Enkephalin release into the ventral tegmental area in response to stress: modulation of mesocorticolimbic dopamine.

Enkephalin-containing neuronal fibers and perikarya, and opioid receptors are present in the A10 dopamine (DA) region, and many studies have implicated enkephalin as a neuromodulator of A10 DA neurons projecting to the prefrontal cortex and certain limbic nuclei. Footshock stress is known to activate the A10 DA neurons projecting to the prefrontal cortex and nucleus accumbens, and the present study was designed to evaluate the possibility that footshock-induced release of enkephalin into the A10 DA region may play a role in activating the DA neurons. Microinjection of the quaternary opioid antagonist, naltrexone methobromide (NMB), into the ventral tegmental area (VTA; subnucleus of the A10 DA region) significantly attenuated the increase in DA metabolism produced by exposure to footshock (0.2 mA; 200 ms on; 800 ms off for 20 min) in the prefrontal cortex and nucleus accumbens. Rats were exposed to footshock for 5, 10 or 20 min and a time-dependent decrease in the level of immunoreactive Met-enkephalin was measured in the midline A10 region, but not in the lateral A10 region. It has been shown that daily exposure to footshock enhances the motor stimulant effect of intra-VTA injection of the enkephalin analogue, [D-Ala2,Met]-enkephalinamide (DALA). Rats were pretreated with an intra-VTA injection of NMB prior to daily exposure to footshock, and it was found that NMB abolished the potentiating effect of daily footshock on subsequent intra-VTA injection of DALA. Taken together, these data indicate that footshock stress enhances the release of enkephalin into the A10 region, and that this enkephalin activates A10 DA neurons projecting to the prefrontal cortex and nucleus accumbens.