Modification of dopamine D(1) receptor knockout phenotype in mice lacking both dopamine D(1) and D(3) receptors.

Experimental evidence suggests that dopamine D(1) and D(3) receptors may interact in an opposing or synergistic fashion. To investigate interactions between both receptors in behaviour, we have used dopamine D(1) and D(3) receptor knockout mice to generate mice lacking both receptors. D(1)(-/-)D(3)(-/-) mice were viable, fertile and showed no gross morphological abnormalities. In an open field, they exhibited lower activity than wild-type, D(1)(-/-) and D(3)(-/-) mice. D(1)(-/-)D(3)(-/-) mice performed equally poorly in the rotarod and Morris water maze tasks as their D(1)(-/-) littermates. Basal locomotor activity and anxiety-like behaviour were normal in D(1)(-/-)D(3)(-/-) mice. Combined deletion of both receptors abolished the exploratory hyperactivity and anxiolytic-like behaviour of dopamine D(3) receptor mutant phenotype and further attenuated the low exploratory phenotype of D(1)(-/-) mice. These results imply an interaction of both receptors in the expression of exploratory behaviour in a novel environment, and the need for the presence of intact dopamine D(1) receptor for the expression of certain behaviours manifested in dopamine D(3) receptor mutant phenotype. In addition, dopamine D(1) receptor, but not dopamine D(3) receptor, is involved in the ability to perform on the rotarod and spatial learning.

Physicochemical differences in the AH receptors of the most TCDD-susceptible and the most TCDD-resistant rat strains.

Long-Evans rats (strain Turku AB; L-E) are at least 1000-fold more sensitive (LD50 about 10 microg/kg) to the acute lethal effects of 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) than are Han/Wistar (Kuopio; H/W) rats (LD50 > 9600 microg/kg). The AH receptor (AHR) is believed to mediate the toxic effects of TCDD and related halogenated aromatic hydrocarbons. We compared the AHRs of L-E and H/W rats to determine if there were any structural or functional receptor differences that might be related to the dramatic difference in the sensitivity of these two strains to the lethal effects of TCDD. Cytosols from liver and lung of the sensitive L-E rats contained about twofold higher levels of specific binding sites for [3H]TCDD than occurred in H/W rats; the Kd for binding of [3H]TCDD to AHR in hepatic cytosols was similar between the two strains. Addition of the oxyanions, molybdate or tungstate (20 mM), had little effect upon ligand binding to AHR in hepatic cytosols from L-E rats whereas in cytosols from H/W rats these agents substantially diminished or totally abolished TCDD binding. The AHR in H/W cytosols also lost ligand-binding function when NaCl (20 to 400 mM) was added to the buffer whereas, in cytosols from L-E rats, the addition of 400 mM NaCl caused the receptor complex to shift from 9S to 6S during velocity sedimentation but did not destroy ligand binding function. AHR from hepatic cytosol of both the L-E and H/W rats could be transformed to the DNA-binding state in the presence of TCDD or other dioxin congeners as assessed by gel mobility shift assays. The most dramatic difference in AHR properties between L-E and H/W rats is molecular mass. Immunoblotting of cytosolic proteins revealed that the AHR in L-E rats has an apparent mass of approximately 106 kDa, similar to the mass of the receptor previously reported in several other common laboratory rat strains. In contrast, the mass of the AHR in H/W rats is approximately 98 kDa, significantly smaller than the mass of receptor reported in any other rat strains. F1 offspring of a cross between L-E and H/W rats expressed both the 106- and the 98-kDa protein. There was no apparent difference in the mass of the AHR nuclear translocator protein (ARNT) between the two strains, but the hepatic concentration of ARNT was about three times as high in L-E as in H/W rats. It will be interesting to find out how the altered structure of the AHR in H/W rats is related to their remarkable resistance to the lethal effects of TCDD.