Endocannabinoid signaling dynamics probed with optical tools

Intercellular signaling dynamics critically influence the functional roles that the signals can play. Small lipids are synthesized and released from neurons, acting as intercellular signals in regulating neurotransmitter release, modulating ion channels on target cells, and modifying synaptic plasticity. The repertoire of biological effects of lipids such as endocannabinoids (eCBs) is rapidly expanding, yet lipid signaling dynamics have not been studied. The eCB system constitutes a powerful tool for bioassaying the dynamics of lipid signaling. The eCBs are synthesized in, and released from, postsynaptic somatodendritic domains that are readily accessible to whole-cell patch electrodes. The dramatic effects of these lipid signals are detected electrophysiologically as CB1-dependent alterations in conventional synaptic transmission, which therefore serve as a sensitive reporter of eCB actions. We used electrophysiological recording, photolytic release of caged glutamate and a newly developed caged AEA (anandamide), together with rapid [Ca2+]i measurements, to investigate the dynamics of retrograde eCB signaling between CA1 pyramidal cells and GABAergic synapses in rat hippocampus in vitro. We show that, at 22 degrees C, eCB synthesis and release must occur within 75-190 ms after the initiating stimulus, almost an order of magnitude faster than previously thought. At 37 degrees C, the time could be < 50 ms. Activation of CB1 and downstream processes constitute a significant fraction of the total delay and are identified as major rate-limiting steps in retrograde signaling. Our findings imply that lipid messenger dynamics are comparable with those of metabotropic neurotransmitters and can modulate neuronal interactions on a similarly fast time scale.

Regulation of IPSP Theta Rhythm by Muscarinic Receptors and Endocannabinoids in Hippocampus

Theta rhythms are behaviorally relevant electrical oscillations in the mammalian brain, particularly the hippocampus. In many cases, theta oscillations are shaped by inhibitory postsynaptic potentials (IPSPs) that are driven by glutamatergic and/or cholinergic inputs. Here we show that hippocampal theta rhythm IPSPs induced in the CA1 region by muscarinic acetylcholine receptors independent of all glutamate receptors can be briefly interrupted by action potential–induced, retrograde endocannabinoid release. Theta IPSPs can be recorded in CA1 pyramidal cell somata surgically isolated from CA3, subiculum, and even from their own apical dendrites. These results suggest that perisomatic-targeting interneurons whose output is subject to inhibition by endocannabinoids are the likely source of theta IPSPs. Interneurons having these properties include the cholecystokinin-containing cells. Simultaneous recordings from pyramidal cell pairs reveal synchronous theta-frequency IPSPs in neighboring pyramidal cells, suggesting that these IPSPs may help entrain or modulate small groups of pyramidal cells.

Endocannabinoid identification in the brain: studies of breakdown lead to breakthrough, and there may be NO hope

Endocannabinoids are a class of fatty acid derivatives defined by their ability to interact with the specific cannabinoid receptors that were originally identified as the targets of Delta9-tetrahydocannabinol (Delta9-THC), the psychoactive component of cannabis. Endocannabinoids have been implicated in a growing number of important physiological and behavioral events. A full understanding of the functions of endocannabinoids will involve knowing which ones are active, and how they are produced, during any given physical event. However, studying these small lipids in the brain presents many technical challenges. New selective pharmacological tools promise to be very useful in unraveling the complexities of endocannabinoid signaling, but parallel developments from the investigation of the cellular neurophysiology of the endocannabinoid systems highlight the difficulties remaining.

Multiple mechanisms of endocannabinoid response initiation in hippocampus

Endocannabinoids (eCBs) act as retrograde messengers at inhibitory synapses of the hippocampal CA1 region. Current models place eCB synthesis in the postsynaptic pyramidal cell and the site of eCB action at cannabinoid receptors located on presynaptic interneuron terminals. Four responses at the CA1-interneuron synapse are attributed to eCBs: depolarization-induced suppression of inhibition (DSI), G-protein-coupled receptor-mediated enhancement of DSI (DeltaDSI), persistent suppression of evoked inhibitory postsynaptic currents (eIPSCs), and finally, mGluR-dependent long-term depression (iLTD). It has been proposed that all are mediated by the eCB, 2-arachidonoyl glycerol, yet there is evidence that DSI does not arise from the same underlying biochemical processes as the other responses. In view of the increasing importance of eCB effects in the brain, it will be essential to understand the mechanisms by which eCB effects are produced. Our results reveal new differences in the biochemical pathways by which the eCB-dependent responses are initiated. Both U73122, a phospholipase C antagonist, and RHC-80267, a diacylglycerol (DAG) lipase antagonist, prevented eCB-dependent iLTD induction by 3,5-dihydroxyphenylglycine (DHPG). However, mAChR activation does not cause eCB-dependent iLTD. Neither enzyme inhibitor affects DSI, and persistent eCB-dependent eIPSC suppression induced by either mGluRs or mAChRs is unaffected by U73122. On the other hand, inhibition of DAG lipase prevents persistent eCB-dependent eIPSC suppression triggered by mAChRs. The results show that the biochemical pathways for the various eCB-dependent responses differ and might therefore be independently manipulated.

Endocannabinoids and their implications for epilepsy

This review covers the main features of a newly discovered intercellular signaling system in which endogenous ligands of the brain's cannabinoid receptors, or endocannabinoids, serve as retrograde messengers that enable a cell to control the strength of its own synaptic inputs. Endocannabinoids are released by bursts of action potentials, including events resembling interictal spikes, and probably by seizures as well. Activation of cannabinoid receptors has been implicated in neuroprotection against excitotoxicity and can help explain the anticonvulsant properties of cannabinoids that have been known since antiquity.

Retrograde endocannabinoid regulation of GABAergic inhibition in the rat dentate gyrus granule cell

The dentate gyrus is a key input gateway for the hippocampus, and dentate function is potently regulated by GABAergic inhibition. GABAergic inhibition is plastic and modulated by many factors. Cytoplasmic calcium ([Ca(+)](i)) is one of these factors, and its elevation inhibits GABA-mediated transmission in the hippocampus including the dentate gyrus granule cells (DGCs). We examined whether the [Ca(+)](i)-dependent decrease of GABA(A) receptor-mediated inhibitory postsynaptic current (IPSC) is explained by the retrograde suppression of GABA release caused by the depolarization-induced elevation of [Ca(+)](i) in DGCs (DSI: depolarization-induced suppression of inhibition). Repeated brief depolarizations or a single long depolarization inhibited spontaneous IPSCs with amplitudes over 25 pA for up to a minute, and reduced the amplitude of IPSCs evoked by direct stimulation in the molecular layer, suggesting that DGCs are susceptible to DSI. The magnitude of DSI correlated linearly with the duration of depolarization, and so did the increase of [Ca(+)](i). DSI was blocked by intrapipette application of BAPTA. In addition, bath application of thapsigargin and ryanodine, and intrapipette application of ryanodine and ruthenium red reduced the [Ca(+)](i) increase caused by the DSI-inducing depolarization, and substantially reduced the magnitude of DSI. Finally, the cannabinoid receptor agonists, CP55,942 and WIN55,212-2, mimicked DSI and prevented further IPSC reduction by DSI. DSI was blocked by the antagonist, SR141716A. We conclude that GABAergic inhibition in DGCs is subject to endogenous cannabinoid (eCB)-mediated retrograde regulation, and this process involves a depolarization-initiated release of Ca(+) from ryanodine-sensitive stores. Our findings suggest eCBs probably have physiological functions in the regulation of GABAergic plasticity in the dentate gyrus.

Inhibition of cyclooxygenase-2 potentiates retrograde endocannabinoid effects in hippocampus

In hippocampal pyramidal cells, a rise in Ca(2+) releases endocannabinoids that activate the presynaptic cannabinoid receptor (CB1R) and transiently reduce GABAergic transmission-a process called depolarization-induced suppression of inhibition (DSI). The mechanism that limits the duration of endocannabinoid action in intact cells is unknown. Here we show that inhibition of cyclooxygenase-2 (COX-2), not fatty acid amide hydrolase (FAAH), prolongs DSI, suggesting that COX-2 limits endocannabinoid action.

Novel form of LTD induced by transient, partial inhibition of the Na,K-pump in rat hippocampal CA1 cells

We tested the hypothesis that transient, partial inhibition of the Na,K-pumps could produce lasting effects on synaptic efficacy in brain tissue by applying a low concentration of the ouabain analogue, dihydroouabain (DHO), to hippocampal slices for 15 min and studying the effects on field excitatory postsynaptic potentials (fEPSPs). DHO caused a suppression of fEPSPs during the application period, but this recovered only partially, to approximately 80% of control levels, after washout lasting as long as 2 h. The lasting suppression had several properties in common with low-frequency stimulation induced long-term depression (LFS-LTD), including an ability to depotentiate long-term potentiated responses. However, DHO-LTD was insensitive to blockade of N-methyl-d-aspartate or mGlu receptors or to inhibitors of protein kinase C or p38 MAP kinase. DHO-LTD did not co-occlude with LFS-LTD and therefore appears to represent a novel form of LTD. Interestingly, DHO-LTD could be prevented by pretreating slices with iberiotoxin, the selective blocker of large, Ca(2+)-dependent K+ channels ("big K," BK channels), although this toxin did not affect basal fEPSPs. Certain pathological conditions, including hypoxia and ischemia, are associated with a decrease in Na,K-pump activity and hence DHO-LTD may serve as a model for the effects on neuronal function in these conditions.

Regulation of exocytosis from single visualized GABAergic boutons in hippocampal slices

Regulation of GABA release is crucial for normal brain functioning, and GABAA-mediated IPSCs are strongly influenced by repetitive stimulation and neuromodulation. However, GABA exocytosis has not been examined directly in organized tissue. Important issues remain outside the realm of electrophysiological techniques or are complicated by postsynaptic factors. For example, it is not known whether all presynaptic modulators affect release from all boutons in the same way, or whether modulator effects depend on the presence of certain types of voltage-gated calcium channels (VGCCs). To address such issues, we used confocal imaging and styryl dyes to monitor exocytosis from identified GABAergic boutons in organotypic hippocampal slice cultures. Repetitively evoked IPSCs declined more rapidly and completely than exocytosis, suggesting that depletion of filled vesicles cannot fully account for IPSC depression and underscoring the usefulness of directly imaging exocytosis. Stimulation at 10 Hz produced a transient facilitation of exocytosis that was dependent on L-type VGCCs. Using specific toxins, we found that release mediated via N-type and P-type VGCCs had similar properties. Neither baclofen nor a cannabinoid receptor agonist, CP55940, affected all boutons uniformly; they slowed release from some but completely prevented detectable release from others. Increasing stimulus frequency overcame this blockade of release. However, baclofen and CP55940 did not act identically, because only baclofen reduced facilitation and affected bouton releasing via P/Q-type VGCCs. Direct observation thus revealed novel features of GABAergic exocytosis and its regulation that would have been difficult or impossible to detect electrophysiologically. These features advance the understanding of the regulation of synapses and networks by presynaptic inhibition.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

Activation of muscarinic acetylcholine receptors enhances the release of endogenous cannabinoids in the hippocampus

Endogenous cannabinoids (endocannabinoids) are endogenous compounds that resemble the active ingredient of marijuana and activate the cannabinoid receptor in the brain. They mediate retrograde signaling from principal cells to both inhibitory ["depolarization-induced suppression of inhibition" (DSI)] and excitatory ("depolarization-induced suppression of excitation") afferent fibers. Transient endocannabinoid release is triggered by voltage-dependent Ca(2+) influx and is upregulated by group I metabotropic glutamate receptor activation. Here we show that muscarinic acetylcholine receptor (mAChR) activation also enhances transient endocannabinoid release (DSI) and induces persistent release. Inhibitory synapses in the rat hippocampal CA1 region of acute slices were studied using whole-cell patch-clamp techniques. We found that low concentrations (0.2-0.5 microm) of carbachol (CCh) enhanced DSI without affecting basal evoked IPSCs (eIPSCs) by activating mAChRs on postsynaptic cells. Higher concentrations of CCh (> or =1 microm) enhanced DSI and also persistently depressed basal eIPSCs, mainly by releasing endocannabinoids. Persistent CCh-induced endocannabinoid release did not require an increase in [Ca2+]i but was dependent on G-proteins. Although they were independent at the receptor level, muscarinic and glutamatergic mechanisms of endocannabinoid release shared intracellular machinery. Replication of the effects of CCh by blocking acetylcholinesterase with eserine suggests that mAChR-mediated endocannabinoid release is physiologically relevant. This study reveals a new role of the muscarinic cholinergic system in mammalian brain.

Mechanisms of neuronal hyperexcitability caused by partial inhibition of Na+-K+-ATPases in the rat CA1 hippocampal region

Extra- and intracellular records were made from rat acute hippocampal slices to examine the effects of partial inhibition of Na(+)-K(+)-ATPases (Na(+)-K(+) pumps) on neuronal hyperexcitability. Bath application of the low-affinity cardiac glycoside, dihydroouabain (DHO), reversibly induced interictal-like epileptiform bursting activity in the CA1 region. Burst-firing was correlated with inhibition of the pumps, which was assayed by changes in [K(+)](o) uptake rates measured with K(+)-ion-sensitive microelectrodes. Large increases in resting [K(+)](o) did not occur. DHO induced a transient depolarization (5-6 mV) followed by a long-lasting hyperpolarization (approximately 6 mV) in CA1 pyramidal neurons, which was accompanied by a 30% decrease in resting input resistance. Block of an electrogenic pump current could explain the depolarization but not the hyperpolarization of the membrane. Increasing [K(+)](o) from 3 to 5.5 mM minimized these transient shifts in passive membrane properties without preventing DHO-induced hyperexcitability. DHO decreased synaptic transmission, but increased the coupling between excitatory postsynaptic potentials and spike firing (E-S coupling). Monosynaptic inhibitory postsynaptic potential (IPSP) amplitudes declined to approximately 25% of control at the peak of bursting activity; however, miniature TTX-resistant inhibitory postsynaptic current amplitudes were unaffected. DHO also reduced the initial slope of the intracellular excitatory postsynaptic potential (EPSP) to approximately 40% of control. The conductances of pharmacologically isolated IPSPs and EPSPs in high-Ca/high-Mg-containing saline were also reduced by DHO by approximately 50%. The extracellular fiber volley amplitude was reduced by 15-20%, suggesting that the decrease in neurotransmission was partly due to a reduction in presynaptic fiber excitability. DHO enhanced a late depolarizing potential that was superimposed on the EPSP and could obscure it. This potential was not blocked by antagonists of NMDA receptors, and blockade of NMDA, mGlu, or GABA(A) receptors did not affect burst firing. The late depolarizing component enabled the pyramidal cells to reach spike threshold without changing the actual voltage threshold for firing. We conclude that reduced GABAergic potentials and enhanced E-S coupling are the primary mechanisms underlying the hyperexcitability associated with impaired Na(+)-K(+) pump activity.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

Endocannabinoids facilitate the induction of LTP in the hippocampus

Exogenous cannabinoids disrupt behavioral learning and impede induction of long-term potentiation (LTP) in the hippocampus, yet endogenous cannabinoids (endocannabinoids) transiently suppress inhibitory post-synaptic currents (IPSCs) by activating cannabinoid CB1 receptors on GABAergic interneurons. We found that release of endocannabinoids by a rat CA1 pyramidal cell during this depolarization-induced suppression of inhibition (DSI) enabled a normally ineffective train of excitatory post-synaptic currents (EPSCs) to induce LTP in that cell, but not in neighboring cells. By showing that endocannabinoids facilitate LTP induction and help target LTP to single cells, these data shed new light on the physiological roles of endocannabinoids and may lead to a greater understanding of their effects on behavior and potential clinical use.

Random response fluctuations lead to spurious paired-pulse facilitation

We studied paired-pulse depression (PPD) of GABA(A)ergic IPSCs under conditions of reduced transmitter release (caused by Cd(2+), baclofen, or reduced stimulus intensity) with whole-cell voltage clamp in CA1 pyramidal cells in vitro. The use-dependent model of paired-pulse responsiveness holds that a decrease in the probability of neurotransmitter release during the first stimulus will cause predictable changes in the paired-pulse ratio (PPR, the amplitude of the second IPSC divided by that of the first). However, the applicability of the use-dependent model to inhibitory synapses is controversial. Our results are inconsistent with this model, but are consistent with the hypothesis that random fluctuations in response size significantly influence PPR. PPR was sensitive to the extracellular stimulus intensity in all conditions. Changes in PPR were not correlated with changes in the first IPSC, but were correlated with changes in variability of the PPRs of individual traces. We show that spurious paired-pulse facilitation (PPF) can result from averaging randomly fluctuating PPRs because the method of calculating PPR as the mean of individual PPRs is biased in favor of high values of PPR. Spurious PPF can mask the intrinsic paired-pulse property of the synapses. Calculating PPR as the mean of the second response divided by the mean of the first avoids the error. We discuss a simple model that shows that spurious PPF depends on both the number of synapses recruited for release and the probability of release at each release site. The random factor can reconcile some conflicting published conclusions.

Metabotropic glutamate receptors drive the endocannabinoid system in hippocampus

Endocannabinoids are key intercellular signaling molecules in the brain, but the physiological regulation of the endocannabinoid system is not understood. We used the retrograde signal process called depolarization-induced suppression of inhibition (DSI) to study the regulation of this system. DSI is produced when an endocannabinoid released from pyramidal cells suppresses IPSCs by activating CB1R cannabinoid receptors located on inhibitory interneurons. We now report that activation of group I metabotropic glutamate receptors (mGluRs) enhances DSI and that this effect is blocked by antagonists of both mGluRs and of CB1R. We also found that DSI is absent in CB1R knock-out (CB1R(-/-)) mice, and, strikingly, that mGluR agonists have no effect on IPSCs in these mice. We conclude that group I mGluR-induced enhancement of DSI, and suppression of IPSCs, is actually mediated by endocannabinoids. This surprising result opens up new approaches to the investigation of cannabinoid actions in the brain.

Spectrins in developing rat hippocampal cells

We studied the spectrins in developing hippocampal tissue in vivo and in vitro to learn how they contribute to the organization of synaptic and extrasynaptic regions of the neuronal plasma membrane. beta-Spectrin, but not beta-fodrin or alpha-fodrin, increased substantially during postnatal development in the hippocampus, where it was localized in neurons but not in astrocytes. Immunoprecipitations from neonatal and adult hippocampal extracts suggest that while both beta-spectrin and beta-fodrin form heteromers with alpha-fodrin, oligomers containing all three subunits are also present. At the subcellular level, beta-fodrin and alpha-fodrin were present in the cell bodies, dendrites, and axons of pyramidal-like neurons in culture, as well as in astrocytes. beta-Spectrin, by contrast, was absent from axons but present in cell bodies and dendrites, where it was organized in a loose, membrane-associated meshwork that lacked alpha-fodrin. A similar meshwork was also apparent in pyramidal neurons in vivo. At some dendritic spines, alpha-fodrin was present in the necks but not in the heads, whereas beta-spectrin was present at significant levels in the spine heads. The presence of significant amounts of beta-spectrin without an accompanying alpha-fodrin subunit was confirmed by immunoprecipitations from extracts of adult hippocampus. Our results suggest that the spectrins in hippocampal neurons can assemble to form different membrane-associated structures in distinct membrane domains, including those at synapses.

Heterogeneous susceptibility of GABA(A) receptor-mediated IPSCs to depolarization-induced suppression of inhibition in rat hippocampus

Depolarization-induced suppression of inhibition (DSI) in central neurons is mediated by a transient reduction of [gamma]-aminobutyric acid (GABA) release from interneurons. DSI is induced by a retrograde signal emitted from principal cells. We used electrophysiological recordings from CA1 neurons of the rat hippocampal slice to test the hypothesis that only certain classes of interneurons are susceptible to DSI. DSI of action potential-dependent, spontaneous, inhibitory postsynaptic currents (sIPSCs) in hippocampus is facilitated by carbachol (3 microM), which increases the occurrence of large sIPSCs. Besides carbachol, noradrenaline (norepinephrine; 10 microM), or elevated extracellular potassium (8 mM), could abruptly increase the occurrence of large sIPSCs and DSI in many cases. DSI appeared and disappeared concomitantly with the onset and offset of these large sIPSCs. In contrast, application of AP-5 and CNQX often markedly increased baseline sIPSC activity without enhancing DSI. A brief train of extracellular electrical stimulation could trigger the onset of prolonged, repetitive IPSC activity that was susceptible to DSI. The magnitude of DSI of single evoked IPSCs (eIPSCs) in a given pyramidal cell could be altered by changes in stimulus strength, but there was no simple relationship between stimulus strength and DSI. Baclofen (0.5-5 microM) eliminated the increase in sIPSC activity and DSI induced by carbachol. A GABA(B)receptor antagonist, CGP 35348, reversed the effects of baclofen. Carbachol-induced sIPSCs had relatively rapid rise and decay phases. There was no marked distinction between DSI-susceptible and non-susceptible sIPSCs. Nevertheless, two kinetically distinct components of the eIPSC could be distinguished by their decay times. DSI reduced GABA(A),(fast) without affecting GABA(A),(slow). Furosemide (frusemide), which blocks only GABA(A),(fast), reduced the eIPSC and occluded DSI. The data suggest that, with respect to DSI, there are at least three functionally distinct types of IPSCs. Two types (one susceptible to DSI and one not) have relatively rapid kinetics are probably made by perisomatic synapses. A third, slow IPSC, which is insensitive to DSI, may be produced by distal dendritic synapses.

Differential effects of the group II mGluR agonist, DCG-IV, on depolarization-induced suppression of inhibition in hippocampal CA1 and CA3 neurons

We investigated the role of metabotropic glutamate receptors in the mediation of depolarization-induced suppression of inhibition (DSI), using whole-cell electrophysiological techniques in rat hippocampal slice preparation. In a previous work, we showed that a retrograde signal travels from CA1 pyramidal cells to GABA interneurons and prevents them from releasing GABA for tens of seconds at 30 degrees C. The resulting suppression of inhibition is DSI. The retrograde signal appeared to be glutamate, or a glutamate analog, which acted on group I metabotropic receptors on the interneurons. It is not known if DSI occurs in hippocampal subregions besides CA1. If DSI does occur in other regions, it will be important to know if the role of metabotropic glutamate receptors (mGluRs) in mediating DSI is the same everywhere. The distribution of mGluR subtypes varies among hippocampal subregions. In the CA3 region, unlike CA1, group II mGluRs are prevalent. It was possible, therefore, that in CA3, the group II mGluRs would mediate DSI. We have begun to investigate these issues. We now report that: 1) DSI does occur in CA3. 2) Carbachol induces IPSC activity that can be recorded in CA1 and CA3a. This carbachol-induced activity can be reduced by the selective group II mGluR agonist, DCG-IV, and by DSI. 3) Evoked IPSCs in CA3a, but not in CA1, can be reduced by DCG-IV; hence the interneurons activated by carbachol may reside in CA3a. 4) Despite the group II mGluR agonist sensitivity of CA3a interneurons, DSI in this region is not affected by a group II mGluR antagonist, CPPG, and therefore does not appear to be mediated by group II mGluRs.

Direct depolarization and antidromic action potentials transiently suppress dendritic IPSPs in hippocampal CA1 pyramidal cells

Whole-cell current-clamp recordings were made from distal dendrites of rat hippocampal CA1 pyramidal cells. Following depolarization of the dendritic membrane by direct injection of current pulses or by back-propagating action potentials elicited by antidromic stimulation, evoked gamma-aminobutyric acid-A (GABA(A)) receptor-mediated inhibitory postsynaptic potentials (IPSPs) were transiently suppressed. This suppression had properties similar to depolarization-induced suppression of inhibition (DSI): it was enhanced by carbachol, blocked by dendritic hyperpolarization sufficient to prevent action potential invasion, and reduced by 4-aminopyridine (4-AP) application. Thus DSI or a DSI-like process can be recorded in CA1 distal dendrites. Moreover, localized application of TTX to stratum pyramidale blocked somatic action potentials and somatic IPSPs, but not dendritic IPSPs or DSI induced by direct dendritic depolarization, suggesting DSI is expressed in part in the dendrites. These data extend the potential physiological roles of DSI.

Calcium dependence of depolarization-induced suppression of inhibition in rat hippocampal CA1 pyramidal neurons

1. We made whole-cell recordings from CA1 pyramidal cells in the rat hippocampal slice preparation to study the calcium (Ca2+) dependence of depolarization-induced suppression of inhibition (DSI). DSI is a retrograde signalling process in which voltage-dependent Ca2+ influx into a pyramidal cell leads to a transient decrease in the release of GABA from interneurons. 2. To investigate the Ca2+ dependence of DSI without altering extracellular divalent cations, we varied the type and amount of Ca2+ chelator (EGTA or BAPTA) in the recording pipette (keeping the chelator : Ca2+ ratio constant). Evoked inhibitory postsynaptic currents (IPSCs) were induced in the presence of antagonists of ionotropic glutamate receptors. DSI was induced by depolarizing voltage steps, lasting from 0.025 to 5 s, to 0 mV. 3. DSI was directly dependent on the duration of the voltage step used to induce it, from threshold up to a maximal value of IPSC suppression, whether EGTA or BAPTA was used, and whether their concentrations were 0.1, 0.5 or 2 mM. For instance, a voltage step lasting 1.37 s produced half-maximal DSI with 2 mM BAPTA, but with 0. 1 mM BAPTA, half-maximal DSI was achieved with a step lasting 0.186 s. Peak DSI was the same in all cases, and DSI was blocked with either 10 mM EGTA or BAPTA in the pipette. Bath application of carbachol could overcome the block of DSI by 10 mM EGTA but not by 10 mM BAPTA. 4. We calculated that a voltage step lasting approximately 100 ms would be necessary to activate half-maximal DSI in the absence of exogenous Ca2+ buffers. 5. Log-log plots of calculated total Ca2+ influx, estimated from time integrals of Ca2+ currents, versus DSI yielded a straight line with a slope of approximately 1, and increasing extracellular [Ca2+] from 2.5 to 5 mM did not change the slope. 6. The time course of decay of DSI was well described by an exponential function with a time constant of approximately 20 s and was not affected by changes in either concentration or type of Ca2+ buffer. 7. The data suggest that, in its Ca2+ dependence, DSI more closely resembles the slow release of neuropeptides and hormones than it does the process of fast release of many neurotransmitters.

Evidence for endogenous excitatory amino acids as mediators in DSI of GABA(A)ergic transmission in hippocampal CA1

Depolarization-induced suppression of inhibition (DSI) is a process whereby brief approximately 1-s depolarization to the postsynaptic membrane of hippocampal CA1 pyramidal cells results in a transient suppression of GABA(A)ergic synaptic transmission. DSI is triggered by a postsynaptic rise in [Ca(2+)](in) and yet is expressed presynaptically, which implies that a retrograde signal is involved. Recent evidence based on synthetic metabotropic glutamate receptor (mGluR) agonists and antagonists suggested that group I mGluRs take part in the expression of DSI and raised the possibility that glutamate or a glutamate-like substance is the retrograde messenger in hippocampal CA1. This hypothesis was tested, and it was found that the endogenous amino acids L-glutamate (L-Glu) and L-cysteine sulfinic acid (L-CSA) suppressed GABA(A)-receptor-mediated inhibitory postsynaptic currents (IPSCs) and occluded DSI, whereas L-homocysteic acid (L-HCA) and L-homocysteine sulfinic acid (L-HCSA) did not. Activation of metabotropic kainate receptors with kainic acid (KA) reduced IPSCs; however, DSI was not occluded. When iontophoretically applied, both L-Glu and L-CSA produced a transient IPSC suppression similar in magnitude and time course to that observed during DSI. Both DSI and the actions of the amino acids were antagonized by (S)-alpha-methyl-4-carboxyphenylglycine ([S]-MCPG), indicating that the effects of the endogenous agonists were produced through activation of mGluRs. Blocking excitatory amino acid transport significantly increased DSI and the suppression produced by L-Glu or L-CSA without affecting the time constant of recovery from the suppression. Similar to DSI, IPSC suppression by L-Glu or L-CSA was blocked by N-ethylmaleimide (NEM). Moreover, paired-pulse depression (PPD), which is unaltered during DSI, is also not significantly affected by the amino acids. Taken together, these results support the glutamate hypothesis of DSI and argue that L-Glu or L-CSA are potential retrograde messengers in CA1.

Muscarinic facilitation of the occurrence of depolarization-induced suppression of inhibition in rat hippocampus

Depolarization-induced suppression of inhibition is a transient decrease in GABAergic input to a hippocampal pyramidal cell following a brief depolarization of that cell. When recorded under whole-cell voltage clamp, monosynaptic, bicuculline-sensitive, GABA(A)-mediated currents are suppressed for a period lasting up to 1 min in response to a retrograde signal released by the pyramidal cell. The depolarization-induced suppression of inhibition process affects spontaneous, action-potential-dependent inhibitory postsynaptic currents, but suppression of these currents is seldom observed in the absence of carbachol, a cholinergic agonist. Because of the central roles played by cholinergic and GABAergic transmission in the regulation of hippocampal rhythmic activity, it will be important to understand the mechanism by which carbachol facilitates the appearance of depolarization-induced suppression of inhibition. As preliminary steps in the investigation of cholinergic actions on depolarization-induced suppression of inhibition, it is necessary to determine which cholinergic receptors are involved and the degree to which activation of these receptors is required for depolarization-induced suppression of inhibition. Nicotine did not mimic the effects of carbachol, and mecamylamine, a nicotinic receptor antagonist, did not block them. In contrast, the actions of carbachol were abolished by atropine and other muscarinic receptor antagonists. The actions of antagonists with relative selectivities for various subtypes of muscarinic receptors [4-diphenylacetoxy-N-methylpiperidine methiodide, pirenzepine, 11-([2-1-piperidinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzod iaz epine-6-one] suggested that cholinergic facilitation of the occurrence of depolarization-induced suppression of inhibition is likely to be mediated through muscarinic receptors of the M1 or M3 rather than M2 subtype. Despite its potent facilitation of the occurrence of depolarization-induced suppression of inhibition, muscarinic stimulation was not required for expression of depolarization-induced suppression of inhibition. Occasionally, depolarization-induced suppression of inhibition of spontaneous inhibitory postsynaptic currents occurred in the absence of carbachol and could not be blocked by atropine, and hence was not likely to be mediated by endogenous acetylcholine. Also, depolarization-induced suppression of inhibition of monosynaptically evoked inhibitory postsynaptic currents occurred without carbachol perfusion, and this was also insensitive to atropine. Therefore, the mechanism of depolarization-induced suppression of inhibition is not dependent on muscarinic receptor activation. Nevertheless, in vivo, septal cholinergic input to the hippocampus may provide the necessary activation of interneurons to allow depolarization-induced suppression of inhibition to occur.