Dendritic morphology and its effects on the amplitude and rise-time of synaptic signals in hippocampal CA3 pyramidal cells

Detailed anatomical analysis and compartmental modeling techniques were used to study the impact of CA3b pyramidal cell dendritic morphology and hippocampal anatomy on the amplitude and time course of dendritic synaptic signals. We have used computer-aided tracing methods to obtain accurate three-dimensional representations of 8 CA3b pyramidal cells. The average total dendritic length was 6,332 +/- 1,029 microns and 5,062 +/- 1,397 microns for the apical and basilar arbors, respectively. These cells also exhibited a rough symmetry in their maximal transverse and septotemporal extents (311 +/- 84 microns and 269 +/- 106 microns). From the calculated volume of influence (the volume of the neuropil from which the dendritic structures can receive input), it was found that these cells show a limited symmetry between their proximal apical and basilar dendrites (2.1 +/- 1.2 x 10(6) microns 3 and 3.5 +/- 1.1 x 10(6) microns 3, respectively). Based upon these data, we propose that the geometry of these cells can be approximated by a combination of two cones for the apical arbor and a single cone for the basilar arbor. The reconstructed cells were used to build compartmental models and investigate the extent to which the cellular anatomy determines the efficiency with which dendritic synaptic signals are transferred to the soma. We found that slow, long lasting signals show only approximately a 50% attenuation when they occur in the most distal apical dendrites. However, synaptic transients similar to those seen in fast glutamatergic transmission are transferred much less efficiently, showing up to a 95% attenuation. The relationship between the distance along the dendrites and the observed attenuation for a transient is described simply by single exponential functions with parameters of 195 and 147 microns for the apical and basilar arbors respectively. In contrast, there is no simple relation that describes how a transient is attenuated with respect to these cells' stratified inputs. This lack of a simple relationship arises from the radial orientation of the proximal apical and basilar dendrites. When combined, the anatomical and modeling data suggest that a CA3b cell can be approximated in three dimensions as the combination of three cones. The amplitude and time-course for a synaptic transient can then be predicted using two simple equations.

NMDA receptor-dependent LTD in different subfields of hippocampus in vivo and in vitro

In simulations with artificial neural networks, efficient information processing and storage has been shown to require that the strength of connections between network elements has the capacity to both increase and decrease in a use-dependent manner. In contrast to long-term potentiation (LTP) of excitatory synaptic transmission, activity-dependent long-term depression (LTD) has been difficult to demonstrate in forebrain in vivo. Theoretical arguments indicate that coincidence of presynaptic excitation and low-magnitude postsynaptic activation are the necessary prerequisites for LTD induction. Here we report that stimulation paradigms which cause 1) sufficient excitation to result in NMDA receptor activation and simultaneously 2) attenuate the level of postsynaptic activation by recruitment of GABAA receptor-mediated inhibition consistently produce LTD of commissural input to area CA1 in the hippocampus of anesthetized adult rats, and of the perforant path input to the dentate gyrus in the hippocampus of anesthetized and unanesthetized adult rabbits. A functionally similar pre- and postsynaptic activation pattern applied to the hippocampal slice preparation by injecting hyperpolarizing current into the postsynaptic cell during NMDA receptor-mediated excitation also was effective in consistently inducing LTD. Results of studies in vitro show that Ca2+ influx through the NMDA channel is necessary for the induction of LTD, and moreover, that NMDA receptors also participate in the expression of LTD. Our findings demonstrate a general mechanism for the implementation of a theoretically derived learning rule in adult forebrain in vivo and in vitro and provide justification for the inclusion of use-dependent decreases of connection weights in formal models of cognitive processing.

Posttetanic potentiation and presynaptically induced long-term potentiation at the mossy fiber synapse in rat hippocampus

A form of long-term potentiation (LTP) is induced at the mossy fiber (MF) synapse in the hippocampus by high-frequency presynaptic stimulation (HFS). It is generally accepted that induction of this form of LTP (MF LTP) does not depend on postsynaptic Ca2+ current gated by N-methyl-D-aspartate receptors, but it has remained controversial whether induction depends on postsynaptic depolarization and voltage-gated entry of Ca2+. There are also contradictory data on the time course of both LTP and post-tetanic potentiation (PTP), a shorter duration form of potentiation observed at MF synapses immediately following HFS. It has been proposed that some of these differences in results may have arisen because of difficulties in isolating monosynaptic responses to MF input. In the present study, whole cell recording was used to observe excitatory postsynaptic currents (EPSCs) elicited in CA3 pyramidal cells by input from MFs. Postsynaptic cells were dialyzed with 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and F- to inhibit postsynaptic mechanisms that required Ca2+, cells were under voltage clamp during HFS, and conditions were selected to minimize the likelihood of polysynaptic contamination. Under these conditions, HFS nevertheless induced robust LTP (mean magnitude, 62%). The possibility that EPSCs were contaminated by polysynaptic components was investigated by exposing the slices to a suppressing medium (one that partially blocked neurotransmission). EPSC waveforms did not change shape during suppression, indicating that contamination was absent. The LTP observed always was accompanied by prominent PTP that lasted through the first 5 to 15 min following HFS (mean decay time constant, 3.2 min). Induction of this LTP was not cooperative; there was no relationship between the size of responses and the magnitude of the LTP induced. LTP magnitude also was unrelated to the extent to which postsynaptic cells depolarized during HFS. These results show that high rates of presynaptic MF activity elicit robust LTP whether or not there is accompanying postsynaptic depolarization or increase in the concentration of postsynaptic Ca2+. High-frequency MF activity also results in a PTP that is unusually large and long.

Major differences between long-term potentiation and ACPD-induced slow onset potentiation in hippocampus

We examined the effects of a long-lasting application of the selective metabotropic glutamate receptor (mGluR) agonist 1S-3R, 1-amino cyclopentane-1,3-dicarboxylic acid (ACPD) on synaptic potentials recorded from the CA1 and CA3 subfields in hippocampal slices maintained in a superfusion slice chamber. In 25% of the slices, ACPD generated an slow onset potentiation (SOP) of population EPSPs (pEPSPs) in CA1. In contrast to long-term potentiation (LTP) induced by a tetanic train, SOP was accompanied by an increase in the magnitude of the presynaptic fiber volley. Potentiation of the isolated afferent volley suggests that the expression of SOP is due to a recruitment of additional presynaptic fibers by the test stimulus caused by the persistent block of K+ channels by ACPD.

Excitatory stimulation during postsynaptic inhibition induces long-term depression in hippocampus in vivo

1. As part of an effort to evaluate the biological plausibility of theoretically derived principles of synaptic modification, we studied activity-dependent long-term depression (LTD) of glutamatergic transmission in the hippocampus of anesthetized adult rats. Field potentials of CA1 pyramidal cells evoked by single-pulse stimulation (0.1 Hz) of the commissural afferents were recorded before and after paired-pulse stimulation (0.5 Hz) of the same pathway. A train of 150 or 200 paired pulses produced robust LTD of the commissural input to the CA1 pyramidal neurons when the interstimulus interval (ISI) of the pairs was short (25 ms) but not when the ISI was long (1,000 ms). 2. Paired-pulse stimulation with the short but not with the long ISI also was associated with pronounced inhibition of pyramidal cell firing upon the second pulse of a pair, despite the fact that the excitatory input was facilitated with the short-ISI paradigm. The inhibition of pyramidal cell activity was mediated by input to the pyramidal cells from local gamma-aminobutyric acid (GABA)-releasing interneurons activated by commissural fibers and/or CA1 recurrent collaterals, because the inhibition was eliminated by local administration of the selective GABAA receptor antagonist, bicuculline (50 microM), near the recording site. 3. Postsynaptic input from GABAergic interneurons was necessary for the induction of LTD, because short-ISI paired-pulse stimulation failed to produce LTD in the presence of bicuculline. 4. N-methyl-D-aspartate (NMDA) receptor-mediated excitation also was necessary for the induction of LTD, because administration of the selective NMDA receptor antagonist, D-2-amino-5-phosphonvaleric acid (100 microM), near the recording site prevented the development of LTD.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of developing rat genioglossal motoneurons studied in vitro: changes in length, branching pattern, and spatial distribution of dendrites

The aim of this study is to describe the postnatal change in dendritic morphology of those motoneurons in the hypoglossal nucleus that innervate the genioglossus muscle. Forty genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labeled by intracellular injection of neurobiotin in an in vitro slice preparation of the rat brainstem and were reconstructed in three-dimensional space. The number of primary dendrites per GG motoneuron was approximately 6 and remained unchanged with age. The development of these motoneurons from birth to 13-15 days was characterized by a simplification of the dendritic tree involving a decrease in the number of terminal endings and dendritic branches. Motoneurons lost their 6th-8th order branches, in parallel with an elongation of their terminal dendritic branches maintaining the same combined dendritic length. The elongation of terminal branches was attributed to both longitudinal growth and the apparent lengthening caused by resorption of distal branches. The elimination of dendritic branches tended to increase the symmetry of the tree, as revealed by topological analysis. Later, between 13-15 days and 19-30 days, there was a reelaboration of the dendritic arborization returning to a configuration similar to that found in the newborn. The length of terminal branches was shorter at 19-30 days, while the length of preterminal branches did not change, suggesting that the proliferation of branches at 19-30 days takes place in the intermediate parts of terminal branches. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes (hexants). This analysis revealed that GG motoneurons have major components of their dendritic tree oriented in the lateral, medial, and dorsal hexants. Further two-dimensional polar analysis (consisting of eight sectors) revealed a reconfiguration of the tree from birth up to 5-6 days involving resorption of dendrites in the dorsal, dorsomedial, and medial sectors and growth in the lateral sector. Later in development (between 13-15 days and 19-30 days), there was growth in all sectors, but of a greater magnitude in the dorsomedial, medial, and dorsolateral sectors.

Asynchrony of mossy fibre inputs and excitatory postsynaptic currents in rat hippocampus

1. Excitatory postsynaptic currents (EPSCs) were studied by whole-cell voltage-clamp recording (WCR) from pyramidal cells in the CA3 field of rat hippocampal slices. Input from mossy fibres was evoked by stimuli applied to stratum granulosum ('dentate gyrus stimulation'). This often resulted in complex, multi-component EPSCs with rise times as long as 5.0 ms (mean = 2.5 ms). In contrast, individual EPSC components typically had rise times between 0.3 and 1.0 ms. 2. To isolate monosynaptic, mossy fibre-driven EPSC components, slices were exposed to 'suppressing' media that reduced response amplitudes by 64-88%. In five out of six cases, long EPSC rising phases (> 3 ms) retained the same shape during suppression. This implied that EPSCs were driven by asynchronously active mossy fibre inputs. 3. From latencies of antidromically driven granule cell population spikes (GCPSs) a mean conduction velocity of 0.67 m/s was inferred. Conduction distance had practically no correlation with GCPS duration, implying that velocity dispersion was small and did not desynchronize mossy fibre impulses. EPSC components exhibited 'surplus' latency; they occurred 0.9-4.8 ms after latencies expected on the basis of direct conduction distances. 4. Mossy fibre volleys (MFVs) were evoked by dentate gyrus stimulation and studied with neurotransmission disabled. MFV negative phases lasted from 2.5 to 4.5 ms and had multiple components. By comparison, negative phases of Schaffer collateral fibre volleys (SCFVs) were always simple in shape and lasted 1.5 ms or less. MFV components had surplus latencies similar to those of EPSC components. Late MFV components did not require high stimulus intensities. 5. Widespread activation of granule cells occurred when stimuli were applied to single loci in the stratum granulosum. This implies that such stimuli elicit antidromic impulses in hilar collaterals of mossy fibres, which could result in activation of orthodromic impulses in mossy fibre trunks that had not been stimulated directly. After anti-, then orthodromic conduction, impulses would arrive in the CA3 subfield with 'surplus' latency. 6. When cuts were made in the hilus to prevent anti-/orthodromic conduction, MFV durations were reduced, but only to a small extent. This implies that surplus latency and asynchrony arise in part by anti-/orthodromic conduction, and partly by a mechanism that is intrinsic to mossy fibres or their 'giant' boutons. 7. Because of desynchronization of mossy fibre inputs, there probably are significant differences between kinetic properties of averaged, compound mossy fibre EPSCs and those of unitary mossy fibre EPSCs (i.e. currents driven by input from single presynaptic axons).(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro electrophysiology of developing genioglossal motoneurons in the rat

1. Experiments were performed to determine the change in membrane properties of genioglossal (GG) motoneurons during development. Intracellular recordings were made in 127 GG motoneurons from rats postnatal ages 1-30 days. 2. The input resistance (R(in)) and the membrane time constant (t(aum)) decreased between 5-6 and 13-15 days from 84.8 +/- 25.4 (SD) to 47.0 +/- 18.9 M omega (P < 0.01) and from 10.0 +/- 4.2 to 7.3 +/- 3.3 ms (P < 0.05), respectively. During this period, the rheobase (Irh) increased (P < 0.01) from 0.13 +/- 0.07 to 0.27 +/- 0.14 nA, and the percentage of cells exhibiting inward rectification increased from 5 to 40%. Voltage threshold (Vthr) of the action potential remained unchanged postnatally. 3. There was also a postnatal change in the shape of the action potential. Specifically, between 1-2 and 5-6 days, there was a decrease (P < 0.05) in the spike half-width from 2.23 +/- 0.53 to 1.45 +/- 0.44 ms, resulting, in part, from a steepening (P < 0.05) of the slope of the falling phase of the action potential from 21.6 +/- 10.1 to 32.9 +/- 13.1 mV/ms. The slope of the rising phase also increased significantly (P < 0.01) between 1-2 and 13-15 days from 68.4 +/- 31.0 to 91.4 +/- 44.3 mV/ms. 4. The average duration of the medium afterhyperpolarization (mAHPdur) decreased (P < 0.05) between 1-2 (193 +/- 53 ms) and 5-6 days (159 +/- 43 ms). Whereas the mAHPdur was found to be independent of membrane potential, there was a linear relationship between the membrane potential and the amplitude of the medium AHP (mAHPamp). From this latter relationship, a reversal potential for the mAHPamp was extrapolated to be -87 mV. No evidence for the existence of a slow AHP was found in these developing motoneurons. 5. All cells analyzed (n = 74) displayed adaptation during the first three spikes. The subsequent firing pattern was classified into two groups, adapting and nonadapting. Cells at birth were all adapting, whereas all cells but two from animals 13 days and older were nonadapting. At the intermediate age (5-6 days), the minority (27%) was adapting and the majority (73%) was nonadapting. 6. The mean slope of primary range for the first interspike interval (1st ISI) was approximately 90 Hz/nA. This value was similar for both adapting and nonadapting cells and did not change postnatally.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspartate and glutamate mediate excitatory synaptic transmission in area CA1 of the hippocampus

We examined whether L-aspartate (ASP) and L-glutamate (GLU) both function as endogenous neurotransmitters in area CA1 of the rat hippocampus. Radioligand displacement experiments using 3H-DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (3H-AMPA) to label AMPA/kainate receptors and 3H-cis-4-phosphonomethyl-2-piperidine carboxylic acid (3H-CGS-19755) to label NMDA receptors confirmed that GLU (Ki approximately 500 nM) but not ASP (Ki > 1 mM) has high affinity for AMPA/kainate receptors whereas GLU (Ki approximately 250 nM) and ASP (Ki approximately 1.3 microM) both have high affinity for NMDA receptors. Elevating extracellular potassium concentration (50 mM, 1 min) evoked the calcium-dependent release of both ASP (approximately 50% increase) and GLU (approximately 200% increase) from hippocampal slices and from minislices of area CA1. Reducing extracellular glucose concentration (0.2 mM) reduced GLU release, enhanced ASP release, and reduced AMPA/kainate receptor-mediated responses more than NMDA receptor-mediated responses (to 7% and 34% of control, respectively). Fiber volleys, antidromic population spikes, membrane potential, input resistance, and ATP content all were not affected by glucose reduction. Unlike low glucose, the inhibitory neuromodulator adenosine (5 microM), which reduces ASP and GLU release to a similar extent, reduced AMPA/kainate and NMDA receptor-mediated population EPSPs similarly (to 11% and 12% of control, respectively). AMPA/kainate and NMDA receptor-mediated population EPSPs were also similarly reduced by 0.4 microM TTX (to 32% and 22% of control, respectively) and similarly enhanced by 10 microM 4-aminopyridine (to 206% and 248% of control, respectively). Finally, NMDA receptor-mediated EPSCs measured by whole-cell recording decayed faster in low glucose (73 msec vs 54 msec) but not in adenosine (73 msec vs 78 msec). Together, these results confirm that ASP and GLU are both involved in excitatory synaptic transmission at the Schaffer collateral-commissural terminals in area CA1 of the rat hippocampus.

Potassium-induced long-term potentiation in rat hippocampal slices

We observed that a transient increase in extracellular potassium concentration (50 mM for 40 s) was sufficient to induce long-term potentiation (LTP) of synaptic transmission in area CA1 of the hippocampal slice. Potassium-induced potentiation of the Schaffer collateral/commissural synapses demonstrated several features characteristic of tetanus-induced LTP: (1) population excitatory post-synaptic potential (EPSP) amplitudes were enhanced to a similar magnitude (on average 70% above baseline) which (2) lasted for more than 20 min; (3) induction was blocked by bath application of the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (D-APV), and (4) was attenuated by reduction of the concentration of calcium in the extracellular medium. Induction of either potassium-induced LTP or tetanus-induced LTP occluded the subsequent expression of the other. Finally, exposure to high potassium in the absence of electrical stimulation was sufficient to induce LTP. Taken together, these data indicate that brief depolarizing stimuli other than tetanus can induce LTP. Because potassium-induced LTP is not restricted to the subset of afferents examined electrophysiologically, such a method could facilitate analyses of the biochemical events underlying both the induction and expression of LTP.

Isolated NMDA receptor-mediated synaptic responses express both LTP and LTD

1. The possibility of use-dependent, long-lasting modifications of pharmacologically isolated N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission was examined by intracellular recordings from granule cells of the hippocampal dentate gyrus in vitro. In the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinaxaline-2,3-dione (CNQX, 10 microM) robust, long-term potentiation (LTP) of NMDA receptor-mediated synaptic potentials was induced by brief, high (50 Hz) and lower (10 Hz) frequency tetanic stimuli of glutamatergic afferents (60 +/- 6%, n = 8, P less than 0.001 and 43 +/- 12%, n = 3, P less than 0.05, respectively). 2. Hyperpolarization of granule cell membrane potential to -100 mV during 50-Hz tetanic stimuli reversibly blocked the induction of LTP (-6 +/- 2%, n = 6, P greater than 0.05) indicating that simultaneous activation of pre- and postsynaptic elements is a prerequisite for potentiation of NMDA receptor-mediated synaptic transmission. In contrast, hyperpolarization of the granule cell membrane potential to -100 mV during 10-Hz tetanic stimuli resulted in long-term depression (LTD) of NMDA receptor-mediated synaptic potentials (-34 +/- 8%, n = 8, P less than 0.01). 3. We also studied the role of [Ca2+]i in the induction of LTP and LTD of NMDA receptor-mediated synaptic responses. Before tetanization, [Ca2+]i was buffered by iontophoretic injections of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). BAPTA completely blocked the induction of LTP (3 +/- 5%, n = 13) and partially blocked LTD (-14.8 +/- 6%, n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of phosphomonoesters on CA1 hippocampal neurons as revealed by a combined electrophysiological and nuclear magnetic resonance study

Phosphomonoesters (PMEs), precursors of membrane phospholipids, are found in high levels in the developing brain and Alzheimer's disease brain. The present study details the neurophysiological and metabolic effects of acute PME elevation on the Fisher 344 rat in vitro hippocampal slice. Two abundant PMEs, phosphoethanolamine (PE) and L-phosphoserine (PS), reliably altered properties of synaptic transmission at the Schaffer collateral/commissural-CA1 cell synapse. Specifically, PE reversibly depressed the amplitude of population EPSPs at millimolar concentrations but had no effect at micromolar concentrations. PS had biphasic effects on population EPSPs, inducing first a reduction followed by an enhancement of response amplitude. In contrast to PE, the effects of PS were not reversible; population EPSPs were augmented during the wash of PS, and the CA3 region generated evoked (but not spontaneous) epileptiform discharges. 31P nuclear magnetic resonance spectroscopy revealed enhanced slice uptake of PS compared to PE. There was no significant effect of PE on slice high-energy phosphates but incubation with PS significantly lowered slice phosphocreatine (PCr) and ATP concentrations. These observations indicate that the slice uptake of PS could be energy requiring and the enhanced response amplitude observed at 5 mM PS also could produce a drain on high-energy phosphates. Possible modes of PME action on hippocampal physiology are discussed.

Heterosynaptic correlates of long-term potentiation induction in hippocampal CA3 neurons

Previous studies have demonstrated that tetanization of hippocampal mossy fibers induces a long-term potentiation of non-tetanized (heterosynaptic) non-mossy fiber afferents (Schaffer collateral/commissural and fimbrial fibers). Tetanization of these non-mossy fiber afferents, in contrast, does not induce mossy fiber long-term potentiation, but induces a long-term depression of mossy fiber responses (Bradler and Barrionuevo, Synapse 4, 132-142, 1989). The synaptic activity necessary to evoke these heterosynaptic alterations of efficacy is not known. Specifically, the dependence of heterosynaptic efficacy on the activation of N-methyl-D-aspartate receptors has not been assessed. In addition, the capability of different afferents to CA3 neurons to support alterations in heterosynaptic efficacy remains largely unknown. In the present study, heterosynaptic alterations of efficacy in the rat did not require the activation of N-methyl-D-aspartate receptors. Mossy fibers supported N-methyl-D-aspartate receptor-independent heterosynaptic long-term depression, and N-methyl-D-aspartate receptor-independent long-term potentiation. In contrast, non-mossy fiber afferents expressed N-methyl-D-aspartate receptor-independent heterosynaptic long-term potentiation induced by a mossy fiber tetanus, and an N-methyl-D-aspartate receptor-independent long-term depression, in addition to N-methyl-D-aspartate receptor-dependent homosynaptic long-term potentiation. The possibility that non-N-methyl-D-aspartate receptor activity in non-tetanized afferents is necessary for heterosynaptic long-term potentiation induction is discussed. Heterosynaptic long-term depression was induced in the absence of homosynaptic long-term potentiation, suggesting that these concomitant forms of synaptic plasticity rely on different mechanisms.

Conductance mechanism responsible for long-term potentiation in monosynaptic and isolated excitatory synaptic inputs to hippocampus

The biophysical mechanisms underlying long-term potentiation (LTP) were investigated in identifiable and monosynaptic excitatory inputs to hippocampal neurons. The results provide the first insights into the conductance changes that are responsible for the expression of LTP. Both current- and voltage-clamp measurements of the mossy fiber synaptic response in pyramidal neurons of region CA3 were made with a single-electrode-clamp system. The excitatory postsynaptic response was pharmacologically isolated by bathing hippocampal slices in saline containing 10 microM picrotoxin, which blocks the synaptic inhibition that normally accompanies the experimentally evoked mossy fiber response. LTP was induced by tetanically stimulating the mossy fiber input for 1 s at 100 Hz. Before and 20 min to 1 h after inducing LTP, we attempted to measure the mean excitatory postsynaptic potential (EPSP) amplitude, intrasomatic current-voltage relationship to a step (RN) or alpha function (AN) current waveform, membrane time constant (tau m), spike threshold (T50), peak excitatory postsynaptic current amplitude (IP), synaptic conductance increase (delta G), and synaptic reversal potential (VR); but adequate assessments of all eight of these were not always obtained for every cell that was studied. The induction of LTP increased the mean (+/- SE) EPSP amplitude form 10.5 +/- 1.4 mV during the control period to 16.8 +/- 2.4 mV after the induction of LTP (n = 14; P less than 0.05). This change was not accompanied by increases in the mean value of RN (63 +/- 11 M omega before and 61 +/- 11 M omega after induction; n = 8; P greater than 0.05); AN, which approximates the effective synaptic input resistance at the soma (10.0 +/- 1.50 M omega before and 10.5 +/- 1.60 M omega after; n = 10; P greater than 0.05); or tau m (22 +/- 2 ms before and 20 +/- 2 ms after; n = 8; P greater than 0.05). There was no significant change in T50, which was also assessed with an alpha function current waveform (1.48 +/- 0.11 nA before and 1.49 +/- 0.10 nA after; n = 6; P greater than 0.05). The mean value of IP increased from 1.1 +/- 0.2 nA during the control period to 1.8 +/- 0.3 nA after inducing LTP (n = 15; P less than 0.05). Similarly, delta G increased from 30 +/- 4 nS before to 47 +/- 4 nS after induction (n = 10; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysiological changes in the in vitro rat hippocampus following chronic lithium administration

The effect of chronic lithium exposure on the electrophysiological responses of the Schaffer collateral-commissural (SCC) input to the CA1 pyramidal neurons in the in vitro hippocampus was investigated. Experimental animals were intubated intragastrically with lithium carbonate (150 mg/kg) for 3-4 weeks. This treatment produced lithium levels in serum and hippocampus of 0.3-1.3 mM. During the recording period, the hippocampal slice retained a stable lithium concentration of 53% of the initial value. Chronic lithium exposure had a depressive effect on input/output relationships, paired-pulse facilitation and strength of orthodromic inhibition. The antidromic inhibition was virtually intact. No obvious differences were found between control and lithium slices in amplitude, latency or waveforms, of synaptic and antidromic extracellular potentials. These findings are compatible with a major action of lithium on the excitability of SCC axons and synaptic terminals.

Associative long-term potentiation in hippocampal slices

Interactions between two excitatory monosynaptic inputs to hippocampal neurons of the CA1 region were examined in the in vitro slice. By adjusting the strengths of the electrical stimuli delivered to the two input pathways, one was made to generate a weak and the other a strong synaptic response. Simultaneous tetanic stimulation of both input pathways resulted in a subsequent long-term enhanced synaptic efficacy in the weak input under conditions in which the same tetanic stimulation of either input alone failed to have this effect. This form of long-term synaptic potentiation (LTP), known as associative LTP, was shown in some cases to last hours without decrement. The plastic changes were localized within the CA1 region and appear to reside in the pre- or postsynaptic elements of the monosynaptic excitatory input to the pyramidal neurons. The increased synaptic efficacy could not be accounted for by any of several measured postsynaptic passive membrane properties.

Intracellular injections of EGTA block induction of hippocampal long-term potentiation

Hippocampal long-term potentiation (LTP) is a remarkably stable facilitation of synaptic responses resulting from very brief trains of high-frequency stimulation. Because of its persistence and modest induction conditions, LTP represents a promising candidate for a substrate of memory. Some progress has been made in localizing the changes responsible for the effect; for example, it has been shown that LTP is not accompanied by changes in the fibre volleys of the test afferents or by generalized alterations of the dendrites of their target cells. However, it is unknown whether the potentiation is due to pre- or postsynaptic changes and there is evidence in favour of each (for example, see refs 5, 6). We now report that intracellular injections of the calcium chelator EGTA block the development of LTP. These results strongly suggest that LTP is caused by a modification of the postsynaptic neurone and that its induction depends on the level of free calcium.

Lateral geniculate nucleus unitary discharge in sleep and waking: state- and rate-specific aspects

The relationship between behavioral state, discharge pattern, and discharge rate was investigated in 26 lateral geniculate nucleus (LGN) units recorded in cats in the dark during waking (W), synchronized sleep (S), and desynchronized sleep (D). A distinctive state-dependent discharge pattern was the presence of stereotyped bursts of 2-7 spikes that occurred in 63% of the units. These bursts were most frequent in S, much less frequent in D, and rarely occurred in W. Lack of association with discharge rate changes between states showed the bursting to be a true state-dependent phenomenon. A burst consisted of 2-7 spikes, with each successive interspike interval being longer than the preceding one; in the 200 ms prior to burst occurrence, discharge probability decreased markedly. This structure of burst organization suggested a model of generation wherein each burst was caused by a unitary event of varying intensity, perhaps a rebound following a hyperpolarization. Spectral and autocorrelational analyses showed bursts occurred rhythmically in three cells at a frequency of 3-4 Hz and in two cells at a frequency of 10-12 Hz, indicating a possible linkage with slow-wave generators. While the number of bursts in the various behavioral states was a state-dependent phenomena, other aspects of discharge pattern were shown to be rate dependent. To evaluate discharge pattern apart from the occurrence of bursts, a "primary event spike train" was formed; this consisted of individual spikes and the first spike of each burst. This analysis showed that, within S, the probability of burst occurrence was highest when the primary spike rate was low. Quantitative analyses showed that first-order pattern measures (the form of the interspike interval histogram, IH) were dependent on the mean interspike interval (ISI, the inverse of mean rate). This association explained 83-89% of the variance in a power series approximation of IH form. Joint interval histograms (JIH) were used to evaluate the signature of bursts and of the form of the primary spike train. As with interval histograms, the main features of the form of the primary spike JIH were dependent on the primary spike rate. Thus, we concluded that first- and second-order discharge patterns of primary events were rate dependent and not state dependent. Our data are compatible with a model where in the absence of retinal input, the frequency of LGN primary spikes over behavioral state changes is largely determined by brain stem reticular formation input.(ABSTRACT TRUNCATED AT 400 WORDS)

Vincamine: a psychogeriatric agent blocking synaptic potentiation in hippocampus

The action of vincamine on the physiology of the CA1 region of the in vitro hippocampal slice preparation was investigated. At concentrations of 1, 10 and 100 microM, a five-minute perfusion with vincamine did not affect the synaptically-mediated activation of pyramidal neurons evoked by stimulation of the Schaffer-commissural fiber system. The effect of vincamine on the excitability of the pyramidal neurons was investigated by studying its effect on the antidromically-elicited field potential and the input-output relation of Schaffer-commissural fiber input. No effect on either of the two parameters was seen at a concentration of 100 microM of vincamine. Vincamine did, however, attenuate both the post-tetanic (PTP) and long-term potentiation (LTP) evoked by repetitive stimulation of the Schaffer-commissural fiber system. At a concentration of 100 microM of vincamine, PTP was significantly reduced and LTP was almost completely suppressed.

Negative effects of chronic hemicerebellectomy of epileptiform after-discharges elicited by focal cortical stimulation in baboons (Papio papio)

The influence of chronic hemicerebellectomy on cortical epileptiform after-discharge (AD) induced by focal electrical stimulation was studied in the baboon. These preliminary results include 22 ADs elicited from motor cortex and 22 ADs elicited from premotor cortex before and after hemicerebellectomy. Only full-developed, generalized seizures with postictal silence were considered. EEG morphology, average duration and average current threshold were compared for each set of ictal events. No significant differences were found before and after hemicerebellectomy.