This may later prove valuable to drug development against disorders like schizophrenia that is caused by excessive dopaminergic activity.
The variations in physiologic actions of dopamine are mediated by at least five distinct G protein coupled receptor types (Missale, C., et al, 1998). Kebabian and Calne (1979) distinguished two dopamine receptor types - and - that can be differentiated 'pharmacologically, biologically, physiologically and by their anatomical distribution' (Civelli, O., 2000). Since the analysis is on receptors only they are being discussed here. Subsequent cloning of receptors revealed that they belonged to the supergene family of the G-protein coupled receptors (Civelli, O., 2000). The three subtypes belonging to the -like sub-family are the, and ones that inhibit adenylyl cyclase and activate channels (Missale, C., et al, 1998). The genetic structure of the and vary by tissue types and species' through alternative splicing. The subtype is highly polymorphic. Since the analysis is on receptor activity on recombinant CHO (Chinese hamster ovary) cells it is necessary to point out that-like receptor activity in the peripheral regions is evident mostly in the kidney, vasculature and pituitary where they affect sodium homeostasis, vascular tone and hormone secretion (Missale, C., et al, 1998). More specifically the analysis of the gene structure of the subtype reveals that there are six introns in the receptor-coding region. This generates two main variants - the (short) and (long) receptors - in turn generated by alternative splicing of a 87-bp exon between introns 4 and 5 (Missale, C., et al, 1998). The two isoforms have shown no difference in functional activities though enough research has been conducted to date to signify variance. Nevertheless, they do vary in efficiencies (Civelli, O., 2000) in mediating intracellular activities (Seeman, P., 2000). An inserted stretch of 29 amino acids in the 3rd long intracellular loop differentiates the two isoforms of the receptor subtype (Missale, C., et al, 1998). Structurally, the -like subtypes have a short COOH terminal tail and a larger 3rd intracellular loop, a feature common with other receptors that interact with proteins to inhibit adenylyl cylase. The cystine residue at the end of the COOH terminus probably binds the receptor polypeptides to cellular membrane through palmitoylation (Civelli, O., 2000).
The cloning strategy adopted to identify receptors was simple. Researchers understood that the protein coding sequence known to exists among G-protein coupled receptors could be used to probe for the receptor