Bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice

Modulating dopamine release by optogenetics in transgenic mice reveals terminal dopaminergic dynamics

Dopamine (DA) release and uptake dynamics in the nucleus accumbens (NAc) have important implications for neurological diseases and mammalian animal behaviors. We demonstrate here the use of cell-type-specific optogenetic targeting in conjunction with fast-scan cyclic voltammetry applied to brain slices prepared from specifically tailored transgenic mice, which conditionally express channelrhodopsin-2 (ChR2) through dopamine transporter (DAT)-Cre. Terminal dopaminergic dynamics and the direct manipulation of induced DA release level by controlling light intensity, pulse width, and the shape of stimulation waveforms were studied. Effective cell terminal-targeting optogenetic induction of DA release at physiological levels in NAc is demonstrated and discussed. It was found that delivering more light energy by increasing stimulation intensity and length is not the only way to control DA release; the temporal shape of the stimulus waveform at light onset is also critically related to induced DA concentrations. In addition, DA uptake dynamics as well as the recovery of the presynaptic releasable DA pool are studied and modeled. More broadly, our experimental findings provide important further evidence for effectively applying optogenetics to induce neurotransmitter release in the behaviorally relevant region of the brain in a highly cell-type selective context.

VIP enhances both pre- and postsynaptic GABAergic transmission to hippocampal interneurones leading to increased excitatory synaptic transmission to CA1 pyramidal cells

Vasoactive intestinal peptide (VIP) is present in the hippocampus in three subtypes of GABAergic interneurones, two of which innervate preferentially other interneurones, responsible for pyramidal cell inhibition. We investigated how pre- and postsynaptic modulation of GABAergic transmission (to both pyramidal cells and interneurones) by VIP could influence excitatory synaptic transmission in the CA1 area of the hippocampus. VIP (0.1-100 nM) increased [(3)H]GABA release from hippocampal synaptosomes (maximum effect at 1 nM VIP; 63.8 +/- 4.0%) but did not change [(3)H]glutamate release. VIP (0.3-30 nM) enhanced synaptic transmission in hippocampal slices (maximum effect at 1 nM VIP; field excitatory postsynaptic potentials (epsp) slope: 23.7 +/- 1.1%; population spike amplitude: 20.3 +/- 1.7%). The action on field epsp slope was fully dependent on GABAergic transmission since it was absent in the presence of picrotoxin (50 microM) plus CGP55845 (1 microM). VIP (1 nM) did not change paired-pulse facilitation but increased paired-pulse inhibition in CA1 pyramidal cells (16.0 +/- 0.9%), reinforcing the involvement of GABAergic transmission in the action of VIP. VIP (1 nM) increased muscimol-evoked inhibitory currents by 36.4 +/- 8.7% in eight out of ten CA1 interneurones in the stratum radiatum. This suggests that VIP promotes increased inhibition of interneurones that control pyramidal cells, leading to disinhibition of synaptic transmission to pyramidal cell dendrites. In conclusion, concerted pre- and postsynaptic actions of VIP lead to disinhibition of pyramidal cell dendrites causing an enhancement of synaptic transmission.

GABA(B) receptor-mediated heterosynaptic depression of excitatory synaptic transmission in rat frontal neocortex

Neocortical synapses display several forms of short-term plasticity including paired-pulse facilitation and depression. The mechanisms underlying this diversity are unclear. Synaptic currents in response to paired stimulation were recorded from layer II/III pyramidal neurons in rat frontal neocortical slices using the whole-cell patch-clamp method. Both paired-pulse facilitation (PPF) and paired-pulse depression (PPD) were observed in control saline. In the presence of 10 microM bicuculline (BIC), prominent PPD was consistently elicited. The maximal depression of the second EPSC occurred around 100 ms although PPD was still observed at intervals up to 1500 ms. Manipulations that reduced the probability of transmitter release significantly affected PPD. Both conditioning (C)- and test (T)-EPSCs were reduced when the extracellular Ca(2+) concentration was lowered from 3 to 1 mM. The decrease was greater in the C-EPSC resulting in a decrease in PPD. The gamma-aminobutyric acid (GABA)(B) receptor agonist baclofen (10 microM) reduced the amplitude of both evoked EPSCs and changed PPD to PPF. In the presence of the GABA(B) antagonists 2(OH)-saclofen (200-400 microM) or SCH50911 (10 microM), PPF was commonly observed. The metabotropic glutamate receptor antagonist MCPG (500 microM) had no effect on neocortical PPD. Brief stimulus trains induced a progressive depression that was insensitive to GABA(B) antagonists. Paired-pulse depression of excitatory synaptic transmission is a prominent phenomenon in frontal neocortex. At least two components of depression were observed. They may play an important role in regulating the balance between excitation and inhibition, therefore maintaining stability in cortical circuits.

Weaker synaptic inhibition in CA1 region of ventral compared to dorsal rat hippocampal slices

Extracellular and intracellular recordings were made from slices taken from the dorsal (DH) and ventral (VH) part of rat hippocampus. Using paired-pulse stimulation of Schaffer collaterals, at different interpulse intervals (IPIs), and records of the population spike (PS) we found that the strength and duration of paired-pulse inhibition was much weaker in VH compared to DH slices: at the IPI of 10 ms the decrease of PS in VH (40%) was significantly smaller compared to that in DH slices (76%), while at 20 ms the decrease of PS in DH slices (60%) corresponded to facilitation in VH slices. Moreover, the amplitude and duration of intracellularly recorded fast inhibitory postsynaptic potentials (fast-IPSPs) were found significantly smaller in VH (5.2+/-0.6 mV, 54.8+/-5.8 ms) than in DH (11.2+/-1.1 mV, 105+/-10 ms) neurons. The smaller and shorter fast-IPSP recorded in VH neurons may at least in part explain the results in paired-pulse inhibition. The demonstrated weaker inhibition may underlie the higher propensity of the ventral hippocampus for epileptiform activity.

Long-term potentiation and adenosine sensitivity are unchanged in the AS/AGU protein kinase Cgamma-deficient rat

The AS/AGU rat is a spontaneously occurring mutation which exhibits locomotor abnormalities, reduced tyrosine hydroxylase levels in substantia nigra and lower extracellular levels of dopamine, making it a valuable model for some human locomotor disorders, and spontaneous chronic degeneration. The molecular defect is an absence of protein kinase Cgamma (PKCgamma), an enzyme suggested to play a role in synaptic plasticity. We have therefore examined long-term potentiation (LTP) in hippocampal slices from the mutant animals compared with the normal control strain of Albino Swiss rat. In the CA1 region, LTP was of the same magnitude in mutant and control animals, and the presynaptic inhibitory effects of adenosine were unchanged in naïve slices or following LTP. Paired-pulse inhibition and facilitation were normal. It is concluded that the absence of PKCgamma in this strain does not modify synaptic plasticity or presynaptic sensitivity to adenosine.

Time window of autoreceptor-mediated inhibition of limbic and striatal dopamine release

Forebrain dopamine release is under the local control of D2 family (D2 and D3) autoreceptors. In this study, autoreceptor-mediated modulation of forebrain dopamine release was investigated using amperometry in brain slices following local electrical stimulation. 350 microm-thick slices of nucleus accumbens or dorsolateral neostriatum were prepared from male Wistar rats (150-200 g) and superfused with artificial cerebrospinal fluid at 32 degrees C. Dopamine release was evoked by electrical pulses (0.1 ms, 10 mA) across bipolar tungsten stimulating electrodes and measured at carbon fibre microelectrodes using fixed potential amperometry (+300 mV vs. Ag/AgCl). Peak dopamine release on stimulation (single pulse) was 0.75 microM (neostriatum) and 1.37 microM (nucleus accumbens). Metoclopramide (1 microM) had no significant effect on DA efflux from a single pulse in either region. Using paired pulse stimuli, dopamine release on the second pulse varied according to the interval between the two pulses. At very long intervals (>20 sec), dopamine release was similar to that for the first pulse. At shorter intervals, dopamine efflux was attenuated. Metoclopramide had no effect on second pulse dopamine release when the pulse was applied at short (<0.1 sec) or long (>5.0 sec) intervals after the first. At intermediate intervals, metoclopramide significantly increased second pulse dopamine release. The peak dopamine autoreceptor effect occurred at approximately 550 ms in neostriatum and approximately 700 ms in nucleus accumbens. The onset time is due both to diffusion of dopamine from the release sites to the autoreceptors and receptor-effector mechanisms. These findings may have implications for the local control of forebrain dopamine function in physiological and pathological states.

Suppression of presynaptic responses to adenosine by activation of NMDA receptors

The interactions between adenosine and NMDA receptors has been investigated using the paired-pulse paradigm in hippocampal slices. This technique allows the study of drug effects specifically at presynaptic terminals. The inhibitory effect of adenosine on population spikes, and the decrease of paired-pulse inhibition assessed using either population spikes or population excitatory postsynaptic potentials, were suppressed by performing the experiments in magnesium-free medium, or by superfusion of the slices with N-methyl-D-aspartate (NMDA) at a concentration (4 microM) which did not itself affect potential size. The suppressant effect of NMDA was prevented by 2-amino-5-phosphonopentanoic acid. All these interactions were still seen in the presence of bicuculline methobromide, 30 microM. Neither alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) nor kainate produced a suppression of adenosine responses. The presence of NMDA did not modify the effects of baclofen on population potentials or paired-pulse inhibition. Activating NMDA receptors by the induction of long-term potentiation or by superfusion with glycine also reduced significantly the effects of adenosine on population spikes and paired-pulse interactions. Increasing population potential size by a mechanism which did not involve the activation of NMDA receptors (increasing stimulus strength) did not change sensitivity to adenosine. When adenosine receptor-selective agonists were tested, it was found that NMDA did not modify the inhibitory effect of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine, but did enhance the excitatory effect of the adenosine A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). The combined response to NMDA and CGS21680 was prevented by the adenosine A(2A) receptor selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). It is concluded that NMDA receptor activation can suppress neuronal sensitivity to adenosine by acting at presynaptic sites, and that this interaction results from an increase in the excitatory action of adenosine A(2A) receptors, rather than a depression of A(1) receptor function.

Polysynaptic excitatory pathways induce heterosynaptic depression in the rat auditory cortex

Short-term plasticity, the effect of a preceding synaptic response on the following response in a pair, in layers II/III of the rat auditory cortex slice after application of repetitive stimuli to layer IV, was investigated using a multichannel extracellular recording system. Paired-pulse depression, which is induced to a moderate degree in standard artificial cerebrospinal fluid, was markedly facilitated in the presence of bicuculline, a GABA(A) receptor antagonist, concurrent with the emergence of a polysynaptic component of the EPSP (polyEPSP) in the first response in a pair. This depression (bicuculline-facilitated synaptic depression, BFSD) was maximal at the minimum interval tested (50 ms), reduced as the interval was increased, and persisted beyond an interval of 2 s. The occurrence of BFSD was dependent on the presence of a polyEPSP regardless of the presence of the monosynaptic component of the EPSP, indicating that BFSD is induced by a heterosynaptic mechanism. D-AP5, an NMDA receptor antagonist, partially eliminated polyEPSPs and reversed BFSD. These results suggest that activation of polysynaptic excitatory pathways induces a heterosynaptic depression in the range of a few seconds and that NMDA receptor activity is involved in this heterosynaptic depression.

Rapid activation of GABAergic interneurons and possible calcium independent GABA release in the mormyrid electrosensory lobe

The primary afferent fibers from the electroreceptors of mormyrid electric fish terminate centrally in the granular layer of the electrosensory lobe (ELL). This study examines the excitatory and inhibitory processes that take place in this layer using an in vitro slice preparation and field potentials evoked by stimulation of primary afferent fibers in the deep fiber layer of ELL. The postsynaptic response to stimulation of the afferent fibers was still present after blocking chemical transmission in three different ways: by adding glutamate receptor antagonists to the medium, by substituting a nominally calcium-free medium for normal medium, and by blocking calcium channels with cadmium. Blockade of chemical transmission was demonstrated by disappearance of control responses to parallel fiber stimulation. The continued presence of a postsynaptic response in the absence of chemical excitation is consistent with previous anatomic and physiological evidence for electrical synapses between afferent fibers and granular cells in ELL. Granular cell activation by primary afferent fibers was followed by a powerful, short-latency inhibition mediated by GABA and GABA(A) receptors, as indicated by a large increase in the postsynaptic response to afferent fiber stimulation following application of the GABA(A) receptor antagonist, bicuculline. Bicuculline caused a marked increase of the postsynaptic response even after chemical synaptic excitation had been blocked by glutamate receptor antagonists, by a calcium-free medium, or by cadmium. Thus activation of the inhibitory interneurons responsible for GABA release did not require chemical excitation. Nonchemical excitation of the inhibitory interneurons could be mediated either by electrical synapses between afferent fibers and inhibitory interneurons, or by nonsynaptic activation of the large GABAergic terminals that are known to be present on granular cells. The marked increase of the postsynaptic response caused by bicuculline in a calcium-free medium or in the presence of cadmium suggests that the release of GABA by inhibitory terminals was not entirely dependent on calcium influx. This effect of bicuculline on the postsynaptic response in a calcium-free medium or in the presence of cadmium was markedly reduced by prior addition of the GABA transporter antagonist, nipecotic acid. Thus calcium-independent release of GABA may occur in ELL and may be partly dependent on reversal of a GABA transporter. Rapid and powerful inhibition at the first stage in the processing of electrosensory information could serve to enhance the small differences in latency among afferent fibers that appear to encode small differences in stimulus intensity.

Neocortical hyperexcitability after GABA withdrawal in vitro

The sharp interruption of the intracortical instillation of exogenous gamma-aminobutyric acid (GABA), generates an epileptic focus in mammals. Seizures elicited by GABA withdrawal last several days or weeks. The present work reports that GABA withdrawal-induced hyperexcitability can be produced in vitro: a sudden withdrawal of GABA (5 mM; 120 min) or benzodiazepine (60 microM flunitrazepam) from the superfusion, induced a gradual increase in the amplitude of the evoked population spike (PS) recorded on neocortical slices. PS enhancement reached 150% above the control value 2.5 h after GABA withdrawal. GABA withdrawal-induced hyperexcitability was facilitated by progesterone. PS enhancement induced by GABA withdrawal was associated with an impairment of GABA transmission occurring before epileptiform discharges were fully established. Paired pulse inhibition and evoked [3H]-GABA release appear decreased; suggesting that cortical hyperexcitability as a result of GABA withdrawal involves pre-synaptic changes. Specific muscimol binding decreased during GABA superfusion but recovered after GABA withdrawal. However, the sensitivity of the post-synaptic response to 3alpha-OH-5alpha-pregnan-20-one or allopregnanolone (alloP) was enhanced after GABA withdrawal, suggesting a functional change in the GABA(A) receptors. The changes described may be the cellular correlates of the withdrawal syndromes appearing after interruption of the administration of GABA(A) receptor agonists.

Occlusive responses to adenosine A1 receptor and muscarinic M2 receptor activation on hippocampal presynaptic terminals

There is substantial evidence for an interaction between adenosine A1 and muscarinic M1/M3 receptors in some tissues, either at the level of the receptors themselves or at the associated transduction system. We have now addressed the question of whether there is a similar interaction between A1 and presynaptic M2 receptors in the hippocampus. The effects of cyclopentyladenosine (CPA) were studied alone or in combination with the M2 receptor agonist oxotremorine-M. The ability of both to depress synaptic transmission presynaptically at the concentrations used was confirmed using paired-pulse inhibition. When combined at a range of concentrations, the effects of the two agents were less than additive, suggesting that they are acting by a common transduction system. The results indicate that the modulatory, antagonistic effects of A1 adenosine receptors are exerted not only on postjunctional M1/M3 receptors but also at M2 presynaptic receptors.

Electrophysiological evidence for agonist properties of flumazenil, a benzodiazepine receptor antagonist, in rat hippocampus slices

The purpose of this study was to determine the ability of the putative benzodiazepine antagonist flumazenil to modulate the excitatory synaptic responses recorded from rat hippocampus slices. The benzodiazepine agonist clonazepam was demonstrated to depress the CA1 population spike. This effect was attributed to an enhancement of GABA efficacy after its electrically-elicited release from local inhibitory circuitry. As an unexpected effect, flumazenil failed to antagonize this depressing effect. Moreover, flumazemil was observed to significantly depress, on its own, the magnitude of the evoked response to the activation of the excitatory afferents. This intrinsic depressant activity of flumazenil suggests that flumazenil acts 'in vitro' as an agonist at the benzodiazepine receptors, and is consistent with some previously reported atypical effects of flumazenil 'in vivo'.

The contribution of adenosine to paired-pulse inhibition in the normal and disinhibited hippocampal slice

The effects of the adenosine receptor antagonist 1,3-dimethyl-8-cyclopentylxanthine (cyclopentyltheophylline) and the enzyme adenosine deaminase have been examined on paired-pulse inhibition between orthodromic evoked field potentials in the CA1 region of the normal and disinhibited hippocampal slice. In the presence of the GABAA receptor antagonist (-)-bicuculline methobromide, cyclopentyltheophylline suppressed homosynaptic paired-pulse inhibition between stimuli 300 ms apart. Slices treated with (-)-bicuculline and cyclopentyltheophylline together tended to develop spontaneous burst potentials. In slices in which a surgical cut isolated the CA1 and CA3 areas, thereby preventing the development of bursts in CA1, the effect on paired-pulse inhibition was lessened but was still apparent. Adenosine deaminase, in the presence of (-)-bicuculline showed the same effect as cyclopentyltheophylline, decreasing substantially the amount of paired-pulse inhibition. These results suggest that adenosine may contribute to homosynaptic paired-pulse inhibition in disinhibited slices. For comparison, we also examined the effect of cyclopentyltheophylline in normal ((-)-bicuculline-free) slices. At 100 nM, cyclopentyltheophylline increased reversibly the size of orthodromically evoked synaptic population potentials in the CA1 region of the slices and also reduced reversibly the degree of homosynaptic paired-pulse inhibition between two stimuli delivered only 30 ms apart. This suggests that adenosine may also contribute to shorter latency paired-pulse inhibition in the normal hippocampal slice.

Comparative sensitivity to adenosine of paired-pulse inhibition and single field potentials in the rat hippocampus

If excitatory terminals onto inhibitory interneurones were more sensitive to adenosine than excitatory terminals onto pyramidal cells in the hippocampus it might explain the effect of adenosine to decrease paired-pulse inhibition and account for reported excitatory effects of low concentrations of adenosine. We have compared the concentration-response relationships for the effect of adenosine on single evoked field potentials and on paired-pulse inhibition in the CA1 area of the rat hippocampal slice in order to test this hypothesis. Adenosine caused a concentration-dependent decrease in both single evoked population spike size and in paired-pulse inhibition between potentials. The concentration-response relationships for both effects was very similar, ruling out the possibility that excitatory terminals onto inhibitory interneurones are more sensitive to adenosine than excitatory terminals onto pyramidal cells, and suggesting that the receptors located at the two sites may be indistinguishable.

Interaction of the beta-carboline harmaline with a GABA-benzodiazepine mechanism: an electrophysiological investigation on rat hippocampal slices

An interaction of harmaline (HA), a beta-carboline, with benzodiazepine (Bzd) receptors, has been reported. HA perfusion induced a similar, although less potent, depressing effect as clonazepam (CLO) on the amplitude of the population spikes (PS) evoked by Schaffer collateral stimulation in the CA1 area of rat hippocampal slices. The suppressant effect of both CLO and HA on PS amplitude was reversed by simultaneous perfusion of the GABA antagonist picrotoxin. These results suggest that HA acts as a weak or partial agonist at Bzd receptors.

Modulation by adenosine of a neuronal inhibitory interaction in the rat hippocampus

Adenosine is acknowledged to have a primarily inhibitory function in the central nervous system, but is believed to have little effect on inhibitory neurones themselves. It is, however, difficult to determine the effect of adenosine on inhibitory synaptic potentials since adenosine directly depresses evoked potentials and, in the presence of bicuculline to block GABAA-mediated inhibition, the bicuculline-resistant fraction of paired-pulse inhibition (ppi) is greater between pairs of small potentials than between pairs of larger potentials. Here, adenosine increased bicuculline-resistant ppi when stimulus strength was constant between adenosine and control but ppi of responses in adenosine was markedly less than ppi of control responses of the same size. Adenosine had less effect on the size of 'conditioned' potentials than on control potentials. It is concluded that adenosine can reduce the bicuculline-resistant fraction of paired-pulse inhibition in the hippocampus. Further quantitative comparison of the effects of adenosine on ppi and on single evoked potentials excluded a difference in the potency of adenosine at excitatory and inhibitory terminals as an explanation for this activity. The results suggest that adenosine may diminish bicuculline-resistant paired-pulse inhibition by enhancing a simultaneous facilitatory component of the neuronal responses.

Effect of adenosine on bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice

This study extends previous investigations into the effect of adenosine on bicuculline-resistant paired-pulse inhibition between field potentials evoked 300 ms apart in the CA1 area of the rat hippocampal slice. A direct assessment of the effect of adenosine on paired-pulse inhibition is complicated by the facts that adenosine directly depresses evoked potentials and bicuculline-resistant paired-pulse inhibition is greater between pairs of small potentials than between pairs of larger potentials. Adenosine increased bicuculline-resistant paired-pulse inhibition when stimulus strength was constant between adenosine and control but paired-pulse inhibition of responses in adenosine was markedly less than paired-pulse inhibition of control responses of the same size. Furthermore, adenosine decreased the size of conditioned potentials to a significantly lesser extent than unpaired potentials of the same initial size. Taken together the results indicate that adenosine can decrease bicuculline-resistant paired-pulse inhibition in the hippocampus. A possible mechanism for this effect is that adenosine is suppressing transmission at excitatory terminals onto interneurones which would suggest that these receptors are more sensitive to adenosine than those on the Schaffer collateral/CA1 pyramidal cell synapses. In this case adenosine should reduce paired-pulse inhibition at lower concentrations than are required for depression of single evoked potentials. A comparison of the concentration-response relationships for the effects of adenosine on paired-pulse inhibition and on single evoked potentials ruled out greater sensitivity of adenosine receptors at excitatory terminals onto interneurones as an explanation for adenosine's action on bicuculline-resistant paired-pulse inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Release and actions of adenosine in the central nervous system

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mumol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established, though the nucleotide is probably released partly directly, as a result of raised intracellular levels, and partly as nucleotides, which are subsequently hydrolysed. Once in the extracellular medium, adenosine markedly inhibits the release of excitatory neurotransmitters and modulatory peptides and has direct inhibitory effects on postsynaptic excitability via A1 receptors. A population of A2 receptors may mediate depolarization and enhanced transmitter release. Adenosine also modulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observed in conscious animals. As a result of their many actions, adenosine analogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.