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.

Regulation of synaptic strength by protein phosphatase 1.

We investigated the role of postsynaptic protein phosphatase 1 (PP1) in regulating synaptic strength by loading CA1 pyramidal cells either with peptides that disrupt PP1 binding to synaptic targeting proteins or with active PP1. The peptides blocked synaptically evoked LTD but had no effect on basal synaptic currents mediated by either AMPA or NMDA receptors. They did, however, cause an increase in synaptic strength following the induction of LTD. Similarly, PP1 had no effect on basal synaptic strength but enhanced LTD. In cultured neurons, synaptic activation of NMDA receptors increased the proportion of PP1 localized to synapses. These results suggest that PP1 does not significantly regulate basal synaptic strength. Appropriate NMDA receptor activation, however, allows PP1 to gain access to synaptic substrates and be recruited to synapses where its activity is necessary for sustaining LTD.

Regulation of AMPA receptor endocytosis by a signaling mechanism shared with LTD.

The endocytosis of AMPA receptors is thought to be important in the expression of long-term depression (LTD) triggered by NMDA receptor activation. Although signaling pathways necessary for LTD induction have been identified, those responsible for the regulated internalization of AMPA receptors are unknown. Here we show that activation of NMDA receptors alone can trigger AMPA receptor endocytosis through calcium influx and activation of the calcium-dependent protein phosphatase calcineurin. A distinct signaling mechanism mediates the AMPA receptor endocytosis stimulated by insulin. These results demonstrate that although multiple signaling pathways can induce AMPA receptor internalization, NMDA receptor activation enhances AMPA receptor endocytosis via a signaling mechanism required for the induction of LTD.

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.

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.

Evidence for metabotropic glutamate receptor activation in the induction of depolarization-induced suppression of inhibition in hippocampal CA1.

Depolarization-induced suppression of inhibition (DSI) is a transient reduction of GABAA receptor-mediated IPSCs that is mediated by a retrograde signal from principal cells to interneurons. Using whole-cell recordings, we tested the hypothesis that mGluRs are involved in the DSI process in hippocampal CA1, as has been proposed for cerebellar DSI. Group II mGluR agonists failed to affect either evoked monosynaptic IPSCs or DSI, and forskolin, which blocks cerebellar DSI, did not affect CA1 DSI. Group I and group III mGluR agonists reduced IPSCs, but only group I agonists occluded DSI. (S)-MCPG blocked (1S,3R)-ACPD-induced IPSC suppression and markedly reduced DSI, whereas group III antagonists had no effect on DSI. Many other similarities between DSI and the (1S,3R)-ACPD-induced suppression of IPSCs also were found. Our data suggest that a glutamate-like substance released from pyramidal cells could mediate CA1 DSI by reducing GABA release from interneurons via the activation of group I mGluRs.

GABA-ergic transmission in deep cerebellar nuclei.

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

N-ethylmaleimide blocks depolarization-induced suppression of inhibition and enhances GABA release in the rat hippocampal slice in vitro.

Regulation of synaptic, GABAA receptor-mediated inhibition is a process of critical importance to normal brain function. Recently, we have described a phenomenon in hippocampus of a transient, yet marked, decrease in spontaneous, GABAA receptor-mediated IPSCs after depolarization activated Ca2+ influx into a pyramidal cell. This process, depolarization-induced suppression of inhibition (DSI), is absent in hippocampal cells that previously had been exposed to pertussis toxin in vivo, implicating a G-protein in the DSI process. To circumvent the problem that a single cell cannot be studied before and after G-protein block using the pertussis toxin pretreatment method, we have used the sulfhydryl alkylating agent N-ethylmaleimide (NEM), which blocks pertussis toxin-sensitive G-proteins, to determine whether acute inhibition of G-proteins can eliminate DSI of spontaneous IPSCs (sIPSCs). In whole-cell recordings from CA1 pyramidal cells that were first determined to express DSI, we have found that NEM does block DSI of sIPSCs. We also report that DSI of monosynaptic, evoked IPSCs is blocked by NEM, suggesting that a similar mechanism underlies both forms of DSI. It was of interest that DSI was abolished at a time when NEM had increased, not decreased, GABA transmission. Indeed, NEM greatly increased quantal GABA release by a Ca(2+)-independent mechanism, an observation with potentially important implications for understanding synaptic GABA release.

Sr2+ supports depolarization-induced suppression of inhibition and provides new evidence for a presynaptic expression mechanism in rat hippocampal slices.

1. We studied the transient suppression of evoked GABAA ergic inhibitory postsynaptic currents (eIPSCs) that follows brief membrane depolarization in rat CA1 hippocampal pyramidal cells, a process called depolarization-induced suppression of inhibition (DSI). We used whole-cell patch electrodes filled with a CsCl-based solution to voltage clamp the currents. All experiments were done in the presence of 50 microM 2-amino-5-phosphonovaleric acid (APV) and 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to block ionotropic glutamate-induced currents and polysynaptic transmission in the slice preparation. 2. Substituting strontium (Sr2+) for extracellular calcium (Ca2+) led to the appearance of numerous 'asynchronous' small IPSCs following an eIPSC. These asynchronous IPSCs were indistinguishable from TTX-insensitive quantal IPSCs. 3. Although somewhat less effective than Ca2+, Sr2+ was capable of supporting DSI, and both asynchronous and synchronous IPSCs were blocked by the DSI process. 4. During DSI, quantal content of eIPSCs, but not quantal size, was significantly reduced. 5. Sr2+ converted paired-pulse depression (PPD) of eIPSCs to a paired-pulse facilitation (PPF), presumably by altering the probability of release at inhibitory nerve terminals. DSI had no effect on either PPD or PPF. 6. The results show that Sr2+ induces asynchronous release of GABA as it does of other neurotransmitters and changes the probability of release at GABAA ergic terminals as well. Most importantly, the results support the hypothesis that, despite being induced postsynaptically, DSI is expressed presynaptically as a decrease in GABA release, possibly by acting at a site other than the Ca(2+)-dependent release step.

Retrograde signalling in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells.

1. We have investigated the phenomenon of 'depolarization-induced suppression of inhibition' (DSI) using whole-cell voltage-clamp techniques in Ca1 pyramidal cells of rat hippocampal slices. DSI was induced by eliciting voltage-dependent calcium (Ca2+) currents with 1 s voltage steps of +60 to +90 mV from the holding potential. DSI was apparent as a reduction in synaptic GABAA responses for a period of about 1 min following the voltage step. 2. TTX-sensitive spontaneous IPSCs (sIPSCs) were susceptible to DSI, while TTX-resistant miniature inhibitory postsynaptic current (mIPSCs) were not. Miniature IPSCs are ordinarily infrequent and independent of external Ca2+ in the CA1 region. To increase the frequency of mIPSCs and to induce a population of Ca(2+)-sensitive mIPSCs, we increased the bath K+ concentration to 15 mM. The increased mIPSCs were also insensitive to DSI, however. 3. T whole-cell pipette-filling solution contained 5 mM 2(triethylamino-N-(2,6-dimethyl-phenyl)acetamide (QX-314) to block voltage-dependent Na+ currents and caesium to block K+ currents. Nevertheless, bath application of 50 microM 4-aminopyridine (4-AP) or 250 nM veratridine both clearly reduced DSI, evidently by acting at presynaptic sites. 4. The amplitudes of monosynaptically evoked IPSCs (elicited in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 microM 2-amino-5-phosphonovaleric acid (APV)) were dramatically reduced during the DSI period. Weak stimulation produced small IPSCs and occasional 'failures' of transmission during the control period. The percentage of failures increased markedly during the DSI period. Moderate-intensity stimulation produced larger IPSCs that were often composed of distinguishable multiquantal components. All-or-none failures of multiquantal IPSC components also occurred during DSI. 5. The degree of paired-pulse IPSC depression did not change during DSI, whereas it was decreased, as expected, by baclofen. 6. We conclude that the data represent novel evidence that DSI is mediated by a retrograde signalling process possibly involving presynaptic axonal conduction block.

Postsynaptic mechanisms underlying long-term depression of GABAergic transmission in neurons of the deep cerebellar nuclei.

1. The mechanisms underlying long-term depression (LTD) of gamma-aminobutyric acid-A (GABAA) receptor-mediated synaptic transmission induced by 10-Hz stimulation of the inhibitory afferents were investigated using perforated and whole cell voltage-clamp recordings from neurons of the deep cerebellar nuclei (DCN). 2. LTD of inhibitory postsynaptic currents (IPSCs) was reliably induced when the 10-Hz stimulation was delivered under current-clamp conditions where the postsynaptic neuronal membrane was allowed to depolarize. 3. Currents elicited by local applications of the GABAA receptor agonist, 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol hydrochloride (THIP) were also depressed during LTD. 4. LTD could be induced heterosynaptically and did not require the activation of GABAA receptors during the 10-Hz stimulation. 5. In cells loaded with QX-314 and superfused with media containing 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2-amino-5-phosphonovaleric acid (APV), a series of depolarizing pulses (50 mV, 200 ms) induced a sustained depression of the IPSC. However, this was not observed in cells recorded with high bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)-containing pipette solutions or when they were exposed to the L-type Ca2+ channel antagonist, nitrendipine. 6. The 10-Hz-induced LTD was also inhibited by BAPTA and was significantly reduced when DCN cells were loaded with microcystin LR or treated with okadaic acid, both inhibitors of protein phosphatases. 7. These results indicate that increases in postsynaptic [Ca2+] and phosphatase activity can reduce the efficacy of GABAA receptor-mediated synaptic transmission.

Tetanus-induced potentiation of inhibitory postsynaptic potentials in hippocampal CA1 neurons.

In the present study, tetanus-induced potentiation of inhibitory postsynaptic potentials (IPSPs), previously described by our laboratory, was further investigated in guinea pig hippocampal CA1 neurons. Tetanic stimulation of the stratum radiatum induced a long-term potentiation of the excitatory postsynaptic potential and a potentiation of the gamma-aminobutyric acid A (GABAA) receptor mediated fast IPSP without enhancing the GABAB receptor-mediated slow IPSP. During the potentiation, IPSPs evoked by stimulation of the alveus were unaffected. When slices were superfused with DL-2-amino-5-phosphonovaleric acid (an N-methyl-D-aspartate, NMDA, antagonist) and 6-cyano-7-nitroquinoxaline-2,3-dione (a non-NMDA glutamate antagonist), the potentiation of the monosynaptic fast IPSP could still be induced and maintained, suggesting that polysynaptic influences were unnecessary for this process. Finally, since the potentiation was observed in CA1 neurons in which BAPTA or K-252b was injected, this form of plasticity does not appear to be dependent on a rise in intracellular [Ca2+] or protein kinase C (PKC) activity. These results indicate that tetanic stimulation of the stratum radiatum induces a potentiation of GABAergic fast IPSPs in CA1 neurons. The potentiation may be localized to the GABAergic synapse on CA1 neurons.

Pharmacological characterization of pre- and postsynaptic GABAB receptors in the deep nuclei of rat cerebellar slices.

Whole-cell current-and voltage-clamp recordings were made from deep nuclear neurons in cerebellar slices from seven- to nine-day-old rats. Baclofen, a GABAB agonist, produced a slow postsynaptic hyperpolarization associated with a decrease in input resistance. The hyperpolarization was G-protein-dependent, blocked by intracellular Cs+ and antagonized by CGP 35348, a GABAB antagonist. In dialysed neurons recorded with Cs+ -containing pipettes, baclofen suppressed deep nuclear neuronal inhibitory postsynaptic potentials and inhibitory postsynaptic currents evoked by electrical stimulations of the Purkinje cell axons. This effect was blocked by CGP 35348, indicating that the suppressions were mediated by presynaptic GABAB receptors. The inability of CGP 35348 or uptake inhibitors (nipecotic acid and NO-711) to alter the decay of inhibitory postsynaptic currents evoked by maximal stimulation suggested that GABAB receptors are not activated by the stimulation of the GABAergic input. Paired-pulse depression of inhibitory postsynaptic currents was not blocked by CGP 35348. Moreover, neither uptake inhibitors nor CGP 35348 produced any significant changes to the whole-cell current produced by a tetanic stimulation of Purkinje cell axons, suggesting that GABAB autoreceptors were also not activated by endogenous GABA release. Our findings indicate that while pre- and postsynaptic GABAB receptors are present in the deep nuclei of the rat cerebellum, they are not activated by electrical stimulation of the Purkinje cell axons.

Presynaptic actions of glutamate receptor agonists in the CA1 region of rat hippocampus in vitro.

A grease-gap recording technique which allows the monitoring of presynaptic d. c. potentials without contamination of potentials from postsynaptic elements was used to examine presynaptic actions of glutamate agonists in the CA1 region of rat hippocampus. Presynaptic depolarizations through the activation of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)- and 2-amino-5-phosphonovaleric acid (APV)-sensitive receptors could be induced by applied agonists. In addition, the N-methyl-D-aspartate (NMDA)-induced depolarization was smaller in the presence of extracellular Mg2+ suggesting some similarity to postsynaptic NMDA receptors. The (1S,3R)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD)-induced depolarization was antagonized by L-2-amino-3-phosphonopropionic acid (L-AP3) but was also sensitive to APV+CNQX, creating ambiguity as to the type of receptors involved. These results suggest that the activation of glutamate autoreceptors leads to a presynaptic depolarization.

Presynaptic actions of GABA and baclofen in CA1 region of the guinea-pig hippocampus in vitro.

The presynaptic actions of GABA and (+/-) baclofen on the stratum radiatum in the CA1 region of guinea-pig hippocampal slices were investigated using a modified grease-gap recording technique. D.c. potential shifts were recorded in response to varying concentrations of GABA and (+/-) baclofen. In Ca(2+)-free media containing tetrodotoxin, bath applications of GABA (2.5 microM to 20 mM) produced depolarizations which were concentration-dependent. Maximum depolarization was attained with 10 mM GABA. Superfusion of (+/-) baclofen (0.125-500 microM) produced a concentration-dependent hyperpolarization which peaked at a concentration of 250 microM. The GABA-induced depolarization but not the (+/-) baclofen-induced hyperpolarization was depressed by the GABAA antagonists bicuculline and picrotoxinin. The (+/-) baclofen-induced hyperpolarization but not the GABA-induced depolarization was suppressed by CGP 35,348, a GABAB antagonist. In the presence of bicuculline, GABA (0.5-5.0 mM) occasionally caused a hyperpolarization which could be blocked by CGP 35,348. These results indicate that the primary presynaptic action of GABA on the d.c. potential in the CA1 region of the hippocampus is to produce a GABAA receptor-mediated depolarization, while (+/-) baclofen induces a GABAB receptor-mediated hyperpolarization. The grease-gap d.c. potential recording technique, described in this paper, is expected to be useful in examining changes in the membrane potentials of presynaptic terminals.

Long-term depression of IPSPs in rat deep cerebellar nuclei.

Stimulation of the white matter between the cerebellar cortex and the deep cerebellar nuclei in rat cerebellar slices, evoked inhibitory postsynaptic potentials (IPSPs) in deep nuclear neurones. These IPSPs had reversal potentials close to -75 mV and were blocked by picrotoxinin. Stable IPSPs were evoked when the white matter was stimulated at 0.033 Hz; however, at frequencies > 0.2 Hz, the synaptic transient was suppressed. Paired-pulse depression of the IPSP occurred at inter-pulse intervals of 100-400 ms. Subsequent to stimulation at 0.2-5 Hz, a short-term depression of the IPSP occurred while a tetanic stimulation at 100 Hz caused a long-term depression. These results indicate that IPSPs in the deep cerebellar nuclei undergo activity-mediated plasticity.

Blockade of hippocampal long-term potentiation by saccharin.

Population spikes, population excitatory postsynaptic potentials and intracellular excitatory postsynaptic potentials were recorded in the CA1 area of guinea-pig hippocampal slices in response to low frequency stimulation of the stratum radiatum. Tetanic stimulation of the same afferents during an application of saccharin (10 mM, 10 min) failed to induced a long-term potentiation of the population spike, population excitatory postsynaptic potential and intracellularly recorded excitatory postsynaptic potential. A post-tetanic application of saccharin did not prevent long-term potentiation of the population spike from developing. Saccharin did not change the input resistance, the membrane potential or the ability to induce action potentials in the CA1 neurons. The slope of the intracellular excitatory postsynaptic potentials recorded in normal medium, in normal medium containing 2-amino-5-phosphonovalerate, or in Mg(2+)-free medium containing 6-cyano-7-nitroquinoxaline-2,3-dione was not significantly altered by saccharin. The depolarizations of CAI neurons produced by superfusion of N-methyl-D-aspartate or during a brief tetanic stimulation of the stratum radiation were also not altered by the drug. It therefore appears that saccharin blocks the induction of long-term potentiation by a mechanism that does not involve a blockade of N-methyl-D-aspartate receptors. Application of fluid samples collected from rabbit neocortical surface during a tetanic stimulation of the neocortex caused neurite growth in PC-12 cells, suggesting that growth-related substances were present in the collected samples. If these samples were superfused onto hippocampal slices, long-term potentiation developed. If however, the samples were co-applied with saccharin, neither neurite growth in PC-12 cells nor long-term potentiation in hippocampal slices was observed, raising the possibility that growth-related substances are involved in long-term potentiation.

Chelation of postsynaptic Ca2+ facilitates long-term potentiation of hippocampal IPSPs.

In guinea pig hippocampal slices, a tetanic stimulation of the stratum radiatum caused long-term potentiation (LTP) of the excitatory postsynaptic potential (EPSP) but not of the GABAB receptor-mediated slow inhibitory postsynaptic potential (IPSP) in the CA1 neurons. In neurons in which Ca2+ was chelated with 1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetra-acetic acid (BAPTA) or ethylene-bis(oxyethylenenitrilo)tetra-acetic acid (EGTA), tetanic stimulation of the stratum radiatum caused LTP of the slow IPSP but not of the EPSP. These results indicate that a reciprocal relationship exists between LTP of the EPSP and LTP of the slow IPSP as far as the involvement of the postsynaptic Ca2+ is concerned.

Studies on substances that induce long-term potentiation in guinea-pig hippocampal slices.

Fluids were collected from the rabbit neocortex during a tetanic stimulation of the cortical surface. When these samples from the neocortex were applied on the guinea-pig hippocampal slices, only those fractions containing substances with molecular weights less than 3000, 3000-10,000 and greater than 50,000 and not with other molecular weights, could induce long-term potentiation of population spikes in the CAI area in response to stratum radiatum stimulation. Intracellular recordings from the CAI neurons revealed that the long-term potentiation-inducing substances increased the excitatory postsynaptic potential without changing the membrane potential and the input resistance of these cells. A pretreatment of the rabbits with MK-801 prevented the release of the long-term potentiation-inducing substances. 2-Amino-5-phosphonovalerate was unable to block the long-term potentiation-inducing action of the substances from the rabbit neocortex. Gel-electrophoresis of the substances collected from the rabbit neocortex revealed the presence of an acidic peptide with a molecular weight of about 69,000. These results indicate that tetanic stimulation of rabbit neocortex results in a release of substances with molecular weights of less than 3000, 3000-10,000 and greater than 50,000 that could induce long-term potentiation in guinea-pig hippocampal slices. The release, but not the long-term potentiation-inducing action, of these substances appears to depend on the activation of N-methyl-D-aspartate receptors. The long-term potentiation-inducing substance in the greater than 50,000 mol. wt fraction may be an acidic peptide with a molecular weight of about 69,000.

Modulation of the induction of long-term potentiation in the hippocampus.

High frequency stimulation in the guinea pig hippocampus or on the rabbit cerebral cortical surface results in a release of substances that produce, when applied, LTP in the CA1 area of guinea pig hippocampal slices. The substances collected from the neocortex also induce neurite growth in PC-12 cells. The samples collected during the tetanic stimulation of the neocortex contained increased concentrations of glycine, and various other amino acids that are being identified, as well as peptides. Whether the release of the substances is from neurons or from glia is being investigated. Tetanic stimulations of stratum radiatum in the guinea pig hippocampus that induce LTP in CA1 neurons also cause large and prolonged depolarization of glial cells in the CA1 apical dendritic area. Artificial depolarization of glial cells during the activation of stratum radiatum results in LTP of the CA1 neuronal EPSP. It is, therefore, suggested that glial depolarization is involved as one of the steps in the induction of LTP. We speculate that the depolarization results in the release of substances into the extracellular space and that these substances are involved in directly or indirectly modulating the NMDA receptor-coupled channels as well as in producing trophic effects to induce structural changes in the synapses that are thought to be associated with the establishment and maintenance of LTP.