Quantal analysis of inhibitory synaptic transmission in the dentate gyrus of rat hippocampal slices: a patch-clamp study

Serotonin facilitates GABAergic transmission in the CA1 region of rat hippocampus in vitro

1. The effect of serotonin on inhibitory synaptic transmission was examined in forty-one CA1 pyramidal neurones using intracellular voltage recordings in vitro. 2. Serotonin (20-50 microM) increased the synaptic noise of most (85%) neurones loaded with chloride (n = 33). The duration of this effect was enhanced with increasing concentrations of serotonin and was fully reversible within 5 min. When serotonin was applied at short intervals (less than 10 min), fading of the response was observed. 3. The effect of serotonin on synaptic noise persisted in the presence of the glutamate NMDA and non-NMDA antagonists, APV (100 microM) and CNQX (10 microM), but it was blocked (n = 5) by a GABAA antagonist, bicuculline (10 microM). 4. The increase in inhibitory synaptic events resulted from an enhanced frequency of unitary IPSPs from 4.6 +/- 3.8 Hz in control to 17.2 +/- 12.5 Hz (n = 5) in serotonin, especially of large events. Serotonin caused no change in the amplitude and frequency of miniature synaptic events recorded in the presence of TTX (n = 5). The mean amplitude of unitary inhibitory postsynaptic potentials (IPSPs) increased from 1.37 +/- 0.35 mV in control to 3.67 +/- 1.38 mV in serotonin. The coefficient of variation of unitary IPSPs increased from 0.40 +/- 0.11 in control to 0.74 +/- 0.23 in serotonin when quantal size appeared unchanged. 5. The 5-HT3 agonist 2-methyl-serotonin (52 microM, n = 4) partially mimicked the effect of serotonin, increasing the inhibitory noise without affecting the pyramidal neurone conductance. The serotonin-induced facilitation of unitary IPSPs was blocked by the 5-HT3 antagonists ICS 205-930 (1-90 nM, n = 3) and metoclopramide (30 microM, n = 1). 6. These results suggest that serotonin directly excites GABAergic interneurones acting on a 5-HT3 receptor and consequently increasing the frequency of inhibitory synaptic events recorded in CA1 pyramidal cells.

Synaptic localization of adrenergic disinhibition in the rat hippocampus

Norepinephrine is an endogenous neurotransmitter that reduces synaptic inhibition onto pyramidal neurons in the hippocampus by an action at an alpha-adrenergic receptor. The physiological mechanism of this disinhibition was previously not known, except that it occurred at a site presynaptic to the inhibited pyramidal cell. In this paper we present evidence that adrenergic disinhibition is restricted to the early phase of the evoked inhibitory postsynaptic potential in area CA1 of the hippocampus. The locus of disinhibition does not appear to reside in the interneuronal terminal, axon, or cell body. Instead, adrenergic agonists appear to reduce evoked synaptic inhibition by depressing excitatory synapses that activate the interneuron.

Learning-related synaptic plasticity: LTP and LTD

The past several years have seen studies of synaptic plasticity in both invertebrate and vertebrate nervous systems come of age and lead to important new findings. In particular, current evidence points to a possible presynaptic site for long-term potentiation and the involvement of a retrograde messenger from the postsynaptic neuron. Recent advances in both cerebellar and cortical forms of long-term depression are also discussed.

Calcium entry increases the sensitivity of cerebellar Purkinje cells to applied GABA and decreases inhibitory synaptic currents

The sensitivity to GABA of Purkinje cells in thin cerebellar slices was examined by recording either spontaneous inhibitory synaptic currents or ionic currents elicited by local GABA applications. The effects of Ca2+ entry induced by depolarizing voltage pulses were opposite for the two types of currents. Currents due to exogenous GABA applications were increased by a train of voltage pulses. This potentiation was transient with an average half recovery period of 3.7 min. Spontaneous synaptic currents were reduced by depolarizing voltage pulses, with a half recovery time of about 20 s. The inhibition was largely explained by a decrease of the frequency of synaptic events, suggesting that the primary location of the effect was presynaptic. Thus, a Ca2+ rise increases the sensitivity of Purkinje cells to GABA and induces a retrograde inhibition of presynaptic terminals. The latter effect may be due to a diffusible Ca2(+)-dependent messenger.

Patch clamp analysis of excitatory synaptic currents in granule cells of rat hippocampus

1. Excitatory postsynaptic potentials (EPSPs) and their underlying currents (EPSCs) were recorded from dentate granule cells in thin hippocampal slices of rats using the tight-seal whole-cell recording technique. 2. At resting membrane potentials (ca -60 to -70 mV), the EPSCs clearly consisted of a dominant fast and a smaller slow component. The slow EPSC component markedly increased with depolarization. This resulted in a region of negative slope conductance (between -50 and -30 mV) in the peak current-voltage (I-V) relation of the dual-component EPSC in most neurones. The EPSCs reversed entirely at -1.2 +/- 2.8 mV (n = 15). 3. Using selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors, two pharmacologically distinct components of the natural EPSCs were isolated. The non-NMDA EPSCs displayed a linear I-V relation. Their rise times (0.5-1.9 ms) were independent of membrane voltage but seemed to depend critically on the precise dendritic location of the synapse. Their decay was approximated by a single exponential with a time constant ranging from 3 to 9 ms. The time course of these EPSCs was independent of changes in extracellular Mg2+. 4. The NMDA EPSCs displayed a non-linear I-V relation. At resting membrane potentials their peak amplitudes were 20 pA and increased steadily with depolarization to -30 mV. At membrane voltages positive to -30 mV the peak I-V relation was linear. The rise times of NMDA EPSCs ranged from 4 to 9 ms and were insensitive to membrane voltage. 5. The NMDA EPSCs decayed biexponentially. Both time constants, tau f and tau s, increased with depolarization in an exponential manner, tau s being more voltage dependent than tau f. Lowering extracellular Mg2+ slightly reduced both rate constants but did not completely abolish their voltage sensitivity. 6. Bath application of NMDA to outside-out patches from granule cells induced single channel currents of 52 pS in nominally Mg(2+)-free solutions. They displayed a burst-like single-channel activity with clusters of bursts lasting several hundreds of milliseconds. Currents through single NMDA receptor channels reversed around 0 mV. 7. The fractional contributions of NMDA and non-NMDA components to peak currents and synaptic charge transfer were assessed. At resting membrane potential the NMDA EPSC component accounted for 23% of the peak current and for 64% of the synaptic charge transfer. The contribution of the NMDA EPSC component to the synaptic charge transfer strongly increased with small depolarizations from rest.

Quantal analysis of excitatory synaptic action and depression in hippocampal slices

Quantal analysis can provide a quantitative description of important aspects of chemical synaptic transmission and its modification. The technique has recently been applied to excitatory synapses within the hippocampus, especially the form of synaptic plasticity known as long-term potentiation. However, these attempts have met with only limited success, in that the individual quantal amplitudes making up the synaptic response generally could not be resolved. Here we have paid attention to the possible instability of the quantal fluctuation pattern over time. We were able to resolve individual quantal component amplitudes for a high proportion of the experiments, and so demonstrate the quantal nature of excitatory transmission in the CA1 region of the hippocampus. Mean quantal amplitudes for individual excitatory postsynaptic potentials were 84-197 microV, with a mean of 131 +/- 29 microV. For periods during which the fluctuation pattern was stable, the variance associated with individual quantal amplitudes was low. We have also used quantal analysis to show that synaptic depression following prolonged stimulation at these synapses is primarily a presynaptic phenomenon.

Patch clamp analysis of excitatory synapses in mammalian spinal cord slices

Excitatory synaptic transmission to visually identified alpha-moto neurones was studied in thin slice preparations of the neonatal rat spinal cord. Excitatory postsynaptic currents (EPSCs) elicited by stimulation of intraspinal presynaptic fibres were recorded using the whole-cell patch clamp technique, following blockade of inhibitory transmission by bath application of strychnine and bicuculline. The EPSCs could be separated pharmacologically into N-methyl-D-aspartate- (NMDA) and non-NMDA-receptor-mediated components, where the contribution of the NMDA-mediated component was significant only at holding potentials more positive than -50 mV. Graded stimulation of intraspinal fibres showed that the NMDA- and the non-NMDA-mediated EPSCs were evoked by activation of presynaptic fibres with similar sensitivities to the stimulation intensity, suggesting that the same presynaptic fibres released the excitatory amino-acid (EAA) activating the two sub-sets of receptors. Studies of the amplitude fluctuations of EPSCs elicited by stimulation of a presumed single fibre revealed similar proportions of transmission failures and similar distributions of both the NMDA- and the non-NMDA-mediated components. These similarities suggest that the EAA transmitter activating the two sub-types of receptors is released from the same set of synaptic boutons and that the receptors are therefore post-synaptically co-localized. In addition the gamma aminobutyric acidB (GABAB) receptor agonist L-baclofen, which is known to decrease transmitter release, changed the amplitude distributions of non-NMDA- and NMDA-receptor-mediated EPSCs into unimodal distributions without affecting the amplitude of the presumed unitary event. The similarity between the transmitter release profiles of the two EAA components further supports the notion of postsynaptic receptor co-localization.