Modulation of calcium channels by group I and group II metabotropic glutamate receptors in dentate gyrus neurons from patients with temporal lobe epilepsy

N-Type Ca Channel in Epileptic Syndromes and Epilepsy: A Systematic Review of Its Genetic Variants

N-type voltage-gated calcium channel controls the release of neurotransmitters from neurons. The association of other voltage-gated calcium channels with epilepsy is well-known. The association of N-type voltage-gated calcium channels and pain has also been established. However, the relationship between this type of calcium channel and epilepsy has not been specifically reviewed. Therefore, the present review systematically summarizes existing publications regarding the genetic associations between N-type voltage-dependent calcium channel and epilepsy.

mGluR1 enhances efferent inhibition of inner hair cells in the developing rat cochlea

KEY POINTS

Spontaneous activity of the sensory inner hair cells shapes maturation of the developing ascending (afferent) auditory system before hearing begins. Just before the onset of hearing, descending (efferent) input from cholinergic neurons originating in the brainstem inhibit inner hair cell spontaneous activity and may further refine maturation. We show that agonist activation of the group I metabotropic glutamate receptor mGluR1 increases the strength of this efferent inhibition by enhancing the presynaptic release of acetylcholine. We further show that the endogenous release of glutamate from the inner hair cells may increase the strength of efferent inhibition via the activation of group I metabotropic glutamate receptors. Thus, before the onset of hearing, metabotropic glutamate signalling establishes a local negative feedback loop that is positioned to regulate inner hair cell excitability and refine maturation of the auditory system.

ABSTRACT

Just before the onset of hearing, the inner hair cells (IHCs) receive inhibitory efferent input from cholinergic medial olivocochlear (MOC) neurons originating in the brainstem. This input may serve a role in the maturation of the ascending (afferent) auditory system by inhibiting spontaneous activity of the IHCs. To investigate the molecular mechanisms regulating these IHC efferent synapses, we combined electrical stimulation of the efferent fibres with patch clamp recordings from the IHCs to measure efferent synaptic strength. By examining evoked responses, we show that activation of metabotropic glutamate receptors (mGluRs) by general and group I-specific mGluR agonists enhances IHC efferent inhibition. This enhancement is blocked by application of a group I mGluR1-specific antagonist, indicating that enhancement of IHC efferent inhibition is mediated by group I mGluRs and specifically by mGluR1s. By comparing spontaneous and evoked responses, we show that group I mGluR agonists act presynaptically to increase neurotransmitter release without affecting postsynaptic responsiveness. Moreover, endogenous glutamate released from the IHCs also enhances IHC efferent inhibition via the activation of group I mGluRs. Finally, immunofluorescence analysis indicates that the efferent terminals are sufficiently close to IHC glutamate release sites to allow activation of mGluRs on the efferent terminals by glutamate spillover. Together, these results suggest that glutamate released from the IHCs activates group I mGluRs (mGluR1s), probably present on the efferent terminals, which, in turn, enhances release of acetylcholine and inhibition of the IHCs. Thus, mGluRs establish a local negative feedback loop positioned to regulate IHC activity and maturation of the ascending auditory system in the developing cochlea.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Blockade of group II metabotropic glutamate receptors produces hyper-locomotion in cocaine pre-exposed rats by interactions with dopamine receptors

It was previously reported that blockade of group II metabotropic glutamate receptors (mGluRs) produces hyper-locomotion in rats previously exposed to amphetamine, indicating that group II mGluRs are well positioned to modulate the expression of behavioral sensitization by amphetamine. The present study further examined the locomotor activating effects of specific blockade of these receptors after cocaine pre-exposures. First, rats were pre-exposed to seven daily injections of cocaine (15mg/kg, IP). When challenged the next day with an injection of either saline or the group II mGluR antagonist LY341495 (0.5, 1.0 or 2.5mg/kg, IP), they produced hyper-locomotor activity, measured by infrared beam interruptions, to LY341495 compared to saline in a dose-dependent manner. Second, rats were pre-exposed to either saline or seven daily injections of cocaine (15mg/kg, IP). Three weeks later, when they were challenged with an injection of either saline or LY341495 (1.0mg/kg, IP), only rats pre-exposed to cocaine produced hyper-locomotor activity to LY341495 compared to saline. These effects, however, were not present when dopamine D1 (SCH23390; 5 or 10microg/kg), but not D2 (eticlopride; 10 or 50microg/kg), receptor antagonist was pre-injected, indicating that this cocaine-induced hyper-locomotor activity to LY341495 may be mediated in dopamine D1 receptor-dependent manner. These results suggest that group II mGluRs may be adapted to interact with dopaminergic neuronal signaling in mediating the sensitized locomotor activity produced by repeated cocaine pre-exposures.

Blockade of group II metabotropic glutamate receptors in the nucleus accumbens produces hyperlocomotion in rats previously exposed to amphetamine

The neurotransmitter glutamate is known to participate in both the induction and expression of locomotor sensitization by psychostimulant drugs like amphetamine. Previously, it was reported that subtype nonselective blockade of metabotropic glutamate receptors (mGluRs) in the nucleus accumbens (NAcc) produces hyperlocomotion in rats previously exposed to amphetamine. The present experiments examined whether group II mGluRs may contribute to this effect. Rats in different groups were administered five injections of either saline or amphetamine (1.0 mg/kg, i.p.), one injection given every third day. Two weeks later, they were tested for 2 h following an injection of either saline or the group II mGluR antagonist LY341495. In one experiment, test injections were administered systemically (saline or LY341495, 1.0 mg/kg, i.p.). Rats previously exposed to amphetamine showed a greater locomotor response to LY341495 on the test compared to controls previously exposed to saline. This hyperlocomotor response was absent in rats tested with a combination of LY341495 and the group II mGluR agonist LY379268 (1.0 mg/kg, i.p.). In a second experiment, different rats were tested following microinjections into the NAcc (saline or LY341495, 0.1, 10 or 100 microg/0.5 microl/side). Again, rats previously exposed to amphetamine showed a greater dose-dependent locomotor response to LY341495 on the test relative to saline-exposed controls. Locomotor activity in saline-exposed rats challenged with LY341495 did not differ from that observed in rats previously exposed and tested with saline in either experiment. These results indicate that group II mGluRs, particularly those found in the NAcc, are well positioned to modulate the expression of locomotor sensitization by amphetamine.

Activation of group II mGlu receptors inhibits voltage-gated Ca2+ currents in myenteric neurons

The enteric nervous system (ENS) contains functional ionotropic and group I metabotropic glutamate (mGlu) receptors. In this study, we determined whether enteric neurons express group II mGlu receptors and the effects of mGlu receptor activation on voltage-gated Ca(2+) currents in these cells. (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC), a group II mGlu receptor agonist, reversibly suppressed the Ba(2+) current in myenteric neurons isolated from the guinea pig ileum. Significant inhibition was also produced by L-glutamate and the group II mGlu receptor agonists, (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I), with a rank order potency of 2R,4R-APDC > DCG-IV > L-glutamate > L-CCG-I, and was reduced by the group II mGlu receptor antagonist LY-341495. Pretreatment of neurons with pertussis toxin (PTX) reduced the action of mGlu receptor agonists, suggesting participation of G(i)/G(o) proteins. Finally, omega-conotoxin GVIA blocked current suppression by DCG-IV, suggesting modulation of N-type calcium channels. mGlu2/3 receptor immunoreactivity was displayed by neurons in culture and in the submucosal and myenteric plexus of the ileum. A subset of these cells displayed a glutamatergic phenotype as shown by the expression of vesicular glutamate transporter 2. These results provide the first evidence for functional group II mGlu receptors in the ENS and show that these receptors are PTX sensitive and negatively coupled to N-type calcium channels. Inhibition of N-type calcium channels produced by activation of group II mGlu receptors may modulate enteric neurotransmission.

Group II metabotropic glutamate receptors modulate extracellular glutamate in the nucleus accumbens

The regulation of extracellular glutamate in the nucleus accumbens by group II metabotropic glutamate receptors (mGluR2/3) was examined in vivo. Stimulation of mGluR2/3 with 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or N-acetylaspartylglutamate reduced extracellular glutamate levels. Conversely, blockade of mGluR2/3 by LY143495 or (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate, an effect antagonized by the coadministration of APDC. These effects likely involve both vesicular and nonvesicular glutamate, because the increase in glutamate by APICA or the decrease by APDC was prevented by blocking N-type calcium channels and the release of glutamate after potassium-induced membrane depolarization was antagonized by APDC. In addition, blockade of the cystine-glutamate exchange, a major nonvesicular source of extracellular glutamate, by (S)-4-carboxyphenylglycine blocked the effects induced by either APDC or APICA. However, blockade of Na(+) channels by tetrodotoxin or Na(+)-dependent glutamate transporters by DL-threo-beta-benzyloxyaspartate failed to affect the alterations in extracellular glutamate by APICA or APDC, respectively. Group II mGluRs are G(i)-coupled and coperfusion with the cAMP-dependent protein kinase (PKA) activator Sp-cAMPS blocked the reduction in glutamate by APDC and the PKA inhibitor Rp-cAMPS prevented the elevation in glutamate by APICA. Taken together, these data support three conclusions: 1) group II mGluRs regulate both vesicular and nonvesicular release of glutamate in the nucleus accumbens, 2) there is tonic in vivo stimulation of mGluR2/3 by endogenous glutamate, and 3) modulation of group II mGluRs of extracellular glutamate is Ca(2+)- and PKA-dependent.

Repeated cocaine administration attenuates group I metabotropic glutamate receptor-mediated glutamate release and behavioral activation: a potential role for Homer

The present study aimed to characterize a functional role for group I metabotropic glutamate receptors (mGluRs) in the nucleus accumbens and the capacity of repeated cocaine to elicit long-term changes in group I mGluR function. Reverse dialysis of the group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) into the nucleus accumbens resulted in an increase in extracellular glutamate levels that was mediated by the mGluR1 subtype and depended on voltage-dependent Na(+) and Ca(2+) conductance. At 3 weeks after discontinuing 1 week of daily cocaine injections, the capacity of DHPG to induce glutamate release was markedly reduced. Similarly, DHPG induced an mGluR1-dependent increase in locomotor activity after microinjection into the nucleus accumbens that was significantly blunted 3 weeks after repeated cocaine administration. Signaling through group I mGluRs is regulated, in part, by Homer proteins, and it was found that the blunting of group I mGluR-induced glutamate release and motor activity after repeated cocaine was associated with a reduction in Homer1b/c protein that was selective for the medial nucleus accumbens. These data show that repeated cocaine produces an enduring inhibition of the neurochemical and behavioral consequences of stimulating mGluR1 that is accompanied by changes in the mGluR scaffolding apparatus.

The process of epileptogenesis: a pathophysiological approach

Several recent advances have contributed to our understanding of the processes associated with mesial temporal lobe epilepsy in humans and in experimental animal models. Common pathological features between the human condition and the animal models may indicate a fundamental involvement of the given pathology in the process of epileptogenesis.