ABSENCE OF D-2S DOPAMINE-RECEPTOR IN THE PROLACTIN-SECRETING MMQ PITUITARY CLONE - CHARACTERIZATION OF A WILD D-2L RECEPTOR-COUPLED TO NATIVE TRANSDUCTION MECHANISMS

We used the PCR amplification technique in an attempt to characterize further the dopamine D-2L receptor expressed in the prolactin-secretin g pituitary MMQ cell clone, derived from the prolactin- and ACTH-secre ting Buffalo rat 7315a pituitary tumour. By semiquantitative PCR ampli fication we were unable to detect the mRNA encoding the D-2S receptor isoform, which derives from the well-known process of alternative spli cing, producing two D-2 receptor subtypes (D-2L and D-2S) in such tiss ues as the anterior pituitary and the corpus striatum. Although the ph armacology of the D-2 receptor has been established in many studies on both native receptors and transfected receptor isoforms, because of t he lack of tissues naturally expressing only one receptor isoform, MMQ cells represent the first example of cells uniquely or prevalently ex pressing only the D-2L receptor, conceivably coupled to its native tra nsduction mechanisms. These considerations prompted us to evaluate the pharmacology and the second messenger systems known to be modulated b y dopamine. Scatchard analysis of [H-3]spiperone binding resulted in a linear plot, consistent with the existence of a single class of bindi ng sites, with a K-d of 0.055 +/- 0.002 nM and a B-max of 27 +/- 3.5 f mol/mg protein. Competition experiments confirmed the GTP-dependence a nd the order of potency for agonist and antagonist ligands consistent with binding to a D-2 receptor. The inhibitory effects of dopamine on adenylyl cyclase activity, inositol phosphate production and intracell ular free calcium concentrations, the latter presumably via the openin g of K+ channels, and prolactin secretion, as well as the reversal of the effect by the D-2-selective antagonist (-)sulpiride and pretreatme nt with pertussis toxin, are consistent with the known biological acti ons of dopamine at D-2 receptors. Based on our observations, the MMQ c ell line can be considered a useful tool for investigating ligand-rece ptor interactions to develop new selective dopaminergic D-2L ligands f or the therapy of dopamine-related disorders such as schizophrenia, de pression, Parkinson's disease and drug addiction.