Role for dopamine neurons of the rostral linear nucleus and periaqueductal gray in the rewarding and sensitizing properties of heroin

There is a mesencephalic dopaminergic network outside the ventral tegmental area (VTA), including structures such as the rostral linear nucleus (RLi) and periaqueductal gray (PAG). These nuclei project to neural areas implicated in reinforcing effects of drugs, indicating that they could participate in opiate reward. The objectives were to study the morphological characteristics of the dopamine network of the RLi/PAG region, and to discern its role on rewarding and sensitizing effects of heroin in rats, following dopamine depletion or local injection of dopaminergic antagonists. The findings indicated that this network is composed of small cells in the RLi/ventral PAG, large multipolar dopamine PAG neurons, and periaqueductal PAG neurons. Following repeated heroin, large PAG neurons and small RLi/ventral PAG cells (not periaqueductal neurons) were activated, since tyrosine-hydroxylase was adaptively induced, without changes in protein kinase A alpha. After dopamine depletion, small RLi/ventral PAG neurons and large cells of the PAG (not periaqueductal ones) were selectively affected by the neurotoxin. Dopamine neurons of the nearby VTA and dorsal raphe were not affected, as revealed by cell counting. After lesion, 'anxiety-like' responses and basal locomotion were not altered. However, conditioned place preference to heroin was found to be abolished, as well as heroin-induced motor sensitization. Following infusions of dopaminergic antagonists into RLi/PAG, D-2 (not D-1) receptor blocking dose-dependently abolished heroin-induced reward. The present study provides evidence that dopamine neurons of the RLi/PAG region (excluding PAG periaqueductal cells) show adaptive biochemical changes after heroin, and mediate the rewarding and sensitizing effects of this drug. D-2 dopamine receptors within the RLi/PAG region participate in these effects.