The purpose of this project is to improve our knowledge of the mechanisms of action of neuropsychoactive drugs, particularly those used to treat anxiety, sleep disturbances and depression. Most of these drugs act in the brain by potentiating the function of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by binding on the same receptor complex (GABA-A) or by inhibiting reuptake into the presynaptic nerve terminal of another chemical messenger between neurons, serotonin (5-HT). Many drugs of the first group produce tolerance and dependence by mechanisms, which are not completely understood. Since GABA-A receptors in the brain consist of various polypeptide subunits and therefore represent a very heterogeneous population of receptors, we plan to perform our studies on the recombinant receptors expressed in human embryonic kidney (HEK 293) cells. The advantages of cultured cells include a relatively homogenous cell population that can be well characterized with respect to receptor subunit composition, structure and function. However, reports related to prolonged exposure of these cells to drugs are few. The cells will be chronically exposed to drugs, mostly ligands of the benzodiazepine binding sites, and the changes in the receptor number and affinity, in mRNA encoding receptor subunits, in subunit protein levels and in the function of receptors will be followed. The results will be compared with those obtained on primary cultures of cerebellar neurons. Comorbidity of epilepsy and depression is not rare. Stress can affect both depression and seizures. Therefore, it is important to know whether an antidepressant drug has pro- or anticonvulsant properties and whether these as well as antidepressant properties will be modified by stress. To find out whether stress modifies the action of antidepressant drugs, often claimed to have proconvulsant action, and the effects of benzodiazepines, known to be anticonvulsants, the ability of these drugs to affect the susceptibility to convulsions will be tested on experimental animals (mice, rats) under control and stressful conditions. The effect of stress on 5-HT-mediated behaviour, and the role of 5-HT receptors in the control of seizures will also be evaluated. The project is important for the elucidation of the mechanism of action of neuropsychoactive drugs, particularly in relation to their unwanted effects, and therefore has a clinical relevance.