Microglial and neurochemical mechanisms of seizure-induced central autonomic cardiovascular dysfunction

Epilepsy is a chronic paroxysmal neurological disorder characterised by seizures. Central autonomic cardiovascular dysfunction is established as a major cause of sudden unexpected death in epilepsy (SUDEP). Cardiovascular autonomic function is controlled by nuclei present in the brainstem, including the rostral ventrolateral medulla (RVLM). The RVLM gives projection to presympathetic neurons in the intermediolateral cell column (IML) of the spinal cord, which in turn controls the sympathetic nerve activity (SNA) of post-ganglionic neurons and thus the cardiovascular system. Microglia are the principal resident immune cells of the CNS that react to pathological or physiological disturbances in the brain, such as seizures, to produce pro-inflammatory or anti-inflammatory effects. Levels of the peptide pituitary adenylate cyclase-activating polypeptide (PACAP) increase during and after seizures, and it has neuroprotective as well as sympathoexcitatory properties, while glutamate, which is the principal cardiovascular autonomic neurotransmitter in the RVLM, plays an important role in the development of seizures. The major aims of this thesis were to investigate the role of microglia, PACAP and glutamate in the RVLM and/or IML in the progression of seizure-induced autonomic cardiovascular dysfunction during acute and chronic epilepsy in rats. The findings suggest that: 1) Kainic acid (KA)-induced acute seizures cause significant anddose-dependent increase in SNA, mean arterial pressure and heart rate and a prolongation of the QT interval in the ECG. 2) Intrathecal (IT) infusion of PACAP antagonist(PACAP(6-38)) or microglial antagonist (minocycline and doxycycline) worsens the cardiovascular responses of acute seizures, whereas IT PACAP agonist (PACAP-38) has no significant effect. 3) Acute seizure-induced cardiovascular responses, including prolongation of the QT interval, are driven by activation of glutamatergic receptors in the RVLM as these effects are abolished with glutamate receptor antagonist (kynurenic acid) microinjection. The activity of PACAP and microglia in the RVLM do not alter SNA, but mediate prolongation of the QT interval. 4) In the vicinity of the RVLM neurons, microglia are in a surveillance state with no change in their number of anti-inflammatory M2 phenotype during acute seizures. 5) In rats with KA-induced chronic temporal lobe epilepsy, spontaneous seizures cause significant tachycardia with long-lasting prolongation of QT interval. The antagonism of microglial activation, but not PACAP, at the level of the IML significantly reduces SNA and proarrhythmogenic effects of chronic seizure activity. Neither PACAP nor microglia regulate baroreflex or peripheral and central chemoreflex responses during chronic epilepsy in rats. 6) In chronic epileptic rats, microglia are in surveillance state and their number of anti-inflammatory M2 phenotype remains unchanged in the vicinity of the RVLM neurons.