Dorsal raphe nucleus acetylcholine-mediated neurotransmission modulates post-ictal antinociception: The role of muscarinic and nicotinic cholinergic receptors

GABAergic and glutamatergic inputs to the medulla oblongata and locus coeruleus noradrenergic pathways are critical for seizures and postictal antinociception neuromodulation

We investigated the participation of the nucleus of the tractus solitarius (NTS) in tonic‒clonic seizures and postictal antinociception control mediated by NMDA receptors, the role of NTS GABAergic interneurons and noradrenergic pathways from the locus coeruleus (LC) in these phenomena. The NTS-lateral nucleus reticularis paragigantocellularis (lPGi)-LC pathway was studied by evaluating neural tract tracer deposits in the lPGi. NMDA and GABAergic receptors agonists and antagonists were microinjected into the NTS, followed by pharmacologically induced seizures. The effects of LC neurotoxic lesions caused by DSP-4, followed by NTS-NMDA receptor activation, on both tonic‒clonic seizures and postictal antinociception were also investigated. The NTS is connected to lPGi neurons that send outputs to the LC. Glutamatergic vesicles were found on dendrites and perikarya of GABAergic interneurons in the NTS. Both tonic‒clonic seizures and postictal antinociception are partially dependent on glutamatergic-mediated neurotransmission in the NTS of seizing rats in addition to the integrity of the noradrenergic system since NMDA receptor blockade in the NTS and intrathecal administration of DSP-4 decrease the postictal antinociception. The GABAA receptor activation in the NTS decreases both seizure severity and postictal antinociception. These findings suggest that glutamatergic inputs to NTS-GABAergic interneurons, in addition to ascending and descending noradrenergic pathways from the LC, are critical for the control of both seizures and postictal antinociception.

The diverse role of the raphe 5-HTergic systems in epilepsy

The raphe nuclei comprise nearly all of 5-hydroxytryptaminergic (5-HTergic) neurons in the brain and are widely acknowledged to participate in the modulation of neural excitability. "Excitability-inhibition imbalance" results in a variety of brain disorders, including epilepsy. Epilepsy is a common neurological disorder characterized by hypersynchronous epileptic seizures accompanied by many psychological, social, cognitive consequences. Current antiepileptic drugs and other therapeutics are not ideal to control epilepsy and its comorbidities. Cumulative evidence suggests that the raphe nuclei and 5-HTergic system play an important role in epilepsy and epilepsy-associated comorbidities. Seizure activities propagate to the raphe nuclei and induce various alterations in different subregions of the raphe nuclei at the cellular and molecular levels. Intervention of the activity of raphe nuclei and raphe 5-HTergic system with pharmacological or genetic approaches, deep brain stimulation or optogenetics produces indeed diverse and even contradictory effects on seizure and epilepsy-associated comorbidities in different epilepsy models. Nevertheless, there are still many open questions left, especially regarding to the relationship between 5-HTergic neural circuit and epilepsy. Understanding of 5-HTergic network in a circuit- and molecule-specific way may not only be therapeutically relevant for increasing the drug specificity and precise treatment in epilepsy, but also provide critical hints for other brain disorders with abnormal neural excitability. In this review we focus on the roles of the raphe 5-HTergic system in epilepsy and epilepsy-associated comorbidities. Besides, further perspectives about the complexity and diversity of the raphe nuclei in epilepsy are also addressed.

Nicotinic and muscarinic cholinergic receptors are recruited by acetylcholine-mediated neurotransmission within the locus coeruleus during the organisation of post-ictal antinociception

Post-ictal antinociception is characterised by an increase in the nociceptive threshold that accompanies tonic and tonic-clonic seizures (TCS). The locus coeruleus (LC) receives profuse cholinergic inputs from the pedunculopontine tegmental nucleus. Different concentrations (1μg, 3μg and 5μg/0.2μL) of the muscarinic cholinergic receptor antagonist atropine and the nicotinic cholinergic receptor antagonist mecamylamine were microinjected into the LC of Wistar rats to investigate the role of cholinergic mechanisms in the severity of TCS and the post-ictal antinociceptive response. Five minutes later, TCS were induced by systemic administration of pentylenetetrazole (PTZ) (64mg/kg). Seizures were recorded inside the open field apparatus for an average of 10min. Immediately after seizures, the nociceptive threshold was recorded for 130min using the tail-flick test. Pre-treatment of the LC with 1μg, 3μg and 5μg/0.2μL concentrations of both atropine and mecamylamine did not cause a significant effect on seizure severity. However, the same treatments decreased the post-ictal antinociceptive phenomenon. In addition, mecamylamine caused an earlier decrease in the post-ictal antinociception compared to atropine. These results suggest that muscarinic and mainly nicotinic cholinergic receptors of the LC are recruited to organise tonic-clonic seizure-induced antinociception.