Effects of 6-cyano-7-nitroquinoxaline-2,3-dione on nicotinic receptor subunit transcript expression in the rat brain

Nicotine Elicits Convulsive Seizures by Activating Amygdalar Neurons

Nicotinic acetylcholine (nACh) receptors are implicated in the pathogenesis of epileptic disorders; however, the mechanisms of nACh receptors in seizure generation remain unknown. Here, we performed behavioral and immunohistochemical studies in mice and rats to clarify the mechanisms underlying nicotine-induced seizures. Treatment of animals with nicotine (1–4 mg/kg, i.p.) produced motor excitement in a dose-dependent manner and elicited convulsive seizures at 3 and 4 mg/kg. The nicotine-induced seizures were abolished by a subtype non-selective nACh antagonist, mecamylamine (MEC). An α7 nACh antagonist, methyllycaconitine, also significantly inhibited nicotine-induced seizures whereas an α4β2 nACh antagonist, dihydro-β-erythroidine, affected only weakly. Topographical analysis of Fos protein expression, a biological marker of neural excitation, revealed that a convulsive dose (4 mg/kg) of nicotine region-specifically activated neurons in the piriform cortex, amygdala, medial habenula, paratenial thalamus, anterior hypothalamus and solitary nucleus among 48 brain regions examined, and this was also suppressed by MEC. In addition, electric lesioning of the amygdala, but not the piriform cortex, medial habenula and thalamus, specifically inhibited nicotine-induced seizures. Furthermore, microinjection of nicotine (100 and 300 μg/side) into the amygdala elicited convulsive seizures in a dose-related manner. The present results suggest that nicotine elicits convulsive seizures by activating amygdalar neurons mainly via α7 nACh receptors.

Parallels between behavioral and neurochemical variability in the rat vacuous chewing movement model of tardive dyskinesia

The widely accepted rat vacuous chewing movement model for tardive dyskinesia could be more fully mined through greater focus on individual variability in vulnerability to this neuroleptic-induced behavior. We have examined parallels between behavioral and neurobiological variability within a cohort in order to evaluate the role that neurobiological factors might play in determining susceptibility to tardive dyskinesia. Inter-observer reliability and individual consistency across time, in both spontaneous and neuroleptic-induced vacuous chewing movements, were empirically demonstrated. While this behavior increased across 8 months of observation in both vehicle controls and haloperidol-treated rats, pre-treatment baselines were predictive of final levels across individuals only in the vehicle control group, not the haloperidol-treated group. Haloperidol-induced elevations in neostriatal D2 and GAD(67) mRNA were not correlated with individual variability in haloperidol-induced vacuous chewing movements. Ambient noise during the observations was found to exacerbate chronic haloperidol-induced, but not spontaneous vacuous chewing movements. Significant correlations were found among the haloperidol-treated rats between nigral and tegmental GAD(67) and tegmental α7 mRNA levels, measured by in situ hybridization histochemistry, and vacuous chewing movements, specifically in the noisy conditions. Variability in these secondary responses to primary striatal dopamine and GABA perturbations may play a role in determining vulnerability to vacuous chewing movements, and by analogy, tardive dyskinesia. Both the differential predictive value of baseline vacuous chewing movements and the differential effect of noise, between controls and haloperidol-treated rats, add to evidence that haloperidol-induced vacuous chewing movements are regulated, in part, by different mechanisms than those mediating spontaneous vacuous chewing movements.

The Magnitude of α7 Nicotinic Receptor Currents in Rat Hippocampal Neurons Is Dependent upon GABAergic Activity and Depolarization

Hippocampal alpha7(*) nicotinic acetylcholine receptors modulate the release of GABA and glutamate. The control of functional receptor pools by cell firing or synaptic activity could therefore allow for a local adjustment of the sensitivity to cholinergic input upon changes in neuronal activity. We first investigated whether tonic depolarization or cell firing affected the function of alpha7(*). The amplitude of alpha7(*)-gated whole-cell currents in cultured rat hippocampal neurons exposed to high-extracellular K(+) (40 mM KCl) for 24 to 48 h increased 1.3 to 5.5 times. The proportion of alpha7(*)-responsive neurons (99%), the potency of acetylcholine, and the sensitivity to nicotinic antagonists were all unaffected. In contrast, block of spontaneous cell firing with tetrodotoxin for 24 h led to a 37% reduction in mean current amplitude. Reduced alpha7(*) responses were seen after a 24-h blockade of N-type calcium channels but not of L-type calcium channels, N-methyl-d-aspartate (NMDA), or non-NMDA receptor channels, protein kinase C, or calcium-calmodulin kinases II and IV. The N-type or L-type calcium channel antagonists omega-conotoxin GVIA and nifedipine did not prevent the current-potentiating effect of KCl. The GABA(A) antagonist picrotoxin led to a 44% reduction of the currents, despite increasing action potential firing, and also reversed the potentiating effect of KCl. Treatment with GABA, midazolam, or a GABA uptake blocker led to increased currents. These data indicate that alpha7(*)-gated currents in hippocampal neurons are regulated by GABAergic activity and suggest that depolarization-induced GABA release may underlie the effect of increased extracellular KCl.