A common feature of abused drugs such as heroin and cocaine is that their use by humans tends to become habit forming. Similar behavior has also been observed in rats and mice, who will self-administer opiates or cocaine repeatedly if given the opportunity. The repeated self- administration of these drugs is thought to be primarily due to their strong positive reinforcing properties. Efforts to identify the neuroanatomical substrates that may be involved in an animal's repeated administration of heroin or cocaine has focused on lesioning of specific brain nuclei, microdialysis and neuropharmacological studies. The current interpretation of this literature is that mesocortical-mesolimbic dopamine neurons are responsible for mediating the positive reinforcing properties of these drugs. Originating in the ventral tegmental area (VTA), these neurons project primarily to the frontal cortex, olfactory tubercle, amygdala, septum and nucleus accumbens. Although each of these brain areas may be important with respect to some aspect of drug action, it is the VTA's dopaminergic input to the nucleus accumbens that has attracted the most attention. This interest is founded in the profound effects that both lesioning and neuropharmacological manipulations of dopamine in the nucleus accumbens which have on the animal's behavioral response to opiates and cocaine. Since dopamine's effects are thought to be mediated by a family of G protein-coupled receptors, collectively referred to as D1-like and D2-like, and the dopamine transporter, there is considerable interest in elucidating the role that each of these proteins may play in drug reward. Unfortunately, nothing is known about the relative contributions that each of the three D2-like receptors, D2, D3 and D4 make to an animal's overall behavior. Therefore, to examine the relative participation of each of the three D2-like receptors in behaviors related to drug abuse we propose to develop an in vivo model system. Through the use of gene targeting by homologous recombination in embryonic stem cells and transgenic mouse technology we are attempting to develop mice deficient in dopamine D2 and D4 receptors. Once these strains have been established they will be crossed in an attempt to generate a third strain expressing only D3 receptors (D2rec-/ D4rec-). The successful targeting of the dopamine D2 and D4 receptor genes in the transgenic mice will initially be evaluated, with respect to wild type mice, four ways. First, in situ hybridization analysis of D2 and D4 receptor mRNA expression will be performed on pre- and postnatal rec- mice. Second, competition binding assays will be performed on brain membranes from the D2 and D4 rec- mice. Next, tissues derived from these animals will be used in second messenger assays. Finally, stereotypic behavior, conditioned place preference and locomotor activity of the rec- animals will be assessed relative to controls following drug treatment and lesioning. The availability of these mice will be a valuable resource for many different in vivo studies of D2-like receptor involvement in drug sensitivity and dependence.