DRUG ADDICTION: DUAL 5-HT INNERVATION OF LIMBIC SYSTEM

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In addition to the role of dopamine (DA) release in reward, recent evidence suggests that 5-HT released in the nucleus accumbens (NAc) may contribute to the addictive effects of stimulant drugs. Little is known about the anatomic organization of 5-HT projections to the NAc and such data are needed to understand the function of this system. In order to elucidate the role of 5-HT neurons in addiction and in drug- induced release of neuro-transmitter, this project will analyze the origin of 5-HT projections to the NAc and to other limbic structures with attention to sets of 5-HT axons that differ in transporter expression. Many psychostimulant drugs act via monoaminergic transporters and are differentially toxic to subsets of 5-HT and DA axons in forebrain. To identify factors that mediate the pharmacologic and toxic effects of stimulant drugs, we will study the vulnerability of 5-HT and DA axons to p- chloroamphetamine (PCA) and to methamphetamine (Meth). We have identified two separate types of 5-HT axons in the NAc, one of which densely innervates the caudal NAc shell, lacks the transporter (SERT) and is highly drug-resistant. A similar dual 5-HT innervation in prefrontal (PFC) and in entorhinal cortex will also be analyzed. (1) This project will investigate (a) the neuronal origins of 5-HT axons that innervate NAc, PFC and entorhinal cortex; (b) the expression of SERT mRNA and protein in the cell bodies of origin; and (c) whether the same 5-HT neurons send drug-resistant axon collaterals to multiple limbic target areas. (2) The NAc shell and entorhinal cortex will be analyzed by EM to establish whether these regions receive a specialized projection of varicose 5-HT axons (lacking SERT) with novel synaptic properties. The selective 5-HT neurotoxin, PCA, will be used to separate the two 5-HT axon types for ultrastructural analysis. This study should contribute to understanding the mechanisms of drug-induced 5-HT release and the role of 5-HT in addiction. We propose that the duality of 5-HT projections plays an important role in addiction, reward and affective state control. Natural, physiologic release of 5-HT resulting from impulse conduction is independent of SERT and should occur in all 5-HT axons, whereas drug-induced release requires the transporter and is more restricted. Therefore, we postulate that physiologic excitation of raphe neurons is likely to produce a different regional pattern of 5-HT release than does drug-induced release, a difference that may underlie the addictive effects of drugs.