Ventral posteromedial nucleus of the thalamus as a neuroanatomical target for the anticephalalgic action of valproic acid and metamizole

Abstract

Introduction. Valproic acid and metamizole are widely used in the treatment of primary headaches. However, despite the drugs’ proven clinical efficacy, the neurophysiological mechanisms underlying their therapeutic action are poorly understood. The ventral posteromedial nucleus of the thalamus (VPM) serves as the highest suprasegmental relay center for transmitting ascending nociceptive information from the cranial structures to the cerebral cortex and is considered as a potential neuroanatomical target for anticephalalgic drugs.The objective of the study was to evaluate the action of valproate and metamizole on the excitability of the VPM thalamic neurons.Methods and materials. In acute controlled experiments on anesthetized rats (n=31), we studied the action of cumulative dosing (three fixed dose injections every 30 minutes) of intravenously administered valproate (n=8, 100 mg/kg x 3) or metamizole (n=16,150 mg/kg · 3) on extracellularly recorded background and dura mater electrical stimulation-evoked activities of the convergent VPM neurons. Results. In all experiments, intravenous valproate significantly inhibited both background activity of the VPM neurons and their responses to dural electrostimulation. Metamizole had a similar action in more than half of the cases, whereas 44 % of the examined thalamic cells (7 out of 16) did not react to its infusion.Conclusion. One of the main neurophysiological mechanisms underlying anticephalalgic action of valproic acid and metamizole may be inhibition of the nociceptive transmission at the thalamic level due to the drugs-induced suppression of the meningeal-sensitive VPM neurons excitability.