Muscarinic M1 receptor and cannabinoid CB1 receptor do not modulate paraoxon-induced seizures

The Relationship between Muscarinic and Cannabinoid Receptors in Neuronal Excitability and Epilepsy: A Review

Background

Of the seventy million people who suffer from epilepsy, 40 percent of them become resistant to more than one antiepileptic medication and have a higher chance of death. While the classical definition of epilepsy was due to the imbalance between excitatory glutamatergic and inhibitory γ-aminobutyric acid (GABA)-ergic signalling, substantial evidence implicates muscarinic receptors in the regulation of neural excitability.

Summary

Cannabinoids have shown to reduce seizure activity and neuronal excitability in several epileptic models through the activation of muscarinic receptors with drugs which modulate their activity. Cannabinoids also have been effective in reducing antiepileptic activity in pharmaco-resistant individuals; however, the mechanism of its effects in temporal lobe epilepsy is not clear.

Key Messages

This review seeks to elucidate the relationship between muscarinic and cannabinoid receptors in epilepsy and neural excitability.

Forebrain Cholinergic Signaling Regulates Innate Immune Responses and Inflammation

The brain regulates physiological functions integral to survival. However, the insight into brain neuronal regulation of peripheral immune function and the neuromediator systems and pathways involved remains limited. Here, utilizing selective genetic and pharmacological approaches, we studied the role of forebrain cholinergic signaling in the regulation of peripheral immune function and inflammation. Forebrain-selective genetic ablation of acetylcholine release and vagotomy abolished the suppression of serum TNF by the centrally-acting cholinergic drug galantamine in murine endotoxemia. Selective stimulation of acetylcholine action on the M1 muscarinic acetylcholine receptor (M1 mAChR) by central administration of the positive allosteric modulator benzyl quinolone carboxylic acid (BQCA) suppressed serum TNF (TNFα) levels in murine endotoxemia. This effect was recapitulated by peripheral administration of the compound. BQCA also improved survival in murine endotoxemia and these effects were abolished in M1 mAChR knockout (KO) mice. Selective optogenetic stimulation of basal forebrain cholinergic neurons innervating brain regions with abundant M1 mAChR localization reduced serum TNF in endotoxemic mice. These findings reveal that forebrain cholinergic neurons regulate innate immune responses and inflammation, suggesting the possibility that in diseases associated with cholinergic dysfunction, including Alzheimer's disease this anti-inflammatory regulation can be impaired. These results also suggest novel anti-inflammatory approaches based on targeting forebrain cholinergic signaling in sepsis and other disorders characterized by immune dysregulation.

Characterization of acute intrastriatal effects of paraoxon on in vivo dopaminergic neurotransmission using microdialysis in freely moving rats

Paraoxon (POX) is an extremely neurotoxic organophosphorous compound (OP) which main toxic mechanism is the irreversible inhibition of cholinesterase. Although the cholinergic system has always been linked as responsible for its acute effects, experimental studies have suggested that the dopaminergic system also may be a potential target for OPs. Based on this, in this study, the acute intrastriatal effects of POX on dopaminergic neurotransmission were characterized in vivo using brain microdialysis in freely moving rats. In situ administration of POX (5, 25 and 50 nmol, 60 min) significantly increased the striatal dopamine overflow (to 435 ± 79%, 1066 ± 120%, and 1861 ± 332%, respectively), whereas a lower concentration (0.5 nmol) did not affect dopamine levels. Administration of POX (25 nmol) to atropine (15 nmol) pretreated animals, produced an increase in dopamine overflow that was ∼63% smaller than those observed in animals not pretreated. Administration of POX (25 nmol) to mecamylamine (35 nmol) pretreated animals did not significantly affect the POX-induced dopamine release. Our results suggest that acute administration of POX increases the dopamine release in a concentration-dependent way, being this release dependent on acetylcholinesterase inhibition and mediated predominantly by the activation of striatal muscarinic receptors, once the muscarinic antagonist atropine partially blocks the POX-induced dopamine release.

The M1 Muscarinic Receptor Antagonist VU0255035 Delays the Development of Status Epilepticus after Organophosphate Exposure and Prevents Hyperexcitability in the Basolateral Amygdala

Exposure to organophosphorus toxins induces seizures that progress to status epilepticus (SE), which can cause brain damage or death. Seizures are generated by hyperstimulation of muscarinic receptors, subsequent to inhibition of acetylcholinesterase; this is followed by glutamatergic hyperactivity, which sustains and reinforces seizure activity. It has been unclear which muscarinic receptor subtypes are involved in seizure initiation and the development of SE in the early phases after exposure. Here, we show that pretreatment of rats with the selective M₁ receptor antagonist, VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide], significantly suppressed seizure severity and prevented the development of SE for about 40 minutes after exposure to paraoxon or soman, suggesting an important role of the M₁ receptor in the early phases of seizure generation. In addition, in in vitro brain slices of the basolateral amygdala (a brain region that plays a key role in seizure initiation after nerve agent exposure), VU0255035 blocked the effects produced by bath application of paraoxon-namely, a brief barrage of spontaneous inhibitory postsynaptic currents, followed by a significant increase in the ratio of the total charge transferred by spontaneous excitatory postsynaptic currents over that of the inhibitory postsynaptic currents. Furthermore, paraoxon enhanced the hyperpolarization-activated cation current I_h in basolateral amygdala principal cells, which could be one of the mechanisms underlying the increased glutamatergic activity, an effect that was also blocked in the presence of VU0255035. Thus, selective M₁ antagonists may be an efficacious pretreatment in contexts in which there is risk for exposure to organophosphates, as these antagonists will delay the development of SE long enough for medical assistance to arrive.