Antagonists D1 and D2 of Dopamine Receptors Determine Different Mechanisms of Neuroprotective Action in the Frog Vestibular.

The high functional plasticity of the glutamatergic synapse of the vestibular epithelium is supported by a delicate balance of excitatory and inhibitory interactions. One of the peptides co-localized with acetylcholine (ACh) in the efferent fibers is dopamine. Using external perfusion of the synaptic zone and multiunit recording of an afferent fibers activity the effect of dopamine antagonists on the background firing rate of the semicircular canal sensory fibers was studied on the isolated frog vestibular. The present research revealed that the dopamine receptor antagonist (D1) SCH-23390 significantly reduced the level of background activity at high concentrations. In contrast, D2 antagonist eticlopride caused positive-negative answer of the background activity of the sensory fibers in dose-depending manner. The data confirm the hypothesis that dopamine, being tonic released from the efferent fibers, realizes neuroprotective inhibitory control over the afferent glutamatergic synapse activity in the vestibular epithelium via D1 and D2 receptors.

Soluble Guanylate Cyclase As the Key Enzyme in the Modulating Effect of NO on Metabotropic Glutamate Receptors.

The synaptic plasticity of the afferent synapse of the vestibular apparatus is defined by the dynamic interaction of ionotropic and metabotropic glutamate receptors and the modulators of synaptic transmission. It was shown that nitric oxide modulates iGluR responses. In this paper, the effect of NO on the function of the afferent synapse mGluR was investigated. Inhibitor of nitric oxide synthase lowered the level of background activity but increased the amplitude of the responses of groups I and II mGluR agonist ACPD. Donor NO SNAP increased the level of background activity. Short-term perfusion of the synaptic region with low concentrations of SNAP led to a decrease in the amplitude of the answers of mGluR agonists ACPD and DHPG. The inhibitory effect of the NO donor was eliminated under blockade of soluble guanylate cyclase with a specific inhibitor ODQ. A prolonged application of NO did not cause a statistically significant change in the amplitude of the ACPD response. However, SNAP at concentrations of 10 and 100 µM increased the amplitude of the mGluR agonist responses 30 and 15 minutes, respectively, after termination of the NO donor exposure. The obtained data show the multidirectional effect of NO on the function of mGluR and testify to the existence of a complex modulating mechanism of the afferent flow from vestibular organs to the central nervous system.

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.

[Metabotropic glutamate receptors in the mechanisms of plasticity of central nervous system of the honeybee Apis mellifera].

Localization of metabotropic glutamate receptors (MGR) in head ganglion of honeybee Apis mellifera, and mechanisms of participation of activated MGR in CNS plasticity are investigated by means of complex approach using immunochemical, electrophysiological and behavioral methods. Influense of MGR activation on GABAergic system and ionotropic glutamate receptors (IGR) of AMPA- and NMDA-subtypes in studied. MGRa are revealed in lateral and medial calices of mushroom bodies. The inhibiting influence of MGR on AMPA- and NMDA receptors is shown using method of conditioned reflex. Previous activation of MGR neutralizes the inhibiting effect of GABA. Modulating role of heterogeneous MGR population in mechanisms of CNS plasticity on the level of glutamate-ergic synapse, and at interaction with GABAergic system is discussed.

Dopaminergic modulation of afferent synaptic transmission in the semicircular canals of frogs.

Using multiunit recording of action potentials from the whole nerve with the aid of external perfusion, we investigated the effects of dopamine (DOP) agonists that are involved in modulatory actions on synaptic transmission in the isolated labyrinth preparations of frogs. The external application of DOP (0.1-1 mM), the D(1) agonist chloro-APB hydrobromide (CAPB, 50-100 microM) and the D2 agonist quinerolane (QUI, 50-100 microM) induced a dose-dependent and reversible decline in the resting discharge frequency. In this concentration range, the potency of applied CAPB considerably exceeded that of QUI. AMPA, NMDA and ACPD responses were inhibited by the D1 and D2 agonists, implicating both subtypes of DOP receptors in the modulation of both ionotropic and metabotropic glutamate receptors. The inhibitory action of the DOP agonists on L-glutamate responses persisted in a high Mg2+ solution in conditions of selective activation of the postsynaptic membrane. The results obtained suggest that DOP may interact with both D1 and D2 receptor subtypes, most likely located postsynaptically on the afferent nerve fibers. This dopaminergic control mechanism may result in the reduction of the activated firing rate, thus preventing over-excitation and excitotoxic injury of the afferent dendrites after the external application of L-glutamate and excessive receptor stimulation.

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.

[Comparative analysis of the effect of endogenous antibiotic defensin NP-1 and aminoglycoside antibiotic gentamicin on synaptic transmission in receptors of the frog vestibular apparatus].

The effect of human and rabbit neutrophilic defensins NP-1 and amonoglycoside antibiotic gentamicin on the synaptic transmission in the afferent synapse of isolated vestibular apparatus of the frog has been comparatively studied. Both defensins proved active in the concentration range of 0.0001 to 1 nM and efficiently decreased the impulse frequency in the afferent nerve fibers in a concentration-dependent manner. No significant differences in the efficiency of rabbit and human defensin NP-1 have been revealed in these experiments. Gentamicin also had an inhibitory effect on the afferent discharge in the concentration range of 10-500 microM (0.5-25 mg/kg). The inhibitory effect of gentamicin on the impulse activity of the vestibular nerve was observed at therapeutic doses. The excitatory effect of the putative neurotransmitter L-glutamate was considerably inhibited by defensin NP-1. These findings suggest that the mechanism of defensin action involves a modification of the synaptic transmission the hair receptor and is mediated by L-glutamate.

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

Using immunocytochemistry and multiunit recording of afferent activity of the whole vestibular nerve, we investigated the role of metabotropic glutamate receptors (mGluR) in the afferent neurotransmission in the frog semicircular canals (SCC). Group I (mGluR1alpha) and group II (mGluR2/3) mGluR immunoreactivities were distributed to the vestibular ganglion neurons, and this can be attributed to a postsynaptic locus of metabotropic regulation of rapid excitatory transmission. The effects of group I/II mGluR agonist (1S,3R)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) and antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) on resting and chemically induced afferent activity were studied. ACPD (10-100 microM) enhanced the resting discharge frequency. MCPG (5-100 microM) led to a concentration-dependent decrease of both resting activity and ACPD-induced responses. If the discharge frequency had previously been restored by L-glutamate (L-Glu) in high-Mg2+ solution, ACPD elicited a transient increase in the firing rate in the afferent nerve suggesting that ACPD acts on postsynaptic receptors. The L-Glu agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA), were tested during application of ACPD. AMPA- and NMDA-induced responses were higher in the presence than absence of ACPD, implicating mGluR in the modulation of ionotropic glutamate receptors. These results indicate that activation of mGluR potentiates AMPA and NMDA responses through a postsynaptic interaction. We conclude that ACPD may exert modulating postsynaptic effects on vestibular afferents and that this process is activity-dependent.

Central non-NMDA receptors of a honey bee with hereditary kynurenine deficiency.

Pharmacological characteristics of non-NMDA receptors involved in associative learning were studied in wild honey bees (normal) and carriers of snow laranja mutation (kynurenine deficiency) by pharmacological analysis and behavioral criteria. The effects of systemic injections of non-NMDA receptor agonists (AMPA, kainic, quisqualic, and domoic acids), AMPA receptor antagonist (NS257-HCl), and AMPA receptor modulator (cyclothiaside) on retention of conditioned reflexes in short-term memory (1 min after the end of learning) were studied. The pharmacological characteristics of non-NMDA receptors were changed in snow laranja mutants with kynurenine deficiency.

[Absence of the effect of opioid peptides on muscarine receptors in the frog vestibular apparatus]. / Otsutstvie vliianiia opioidnykh peptidov na muskarinovye retseptory vestibuliarnogo apparata liagushki.

We studied the effects of opioid peptide leu-enkephaline, a specific antagonist of acetylcholine receptors atropine, and non-selective opiate antagonist naloxone on synaptic transmission and responses evoked by acetylcholine in semicircular organs of the frog. A decrease in frequency of acetylcholine (0.1-5.0 microM) responses under leu-enkephaline (10 nM) id not differ from the frequency decline induced by leu-enkephaline alone. Atropine (1 microM) left the response to leu-enkephaline intact while blocking the excitatory effect of acetylcholine. No modification of the acetylcholine response under leu-enkephaline was observed in the presence of naloxone (1 microM). The findings suggest that no interaction exists between the acetylcholine-mediated excitatory action on resting activity in the isolated semicircular canal preparation and the suppressive action of leu-enkephaline.

Lack of evidence of an interaction between leu-enkephalin and muscarinic-like responses in the frog semicircular canal.

Using multiunit recording of action potentials from the whole vestibular nerve, we studied whether opioid peptide leu-enkephalin (Enk) may modulate muscarinic-like responses in semicircular organs of the frog. When acetylcholine (ACh) (0.1-1 mM) was applied with 1 nM Enk, the maximal frequency increase induced by ACh was reduced.However,the frequency decrease of ACh responses under Enk did not differ from the frequency decline in basal spike discharge induced by Enk alone. Administration of atropine (1 microM) left the response to Enk intact and blocked the excitatory effect of ACh. No modification of the ACh response under Enk was observed in the presence of the non-selective opioid receptor antagonist naloxone (10 microM). This study suggests that no interaction exists between the ACh-mediated excitatory action on resting activity in the isolated semicircular canal preparation and the suppressive action of Enk.

Effects of dalargin on excitation induced by L-glutamate agonists in the frog vestibular organs.

We have used an electrophysiological approach to investigate the action of a synthetic analog of leu-enkephalin dalargin (DAL) on chemically induced afferent activity in the frog vestibular organs. Administration of 5.0 microM kainic acid (KA), 5.0 microM (AMPA) and 50 microM NMDA produced an increase in the frequency of the resting discharge. Firing evoked by KA, AMPA or NMDA could be depressed by administration of 1 nM Dal by 55.5 +/- 9.9% (n = 10, p < 0.05), 64.5 +/- 11.2% (n = 13, p < 0.05) and 21.3 +/- 11.1% (n = 14, p = 0.051), respectively. Thus, the frequency decrease under NMDA was statistically non-significant. These results show that non-NMDA, but not NMDA subtypes of receptors are mostly involved in opioid action at the vestibular organs of the frog.

[NMDA-Receptors in the central nervous system of the honey bee with kynurenine deficiency]. / NMDA-Retseptory tsentral'noi nervnoi sistemy medonosnoi pchely v usloviiakh defitsita kinureninov.

Effects of some drugs on the short-term memory were tested. In kynurenine deficit, mutant snow laranja manifested the same pharmacological profile and 10 to 100-fold enhancement of sensitivity of the NMDA receptors' different sites. The data obtained suggest that the gene controlling the key enzyme activity of the kynurenine pathway of tryptophan metabolism is involved in regulation of the CNS NMDA receptors' functional condition. The kynurenines may be classified as endogenous modulators of the NMDA receptor sensitivity.

Opioid peptides as possible neuromodulators of the afferent synaptic transmission in the frog semicircular canal.

Vestibular receptors of the frog, Rana temporaria, were examined for the effect of bath-applied opioid peptide leu-enkephalin, its synthetic analogue dalargin and the specific opiate antagonist naloxone. Multiunit afferent activity of the whole vestibular nerve was recorded in an in vitro preparation. Leu-enkephalin (0.005-100 nM) and dalargin (0.1-100 nM) depress the resting discharge frequency. Naloxone (10 nM-1 microM) antagonizes responses induced by leu-enkephalin and dalargin that suggests a specific action of opioid peptides. Leu-enkephalin and delargin inhibit the excitatory action of L-glutamate. The effects of opioid peptides on L-glutamate-induced responses are unaffected by Co2+ block of transmitter release from hair cells that could speak in favour of the postsynaptic nature of these responses. At the same time, the other possible site of action of opioid peptides, such as efferent system, can not be excluded. The results indicate that opiate receptors are present in hair cells and that the neurotransmitter L-glutamate is involved in opiate action at the peripheral vestibular system of the frog. We suggest that opioid peptides may act as a neuromodulator in this system.

Analysis of L-[3H]-glutamate radioligand binding to plasma membranes of the ampullae of Lorenzini of skates.

Radioligand binding analysis of L-[3H]-glutamate to plasma membranes of the ampullae of Lorenzini in the skate revealed one type of binding site with KD = 286 nM and Bmax = 2.1 pmol mg-1 protein. It was revealed that Na+, K+, Ca2+ and Mg2+ inhibited the L-[3H]-glutamate binding, while Cl- had no effect. The glutamate agonists kainate, quisqualate and N-methyl-D-aspartate (NMDA) were shown to inhibit glutamate binding in a dose-dependent manner. The results obtained confirm the electrophysiological observations suggesting the existence of three types of glutamate receptor at afferent synapses of the ampullae of Lorenzini of skates.

Actions of dalargin upon single unit activity in the ampullae of Lorenzini of the skate Raja clavata.

In the present study we have shown by single afferent unit recording in electroreceptors of skates (the ampullae of Lorenzini) that the synthetic analogue of leu-enkephalin, dalargin (DAL) at concentrations between 10(-6)-10(-10) M cause a concentration-dependent decrease in the resting discharge frequency as well as a decrease in stimulus evoked responses. The specific opiate antagonist naloxone (NAL, 10(-6) M) antagonizes responses induced by DAL. DAL depresses the excitatory action of L-glutamate (L-GLU). The data obtained speak in favour of the presence of opiate receptors at the synaptic membrane of the ampullae of Lorenzini.