Differential effects of opioid peptides administered intracerebrally in loci of self-stimulation reward of lateral hypothalamus and ventral tegmental area--substantia nigra.

Leu-enkephalin, leu-enkephalinamide, ala-leu-enkephalin, met-enkephalin and dynorphin-A[1-13] were administered in microinjection into one of the self-stimulation sites of SN-VTA or MFB-LH and the electrical self-stimulation (SS) of the injected site and of the second site was recorded. The study revealed that the leu-enkephalin and the leu-enkephalinamide inhibited the SS of SN-VTA and produced no effect on the SS of MFB-LH, when administered into these sites. The MFB-LH injection, however, facilitated the SS of SN-VTA. The effect of ala-leu-enkephalin injection in MFB-LH was similar to the above, but the effect of the injection in SN-VTA was different in that it caused the facilitation of its SS and not the depression as seen with leu-enkephalin. Met-enkephalin injections in the two regions caused no direct or indirect changes of the SS of the regions. Dynorphin injection in SN-VTA facilitated its SS, like the injection of ala-leu-enk, but dynorphin injections in MFB-LH produced no effects. The results essentially demonstrate the differences in the effects of the different opioids in the reward system of the SN-VTA, and it is discussed that these differences are probably due to the preferences in the types of the receptors upon which these opioids act in the SN-VTA neuronal organisation. The results also demonstrate the major difference in the organisation of the reward substrate of the MFB-LH from that of the SN-VTA, as the effects of the opioids in the MFB-LH are markedly different or none compared to the effects in the SN-VTA.