Synaptic inhibition regulates associative interactions between afferents during the induction of long-term potentiation and depression

Phrenic motoneuron structural plasticity across models of diaphragm muscle paralysis

Structural plasticity in motoneurons may be influenced by activation history and motoneuron-muscle fiber interactions. The goal of this study was to examine the morphological adaptations of phrenic motoneurons following imposed motoneuron inactivity while controlling for diaphragm muscle inactivity. Well-characterized rat models were used including unilateral C2 spinal hemisection (SH; ipsilateral phrenic motoneurons and diaphragm muscle are inactive) and tetrodotoxin phrenic nerve blockade (TTX; ipsilateral diaphragm muscle is paralyzed while phrenic motoneuron activity is preserved). We hypothesized that inactivity of phrenic motoneurons would result in a decrease in motoneuron size, consistent with a homeostatic increase in excitability. Phrenic motoneurons were retrogradely labeled by ipsilateral diaphragm muscle injection of fluorescent dextrans or cholera toxin subunit B. Following 2 weeks of diaphragm muscle paralysis, morphological parameters of labeled ipsilateral phrenic motoneurons were assessed quantitatively using fluorescence confocal microscopy. Compared to controls, phrenic motoneuron somal volumes and surface areas decreased with SH, but increased with TTX. Total phrenic motoneuron surface area was unchanged by SH, but increased with TTX. Dendritic surface area was estimated from primary dendrite diameter using a power equation obtained from three-dimensional reconstructed phrenic motoneurons. Estimated dendritic surface area was not significantly different between control and SH, but increased with TTX. Similarly, TTX significantly increased total phrenic motoneuron surface area. These results suggest that ipsilateral phrenic motoneuron morphological adaptations are consistent with a normalization of motoneuron excitability following prolonged alterations in motoneuron activity. Phrenic motoneuron structural plasticity is likely more dependent on motoneuron activity (or descending input) than muscle fiber activity.

Multiple shared mechanisms for homeostatic plasticity in rodent somatosensory and visual cortex

We compare the circuit and cellular mechanisms for homeostatic plasticity that have been discovered in rodent somatosensory (S1) and visual (V1) cortex. Both areas use similar mechanisms to restore mean firing rate after sensory deprivation. Two time scales of homeostasis are evident, with distinct mechanisms. Slow homeostasis occurs over several days, and is mediated by homeostatic synaptic scaling in excitatory networks and, in some cases, homeostatic adjustment of pyramidal cell intrinsic excitability. Fast homeostasis occurs within less than 1 day, and is mediated by rapid disinhibition, implemented by activity-dependent plasticity in parvalbumin interneuron circuits. These processes interact with Hebbian synaptic plasticity to maintain cortical firing rates during learned adjustments in sensory representations.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Hyperexcitability of the CA1 hippocampal region during epileptogenesis

Temporal Lobe Epilepsy (TLE) is often preceded by a latent (seizure-free) period during which complex network reorganizations occur. In experimental epilepsy, network hyperexcitability is already present during the latent period, suggesting a modification of information processing. The purpose of this study was to assess the input/output relationship in the hippocampal CA1 region during epileptogenesis. Field recordings in strata pyramidale and radiatum were used to measure the output of CA1 pyramidal cells as a function of the synaptic inputs they receive following the stimulation of Shaffer collaterals in slices obtained from sham and pilocarpine-treated animals during the latent and chronic periods. We show that there is a transient increase of the input and output field responses during the latent period as compared to sham and epileptic animals. The coupling between excitatory inputs and cell firing was also increased during the latent period. This increase persisted in epileptic animals, although to a lesser extent. We also confirm that paired-pulse facilitation occurs before the chronic phase. The present data further support the view that hyperexcitability is present at an early stage of epileptogenesis. Network output is more facilitated during the latent than during the chronic period. Hyperexcitability may participate to epileptogenesis, but it is not sufficient in itself to produce seizures.

Differential modulation of CA1 and dentate gyrus interneurons during exploration of novel environments

Parallel recordings of hippocampal principal cells and interneurons were obtained as rats foraged in familiar and adjacent, novel environments. Firing rates of each cell type were assessed as a function of spatial location. Many CA1 interneurons exhibited large decreases in activity in the novel compared with the familiar environment. Dentate gyrus interneurons, however, were much more likely to exhibit large increases in firing in the novel environment. Neither effect was correlated with basic interneuron discharge properties such as degree of theta modulation, baseline firing rate or degree of spatially modulated discharge. Both CA1 and dentate gyrus interneuron rate changes extended into regions of the familiar environment bordering the novel environment. Principal cells in CA1 and dentate gyrus exhibited similar patterns of place specific activity each being indicative of incorporation of novel spatial information into the spatial representation of the familiar environment. The data indicate that inhibitory networks in the CA1 and dentate gyrus areas are modulated in a divergent fashion during the acquisition of novel spatial information and that interneuron activities can be used to detect those regions of an environment subject to redistribution of principal cell spatial activity patterns.

Reduced synaptic plasticity in the lateral perforant path input to the dentate gyrus of aged C57BL/6 mice

Hippocampal slices obtained from C57BL/6 mice (3-25 mo) were used to investigate the effects of aging on excitatory postsynaptic potentials (EPSPs) elicited in dentate gyrus with lateral perforant path stimulation. The maximal amplitude of the EPSP, as well as the degree of paired-pulse facilitation, was significantly reduced in animals aged 12 mo or more compared with younger animals (<12 mo). Although all animals showed equivalent short-term potentiation (STP) in response to high-frequency stimulation, this did not translate into a long-lasting increase in synaptic efficacy in the older animals. A significant degree of long-term potentiation (LTP) of synaptic efficacy was only observed in animals <12 mo of age when measured 30 min after induction. Blocking GABAA-mediated inhibition significantly enhanced STP in younger and older animals; however, a significant degree of LTP was again only observed in slices taken from younger animals. These data indicate that the lateral perforant path input to the dentate gyrus is altered by the aging process, and that this results in a reduction in the capacity of this input to exhibit long-lasting synaptic plasticity.

Impaired dendritic spine maturation in GABAA receptor α1 subunit knock out mice

In this study we investigated the functional implications of GABAA receptor alpha1 subunit deletion on dendritic arborization and spine maturation in the visual cortex. This subunit is normally strongly upregulated during early postnatal development. Our main finding is that mice lacking the GABAA receptor alpha1 subunit displayed an increased density of dendritic filopodia during the second and third postnatal weeks. However, there was a concomitant decreased density of mature mushroom-shaped spines, which became more pronounced in adults. In contrast, dendritic arborization was not altered in these mice. We propose that an increased efficacy of the inhibitory synaptic transmission in the alpha1 knock out mice may lead to an enhancement of the outgrowth of filopodia around eye opening, but to a failure in spine maturation at later stages.

Long-term potentiation/depotentiation are accompanied by complex changes in spontaneous unit activity in the hippocampus

Typically, long-term potentiation (LTP) has been assessed as long-lasting changes in field potentials or intracellularly recorded postsynaptic potentials evoked by activation of a set of afferents. In the present experiment, we determined changes in spontaneous unit activity in the dentate gyrus (DG) following high-frequency (HFS) or low-frequency stimulation (LFS) of the medial perforant pathway. Experiments were performed in anesthetized rats. Field potentials and unit recordings were obtained alternatively from the same recording electrode. Of 39 single units isolated (from 25 independent sessions), the spontaneous discharges of 13 units (33%) increased, while 7 units (18%) decreased their discharges following HFS that induced significant LTP of the field potentials. Such opposing modulations of unit discharges following HFS were observed on simultaneously recorded units. LFS applied following HFS also induced bi-directional effects on unit discharges. Of 20 single units isolated from a subset of recordings (12 experiments) to which LFS was applied, 6 units increased and 4 units decreased their discharges. LFS produced a long-lasting (>20 min) depotentiation, to the baseline level, on field potentials in four recording cases. The autocorrelation functions indicated that the isolated unit discharges were comparable to those of the putative DG granule cells and interneurons, shown in previous studies. The results suggest that changes in synaptic efficacy following HFS or LFS produce rather dynamic changes in cell activity in the DG.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

Enhanced synaptic transmission and reduced threshold for LTP induction in fyn-transgenic mice

To elucidate the physiological role of Fyn, we analysed the properties of synaptic transmission and synaptic plasticity in hippocampal slices of mice overexpressing either wild-type Fyn (w-Fyn) or its constitutively active mutant (m-Fyn). These fyn-transgenes were driven by the calcium/calmodulin-dependent protein kinase II alpha promoter which turned on in the forebrain neurons including hippocampal pyramidal cells and in late neural development. In the hippocampal slices expressing m-Fyn the paired-pulse facilitation was reduced and the basal synaptic transmission was enhanced. A weak theta-burst stimulation, which was subthreshold for the induction of long-term potentiation (LTP) in control slices, elicited LTP in CA1 region of the slices expressing m-Fyn. When a relatively strong stimulation was applied, the magnitude of LTP in m-Fyn slices was similar to that in control slices. By contrast, the basal synaptic transmission and the threshold for the induction of LTP were not altered in the slices overexpressing wild-type Fyn. To examine the effect of expression of m-Fyn on GABAergic inhibitory system, we applied bicuculline, a GABAA receptor blocker, to the hippocampal slices. The ability of bicuculline to enhance excitatory postsynaptic potentials was attenuated in slices expressing m-Fyn, suggesting that the overexpression of m-Fyn reduced the GABAergic inhibition. The enhancement of synaptic transmission and the reduction of GABAergic inhibition may contribute to the enhanced seizure susceptibility in the mice expressing m-Fyn. Thus, these results suggest that regulation of Fyn tyrosine kinase activity is important for both synaptic transmission and plasticity.

Age-associated changes in Ca(2+)-dependent processes: relation to hippocampal synaptic plasticity

Altered calcium (Ca2+) homeostasis is thought to play a key role in aging and neuropathology resulting in memory deficits. Several forms of hippocampal synaptic plasticity are dependent on Ca2+, providing a potential link between altered Ca2+ homeostasis and memory deficits associated with aging. The current study reviews evidence for Ca2+ dysregulation during aging which could interact with Ca(2+)-dependent synaptic plasticity. The authors suggest that changes in Ca2+ regulation could adjust the thresholds for synaptic modification, favoring processes for depression of synaptic strength during aging.

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex. J. Neurophysiol. 78: 2546-2559, 1997. Physiological analysis in the companion paper demonstrated that gamma-aminobutyric acid-A (GABAA)-mediated inhibition in piriform cortex is generated by circuits that are largely independent in apical dendritic and somatic regions of pyramidal cells and that GABAA-mediated inhibitory postsynaptic currents (IPSCs) in distal dendrites have a slower time course than those in the somatic region. This study used modeling methods to explore these characteristics of GABAA-mediated inhibition with respect to regulation of the N-methyl--aspartate (NMDA) component of excitatory postsynaptic potentials. Such regulation is relevant to understanding NMDA-dependent long-term potentiation (LTP) and the integration of repetitive synaptic inputs that can activate the NMDA component as well as pathological processes that can be activated by overexpression of the NMDA component. A working hypothesis was that the independence and differing properties of IPSCs in apical dendritic and somatic regions provide a means whereby the NMDA component and other dendritic processes can be controlled by way of GABAergic tone without substantially altering system excitability. The analysis was performed on a branched compartmental model of a pyramidal cell in piriform cortex constructed with physiological and anatomic data derived by whole cell patch recording. Simulations with the model revealed that NMDA expression is more effectively blocked by the slow GABAA component than the fast. Because the slow component is present in greater proportion in apical dendritic than somatic regions, this characteristic would increase the capacity of dendritic IPSCs to regulate NMDA-mediated processes. The simulations further revealed that somatic-region GABAergic inhibition can regulate the generation of action potentials with little effect on the NMDA component generated by afferent fibers in apical dendrites. As a result, if expression of the NMDA component or other dendritic processes were enabled by selective block of dendritic inhibition, for example, by centrifugal fiber systems that may regulate learning and memory, the somatic-region IPSC could preserve system stability through feedback regulation of firing without counteracting the effect of the dendritic-region block. Simulations with paired inputs revealed that the dendritic GABAA-mediated IPSC can regulate the extent to which a strong excitatory input facilitates the NMDA component of a concurrent weak input, providing a possible mechanism for control of "associative LTP" that has been demonstrated in this system. Postsynaptic GABAB-mediated inhibition had less effect on the NMDA component than either the fast or slow GABAA components. Depolarization from a concomitant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component also was found to have comparatively little effect on current through the NMDA channel because of its brief time course.

Quantifying the role of inhibition in associative long-term potentiation in dentate granule cells with computational models

In the dentate gyrus, coactivation of a mildly strong ipsilateral perforant path (pp) input with a weak contralateral pp input will not induce associative long-term potentiation in the weak input path unless both inputs project to the same part of the molecular layer. This "spatial convergence requirement" is thought to arise from either voltage attenuation between input locations or inhibition. Simulations with a detailed model of a dentate granule cell were performed to rule out voltage attenuation and to quantify the inhibition necessary to obtain the spatial convergence requirement. Strong lateral and weak medial or strong medial and weak lateral pp input were activated eight times at 400 Hz. Calcium current through N-methyl-D-aspartate receptor channels and subsequent changes in calcium concentration and the concentration of calmodulin bound with four calcium ions ([Cal-Ca4]) in the spine head were computed for a medial and a lateral pp synapse. To satisfy the spatial convergence requirement, peak [Cal-Ca4] had to be much larger in the strongly activated path synapse than in the weakly activated path synapse. With no inhibition in the model, differences in peak [Cal-Cal4] at the two synapses were small, ruling out voltage attenuation as the explanation of the spatial convergence requirement. However, with shunting inhibition, modeled by reducing membrane resistivity to 1,600 omega cm2 in the distal two-thirds of the dendritic tree, peak [Cal-Ca4] was 3-5 times larger in the strongly activated path synapse than in the weakly activated path synapse. The magnitude of shunting inhibition was varied to determine the level that maximized this difference in peak [Cal-Ca4]. For strong lateral and weak medial pp input, the optimal level was one that prevented the cell from firing an action potential. For strong medial and weak lateral pp input, the optimal level was one at which the cell fired two action potentials. The distribution of shunting inhibition that best satisfied the spatial convergence requirement was inhibition on the distal two-thirds of the dendritic tree with or without inhibition at the soma, with inhibition stronger in the distal third than in the middle third. It was estimated that the number of inhibitory synapses involved in the shunting inhibition should be 25-50% of the number of excitatory synapses activated by the eight-pulse, 400-Hz tetanus. This number could be 20-50% of the total number of inhibitory synapses in the distal two-thirds of the dendritic tree. The addition of a single inhibitory synapse on a dendrite had a significant effect on peak spine head [Cal-Ca4] in nearby spines. Inhibitory synapses had to be activated four or more times at 100 Hz for effective shunting to take place, and the inhibition had to begin no later than 2-5 ms after the first excitatory input. The results suggest that inhibition can isolate potentiated synapses to particular dendritic domains and that the location of activated inhibitory synapses may affect potentiation of individual synapses on individual dendrites.

Effect of GABA(B) receptors on synaptic interactions in dentate gyrus granule neurons of the rat

Dendritic arborization permits convergence of synaptic inputs and their integration in single neurons. The granule neuron in the dentate gyrus represents a relatively simple example where anatomically and functionally distinct medial and lateral perforant pathways terminate on different regions of the dendritic tree. High-frequency stimulation of either pathway alone results in the induction of long-term potentiation. However, whether the potentiated synapses in different parts of the dendrites interact is not known. In this study we have compared long-term potentiation and synaptic interactions in the lateral and medial perforant pathways in the "disinhibited" hippocampal slice preparation in the presence of the GABA(A) receptor blocker bicuculline. The data show that the magnitude of long-term potentiation induced by tetanic stimulation was similar in both pathways, but differences between the two pathways were revealed after two or more tetanizations. A significantly smaller capacity for further long-term potentiation in the lateral, as compared to the medial, perforant pathway was found and can be attributed to stronger postsynaptic GABA(B) inhibition in distal dendrites of granule neurons. Blockade of GABA(B) inhibition with CGP36742 (100 microM) unmasked additional long-term potentiation in the lateral pathway. Presynaptically, GABA(B) receptors produced a short-lasting heterosynaptic depression in the medial pathway, which was reduced by CGP36742. Coincident activation of the two pathways boosted long-term potentiation only in the medial pathway. We propose that the interactions between the two pathways are orchestrated to maximize associative long-term potentiation in the medial pathway; this may be important for types of learning attributed to the hippocampus.

Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition

Opioid peptides costored with glutamate have emerged as powerful regulators of long-term potentiation (LTP) induction in several hippocampal pathways. The objectives of the present study were twofold: (1) to identify which opioid receptor types (mu, delta, or kappa) regulate LTP induction at lateral perforant path-granule cell synapses and (2) to test the hypothesis that endogenous opioids regulate LTP induction via modulation of GABAergic inhibition. LTP of lateral perforant path-evoked field EPSPs was induced selectively by high-frequency stimulation applied to the outer third of the molecular layer of the dentate gyrus of rat hippocampal slices. No changes in medial perforant path responses occurred. LTP was blocked when high-frequency stimulation was applied in the presence of the mu receptor antagonist CTAP, the selective delta-1 receptor antagonist BNTX, or the delta-1 and delta-2 receptor antagonist naltrindole. By contrast, the kappa-1 opioid receptor antagonist NBNI had no effect on LTP induction. The role of GABAergic inhibition was investigated by comparing the effect of naloxone on LTP induction in slices maintained in standard buffer and picrotoxin-containing buffer. Naloxone blocked LTP in standard buffer, whereas normal LTP was induced in picrotoxin-treated, disinhibited slices. Finally, NMDA receptor blockade completely inhibited LTP in both standard and disinhibited slices. The results show that mu and delta-1 opioid receptors regulate LTP induction and that this mechanism critically depends on GABAergic inhibition. A key issue then becomes how endogenous opioids fine-tune the activity of intact inhibitory networks in the dentate gyrus, effectively gating synaptic plasticity in specific dendritic strata.

Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition

Opioid peptides costored with glutamate have emerged as powerful regulators of long-term potentiation (LTP) induction in several hippocampal pathways. The objectives of the present study were twofold: (1) to identify which opioid receptor types (mu, delta, or kappa) regulate LTP induction at lateral perforant path-granule cell synapses and (2) to test the hypothesis that endogenous opioids regulate LTP induction via modulation of GABAergic inhibition. LTP of lateral perforant path-evoked field EPSPs was induced selectively by high-frequency stimulation applied to the outer third of the molecular layer of the dentate gyrus of rat hippocampal slices. No changes in medial perforant path responses occurred. LTP was blocked when high-frequency stimulation was applied in the presence of the mu receptor antagonist CTAP, the selective delta-1 receptor antagonist BNTX, or the delta-1 and delta-2 receptor antagonist naltrindole. By contrast, the kappa-1 opioid receptor antagonist NBNI had no effect on LTP induction. The role of GABAergic inhibition was investigated by comparing the effect of naloxone on LTP induction in slices maintained in standard buffer and picrotoxin-containing buffer. Naloxone blocked LTP in standard buffer, whereas normal LTP was induced in picrotoxin-treated, disinhibited slices. Finally, NMDA receptor blockade completely inhibited LTP in both standard and disinhibited slices. The results show that mu and delta-1 opioid receptors regulate LTP induction and that this mechanism critically depends on GABAergic inhibition. A key issue then becomes how endogenous opioids fine-tune the activity of intact inhibitory networks in the dentate gyrus, effectively gating synaptic plasticity in specific dendritic strata.

Cooperative interactions in the induction of long-term potentiation and depression of synaptic excitation between hippocampal CA3-CA1 cell pairs in vitro

The requirement for cooperative interactions between multiple synaptic inputs in the induction of long-term potentiation (LTP) and long-term depression (LTD) has been tested at Schaffer collateral synapses with paired recordings from monosynaptically coupled CA3-CA1 cell pairs in rat hippocampal slice cultures. Tetanization of single presynaptic neurons at 50 Hz (repeated 5-7 times for 300-500 ms each) induced only a transient potentiation (< 3 min) of excitatory postsynaptic potentials (EPSPs). Persistent potentiation (> 15 min) was induced only when single presynaptic action potentials were synchronously paired with directly induced postsynaptic depolarizing pulses (repeated 50-100 times). Tetanus-induced potentiation of extracellularly evoked EPSPs lasting > 4 min could only be obtained if the EPSP was > 4 mV. Because unitary EPSP amplitudes average approximately 1 mV, we conclude that high-frequency discharge must occur synchronously] in 4-5 CA3 cells for LTP to be induced in a common postsynaptic CA1 cell. Asynchronous pairing of presynaptic action potentials with postsynaptic depolarizing current pulses (preceding each EPSP by 800 ms) depressed both naive and previously potentiated unitary EPSPs. Likewise, homosynaptic LTD of unitary EPSPs was induced when the presynaptic cell was tetanized at 3 Hz for 3 min, regardless of their amplitude (0.3-3.2 mV). Homosynaptic LTD of extracellularly evoked Schaffer collateral EPSPs < 4 mV could be induced if no inhibitory postsynaptic potential was apparent, but was prevented by eliciting a large inhibitory postsynaptic potential or by injection of hyperpolarizing current in the postsynaptic cell. We conclude that cooperative interactions among multiple excitatory inputs are not required for induction of homosynaptic LTD of unitary EPSPs.

GABAergic neurons in rat nuclei of solitary tracts receive inhibitory-type synapses from amygdaloid efferents lacking detectable GABA-immunoreactivity

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter in both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connected by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in the amygdaloid efferents or their targets in the mNTS, we combined peroxidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNTS. By light microscopy, peroxidase labeling for either PHA-L or BDA was seen in varicose processes, whereas immunogold-silver labeling for GABA was detected in perikarya and processes throughout the rostrocaudal mNTS. The intermediate mNTS at the level of the area postrema, a region receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde labeling was observed exclusively in unmyelinated axons and axon terminals. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets of the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received convergent input from terminals that were intensely GABA-immunoreactive. We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our results indicate that the inhibition of these GABAergic neurons may be further augmented by release of GABA from other converging terminals in the mNTS.

Morphology and soma-dendritic distribution of synaptic endings from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) on motoneurons in the oculomotor and trochlear nuclei in the cat

The morphology and soma-dendritic distribution of anterograde biocytin-labelled rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) synaptic endings in the oculomotor and trochlear nuclei have been examined by electron microscopy by using both preembedding immunoperoxidase and postembedding immunogold methods. The results indicate that three morphological types of riMLF synaptic endings are distinguishable on the basis of synaptic vesicle morphology (spheroidal, pleiomorphic, or ellipsoidal) and postsynaptic membrane specializations (asymmetrical or symmetrical). All three morphological types of riMLF synaptic endings establish synaptic connections predominantly with dendrites. Synaptic endings that contain ellipsoidal synaptic vesicles have a more proximal soma-dendritic distribution than those that contain either spheroidal or pleiomorphic synaptic vesicles. Furthermore, all three morphological types of synaptic endings are encountered in the same motoneuron subdivisions of the oculomotor and trochlear nuclei in the same experiments. The findings suggest that subregions of the riMLF contain coexistent populations of excitatory and inhibitory premotor neurons that are related to opposite directions of vertical saccadic eye movements but that project to the same motoneuron subgroups on the ipsilateral side. Both the morphology and the mode, pattern, and soma-dendritic distribution of saccade-related riMLF synaptic endings that establish synaptic connections with vertical motoneurons differ from those of excitatory and inhibitory second-order vertical vestibular synaptic endings. These differences in the synaptic organization of riMLF and second-order vestibular inputs to oculomotor and trochlear motoneurons may be related to differences in the information transferred by each source, the riMLF input conveying eye-velocity signals, and the vestibular input conveying eye-position signals.

Associative long-term depression in the hippocampus in vitro

An association between the test and conditioning stimuli is critical for determining the nature of their interaction during learning and memory. Two experimental protocols which result in the induction of associative forms of long-term depression (LTD) at Schaffer collateral synapses onto CA1 pyramidal cells in vitro are reviewed in this article. The requirements for the induction of LTD with these protocols are discussed, as well as the relationship between these forms of associative LTD and so-called homosynaptic LTD. In particular, the biological basis of the experimentally demonstrated necessity for temporal and spatial conjunction between the test and conditioning stimuli is examined.

Homosynaptic LTD and depotentiation: do they differ in name only?

Long-term depression (LTD) now occupies a major place in theories of the cellular basis of learning and memory and other nervous system phenomena involving persistent changes in synaptic responsiveness. LTD can be induced using a variety of stimulation paradigms. Homosynaptic LTD in this review refers to a depression of basal responses that is restricted to the pathway that has been stimulated by a low-frequency (1 Hz) stimulus train. Despite the intensive interest in LTD, there has been controversy about the ease with which LTD can be induced and reports range from no success to routine success. There has been much less controversy about a related form of response depression now called "depotentiation" which shares many similarities with LTD. Depotentiation is the response reduction that affects, not the basal responses affected by LTD, but responses that have been increased by the process of long-term potentiation (LTP). LTD and depotentiation can be induced by similar stimulation and have many biochemical properties in common, but it has not been clear whether or not they represent the same phenomenon, in part because it often occurs that the same preparation that does not undergo LTD readily expresses depotentiation. We review work that indicates that the major differences between LTD and depotentiation involve age-dependence, the need for priming stimulation and sensitivity to GABA receptor antagonists. We present a hypothetical model that can reconcile the apparent disparities between LTD and depotentiation.

Cooperative interactions among afferents govern the induction of homosynaptic long-term depression in the hippocampus

Prolonged periods of low-frequency stimulation have been shown to produce a robust, long-term synaptic depression (LTD) in both hippocampus and visual cortex. In the present study we have examined the extent to which interactions among afferents govern the induction of homosynaptic LTD in young-adult rats in hippocampal region CA1 in vitro. Field excitatory postsynaptic potentials were assessed before and after conditioning stimulation consisting of two 10-min trains of low-frequency stimulation (LFS; 1 Hz) of the Schaffer collateral/commissural pathway. LFS at an intensity producing a 0.5-mV response did not produce significant synaptic depression. However, LFS administered at a higher intensity resulted in significant input-specific LTD of a 0.5-mV test response. Picrotoxin, which also facilitates depolarization of CA1 neurons, significantly enhanced the magnitude of LTD after LFS at 0.5 mV. In addition, LFS at 0.5 mV in normal perfusion medium (no picrotoxin) produced only small changes in synaptic efficacy when either of two converging pathways was conditioned separately but produced a robust LTD when both pathways were conditioned simultaneously. This cooperative LTD was reversibly blocked by prior administration of 100 microM DL-aminophosphonovaleric acid but not by 20 microM nimodipine. Taken together, these results suggest that cooperative interactions among afferents contribute to voltage-dependent processes underlying the induction of homosynaptic LTD.

Amygdala efferents form inhibitory-type synapses with a subpopulation of catecholaminergic neurons in the rat Nucleus tractus solitarius

The central nucleus of the amygdala (CNA) integrates visceral responses to stress partially through efferent projections to portions of the medial nuclei of the solitary tracts (mNTS) containing catecholaminergic neurons. To determine anatomical sites for CNA modulation of these neurons, immunoperoxidase detection of anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) was combined with immunogold-silver labeling of the catecholamine-synthesizing enzyme, tyrosine hydroxylase, in adult rat mNTS. From 350 anterogradely labeled terminals identified within the intermediate mNTS, 30% formed symmetric, inhibitory-type synapses and the remainder lacked recognized junctions as seen within a single plane of section. Of the terminals forming symmetric synapses, 16% were presynaptic to tyrosine hydroxylase immunoreactive dendrites and the remainder to unlabeled dendrites. The level of tyrosine hydroxylase immunoreactivity as assessed by density of gold-silver particles was significantly lower in dendrites receiving synaptic input from CNA efferents as compared with dendrites of the same sizes (2.0 microns 2 in mean area) which received synapses from unlabeled terminals or lacked recognizable synaptic inputs. When separately examined without regard to afferent input, the medium- and larger-sized dendrites having mean cross-sectional areas of 1-3 microns 2 also contained significantly less tyrosine hydroxylase immunoreactivity than small (< 1 micron 2) dendrites. These results suggest that CNA efferents to the mNTS inhibit non-catecholamine-containing neurons and a subpopulation of catecholaminergic neurons distinguished by their low levels of tyrosine hydroxylase. The findings also indicate that small, presumably more distal, dendrites in the intermediate mNTS may synthesize and/or release catecholamines.

Different mechanisms for use-dependent depression of two GABAA-mediated IPSCs in rat hippocampus

1. The mechanisms involved in the use-dependent depression of GABAA,fast and GABAA,slow, two GABAA-mediated IPSCs in the rat hippocampal slice preparation, were investigated by observing the effects of paired-pulse depression and of baclofen and CGP 35348 on monosynaptic inhibitory currents recorded from CA1 pyramidal neurons. 2. The second of a pair of evoked responses that consisted of both inhibitory components was depressed and decayed more rapidly compared to the first at an interpulse interval (IpI) of 200 ms. This effect was due to a decrease in the amplitude of GABAA,slow, with no effect on the time constant of decay or on the amplitude or time constant of GABAA,fast. 3. The time course of paired-pulse depression of both components at IpIs ranging from 5 to 2560 ms was compared. GABAA,slow was depressed maximally by 55% at IpIs of 80-160 ms. GABAA,fast was depressed maximally by 38% at 5 ms, and recovered exponentially with a time constant of 130 ms. 4. GABAA,slow was more sensitive than GABAA,fast to depression by baclofen. GABAA,slow was susceptible to complete block, with an ED50 of approximately 200 nM for (+/-)-baclofen and 100 nM for the active enantiomer, (R)-(+)-baclofen. GABAA,fast was blocked by only 50% by the highest concentrations of baclofen tested (10-100 microM (R)-(+)-baclofen), with an ED50 of approximately 2 microM for (+/-)-baclofen and 1 microM for (R)-(+)-baclofen. Paired-pulse depression of GABAA,fast was not occluded by 10 or 100 microM (R)-(+)-baclofen. 5. The GABAB antagonist CGP 35348 (0.4-1 mM), prevented paired-pulse depression of GABAA,slow at IpIs of 160 to 200 ms, and reversed the depression of GABAA,fast by baclofen, but had no effect on paired-pulse depression of GABAA,fast at IpIs of 20 to 40 ms. 6. It is concluded that use-dependent depression of GABAA,slow, but not GABAA,fast, is mediated by a presynaptic GABAB receptor. It is speculated that use-dependent depression of GABAA,fast, which occurs only over a much faster time scale, may be due to rapid postsynaptic GABAA receptor desensitization.

Reduction of the threshold for long-term potentiation by prior theta-frequency synaptic activity

Activation of the lateral perforant path input to the dentate gyrus with theta-patterned conditioning trains produced LTP of synaptic efficacy that changed in magnitude as an inverted U-shaped function of the number of trains. The LTP induction function was not fixed, however, and could be shifted to the left by administering 5 Hz "priming" stimulation to the lateral path 10 min prior to the conditioning trains. The priming effect was input specific and selective to a narrow window of stimulus frequencies. The shift to the left of the LTP induction function by priming stimulation was blocked by the muscarinic receptor antagonist atropine sulphate. Nimodipine, an antagonist of L-type voltage-sensitive calcium channels, did not mimic the priming effect but instead produced a general facilitation of LTP induction. These data demonstrate that the degree to which LTP is induced in the lateral path is a non-linear function of afferent activity, and that this function, including LTP threshold, can be shifted to the left by prior synaptic activity at hippocampal theta-rhythm frequencies.