Facilitation of self-stimulation behavior following intracerebral microinjections of opioids into the ventral tegmental area.

The intracerebral microinjection technique was used to localize sites in the brain where morphine facilitated the self-stimulation rate at hypothalamic electrode sites. Bilateral injections of morphine (2 x 1 microgram) into the ventral tegmental area and substantia nigra produced the strongest enhancement at the shortest latencies. At these sites, bilateral injections of 200 ng of morphine also produced a significant enhancement whereas a dose of 50 ng was below threshold for the rate increasing effect. The enhancement by morphine was effectively antagonized by naloxone (5 mg/kg). When injected bilaterally into the same area, D-Ala2-Met5-enkephalinamide (2 x 1 microgram) also induced a strong enhancement of self-stimulation lasting for 70 minutes. A possible dopaminergic substrate for the opiate induced behavioral stimulation is discussed.