Immunocytochemical and pharmacological characterization of metabotropic glutamate receptors of the vestibular end organs in the frog

Pharmacological characterization and differential expression of NMDA receptor subunits in the chicken vestibular system during development

Previous studies demonstrated that in vitro preparations of the isolated vestibular system of diverse animal species still exhibit stable resting electrical activity and mechanically evoked synaptic transmission between hair cells and primary afferent endings. However, there are no reports related to their neurodevelopment. Therefore, this research aimed to examine whether NMDA receptors mediate these electrical signals in an isolated preparation of the chicken vestibular system at three developmental stages, E15, E18, and E21. We found that the spontaneous and mechanically evoked discharges from primary afferents of the posterior semicircular canal were modulated by agonists NMDA and glycine, but not by the agonist d-serine applied near the synapses. Moreover, the individually applied by bath perfusion of three NMDA receptor antagonists (MK-801, ifenprodil, and 2-naphthoic acid) or high Mg²⁺ decreased the resting discharge rate, the NMDA response, and the discharge rate of mechanically evoked activity from these primary afferents. Furthermore, we found that the vestibular ganglion shows a stage-dependent increase in the expression of NMDA receptor subunits GluN1, GluN2 (A-C), and GluN3 (A-B), being greater at E21, except for GluN2D, which was inversely related to the developmental stage. However, in the crista ampullaris, the expression pattern remained constant throughout development. This could suggest the possible existence of presynaptic NMDA receptors. Our results highlight that although the NMDA receptors are functionally active at the early embryonic stages of the vestibular system, NMDA and glycine reach their mature functionality to increase NMDA responses close to hatching (E21).

Molecular Characterization and Expression Analysis of Putative Class C (Glutamate Family) G Protein-Coupled Receptors in Ascidian Styela clava

In this study, we performed the genome-wide domain analysis and sequence alignment on the genome of Styela clava, and obtained a repertoire of 204 putative GPCRs, which exhibited a highly reduced gene number compared to vertebrates and cephalochordates. In this repertoire, six Class C GPCRs, including four metabotropic glutamate receptors (Sc-GRMs), one calcium-sensing receptor (Sc-CaSR), and one gamma-aminobutyric acid (GABA) type B receptor 2-like (Sc-GABABR2-like) were identified, with the absence of type 1 taste and vomeronasal receptors. All the Sc-GRMs and Sc-CaSR contained the typical "Venus flytrap" and cysteine-rich domains required for ligand binding and subsequent propagation of conformational changes. In swimming larvae, Sc-grm3 and Sc-casr were mainly expressed at the junction of the sensory vesicle and tail nerve cord while the transcripts of Sc-grm4, Sc-grm7a, and Sc-grm7b appeared at the anterior trunk, which suggested their important functions in neurotransmission. The high expression of these Class C receptors at tail-regression and metamorphic juvenile stages hinted at their potential involvement in regulating metamorphosis. In adults, the transcripts were highly expressed in several peripheral tissues, raising the possibility that S. clava Class C GPCRs might function as neurotransmission modulators peripherally after metamorphosis. Our study systematically characterized the ancestral chordate Class C GPCRs to provide insights into the origin and evolution of these receptors in chordates and their roles in regulating physiological and morphogenetic changes relevant to the development and environmental adaption.

Metabotropic glutamate receptors as targets of neuromodulatory influence of nitric oxide

A possible effect of nitric oxide (NO) on metabotropic glutamate receptor (mGluR) function in the amino acid afferent synapse was investigated in the isolated labyrinth of the frog Rana temporaria. The modification of the amplitude of responses of metabotropic glutamate receptor agonist trans-ACPD was analyzed during bath applied NO donor S-nitroso-N-acetyl-DL-penicillamine SNAP (0.1-100 μM) or nitric oxide synthase inhibitor L-NAME. It was shown that NO donor SNAP (1 μM) inhibited mGluR induced responses, and the inhibitor of NO-synthase L-NAME (100 μM) increased the amplitude of trans-ACPD evoked answers. The results suggest that NO can depress mGluR function due to modulation of functions of the endoplasmic reticulum channels.

The role of defensins in the excitability of the peripheral vestibular system in the frog: Evidence for the presence of communication between the immune and nervous systems

Defensins are one of the major groups of endogenous peptides that are considered to be important antibiotic-like effectors of host innate and adaptive antimicrobial immunity. The current study investigated the electrophysiological effects of externally applied human and rabbit defensins (HNP-1 and RNP-1, correspondingly) on afferent neurotransmission in the frog semicircular canals (SCC). Application of HNP-1 and RNP-1 induces a concentration-dependent decrease in resting activity. Threshold concentrations for both substances were of the order of 0.0001 nM. The firing evoked by L-glutamate (L-Glu) and its agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) and (1S, 3R)-1-aminocyclopentane-trans-1,3-dicarboxilic acid (ACPD) could be inhibited by HNP-1, suggesting that defensins exert inhibitory control over both ionotropic and metabotropic glutamate receptors. HNP-1 considerably inhibited the L-glutamate/high Mg2+ -induced increase in frequency, thus, demonstrating its postsynaptic site of action. Acetylcholine (ACh) responses under HNP-1 did not differ from the frequency increase induced by ACh alone, and the ACh antagonist atropine left the response to HNP-1 intact. The specific opioid receptor antagonist naloxone (Nal) antagonized the inhibitory response evoked by HNP-1. The results obtained support the evidence for the recruitment of defensins in communication between the immune and nervous systems, and on the potential of sensory receptors to participate in the inflammatory response.