Tyrosine receptor kinase B receptor activation reverses the impairing effects of acute nicotine on contextual fear extinction.

Anxiety and stress disorders have been linked to deficits in fear extinction. Our laboratory and others have demonstrated that acute nicotine impairs contextual fear extinction, suggesting that nicotine exposure may have negative effects on anxiety and stress disorder symptomatology. However, the neurobiological mechanisms underlying the acute nicotine-induced impairment of contextual fear extinction are unknown. Therefore, based on the previous studies showing that brain-derived neurotrophic factor is central for fear extinction learning and acute nicotine dysregulates brain-derived neurotrophic factor signaling, we hypothesized that the nicotine-induced impairment of contextual fear extinction may involve changes in tyrosine receptor kinase B signaling. To test this hypothesis, we systemically, intraperitoneally, injected C57BL/6J mice sub-threshold doses (2.5 and 4.0 mg/kg) of 7,8-dihydroxyflavone, a small-molecule tyrosine receptor kinase B agonist that fully mimics the effects of brain-derived neurotrophic factor, or vehicle an hour before each contextual fear extinction session. Mice also received injections, intraperitoneally, of acute nicotine (0.18 mg/kg) or saline 2-4 min before extinction sessions. While the animals that received only 7,8-dihydroxyflavone did not show any changes in contextual fear extinction, 4.0 mg/kg of 7,8-dihydroxyflavone ameliorated the extinction deficits in mice administered acute nicotine. Overall, these results suggest that acute nicotine-induced impairment of context extinction may be related to a disrupted brain-derived neurotrophic factor signaling.

N-methyl-D-aspartate receptors and amnesia in mice with depression-like state.

We studied the effect of activation (N-methyl-D-aspartic acid and D-cycloserine) and blockade (dizocilpine and 7-chlorokynurenic acid) of N-methyl-D-aspartate receptors on the development of amnesia in intact and depressive mice under conditions of conditioned passive avoidance response. Agonists and antagonists of N-methyl-D-aspartate receptors produce a strong antiamnesic effect in mice with behavioral despair. In intact animals, only N-methyl-D-aspartic acid and D-cycloserine improved passive avoidance performance.

Gamma-aminobutyric acidergic and glycinergic inputs shape coding of amplitude modulation in the chinchilla cochlear nucleus.

Amplitude modulation is a prominent acoustic feature of biologically relevant sounds, such as speech and animal vocalizations. Enhanced temporal coding of amplitude modulation signals is found in certain dorsal and posteroventral cochlear nucleus neurons when they are compared to auditory nerve. Although mechanisms underlying this improved temporal selectivity are not known, involvement of inhibition has been suggested. gamma-Aminobutyric acid- and glycine-mediated inhibition have been shown to shape the dorsal cochlear nucleus and posteroventral cochlear nucleus response properties to other acoustic stimuli. In the present study, responses to amplitude modulation tones were obtained from chinchilla dorsal cochlear nucleus and posteroventral cochlear nucleus neurons. The amplitude modulation carrier was set to the neuron's characteristic frequency and the modulating frequency varied from 10 Hz. Rate and temporal modulation transfer functions were compared across neurons. Bandpass temporal modulation transfer functions were observed in 74% of the neurons studied. Most cochlear nucleus neurons (90%) displayed flat or lowpass rate modulation transfer functions to amplitude modulation signals presented at 2540 dB (re: characteristic frequency threshold). The role of inhibition in shaping responses to amplitude modulation stimuli was examined using iontophoretic application of glycine or gamma-aminobutyric acidA receptor agonists and antagonists. Blockade of gamma-aminobutyric acidA or glycine receptors increased stimulus-evoked discharge rates for a majority of neurons tested. Synchronization to the envelope was reduced, particularly at low and middle modulating frequencies, with temporal modulation transfer functions becoming flattened and less bandpass in appearance. Application of glycine, gamma-aminobutyric acid or muscimol increased the modulation gain over the low- and mid-modulation frequencies and reduced the discharge rate across envelope frequencies for most neurons tested. These findings support the hypothesis that glycinergic and gamma-aminobutyric acidergic inputs onto certain dorsal cochlear nucleus and posteroventral cochlear nucleus neurons play a role in shaping responses to amplitude modulation stimuli and may be responsible for the reported preservation of amplitude modulation temporal coding in dorsal cochlear nucleus and posteroventral cochlear nucleus neurons at high stimulus intensities or in background noise.

Protective effect of riluzole on excitatory amino acid-mediated neurotoxicity in motoneuron-enriched cultures.

Excitatory amino acid-mediated neurotoxicity was investigated in motoneuron-enriched cultures from fetal rats at 12-14 days of gestation. The cultures were mainly composed of differentiated motoneurons identified by choline acetyl transferase and calcitonin gene-related peptide (CGRP) immunoreactivity. Addition of glutamate (600 microM) to the conditioned medium induced no acute neuronal swelling. However, it was followed by a widespread neuronal degeneration over the next 24 h, accounting for 77% of the total cell number. Glutamate toxicity was dose dependent, with an EC50 around 300 microM. Treatment for 24 h with the agonists, N-methyl-D-aspartate (NMDA, 100 microM), kainate (500 microM) or RS-alpha-amino-3-hydroxy-5-methyl-4-isoxalopropionate (AMPA, 10 microM), also induced a significant cell loss. Riluzole (2 amino 6-trifluoromethoxybenzothiazole), a compound known to interfere with glutamatergic transmission pre- and postsynaptically, significantly reduced glutamate and NMDA neurotoxicity in a dose-dependent manner. These results suggest that a prolonged activation of one or more subtypes of ionotropic excitatory amino acid receptors can lead to motoneuron degeneration in vitro, and provide direct experimental evidence supporting the neuroprotective effect of riluzole in cultured motoneurons.

Hyperekplexia mutations of the glycine receptor unmask the inhibitory subsite for beta-amino-acids.

beta-Alanine and taurine are agonists of the glycine receptor (GlyR) which, at low concentrations, antagonize the action of the principal agonist glycine. We analysed the potency of these ligands on alpha 1 subunits mutated at residue R271. GlyRs formed from alpha 1R271K subunits showed a reduction of beta-alanine and taurine affinities and maximal inducible currents; the mutants alpha 1R271Q and alpha 1R271L associated with human hyperekplexia gave no responses to these ligands. Inhibition of glycine-evoked currents by beta-alanine and taurine, however, was similar for all mutant GlyRs. These data are consistent with the existence of two subdomains within the ligand binding region of the GlyR, an agonistic one, which depends on arginine 271, and an antagonistic subsite, which is not connected to this residue.

Postsynaptic modifications in long-term facilitation in Aplysia: upregulation of excitatory amino acid receptors.

Long-term sensitization of the gill and siphon withdrawal in Aplysia is accompanied by facilitation of sensorimotor synaptic connections that depends on new protein synthesis. This phenomenon has been previously shown to involve presynaptic growth. At the postsynaptic level, a reorganization should occur to parallel the formation of new synaptic contacts. We show here that 24 hr following an application of 5-HT, which produces long-term synaptic facilitation (LTF), the response of the motoneuron to an excitatory amino acid agonist of the synaptic receptors is increased. General inhibition of protein synthesis with anisomycin blocks this enhancement. Inhibiton of protein synthesis limited to the postsynaptic neuron by intracellular injection of gelonin, a ribosome-inactivating toxin, also blocks the increase in the response to the agonist but fails to block 24 hr LTF. These results are compatible with a model of LTF that involves coordinate pre- and postsynaptic changes. The latter may include an upregulation of functional postsynaptic receptors. These may not be initially required for LTF measured at a 24 hr time point, but could become necessary for later stages of LTF. An increase in the number of functional postsynaptic receptors in a reserve pool may also prime the postsynaptic neuron for subsequent learning-associated plasticity.

Inhibition of sympathetic preganglionic neurons by spinal glycinergic interneurons.

Intracellular and whole-cell patch-clamp recordings were obtained from sympathetic preganglionic neurons in rat spinal cord slices. Perfusion of selective ionotropic and metabotropic excitatory amino acid agonists induced depolarizing responses in all neurons. In approximately 20% of neurons the application of these agonists also evoked inhibitory postsynaptic potentials. The application of the ionotropic receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5-40 microM) blocked the inhibitory postsynaptic potential discharges induced by (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (0.5-50 microM) and N-methyl-D-aspartate (0.5-50 microM), but failed to block the inhibitory postsynaptic potentials induced by quisqualate (0.5-50 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (10-200 microM). Similar inhibitory postsynaptic potentials were seen to occur spontaneously or could be evoked by electrical stimulation of the dorsal horn. The application of tetrodotoxin blocked the spontaneous and evoked inhibitory postsynaptic potential, indicating that they result from activity-dependent release of neurotransmitter. Strychnine antagonized all inhibitory postsynaptic potentials suggesting that they were mediated via glycine receptors. The reversal potential of the inhibitory postsynaptic potentials was -65 mV for intracellular and -55 mV for whole-cell recordings. This latter value is close to the reversal potential for chloride, suggesting that the inhibitory postsynaptic potentials were mediated by a chloride conductance. Perfusion of glycine (0.1-1 mM) induced inhibitory hyperpolarizing responses in the majority of neurons. This hyperpolarizing response was associated with a reduction in neuronal input resistance, persisted in the presence of tetrodotoxin, was blocked by strychnine and reversed at -55 mV. In some neurons, glycine induced a membrane depolarization and increased the rate of spontaneous action potential firing. This excitatory effect of glycine was blocked by tetrodotoxin, showed voltage dependency and was less sensitive to strychnine than the glycine-induced inhibitory response. We conclude from these data that spinal interneurons which synapse with sympathetic preganglionic neurons can be activated through multiple subtypes of excitatory amino acid receptor, including both ionotropic and metabotropic receptors. These interneurons release glycine to evoke inhibitory postsynaptic potentials which are mediated via a strychnine-sensitive glycine receptor coupled to a chloride conductance.

Effect of excitatory amino acid receptor agonists on secretion of growth hormone as assessed by the reverse hemolytic plaque assay.

Recent findings indicate that excitatory amino acids (EAAs) can modulate growth hormone (GH) secretion in several mammalian species in vivo and in vitro. In this study, we examined the effects of EAA receptor antagonists [N-methyl-D,L-aspartate (NMDA), kainic acid, L-glutamate] on GH secretion by the reverse hemolytic plaque assay (RHPA). Anterior pituitary cells of adult male Sprague-Dawley rats were enzymatically dispersed and subjected to RHPA. EAA receptor agonists increased the mean plaque area in a dose-dependent manner: the maximal increase was observed at 10 microM and increased the fraction of somatotrophs forming large plaques. NMDA (10 microM) did not increase the mean plaque area in the presence of the NMDA receptor antagonists 10 microM AP-7 and 10 microM MK-801. Coincubation of kainic acid with the non-NMDA receptor antagonist CNQX blocked the kainic-acid-stimulated increase in GH secretion. The addition of MK-801, AP-7 or CNQX to glutamate caused a partial reduction of the mean plaque area. Ten micromoles per liter glutamate with 10 nM GH-releasing hormone (GHRH) produced an additive effect on GHRH-induced GH release. Somatostatin suppressed the stimulatory action of glutamate. We speculate that glutamate plays a role in the regulation of GH secretion.