N- and L-type calcium channel involvement in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells

Endogenous cannabinoid as a retrograde messenger from depolarized postsynaptic neurons to presynaptic terminals

Cannabinoid receptors are the molecular targets for the active component Delta(9)-tetrahydrocannabinol of marijuana and hashish, and constitute a major family of G protein-coupled seven-transmembrane-domain receptors. They consist of type 1 (CB1) and type 2 (CB2) receptors of which the CB1 is rich in various regions of the CNS. Accumulated evidence suggests that endogenous cannabinoids function as diffusible and short-lived intercellular messengers that modulate synaptic transmission. Recent studies have provided strong experimental evidence that endogenous cannabinoids mediate signals retrogradely from depolarized postsynaptic neurons to presynaptic terminals to suppress subsequent neurotransmitter release, driving the synapse into an altered state. In hippocampal neurons, depolarization of postsynaptic neurons and resultant elevation of [Ca(2+)](i) lead to transient suppression of inhibitory transmitter release (depolarization-induced suppression of inhibition, DSI). In cerebellar Purkinje cells, on the other hand, depolarization-induced elevation of [Ca(2+)](i) causes transient suppression of excitatory transmitter release (depolarization-induced suppression of excitation, DSE). DSI and DSE appear to share the same properties and may be a general and important mechanism by which the postsynaptic neuronal activity can influence the amount of transmitter release.

Photolysis-induced suppression of inhibition in rat hippocampal CA1 pyramidal neurons

1. Whole cell patch clamp recording, Ca(2+) measurement with ratiometric fluorescent dyes and photolysis of caged Ca(2+) were combined to investigate the depolarization- and photolysis-induced suppression of inhibition (DSI and PSI) in rat hippocampal CA1 pyramidal cells. 2. A 5-s depolarization from -70 mV to 0 mV or a 6-s photolysis of nitrophenyl-EGTA (NPE) in cell bodies could each depress the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) and the amplitude of evoked IPSCs while elevating intracellular Ca(2+) concentration ([Ca(2+)](i)). 3. Within a cell the elevation of [Ca(2+)](i) induced by depolarization was inversely related to that induced by photolysis, suggesting that higher [NPE] is more effective in releasing caged Ca(2+) but also increases buffer capacity to reduce [Ca(2+)](i) rises caused by Ca(2+) influx through voltage-dependent Ca(2+) channels. 4. Both DSI and PSI were linearly related to [Ca(2+)](i), with a 50 % reduction in transmission occurring at about 3.6--3.9 microM. 5. [Ca(2+)](i) recovered more quickly than DSI, indicating that the duration of DSI is not set simply by the duration of [Ca(2+)](i) elevation, but rather entails other rate-limiting processes. 6. We conclude that DSI is activated by micromolar [Ca(2+)](i) acting far from sites of Ca(2+) entry through channels in the plasma membrane.

Endogenous cannabinoids mediate retrograde signalling at hippocampal synapses

Marijuana affects brain function primarily by activating the G-protein-coupled cannabinoid receptor-1 (CB1), which is expressed throughout the brain at high levels. Two endogenous lipids, anandamide and 2-arachidonylglycerol (2-AG), have been identified as CB1 ligands. Depolarized hippocampal neurons rapidly release both anandamide and 2-AG in a Ca2+-dependent manner. In the hippocampus, CB1 is expressed mainly by GABA (gamma-aminobutyric acid)-mediated inhibitory interneurons, where CB1 clusters on the axon terminal. A synthetic CB1 agonist depresses GABA release from hippocampal slices. These findings indicate that the function of endogenous cannabinoids released by depolarized hippocampal neurons might be to downregulate GABA release. Here we show that the transient suppression of GABA-mediated transmission that follows depolarization of hippocampal pyramidal neurons is mediated by retrograde signalling through release of endogenous cannabinoids. Signalling by the endocannabinoid system thus represents a mechanism by which neurons can communicate backwards across synapses to modulate their inputs.

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.

The retrograde inhibition of IPSCs in rat cerebellar purkinje cells is highly sensitive to intracellular Ca2+

The Ca2+-dependent retrograde inhibition of inhibitory postsynaptic currents (depolarization-induced-suppression of inhibition; DSI) was investigated using fura-2 Ca2+ measurements and whole-cell patch-clamp recordings in rat cerebellar Purkinje cells. DSI was studied in cells loaded with different concentrations of the Ca2+ chelators BAPTA and EGTA. A concentration of 40 mM BAPTA was required to significantly interfere with DSI, whereas 10 mM BAPTA was almost ineffective. 40 mM EGTA reduced DSI, but was less effective than 40 mM BAPTA. Ratiometric Ca2+ measurements indicated that the extent of DSI depended critically on the changes in intracellular calcium ([Ca2+]i). The relationship between DSI and peak Delta[Ca2+]i could be approximated by a hyperbolic function, with apparent half-saturation concentrations of 200 and 40 nM for dendritic and somatic [Ca2+]i, respectively. It is suggested that DSI is due to somatodendritic exocytosis of a retrograde messenger, and that this exocytosis is highly sensitive to [Ca2+]i.

Heterosynaptic expression of depolarization-induced suppression of inhibition (DSI) in rat hippocampal cultures

Depolarization-induced suppression of inhibition (DSI) is a transient suppression of the inhibitory synaptic transmission, observed in the hippocampus and the cerebellum, upon postsynaptic depolarization. Using rat hippocampal cultures, we examined whether DSI is confined to the inhibitory synapses on the depolarized neuron or, if DSI can spread to those on neighboring non-depolarized neurons. Whole-cell recordings were performed in 108 neuronal pairs with the following synaptic responses. Stimulation of one neuron evoked the inhibitory autaptic currents (IACs) recurrently in that neuron and also elicited the inhibitory postsynaptic currents (IPSCs) orthodromically in the other neuron. In 38 of 108 pairs, the postsynaptic depolarization caused transient suppression of IPSCs (homosynaptic DSI). In 11 of the 38 pairs exhibiting the homosynaptic DSI, the depolarization also induced suppression of IACs (heterosynaptic DSI). The heterosynaptic DSI, like the homosynaptic DSI, depended on depolarizing pulse duration and was blocked by a phorbol ester. These results suggest that DSI can spread to the synapses on a neighboring non-depolarized neuron in rat hippocampal cultures.

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.

Long-term potentiation of GABAergic synaptic transmission in neonatal rat hippocampus

1. The plasticity of GABAergic synapses was investigated in neonatal rat hippocampal slices obtained between postnatal days 3 and 6 using intracellular recording techniques. Ionotropic glutamate receptor antagonists were present throughout the experiments to isolate GABAA receptor-mediated postsynaptic potentials (GABAA PSPs) or currents (GABAA PSCs). 2. Repetitive depolarizing pulses (20 pulses, 0.5 s duration, at 0.1 Hz, each pulse generating 4-6 action potentials) induced a long-term potentiation in the slope and amplitude of the evoked GABAA PSPs and GABAA PSCs. 3. Long-term potentiation was prevented by intracellular injection of the calcium chelator BAPTA (50 mM), or when the voltage-dependent calcium channels blockers Ni2+ (50 microM) and nimodipine (10 microM) were bath applied. 4. Repetitive depolarizing pulses induced a persistent (over 1 h) increase in the frequency of spontaneous GABAA PSCs. 5. Repetitive depolarizing pulses induced a long-lasting increase in the frequency of miniature GABAA PSCs, without altering their amplitude or decay-time constant. 6. It is concluded that the postsynaptic activation of voltage-dependent calcium channels leads to a long-term potentiation of GABAergic synaptic transmission in neonatal rat hippocampus. This form of plasticity is expressed as an increase in the probability of GABA release or in the number of functional synapses, rather than as an upregulation of postsynaptic GABAA receptor numbers or conductance at functional synapses.