Activation of Group II and Group III metabotropic glutamate receptors by endogenous ligand(s) and the modulation of synaptic transmission in the superficial superior colliculus

mGlu3 receptor regulates microglial cell reactivity in neonatal rats

BACKGROUND

Perinatal inflammation is a key factor of brain vulnerability in neonates born preterm or with intra-uterine growth restriction (IUGR), two leading conditions associated with brain injury and responsible for neurocognitive and behavioral disorders. Systemic inflammation is recognized to activate microglia, known to be the critical modulators of brain vulnerability. Although some evidence supports a role for metabotropic glutamate receptor 3 (mGlu3 receptor) in modulation of neuroinflammation, its functions are still unknown in the developing microglia.

METHODS

We used a double-hit rat model of perinatal brain injury induced by a gestational low-protein diet combined with interleukin-1β injections (LPD/IL-1β), mimicking both IUGR and prematurity-related inflammation. The effect of LPD/IL-1β on mGlu3 receptor expression and the effect of mGlu3 receptor modulation on microglial reactivity were investigated using a combination of pharmacological, histological, and molecular and genetic approaches.

RESULTS

Exposure to LPD/IL-1β significantly downregulated Grm3 gene expression in the developing microglia. Both transcriptomic analyses and pharmacological modulation of mGlu3 receptor demonstrated its central role in the control of inflammation in resting and activated microglia. Microglia reactivity to inflammatory challenge induced by LPD/IL-1β exposure was reduced by an mGlu3 receptor agonist. Conversely, both specific pharmacological blockade, siRNA knock-down, and genetic knock-out of mGlu3 receptors mimicked the pro-inflammatory phenotype observed in microglial cells exposed to LPD/IL-1β.

CONCLUSIONS

Overall, these data show that Grm3 plays a central role in the regulation of microglial reactivity in the immature brain. Selective pharmacological activation of mGlu3 receptors may prevent inflammatory-induced perinatal brain injury.

(2R,6R)-hydroxynorketamine exerts mGlu2 receptor-dependent antidepressant actions

Currently approved antidepressant drugs often take months to take full effect, and ∼30% of depressed patients remain treatment resistant. In contrast, ketamine, when administered as a single subanesthetic dose, exerts rapid and sustained antidepressant actions. Preclinical studies indicate that the ketamine metabolite (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a rapid-acting antidepressant drug candidate with limited dissociation properties and abuse potential. We assessed the role of group II metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) in the antidepressant-relevant actions of (2R,6R)-HNK using behavioral, genetic, and pharmacological approaches as well as cortical quantitative EEG (qEEG) measurements in mice. Both ketamine and (2R,6R)-HNK prevented mGlu_2/3 receptor agonist (LY379268)-induced body temperature increases in mice lacking the Grm3, but not Grm2, gene. This action was not replicated by NMDA receptor antagonists or a chemical variant of ketamine that limits metabolism to (2R,6R)-HNK. The antidepressant-relevant behavioral effects and 30- to 80-Hz qEEG oscillation (gamma-range) increases resultant from (2R,6R)-HNK administration were prevented by pretreatment with an mGlu_2/3 receptor agonist and absent in mice lacking the Grm2, but not Grm3 ^-/-, gene. Combined subeffective doses of the mGlu_2/3 receptor antagonist LY341495 and (2R,6R)-HNK exerted synergistic increases on gamma oscillations and antidepressant-relevant behavioral actions. These findings highlight that (2R,6R)-HNK exerts antidepressant-relevant actions via a mechanism converging with mGlu₂ receptor signaling and suggest enhanced cortical gamma oscillations as a marker of target engagement relevant to antidepressant efficacy. Moreover, these results support the use of (2R,6R)-HNK and inhibitors of mGlu₂ receptor function in clinical trials for treatment-resistant depression either alone or in combination.

GABA mechanisms of the nucleus of the solitary tract regulates the cardiovascular and sympathetic effects of moxonidine

The antihypertensive drugs moxonidine and clonidine are α2-adrenoceptor and imidazoline (I1) agonists. Previous results from our laboratory have shown that moxonidine can act in the commissural nucleus of the solitary tract (commNTS). In addition, some studies have shown that GABA or glutamate receptor blockade in the RVLM blunted the hypotension produced by these antihypertensive agents in spontaneously hypertensive rats. Therefore, in the present study we verify whether the cardiovascular and sympathetic effects produced by moxonidine in the commNTS are dependent on GABAergic or glutamatergic mechanisms. Mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (sSNA) were recorded in urethane-anesthetized, and artificially-ventilated male Wistar rats (250-350 g). Injection of the GABAA antagonist bicuculline (25 pmol/50 nL) into the commNTS reduced the hypotension as well as the sympathoinhibition elicited by moxonidine. Prior injection of the glutamate receptor antagonist kynurenic acid (2.5 nmol/50 nL) into the commNTS was not effective in reducing the hypotension and sympathoinhibition elicited by moxonidine. Therefore, we conclude that the hypotensive and sympathoinhibitory effects elicited by microinjection of moxonidine into the commNTS are dependent on GABA receptors, but not ionotropic glutamate receptors.

Regulation of Hypothalamic Presympathetic Neurons and Sympathetic Outflow by Group II Metabotropic Glutamate Receptors in Spontaneously Hypertensive Rats

Increased glutamatergic input in the hypothalamic paraventricular nucleus (PVN) plays an important role in the development of hypertension. Group II metabotropic glutamate receptors are expressed in the PVN, but their involvement in regulating synaptic transmission and sympathetic outflow in hypertension is unclear. Here, we show that the group II metabotropic glutamate receptors agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) produced a significantly greater reduction in the frequency of spontaneous and miniature excitatory postsynaptic currents and in the amplitude of electrically evoked excitatory postsynaptic currents in retrogradely labeled spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs) than in normotensive control rats. DCG-IV similarly decreased the frequency of GABAergic inhibitory postsynaptic currents of labeled PVN neurons in the 2 groups of rats. Strikingly, DCG-IV suppressed the firing of labeled PVN neurons only in SHRs. DCG-IV failed to inhibit the firing of PVN neurons of SHRs in the presence of ionotropic glutamate receptor antagonists. Lowering blood pressure with celiac ganglionectomy in SHRs normalized the DCG-IV effect on excitatory postsynaptic currents to the same level seen in control rats. Furthermore, microinjection of DCG-IV into the PVN significantly reduced blood pressure and sympathetic nerve activity in SHRs. Our findings provide new information that presynaptic group II metabotropic glutamate receptor activity at the glutamatergic terminals increases in the PVN in SHRs. Activation of group II metabotropic glutamate receptors in the PVN inhibits sympathetic vasomotor tone through attenuation of increased glutamatergic input and neuronal hyperactivity in SHRs.

Changes in the modulation of retinocollicular transmission through group III mGluRs long after an increase in intraocular pressure in a rat model of glaucoma

Metabotropic glutamate receptors (mGluRs) have been shown to be involved in the modulation of retinocollicular neurotransmission. In glaucoma, retinal ganglion cells (RGCs) degenerate, which may have an implication on this transmission as the superior colliculus is their major central target in the much-used rodent models of the disease. We have investigated this using an in vitro slice preparation of the superior colliculus by eliciting field excitatory postsynaptic potentials (fEPSPs) through optic tract stimulation in a rat ocular hypertension model of glaucoma. Application of the group III mGluR agonist L-AP4 reduced the peak amplitude of the fEPSP in superior colliculus slices through presynaptic mechanisms as previously shown in our lab. At 3 and 16 weeks after surgery, there were no significant differences in the effect of L-AP4 on fEPSP peak amplitude in the superior colliculus slices receiving input from the glaucomatous eyes [elevated intraocular pressure (IOP)] compared to those with input from the unoperated eyes (normal IOP). However, at 32 weeks, the fEPSP peak amplitude was reduced to a significantly greater degree during L-AP4 application in the elevated IOP slices compared to normal IOP slices. At all time points, there were no significant changes in the baseline amplitudes of fEPSPs or the stimulus intensities required to evoke fEPSPs. These results suggest that the modulation of synaptic transmission through group III mGluRs on RGC terminals to the superior colliculus is changed at later stages due to RGC degeneration through IOP elevation. These changes may be compensatory changes possibly through plasticity in the RGC terminals of surviving cells, which may be due to increases in the numbers of group III mGluRs. This result may have implications on further treatment studies carried out using these models of glaucoma as changes in the central visual system may need to be considered along with the retinal changes that occur.

Modulation of GABAergic inhibition in the rat superior colliculus by a presynaptic group II metabotropic glutamate receptor

Previous work has indicated that metabotropic glutamate receptors (mGluRs) modulate visual responses of superior colliculus (SC) neurones in vivo in a variety of ways, in a manner that can be dependent upon visual stimulus properties. How this occurs remains unclear. In this study we aimed to determine how activation of mGluR2 and mGluR3 receptors (Group II) might modulate visual responses, by using field potential and whole-cell patch clamp recording techniques in rat SC slice. Stimulation within the superficial layers of the SC, in the presence of ionotropic glutamate receptor antagonists, evoked IPSCs that were blocked by bicuculline indicating that they are mediated via GABAA receptors. It is likely that these IPSCs were of heterogeneous origin as they showed substantial variation in paired-pulse behaviour. Nevertheless, activation of Group II mGluRs with the group-selective agonist LY354740 (300 nM, bath application) resulted in a reduction of these IPSCs (to 56% of control amplitude), and this was associated with a decrease in paired-pulse depression. At the same concentration, LY354740 did not reduce the EPSC or field-EPSP evoked by stimulation of the retinal input to the SC. The effects of LY354740 on IPSCs were not mimicked by the mGluR3-selective agonist N-acetyl-aspartyl-glutamate (NAAG, 200-500 microM). Stimulation of IPSCs with trains of impulses (10 at 20 Hz) in order to mimic natural activation patterns resulted in sequences of IPSCs that were reduced in amplitude towards the end of the stimulus train. Application of the Group II antagonist LY341495 (100 nM) under these conditions resulted in an increase in later IPSCs in a third of neurones tested. These findings indicate that mGluR2 (but not mGluR3) can selectively modulate GABAergic inhibition in SC, probably via a presynaptic mechanism. Furthermore, these receptors may be activated by synaptically released transmitter during patterns of activation similar to those seen during visual processing. Thus mGluR2 receptors could have a function in activity-dependent modulation of inhibitory processing during visual responses.

Effects of mGlu2 or mGlu3 receptor deletions on mGlu2/3 receptor agonist (LY354740)-induced brain c-Fos expression: Specific roles for mGlu2 in the amygdala and subcortical nuclei, and mGlu3 in the hippocampus

LY354740 is a potent and selective mGlu2/3 receptor agonist with activity in models of psychiatric disorders (anxiety, psychosis), and early clinical studies in anxiety patients. However, the specific receptor subtypes and brain regions which mediate mGlu2/3 receptor agonist pharmacology/efficacy are not well understood. Here we investigate the effects of deleting mGlu2 or mGlu3 receptors on basal and LY354740-regulated c-Fos expression in mouse brain using mGlu2 or mGlu3 knockout mice. Consistent with our earlier findings, LY354740 administration (20 mg/kg, i.p.) to wild-type mice increased c-Fos expression in specific limbic (central amygdala, bed nucleus of the stria terminalis, midline thalamic nuclei) and non-limbic (thalamic dorsolateral geniculate nucleus, superior colliculus, Edinger-Westphal) structures, while modestly suppressing hippocampal c-Fos expression. The LY354740-induced increases in c-Fos expression in all the above regions were abolished by mGlu2, but not mGlu3, receptor deletion. Interestingly, basal c-Fos expression was significantly increased in the hippocampus of mGlu3, but not mGlu2, receptor knockouts compared to wild-type mice. Moreover, this increase was not suppressed by LY354740, such that in the CA3 region LY354740 now increased c-Fos expression in the mGlu3 knockouts. These results demonstrate that the LY354740-induced increases of c-Fos expression in specific brain regions, including the central and extended amygdala are specifically linked to mGlu2 receptors, and LY354740 suppressions of neuronal activity in the hippocampus are linked to mGlu3 receptors.

Modulation of excitatory synaptic transmission by endogenous glutamate acting on presynaptic group II mGluRs in rat substantia nigra compacta

Excitatory synaptic inputs from the subthalamic nucleus (STN) have been proposed to underlie burst firing of substantia nigra pars compacta (SNc) dopamine (DA) neurons in Parkinson's disease. Given the potential importance of the STN-SNc synapse in health and disease, our goal was to study how transmission at this synapse is regulated. We tested the hypothesis that neurotransmission at STN-SNc synapses is tonically inhibited by endogenous glutamate acting on presynaptic group II metabotropic glutamate receptors (mGluRs). By using whole-cell recording techniques in brain slices, we examined the effect of LY341495, a mGluR antagonist that is most potent at group II mGluRs, on excitatory postsynaptic currents (EPSCs) that either were evoked in SNc DA neurons by stimulation of the STN or were spontaneously occurred in the presence of tetrodotoxin (miniature EPSCs; mEPSCs). LY341495 increased the evoked EPSC amplitude and mEPSC frequency without changing mEPSC amplitude. In contrast, the group III mGluR antagonist UBP1112 failed to increase the evoked EPSC amplitude. An elevation of extracellular glutamate concentration by a glutamate transporter inhibitor, TBOA, suppressed the evoked EPSCs. LY341495, but not UBP1112, partially reversed the TBOA action. The modulations of EPSCs by TBOA and LY341495 were associated with changes in paired-pulse facilitation ratio. Furthermore, TBOA decreased mEPSC frequency, which was partially reversed by LY341495, without affecting mEPSC amplitude. The results indicate that presynaptic group II mGluRs at STN-SNc synapses appear to be partially activated by a basal level of extracellular glutamate and able to sense the change in extracellular glutamate concentration, subsequently modulating synaptic glutamate release.

Modulation of retino-collicular transmission by Group III metabotropic glutamate receptors at different ages during development

Group III metabotropic glutamate receptors (especially mGlu4, mGlu7, mGlu8) are thought to be involved in modulating visual processing in the adult superior colliculus, a major termination site of retinal input in the rodent brain. We have investigated this role by making field EPSP recordings in response to optic tract stimulation in superior colliculus slices taken from rats aged from P14 to P180. Application of the Group III agonist L-AP4 at a concentration (10 microM) effective to activate mGlu4 and mGlu8 receptors, but not mGlu7 receptors, resulted in reductions of the field EPSP in all ages, although the effect was greatest in slices taken from P14 rats. Increasing the L-AP4 concentration to 100 microM so as to also activate mGlu7 receptors resulted in further field EPSP reductions. Similar reductions were seen in the combined presence of the GABA antagonists picrotoxin and CGP55845A, indicating a lack of involvement of GABAergic mechanisms in the action of L-AP4. Pairing of optic tract stimuli (20 ms separation) resulted in paired-pulse depression at all ages. L-AP4 was found to reduce paired-pulse depression at both 10 microM and 100 microM in slices from all ages of rat. The results of this study suggest that mGlu4/mGlu8 and mGlu7 receptors modulate retino-tectal transmission via a presynaptic mechanism, and that these effects are greatest in young animals. This is the first demonstration of a functional change in Group III receptor effect with aging, and this would be consistent with developmental regulation of these receptors.

Melatonin decreases calcium levels in retinotectal axons of Xenopus laevis by indirect activation of group III metabotropic glutamate receptors

Melatonin is a neuromodulator that binds to receptors in the retinotectal laminae of the amphibian optic tectum. The effect of melatonin on calcium dynamics in Xenopus retinotectal axons was investigated by imaging retinotectal axons labeled with the fluorescent indicator Fluo-4. Melatonin exerted an inhibitory influence on depolarization-evoked calcium increases, and the melatonin receptor antagonist 4-P-PDOT blocked this effect. Blockade of group III metabotropic receptors (mGluRs) counteracted the effect of melatonin on retinotectal axons. Application of the group II/group III mGluR antagonist MSPG or the group III-selective antagonist MSOP abolished the effect of melatonin. Conversely, this effect was not significantly affected by the group I mGluR antagonist LY367385 nor by EGLU or LY341495 at concentrations that specifically inhibit group II mGluRs. Furthermore, a higher concentration of LY341495 that affects group III mGluRs inhibited the effect of melatonin. The data therefore support the hypothesis that, in retinotectal axons, melatonin reduces cAMP levels, thereby relieving PKA-induced inhibition of group III mGluRs; the newly activated mGluRs in turn inhibit voltage-sensitive calcium channels, leading to a decrease in Ca2+ concentrations. The role of GABA(C) receptors in retinotectal responses was also evaluated. GABA(C) receptor antagonists did not block the effects of melatonin but instead were additive. Moreover, while other studies have shown that in Xenopus tectal cells, GABA(C) receptors mediate inhibition, in retinotectal axons, the opposite appears to occur since depolarization-evoked calcium rises in retinotectal axons were inhibited by GABA(C) receptor blockade. This result suggests that activation of GABA(C) receptors produces an increase in the synaptic excitability of retinotectal axon terminals.