Dual effect of GABA on descending monosynaptic excitatory postsynaptic potential in frog lumbar motoneurons

Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulating ipsilateral ventrolateral column (VLC) in the thoracic section were recorded in lumbar motoneurons within the isolated spinal cord of the frog Rana ridibunda. Bath application of the selective GABAB receptor agonist (-)-baclofen (0.05 mM) caused a reduction in the peak amplitude of VLC EPSP. Baclofen did not cause any consistent change in the membrane potential or in the EPSP waveform within frog motoneurones. The selective GABA(B) receptor antagonist saclofen (0.1 mM) completely blocked the effect of (-)-baclofen on VLC EPSP. A decrease in VLC EPSP peak amplitude was also observed during GABA (0.5 mM) application. Unlike (-)-baclofen, inhibition of VLC EPSP induced by GABA was accompanied by a shortening of the EPSP time course and a reduction in membrane input resistance within lumbar motoneurons. The decrease in VLC EPSP peak amplitude induced by (-)-baclofen and GABA was accompanied by an increase in the paired-pulse facilitation. These data provide evidence for a dual pre- and postsynaptic GABAergic inhibition of the VLC monosynaptic EPSP in lumbar motoneurons within the frog spinal cord.     

Presynaptic modulation by GABAB receptors of glutamatergic excitation and GABAergic inhibition of neostriatal neurons.

1. The present experiments investigated the effects of gamma-amino-butyric acidB (GABAB) receptor stimulation on the excitatory and inhibitory responses of neostriatal neurons evoked by stimulation of the subcortical white matter in a rat neostriatal slice preparation. 2. Intracellular recordings showed that single-impulse stimulation of the corpus callosum evoked monosynaptic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) that were attenuated by the GABAB receptor agonist, p-chlorophenyl-GABA (baclofen, 0.5-10 microM) in a concentration-dependent manner. Baclofen also blocked the GABAA-mediated inhibition of neostriatal cell responses, which were revealed by paired-impulse stimulation of the subcortical white matter. Both of these effects persisted in slices in which the anterior cortex was removed, indicating that the site of action for baclofen was intrinsic to the neostriatum. The GABAB antagonist 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulfonic acid (saclofen, 250-500 microM) reversed the depressant actions of baclofen on both the excitatory and inhibitory responses of neostriatal cells. 3. Concentrations of baclofen as high as 100 microM, which markedly attenuated EPSP amplitude, did not exert direct effects on resting membrane potential, current-voltage relationship, input resistance, or spike threshold and thus appeared to have no postsynaptic effect on the population of neurons recorded. 4. These results indicate that, in contrast to other regions of the CNS, the depressant effects of baclofen on glutamate-dependent EPSPs are mediated exclusively through GABAB receptors located presynaptically on the terminals of glutamatergic afferents.(ABSTRACT TRUNCATED AT 250 WORDS)     

The actions of 2-hydroxy-saclofen at presynaptic GABAB receptors in the rat hippocampus

The actions of 2-hydroxy-saclofen (2-OH-S), a recently developed analog of baclofen, were studied at presynaptic GABAB receptors in the rat hippocampal slice. Baclofen (0.5-20 microM) reduces the amplitude of excitatory postsynaptic potentials (EPSPs) recorded from hippocampal CA1 pyramidal neurons. In the presence of 200-500 microM 2-OH-S, the synaptic depressant action of baclofen is significantly reduced. These data show that 2-OH-S is an effective antagonist at presynaptic GABAB receptors on excitatory terminals in the hippocampus.     

Phase-dependent presynaptic modulation of mechanosensory signals in the locust flight system

In the locust flight system, afferents of a wing hinge mechanoreceptor, the hindwing tegula, make monosynaptic excitatory connections with motoneurons of the elevator muscles. During flight motor activity, the excitatory postsynaptic potentials (EPSPs) produced by these connections changed in amplitude with the phase of the wingbeat cycle. The largest changes occurred around the phase where elevator motoneurons passed through their minimum membrane potential. This phase-dependent modulation was neither due to flight-related oscillations in motoneuron membrane potential nor to changes in motoneuron input resistance. This indicates that modulation of EPSP amplitude is mediated by presynaptic mechanisms that affect the efficacy of afferent synaptic input. Primary afferent depolarizations (PADs) were recorded in the terminal arborizations of tegula afferents, presynaptic to elevator motoneurons in the same hemiganglion. PADs were attributed to presynaptic inhibitory input because they reduced the input resistance of the afferents and were sensitive to the gamma-aminobutyric acid antagonist picrotoxin. PADs occurred either spontaneously or were elicited by spike activity in the tegula afferents. In summary, afferent signaling in the locust flight system appears to be under presynaptic control, a candidate mechanism of which is presynaptic inhibition.     

Factors that control amplitude of EPSPs in dendritic neurons

We have used a computer-based mathematical model of alpha-motoneurons and of group Ia synaptic input to them, based on anatomical and electrophysiological data from the cat spinal cord, in order to examine the effects of variations in neuron size and input resistance and of conductance magnitude and duration on the generation of excitatory postsynaptic potentials (EPSPs). The first set of calculations were designed to test the possible role of nonlinear EPSP summation in producing a differential distribution of posttetanic potentiation of group Ia EPSPs, described in the preceding paper (25; see also Refs. 26, 27). The results suggest that the negative correlations observed between the degree of posttetanic potentiation of Ia EPSPs and initial (pretetanic) EPSP amplitude as well as with the input resistance of the postsynaptic motoneurons can be explained in part by the inherent non-linearity between conductance change and the resultant potential change at chemical synapses. In a second set of calculations, we used the same model system to evaluate the effects produced by variations in neuronal membrane area, input resistance, and specific membrane resistivity, as well as of the density of excitatory synaptic input on the peak amplitude of EPSPs. With parameters constrained to match the properties of alpha-motoneurons and group Ia synaptic input, EPSP amplitudes were most sensitive to changes in synaptic density and were much less sensitive to alterations in neuron input resistance and specific membrane resistivity when synaptic density was constant.     

Effects of ethanol and other drugs on excitatory and inhibitory neurotransmission in the crayfish.

1. Crayfish exposed to 434 mM ethanol (EtOH) showed signs of hyperactivity within 0.5-2 h, at which times crayfish hemolymph EtOH concentration had reached 60-90 mM. 2. A 10-min exposure to 60-90 mM EtOH reduced presynaptic inhibition of excitatory postsynaptic currents (EPSCs) at the crayfish opener neuromuscular junction (NMJ) in vitro but did not significantly alter excitatory neurotransmission. The same concentrations of EtOH did not alter other potentials or currents associated with inhibition at this synapse, such as presynaptic inhibitory potentials (PIPs), inhibitory postsynaptic potentials (IPSPs), and inhibitory postsynaptic currents (IPSCs). 3. Intermediate EtOH concentrations (120-180 mM) applied for 10 min in vitro reduced the amplitude of excitatory postsynaptic potentials (EPSPs) by decreasing the membrane resistance of opener muscle fibers and by reducing the amplitude of EPSCs. 4. High EtOH concentrations (434 mM) applied for 10 min in vitro had yet greater depressive effects on measures of postsynaptic properties described above. The time course of EPSCs was also significantly reduced. In addition, presynaptic properties such as action-potential (AP) amplitude and frequency of spontaneous release of neurotransmitter were reduced by 434 mM EtOH. 5. Presynaptic inhibition, gamma-aminobutyric acid (GABA; 250-500 microM), muscimol (50 microM), and baclofen (75 microM) all reduced the depolarizing afterpotential of APs in the excitor axon and reduced EPSPs in opener muscle fibers. GABA (500 microM) and baclofen (75 microM) significantly reduced presynaptic AP amplitudes, whereas presynaptic inhibition, GABA (250 microM), and muscimol (50 microM) had no effect on AP amplitude. Bicuculline (250-500 microM), a GABAA antagonist, did not entirely eliminate presynaptic inhibition, whereas picrotoxin (50 microM), another GABAA antagonist, completely removed presynaptic inhibition. Thus presynaptic inhibitory mechanisms may involve both GABAA and GABAB receptors on the opener excitor axon. 6. Our data suggest that the behavioral hyperactivity seen at hemolymph EtOH concentrations of 60-90 mM is not accompanied by a change in excitatory synaptic transmission observed at the opener NMJ. Rather, crayfish hyperactivity may be due to depressive effects of EtOH on inhibitory synapses in the CNS similar to the disinhibition evoked by EtOH at the opener NMJ.     

Primary afferents evoke excitatory amino acid receptor-mediated EPSPs that are modulated by presynaptic GABAB receptors in lamprey.

1. The primary afferent neurons (dorsal cells) are of two types in lamprey, which are fast (touch) and slowly adapting (pressure), respectively. Intracellular stimulation of such sensory neurons evokes mono- and polysynaptic excitatory postsynaptic potentials (EPSPs) in spinobulbar neurons (giant interneurons) and in unidentified interneurons. Paired intracellular recordings between identified sensory cells and spinobulbar neurons made it possible to study the synaptic transmission in detail. It is shown that both touch and pressure primary afferents utilize excitatory amino acid (EAA) transmission and, furthermore, that these effects are subject to a presynaptic GABAB receptor modulation. 2. The monosynaptic mixed electrical and chemical EPSPs in giant interneurons had a mean peak amplitude of 3.2 +/- 1.3 (SD) mV, a time to peak of 4.7 +/- 1.2 ms, and a duration at one-half peak amplitude of 9.4 +/- 3.2 ms. Corresponding results were obtained with dorsal root or dorsal column stimulation. Seventy percent of the fast-adapting dorsal cells of the "touch" type evoked monosynaptic mixed EPSPs in giant interneurons, whereas only 3% of the slowly adapting "pressure" dorsal cells did. 3. The chemical part of the monosynaptic EPSPs evoked in giant interneurons was, in all cases tested, blocked by application of EAA antagonists, like the nonselective antagonist kynurenic acid (KYAC; 2 mM). The selective kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) had a similar effect, whereas the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphono-5-valeric acid (AP-5; 200-400 microM) did not change the EPSP, even in the absence of magnesium ions. 4. The monosynaptic excitatory synaptic transmission was modulated by application of the selective GABAB receptor agonist L-baclofen (5-10 mM local droplet application or 100-1,000 microM bath applied) or by gamma-aminobutyric acid (GABA; 100-1,000 microM), also when GABAA receptor-evoked effects were blocked by bicuculline (10 microM). L-baclofen or GABA in combination with bicuculline did not evoke any effects in the postsynaptic neuron on membrane potential, input resistance, or spike threshold. Therefore the effects of the GABAB receptor activation most likely occurs at the presynaptic afferent level. 5. In conclusion, the monosynaptic excitation from skin mechanoreceptors evoked in spinobulbar neurons is mediated by EAA receptors of the kainate/AMPA type. GABAB receptor activation causes a depression of this EPSP, most likely because of a presynaptic action. GABA interneurons are known to form close appositions on sensory axons in the lamprey.     

Postsynaptic potentials mediated by excitatory and inhibitory amino acids in interneurons of stratum pyramidale of the CA1 region of rat hippocampal slices in vitro.

1. Because interneurons of stratum pyramidale partly mediate the feed-forward inhibition of pyramidal cells, intracellular postsynaptic potentials (PSPs) evoked by activation of afferent fibers were examined in 32 nonpyramidal cells of stratum pyramidale of the CA1 region of rat hippocampal slices. 2. Electrical stimulation of stratum radiatum at the CA1-CA3 border elicited, in interneurons, PSPs that were composed of four components: a fast excitatory postsynaptic potential (EPSP), an early inhibitory postsynaptic potential (IPSPA), a late IPSPB, and in some cells a delayed, slower EPSP. These synaptic potentials summated and elicited single action potentials in 57% of cells (17/30) and burst of action potentials (2-10) in the remaining 43%. 3. The fast EPSP was observed in all cells, and the mean stimulation intensity at its threshold was 53.4 microA. Its amplitude increased with membrane hyperpolarization, and it was associated with a 45.4% decrease in cellular input resistance. The fast EPSP always elicited an action potential at short latencies (3.6-6.4 ms poststimulation). It was reversibly reduced by 6-cyano-7-nitroquinoxaline-2,3- dione (CNQX), a blocker of non-N-methyl-D-aspartate (non-NMDA) excitatory amino acid receptors. 4. The IPSPA was observed in 28/32 cells, and the mean intensity of stimulation was 57.6 microA at its threshold. The mean latency of its peak amplitude was 17.4 ms. The mean equilibrium potential (Erev) was -72.8 mV, and it was associated with a 38.9% decrease in cellular input resistance. IPSPA was blocked by the GABAA antagonist bicuculline. 5. The IPSPB was seen in 29/32 cells, and the mean intensity of stimulation at its threshold was 80.3 microA. Its latency to peak was 130.6 ms, its Erev was -107.6 mV, and it was associated with a small (7.6%) decrease in cellular input resistance. IPSPB was blocked by the GABAB antagonist phaclofen. 6. In 11/32 cells a slower EPSP was also observed. Its mean latency to peak was 53.3 ms, and the mean intensity of stimulation at its threshold was 89.4 microA. In two cells its amplitude decreased with membrane hyperpolarization, and its was associated with a 6.8% increase in cellular input resistance. In 8 of 13 cells showing burst responses, this slow EPSP was present. 7. Both EPSPs and IPSPs were sensitive to repetitive stimulation. The amplitude of the fast EPSP was potentiated during paired-pulse stimulation at interstimulus intervals between 30 and 200 ms and occasionally depressed at intervals of 10-20 ms.(ABSTRACT TRUNCATED AT 400 WORDS)     

Presynaptic GABAB receptors modulate thalamic excitation of inhibitory and excitatory neurons in the mouse barrel cortex

Cortical inhibition plays an important role in the processing of sensory information, and the enlargement of receptive fields by the in vivo application of GABAB receptor antagonists indicates that GABAB receptors mediate some of this cortical inhibition. Although there is evidence of postsynaptic GABAB receptors on cortical neurons, there is no evidence of GABAB receptors on thalamocortical terminals. Therefore to determine if presynaptic GABAB receptors modulate the thalamic excitation of layer IV inhibitory neurons and excitatory neurons in layers II-III and IV of the somatosensory "barrel" cortex of mice, we used a thalamocortical slice preparation and patch-clamp electrophysiology. Stimulation of the ventrobasal thalamus elicited excitatory postsynaptic currents (EPSCs) in cortical neurons. Bath application of baclofen, a selective GABAB receptor agonist, reversibly decreased AMPA receptor-mediated and N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs in inhibitory and excitatory neurons. The GABAB receptor antagonist, CGP 35348, reversed the inhibition produced by baclofen. Blocking the postsynaptic GABAB receptor-mediated effects with a Cs+ -based recording solution did not affect the inhibition, suggesting a presynaptic effect of baclofen. Baclofen reversibly increased the paired-pulse ratio and the coefficient of variation, consistent with the presynaptic inhibition of glutamate release. Our results indicate that the presynaptic activation of GABAB receptors modulates thalamocortical excitation of inhibitory and excitatory neurons and provide another mechanism by which cortical inhibition can modulate the processing of sensory information.     

NT-3 evokes an LTP-like facilitation of AMPA/kainate receptor-mediated synaptic transmission in the neonatal rat spinal cord

Neurotrophin-3 (NT-3) is a neurotrophic factor required for survival of muscle spindle afferents during prenatal development. It also acts postsynaptically to enhance the monosynaptic excitatory postsynaptic potential (EPSP) produced by these fibers in motoneurons when applied over a period of weeks to the axotomized muscle nerve in adult cats. Similar increases in the amplitude of the monosynaptic EPSP in motoneurons are observed after periodic systemic treatment of neonatal rats with NT-3. Here we show an acute action of NT-3 in enhancing the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptor-mediated fast monosynaptic EPSP elicited in motoneurons by dorsal root (DR) stimulation in the in vitro hemisected neonatal rat spinal cord. The receptor tyrosine kinase inhibitor K252a blocks this action of NT-3 as does the calcium chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) injected into the motoneuron. The effect of NT-3 resembles long-term potentiation (LTP) in that transient bath application of NT-3 to the isolated spinal cord produces a long-lasting increase in the amplitude of the monosynaptic EPSP. An additional similarity is that activation of N-methyl-D-aspartate (NMDA) receptors is required to initiate this increase but not to maintain it. The NMDA receptor blocker MK-801, introduced into the motoneuron through the recording microelectrode, blocks the effect of NT-3, indicating that NMDA receptors in the motoneuron membrane are crucial. The effect of NT-3 on motoneuron NMDA receptors is demonstrated by its enhancement of the depolarizing response of the motoneuron to bath-applied NMDA in the presence of tetrodotoxin (TTX). The potentiating effects of NT-3 do not persist beyond the first postnatal week. In addition, EPSPs with similar properties evoked in the same motoneurons by stimulation of descending fibers in the ventrolateral funiculus (VLF) are not modifiable by NT-3 even in the initial postnatal week. Thus, NT-3 produces synapse-specific and age-dependent LTP-like enhancement of AMPA/kainate receptor-mediated synaptic transmission in the spinal cord, and this action requires the availability of functional NMDA receptors in the motoneuron.     

Analysis of Ia-inhibitory synaptic input to cat spinal motoneurons evoked by vibration of antagonist muscles.

1. Steady-state inhibitory postsynaptic potentials (IPSPs) were evoked in tibialis anterior and extensor digitorum longus motoneurons of the cat by using tendon vibration to activate Ia-afferent fibers from the antagonist medial gastrocnemius muscle. 2. The effective synaptic currents (IN) underlying the steady-state IPSPs were measured by the use of a modified voltage-clamp technique. The amplitudes of the effective synaptic currents (1.62 +/- 0.66 nA, mean +/- SD; n = 20) extended over a fivefold range (0.5-2.7 nA) but were not correlated with the intrinsic properties of the motoneurons or with putative motor unit type. 3. We calculated the synaptic conductance (GS) underlying the steady-state Ia IPSPs from measurements of motoneuron input conductance during the activation of the Ia synaptic input. As was expected from Ohm's law, the Ia-inhibitory GS and IN were correlated (r = 0.49; P less than 0.05). Like IN, GS (175 +/- 202 nS, mean +/- SD; n = 20) was not correlated with the intrinsic properties of the motoneurons. 4. As has been reported previously for transient Ia IPSPs, the amplitudes of the steady-state IPSPs were correlated with motoneuron input resistance (r = 0.74; P less than 0.001) and homonymous Ia excitatory postsynaptic synaptic potential (EPSP) amplitude (r = 0.72; P less than 0.001). 5. The amplitudes of the steady-state Ia IPSPs and the homonymous Ia EPSPs were plotted on logarithmic axes. The slope (0.59) was significantly less than 1, which indicates that the gradient of Ia inhibition across the motoneuron pool is less steep than that of Ia excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.     

Influence of synaptic identity on single-Ia-afferent connectivity and EPSP amplitude in the adult cat: homonymous versus heteronymous connections.

1. This study makes use of the pattern of synaptic connections between motoneurons and Ia afferents of triceps surae muscles in the cat to test the relative importance of synaptic identity, neuronal size, and neuronal topography as determinants of Ia-afferent connectivity and excitatory postsynaptic potential (EPSP) amplitude. 2. The synaptic actions of single-Ia medial gastrocnemius (MG) afferents were measured by intracellular recording in MG and lateral gastrocnemius (LG) motoneurons. The spike-triggered averaging technique was used to measure EPSPs generated by homonymous or heteronymous Ia afferents and motoneurons, i.e., neurons supplying the same or different muscles, respectively. In agreement with earlier studies, the pooled sample showed that the number of functional connections and the size of EPSPs were both significantly greater for homonymous than for heteronymous neurons. 3. Afferent conduction velocity, motoneuron conduction velocity, rheobase current, and position of the motoneuron relative to the spinal cord afferent entry were all correlated with EPSP amplitude, but the amplitude difference between homonymous and heteronymous connections remained significant after the statistical removal analysis of covariance (ANCOVA) of the contribution of these variables. Stepwise multiple-regression analysis showed that synaptic identity explained the greatest fraction of the variance in EPSP amplitude (9%), with significant but smaller fractions accounted for by rheobase current or motoneuron conduction velocity. 4. In a separate experiment, the monosynaptic affects from both homonymous and heteronymous single-Ia afferents were examined in each of 88 MG or LG motoneurons. The single-Ia afferents used in this portion of the study were sampled from both MG and LG muscles and selected for similar conduction velocities and spinal cord entry points.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presynaptic GABA(B) receptors inhibit synaptic inputs to rat subthalamic neurons.

Effects of baclofen on synaptic transmission were studied in rat subthalamic neurons using whole-cell patch clamp recording from brain slices. Focal electrical stimulation of the brain slice evoked GABAergic inhibitory postsynaptic currents and glutamatergic excitatory postsynaptic currents. Baclofen reduced the amplitude of evoked inhibitory postsynaptic currents in a concentration-dependent manner with an IC(50) of 0.6+/-0.2 microM. Evoked excitatory postsynaptic currents were also reduced by baclofen concentration-dependently (IC(50) of 1.6+/-0.2 microM), but baclofen was more potent at reducing the GABA(A) receptor inhibitory postsynaptic currents. The GABA(B) receptor antagonist CGP 35348 blocked these inhibitory effects of baclofen on evoked inhibitory and excitatory postsynaptic currents. Baclofen increased the paired-pulse ratios of evoked inhibitory and excitatory postsynaptic currents. Furthermore, baclofen reduced the frequency of spontaneous miniature excitatory postsynaptic currents, but had no effect on their amplitude.These results provide evidence for presence of presynaptic GABA(B) receptors that modulate both GABA and glutamate release from afferent terminals in the subthalamus.     

Analysis of individual Ia-afferent EPSPs in a homonymous motoneuron pool with respect to muscle topography

The spike-triggered averaging technique (26) was used to determine whether the synaptic input from medial gastrocnemius (MG) Ia-afferent fibers to homonymous motoneurons is "topographically weighted" (22) by means of differences in projection frequency, excitatory postsynaptic potential (EPSP) amplitude, or a combination of both factors. Motoneurons were classified as either "same branch" or "other branch," depending on whether a Ia-afferent fiber and motor axon were contained in the same or different intramuscular nerve branches. No difference was found in the projection frequency of Ia-afferents to the same branch and other branch motoneurons (95 versus 94%, respectively). The mean EPSP amplitude was larger in the same branch group of motoneurons (92 +/- 8 (SE) microV; n = V; n = 97) than in the other branch group (77 +/- 7 microV; n = 79). This difference was most striking in high-rheobase (greater than or equal to 10 nA) motoneurons, for which the mean EPSP amplitude in the same branch group was 82 +/- 12 microV (n = 48), whereas that in the other branch group was 52 +/- 5 microV (n = 37). In 60 cases it was possible to compare the EPSPs produced by a same branch afferent and an other branch afferent in the same motoneuron. The same branch afferent produced the larger EPSP in 73% (44/60) of the cases. Moreover, the mean ratio of the same branch to the other branch EPSP amplitudes was 1.7, which was both statistically significant and consistent with analogous results from our preceding study of aggregate EPSPs (22). Mean rise times and half-widths of EPSPs in the same branch group were not significantly different from those in the other branch group. Furthermore, no significant differences in rise times or half-widths between the two groups were evident when motoneurons were segregated according to their rheobase values. This suggests that the segregation of Ia-afferent and motor axons across the intramuscular nerve branches is not reflected in the locations of Ia terminals on the motoneuron somadendritic surface and that other factors must account for observed EPSP amplitude differences. Our data suggest that the topographic weighting of homonymous Ia-afferent input to cat MG motoneurons is mediated by a gradient of EPSP amplitude rather than by a gradient of Ia connectivity and also suggest that the effect is most prominent in high-rheobase motoneurons.     

The effect of axotomy on posttetanic potentiation of group Ia synapses in the cat

Posttetanic potentiation (PTP) of composite Ia excitatory postsynaptic potentials (EPSPs) has been studied in normal cat alpha-motoneurons and in motoneurons axotomized 2-3 wk earlier by ventral root section. The maximal amount of PTP of EPSP amplitude (expressed relative to unpotentiated amplitude) was considerably less in the axotomized population compared with the normal population. The decrease in PTP provoked by axotomy occurs in association with a postaxotomy increase of input resistance, the net effect being that PTP in axotomized cells was much the same as that observed by others in normal motoneurons possessing similarly high input resistance. In agreement with previous results, EPSP peak amplitudes were decreased after axotomy. This decrease seemed to be largely related to an absence of the largest EPSPs, since otherwise the EPSP distributions of normal and axotomized motoneurons showed considerable overlap. It is suggested that the observed decrease in PTP after axotomy is related to a change in synaptic release properties and not secondary to changes in the electrical properties of motoneurons. A previous analysis has suggested that axotomy causes an alteration of the distribution of passive electrical properties among motoneurons such that axotomized cells resemble normal high-resistance motoneurons. The present results suggest that axotomy may affect the distribution of Ia synaptic release properties in a similar manner, since PTP in axotomized motoneurons resembles that observed in normal high-resistance motoneurons.     

The physiological role of pre- and postsynaptic GABAB receptors in membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus

In the inferior colliculus (IC), GABAergic inhibition mediated by GABA_A receptors has been shown to play a significant role in regulating physiological responses, but little is known about the physiological role of GABA_B receptors in IC neurons. In the present study, we used whole-cell patch clamp recording in vitro to investigate the effects of activation of GABA_B receptors on membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus (ICD). Repetitive stimulation of GABAergic inputs to ICD neurons at high frequencies could elicit a slow and long-lasting postsynaptic response, which was reversibly abolished by the GABA_B receptor antagonist, CGP 35348. The results suggest that postsynaptic GABA_B receptors can directly mediate inhibitory synaptic transmission in ICD. The role of postsynaptic GABA_B receptors in regulation of membrane excitability was further investigated by application of the GABA_B receptor agonist, baclofen. Baclofen hyperpolarized the cell, reduced the membrane input resistance and firing rate, increased the threshold for generating action potentials (APs), and decreased the amplitude of the AP and its associated after-hyperpolarization. The Ca²⁺-mediated rebound depolarization following hyperpolarization and the depolarization hump at the beginning of membrane depolarization were also suppressed by baclofen. In voltage clamp experiments, baclofen induced inward rectifying K⁺ current and reduced low- and high-threshold Ca²⁺ currents, which may account for the suppression of membrane excitability by postsynaptic GABA_B receptors. Application of baclofen also reduced excitatory synaptic responses mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and inhibitory synaptic responses mediated by GABA_A receptors. Baclofen increased the ratios of 2nd/1st excitatory and inhibitory postsynaptic currents to paired-pulse stimulation of the synaptic inputs. These results suggest that fast glutamatergic and GABAergic synaptic transmission in ICD can be modulated by presynaptic GABA_B receptors.     

Modulation of synaptic transmission at Ia-afferent fiber connections on motoneurons during high-frequency stimulation: role of postsynaptic target.

1. High-frequency stimulation of single group Ia-fibers results in modulation of excitatory postsynaptic potential (EPSP) amplitude recorded in target motoneurons. This can be either positive (EPSP amplitude increases in response to successive stimuli in the high-frequency burst) or negative (decrease in EPSP amplitude). We have investigated whether the magnitude of modulation is associated with the stimulated afferent, the responding motoneuron, or the amplitude of the EPSP. 2. In agreement with previous findings, we found that positive modulation tends to occur at connections generating small EPSPs and negative modulation, at those producing large EPSPs. Because large EPSPs generally are evoked in motoneurons with low values of rheobase, we found, as anticipated, that connections on low rheobase motoneurons are prone to negative modulation during high-frequency stimulation, whereas those on high rheobase motoneurons (which tend to generate small EPSPs) are prone to positive modulation. 3. In experiments where the projection of multiple afferents to a single motoneuron was studied, we found that amplitude modulation was similar despite differences in EPSP amplitude. Thus in a given motoneuron there is no relationship between modulation and amplitude, in contrast to the existence of such a relationship in the population of connections as a whole. 4. In the converse experiments where the projection of single afferents to multiple motoneurons was studied, we found marked variability in the modulation patterns with clear indications that amplitude and modulation are correlated as in the entire population of Ia/motoneuron connections. 5. We tested the constancy of modulation patterns evoked in a given motoneuron by comparing the modulation patterns evoked in motoneurons by single fibers, and by stimulation of the heteronymous nerve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Computer simulation of group Ia EPSPs using morphologically realistic models of cat alpha-motoneurons

1. Morphological and electrophysiological data on the electrotonic structure of six triceps surae alpha-motoneurons and on the number and location of 202 Group Ia synapses making contact with ankle extensor motoneurons, previously obtained in this laboratory, were used to construct computer models to examine the generation of composite monosynaptic Group Ia excitatory postsynaptic potentials (EPSPs). 2. A total of 300 active synapses, each generating conductance transients based on voltage-clamp data and having activation times temporally dispersed (range approximately 1.3 ms) according to the conduction velocity profile of Group Ia-afferents, were used to generate composite EPSPs. 3. The shape indexes (foot-to-peak rise times and half widths) of simulated EPSPs matched those of experimentally observed Ia EPSPs reasonably well, although the rise times were, on average, approximately 0.25 ms longer in the simulated EPSPs. This may indicate that the effective temporal dispersion of actual Group Ia monosynaptic EPSPs is less than that the temporal asynchrony used in the simulations. 4. The peak amplitudes of simulated composite EPSPs (6-14 mV), as well as EPSPs produced by single somatic synapses (80-300 microV), were comparable to those found in experimental data. 5. Simulated EPSPs in motoneuron models with two forms of nonuniform Rm distribution ("step" increase from low values of Rm on the soma to much higher but uniform values in the dendrites, versus gradual monotonic "sigmoidal" increases from soma to distal dendrites) were similar in shape and amplitude. This prevented choosing one or the other Rm model as more "correct." 6. Transmembrane voltages at synaptic sites in motoneuron dendrites during generation of composite Ia EPSPs had peak amplitudes less than twice those of the somatic EPSP. The amount of nonlinearity during EPSP production was assessed by making the delivery of synaptic current independent of the local transmembrane voltage. This non-linearity was modest (less than 10%) during composite EPSP generation, consistent with previous experimental evidence. 7. The local voltages produced in various parts of different dendrites during composite EPSP generation depended on the number and location of active synapses and on the electrotonic structure of the particular dendrite. The results show that dendrites that project in different directions away from the motoneuron soma could, in principle, exhibit different degrees of interaction between Ia and other synaptic inputs. 8. Although produced by the same number of active synapses, the simulated composite Ia EPSPs varied over a two-fold range of peak amplitude in relation to motor-unit type, cell input resistance, and cell size (total membrane area).(ABSTRACT TRUNCATED AT 400 WORDS)     

Voltage threshold and excitability among variously sized cat hindlimb motoneurons

Intracellular recording has been performed to examine whether any differences in apparent initial-segment voltage threshold exist between types F and S cat triceps surae motoneurons. Voltage threshold was estimated using orthodromic action potentials initiated by large, monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by dorsal root stimulation. No significant differences in voltage threshold could be detected between types F and S motoneurons. Further, voltage thresholds did not covary with motoneuron input resistance, afterhyperpolarization duration, or the twitch contraction time of functionally isolated muscle units. Significant positive correlations were observed between voltage threshold and the motoneuron resting potential. Utilizing a compartmental neuron model, a theoretical analysis has been performed that examines the influence of specific passive membrane properties on current threshold for action potentials initiated by large, monosynaptic EPSPs. This analysis indicates that total membrane capacitance will be the primary determinant of these thresholds. Further analysis of available data suggests that active membrane properties will play a minimal role in setting these thresholds. Since specific membrane capacitance is likely to be similar among cat motoneurons, it is concluded that only size or surface area-related current threshold differences will exist among these cells for activation with brief currents such as those underlying large EPSPs. For motoneurons thus activated, it is suggested that variations in the excitatory/inhibitory balance or density of synaptic input would be the major mechanisms for producing differential recruitment thresholds among the motoneuron population. Other available evidence is discussed that indicates that factors intrinsic to the motoneurons themselves will contribute to the setting of functional recruitment thresholds for activation with longer duration currents.