LTP regulates burst initiation and frequency at mossy fiber-granule cell synapses of rat cerebellum: experimental observations and theoretical predictions

Long-term potentiation (LTP) is a synaptic change supposed to provide the cellular basis for learning and memory in brain neuronal circuits. Although specific LTP expression mechanisms could be critical to determine the dynamics of repetitive neurotransmission, this important issue remained largely unexplored. In this paper, we have performed whole cell patch-clamp recordings of mossy fiber-granule cell LTP in acute rat cerebellar slices and studied its computational implications with a mathematical model. During LTP, stimulation with short impulse trains at 100 Hz revealed earlier initiation of granule cell spike bursts and a smaller nonsignificant spike frequency increase. In voltage-clamp recordings, short AMPA excitatory postsynaptic current (EPSC) trains showed short-term facilitation and depression and a sustained component probably generated by spillover. During LTP, facilitation disappeared, depression accelerated, and the sustained current increased. The N-methyl-d-aspartate (NMDA) current also increased. In agreement with a presynaptic expression caused by increased release probability, similar changes were observed by raising extracellular [Ca(2+)]. A mathematical model of mossy fiber-granule cell neurotransmission showed that increasing release probability efficiently modulated the first-spike delay. Glutamate spillover, by causing tonic NMDA and AMPA receptor activation, accelerated excitatory postsynaptic potential (EPSP) temporal summation and maintained a sustained spike discharge. The effect of increasing neurotransmitter release could not be replicated by increasing receptor conductance, which, like postsynaptic manipulations enhancing intrinsic excitability, proved very effective in raising granule cell output frequency. Independent regulation of spike burst initiation and frequency during LTP may provide mechanisms for temporal recoding and gain control of afferent signals at the input stage of cerebellar cortex.     

Impairment of neocortical long-term potentiation in mice deficient of endothelial nitric oxide synthase

The role of the possible retrograde messenger nitric oxide (NO) in the induction of long-term potentiation (LTP) was studied in supragranular layers of somatosensory cortical slices obtained from adult mice. High-frequency stimulation produced a slowly rising, long-lasting (50 min) and significant (P < 0.001) increase in the extracellular synaptic response by 23%. The induction of LTP was independent from activation of N-methyl-D-aspartate (NMDA) receptors, but prevented by bath application of NG-nitro-L-arginine methyl ester (L-NAME), indicating that one or several of the different NO synthases (NOS) produced NO within the postsynaptic neuron. No LTP could be induced in knockout mice lacking the endothelial NOS (eNOS) isoform. These data suggest that eNOS is involved in an NMDA receptor-independent form of LTP in the rodent cerebral cortex.     

Presynaptic current changes at the mossy fiber-granule cell synapse of cerebellum during LTP

The involvement of presynaptic mechanisms in the expression of long-term potentiation (LTP), an enhancement of synaptic transmission suggested to take part in learning and memory in the mammalian brain, has not been fully clarified. Although evidence for enhanced vesicle cycling has been reported, it is unknown whether presynaptic terminal excitability could change as has been observed in invertebrate synapses. To address this question, we performed extracellular focal recordings in cerebellar slices. The extracellular current consisted of a pre- (P(1)/N(1)) and postsynaptic (N(2)/SN) component. In ~50% of cases, N(1) could be subdivided into N(1a) and N(1b). Whereas N(1a) was part of the fiber volley (P(1)/N(1a)), N(1b) corresponded to a Ca(2+)-dependent component accounting for 40-50% of N(1), which could be isolated from individual mossy fiber terminals visualized with fast tetramethylindocarbocyanine perchlorate (DiI). The postsynaptic response, given its timing and sensitivity to glutamate receptor antagonists [N(2) was blocked by 10 microM [1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) and SN by 100 microM APV +50 microM 7-Cl-kyn], corresponded to granule cell excitation. N(2) and SN could be reduced by 1) Ca(2+) channel blockers, 2) decreasing the Ca(2+) to Mg(2+) ratio, 3) paired-pulse stimulation, and 4) adenosine receptor activation. However, only the first two manipulations, which modify Ca(2+) influx, were associated with N(1) (or N(1b)) reduction. LTP was induced by theta-burst mossy fiber stimulation (8 trains of 10 impulses at 100 Hz separated by 150-ms pauses). Interestingly, during LTP, both N(1) (or N(1b)) and N(2)/SN persistently increased, whereas P(1) (or P(1)/N(1a)) did not change. Average changes were N(1) = 38.1 +/- 31.9, N(2) = 49.6 +/- 48.8, and SN = 59.1 +/- 35.5%. The presynaptic changes were not observed when LTP was prevented by synaptic inhibition, by N-methyl-D-aspartate and metabotropic glutamate receptor blockage, or by protein kinase C blockage. Moreover, the presynaptic changes were sensitive to Ca(2+) channel blockers (1 mM Ni(2+) and 5 microM omega-CTx-MVIIC) and occluded by K(+) channel blockers (1 mM tetraethylammmonium). Thus regulation of presynaptic terminal excitability may take part in LTP expression at a central mammalian synapse.     

An NMDA receptor/nitric oxide cascade is involved in cerebellar LTD but is not localized to the parallel fiber terminal

Long-term depression (LTD) of the parallel fiber-Purkinje cell synapse in the cerebellum is a cellular model system that has been suggested to underlie certain forms of motor learning. Induction of cerebellar LTD requires a postsynaptic kinase limb involving activation of mGluR1, protein kinase Calpha (PKCalpha), and phosphorylation of ser-880 on the AMPA receptor subunit GluR2. Several lines of evidence have also implicated a complementary phosphatase limb in which N-methyl-d-aspartate (NMDA) receptor-mediated Ca(2+) influx activates neuronal nitric oxide synthase (nNOS), the ultimate consequences of which are mediated by nitric oxide (NO), cGMP, and inhibition of postsynaptic protein phosphatases. However, the cellular localization of an NMDA/NO cascade has been complicated by the fact that neither functional NMDA receptors nor nNOS are expressed in Purkinje cells. This has lead to a proposal in which NMDA receptors activate nNOS in parallel fibers. Here, we confirm that pharmacological blockade of NMDA receptor or NO signaling blocks induction of LTD. However, no evidence was found for functional NMDA receptors in parallel fiber terminals: blockade of NMDA receptors did not alter either presynaptic Ca(2+) transients or the frequency of miniature excitatory postsynaptic currents. NMDA receptor blockade did abolish a slow depolarization evoked by burst stimulation of parallel fiber-stellate cell synapses. The application of NMDA evoked a Ca(2+) transient in stellate cell terminals but not in parallel fiber terminals. These results are consistent with the hypothesis that an NMDA receptor/NO cascade involved in cerebellar LTD is localized to interneurons rather than parallel fibers.     

Separate mechanisms of long-term potentiation in two input systems to CA3 pyramidal neurons of rat hippocampal slices as revealed by the whole-cell patch-clamp technique.

Excitatory synaptic transmission from two input systems to hippocampal CA3 pyramidal neurons was investigated by the whole-cell patch-clamp technique for thin slice preparation, with special reference to long-term potentiation (LTP) in these systems. Excitatory postsynaptic currents (EPSCs) evoked by fimbrial stimulation consisted of two components; one was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the other was persistent at depolarized membrane potentials and blocked by D-2-amino-5-phosphonovalerate (D-AP5). The contribution of the D-AP5-sensitive component to EPSCs evoked by stimulation of mossy fibers was much less than that to fimbrial EPSCs. High-frequency stimulation of afferent fibers, under current-clamp conditions, elicited LTP. Bath application of D-AP5 blocked the induction of LTP in the fimbrial but not in the mossy fiber synapses. Induction of fimbrial LTP was completely blocked by 10 mM BAPTA applied intracellularly. In contrast, mossy fiber LTP was not blocked by 10 mM BAPTA. Furthermore, mossy fiber LTP, but not fimbrial LTP, was elicited by high-frequency stimulation under voltage-clamp (-80 mV) conditions. These results suggest that activation of NMDA receptors, increase in postsynaptic [Ca2+]i, and postsynaptic membrane depolarization are required for the induction of fimbrial but not for mossy fiber LTP.     

Multiple forms of LTP in hippocampal CA3 neurons use a common postsynaptic mechanism.

We investigated long-term potentiation (LTP) at mossy fiber synapses on CA3 pyramidal neurons in the hippocampus. Using Ca2+ imaging techniques, we show here that when postsynaptic Ca2+ was sufficiently buffered so that [Ca2+]i did not rise during synaptic stimulation, the induction of mossy fiber LTP was prevented. In addition, induction of mossy fiber LTP was suppressed by postsynaptic injection of a peptide inhibitor of cAMP-dependent protein kinase. Finally, when ionotropic glutamate receptors were blocked, LTP depended on the postsynaptic release of Ca2+ from internal stores triggered by activation of metabotropic glutamate receptors. These results support the conclusion that mossy fiber LTP and LTP at other hippocampal synapses share a common induction mechanism involving an initial rise in postsynaptic [Ca2+].     

Regulatory role of nitric oxide over extracellular taurine in the hippocampus of freely moving rats

We have studied the effects of drugs which manipulate nitric oxide (NO) levels as well the effect of N-methyl-d-aspartate (NMDA) infusion on extracellular taurine in rat hippocampus using in vivo microdialysis. The NO donor S-nitroso-N-acetylpenicillamine (SNAP) increased dialysate taurine in a concentration-dependent manner, and this effect was blocked by the inhibitor of soluble guanylate cyclase1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ). NMDA (100 microM) increased hippocampal taurine release, an effect that was reversed by the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5; 10 microM). The non-selective nitric oxide synthase (NOS) inhibitor N-nitro-l-arginine methyl ester (L-NAME; 100 microM and 1.0 mM) increased extracellular taurine in a concentration-dependent manner while 7-nitroindazole (7-NI), a relatively selective neuronal NOS (nNOS) inhibitor, at the same concentrations decreased extracellular taurine. L-NAME (1.0 mM) infused prior to NMDA did not alter the effect of NMDA on extracellular taurine having an effect essentially identical to that seen with L-NAME infused alone. In contrast, when 7-NI was infused for 30 min prior to NMDA, taurine levels were no longer increased above basal. This suggests to us that taurine efflux is mediated by two different mechanisms: an NMDA-evoked, 7-NI-sensitive pathway which may be dependent on cyclic guanosine monophosphate formation, and an L-NAME-modulated mechanism which presumably involves other members of the NOS group of enzymes than nNOS alone.     

Evidence for nitric oxide synthase inhibitor-sensitive and insensitive hippocampal synaptic potentiation.

1. Nitric oxide (NO) has been proposed as a retrograde messenger, mediating the postsynaptic to presynaptic transfer of the effects of conditioning stimulation, responsible for the initiation of hippocampal long-term potentiation (LTP). To further test this hypothesis, we inhibited nitric oxide synthase (NOS) to determine whether synaptic potentiation produced by different conditioning stimulus patterns and intensities was differentially affected by reduction of stimulation-dependent NO production. 2. Synaptic potentiation was produced in hippocampal slices from young F-344 rats by two different conditioning stimulation protocols. Conditioning stimuli were delivered to the Schaffer-collateral commissural system, and moderate levels of potentiation of the population excitatory postsynaptic potential (EPSP) in area CA1 were produced by a single 100 Hz, 1-s conditioning train delivered at half-maximal stimulus intensity. Higher levels of potentiation of the population EPSP were obtained by delivering two 100 Hz, 1-s conditioning stimulus trains, with a 60-s intertrain interval, at high stimulus currents. 3. Application of the nitric oxide synthase inhibitors NG-nitro-L-arginine (NOARG; 0.1-200 microM) and NG-monomethyl-L-arginine (NMMA; 100 microM) produced no significant direct effects on synaptic responses. 4. In slices that received a single conditioning stimulus train, both NOARG and NMMA were ineffective in blocking or reducing potentiation at concentrations between 0.1 and 200 microM. In slices receiving the more intense pair of conditioning stimulus trains, levels of potentiation in control slices were higher, and there was a very significant reduction by both NOARG (50 and 100 microM) and NMMA (100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Synapses between parallel fibres and stellate cells express long-term changes in synaptic efficacy in rat cerebellum

Various forms of synaptic plasticity underlying motor learning have already been well characterized at cerebellar parallel fibre (PF)–Purkinje cell (PC) synapses. Inhibitory interneurones play an important role in controlling the excitability and synchronization of PCs. We have therefore tested the possibility that excitatory synapses between PFs and stellate cells (SCs) are also able to exhibit long-term changes in synaptic efficacy. In the present study, we show that long-term potentiation (LTP) and long-term depression (LTD) were induced at these synapses by a low frequency stimulation protocol (2 Hz for 60 s) and that pairing this low frequency stimulation protocol with postsynaptic depolarization induced a marked shift of synaptic plasticity in favour of LTP. This LTP was cAMP independent, but required nitric oxide (NO) production from pre- and/or postsynaptic elements, depending on the stimulation or pairing protocol used, respectively. In contrast, LTD was not dependent on NO production but it required activation of postsynaptic group II and possibly of group I metabotropic glutamate receptors. Finally, stimulation of PFs at 8 Hz for 15 s also induced LTP at PF–SC synapses. But in this case, LTP was cAMP dependent, as was also observed at PF–PC synapses for presynaptic LTP induced in the same conditions. Thus, long-term changes in synaptic efficacy can be accomplished by PF–SCs synapses as well as by PF–PC synapses, suggesting that both types of plasticity might co-operate during cerebellar motor learning.     

Recurrent mossy fiber pathway in rat dentate gyrus: synaptic currents evoked in presence and absence of seizure-induced growth

A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant.     

Presynaptic kainate receptors impart an associative property to hippocampal mossy fiber long-term potentiation

Hippocampal mossy fiber synapses show an unusual form of long-term potentiation (LTP) that is independent of NMDA receptor activation and is expressed presynaptically. Using receptor antagonists, as well as receptor knockout mice, we found that presynaptic kainate receptors facilitate the induction of mossy fiber long-term potentiation (LTP), although they are not required for this form of LTP. Most importantly, these receptors impart an associativity to mossy fiber LTP such that activity in neighboring mossy fiber synapses, or even associational/commissural synapses, influences the threshold for inducing mossy fiber LTP. Such a mechanism greatly increases the computational power of this form of plasticity.     

Mechanisms underlying LTP of inhibitory synaptic transmission in the deep cerebellar nuclei

Whole-cell recordings were used to investigate long-term potentiation of inhibitory synaptic currents (IPSCs) in neurons of deep cerebellar nuclei (DCN) in slices. IPSCs were evoked by electrical stimulation of the white matter surrounding the DCN in the presence of non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (20 microM). High-frequency stimulation induced a long-term potentation (LTP) of the IPSC amplitude without changing its reversal potential, rise time, and decay-time constant. This LTP did not require the activation of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors but depended on the activation of NMDA receptors. LTP of IPSCs in DCN neurons could also be induced by voltage-depolarizing pulses in postsynaptic neurons and appeared to depend on an increase in intracellular calcium as the LTP was blocked when the cells were loaded with a calcium chelator, 1,2-bis-(2-amino-phenoxy)-N,N,N', N'-tetraacetic acid (BAPTA, 10 mM). LTP of IPSCs was accompanied by an increase in the frequency of spontaneous IPSCs and miniature IPSCs (recorded in the presence of tetrodotoxin 1 microM), but there was no significant change in their amplitude. In addition, during the LTP, the amplitude of response to exogenously applied GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride was increased. Intracellular application of tetanus toxin, a powerful blocker of exocytosis, in DCN neuron prevented the induction of LTP of IPSCs. Our results suggest that the induction of LTP of IPSCs in the DCN neurons likely involves a postsynaptic locus. Plasticity of inhibitory synaptic transmission in DCN neurons may play a crucial role in cerebellar control of motor coordination and learning.     

NMDA receptor-dependent metaplasticity at hippocampal mossy fiber synapses

Hippocampal mossy fiber synapses have been reported to lack NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of AMPA excitatory postsynaptic currents (EPSCs), unlike conventional glutamatergic synapses. An explanation for this difference may reside in the relatively low number of NMDARs at these synapses. Because mossy fiber synapses display LTP selective for NMDARs, we examined whether this would affect the plasticity rules at mossy fiber-CA3 synapses in mouse hippocampal slices. We found that LTP of NMDARs serves as a metaplastic switch making mossy fiber synapses competent for generating NMDAR-dependent LTP of AMPA EPSCs.     

Evidence for involvement of group II/III metabotropic glutamate receptors in NMDA receptor-independent long-term potentiation in area CA1 of rat hippocampus

Previous studies implicated metabotropic glutamate receptors (mGluRs) in N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation (LTP) in area CA1 of the rat hippocampus. To learn more about the specific roles played by mGluRs in NMDA receptor-independent LTP, we used whole cell recordings to load individual CA1 pyramidal neurons with a G-protein inhibitor [guanosine-5'-O-(2-thiodiphosphate), GDPbetaS]. Although loading postsynaptic CA1 pyramidal neurons with GDPbetaS significantly reduced G-protein dependent postsynaptic potentials, GDPbetaS failed to prevent NMDA receptor- independent LTP, suggesting that postsynaptic G-protein-dependent mGluRs are not required. We also performed a series of extracellular field potential experiments in which we applied group-selective mGluR antagonists. We had previously determined that paired-pulse facilitation (PPF) was decreased during the first 30-45 min of NMDA receptor-independent LTP. To determine if mGluRs might be involved in these PPF changes, we used a twin-pulse stimulation protocol to measure PPF in field potential experiments. NMDA receptor-independent LTP was prevented by a group II mGluR antagonist [(2S)-alpha-ethylglutamic acid] and a group III mGluR antagonist [(RS)-alpha-cyclopropyl-4-phosphonophenylglycine], but was not prevented by other group II and III mGluR antagonists [(RS)-alpha-methylserine-O-phosphate monophenyl ester or (RS)-alpha-methylserine-O-phosphate]. NMDA receptor-independent LTP was not prevented by either of the group I mGluR antagonists we examined, (RS)-1-aminoindan-1,5-dicarboxylic acid and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. The PPF changes which accompany NMDA receptor-independent LTP were not prevented by any of the group-selective mGluR antagonists we examined, even when the LTP itself was blocked. Finally, we found that tetanic stimulation in the presence of group III mGluR antagonists lead to nonspecific potentiation in control (nontetanized) input pathways. Taken together, our results argue against the involvement of postsynaptic group I mGluRs in NMDA receptor-independent LTP. Group II and/or group III mGluRs are required, but the specific details of the roles played by these mGluRs in NMDA receptor-independent LTP are uncertain. Based on the pattern of results we obtained, we suggest that group II mGluRs are required for induction of NMDA receptor-independent LTP, and that group III mGluRs are involved in determining the input specificity of NMDA receptor-independent LTP by suppressing potentiation of nearby, nontetanized synapses.     

Distinct glutamate receptors govern differential levels of nitric oxide production in a layer-specific manner in the rat cerebellar cortex

To evaluate roles of nitric oxide (NO) in neural functions, it is critical to know how neural inputs activate neuronal NO synthase in individual sites. Although NMDA receptor-dependent mechanism well explains postsynaptic, robust NO production, this sole mechanism does not explain some aspects of NO production in the brain, such as the low-level production of NO and the mechanism for presynaptic NO production. We hypothesized that the glutamate receptor involved in NO production is site-specific and controls the initial NO concentration in each site. We visualized NO production mediated by NMDA, AMPA and type-1 metabotropic glutamate (mGlu-1) receptors in rat cerebellar slices and granule cells in culture, with an NO-specific fluorescent indicator, diaminofluorescein-2. AMPA receptor, but not NMDA or mGlu-1 receptor, was responsible for NO production at parallel fiber terminals, which was blocked by CNQX, tetrodotoxin or voltage-dependent calcium channel blockers. More numbers of electrical stimulation were required for NO production in the molecular layer than in other layers, suggesting that AMPA receptor activation generates NO at lower concentrations through a remote interaction with NO synthase. Although Purkinje cell does not express NO synthase, we detected NO production in Purkinje cell layer following electrical stimulation in the white matter at 50 Hz, but not at 10 Hz. This NO production was tetrodotoxin-sensitive, suggesting occurrence in the basket cell terminals, and required synergistic activation of mGlu-1 and NMDA receptors. In the granule cell layer, activation of AMPA or mGlu-1 receptor produced NO uniformly, while NMDA receptor activation produced NO in discontinuous areas of this layer. Thus, distinct glutamate receptors, including non-NMDA receptors, govern occurrence and level of NO production in a layer-specific manner.     

Noradrenergic enhancement of long-term potentiation at mossy fiber synapses in the hippocampus

1. We tested several hypotheses related to the modulation of long-term potentiation (LTP) by norepinephrine (NE) at the mossy fiber synapses in the rat hippocampal slice preparation using extracellular and intracellular recording techniques. 2. NE exerted frequency-dependent effects on mossy fiber synaptic transmission. It had little effect on extracellular population excitatory postsynaptic potentials (pEPSPs) sampled during low-frequency stimulation, whereas it had marked effects on the duration, magnitude, and probability of induction of LTP at these synapses. 3. The beta-adrenoceptor agonist isoproterenol mimicked all of the effects of NE, whereas the beta-adrenoceptor antagonists propranolol and timolol reversibly blocked the induction of LTP, suggesting the effects of NE are mediated by a beta-adrenoceptor and that beta-adrenoceptor activation may be an important constituent for the expression of LTP at these synapses. 4. Frequency-dependent effects of NE and isoproterenol on mossy fiber pEPSPs were also observed in the presence of the gamma-aminobutyric acid (GABA) antagonist, picrotoxin, suggesting that NE can enhance LTP by a mechanism that does not depend on intact inhibition. However, propranolol did not block LTP in these disinhibited slices and did not affect LTP magnitude. 5. The adenylate cyclase activator forskolin augmented pEPSPs sampled during low-frequency stimulation in disinhibited slices and significantly enhanced LTP. Forskolin, however, did not produce LTP in the absence of tetanic stimulation. This supports the hypothesis that NE and isoproterenol augment features of LTP by stimulating adenosine 3',5'-cyclic monophosphate (cAMP) production and that cAMP plays a modulatory role in the induction of LTP. 6. The postsynaptic injection of the cAMP analogue 8-bromoadenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) significantly increased the probability of induction of LTP measured intracellularly under voltage-clamp conditions with intact inhibition. An analysis of the inhibitory synaptic slope conductance during these experiments indicated that changes in this measure could neither account for the increase in mossy fiber synaptic slope conductance in those cells that displayed it nor account for the group differences in this variable. 7. The amplitude and duration of the postsynaptic depolarization during tetanic stimulation in the cells that displayed LTP in the 8-bromo-cAMP-injected group were significantly greater than in the cells that did not display LTP in the adenosine 5'-monophosphate-injected group.(ABSTRACT TRUNCATED AT 400 WORDS)     

Presynaptically expressed long-term depression at cerebellar parallel fiber synapses

Plasticity at synapses between parallel fiber (PF) and Purkinje neurons (PN) is widely accepted as a cellular model for certain forms of cerebellar learning. Although PF–PN synapses are known to express bidirectional long-term plasticity at the postsynaptic site, long-term plasticity at the presynaptic site is currently limited to potentiation of the synapses. In this paper, we report on presynaptically expressed PF long-term depression (preLTD) that is observed when presynaptically expressed PF long-term potentiation (preLTP) is pharmacologically prevented. PF preLTD is most efficiently induced by 4 Hz PF stimulation and requires activation of cannabinoid CB1 receptors. Our results indicate that, during preLTD induction, endocannabinoids are released in an NMDA receptor-dependent, but not mGlu1 receptor-dependent, fashion. We conclude that bidirectional plasticity mechanisms exist for both presynaptic and postsynaptic components of cerebellar learning.     

Mossy fiber-granule cell synapses in the normal and epileptic rat dentate gyrus studied with minimal laser photostimulation.

Dentate granule cells become synaptically interconnected in the hippocampus of persons with temporal lobe epilepsy, forming a recurrent mossy fiber pathway. This pathway may contribute to the development and propagation of seizures. The physiology of mossy fiber-granule cell synapses is difficult to characterize unambiguously, because electrical stimulation may activate other pathways and because there is a low probability of granule cell interconnection. These problems were addressed by the use of scanning laser photostimulation in slices of the caudal hippocampal formation. Glutamate was released from a caged precursor with highly focused ultraviolet light to evoke action potentials in a small population of granule cells. Excitatory synaptic currents were recorded in the presence of bicuculline. Minimal laser photostimulation evoked an apparently unitary excitatory postsynaptic current (EPSC) in 61% of granule cells from rats that had experienced pilocarpine-induced status epilepticus followed by recurrent mossy fiber growth. An EPSC was also evoked in 13-16% of granule cells from the control groups. EPSCs from status epilepticus and control groups had similar peak amplitudes ( approximately 30 pA), 20-80% rise times (approximately 1.2 ms), decay time constants ( approximately 10 ms), and half-widths (approximately 8 ms). The mean failure rate was high (approximately 70%) in both groups, and in both groups activation of N-methyl-D-aspartate receptors contributed a small component to the EPSC. The strong similarity between responses from the status epilepticus and control groups suggests that they resulted from activation of a similar synaptic population. No EPSC was recorded when the laser beam was focused in the dentate hilus, suggesting that indirect activation of hilar mossy cells contributed little, if at all, to these results. Recurrent mossy fiber growth increases the density of mossy fiber-granule cell synapses in the caudal dentate gyrus by perhaps sixfold, but the new synapses appear to operate very similarly to preexisting mossy fiber-granule cell synapses.     

Non-competitive antagonists of NMDA and AMPA receptors decrease seizure-induced c-fos protein expression in the cerebellum and protect against seizure symptoms in adult rats

The aim of the present study was to examine the role of ionotropic glutamate receptors in the cerebellum during generalized seizures. Epileptic neuronal activation was evaluated through the immunohistochemical detection of c-fos protein in the cerebellar cortex. Generalized seizures were precipitated by the intraperitoneal injection of 4-aminopyridine. The animals were pretreated with the NMDA receptor antagonists MK-801 (2?mg/kg), amantadine (50?mg/kg), and the AMPA receptor antagonist GYKI 52466 hydrochloride (50?mg/kg). Two hours after 4-aminopyridine injection, the number of c-fos immunostained cell nuclei was counted in serial immunohistochemical sections of the cerebellar vermis. The number of c-fos immunostained cell nuclei in the granular layer decreased significantly in animals pretreated with the glutamate receptor antagonists compared to the untreated animals having convulsion. We can conclude that mossy fiber stimulation exerts its seizure-generating action mainly through the ionotropic glutamate receptors of the mossy fiber synapses. Both NMDA and AMPA receptor antagonists are effective in reducing glutamate-mediated postsynaptic effects in the cerebellar cortex.