Abstracts

RATIONALE: Valproic acid (VPA) has well-established efficacy in the treatment of both generalized and partial seizures, neuropathic pain, migraine, and some psychiatric disorders. Some of the proposed actions for VPA, including augmentation of GABAergic responsiveness or attenuation of NMDA responses, are consistent with its clinical effects, but not its therapeutic effects on non-convulsive absence seizures. The reticular thalamic nucleus (RTN) is a shell-like group of inhibitory neurons that plays a key role in thalamocortical rhythm generation that is relevant to sleep and absence epilepsy. Previous studies support an endogenous anticonvulsant role of neuropeptide Y (NPY, which is coexpressed with GABA in RTN neurons), and we have found that application of exogenous NPY to thalamic slices results in a suppression of epileptiform activity. In this study we investigated whether the anti-epileptic properties of VPA may be in part mediated by increased expression of the putative endogenous anticonvulsive substance NPY.
METHODS: VPA (200 mg/kg in 3mL/kg H2O) was administered to P32 to P47 rats every 8 hours for 4 days via intraperitoneal injections. Control rats received equivalent doses of the structurally related but therapeutically inactive compound n-octanoic acid (OA). Following chronic treatment the animals were sacrificed, perfused and the brains were sectioned and processed for immunohistochemical analysis. Using scanning laser confocal microscopy, we qualitatively surveyed the effects of valproate on NPY levels in rat cortex, thalamus and RTN, and quantitatively examined the effects on neuropeptide Y levels in RTN. For controls, we examined the immunostaining of two other thalamic neuropeptides, VIP and somatostatin. To quantify peptide content, we used the approach of semi-automatically locating and identifying individual neuronal somata via NeuN staining, and then assessing NPY, VIP and SST levels on a per neuron basis.
RESULTS: Chronic valproic acid administration significantly increases the levels of NPY in cortical interneurons and RTN neurons compared to OA treated rats. We found an increase of over 30% in RTN cells, and a prominent expression in RTN axons that was rarely, if ever observed na[iuml]ve or OA treated animals. By contrast, levels of VIP and SST were similar in OA and VPA treated animals. These results were replicated in four separate OA/VPA matched trials.
CONCLUSIONS: The marked specific increase in NPY-like immunoreactivity in RTN cells and cortical interneurons following VPA treatment, along with our recent evidence for an endogenous antiepileptic role of NPY (Y1 receptor antagonists enhance and agonists suppress thalamic epileptiform activity) suggests that the therapeutic effects of valproic acid and related compounds may be due in part to a specific up-regulation of this endogenous antiepileptic peptide in neurons. RTN cells prominently and uniformly express NPY and these data provide further support a role for this neurotransmitter in regulation of thalamocortical seizure activity.
[Supported by: NIH Grants NS06477 and NS34774 from the NINDS.]