Immunoreactivity for the group III receptor subtype mGluR4a in the visual layers of the rat superior colliculus

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.

Kinetics of both synchronous and asynchronous quantal release during trains of action potential-evoked EPSCs at the rat calyx of Held

We studied the kinetics of transmitter release during trains of action potential (AP)-evoked excitatory postsynaptic currents (EPSCs) at the calyx of Held synapse of juvenile rats. Using a new quantitative method based on a combination of ensemble fluctuation analysis and deconvolution, we were able to analyse mean quantal size (q) and release rate (xi) continuously in a time-resolved manner. Estimates derived this way agreed well with values of q and quantal content (M) calculated for each EPSC within the train from ensemble means of peak amplitudes and their variances. Separate analysis of synchronous and asynchronous quantal release during long stimulus trains (200 ms, 100 Hz) revealed that the latter component was highly variable among different synapses but it was unequivocally identified in 18 out of 37 synapses analysed. Peak rates of asynchronous release ranged from 0.2 to 15.2 vesicles ms(-1) (ves ms(-1)) with a mean of 2.3 +/- 0.6 ves ms(-1). On average, asynchronous release accounted for less than 14% of the total number of about 3670 +/- 350 vesicles released during 200 ms trains. Following such trains, asynchronous release decayed with several time constants, the fastest one being in the order of 15 ms. The short duration of asynchronous release at the calyx of Held synapse may aid in generating brief postsynaptic depolarizations, avoiding temporal summation and preserving action potential timing during high frequency bursts.

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.