Pre- and postsynaptic ionotropic glutamate receptors in the auditory system of mammals.

The ionotropic glutamate receptors (iGluRs) concertedly mediate neurotransmission to convey, process, and integrate acoustic information along the auditory pathway. In order to ensure these challenging tasks, the iGluRs are variously expressed in auditory neurons in an age- and site-dependent manner. The subunit compositions of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and N-methyl-D-aspartate receptors (NMDARs) are altered with development, underlying the acceleration in kinetics of excitatory postsynaptic responses. AMPAR desensitization partly affects short-term synaptic plasticity upon repetitive stimuli in subsets of auditory neurons at a given period of maturation. NMDAR activation is required for long-term synaptic plasticity in a cerebellum-like microcircuit within the first auditory brainstem nucleus. Along with their postsynaptic functions, AMPARs and NMDARs fulfill essential roles in presynaptic modulation of auditory neurotransmission. Despite the expression of the kainate and delta receptors, their functions remain unknown. Here this review aims to discuss the diverse distribution and functions of pre- and postsynaptic iGluRs in the peripheral and central auditory systems.

Presynaptic lonotropic receptors.

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Presynaptic alpha7 and non-alpha7 nicotinic acetylcholine receptors modulate [3H]d-aspartate release from rat frontal cortex in vitro.

The presynaptic nicotinic modulation of glutamatergic transmission in the CNS has been associated with activation of the alpha7 subtype of nicotinic acetylcholine receptor (nAChR) in sub-cortical regions, whereas in the frontal cortex, non-alpha7 nAChRs have been implicated. The aim of this investigation was to directly characterise nAChR-evoked release of excitatory amino acids from rat frontal cortex, by monitoring the release of [3H]D-aspartate from superfused synaptosomes or minces. Co-administration of a nAChR agonist with a depolarising stimulus enhanced [3H]D-aspartate release above the effect of depolarising agent alone. This enhancement was blocked by the nicotinic antagonist mecamylamine. Other experiments revealed that in the absence of a depolarising stimulus, the nAChR agonists nicotine, epibatidine and anatoxin-a could evoke the release of [3H]D-aspartate in a Ca2+- and concentration-dependant manner. Differential sensitivity to the alpha7- and beta2*-selective nAChR antagonists alpha-bungarotoxin (alpha-Bgt) and dihydro-beta-erythroidine (DHbetaE) implicated two nAChR subtypes (alpha7 and beta2*), and this was supported by using the subtype-selective agonists choline (10 mM; alpha7 selective, blocked by alpha-Bgt but not by DHbetaE) and 5-Iodo-A-85380 (10 nM; beta2*-selective, blocked by DHbetaE but not by alpha-Bgt). Immunocytochemistry showed that alpha-Bgt labelling was associated with structures immunopositive for vesicular glutamate transporters, in both frontal cortex sections and synaptosome preparations, supporting the presence of alpha7 nAChR on glutamatergic terminals in rat frontal cortex.

Joint explorative analysis of neuroreceptor subsystems in the human brain: application to receptor-transporter correlation using PET data.

Positron emission tomography (PET) has proved to be a highly successful technique in the qualitative and quantitative exploration of the human brain's neurotransmitter-receptor systems. In recent years, the number of PET radioligands, targeted to different neuroreceptor systems of the human brain, has increased considerably. This development paves the way for a simultaneous analysis of different receptor systems and subsystems in the same individual. The detailed exploration of the versatility of neuroreceptor systems requires novel technical approaches, capable of operating on huge parametric image datasets. An initial step of such explorative data processing and analysis should be the development of novel exploratory data-mining tools to gain insight into the "structure" of complex multi-individual, multi-receptor data sets. For practical reasons, a possible and feasible starting point of multi-receptor research can be the analysis of the pre- and post-synaptic binding sites of the same neurotransmitter. In the present study, we propose an unsupervised, unbiased data-mining tool for this task and demonstrate its usefulness by using quantitative receptor maps, obtained with positron emission tomography, from five healthy subjects on (pre-synaptic) serotonin transporters (5-HTT or SERT) and (post-synaptic) 5-HT(1A) receptors. Major components of the proposed technique include the projection of the input receptor maps to a feature space, the quasi-clustering and classification of projected data (neighbourhood formation), trans-individual analysis of neighbourhood properties (trajectory analysis), and the back-projection of the results of trajectory analysis to normal space (creation of multi-receptor maps). The resulting multi-receptor maps suggest that complex relationships and tendencies in the relationship between pre- and post-synaptic transporter-receptor systems can be revealed and classified by using this method. As an example, we demonstrate the regional correlation of the serotonin transporter-receptor systems. These parameter-specific multi-receptor maps can usefully guide the researchers in their endeavour to formulate models of multi-receptor interactions and changes in the human brain.

alpha-bungarotoxin-sensitive nicotinic receptors indirectly modulate [(3)H]dopamine release in rat striatal slices via glutamate release.

Nicotinic agonists elicit the release of dopamine from striatal synaptosomes by acting on presynaptic nicotinic acetylcholine receptors (nAChRs) on dopamine nerve terminals. Both alpha3beta2* and alpha4beta2 nAChR subtypes (but not alpha7* nAChRs) have been implicated. Here, we compared nAChR-evoked [(3)H]dopamine release from rat striatal synaptosome and slice preparations by using the nicotinic agonist anatoxin-a. In the more integral slice preparation, the concentration-response curve for anatoxin-a-evoked [(3)H]dopamine release was best fitted to a two-site model, giving EC(50) values of 241 nM and 5.1 microM, whereas only the higher-affinity component was observed in synaptosome preparations (EC(50) = 134 nM). Responses to a high concentration of anatoxin-a (25 microM) in slices (but not in synaptosomes) were partially blocked by ionotropic glutamate receptor antagonists (kynurenic acid, 6,7-dinitroquinoxaline-2,3-dione) and by alpha7*-selective nAChR antagonists (alpha-bungarotoxin, alpha-conotoxin-ImI, methyllycaconitine) in a nonadditive manner. In contrast, the alpha3beta2-selective nAChR antagonist alpha-conotoxin-MII partially inhibited [(3)H]dopamine release from both slice and synaptosome preparations, stimulated with both low (1 microM) and high (25 microM) concentrations of anatoxin-a. Antagonism by alpha-conotoxin-MII was additive with that of alpha7*-selective antagonists. These data support a model in which alpha7* nAChRs on striatal glutamate terminals elicit glutamate release, which in turn acts at ionotropic glutamate receptors on dopamine terminals to stimulate dopamine release. In addition, non-alpha7* nAChRs on dopamine terminals also stimulate dopamine release. These observations have implications for the complex cholinergic modulation of inputs onto the major efferent neurons of the striatum.

Two populations of kainate receptors with separate signaling mechanisms in hippocampal interneurons.

Consistent with the epileptogenic and deleterious effects of the potent neurotoxin kainate, the activation of kainate receptors reduces the synaptic inhibition induced by the amino acid gamma-aminobutyric acid (GABA). Extrapolating from these data led to the conclusion that kainate receptors are located presynaptically. However, kainate directly depolarizes the inhibitory interneurons, causing them to fire repeatedly. This effect might indirectly decrease the size of inhibitory postsynaptic currents recorded from pyramidal cells and places in doubt the presynaptic location for kainate receptors. Here we show that both effects, membrane depolarization and the reduction of inhibitory potentials, can be dissociated by several means, particularly by the natural agonist of kainate receptors, glutamate. Indeed, when applied at low concentrations, glutamate inhibited GABA release without affecting the firing rate of GABA interneurons. These results indicate that CA1 interneurons contain two populations of kainate receptors, each with different agonist sensitivity and coupled to distinct signaling pathways.

[New approaches to determining the type classification of presynaptic acetylcholine receptors]. / Novye podkhody k opredeleniiu tipovoi prinadlezhnosti presinapticheskikh retseptorov atsetilkholina.

A complex of special tests which took into account the coupling of the specific physiological responses with individual subtypes of muscarinic cholinoceptors, and radioligand analysis showed the predominance of M3- and M2-subtype presynaptic receptors in the rat brain hemispheres. The autoreceptors regulating the presynaptic release of acetylcholine are related mainly to the M2-subtype and account for the smaller part of the mixed population of presynaptic receptors.

Inhibition of noradrenaline release via presynaptic 5-HT1D alpha receptors in human atrium.

In segments of human right atrial appendages preincubated with [3H]noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, we determined the effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on tritium overflow evoked by transmural electrical stimulation (2 Hz). Tritium overflow was inhibited by 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate (RU 24969) and sumatriptan. Yohimbine and oxymetazoline (in the presence of idazoxan) also inhibited tritium overflow. The inhibitory potency of the drugs was significantly correlated with their affinity for 5-HTID receptors in human brain and for cloned human 5-HT1D alpha and 5-HT1D beta receptors, but not with their affinity for 5-HT1B, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT5A, 5-HT5B and 5-HT7 receptors. The potency order 5-CT > 5-HT > 5-MeOT is opposite to the order of affinities reported for 5-HT6 binding sites. The preferential 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (up to 0.3 microM) and the selective 5-HT4 receptor agonist cisapride (up to 1 microM) failed to inhibit tritium overflow. L-694,247, a potent 5-HT1D beta receptor agonist, did not inhibit tritium overflow, but counteracted the inhibitory effect of 5-HT. Ketanserin at a concentration which should block 5-HT1D alpha but not 5-HT1D beta receptors and methiothepin at a concentration which may be assumed to block both 5-HT1D alpha and 5-HT1D beta receptors antagonized the inhibitory effect of 5-HT. Propranolol and ondansetron did not modify the 5-HT-induced inhibition of release. In conclusion, noradrenaline release in human right atrial appendages is inhibited via 5-HT receptors which are located on the noradrenergic axon terminals. These inhibitory presynaptic 5-HT receptors belong to the 5-HT1D subfamily. The ability of ketanserin to antagonize the inhibitory effect induced by activation of these receptors suggests that they can be subclassified as 5-HT1D alpha.

Subclassification of presynaptic 5-HT autoreceptors in the human cerebral cortex as 5-HT1D receptors.

Human cerebral cortical synaptosomes were used to determine the 5-hydroxytryptamine (5-HT) receptor subtype to which the inhibitory presynaptic 5-HT autoreceptor belongs. The synaptosomes preincubated with [3H]5-HT were superfused and tritium overflow was stimulated by high K+. The K(+)-evoked tritium overflow, which was Ca(2+)-dependent but tetrodotoxin-resistant, was concentration-dependently inhibited by the nonselective 5-HT 1D alpha/1D beta receptor agonist, 5-carboxamidotryptamine. Ketanserin at a concentration which should block the 5-HT 1D alpha but not the 5-HT 1D beta receptor failed to antagonize the inhibitory effect of 5-carboxamidotryptamine. In contrast, the nonselective 5-HT 1D alpha/1D beta receptor antagonist, methiothepin, at a concentration which should block both the 5-HT 1D alpha and the 5-HT 1D beta receptor abolished the effect of 5-carboxamidotryptamine. It is concluded that the presynaptic 5-HT autoreceptor, which has previously been classified as 5-HT 1D, belongs to the 5-HT1D beta subtype.