Heterogeneous properties of central lateral and parafascicular thalamic synapses in the striatum

To understand the principles of operation of the striatum it is critical to elucidate the properties of the main excitatory inputs from cortex and thalamus, as well as their ability to activate the main neurons of the striatum, the medium spiny neurons (MSNs). As the thalamostriatal projection is heterogeneous, we set out to isolate and study the thalamic afferent inputs to MSNs using small localized injections of adeno-associated virus carrying fusion genes for channelrhodopsin-2 and YFP, in either the rostral or caudal regions of the intralaminar thalamic nuclei (i.e. the central lateral or parafascicular nucleus). This enabled optical activation of specific thalamic afferents combined with whole-cell, patch-clamp recordings of MSNs and electrical stimulation of cortical afferents, in adult mice. We found that thalamostriatal synapses differ significantly in their peak amplitude responses, short-term dynamics and expression of ionotropic glutamate receptor subtypes. Our results suggest that central lateral synapses are most efficient in driving MSNs to depolarization, particularly those of the direct pathway, as they exhibit large amplitude responses, short-term facilitation and predominantly express postsynaptic AMPA receptors. In contrast, parafascicular synapses exhibit small amplitude responses, short-term depression and predominantly express postsynaptic NMDA receptors, suggesting a modulatory role, e.g. facilitating Ca(2+)-dependent processes. Indeed, pairing parafascicular, but not central lateral, presynaptic stimulation with action potentials in MSNs, leads to NMDA receptor- and Ca(2+)-dependent long-term depression at these synapses. We conclude that the main excitatory thalamostriatal afferents differ in many of their characteristics and suggest that they each contribute differentially to striatal information processing.

Loss of zolpidem efficacy in the hippocampus of mice with the GABAA receptor gamma2 F77I point mutation

Zolpidem is a hypnotic benzodiazepine site agonist with some gamma-aminobutyric acid (GABA)(A) receptor subtype selectivity. Here, we have tested the effects of zolpidem on the hippocampus of gamma2 subunit (gamma2F77I) point mutant mice. Analysis of forebrain GABA(A) receptor expression with immunocytochemistry, quantitative [(3)H]muscimol and [(35)S] t-butylbicyclophosphorothionate (TBPS) autoradiography, membrane binding with [(3)H]flunitrazepam and [(3)H]muscimol, and comparison of miniature inhibitory postsynaptic current (mIPSC) parameters did not reveal any differences between homozygous gamma2I77/I77 and gamma2F77/F77 mice. However, quantitative immunoblot analysis of gamma2I77/I77 hippocampi showed some increased levels of gamma2, alpha1, alpha4 and delta subunits, suggesting that differences between strains may exist in unassembled subunit levels, but not in assembled receptors. Zolpidem (1 microm) enhanced the decay of mIPSCs in CA1 pyramidal cells of control (C57BL/6J, gamma2F77/F77) mice by approximately 60%, and peak amplitude by approximately 20% at 33-34 degrees C in vitro. The actions of zolpidem (100 nm or 1 microm) were substantially reduced in gamma2I77/I77 mice, although residual effects included a 9% increase in decay and 5% decrease in peak amplitude. Similar results were observed in CA1 stratum oriens/alveus interneurons. At network level, the effect of zolpidem (10 microm) on carbachol-induced oscillations in the CA3 area of gamma2I77/I77 mice was significantly different compared with controls. Thus, the gamma2F77I point mutation virtually abolished the actions of zolpidem on GABA(A) receptors in the hippocampus. However, some residual effects of zolpidem may involve receptors that do not contain the gamma2 subunit.

Sprouting of mossy fibers and presynaptic inhibition by group II metabotropic glutamate receptors in pilocarpine-treated rat hippocampal slice cultures

Mossy fibre sprouting (MFS) is a phenomenon observed in the epileptic hippocampus. We have studied MFS, in 7, 14 and 21 day in vitro (DIV) organotypic slice cultures, or in slice cultures treated with pilocarpine (0.5 mM) or pilocarpine and atropine (0.1 mM or 0.5 mM) for 48-72 h at 5 DIV and tested at 21 DIV. Acute application of pilocarpine directly activated hilar neurons and elicited epileptic-like discharges in CA3 pyramids and mossy cells of 5-8 DIV cultures, without causing substantial cell death, as assessed by lactate dehydrogenase measurements. Timm staining revealed increases in MFS in chronic pilocarpine-treated cultures, which was prevented by prior application of atropine. Extracellular synaptic responses were recorded in the granule cell layer and elicited by antidromic mossy fibre stimulation. The GABA(A) antagonist 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid (1 microM) induced a greater increase in the coastline bursting index in pilocarpine-treated cultures than in 21 DIV controls. However, there was no significant increase in the frequency of spontaneous or miniature synaptic events recorded in granule cells from pilocarpine-treated cultures. Granule cells were filled with biocytin and morphometric analysis revealed that the length of axon collaterals in the granule and molecular layer was longer in pilocarpine-treated cultures than in 21 DIV controls. Dual recordings between granule cells and between granule and hilar neurons showed that pilocarpine-treated cultures had a larger proportion of monosynaptic and polysynaptic connections. The group II metabotropic glutamate receptor (mGluR) agonist LY354740 (0.5 microM) suppressed excitatory but not inhibitory monosynaptic currents. LY354740 also inhibited antidromically evoked action currents in granule cells from pilocarpine- and to a lesser extent in pilocarpine and atropine-treated cultures, suggesting that group II mGluRs can reside along the axon and suppress action potential invasion. We provide direct evidence for the development of functional MFS and suggest a novel, axonal mechanism by which presynaptic group II mGluRs can inhibit selected synapses.

Abolition of zolpidem sensitivity in mice with a point mutation in the GABAA receptor gamma2 subunit

Agonists of the allosteric benzodiazepine site of GABAA receptors bind at the interface of the alpha and gamma subunits. Here, we tested the in vivo contribution of the gamma2 subunit to the actions of zolpidem, an alpha1 subunit selective benzodiazepine agonist, by generating mice with a phenylalanine (F) to isoleucine (I) substitution at position 77 in the gamma2 subunit. The gamma2F77I mutation has no major effect on the expression of GABAA receptor subunits in the cerebellum. The potency of zolpidem, but not that of flurazepam, for the inhibition of [3H]flunitrazepam binding to cerebellar membranes is greatly reduced in gamma2I77/I77 mice. Zolpidem (1 microM) increased both the amplitude and decay of miniature inhibitory postsynaptic currents (mIPSCs) in Purkinje cells of control C57BL/6 (34% and 92%, respectively) and gamma2F77/F77 (20% and 84%) mice, but not in those of gamma2F77I mice. Zolpidem tartrate had no effect on exploratory activity (staircase test) or motor performance (rotarod test) in gamma2I77/I77 mice at doses up to 30 mg/kg (i.p.) that strongly sedated or impaired the control mice. Flurazepam was equally effective in enhancing mIPSCs and disrupting performance in the rotarod test in control and gamma2I77/I77 mice. These results show that the effect of zolpidem, but not flurazepam, is selectively eliminated in the brain by the gamma2F77I point mutation.

Long-term Potentiation of NMDA Receptor-mediated EPSP in Guinea-pig Hippocampal Slices

Hippocampal slices from guinea-pigs were used to examine the long-term potentiation (LTP) of the N-methyl-d-aspartate (NMDA)-mediated excitatory postsynaptic potential (EPSP). Intracellular recordings were performed from CA1 pyramidal neurons in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 - 10 microM) and picrotoxin (50 microM). In these experimental conditions test stimuli applied at low frequency (0.1 Hz) to the Schaffer collateral - commissural pathway evoked a prolonged EPSP (150 - 200 ms). To obtain this CNQX-resistant EPSP, stimulus intensities had to be raised above the level required to evoke an EPSP of comparable amplitude in physiological solution. Tetanic stimulation (two trains of 100 Hz, 1 s every 20 s) led to a potentiation of the CNQX-resistant EPSP, and this potentiated response was abolished with d-(-)-2-amino-5-phosphonovaleric acid (50 microM). The potentiation of the NMDA receptor-mediated EPSP was more pronounced for strong than for weak test stimuli, and was suppressed when test EPSPs were evoked during membrane hyperpolarization. These results suggest that NMDA receptor-mediated responses can undergo LTP, and hence can contribute to the maintenance of LTP.

Cannabinoid 1 receptors are expressed by nerve growth factor- and glial cell-derived neurotrophic factor-responsive primary sensory neurones

Expression of the cannabinoid 1 (CB1) receptor and its regulation were studied in the different nociceptive and non-nociceptive sub-populations of cultured primary sensory neurones of adult rats. Bandairaea simplicifolia isolectin B4 (IB4) binding and calcitonin gene-related peptide (CGRP) immunostaining were used to distinguish between the glial cell-derived neurotrophic factor (GDNF)- and nerve growth factor (NGF)-responsive nociceptive and the non-nociceptive primary sensory neurones while a specific CB1 receptor antibody was used to study the expression of the CB1 receptor protein. About half of the total number of primary sensory neurones (47+/-3.2%) cultured for 1 day in the presence of both neurotrophic factors (50 ng/ml each) showed CB1 receptor-like immunostaining, whereas 21.8+/-3.3% and 32.7+/-5.6% of the neurones showed CGRP-like immunopositivity and IB4 binding, respectively. A proportion of the CB1 receptor-like immunopositive neurones was immunostained for CGRP (31.7+/-5%) and IB4 (48.2%+/-7.5), with a minimal (1%) co-expression of CGRP and IB4 binding. About a fifth of the CB1 receptor-like immunopositive neurones did not show either CGRP-like immunostaining or IB4 binding. To find out whether CB1 receptor expression in nociceptive primary sensory neurones is regulated by GDNF or NGF, cultures were grown in the presence or absence of the neurotrophic factors for 7 days. Vanilloid receptor 1 (VR1) immunostaining was used as a control marker to monitor the effect of the neurotrophins. In cultures maintained in the presence of both factors (50 ng/ml each) 51+/-2.6% and 42.4+/-1.2% of the cells showed CB1 receptor-like and VR1-like immunostaining, respectively. In cultures grown for 7 days in the absence of either of the neurotrophic factors the relative number of VR1-like immunopositive cells decreased to 13.4+/-2.7%, whereas the relative number of CB1 receptor-like immunopositive neurones was unchanged (50.6+/-1.1%). Our data suggest that the CB1 receptor is expressed in all of the three major sub-populations of primary sensory neurones and that the CB1 receptor expression is not regulated by either NGF or GDNF.

Cannabinoid 1 receptors are expressed in nociceptive primary sensory neurons

Expression of cannabinoid 1 (CB1) and vanilloid 1 (VR1) receptor proteins was studied in adult, cultured rat dorsal root ganglion neurons. Immunostaining of CB1 receptors alone produced labelling in 57+/-2% of the cultured dorsal root ganglion neurons (n=3 cultures). The area of the labelled cells was between 200 and 800 microm(2) with an average of 527+/-68 microm(2). VR1 immunolabelling revealed immunopositivity in 42+/-6% of the total population of dorsal root ganglion neurons. Cells showing VR1-like immunopositivity had an area between 200 and 600 microm(2). The mean area of the VR1-like immunopositive neurons was 376+/-61 microm(2). Double immunostaining with antisera raised against the CB1 and VR1 receptor proteins, showed a high degree of co-expression between CB1 and VR1 receptors. An average of 82+/-3% of the CB1-like immunopositive cells also showed VR1-like immunoreactivity (n=3 cultures) while 98+/-2% of the VR1-like immunolabelled neurons showed CB1 receptor-like immunostaining (n=3 cultures). Our data suggests that nociceptive primary sensory neurons co-express CB1 and VR1 receptors to a very high degree. We propose that this may provide an anatomical basis for a powerful combination of VR1 mediated excitation and CB1-mediated inhibition of nociceptive responses at central and peripheral terminals of nociceptive primary afferents.

Excitatory synaptic transmission and its modulation by PKC is unchanged in the hippocampus of GAP-43-deficient mice

We compared excitatory synaptic transmission between hippocampal pyramidal cells in dissociated hippocampal cell cultures and in area CA3 of hippocampal slice cultures derived from wild-type mice and mice with a genetic deletion of the presynaptic growth associated protein GAP-43. The basal frequency and amplitude of action potential-dependent and -independent spontaneous excitatory postsynaptic currents were similar in both groups. The probability that any two CA3 pyramidal cells in wild-type or GAP-43 knockout (-/-) slice cultures were synaptically connected was assessed with paired recordings and was not different. Furthermore, unitary synaptic responses were similar in the two genotypes. Bath application of phorbol 12,13-diacetate (0.6-3 microM) elicited a comparable increase in the frequency of miniature excitatory synaptic currents in wild-type and GAP-43 (-/-) cultures. This effect was blocked by the protein kinase C inhibitor, bisindolylmaleimide I (1.2 microM). Finally, 3 microM phorbol 12,13-diacetate potentiated the amplitude of unitary synaptic currents to a comparable extent in wild-type and GAP-43 (-/-) slice cultures. We conclude that GAP-43 is not required for normal excitatory synaptic transmission or the potentiation of presynaptic glutamate release mediated by activation of protein kinase C in the hippocampus.

Miniature synaptic events maintain dendritic spines via AMPA receptor activation

We investigated the influence of synaptically released glutamate on postsynaptic structure by comparing the effects of deafferentation, receptor antagonists and blockers of glutamate release in hippocampal slice cultures. CA1 pyramidal cell spine density and length decreased after transection of Schaffer collaterals and after application of AMPA receptor antagonists or botulinum toxin to unlesioned cultures. Loss of spines induced by lesion or by botulinum toxin was prevented by simultaneous AMPA application. Tetrodotoxin did not affect spine density. Synaptically released glutamate thus exerts a trophic effect on spines by acting at AMPA receptors. We conclude that AMPA receptor activation by spontaneous vesicular glutamate release is sufficient to maintain dendritic spines.

Presynaptic facilitation of synaptic transmission in the hippocampus

Biochemical and genetic characterization of proteins in presynaptic axon terminals have led to models of the biochemical pathways underlying synaptic vesicle docking, activation, and fusion. Several studies have attempted recently to assign a precise physiological role to these proteins. This review deals with some of these studies, concentrating on those performed with hippocampal synapses. It is shown that changes in the state of these presynaptic proteins, together with modifications in Ca2+ dynamics in axon terminals, functionally determine the level of basal synaptic transmission, and underlie pharmacologically induced and activity-dependent facilitation of transmitter release in the central nervous system.

Ca2+ or Sr2+ partially rescues synaptic transmission in hippocampal cultures treated with botulinum toxin A and C, but not tetanus toxin

Botulinum (BoNT/A-G) and tetanus toxins (TeNT) are zinc endopeptidases that cleave proteins associated with presynaptic terminals (SNAP-25, syntaxin, or VAMP/synaptobrevin) and block neurotransmitter release. Treatment of hippocampal slice cultures with BoNT/A, BoNT/C, BoNT/E, or TeNT prevented the occurrence of spontaneous or miniature EPSCs (sEPSCs or mEPSCs) as well as the [Ca2+]o-independent increase in their frequency induced by phorbol ester, 0.5 nM alpha-latrotoxin, or sucrose. [Ca2+]o-independent and -dependent release thus requires that the target proteins of clostridial neurotoxins be uncleaved. In contrast, significant increases in mEPSC frequency were produced in BoNT-treated, but not TeNT-treated, cultures by application of the Ca2+ ionophore ionomycin in the presence of 10 mM [Ca2+]o. The frequency of sEPSCs was increased in BoNT-treated, but not TeNT-treated, cultures by increasing [Ca2+]o from 2.8 to 5-10 mM or by applying 5 mM Sr2+. Large Ca2+ and Sr2+ influxes thus can rescue release after BoNT treatment, albeit less than in control cultures. The nature of the toxin-induced modification of Ca2+-dependent release was assessed by recordings from monosynaptically coupled CA3 cell pairs. The paired-pulse ratio of unitary EPSCs evoked by two presynaptic action potentials in close succession was 0.5 in control cultures, but it was 1.4 and 1.2 in BoNT/A- or BoNT/C-treated cultures when recorded in 10 mM [Ca2+]o. Log-log plots of unitary EPSC amplitude versus [Ca2+]o were shifted toward higher [Ca2+]o in BoNT/A- or BoNT/C-treated cultures, but their slope was unchanged and the maximal EPSC amplitudes were reduced. We conclude that BoNTs reduce the Ca2+ sensitivity of the exocytotic machinery and the number of quanta released.

Organotypic slice cultures: a technique has come of age

Slices of CNS tissue prepared from young rodents can be maintained in culture for many weeks to months. The basic requirements are simple: a stable substratum, culture medium, sufficient oxygenation and incubation at a temperature of about 36 degrees C. Under these conditions, nerve cells continue to differentiate and to develop a tissue organization that closely resembles that observed in situ. Several alternative culturing methods have been developed recently. Slices maintained in stationary culture with the interface method are ideally suited for questions requiring a three-dimensional structure, whereas slices cultured in roller-tubes remain the method of choice for experiments that require optimal optical conditions. In this report, three typical experiments are discussed that illustrate the potential of the slice-culture technique. The first example indicates that, due to their high neuronal connectivity, slice cultures provide a very useful tool for studying the properties of synaptic transmission between monosynaptically coupled cell pairs. The other two studies show how long-term application of substances to slice cultures can be used to examine the consequences of epileptic discharges in vitro, as well as the effects of slowly acting clostridial neurotoxins on synaptic transmission.

Properties of spontaneous miniature GABAA receptor mediated synaptic currents in area CA3 of rat hippocampal slice cultures

Miniature, gamma-aminobutyric acid A receptor mediated inhibitory postsynaptic currents (mIPSCs) were recorded from CA3 pyramidal cells in hippocampal slice cultures using whole-cell techniques in the presence of tetrodotoxin. The kinetics and amplitudes of the mIPSCs were analyzed with the aim of determining whether subclasses of events arising from distinct populations of presynaptic interneurons could be distinguished. Histograms of mIPSC amplitude, rise time constant, and decay time constant were all positively skewed, but discrete subsets of events could not be distinguished. The positive skew did not appear to result from electrotonic filtering of distal synaptic currents because there was no correlation among mIPSC amplitudes and the kinetic parameters. Analysis of the intervals between mIPSCs indicated that each event occurred independently. The analysis of spontaneous mIPSCs does not provide evidence of the innervation of pyramidal cells by heterogeneous interneurons.

Calcium-independent actions of alpha-latrotoxin on spontaneous and evoked synaptic transmission in the hippocampus

1. The black widow spider venom component, alpha-latrotoxin (alpha-LTx) (< 0.5 nM), increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in hippocampal CA3 pyramidal cells 14-fold, without changing their amplitude. 2. This action of alpha-LTx was not affected by application of Ca(2+)-free/ethylene glycol-bis(b-aminoethyl ether)-N,N,N',N'-tetraacetic acid-containing saline, 100 microM Cd2+, or 50 microM Gd3+. The increase in mEPSC frequency was thus not due to an influx of Ca2+ into the axon terminal via voltage-dependent Ca2+ channels or alpha-LTx-induced pores. 3. alpha-LTx did not increase spontaneous release when synaptic transmission had been impaired by botulinum toxin/F. 4. alpha-LTx reduced the amplitude of EPSCs, elicited with stimulation of mossy fibers, without affecting paired-pulse facilitation. 5. The Ca2+ ionophore ionomycin (2-2.5 microM) also enhanced the frequency of mEPSCs, but unlike alpha-LTx, potentiated evoked EPSCs and reduced paired-pulse facilitation. Application of N-methyl-D-aspartate elicited a high frequency of Ca(2+)-dependent, tetrodotoxin-sensitive spontaneous EPSCs, but did not affect evoked EPSC amplitude. Agents that stimulate vesicular release by increasing presynaptic Ca2+ influx thus do not mimic the alpha-LTx-induced depression of evoked EPSCs. 6. We conclude that entry of Ca2+ into presynaptic axon terminals is not responsible for the effects of low concentrations of alpha-LTx on either spontaneous or evoked transmitter release in the hippocampus. 7. Potential presynaptic mechanisms that could mediate the opposing actions of alpha-LTx on spontaneous and evoked transmitter release in the hippocampus (i.e., alpha-LTx-induced ionic pores, depletion of synaptic vesicles, actions on exocytotic proteins) are discussed.

Somatic voltage-gated potassium currents of rat hippocampal pyramidal cells in organotypic slice cultures

1. The dominant voltage-gated K+ currents in the somatic membrane of CA3 pyramidal cells from hippocampal slice cultures were characterized using the cell-attached configuration of the patch-clamp recording method. The kinetics, the voltage dependence of activation and inactivation, and the pharmacological properties of the current were determined from ensemble averages of large numbers of episodes from multichannel patches. 2. Steady-state analysis revealed that this current was half-inactivated at the resting membrane potential (Vr), and fully inactivated when patches were held 40 mV positive to Vr. Inactivation was removed when patches were hyperpolarized by 50 mV from Vr. Inactivation was well described by the Boltzmann equation with a slope factor of 12.6 mV. Removal of inactivation of the peak outward current could be described by a time-dependent monoexponential function with a time constant of the order of 100 ms. In contrast, the time course of inactivation was very slow: a +40 mV depolarization relative to Vr of several seconds was required for complete inactivation of the total outward current. 3. When steady-state inactivation was removed by hyperpolarization, the outward current activated with a threshold 10 mV positive to Vr and was half-activated at a potential 57 mV positive to Vr. The conductance can be described in terms of a single Boltzmann equation with a slope factor of 13.5 mV. Activation and inactivation properties of the somatic conductance produce a small window current between +10 and +20 mV relative to Vr. 4. The outward current activated in a voltage-dependent manner in less than 10 ms with 500 ms depolarizing steps. A kinetic analysis of its decay revealed at least three components, with the following time constants: a fast (17 ms), a slowly (approximately 150 ms), and a very slowly inactivating component (in the range of seconds). 5. External application of 4-aminopyridine (4-AP) induced a dose-dependent block of the peak outward current with an IC50 of 28 microM. The inhibitory effect of 4-AP saturated at a concentration of 200 microM which blocked 80% of the total current. The slowly and very slowly inactivating components of the current were not observed with 20 mM tetraethylammonium (TEA) in the pipette solution. A fast transient ensemble current (mean decay time constant, 24 ms) persisted in the presence of extracellular TEA in 29% of the patches. 6. In summary, at least two distinct voltage-gated K+ currents were present at the somatic level of hippocampal pyramidal cells. The dominant one, which we named IK(AT), is sensitive to micromolar concentrations of 4-AP and millimolar concentrations of TEA, and contributes three kinetic components to the total outward current. The second is TEA insensitive, and contributes only a fast transient component of outward current probably corresponding to the classic A-type K+ current. Intracellular recordings in CA3 pyramidal cells showed that IK(AT) plays an important role in regulating the duration of the action potential.

Presynaptic inhibition of calcium-dependent and -independent release elicited with ionomycin, gadolinium, and alpha-latrotoxin in the hippocampus

1. Presynaptic inhibition of synaptic transmission in the hippocampus was investigated by comparing the effects of several agonists on miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs). 2. The Ca2+ ionophore ionomycin increased the frequency of mEPSCs and mIPSCs but did not affect their amplitude. Ionomycin-induced release required extracellular Ca2+ and was prevented by pretreatment with botulinum neurotoxin serotype F, like evoked synaptic transmission. Unlike evoked transmission, however, this increase did not involve activation of voltage-dependent Ca2+ channels because it was insensitive to Cd2+. 3. Both the lanthanide gadolinium and alpha-latrotoxin produced increases in the frequency of mEPSCs and mIPSCs, but their actions were independent of extracellular Ca2+. 4. Adenosine, the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen, and a mu-opioid receptor agonist strongly reduced the frequency of synaptic currents triggered by all three secretagogues. 5. We conclude that activation of these presynaptic receptors can reduce high frequencies of vesicular glutamate and GABA release by directly impairing transmitter exocytosis. Presynaptic inhibition of gadolinium- and alpha-latrotoxin-induced release indicates that this impairment occurs without changes in intraterminal Ca2+ homeostasis and when vesicle fusion is rendered Ca2+ independent, respectively. 6. The inhibition of ionomycin-induced release provides additional evidence for a direct, neurotransmitter receptor-mediated modulation of the proteins underlying vesicular docking or fusion as an important component of presynaptic inhibition of evoked synaptic transmission.

Presynaptic enhancement of inhibitory synaptic transmission by protein kinases A and C in the rat hippocampus in vitro

The protein kinase C activator phorbol 12,13-dibutyrate (0.5 microM, PDBu) and the protein kinase A activator forskolin (20 microM) each increased evoked monosynaptic inhibitory postsynaptic current (IPSC) amplitude, without affecting its reversal potential, and increased the frequency of miniature IPSCs (mIPSCs), without affecting their amplitude or kinetics, as assessed with whole-cell recording form CA3 pyramidal cells in hippocampal slice cultures. The effects of forskolin and PDBu on both evoked IPSC amplitude and mIPSC frequency were additive and were antagonized by inhibitors of protein kinases A and C, respectively. The kinase activator-induced increases in mIPSC frequency were quantitatively comparable to the increases in evoked IPSC amplitude. The increases in mIPSC frequency were not attenuated by the voltage-dependent calcium channel blocker Cd2+ (100 microM). We conclude that stimulation of protein kinases A and C potentiates hippocampal inhibitory synaptic transmission through independent presynaptic mechanisms of action. Kinase-induced potentiation of spontaneous release does not require modulation of axon terminal Ca2+ channels. This mechanism may also contribute substantially to the potentiation of evoked release.

The beta-carboline derivative DMCM decreases gamma-aminobutyric acid responses and Ca(2+)-mediated K(+)-conductance in rat neocortical neurons in vitro

Electrophysiological recordings from neurons of rat frontal neocortical slices have been used to investigate the action of the beta-carboline methyl-6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate (DMCM), on responses to gamma-aminobutyric acid (GABA) and on the excitability of the neurons. Iontophoretic application of GABA close to the intracellularly recorded cells (resting membrane potential -74 +/- 0.9 mV) elicited a depolarization associated with a decrease of input resistance, mediated by GABAA receptors. Bath application of DMCM (0.1-1 microM) reduced these GABA responses decreasing the affinity of the receptors for GABA. This effect was blocked by the benzodiazepine receptor (BZR) antagonist ZK 93426 (1 microM). DMCM (0.1 microM) also decreased the hyperpolarization that followed a train of action potentials (AHP), mediated by Ca(2+)-dependent K+ conductance, and increased the duration of Ca(2+)-dependent action potentials recorded after blockade of Na+ and K+ conductances. Neither effect was blocked by BZR antagonists. These results indicate that DMCM increases the excitability of neurons not only by reducing the gain of the GABAA/BZR complex, but also by modulating intrinsic membrane mechanisms.

Mechanism of mu-opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro

1. The electrophysiological action of the mu-opioid receptor-preferring agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synaptic transmission has been studied in area CA3 of organotypic rat hippocampal slice cultures. 2. FK 33-824 (1 microM) had no effect on the amplitude of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated EPSPs. 3. FK 33-824 (10 nM to 10 microM) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pharmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 microM caused a mean reduction in the amplitude of the monosynaptic IPSP of 70%. 4. Neither delta- nor kappa-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertussis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu, 0.5 microM). 6. The depression of monosynaptic IPSPs by FK 33-824 was unaffected by extracellular application of the K+ channel blockers Ba2+ or Cs+ (1 mM each). 7. FK 33-824 produced a decrease in the frequency of miniature, action potential-independent, spontaneous inhibitory synaptic currents (mIPSCs) recorded with whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 microM) or of nominally Ca(2+)-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevented by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GABA release process, and not by changes in axon terminal K+ or Ca2+ conductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.

Presynaptic inhibition in the hippocampus

Presynaptic receptors for virtually all transmitters have been identified throughout the nervous system. Recent studies in the hippocampus provide new insights into the mechanisms by which the activation of these receptors leads to presynaptic inhibition of transmitter release, and characterize the second messengers involved in coupling presynaptic receptors to their effectors. Presynaptic receptors also provide a tractable route via which the amount of transmitter release may be selectively regulated in therapeutically useful ways.

Acamprosate (calciumacetylhomotaurinate) decreases postsynaptic potentials in the rat neocortex: possible involvement of excitatory amino acid receptors

Acamprosate (calciumacetylhomotaurinate) is used therapeutically against relapse in weaned alcoholics. In the present study, the mechanism of action was investigated by making intracellular in vitro and extracellular in vivo recordings from rat neocortical neurons. Acamprosate (0.1-1 mM) added to the perfusion fluid in vitro reduced excitatory and inhibitory postsynaptic potentials and the depolarizing responses evoked by iontophoretic application of the excitatory amino acids, L-glutamate, L-aspartate, L-homocysteate and N-methyl-D-aspartate, but did not alter the responses to gamma-aminobutyric acid. Acamprosate decreased electrical excitability without apparently changing membrane potential, input resistance, afterhyperpolarization, or threshold and amplitude of the action potential. In vivo iontophoretic application of acamprosate reduced the extracellularly recorded unit activity elicited by iontophoretically applied L-glutamate, whereas spontaneous discharges remained unaffected. These data suggest that acamprosate reduces the postsynaptic efficacy of excitatory amino acid neurotransmitters and lowers neuronal excitability in the neocortex of the rat.

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.

[The direction of rapid eye movements as an indication of hemispheric asymmetry during REM sleep. II]

The hypothesis of right hemisphere predominance in REM sleep and of an increase in left activity throughout the night have been tested by analyzing the distribution of vertical and of horizontal rapid eye movements (REMs) to the right and to the left during the first and the last REM periods in 5 right-handed subjects. Neither the expected superiority of REMs to the left nor variations along the REM periods were found. For vertical eye movements our data suggest a superiority of upward movements during REM. In waking some empirical evidences suggest a relationship between upward eye movements and right hemisphere functioning although to date no hemispheric model can explain it.

[Hemispheric asymmetries in cortical electrical activity during sleep. I]

The hypothesis of a predominance of the right hemisphere in stage REM as compared to NREM has been tested through a spectral analysis of the EEG recorded from left (T3) and right (T4) temporal sites in 5 young healthy right-handed male subjects. Variations in the asymmetry coefficient R - L/R + L in different sleep stages have been analyzed by one way ANOVAs and Sheffé's tests. The hypothesis of a progressive increase in left hemisphere activity throughout different REM cycles as one approaches final awakenings have been investigated by comparing variations in the asymmetry coefficient for epochs of REM and stage 2 NREM sampled in different phases of the REM cycle. EEG results do not support either the hypothesized stage dependent or cycle dependent variation in EEG activity during sleep. We question whether variations in EEG amplitude and synchronization can be used as indices of hemispheric asymmetries during sleep.