Quantification of D1B (D5) receptors in dopamine D1A receptor-deficient mice

The unavailability of selective D-1A (D-1) or D-1B (D-5) dopamine receptor ligands has prevented the direct localization of binding sites for these receptors. Thus, receptor autoradiography with long exposure times was used to detect minor D-1-like binding sites in the brains of D-1A null mutants. Coronal brain sections were prepared from the caudal portion of the prefrontal cortex of homozygous or heterozygous D-1A knockout mice or wildtype mice, and labeled with the D-1 receptor antagonist [H-3]- SCH23390. Slides were dried, and apposed to film with polymer-calibrated standards for 90 days to allow visualization of any low abundance binding sites. No binding was detected in most regions of homozygote (-/-) mouse brains that have high densities of D-1 binding in wildtype mice (e.g., the striatum, nucleus accumbens, olfactory tubercles or amygdala). Conversely, small, but detectable amounts of D-1-binding were measured in the hippocampus, albeit with a density less than the lowest standard (ca. 20 fmol/mg). Saturation binding of [H-3]-SCH23390 in hippocampal homogenates from homozygous mice confirmed a B-max of 12.3 fmol/mg protein with a K-D of 0.57 nM. The current work demonstrates directly the presence of D-1B (D-5) receptors in hippocampus, and also shows that; the loss of functional D-1A gene products almost completely eliminates detectable D-1-binding sites in striatum, as well as in some regions (e.g., the amygdala) where a non-adenylyl cyclase coupled D-1 receptor has been reported. This indicates that these non-adenylyl cyclase coupled D-1-like receptors represent alternate signaling pathways rather than novel gene products(s). Synapse 39:319-322, 2001. (C) 2001 Wiley-Liss, Inc.