Hyperpolarizing synaptic potentials evoked in CA1 pyramidal cells by glutamate stimulation of interneurons from the oriens/alveus border of rat hippocampal slices. II. Sensitivity to GABA antagonists

Spike-timing-dependent plasticity rewards synchrony rather than causality

Spike-timing-dependent plasticity (STDP) is a candidate mechanism for information storage in the brain, but the whole-cell recordings required for the experimental induction of STDP are typically limited to 1 h. This mismatch of time scales is a long-standing weakness in synaptic theories of memory. Here we use spectrally separated optogenetic stimulation to fire precisely timed action potentials (spikes) in CA3 and CA1 pyramidal cells. Twenty minutes after optogenetic induction of STDP (oSTDP), we observed timing-dependent depression (tLTD) and timing-dependent potentiation (tLTP), depending on the sequence of spiking. As oSTDP does not require electrodes, we could also assess the strength of these paired connections three days later. At this late time point, late tLTP was observed for both causal (CA3 before CA1) and anticausal (CA1 before CA3) timing, but not for asynchronous activity patterns (Δt = 50 ms). Blocking activity after induction of oSTDP prevented stable potentiation. Our results confirm that neurons wire together if they fire together, but suggest that synaptic depression after anticausal activation (tLTD) is a transient phenomenon.

The GABAA receptor-mediated recurrent inhibition in ventral compared with dorsal CA1 hippocampal region is weaker, decays faster and lasts less

Hippocampal functions appear to be segregated along the dorso-ventral axis of the structure. Differences at the cellular and local neuronal network level may be involved in this functional segregation. In this study the characteristics of CA1 recurrent inhibition (RI) were measured and compared between dorsal (DH, n = 95) and ventral (VH, n = 60) hippocampal slices, using recordings of suprathreshold field potentials. RI strength was estimated as the percentile decrease of the population spike (PS) amplitude evoked with an orthodromic stimulus (at the Schaffer collaterals) when preceded by an antidromic stimulus (at the alveus). Varying the interpulse interval (IPI) between the two stimuli, we estimated RI duration. Alvear stimulation produced significant PS suppression in both VH and DH at every IPI tested, from 10 to 270 ms. Moreover, gradually more oblique DH (but not VH) slices displayed increasing RI, which at IPIs < or = 125 ms was reversibly abolished by the GABAA receptor antagonist picrotoxin (10 microM). The GABAA-mediated RI, measured under the blockade of GABAB receptors, was weaker, decayed faster and lasted less in VH compared to DH slices, regardless of the slice orientation. Specifically, in VH compared to DH, the PS suppression at 20 ms was 34.4 +/- 4.5% versus 69.9 +/- 6.5% (P < 0.001), the time constant of RI decay was 29 +/- 2.4 versus 87.5 +/- 13.6 ms (P < 0.01) and the duration was 50 versus 125 ms (P < 0.001). Thus, GABAA-mediated RI may control the CA1 excitatory output less effectively in VH compared to DH. The observed dorso-ventral differences in RI contribute to the longitudinal diversification of the structure and may underlie to some extent the region-specificity of hippocampal functions.

alpha7 nicotinic acetylcholine receptors on GABAergic interneurons evoke dendritic and somatic inhibition of hippocampal neurons

GABAergic interneurons in the hippocampus express high levels of alpha7 nicotinic acetylcholine receptors, but because of the diverse roles played by hippocampal interneurons, the impact of activation of these receptors on hippocampal output neurons (i.e., CA1 pyramidal cells) is unclear. Activation of hippocampal interneurons could directly inhibit pyramidal neuron activity but could also produce inhibition of other GABAergic cells leading to disinhibition of pyramidal cells. To characterize the inhibitory circuits activated by these receptors, exogenous acetylcholine was applied directly to CA1 interneurons in hippocampal slices, and the resulting postsynaptic responses were recorded from pyramidal neurons or interneurons. Inhibitory currents mediated by GABA(A) receptors were observed in 27/131 interneuron/pyramidal cell pairs, but no instances of disinhibition of spontaneous inhibitory events or GABA(B) receptor-mediated responses were observed. Two populations of bicuculline-sensitive GABA(A) receptor-mediated currents could be distinguished based on their kinetics and amplitude. Anatomical reconstructions of the interneurons in a subset of connected pairs support the hypothesis that these two populations correspond to inhibitory synapses located either on the somata or dendrites of pyramidal cells. In 11 interneuron/interneuron cell pairs, one presynaptic neuron was observed that produced strong inhibitory currents in several nearby interneurons, suggesting that disinhibition of pyramidal neurons may also occur. All three types of inhibitory responses (somatic-pyramidal, dendritic-pyramidal, and interneuronal) were blocked by the alpha7 receptor-selective antagonist methyllycaconitine. These data suggest activation of these functionally distinct circuits by alpha7 receptors results in significant inhibition of both hippocampal pyramidal neurons as well as interneurons.

Identification of different putative neuronal subtypes in cultures of the superior region of the hippocampus using electrophysiological parameters

Cultured neurons offer many advantages over a slice preparation for whole-cell patch-clamp studies, such as better control over the environment and space clamp control. However, heterogeneous cultures of neurons present problems in distinguishing the cell type from which recordings are made. The present study uses correlations with data obtained in the hippocampal slice preparation to determine the feasibility of "identifying" different neuronal subtypes in cultures obtained from the superior region of postnatal two- to 13-day-old rat hippocampus. Whole-cell patch-clamp recording in the current-clamp mode after 24-96 h in culture was used to determine if the action potential duration would be a useful criterion in distinguishing cell types. Single action potentials were elicited by a 0.1-0.2 ms, 2-4 nA depolarizing pulse. The average membrane potential and input resistance were -46.8+/-1.2 mV (n = 58) and 576+/-56 Mohms (n = 57), respectively. A frequency distribution of the action potential duration measured at half-maximal amplitude showed four distinct groups of neurons (group 1, 1.36+/-0.03 ms, n = 17; group 2, 2.19+/-0.05 ms, n = 20; group 3, 3.17+/-0.10 ms, n = 16; group 4, 4.36+/-0.13, n = 5). Based on correlations with previous studies using intracellular recording in identified cells in slices, the data suggest that group 1 represents basket cells, group 2 represents vertical cells, group 3 represents a combination of stellate cells and pyramidal cells, and group 4 represents another unidentified class of cells. Further analysis of the fast afterhyperpolarization allows distinction between pyramidal cells and stellate cells in group 3. In contrast to the interneurons in a slice preparation, these cells offer good voltage control and environmental control. Future studies will record from these cells in current-clamp mode to quickly characterize the action potential before switching to voltage-clamp recording to characterize the currents present in the different types of interneurons.

Synaptic connectivity of distinct hilar interneuron subpopulations

Dual intracellular recordings of hilar interneurons and CA3 pyramidal cells were performed in transverse slices of guinea pig hippocampus in the presence of the convulsant compound 4-aminopyridine (4-AP) and ionotropic glutamate receptor antagonists. Under these conditions, interneurons burst fire synchronously, producing synchronized inhibitory postsynaptic potentials (sIPSPs) in pyramidal cells. Three different hilar interneuron subpopulations that contributed to the sIPSP were identified based on their projection properties and morphology. These three types were pyramidal-like stellate interneurons, spheroid interneurons, and oviform interneurons. Physiologically, pyramidal-like stellate interneurons could be differentiated from the other interneuron subpopulations because they generated short synchronized bursts of action potentials coincident with the hyperpolarizing and depolarizing gamma-aminobutyric acid-A (GABAA)-mediated inhibitory postsynaptic potentials (IPSPs) recorded in pyramidal cells. The bursts in pyramidal-like stellate cells were abolished by theGABAA-receptor blocker, bicuculline. In contrast, spheroid interneurons of the dentate-hilus (D-H) border and oviform hilar interneurons exhibited prolonged bicuculline-resistant bursts that occurred coincident with the GABAB pyramidal cell sIPSPs. Pyramidal-like stellate interneurons likely did not contribute to the generation of synchronized GABAB responses in hippocampal pyramidal cells. Spheroid interneurons were unique among these subpopulations of interneurons in that the bicuculline-resistant bursts in spheroid interneurons were sustained by a synaptic depolarization that persisted in the presence of antagonists of ionotropic glutamate, GABAA and GABAB receptors [6-cyano-7-nitroquinoxaline-2,3-dione, 20 microM; 3-3(2-carboxipiperazine-4-yl)propyl-1-phosphonate, 20 microM; bicuculline, 10-15 microM; CGP 55845A, 20 microM]. This novel depolarizing potential reversed between -30 and 0 mV. No noticeable synaptic depolarization sustaining burst firing could be isolated in oviform interneurons, suggesting that firing in this interneuron subpopulation was synchronized by nonsynaptic mechanisms. The results of the present study indicate that the hilar inhibitory circuit is composed of at least three different subpopulations of interneurons, distinguishable by their morphological characteristics and synaptic inputs and outputs. These findings give further support to the hypothesis that there are distinct populations of interneurons producing GABAA and GABAB responses with defined functional roles within the hippocampal inhibitory circuit. Notably, we found that spheroid interneurons were unique among the hilar interneurons studied, in that the synchronized bursts observed in these cells are sustained by a novel ionotropic glutamate and GABA receptor-independent synaptic depolarization.

GABA(B) modulation improves sequence disambiguation in computational models of hippocampal region CA3

Computational models of hippocampal region CA3 were used to study the role of theta rhythm in storage and retrieval of temporal sequences of neuronal activity patterns. Retrieval of multiple overlapping temporal sequences requires a mechanism for disambiguation, e.g., for choosing between two sequences with the same starting pattern but different final patterns (forked sequences). Modulatory input to the hippocampus from the medial septum may enhance the disambiguation of pattern sequences by causing phasic changes in the relative strength of afferent input and recurrent excitation. In the models, the strength of recurrent synaptic transmission is modulated by activation of GABA(B) receptors. Theta frequency inputs from the medial septum cause oscillations in the levels of GABA in the model, producing phasic changes in the strength of synaptic potentials during a theta cycle similar to those observed experimentally (Wyble et al., Soc Neurosci Abstr 1997;23: 197.7). These phasic changes in GABA(B) suppression improve sequence disambiguation in the simulations, as previously shown with analysis of a simpler model (Sohal and Hasselmo, Neural Comp 1998;10:889-902). In addition, tonic changes in levels of cholinergic modulation enhance the storage of forked sequences by preventing a strong influence of recurrent synapses during storage.

Facilitating pyramid to horizontal oriens-alveus interneurone inputs: dual intracellular recordings in slices of rat hippocampus

1. In adult rat hippocampal slices, simultaneous intracellular recordings from pyramidal cells in CA1 and interneurones near the stratum oriens-alveus border revealed excitatory connections that displayed facilitation on repetitive activation in twelve of thirty-six pairs tested. 2. Postsynaptic interneurones were classified as horizontal oriens-alveus interneurones by the pronounced 'sag' in response to hyperpolarizing current injection, high levels of spontaneous synaptic activity and by the morphology of their somata and dendrites, which were confined to stratum oriens-alveus and their axons which projected to stratum lacunosum-moleculare where they ramified extensively, in the region of entorhinal cortex input to CA1. 3. Excitatory postsynaptic potentials (EPSPs) elicited by single pyramidal cells were 0 to 12 mV in amplitude. Mean EPSP amplitude (single spikes) was 0.93 +/- 1. 06 mV at -70 +/- 2.3 mV (n = 10). The rise time was 1.2 +/- 0.5 ms and the width at half-amplitude was 7.5 +/- 4.7 ms. 4. EPSPs fluctuated greatly in amplitude; the mean coefficient of variation was 0.84 +/- 0.37 for the first EPSP and 0.47 +/- 0.24 for the second. Apparent failures of transmission frequently occurred after first presynaptic spikes but less frequently after the second or subsequent spikes in brief trains. 5. EPSPs displayed facilitation at membrane potentials between -80 mV and spike threshold. Second EPSPs within 20 ms of the first were 253 +/- 48 % (range, 152-324 %) of the mean first EPSP amplitude. Third EPSPs within 60 ms were 266 +/- 70 % (range, 169-389 %) and fourth EPSPs within 60-120 ms were 288 +/- 71 % (range, 188-393 %). Both proportions of apparent failures of transmission and coefficient of variation analysis indicated a presynaptic locus for this facilitation.

Theta-like activity in hippocampal formation slices: the effect of strong disinhibition of GABAA and GABAB receptors

The involvement of GABAA and GABAB receptors in neural mechanisms responsible for the production of theta rhythms in hippocampal formation (HPC) slices is addressed in the present study. In a number of papers published in the last decade, we have demonstrated that theta-like activity can be successfully recorded in the limbic cortex maintained in vitro when the cholinergic agonists, acetylcholine, carbachol or muscarine, were added to the bath. Recently, we have also shown a strong GABAA modulation of the cholinergic-induced in vitro theta-like activity. This study presents a report of the first demonstration of in vitro theta-like field responses induced a consequence of simultaneously inhibiting hippocampal GABAA and GABAB receptors. HPC slices (350 microns) were maintained in a gas-liquid interface chamber (35 degrees C). Theta-like activity was induced in the presence of bath perfusion of bicuculline (GABAA antagonist) and 2-hydroxysaclophen (GABAB antagonist). This in vitro induced field response was antagonized both by muscimol (GABAA agonist) and baclophen (GABAB agonist). In addition, the experiments presented here revealed that bicuculline/2-hydroxysaclophen-induced in vitro theta-like activity also had a strong cholinergic M1 involvement: it was abolished by hemicholinium-3 (choline transport blocker) and pirenzepine (specific antagonist of M1 receptor), but not by gallamine (specific antagonist of M2 receptor). The results of the present study provided further evidence for a strong GABAergic/cholinergic interaction in the neural mechanism responsible for production of theta-like activity in the hippocampal formation slices.

Interneurons in area CA1 stratum radiatum and stratum oriens remain functionally connected to excitatory synaptic input in chronically epileptic animals

Past work has demonstrated a reduction of stimulus-evoked inhibitory input to hippocampal CA1 pyramidal cells in chronic models of temporal lobe epilepsy (TLE). It has been postulated that this reduction in inhibition results from impaired excitation of inhibitory interneurons. In this report, we evaluate the connectivity of area CA1 interneurons to their excitatory afferents in hippocampal-parahippocampal slices obtained from a rat model of chronic TLE. Rats were made chronically epileptic by a period of continuous electrical stimulation of the hippocampus, which establishes an acute condition of self-sustained limbic status epilepticus (SSLSE). This period of SSLSE is followed by a development of chronic recurrent spontaneous limbic seizures that are associated with chronic neuropathological changes reminiscent of those encountered in human TLE. Under visual control, whole cell patch-clamp recordings of interneurons and pyramidal cells were obtained in area CA1 of slices taken from adult, chronically epileptic post-SSLSE rats. Neurons were activated by means of electrodes positioned in stratum radiatum. Intrinsic membrane properties, including resting membrane potential, action potential (AP) threshold, AP half-height width, and membrane impedance, were unchanged in interneurons from chronically epileptic (post-SSLSE) tissue compared with control tissue. Single stimuli delivered to stratum radiatum evoked depolarizing excitatory postsynaptic potentials and APs in interneurons, whereas paired-pulse stimulation evoked facilitation of the postsynaptic current (PSC) in both control and post-SSLSE tissue. No differences between interneurons in control versus post-SSLSE tissue could be found with respect to the mean stimulus intensity or mean stimulus duration needed to evoke an AP. A multiple linear regression analysis over a range of stimulus intensities demonstrated that a greater number of APs could be evoked in interneurons in post-SSLSE tissue compared with control tissue. Spontaneous PSCs were observed in area CA1 interneurons in both control and post-SSLSE tissue and were markedly attenuated by glutamatergic antagonists. In conclusion, our data suggest that stimulus-evoked and spontaneous excitatory synaptic input to area CA1 interneurons remains functional in an animal model of chronic temporal lobe epilepsy. These findings suggest, therefore, that the apparent decrease of polysynaptic inhibitory PSPs in CA1 pyramidal cells in epileptic tissue is not due to a deficit in excitatory transmission from Schaffer collaterals to interneurons in stratum radiatum and straum oriens.

Properties of unitary IPSCs in hippocampal pyramidal cells originating from different types of interneurons in young rats

Whole cell recordings were used in hippocampal slices of young rats to examine unitary inhibitory postsynaptic currents (uIPSCs) evoked in CA1 pyramidal cells at room temperature. Loose cell-attached stimulation was applied to activate single interneurons of different subtypes located in stratum oriens (OR), near stratum pyramidale (PYR), and at the border of stratum radiatum and lacunosum-moleculare (LM). uIPSCs evoked by stimulation of PYR and OR interneurons had similar onset latency, rise time, peak amplitude, and decay. In contrast, uIPSCs elicited by activation of LM interneurons were significantly smaller in amplitude and had a slower time course. The mean reversal potential of uIPSCs was -53.1 +/- 2.1 (SE) mV during recordings with intracellular solution containing potassium gluconate. With the use of recording solution containing the potassium channel blocker cesium, the reversal potential of uIPSCs was not significantly different (-58.5 +/- 2.6 mV), suggesting that these synaptic currents were not mediated by potassium conductances. Bath application of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline (25 microM) reversibly blocked uIPSCs evoked by stimulation of all interneuron subtypes. In bicuculline, the mean peak amplitude of uIPSCs recorded with potassium gluconate was reduced to 3.5 +/- 4.4% of control (n = 7). Similarly, with cesium methanesulfonate, the mean amplitude in bicuculline was 2.9 +/- 3.1% of control (n = 13). Application of the GABA(B) receptor antagonist CGP 55845A (5 microM) resulted in a significant and reversible increase in the mean amplitude of uIPSCs recorded with cesium-containing intracellular solution. Thus uIPSCs from all cell types appeared under tonic presynaptic inhibition by GABA(B) receptors. Paired stimulation of individual interneurons at 100- to 200-ms intervals did not result in paired pulse depression of uIPSCs. For individual responses, a significant negative correlation was observed between the amplitude of the first and second uIPSCs. A significant paired pulse facilitation (154.0 +/- 8.0%) was observed when the first uIPSC was smaller than the mean of all first uIPSCs. A small, but not significant, paired pulse depression (90.8 +/- 4.0%) was found when the first uIPSC was larger than the mean of all first uIPSCs. Our results indicate that these different subtypes of hippocampal interneurons generate Cl(-)-mediated GABA(A) uIPSCs. uIPSCs originating from different types of interneurons may have heterogeneous properties and may be subject to tonic presynaptic inhibition via heterosynaptic GABA(B) receptors. These results suggest a specialization of function for inhibitory interneurons and point to complex presynaptic modulation of interneuron function.

Activation of interneurons at the stratum oriens/alveus border suppresses excitatory transmission to apical dendrites in the CA1 area of the mouse hippocampus

The consequences of activation or inactivation of interneurons at the CA1 stratum oriens/ alveus border for signal transmission at the apical dendritic region of pyramidal cells were investigated in slices from mice submerged in a perfusion chamber. A characteristic subpopulation of interneurons with a horizontal dendritic tree in this region, which sends a GABAergic projection to the apical dendrites of CA1 pyramidal cells is strongly excited by metabotropic glutamate receptor activation and receives GABAergic input from vasoactive intestinal polypeptide-containing interneurons. Pressure ejection of glutamate or the metabotropic agonist 1s,3r-aminocyclopentane dicarboxylic acid from micropipettes onto the stratum oriens/alveus border caused a long lasting (more than 90 min) decrease of field-excitatory postsynaptic potentials in the strata radiatum and lacunosum-moleculare. The GABAB antagonist CGP 35348 (100 microM in the perfusion fluid) partially and reversibly blocked this effect. Vasoactive intestinal polypeptide- (0.1 microM in the bath) excited neurons with response and firing properties characteristic for interneurons at the oriens/alveus border. Local pressure application of vasoactive intestinal polypeptide (10 microM) to the alveus region led, after a brief (2 min) and small (10%) increase, to a longer lasting (30-50 min) decrease (by 20-30%) in the slope of the field-excitatory postsynaptic potential in strata radiatum and lacunosum-moleculare. This action was completely blocked by bath application of CGP 35348. Local application of tetrodotoxin in the stratum oriens/alveus region markedly increased the slope of evoked dendritic excitatory postsynaptic potentials, and caused multiple firing of pyramidal cells. Thus, stratum oriens/alveus interneurons have a profound inhibitory effect on signal transmission in the apical dendritic area of CA1, which is, at least in part, mediated by GABAB receptors. It appears that the GABAB receptor-mediated effect in stratum lacunosum-moleculare is produced by vasoactive intestinal polypeptide-sensitive interneurons.

Single axon IPSPs elicited in pyramidal cells by three classes of interneurones in slices of rat neocortex

1. Using dual intracellular recordings in slices of adult rat neocortex, twenty-four IPSPs activated by single presynaptic interneurones were studied in simultaneously recorded pyramidal cells. Fast spiking interneurones inhibited one in four or five of their close pyramidal neighbours. No reciprocal connections were observed. After recordings neurones were filled with biocytin. 2. Interneurones that elicited IPSPs were classified as classical fast spiking (n = 10), as non-classical fast spiking (n = 3, including one burst-firing interneurone), as unclassified, or slow interneurones (n = 8), or as regular spiking interneurones (n = 3), i.e. interneurones whose electrophysiological characteristics were indistinguishable from those of pyramidal cells. 3. All of the seven classical fast spiking cells anatomically fully recovered had aspiny, beaded dendrites. Their partially myelinated axons ramified extensively, varying widely in shape and extent, but randomly selected labelled axon terminals typically innervated somata and large calibre dendrites on electron microscopic examination. One 'autapse' was demonstrated. One presumptive regular spiking interneurone axon made four somatic and five dendritic connections with unlabelled targets. 4. Full anatomical reconstructions of labelled classical fast spiking interneurones and their postsynaptic pyramids (n = 5) demonstrated one to five boutons per connection. The two recorded IPSPs that were fully reconstructed morphologically (3 and 5 terminals) were, however, amongst the smallest recorded (< 0.4 mV). Some connections may therefore involve larger numbers of contacts. 5. Single axon IPSPs were between 0.2 and 3.5 mV in average amplitude at -55 to -60 mV. Extrapolated reversal potentials were between -70 and -82 mV. IPSP time course correlated with the type of presynaptic interneurone, but not with IPSP latency, amplitude, reversal potential, or sensitivity to current injected at the soma. 6. Classical fast spiking interneurones elicited the fastest IPSPs (width at half-amplitude 14.72 +/- 3.83 ms, n = 10) and unclassified, or slow interneurones the slowest (56.29 +/- 23.44 ms, n = 8). Regular spiking interneurone IPSPs had intermediate half-widths (27.3 +/- 3.68 ms, n = 3). 7. Increasingly brief presynaptic interspike intervals increased the peak amplitude of, but not the area under, the summed IPSP. Only at interspike intervals between 10 and 20 ms did IPSP integrals exhibit paired pulse facilitation. Paired pulse depression was apparent at < 10 and 20-60 ms. During longer spike trains, summing IPSPs decayed to a plateau potential that was relatively independent of firing rate (100-250 Hz). Thereafter, the voltage response could increase again. 8. Summed IPSPs elicited by two to fifteen presynaptic spike trains decayed as, or more rapidly than, single-spike IPSPs. Summed IPSPs elicited by > 20 spikes (> 150 Hz), however, resulted in an additional, more slowly decaying component (latency > 50 ms, duration > 200 ms). The possible involvement of GABAB receptors in this component is discussed. 9. It is suggested that three broad classes of interneurones may activate GABAA receptors on relatively proximal portions of neocortical pyramidal neurones. The different time courses of the IPSPs elicited by the three classes may reflect different types of postsynaptic receptor rather than dendritic location. An additional class, burst firing, spiny interneurones appear to activate GABAA receptors on more distal sites.

Spermine increases paired-pulse facilitation in area CA1 of hippocampus in a calcium-dependent manner

The effect of spermine on neurotransmission was studied in area CA1 of the hippocampal slice preparation. Paired-pulse stimulation (20 ms interpulse interval) was delivered to stratum radiatum; the evoked field potential responses were recorded simultaneously from stratum radiatum and from stratum pyramidale. At mM and sub-mM concentrations, spermine decreased the slope of pEPSP in stratum radiatum and the area of the conditioning population spike in stratum pyramidale. Short-latency paired-pulse inhibition of the population spike was converted to facilitation by spermine. These effects of spermine resembled those observed at low calcium concentration. In addition, dose-response and input-output curves determined at various Ca2+ concentrations demonstrated that the depressant effects of spermine were larger at low Ca2+ levels. The results support the notion that spermine competitively blocks presynaptic voltage-sensitive Ca2+ channels, thus causing a decreased release of neurotransmitter. Since spermine is present in brain, it is likely that it is a natural modulator of Ca2+ channels.

Presynaptic depression of inhibitory postsynaptic potentials by metabotropic glutamate receptors in rat hippocampal CA1 pyramidal cells

The effects of the metabotropic glutamate (mGlu) receptor agonists (+/-)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or 1S,3R-ACPD on gamma-aminobutyric acid (GABA)-mediated inhibitory synaptic responses have been investigated in vitro in CA1 pyramidal cells of rat hippocampal slices. Bath application of both agonists depolarized the resting membrane potential and increased membrane resistance. Simultaneously, the afterhyperpolarization induced by a burst of spikes as well as spike accomodation were blocked. Stimulation of the stratum radiatum induced in CA1 pyramidal cells an early excitatory postsynaptic potential (EPSP) followed by a fast GABAA and a slow GABAB-mediated inhibitory postsynaptic potentials (IPSPs). All synaptic responses were dose dependently depressed by mGlu receptor agonists. At low concentration, (+/-)-trans-ACPD (10-100 microM) and 1S,3R-ACPD (10 microM) consistently reduced the EPSP, slightly depressed the fast IPSP but greatly decreased the slow IPSP. Increasing the concentration of mGlu receptor agonists to 200 microM and 50 microM, respectively further depressed the EPSP and dramatically reduced the amplitude of both IPSPs. In the presence of the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) and D-(-)-2-amino-5-phosphonovaleric acid (30 microM), monosynaptically evoked IPSPs were still depressed by mGlu receptor agonists. In the same conditions, the discharge frequency of spontaneous IPSPs which reflect the activity of GABAergic interneurons was enhanced by low doses of mGlu receptor agonists but depressed with higher concentrations. On the other hand, the postsynaptic hyperpolarization and decrease in membrane resistance induced by the GABAB receptor agonist baclofen applied in the bath or by microiontophoresis were not affected by mGlu receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of zinc on hyperpolarizing and depolarizing GABAA synaptic potentials in hippocampal slice cultures

We have examined the changes in GABAA-mediated synaptic potentials recorded from CA3 pyramidal neurons in hippocampal slice cultures following application of zinc (Zn2+). Unlike 4-AP, Zn2+ did not enhance fast hyperpolarizing potentials but primarily enhanced depolarizing GABAA potentials. Zn2+ did not alter the postsynaptic response of pyramidal neurons to pressure applied GABA, consistent with previous reports that Zn2+ enhances the release of GABA from presynaptic terminals. To examine the role of local circuitry in the production of Zn2+ responses, we recorded from cultures maintained for 7-10 days following removal of the dentate and hilus to allow complete degeneration of the mossy fibers (DGX cultures). Zn2+ produced giant depolarizing potentials (GDPs) in DGX cultures that were identical to those in intact cultures. In contrast, the 4-AP response was dramatically altered in DGX cultures. In DGX cultures, Zn2+ co-applied with 4-AP appeared to inhibit the production of fast hyperpolarizing GABAA synaptic potentials produced by 4-AP alone. This inhibition of fast hyperpolarizing potentials suggests that Zn2+ may reduce the release of GABA onto pyramidal cell somata. These observations suggest that Zn2+ enhances GABA release from local circuit neurons that synapse onto pyramidal cell dendrites, and inhibits GABA release onto pyramidal cell somata.

Single axon fast inhibitory postsynaptic potentials elicited by a sparsely spiny interneuron in rat neocortex

Many of the different morphological types of interneurons in mammalian neocortex are presumed to be inhibitory, but to date, conclusive functional data have been lacking. Using paired intracellular recordings in slices of adult rat somatosensory cortex, we present a sparsely spiny, burst firing interneuron that elicits in a simultaneously recorded pyramid a fast inhibitory postsynaptic potential, reversing at -78 mV. Neither inhibitory postsynaptic potential time course, nor paired pulse depression (inter-spike interval 15-120 ms), was affected by addition of the GABAB antagonist/partial agonist 2-OH-Saclofen (250 microM), but increasing extracellular [Ca2+] enhanced inhibitory postsynaptic potential amplitude at low firing rates and increased paired pulse depression at higher rates. Light microscopic examination of the biocytin-filled neurons revealed the presynaptic cell to be a sparsely spiny interneuron and the postsynaptic to be a small pyramidal neuron, both in layer II. Ultrastructural examination of 16 terminals of the presynaptic interneuron revealed that they formed symmetric contacts with unlabelled neurons, four with neuronal somata, 10 with dendritic shafts and two with spine shafts. This, therefore, is the first report of the properties of a single axon inhibitory postsynaptic potential in neocortex resulting from action potentials in an electro-physiologically and morphologically identified interneuron. We propose that at least some of the sparsely spiny, burst firing interneurons inhibit pyramidal neurons via GABAA receptors.

Hyperpolarizing synaptic potentials evoked in CA1 pyramidal cells by glutamate stimulation of interneurons from the oriens/alveus border of rat hippocampal slices. II. Sensitivity to GABA antagonists

The receptor type mediating the inhibitory postsynaptic potentials (glut-IPSPs), recorded in CA1 pyramidal cells, as a result of glutamate stimulation of interneurons in stratum oriens near the alveus (O/A) was assessed and compared to the type mediating recurrent IPSPs evoked by recurrent activation of interneurons through glutamate stimulation of pyramidal cells in stratum pyramidale (PYR). In response to repetitive electrical stimulation, the peak amplitude of both the O/A glut-IPSP and the PYR glut-IPSP was attenuated (n = 5) in parallel to the reduction in amplitude of the early and late components of the electrically evoked response (stimulus-evoked disinhibition). This suggested the involvement of GABAergic receptors and attested that the interneurons activated during glut-IPSPs were also involved in the circuitry of the electrically evoked IPSPs. The local application of the selective GABAA antagonist bicuculline (100-200 microM) to the slice resulted in a significant reduction in the amplitude of both the O/A (by 76.5%; n = 9) and PYR (by 86.2%; n = 5) glut-IPSPs, in parallel to a decrease of the electrically evoked early IPSP, but not of the late IPSP. The presence of the GABAB antagonist 2-hydroxy-saclofen (1 mM) was able to significantly reduce the amplitude of the O/A glut-IPSPs (by 27.5%; n = 7) and of the electrically evoked late IPSP, but not the PYR glut-IPSP (n = 3). Although the application of phaclofen (20 mM) to the slice reduced the amplitude of the O/A glut-IPSPs (n = 3), the reduction was not statistically significant. These results suggest that recurrent IPSPs elicited from activation of interneurons by stimulation of pyramidal cells are mediated solely via GABAA receptors. Inhibitory postsynaptic potentials elicited from stimulation of interneurons in O/A were also mediated mostly by GABAA receptors, but in addition, displayed a minor component mediated by GABAB receptors. Therefore, since a large proportion of interneurons in O/A are recurrently excited by pyramidal cells (Lacaille J-C et al., 1987, J Neurosci 7: 1979-1993), and since recurrent IPSPs appeared mediated by GABAA receptors, a subpopulation of interneurons activated from O/A might exist that do not receive recurrent excitation but can inhibit pyramidal cells via GABAB receptors.

Hyperpolarizing synaptic potentials evoked in CA1 pyramidal cells by glutamate stimulation of interneurons from the oriens/alveus border of rat hippocampal slices. I. Electrophysiological response properties

To examine the inhibitory postsynaptic potentials (IPSPs) elicited in pyramidal cells by interneurons situated at the stratum oriens/alveus border (O/A), glutamate was applied by micropressure to this area during intracellular recordings from CA1 pyramidal cells. Glutamate stimulation evoked IPSPs (glut-IPSPs) of small amplitude (4 mV), delayed peak latency (100-110 ms), and long duration (300-400 ms). Recurrent activation of interneurons via glutamate stimulation of pyramidal cells by local application in stratum pyramidale (PYR) evoked recurrent IPSPs (PYR glut-IPSPs) with similar amplitude and time course as O/A glut-IPSPs. The mean equilibrium potential of O/A glut-IPSPs (-77 mV) was significantly different from that of the PYR glut-IPSPs (-71 mV), however, neither equilibrium potential was significantly different from that of the electrically evoked early IPSP in the same cells. Glutamate-evoked IPSPs elicited from O/A displayed some response reversal (27% reversal) like those evoked from PYR (41% reversal). The early IPSP evoked by electrical stimulation displayed significantly more response reversal (67% reversal) than glut-IPSPs. Both types of glut-IPSPs (O/A and PYR) were associated with moderate increases in membrane conductance (5.9 and 6.6 nS, respectively), which were significantly less than the conductance change associated with the early IPSP (45.8 nS). In interneurons within PYR, glutamate stimulation in PYR readily elicited a flurry of excitatory postsynaptic potentials, whereas glutamate stimulation in O/A elicited IPSPs. The electrophysiological properties of IPSPs elicited in pyramidal cells by glutamate stimulation of interneurons in O/A were similar to those of recurrent IPSPs evoked from PYR. Given that both of these types of glutamate-evoked IPSPs were mostly mediated via GABAA receptor channels (Samulack DD, Lacaille J-C, 1993, Hippocampus 3:345-358), the small differences observed between equilibrium potentials, response reversals, and conductance changes could be due to a more electronically distant location from the soma of the synapses involved in O/A glut-IPSPs as compared to those of recurrent IPSPs elicited from PYR.