Anatomical analysis of frontal cortex sites at which carbachol induces motor seizures in the rat

Cholinergic Signaling, Neural Excitability, and Epilepsy

Epilepsy is a common brain disorder characterized by recurrent epileptic seizures with neuronal hyperexcitability. Apart from the classical imbalance between excitatory glutamatergic transmission and inhibitory γ-aminobutyric acidergic transmission, cumulative evidence suggest that cholinergic signaling is crucially involved in the modulation of neural excitability and epilepsy. In this review, we briefly describe the distribution of cholinergic neurons, muscarinic, and nicotinic receptors in the central nervous system and their relationship with neural excitability. Then, we summarize the findings from experimental and clinical research on the role of cholinergic signaling in epilepsy. Furthermore, we provide some perspectives on future investigation to reveal the precise role of the cholinergic system in epilepsy.

Carbachol and Nicotine in Prefrontal Cortex Have Differential Effects on Sleep-Wake States

The role of the brainstem cholinergic system in the regulation of sleep-wake states has been studied extensively but relatively little is known about the role of cholinergic mechanisms in prefrontal cortex in the regulation of sleep-wake states. In a recent study, we showed that prefrontal cholinergic stimulation in anesthetized rat can reverse the traits associated with anesthesia and restore a wake-like state, thereby providing evidence for a causal role for prefrontal cholinergic mechanisms in modulating level of arousal. However, the effect of increase in prefrontal cholinergic tone on spontaneous sleep-wake states has yet to be demonstrated. Therefore, in this study, we tested the hypothesis that delivery of cholinergic agonists - carbachol or nicotine - into prefrontal cortex of rat during slow wave sleep (SWS) would produce behavioral arousal and increase the time spent in wake state. We show that unilateral microinjection (200 nL) of carbachol (1 mM) or nicotine (100 mM) into prefrontal cortex during SWS decreased the latency to the onset of wake state (p = 0.03 for carbachol, p = 0.03 for nicotine) and increased the latency to the onset of rapid eye movement sleep (p = 0.008 for carbachol, p = 0.006 for nicotine). Although the infusion of 1 mM carbachol increased the time spent in wake state (p = 0.01) and decreased the time spent in SWS (p = 0.01), infusion of 10 or 100 mM nicotine did not produce any statistically significant change in sleep-wake architecture. These data demonstrate a differential role of prefrontal cholinergic receptors in modulating spontaneous sleep-wake states.

Boosting cholinergic activity in gustatory cortex enhances the salience of a familiar conditioned stimulus in taste aversion learning

The cholinergic system is important for learning, memory, and responses to novel stimuli. Exposure to novel, but not familiar, tastes increases extracellular acetylcholine (ACh) levels in insular cortex (IC). To further examine whether cholinergic activation is a critical signal of taste novelty, in these studies carbachol, a direct cholinergic agonist, was infused into IC before conditioned taste aversion (CTA) training with a familiar taste. By mimicking the cholinergic activation generated by novel taste exposure, it was hypothesized that a familiar taste would be treated as novel and therefore a salient target for aversion learning. As predicted, rats infused with the agonist were able to acquire CTAs to familiar saccharin. Effects of carbachol infusion on patterns of neuronal activation during conditioned stimulus-unconditioned stimulus pairing were assessed using Fos-like immunoreactivity (FLI). Familiar taste-illness pairing following carbachol, but not vehicle, induced significant elevations of FLI in amygdala, a region with reciprocal connections to IC that is also important for CTA learning. These results support the view that IC ACh activity provides a critical signal of taste novelty that facilitates CTA acquisition.

Transient impairment of cholinergic function in the rat insular cortex disrupts the encoding of taste in conditioned taste aversion

The muscarinic antagonist scopolamine blocks conditioned taste aversion (CTA) when microinjected bilaterally into the rat insular cortex shortly before the exposure of the rat to a novel taste (the conditioned stimulus, CS) in CTA training. Scopolamine has no effect when microinjected shortly after the exposure to the novel taste or shortly before the application of the malaise-inducing agent (unconditioned stimulus, UCS). Scopolamine does not affect sensory, motor and retrieval mechanisms required for performing the CTA task, and does not block CTA when injected into another cortical area. The effect of scopolamine is independent of the taste used as CS. Furthermore, microinjection of scopolamine into the insular cortex shortly before the pre-exposure to a new taste in a latent inhibition paradigm, impairs the attenuation of CTA by that pre-exposure. Other muscarinic antagonists, pirenzepine and AF DX-116, have an effect similar to that of scopolamine. Comparison of the dose-dependency curves of the muscarinic antagonists suggests a predominant role in CTA for M2 subtype receptors. Carbachol, a muscarinic agonist, also impairs the encoding of taste in the insular cortex, but the results are confounded by the ability of that ligand to induce seizures. Our findings suggest that cholinergic neuromodulation participates in processing the CS in the gustatory cortex in CTA, either by encoding novelty at the cellular level, or by instructing the neural circuits to store the novel taste representation.

Cholinergic mechanisms in generalized seizures: importance of the zona incerta

OBJECTIVE

Stimulation of the central cholinergic system results in generalized epileptic seizures. The goal of this study was to map the epileptogenic effects of the cholinergic agonist, carbachol injected into different sites of the basal forebrain and diencephalon of the rat brain.

METHODS

Carbachol was injected directly into the brain in a dose of 1 or 3 micrograms. Seizures were assessed behaviourally on a five-stage scale with electroencephalographic controls. Seizures at stage 1 were the least severe and those at stage 5 the most severe.

RESULTS

Injections of high dose carbachol (3 micrograms) induced seizures from 40% of all injected brain sites. Injections of low dose carbachol (1 microgram) or isotonic saline into the same brain sites did not cause any behavioural or electrographic seizures. The majority of sites (84%) producing generalized seizures (stage 5) were concentrated in or around the zona incerta.

CONCLUSIONS

Within the anatomical limits of the study, the zona incerta is the area most sensitive to carbachol-induced generalized seizures.

Involvement of M1 muscarinic receptors in the initiation of cholinergically induced epileptic seizures in the rat brain

The present study was designed to determine the types of acetylcholine receptors involved in the initiation of epileptic seizures from the zona incerta and surrounding structures by cholinergic stimulation in rats. Unilateral intracerebral microinjection of the mixed muscarinic and nicotinic agonist carbachol (3 micrograms) produced generalized seizures in 12 of 20 rats studied. Local pretreatment with equimolar doses of acetylcholine receptor antagonists was used as a method of determining the receptor type involved in the initiation of cholinergically induced seizures in the rat diencephalon. Pretreatment with the M1 muscarinic receptor antagonist, pirenzepine (7 micrograms), abolished carbachol-induced seizures in 91% of the animals tested. The M2 muscarinic receptor antagonist, methoctramine (12 micrograms) and the nicotinic receptor antagonist, mecamylamine (3 micrograms), were relatively ineffective in antagonizing seizures in 9% and 27%, respectively. The results suggest that M1 muscarinic receptors are preferentially involved in the initiation of generalized epileptic seizures in the basal diencephalon of the rat.

Histochemical mapping of nitric oxide synthase in the rat brain

The NADPH-diaphorase histochemical technique provides a simple and robust method to stain select populations of neurons throughout the brain. We have recently identified the enzyme responsible for this histochemical reaction to be nitric oxide synthase. This enzyme is responsible for the calcium-dependent synthesis of nitric oxide from arginine. Nitric oxide acts as a novel neural messenger by stimulating soluble guanylyl cyclase thereby increasing the levels of cyclic guanosine 3',5'-monophosphate in target cells. Thus the NADPH-diaphorase histochemical method allows the direct visualization of the neurons which use this novel signal transduction pathway. We now describe the detailed distribution of this enzyme in the rat brain. Our results suggest a widespread role for the nitric oxide-cyclic guanosine monophosphate system in the nervous system.

Autoradiographic analysis of [35S]TBPS binding in entorhinal cortex-kindled rat brains

A quantitative autoradiographic analysis of [35S]t-butylbicyclophosphorothionate (TBPS) binding to the gamma-aminobutyric acid (GABA)-mediated chloride ionophore was carried out in 104 brain areas of entorhinal cortex-kindled and control rats. Subjects were sacrificed either 24 h or 28 days after the last kindled seizure. Kindled subjects in the 24 h group showed reductions in mean [35S]TBPS binding in the lateral nucleus of the amygdala (-31%), the infralimbic cortex (-14%), and the paracentral nucleus of the thalamus (-22%). At 28 days, reductions in binding were observed in the infralimbic cortex (-15%) and the paracentral nucleus of the thalamus (-18%). These data suggest that repeated seizures (kindling) modify the GABA-mediated chloride ionophore, and that in some brain areas related to seizure generalization the modifications are very long lasting.

Controllability of prestimulation of the medial prefrontal cortex determines the facilitation of self-stimulation and kindled seizures

Electrical stimulation of the medial prefrontal cortex (MPC) was administered according to the triadic design typically used to demonstrate learned helplessness. Three groups received either controllable, uncontrollable or no stimulation during the pretreatment phase. The effects of this pretreatment on the acquisition of self-stimulation at the same electrode site were investigated in the second phase of the experiment. Relative to unstimulated controls, both controllable and uncontrollable prestimulation facilitated the acquisition of self-stimulation and produced higher self-stimulation rates. In addition, compared with controllable stimulation, pretreatment with uncontrollable stimulation produced a greater facilitation in self-stimulation rate. The unambiguous demonstration of a behavioural facilitation produced by pretreatment with uncontrollable stimulation is, effectively, the inverse of the typical learned helplessness finding. It was also found, in the second phase of the experiment, that 6 of the 7 rats previously exposed to uncontrollable stimulation developed full class 5 seizures. No behavioural evidence of kindling was seen in any of the other rats or during the prestimulation procedure. These data are interpreted in terms of kindling and stress effects both proximal and distal to the site of stimulation.