Neuroanatomical and neuropharmacological approaches to postictal antinociception-related prosencephalic neurons: 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 regulation of the pedunculopontine tegmental nucleus in sleep-wake states

The pedunculopontine tegmental nucleus (PPTg) plays a vital role in sleep/wake states. There are three main kinds of heterogeneous neurons involved: cholinergic, glutamatergic, and gamma-aminobutyric acidergic (GABAergic) neurons. However, the precise roles of cholinergic, glutamatergic and GABAergic PPTg cell groups in regulating sleep-wake are unknown. Recent work suggests that the cholinergic, glutamatergic, and GABAergic neurons of the PPTg may activate the main arousal-promoting nucleus, thus exerting their wakefulness effects. We review the related projection pathways and functions of various neurons of the PPTg, especially the mechanisms of the PPTg in sleep-wake, thus providing new perspectives for research of sleep-wake mechanisms.

Effect of electrical and chemical (activation versus inactivation) stimulation of the infralimbic division of the medial prefrontal cortex in rats with chronic neuropathic pain

Neuropathic pain (NP) represents a complex disorder with sensory, cognitive, and emotional symptoms. The medial prefrontal cortex (mPFC) takes critical regulatory roles and may change functionally and morphologically during chronic NP. There needs to be a complete understanding of the neurophysiological and psychopharmacological bases of the NP phenomenon. This study aimed to investigate the participation of the infralimbic division (IFL) of the mPFC in chronic NP, as well as the role of the N-methyl-D-aspartic acid receptor (NMDAr) in the elaboration of chronic NP. Male Wistar rats were submitted to the von Frey and acetone tests to assess mechanical and cold allodynia after 21 days of chronic constriction injury (CCI) of the sciatic nerve or Sham-procedure ("false operated"). Electrical neurostimulation of the IFL/mPFC was performed by low-frequency stimuli (20 μA, 100 Hz) applied for 15 s by deep brain stimulation (DBS) device 21 days after CCI. Either cobalt chloride (CoCl2 at 1.0 mM/200 nL), NMDAr agonist (at 0.25, 1.0, and 2.0 nmol/200 nL) or physiological saline (200 nL) was administered into the IFL/mPFC. CoCl2 administration in the IFL cortex did not alter either mechanical or cold allodynia. DBS stimulation of the IFL cortex decreased mechanical allodynia in CCI rats. Chemical stimulation of the IFL cortex by an NMDA agonist (at 2.0 nmol) decreased mechanical allodynia. NMDA at any dose (0.25, 1.0, and 2.0 nmol) reduced the flicking/licking duration in the cold test. These findings suggest that the IFL/mPFC and the NMDAr of the neocortex are involved in attenuating chronic NP in rats.

Neostriatum neuronal TRPV1-signalling mediates striatal anandamide at high concentration facilitatory influence on neostriato-nigral dishinhibitory GABAergic connections

RATIONALE

Several lines of evidence have demonstrated that the cannabinoid type 1 receptor (CB₁) is found in the caudate nucleus and putamen (CPu) in addition to the substantia nigra pars reticulata (SNpr). Here, we investigated the role of endocannabinoid neuromodulation of striato-nigral disinhibitory projections on the activity of nigro-collicular GABAergic pathways that control the expression of unconditioned fear-related behavioural responses elicited by microinjections of the GABA_A receptor selective antagonist bicuculline (BIC) in the deep layers of the superior colliculus (dlSC).

METHODS

Fluorescent neural tract tracers were deposited in either CPu or in SNpr. Wistar rats received injection of vehicle, anandamide (AEA), either at low (50 pmol) or high (100 pmol) concentrations in CPu followed by bicuculline microinjections in dlSC.

RESULTS

Connections between CPu, the SNpr and dlSC were demonstrated. The GABA_A receptor blockade in dlSC elicited panic-like behaviour. AEA at the lowest concentration caused a panicolytic-like effect that was antagonised by the CPu pretreatment with AM251 at 100 pmol. AEA at the highest concentration caused a panicogenic-like effect that was antagonised by the CPu pretreatment with 6-iodonordihydrocapsaicin (6-I-CPS) at different concentrations (0.6, 6, 60 nmol).

CONCLUSION

These findings suggest that while pre-synaptic CB1-signalling subserves an indirect facilitatory effect of AEA on striato-nigral pathways causing panicolytic-like responses through midbrain tectum enhanced activity, post-synaptic TRPV₁-signalling in CPu mediates AEA direct activation of striato-nigral disinhibitory pathways resulting in increasing dlSC neurons activity and a panicogenic-like response. All these actions seem to depend on the interface with the nigro-collicular inhibitory GABAergic pathways.

D2-like receptor activation by intranasal dopamine attenuates fear responses induced by electrical stimulation of the dorsal periaqueductal grey matter, but fails to reduce aversion to pit vipers and T-maze performance

BACKGROUND

Panic-like reactions elicited by electrical stimulation of the dorsal periaqueductal grey matter (ES-dPAG) seem to be regulated by dopamine (DA). We showed that DA applied intranasally (IN) increased escape-behaviour thresholds induced by ES-dPAG of rats, indicating a panicolytic-like effect.

AIMS

We investigated whether IN-DA increases escape-response thresholds induced by ES-dPAG by acting on D₂-like receptors, and whether IN-DA affects escape responses elicited by the presence of a potential predator and by open space and height of the elevated T-maze (ETM) as well as motor performance in the open field (OF) test.

METHODS

Wistar rats exposed to ES-dPAG were treated with Sulpiride (SUL, 40 mg/kg, D₂-like receptor antagonist) previously IN-DA (2 mg/kg). Independent groups of rats treated with IN-DA were submitted to prey versus snake paradigm (PSP), ETM and OF.

RESULTS

Anti-aversive effects of the IN-DA were reduced by SUL pretreatment in the ES-dPAG test. IN-DA did not affect the escape number in the PSP nor the escape latencies in the ETM as well as motor performance in the OF.

CONCLUSIONS/INTERPRETATION

The IN-DA effects in reducing unconditioned fear responses elicited by ES-dPAG seem to be mediated by D₂-like receptors. The lack of effects on panic-related responses in the ETM and PSP may be related to the possibility of avoiding the danger inherent to these models, a defence strategy not available during ES-dPAG. These findings cannot be attributed to motor performance. The decision-making responses to avoid dangerous situations can be orchestrated by supra-mesencephalic structures connected by non-dopaminergic inputs.

Awareness and current knowledge of epilepsy

Epilepsy is a severe neural disorder that affects approximately fifty million individuals globally. Despite the fact that for most of the people with epilepsy, convulsions are better controlled by current accessible antiepileptic medicines, yet there are more than 30% of individuals affected with medically intractable epilepsy and around 30-40% of all patients with epilepsy affected by many adverse reactions and convulsion resistance to the present antiepileptic drugs. Consequently, various scientists attempt to develop new strategies to treat epilepsy, for instance, to find out novel antiepileptic ingredients from traditional medicines. This work aims to present a complete summary of natural medicines prescribed as antiepileptic agents all over the world by ethnic groups and different tribes. We undertook an extensive bibliographic analysis by searching peer reviewed papers and classical textbooks and further consulting well accepted worldwide scientific databases. We carried out PubMed, EMbase and CENTRAL searches by means of terms such as "antiepileptic" and "anti-convulsant" activity of plants. Medicinal plants have been prescribed to treat epilepsy and have been recognized as antiepileptic medicines. In this review, a variety of herbs have been reviewed for thorough studies such as Cuminum cyminum, Butea monosperma, Solanum americanum, Anacyclus pyrethrum, Leonotis leonurus, Elaeocarpus ganitrus and Angelica archangelica. This paper shows that it was high time experimental studies are increased to obtain novel potential active principles from medicinal plants. Plant extracts and their chemical constituents should be further evaluated to clarify their mechanisms of action. This paper provides a solid base upon which to further investigate the clinical efficacy of medicinal plants that are both currently prescribed by physicians as traditional antiepileptic agents, but also could be effective as an antiepileptic drug with further research and study.

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.

Intrinsic connections within the pedunculopontine tegmental nucleus are critical to the elaboration of post-ictal antinociception

This study investigated the intrinsic connections of a key-structure of the endogenous pain inhibitory system, the pedunculopontine tegmental nucleus (PPTN), in post-ictal antinociceptive process through synaptic inactivation of the PPTN with cobalt chloride. Male Wistar rats (n = 6 or 7 per group), weighing 250-280 g, had the tail-flick baseline recorded and were submitted to a stereotaxic surgery for the introduction of a guide-cannula aiming at the PPTN. After 5 days of postoperative recovery, cobalt chloride (1 mM/0.2 µL) or physiological saline (0.2 µL) were microinjected into the PPTN and after 5 min, the tail-withdrawal latency was measured again at 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, and 120 min after seizures evoked by intraperitoneal injection of pentylenetetrazole (64 mg/kg). The synaptic inactivation of PPTN decreased the post-ictal antinociceptive phenomenon, suggesting the involvement of PPTN intrinsic connections in the modulation of pain, during tonic-clonic seizures. These results showed that the PPTN may be crucially involved in the neural network that organizes the post-ictal analgesia.

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5-HT₂A and 5-HT₂C serotonergic receptors in post-ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5-hydroxytryptamine (5-HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post-ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure-induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic-clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA-mediated Cl(-) influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl2 /0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post-ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5-HT2A/2C receptors, decreased tonic-clonic seizure-induced antinociception. Both dmPAG and vlPAG treatment with either the 5-HT2A receptor selective antagonist R-96544 (10 nM/0.2 µL), or the 5-HT2C receptors selective antagonist RS-102221 (0.15 µg/0.2 µL) also decrease post-ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5-HT2A and 5-HT2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post-ictal antinociception.