Diazepam, given postischemia, protects selectively vulnerable neurons in the rat hippocampus and striatum

Abstract

Following cerebral ischemia, certain populations of neurons degenerate. Excessive accumulation of excitatory amino acids in the synaptic cleft, activation of excitatory amino acid receptors, and influx of calcium into neurons play a key role in the development of ischemia-induced neuronal death. We hypothesized that neuroprotection may be achieved by enhancing inhibitory (i.e., gamma-aminobutyric acid, GABA) neurotransmission to offset excitation. Diazepam, a drug that increases GABA-induced chloride channel opening, was administered (10 mg/kg, i.p.) to rats 1 and 2 hr following 15 min of transient global ischemia, when hippocampal GABA levels, increased during ischemia, returned to basal. Rats were maintained normothermic during ischemia and became hypothermic following the injections of diazepam. Four days later, rats were sacrificed and the brains were examined for neuronal degeneration and the presence of GABAA receptors labeled by 35S-t- butylbicyclophosphorothionate (35S-TBPS). There was substantial neuroprotection of striatal neurons and pyramidal neurons in the CA1 area of the hippocampus. In addition, diazepam prevented the loss of 35S-TBPS binding sites in the striatum and in the dendritic fields of the CA1 hippocampus following ischemia. Since hypothermia, itself, is neuroprotective, we determined if hypothermia was required for the ability of diazepam to produce neuroprotection. Diazepam was microinjected into the CA1 hippocampus 1 and 2 hr following ischemia, and rats remained normothermic. Four days later, diazepam still produced substantial protection of hippocampal neurons. Thus, postischemic hypothermia may have contributed to the neuroprotection by diazepam when it was administered systemically, but the neuroprotective effect of diazepam did not require hypothermia. We conclude that delayed enhancement of GABAergic neurotransmission directly at the site of vulnerability following an ischemic event protects the vulnerable neurons from death.

Tacrolimus (FK506) reduces ischemia-induced hippocampal damage in rats: a 7- and 30-day study

“…Such a bimodal distribution of ischemic CA1 damage has also been reported after treatment with different compounds, such as the AMPA receptor antagonist, NBQX (27,28), the benzodiazepine agonist, diazepam (24,29,30), and the partial benzodiazepine agonist, imidazenil (24). Such variability in response is thought to result from a combination of at least two factors, i.e., normal variation of damage in the model, and differences in the real concentration of the drug which reaches the brain in each individual (27).…”

Ischemic insults promote epigenetic reprogramming of μ opioid receptor expression in hippocampal neurons

“…Under physiological conditions, activation of MOR inhibits GABA release from inhibitory interneurons that selectively project onto the somata of pyramidal neurons, thereby enhancing the excitability and output of pyramidal neurons (50). A possible scenario is that ischemia-induced gene silencing of MOR disinhibits GABA release from inhibitory interneurons and attenuates excitability and activity of CA1 pyramidal neurons and reduces excitotoxicity of endogenous glutamate (51). In this light, MOR-1 silencing would represent a failed attempt of postischemic inhibitory interneurons to promote survival of CA1 pyramidal neurons.…”

Suppression of Neuronal Hyperexcitability and Associated Delayed Neuronal Death by Adenoviral Expression of GABACReceptors

“…Our results are consistent with a previous observation that GABAergic neocortical neurons are resistant to NMDA receptor-mediated injury (Tecoma and Choi, 1989). Exogenous administration of diazepam, an allosteric GABA potentiator, also has been shown to reduce postischemic and traumatic neuronal death in vivo (Schwartz et al, 1995;O'Dell et al, 2000). Because generalized augmentation of GABAergic inhibition, whether pharmacologically or by gene targeting, may impair normal synaptic plasticity (Levkovitz et al, 1999), there may be a strong advantage for a regional rather than a general enhancement of inhibition.…”

“…Experimental models of epilepsy, for example, demonstrate a decrease in hippocampal GABA A receptor expression (Friedman et al, 1994;Rice et al, 1996) and function (Gibbs et al, 1997), suggesting that loss of inhibitory control plays a pathogenic role (Mody, 1998). Postischemic and traumatic neuronal death is reduced by the GABA potentiator diazepam (Schwartz et al, 1995;O'Dell et al, 2000). Enhancement of the GABA effect at the receptor is a reasonable strategy for increasing inhibition; this is the mechanism of many clinically useful drugs including anticonvulsants, anxiolytics, and general anesthetics (Hevers and Luddens, 1998).…”