Brain somatostatin: a candidate inhibitory role in seizures and epileptogenesis

Endogenous control of hippocampal epileptogenesis: a molecular cascade involving brain-derived neurotrophic factor and neuropeptide Y

PURPOSE

Seizures increase the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. Because this neurotrophin exerts modulatory effects on hippocampal neuronal excitability, it may play an important role in epileptogenesis initiated in this structure. Moreover BDNF is known to regulate the expression of neuropeptide Y (NPY), which displays modulatory properties on seizure activity. This suggests that the effects of BDNF on epileptogenesis may be mediated by NPY.

METHODS

Adult male rats received a 7-day chronic intrahippocampal infusion of BDNF, BDNF antisense oligodeoxynucleotides, NPY, or anti-NPY immunoglobulin G during kindling of the hippocampus. The long-term regulation of NPY expression by BDNF was also studied by immunohistochemistry and radioimmunoassay.

RESULTS

BDNF applied during the first week of hippocampal stimulation significantly delayed the progression of kindling, an effect that outlasted the end of the infusion by at least 7 days. Conversely, infusion of BDNF antisense oligodeoxynucleotides to reduce the expression of endogenous BDNF in the hippocampus aggravated the electroencephalographic expression of seizures. Chronic infusion of BDNF increased the expression of NPY in the hippocampus, with a time course similar to that of the protective effect of the neurotrophin on kindling. Finally, chronic infusion of NPY in the hippocampus delayed the progression of hippocampal kindling, whereas anti-NPY antibodies had an aggravating effect.

CONCLUSIONS

Our results suggest that the seizure-induced increase in BDNF expression in the hippocampus may constitute an endogenous protective mechanism able to counteract hippocampal epileptogenesis. This protective effect appears to be mediated at least in part through the regulation of NPY expression.

What is GABAergic inhibition? How is it modified in epilepsy?

A deficit of gamma-aminobutyric acid-ergic (GABAergic) inhibition is hypothesized to underlie most forms of epilepsy. Although apparently a straightforward and logical hypothesis to test, the search for a deficit of GABAergic inhibition in epileptic tissue has revealed itself to be as difficult as the quest for the Holy Grail. The investigator faces many obstacles, including the multiplicity of GABAergic inhibitory pathways and the multiplicity of variables that characterize the potency of inhibition within each inhibitory pathway. Perhaps more importantly, there seems to be no consensual definition of GABAergic inhibition. The first goal of this review is to try to clarify the notion of GABAergic inhibition. The second goal is to summarize our current knowledge of the various alterations that occur in the GABAergic pathways in temporal lobe epilepsy. Two important features will emerge: (a) according to the variable used to measure GABAergic inhibition, it may appear increased, decreased, or unchanged; and (b) these modifications are brain area- and inhibitory pathway-specific. The possible functional consequences of these alterations are discussed.

Kindled seizure-evoked somatostatin release in the hippocampus: inhibition by MK-801

The aim of this study was to evaluate the contribution of ionotropic glutamate receptors to kindled seizure-evoked somatostatin release in the hippocampus, using a microdialysis approach. Basal and amygdala stimulation-evoked somatostatin-like immunoreactivity (-LI) release was significantly greater in kindled compared to naive rats. In naive rats, neither hippocampal perfusion with the selective AMPA/kainate receptor antagonist GYKI 52466 nor with the selective NMDA receptor antagonist MK-801 affected behavior, EEG, or somatostatin-LI release. In kindled rats, GYKI 52466 was still devoid of any effect, while MK-801 significantly decreased stimulus-evoked (but not basal) somatostatin-LI efflux. MK-801 produced identical effects when injected i.p. This study provides the first direct evidence that kindled seizure-evoked somatostatin release in the hippocampus is partly NMDA receptor dependent.

Inflammatory cytokines and related genes are induced in the rat hippocampus by limbic status epilepticus

Limbic status epilepticus was induced in rats by unilateral 60-min electrical stimulation of the CA3 region of the ventral hippocampus. As assessed by RT-PCR followed by Southern blot analysis, transcripts of interleukin-1beta, interleukin-6, interleukin-1 receptor antagonist and inducible nitric oxide synthase were significantly increased 2 h after status epilepticus in the stimulated hippocampus. Induction was maximal at 6 h for interleukin-1beta (445%), interleukin-6 (405%) and tumour necrosis factor-alpha (264%) and at 24 h for interleukin-1 receptor antagonist (494%) and inducible nitric oxide synthase (432%). In rats with spontaneous seizures (60 days after status epilepticus), interleukin-1beta mRNA was still higher than controls (241%). Immunocytochemical staining of interleukin-1beta, interleukin-6 and tumour necrosis factor-alpha was enhanced in glia with a time-course similar to that of the respective transcripts. Sixty days after status epilepticus, interleukin-1beta immunoreactivity was increased exclusively in neurons in one third of the animals. Multiple intracerebroventricular injections of interleukin-1 receptor antagonist (0.5 microg/3 microL) significantly decreased the severity of behavioural convulsions during electrical stimulation and selectively reduced tumour necrosis factor-alpha content in the hippocampus measured 18 h after status epilepticus. Thus, the induction of spontaneously recurring seizures in rats involves the activation of inflammatory cytokines and related pro- and anti-inflammatory genes in the hippocampus. These changes may play an active role in hyperexcitability of the epileptic tissue.

Advances in understanding neuronal somatostatin receptors

It has long been considered that somatostatin acts as a neuromodulator in the mammalian central nervous system but its precise physiological roles remain elusive. Early studies to identify somatostatin-binding sites revealed a widespread heterogeneous pattern, especially in the CNS. More recently, a family of somatostatin receptors have been identified, of which five genes (sst(1-5)) have been cloned. In this review, we discuss current data describing the localisation of the five receptor types. Recent progress in understanding their function has been made using high-affinity, selective receptor ligands and transgenic animal technology. Finally, the therapeutic potential for somatostatin receptor-selective compounds as analgesics is considered.