Synchronous and asynchronous exocytosis induced by subthreshold high K+ at Cs(+)-loaded terminals of rat hippocampal neurons

Modulation of neurosecretion and approaches for its multistep analysis

BACKGROUND

Neurosecretion is the multistep process occurring in separate spatial and temporal cellular boundaries which complicates its comprehensive analysis. Most of the research are focused on one distinct stage of synaptic vesicle recycling. Here, we describe approaches for complex analysis of synaptic vesicle (SV) endocytosis and separate steps of exocytosis at the level of presynaptic bouton and highly purified SVs.

METHODS

Proposed fluorescence-based strategies and analysis of neurotransmitter transport provided the advantages in studies of exocytosis steps. We evaluated SV docking/tethering, their Ca²⁺-dependent fusion and release of neurotransmitters gamma-aminobutyric acid (GABA) and glutamate in two animal models.

RESULTS

Approaches enabled us to study: 1) endocytosis/Ca²⁺-dependent release of fluorescent carbon nanodots (CNDs) during stimulation of nerve terminals; 2) the action of levetiracetam, modulator of SV glycoprotein SV2, on fusion competence of SVs and stimulated release of GABA and glutamate; 3) impairments of several steps of neurosecretion under vitamin D₃ deficiency.

CONCLUSIONS

Our algorithm enabled us to verify the method validity for multidimensional analysis of SV turnover. By increasing SV docking and the size of readily releasable pool (RRP), levetiracetam is able to selectively enhance the stimulated GABA secretion in hippocampal neurons. Findings suggest that SV2 regulates RRP through impact on the number of docked/primed SVs.

GENERAL SIGNIFICANCE

Methodology can be widely applied to study the stimulated neurosecretion in presynapse, regulation of SV docking, their Ca²⁺-dependent fusion with target membranes, quantitative analysis of expression of neuron-specific proteins, as well as for testing the efficiency of pre-selected designed neuroactive substances.

A role for presenilin in post-stress regulation: effects of presenilin mutations on Ca2+ currents in Drosophila

It has been shown that presenilin is involved in maintaining Ca2+ homeostasis in neurons, including regulating endoplasmic reticulum (ER) Ca2+ storage. From studies of primary cultures and cell lines, however, its role in stress-induced responses is still controversial. In the present study we analyzed the effects of presenilin mutations on membrane currents and synaptic functions in response to stress using an in vivo preparation. We examined voltage-gated K+ and Ca2+ currents at the Drosophila larval neuromuscular junction (NMJ) with voltage-clamp recordings. Our data showed that both currents were generally unaffected by loss-of-function or Alzheimer's disease (AD) -associated presenilin mutations under normal or stress conditions induced by heat shock (HS) or ER stress. In larvae expressing the mutant presenilins, prolonged Ca2+ tail current, reflecting slower deactivation kinetics of Ca2+ channels, was observed 1 day after stress treatments were terminated. It was further demonstrated that the L-type Ca2+ channel was specifically affected under these conditions. Moreover, synaptic plasticity at the NMJ was reduced in larvae expressing the mutant presenilins. At the behavioral level, memory in adult flies was impaired in the presenilin mutants 1 day after HS. The results show that presenilin function is important during the poststress period and its impairment contributes to memory dysfunction observed during adaptation to normal conditions after stress. Our findings suggest a new stress-related mechanism by which presenilin may be implicated in the neuropathology of AD.

Activation of presynaptic GABAA receptors increases spontaneous glutamate release onto noradrenergic neurons of the rat locus coeruleus

In order to further explore how GABA can modulate the excitability of noradrenergic neurons of the locus coeruleus (LC), we investigated the presence of GABA(A) receptors on glutamatergic nerve terminals and the functional consequences of their activation. We used mechanically dissociated immature rat LC neurons with adherent nerve terminals and patch-clamp recordings of spontaneous excitatory postsynaptic currents. Activation of presynaptic GABA(A) receptors by muscimol facilitated spontaneous glutamate release by activating tetrodotoxin-sensitive Na(+) channels and high-threshold Ca(2+) channels. Bumetanide (10 microM), a potent blocker of Na(+)-K(+)-Cl(-) cotransporter, diminished the muscimol-induced facilitatory action of glutamate release. Our results indicate that the Na(+)-K(+)-Cl(-) cotransporter accumulates Cl(-) inside the nerve terminals so that activation of presynaptic GABA(A) receptors causes depolarization. This GABA(A)-receptor-mediated modulation of spontaneous glutamatergic transmission is another mechanism by which GABA and its analogues can regulate the excitability and activity of noradrenergic neurons in the LC.

Pharmacological dissection of calcium channel subtype-related components of strontium inflow in large mossy fiber boutons of mouse hippocampus

Several subtypes of voltage-dependent calcium channels (VDCCs) are present in the presynaptic terminals. In the mammalian hippocampus, P/Q-, N-, and R- but not L-type VDCCs are involved in the fast transmitter release from large mossy fiber (MF) boutons, which are associated with CA3 pyramidal cell dendrites. We investigated whether L-type VDCCs are indeed absent in these large MF boutons. With the use of Sr2+ as the Ca2+ substitute, the stimulus-evoked Sr2+ increment (delta[Sr2+]pre) was evaluated fluorometrically. Delta[Sr2+]pre appeared to be proportional to Sr2+ inflow through VDCCs and was specifically attenuated by conventional VDCC subtype-selective antagonists. The P/Q-type selective omega-agatoxin IVA (AgTx(IVA)) blocked delta[Sr2+]pre with an IC50 of 28 nM and by 30-35% at its maximum effective concentration of 0.5 microM. The N-type selective omega-conotoxin GVIA (CgTx(GVIA)) blocked delta[Sr2+]pre with an IC50 of 15 nM and by 20-25% at its maximum effective concentration of 1 microM. The R-type selective SNX-482 blocked delta[Sr2+]pre with an IC50 of 79 nM and by 20-25% at its maximum effective concentration of 1 microM. The effects of these toxins did not overlap at their maximum effective concentrations and about 70-80% of delta[Sr +]pre was blocked by the simultaneous exposure to these toxins. delta[Sr2+]pre component that is resistant to AgTx(IVA), CgTx(IVA), and SNX-482 was significantly potentiated by an L-type agonist, (S)-(-)-Bay K8644, and attenuated by an L-type antagonist, nimodipine, suggesting that L-type VDCCs are present in large MF terminals. The L-type agonist, (+/-)-Bay K8644, also potentiated Sr2+ inflow into individual boutons identified as large MF boutons under confocal microscopy. Almost similar results were observed for Ca2+ inflow-dependent fluorescence increments. L-type VDCCs appear to be present in large MF boutons and mediate a substantial Ca2+ inflow into presynaptic terminals during action potentials.

Calcium channel subtypes on single GABAergic presynaptic terminal projecting to rat hippocampal neurons

High voltage-activated Ca(2+) channel subtypes triggering GABA release from nerve terminals (boutons) projecting to rat hippocampal CA1 pyramidal neurons were studied. Evoked GABAergic inhibitory postsynaptic currents (eIPSCs) were recorded in response to focal stimulation of single boutons in mechanically dissociated neurons and in response to stimulation of nerve bundle in slice preparations. Nilvadipine (3 micro M), an L-type Ca(2+) channel blocker, completely inhibited eIPSCs evoked by stimulation of single boutons, but had no effect on eIPSCs evoked by stimulation of nerve bundle at low frequencies. Nilvadipine (3 micro M) did, however, prevent the potentiation of eIPSC amplitude following high-frequency stimulation of nerve bundles in the slice preparation. omega-Conotoxin-GVIA (3 micro M), an N-type Ca(2+) channel blocker, and omega-agatoxin-IVA (0.3 micro M), a P/Q-type Ca(2+) channel blocker, completely inhibited single bouton evoked eIPSCs in 33.3 and 83.3% of recordings, respectively. In response to low-frequency nerve bundle stimulation in the slice preparation, omega-conotoxin-GVIA (3 micro M), omega-agatoxin-IVA (0.1 micro M) both partially reduced eIPSC amplitude, with the residual component being abolished by Cd(2+) (0.1 mM). From these results, the following hypotheses could be drawn. (1). The distribution of P/Q- and N-type channels at a single bouton is nonuniform. (2. When a focal stimulation is applied to a single bouton, L-type channels play a significant role in a generation of an action potential which subsequently activates P/Q- and N-type channels at GABA release sites. (3). Action potentials conducted through axons in the slice preparation are sufficient to depolarize the bouton membrane, even when L-type channels are suppressed.