CELL-TYPE-SPECIFIC EXPRESSION OF CATECHOLAMINE TRANSPORTERS IN THE RAT-BRAIN

The dopamine transporter (DAT) and norepinephrine transporter (NET) te rminate catecholaminergic neurotransmission at synapses by high-affini ty sodium-dependent reuptake take into presynaptic terminals, and are the initial sites of action for drugs of abuse and antidepressants. In the present study, we used in situ hybridization combined with immuno histochemistry to study the distribution of DAT and NET mRNA in the ad ult rat brain. Cells were first immunolabeled with antisera directed a gainst one of the catecholamine-synthetic enzymes, tyrosine hydroxylas e (TH), dopamine-beta-hydroxylase (DBH), or phenylethanolamine-N-methy ltransferase (PNMT), in order to identify dopaminergic, noradrenergic, or epinephrine-containing cells. The immunolabeled cells were subsequ ently assayed for their ability to express catecholamine transporter m RNAs by in situ hybridization using either a rat DAT or NET cRNA probe . All dopaminergic cell groups of the mesencephalon contained high lev els of DAT mRNA but only the A12 and A13 dopaminergic cell groups of t he diencephalon appear to express detectable levels of DAT. All norepi nephrine-containing cell bodies in the brainstem (locus coeruleus and lateral tegmentum) appear to express NET mRNA. In contrast, epinephrin e-containing cell bodies of the brainstem do not appear to express NET mRNA, which raises the possibility that epinephrine may utilize a tra nsporter that is distinct from the other bioactive amines, or may act as an endocrine regulator that does not require rapid reuptake mechani sms. Moreover, the cell-type-specific expression of catecholamine tran sporters suggests that DAT and NET gene expression may be closely link ed to cellular mechanisms that specify transmitter phenotype. The term ination of neurotransmission is a critical component of neural signali ng and depends on the rapid removal of neurotransmitters from the syna ptic cleft. Pharmacological evidence indicates that the action of mono amines at the synapse is terminated predominantly by rapid reuptake in to presynaptic nerve endings via neurotransmitter-specific, high-affin ity, Na+-dependent membrane transporter proteins. The cDNAs encoding d istinct transporter proteins for the monoamines dopamine, norepinephri ne, and serotonin have been cloned, expressed, and characterized in a variety of heterologous systems (Blakely et al., 1991; Giros et al., 1 991; Hoffman et al., 1991; Kilty et al., 1991; Pacholczyk et al., 1991 ; Shimada et al., 1991; Usdin et al., 1991). Although the monoamine tr ansporters share a high degree of sequence homology, they are distingu ished by their monoamine substrate specificities and by their differen tial sensitivities to a wide spectrum of transport antagonists. For ex ample, pharmacological agents that potently inhibit norepinephrine and serotonin transport, such as desmethylimipramine and citalopram, have little effect on the activity of the dopamine transporter (Javitch et al., 1983). Correspondingly, certain inhibitors of dopamine uptake, s uch as the GBR family of compounds and mazindol, pharmacologically dis tinguish the dopamine transporter from other monoamine transporters (J anowsky et al., 1986; Anderson, 1987, 1989). Thus, in addition to play ing an essential role in synaptic transmission, neurotransmitter trans porters are the sites of action for a wide range of drugs with demonst rated therapeutic potential.