Tolerance to the sedative effect of lorazepam correlates with a diminution in cortical release and affinity for glutamate.

Benzodiazepines are anxiolytic, anticonvulsant, sedative and hypnotic compounds usually prescribed on a long-term basis. Chronic treatment with these compounds induces tolerance, which has been extensively attributed to modifications in the GABAergic neurotransmission. However, a compensatory increase in the excitatory response, named as an oppositional response, has also been put forward as a means for explaining such tolerance. Changes in the excitatory neurotransmission have been found in withdrawn rats after a long treatment with benzodiazepines but these modifications have not been conclusively studied during tolerance. In this work we studied several parameters of the glutamatergic neurotransmission in rats made tolerant to the sedative effect of 3 mg/kg (i.p.) of lorazepam (LZ). We found a decrease in the affinity of cortical NMDA receptors for (3)H-glutamate (K(D): 124.4 +/- 13.3 nM in tolerant rats, 71.6 +/- 10.4 nM in controls, P<0.05) together with a decrease in the in vitro 60 mM K(+)-stimulated cortical glutamate release (59+/- 12% vs. 153 +/- 38%, tolerant rats vs. controls, P<0.05). We conclude that tolerance to the sedative effect of LZ correlates with a decreased sensitivity for glutamate that may in turn diminish the cortical response to a chemical stimulus. Our findings constitute an evidence against the oppositional model of pharmacodynamic tolerance in this experimental condition.

Diazepam fails to potentiate GABA-induced chloride uptake and to produce anxiolytic-like action in aged rats.

The pharmacological response to benzodiazepines has been demonstrated to be different in aged individuals in comparison to adults. We studied the age-dependent changes in some of the in vitro and behavioral effects of diazepam in aged (24 months old) rats, comparing them to adults (3 months old). We evaluated the in vitro gamma-aminobutyric acid (GABA)-induced 36Cl- uptake and the diazepam potentiation of GABA-stimulated 36Cl- uptake in microsacs from cerebral cortex of both groups of animals. We found no differences in the GABA-stimulated 36Cl- uptake between adult and aged animals, and diazepam failed to potentiate GABA-induced 36Cl- flux in the aged cortical microsacs. We also examined the effect of 0.03-10 mg of diazepam on locomotor activity in an open-field test and the anxiolytic-like action of diazepam in doses ranging from 0.03 to 1 in a dark-light transition test. We observed no anxiolytic-like action of the drug in the dark-light transition test in the aged rats, while there was a shift to the left in the diminution of locomotor activity evaluated by the open-field test. We conclude that the pharmacodynamic changes observed in cortical GABA(A) receptors in aged rats could partially explain the lack of anxiolytic-like action but not the oversedation evidenced in this group of animals.

Chronic corticosterone impairs inhibitory avoidance in rats: possible link with atrophy of hippocampal CA3 neurons.

The aim of our work was to evaluate the effect of a chronic (22 days) administration of corticosterone, which induces supraphysiological serum levels of the hormone, on an inhibitory avoidance learning in rats (one-trial step-through learning task, footshock: 0.5 mA, 2 s). We also studied hippocampal markers of neuroanatomical CA3 pyramidal neuron atrophy by using the Golgi staining method. Chronic exposure to high CORT serum levels induced a significant impairment of inhibitory avoidance learning. The CORT group also showed hippocampal glucocorticoid receptor (GR) downregulation and the decrease of hippocampal CA3 branch points and total dendritic length in the apical tree that would be causally related with the learning impairment.