Selective effects of MAO inhibition on peripheral benzodiazepine receptor binding in the mouse.

Monoamine Oxidase (MAO) and the peripheral benzodiazepine binding site (PBR) share a close physical proximity to each other in the outer mitochondrial membrane. Furthermore, MAO activity and the density of PBR sites are affected by stress; benzodiazepines may influence stress-induced changes in MAO activity. In view of the close physical association between MAO and the PBR, we examined the effects of chronic administration of selective and nonselective MAO inhibitors to mice on the specific binding of 3H-Ro5-4864 and 3H-PK-11195 to crude membranes prepared from kidney, heart and liver. Chronic MAO inhibition was associated with alterations in PBR binding in all three tissues; however, in heart and liver changes were not detectable with 3H-PK-11195. Perhaps, the ability to discern changes with 3H-Ro5-4864 that are not detectable with 3H-PK-11195 reflects a functional change in the "activity" of the PBR site in heart and liver that is elicited by chronic MAO inhibition and mediated by a change in the "conformation" of the protein that is detected with 3H-Ro5-4864. Importantly, iproniazid, the nonselective MAO inhibitor, caused changes in PBR binding in all three of the tissues.

MK-801 alters the GABAA receptor complex and potentiates flurazepam's antiseizure efficacy.

MK-801 is an uncompetitive allosteric antagonist that interferes with glutamate-gated calcium ion conductance through the NMDA receptor-associated ionophore. In an outbred strain of mouse, MK-801 elicits episodes of explosive "popping" behaviors that may serve as a preclinical screening paradigm for novel antipsychotic medications. This investigation examined the effects of MK-801, at doses associated with the elicitation of popping, on the GABAA receptor complex in cerebral cortex, and flurazepam's ability to antagonize electrically precipitated seizures. Twenty four hours after MK-801 administration, there was an increased density of the radiolabeled antagonist-preferring conformation of the central benzodiazepine binding site and a potentiation of flurazepam's antiseizure efficacy. The data show that interference with NMDA receptor-mediated calcium ion conductance is associated with a relatively selective change in the GABAA receptor complex in cerebral cortex, and has functional behavioral consequences. Moreover, the data provide additional evidence for a delicate balance between GABAergic and glutamatergic transmission. Disturbance of this balance can have behavioral consequences for the animal.

Swim stress selectively alters the specific binding of a benzodiazepine antagonist in mice.

The ability of flurazepam to antagonize the electrical precipitation of tonic hindlimb extension is reduced 24 h after mice are forced to swim for 10 min in cold water (6 degrees C). Presumably, this reduction in flurazepam's antiseizure efficacy reflects an environmental stress-induced modification of the GABAA receptor complex. The current study employed a variety of complementary in vitro approaches to characterize the delayed effects of cold-water swim stress on binding parameters of the GABAA receptor complex that may be associated with flurazepam's reduced antiseizure efficacy. The specific binding of [3H]flunitrazepam and the potentiation of this binding by chloride ions did not change after stress in the cerebral cortex, hippocampus, and cerebellum. Moreover, swim stress did not alter the ability of GABA to inhibit the binding of [35S]t-butylbicyclophosphorothionate (TBPS), a ligand that is a specific biochemical marker of the GABA-associated chloride ionophore, to crude membranes prepared from the cerebral cortex and cerebellum. Swim stress was associated with alterations of the specific binding of [3H]Ro 15-1788, a benzodiazepine receptor antagonist, to crude hippocampal and cerebellar membranes. The results are considered in the context of new insights derived from molecular cloning studies of the GABAA receptor complex.

Reduction of flurazepam's antiseizure efficacy persists after stress.

Twenty-four hours after mice were forced to swim for up to 10 min in cold (6 degrees C) water, the ability of flurazepam to antagonize the electrical precipitation of seizures was reduced. This stress-induced reduction in flurazepam's antiseizure efficacy persisted for at least 72 h; but was absent 1 week after the single session of swim stress. The data may be relevant to stress-related psychiatric disorders and suggest that the therapeutic efficacy of benzodiazepines may be altered after a severe stress.

Effects of milacemide, a glycine prodrug, on ethanol's antiseizure efficacy.

A variety of in vitro data suggest that ethanol interferes with N-methyl-D-aspartate (NMDA)-stimulated calcium ion conductance. This effect occurs at ethanol concentrations in blood associated with acute intoxication in the nontolerant human (less than 50 mM) and may involve its selective action at the strychnine-insensitive glycine binding site on the NMDA receptor complex. Moreover, there are in vitro data showing that glycinergic interventions can attenuate ethanol's inhibitory actions on NMDA-mediated transmission. The relevance of these in vitro findings to the intact animal was tested in an incremental electroconvulsive shock (IECS) paradigm using milacemide, a lipophilic prodrug of glycine. In this paradigm, the influence of milacemide on ethanol's ability to antagonize the electrical precipitation of seizures was tested. Doses of 3.2 and 32.0 mg/kg did not change ethanol's antiseizure efficacy, whereas 320.0 mg/kg potentiated ethanol's antiseizure efficacy. The mechanism of potentiation of ethanol's antiseizure efficacy by milacemide is unknown. Potentiation could result from stimulation of chloride ion conductance in the brainstem by glycine liberated from the lipophilic prodrug and acting at the strychnine-sensitive site. Alternatively, unmetabolized milacemide, which accumulates at the highest administered dose, may antagonize NMDA-mediated neural transmission. The latter explanation would be consistent with a role for receptor-gated calcium ion conductance in the mediation of ethanol's actions.