Genetic knockout and pharmacological blockade studies of the 5-HT7 receptor suggest therapeutic potential in depression

The affinity of several antidepressant and antipsychotic drugs for the 5-HT7 receptor and its CNS distribution suggest potential in the treatment of psychiatric diseases. However, there is little direct evidence of receptor function in vivo to support this. We therefore evaluated 5-HT7 receptors as a potential drug target by generating and assessing a 5-HT7 receptor knockout mouse. No difference in assays sensitive to potential psychotic or anxiety states was observed between the 5-HT7 receptor knockout mice and wild type controls. However, in the Porsolt swim test, 5-HT7 receptor knockout mice showed a significant decrease in immobility compared to controls, a phenotype similar to antidepressant treated mice. Intriguingly, treatment of wild types with SB-258719, a selective 5-HT7 receptor antagonist, did not produce a significant decrease in immobility unless animals were tested in the dark (or active) cycle, rather than the light, adding to the body of evidence suggesting a circadian influence on receptor function. Extracellular recordings from hypothalamic slices showed that circadian rhythm phase shifts to 8-OH-DPAT are attenuated in the 5-HT7 receptor KO mice also indicating a role for the receptor in the regulation of circadian rhythms. These pharmacological and genetic knockout studies provide the first direct evidence that 5-HT7 receptor antagonists should be investigated for efficacy in the treatment of depression.

Adaptation of gamma-aminobutyric acid type A receptors to alcohol exposure: studies with stably transfected cells

We studied the adaptation of gamma-aminobutyric acid type A (GABAA) receptor function to chronic ethanol exposure in cells stably transfected with the following GABAA receptor subunits: alpha-1 beta-2 gamma-2L, alpha-1 beta-2 gamma-2S, alpha-1 beta-3 gamma-2S, alpha-1 beta-1, alpha-5 beta-3 gamma-3 and alpha-6 beta-3 gamma-2S. Chronic exposure to ethanol resulted in a decrease in muscimol-stimulated 36Cl- flux and a decrease in modulation of that flux by ethanol, flunitrazepam, methyl-6,7-4-dimethoxy-4-ethyl-beta-carboline-3-carboxylate and pregnanolone without any change in the modulation by pentobarbital or zinc. Direct activation of the GABAA receptor by pentobarbital was enhanced by chronic ethanol treatment. Reduction of the action of muscimol, ethanol and flunitrazepam differed in the duration and amount of ethanol required to see an effect. Reduction of the action of ethanol of alpha-1 beta-2 gamma-2L cells occurred within 15 min and was near-maximal for 25 mM ethanol, whereas reduction of the actions of muscimol and flunitrazepam actions required hours of exposure and higher concentrations of ethanol. Chronic ethanol exposure produced a reduction in the Emax value for the action of muscimol for all six subunit combinations, but quantification of surface receptors by immunolabeling showed no change in GABAA receptor density. The differences in alcohol sensitivity and time courses for different effects of ethanol indicate multiple mechanisms of adaptation of GABAA receptors. Use of stably transfected cells rules out "subunit substitution" as a mechanism for these changes and points to post-translational changes (e.g., phosphorylation, receptor assembly) as the most likely mechanisms. These in vitro findings are compared with results from in vivo studies.

Ethanol, flunitrazepam, and pentobarbital modulation of GABAA receptors expressed in mammalian cells and Xenopus oocytes

GABAA receptors composed of human alpha 1 beta 2 gamma 2L, alpha 1 beta 2 gamma 2S, alpha 1 beta 3 gamma 2S, alpha 6 beta 3 gamma 2S, and alpha 5 beta 3 gamma 3 subunits as well as bovine alpha 1 beta 1 gamma 2L and alpha 1 beta 1 subunits were stably expressed in mammalian L(tk-) cells and transiently expressed in Xenopus oocytes. Effects of muscimol, ethanol, flunitrazepam, and pentobarbital on receptor function were compared for the two expression systems using a 36Cl- flux assay for cells and an electrophysiological assay for oocytes. Muscimol activated all receptors in both expression systems but was more potent for L(tk-) cells than oocytes; this difference ranged from 2.6-to 26-fold, depending upon subunit composition. The most pronounced differences between receptors and expression systems were found for ethanol. In L(tk-) cells, low (5-50 mM) concentrations of ethanol potentiated muscimol responses only with receptors containing the gamma 2L subunit. In oocytes, concentrations of 30-100 mM produced small enhancements for most subunit combinations. Flunitrazepam enhanced muscimol responses for all receptors except alpha 6 beta 3 gamma 2S and alpha 1 beta 1, and this enhancement was similar for both expression systems. Pentobarbital also enhanced muscimol responses for all receptors, and this enhancement was similar for L(tk-) cells and oocytes, except for alpha 6 beta 3 gamma 2S where the pentobarbital enhancement was much greater in oocytes than cells. The alpha 6 beta 3 gamma 2S receptors were also distinct in that pentobarbital produced direct activation of chloride channels in both expression systems. Thus, the type of expression/assay system markedly affects the actions of ethanol on GABAA receptors and also influences the actions of muscimol and pentobarbital on this receptor. Differences between these expression systems may reflect posttranslational modifications of receptor subunits.

Radiation-reduced hybrids for the myotonic dystrophy locus

The myotonic dystrophy (DM) gene maps to the long arm of human chromosome 19 and is flanked by markers ERCC1 and D19S51. Also mapping to this region is the polio virus receptor gene (PVS). To produce more markers for this interval, we have constructed radiation-reduced hybrids by selecting for the retention of ERCC1 and for the loss of PVS. One of the cell lines produced has been characterized extensively and contains about 2 Mb of human DNA derived exclusively from chromosome 19, and includes ERCC1 and D19S51. Phage libraries constructed from DNA of this cell line have been screened and several new markers identified, including two for which cDNAs have been isolated. These represent candidate genes for DM. The new markers have also been used to extend the long-range restriction map of this region.