet either the 20 s (maximum) or the 7 s (asymptotic) response criterion. Since the ventral tegmental injections did not alter the absolute level of asymptotic performance, the changes in eating threshold associated with these injections are assumed to reflect motivational influences of morphine.