Pilocarpine-Induced Convulsions in Rats: Evidence for Muscarinic Receptor-Mediated Activation of Locus Coeruleus and Norepinephrine Release in Cholinolytic Seizure Development

Abstract

We recently reported that systemic administration of the anticholinesterase, soman, caused rapid depletion of forebrain norepinephrine (NE) in convulsive but not in nonconvulsive rats (18). As neurons in nucleus locus coeruleus (LC) provide the bulk of NE innervation to most of the forebrain and the sole source of NE input to the cortex and the olfactory bulb, soman-induced NE depletion was hypothesized to result from activation of LC neurons. This activation was thought to be due to inhibition of acetylcholinesterase by soman, leading to rapid, sustained accumulation of acetylcholine in LC, causing these cells to fire at a high sustained rate. Support for this hypothesis was provided by neurophysiological findings (21) showing that: (i) Systemic administration of soman in anesthetized rats caused a sustained, fivefold increase in the mean firing rate of LC neurons and (ii) microinjections of soman directly into LC caused a similar increase in the firing rate of LC neurons (21). Soman-induced activation of LC occurred prior to and even in the absence of seizures. As systemic administration of the muscarinic receptor antagonist, scopolamine, rapidly and completely reversed soman-induced activation of LC, it was further hypothesized that activation of LC neurons following soman administration is due to muscarinic receptor stimulation. The rapid release of NE by cholinolytic agents, thus, may play an important role in the initiation and/or maintenance of convulsions. To further test the hypothesis that NE release in soman-intoxicated rats is due to muscarinic activation of LC, we have investigated the effects of the muscarinic receptor agonist, pilocarpine, on NE release and LC discharge. In one set of experiments, rats were injected with a periconvulsive dose of pilocarpine (300 mg/kg, ip); both convulsive and nonconvulsive rats were sacrificed between 1 and 96 h and monoamine levels in the rostral forebrain and olfactory bulb were determined by HPLC with electrochemical detection. NE levels declined substantially only in convulsive rats; forebrain NE levels in convulsive rats rapidly decreased to 50% of control levels at 1 h and to 37% of controls level between 2 and 4 h. The time course and magnitude of these changes were similar to those observed following soman administration in our previous study (18). Recovery of forebrain NE began at 8 h and was complete by 96 h following pilocarpine administration. Neither dopamine (DA) nor serotonin (5-HT) levels were changed in the forebrain and olfactory bulb of either convulsive or nonconvulsive rats. However, both convulsive and nonconvulsive rats exhibited similar increases at 1-4 h in the levels of DA metabolites (3,4-dihydroxyphenylacetic acid, homovanillic acid) and the major metabolite of 5-HT, 5-hydroxyindoleacetic acid. In a second set of experiments, we examined the effect of systemic and local administration of pilocarpine on the firing rate LC neurons in anesthetized rats. Systemic administration of pilocarpine caused a sustained, fivefold increase in mean LC neurons firing rate. Microinjections of pilocarpine directly into LC caused a similar increase in the firing rate of LC neurons. Thus, the increase in LC discharge rate produced by pilocarpine was nearly identical to that of soman. The effects of systemically and intracoerulearly administered pilocarpine on LC discharge were reversed by the muscarinic receptor antagonist, scopolamine. The present neurochemical and electrophysiological results thus support the hypothesis that NE depletion in convulsive rats treated with soman is the result of muscarinic receptor-mediated cholinergic activation of LC neurons.