Glycine conductance changes in chick spinal cord neurons developing in culture

Inhibitory glycinergic neurotransmission in the mammalian auditory brainstem upon prolonged stimulation: short-term plasticity and synaptic reliability

Short-term plasticity plays a key role in synaptic transmission and has been extensively investigated for excitatory synapses. Much less is known about inhibitory synapses. Here we analyze the performance of glycinergic connections between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO) in the auditory brainstem, where high spike rates as well as fast and precise neurotransmission are hallmarks. Analysis was performed in acute mouse slices shortly after hearing onset (postnatal day (P)11) and 8 days later (P19). Stimulation was done at 37°C with 1–400 Hz for 40 s. Moreover, in a novel approach named marathon experiments, a very prolonged stimulation protocol was employed, comprising 10 trials of 1-min challenge and 1-min recovery periods at 50 and 1 Hz, respectively, thus lasting up to 20 min and amounting to >30,000 stimulus pulses. IPSC peak amplitudes displayed short-term depression (STD) and synaptic attenuation in a frequency-dependent manner. No facilitation was observed. STD in the MNTB-LSO connections was less pronounced than reported in the upstream calyx of Held-MNTB connections. At P11, the STD level and the failure rate were slightly lower within the ms-to-s range than at P19. During prolonged stimulation periods lasting 40 s, P19 connections sustained virtually failure-free transmission up to frequencies of 100 Hz, whereas P11 connections did so only up to 50 Hz. In marathon experiments, P11 synapses recuperated reproducibly from synaptic attenuation during all recovery periods, demonstrating a robust synaptic machinery at hearing onset. At 26°C, transmission was severely impaired and comprised abnormally high amplitudes after minutes of silence, indicative of imprecisely regulated vesicle pools. Our study takes a fresh look at synaptic plasticity and stability by extending conventional stimulus periods in the ms-to-s range to minutes. It also provides a framework for future analyses of synaptic plasticity.

Repetitive light stimulation inducing glycine receptor plasticity in the retinal neurons

Neurotransmitter receptor plasticity is a mechanism that can regulate the temporal and intensity encoding of a synapse. While this has been extensively studied as a mechanism of learning, less is known about such processes in sensory systems. This study examines modulation of glycine receptor function at the first synapse in the retina. It was found that horizontal cells, which are postsynaptic to photoreceptors, have glycine receptor currents that are enhanced when internal calcium is elevated. This can be achieved by glutamatergic synaptic input or by activation of voltage-gated calcium channels. When the retina was maintained in a dark-adapted state, the calcium levels in horizontal cells were relatively low. After a series of brief light stimuli, the internal calcium concentration in horizontal cells was elevated, and the glycine currents were faster and greater in amplitude. The increase of internal calcium levels was caused by increased transmitter release from photoreceptors. Thus glycine receptor function is state dependent and can be rapidly altered by synaptic input from photoreceptors. Light stimulation drives glycine receptor plasticity in the retinal neural network.

Benzodiazepines block alpha2-containing inhibitory glycine receptors in embryonic mouse hippocampal neurons

Inhibitory glycine receptors (GlyRs) in the mammalian cortex probably contribute to brain development and to maintaining tonic inhibition. Given their presence throughout the cortex, their modulation likely has important physiological consequences. Although benzodiazepines potentiate gamma-aminobutyric acidA receptors (GABAARs), they may also modulate GlyRs because binding studies initially suggested that they act at GlyRs. Furthermore, their diminished ability to potentiate neonatal GABAARs suggests that they may exert their beneficial clinical effects at another site in the developing brain. Therefore we examined the effect of benzodiazepines on whole cell currents mediated by GlyRs in cultured embryonic mouse hippocampal neurons. First, we determined the GlyR subunit composition in this preparation. Glycine, beta-alanine, and taurine activate strychnine-sensitive chloride currents in a dose-dependent manner. Maximal concentrations of the three agonists produce equal, nonadditive responses as expected of full agonists. The pharmacological properties of the GlyR currents including their pattern of modulation by picrotoxinin, picrotin, and tropisetron indicate that GlyRs consist of alpha2beta heteromers and alpha2 homomers. Reverse transcriptase polymerase chain reaction (RTPCR) studies confirmed the presence of alpha2 and beta subunits. Second, we found that micromolar concentrations of some benzodiazepines, including chlordiazepoxide and nitrazepam, inhibit GlyR currents. Nitrazepam inhibition of GlyRs is noncompetitive, is not voltage dependent, and does not reflect enhanced desensitization. Thus benzodiazepines allosterically inhibit alpha2-containing GlyRs in embryonic mouse hippocampal neurons via a "low"-affinity site.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Contribution of single-channel properties to the time course and amplitude variance of quantal glycine currents recorded in rat motoneurons

The amplitude of spontaneous, glycinergic miniature inhibitory postsynaptic currents (mIPSCs) recorded in hypoglossal motoneurons (HMs) in an in vitro brain stem slice preparation increased over the first 3 postnatal weeks, from 42 +/- 6 pA in neonate (P0-3) to 77 +/- 11 pA in juvenile (P11-18) HMs. Additionally, mIPSC amplitude distributions were highly variable: CV 0.68 +/- 0.05 (means +/- SE) for neonates and 0.83 +/- 0.06 for juveniles. We wished to ascertain the contribution of glycine receptor (GlyR)-channel properties to this change in quantal amplitude and to the amplitude variability and time course of mIPSCs. To determine whether a postnatal increase in GlyR-channel conductance accounted for the postnatal change in quantal amplitude, the conductance of synaptic GlyR channels was determined by nonstationary variance analysis of mIPSCs. It was 48 +/- 8 pS in neonate and 46 +/- 10 pS in juvenile HMs, suggesting that developmental changes in mIPSC amplitude do not result from a postnatal alteration of GlyR-channel conductance. Next we determined the open probability (Popen) of GlyR channels in outside-out patches excised from HMs to estimate the contribution of stochastic channel behavior to quantal amplitude variability. Brief (1 ms) pulses of glycine (1 mM) elicited patch currents that closely resembled mIPSCs. The GlyR channels' Popen, calculated by nonstationary variance analysis of these currents, was approximately 0.70 (0.66 +/- 0.09 in neonates and 0.72 +/- 0.05 in juveniles). The decay rate of patch currents elicited by brief application of saturating concentrations of glycine (10 mM) increased postnatally, mimicking previously documented changes in mIPSC time course. Paired pulses of glycine (10 mM) were used to determine if rapid GlyR-channel desensitization contributed to either patch current time course or quantal amplitude variability. Because we did not observe any fast desensitization of patch currents, we believe that fast desensitization of GlyRs underlies neither phenomenon. From our analysis of glycinergic patch currents and mIPSCs, we draw three conclusions. First, channel deactivation is the primary determinant of glycinergic mIPSC time course, and postnatal changes in channel deactivation rate account for observed developmental changes in mIPSC decay rate. Second, because GlyR-channel Popen is high, differences in receptor number between synapses rather than stochastic channel behavior are likely to underlie the majority of quantal variability seen at glycinergic synapses throughout postnatal development. We estimate the number of GlyRs available at a synapse to be on average 27 in neonate neurons and 39 in juvenile neurons. Third, this change in the calculated number of GlyRs at each synapse may account for the postnatal increase in mIPSC amplitude.

Kinetic analysis of glycine receptor currents in ventral cochlear nucleus

Glycine plays an important role as an inhibitory neurotransmitter in the ventral cochlear nucleus. However, little is known about the kinetic behavior of glycine receptors. The present study examines the kinetics of the native inhibitory glycine receptors in neurons of the ventral cochlear nucleus, using outside-out patches from acutely dissociated cells and a fast flow system. Steps into 1 mM glycine revealed fast phases of desensitization with time constants of 13 and 129 ms, that together produced a 40% reduction in current from the peak response. Slower desensitization phases also were observed. After removal of glycine, currents deactivated with two time constants of 15 and 68 ms, and these rates were independent of the glycine concentration between 0.2 and 1 mM. Recovery from desensitization was slow relative to desensitization itself. These results demonstrate that glycine receptors can exhibit faster rates of desensitization and deactivation than previously reported.

Developmental expression of glycine immunoreactivity and its colocalization with GABA in the embryonic chick lumbosacral spinal cord

The development of immunoreactivity for the putative inhibitory amino acid neurotransmitter glycine was investigated in the embryonic and posthatched chick lumbosacral spinal cord by using postembedding immunocytochemical methods. Glycine immunoreactive perikarya were first observed at embryonic day 8 (E8) both in the dorsal and ventral gray matters. The number of immunostained neurons sharply increased by E10 and was gradually augmented further at later developmental stages. The general pattern of glycine immunoreactivity characteristic of mature animals had been achieved by E12 and was only slightly altered afterward. Most of the immunostained neurons were located in the presumptive deep dorsal horn (laminae IV-VI) and lamina VII, although glycine-immunoreactive neurons were scattered throughout the entire extent of the spinal gray matter. By using some of our previously obtained and published data concerning the development of gamma-aminobutyric acid (GABA)-ergic neurons in the embryonic chick lumbosacral spinal cord, we have compared the numbers, sizes, and distribution of glycine- and GABA-immunoreactive spinal neurons at various developmental stages and found the following marked differences in the developmental characteristics of these two populations of putative inhibitory interneurons. (i) GABA immunoreactivity was expressed very early (E4), whereas immunoreactivity for glycine appeared relatively late (E8) in embryonic development. (ii) In the ventral horn, GABA immunoreactivity declined, whereas immunoreactivity for glycine gradually increased from E8 onward in such a manner that the sum of glycinergic and GABAergic perikarya remained constant during the second half of embryonic development. (iii) Glycinergic and GABAergic neurons showed different distribution patterns in the spinal gray matter throughout the entire course of embryogenesis as well as in the posthatched animal. When investigating the colocalization of glycine and GABA immunoreactivities, perikarya immunostained for both amino acids were revealed at all developmental stages from E8 onward, and the proportions of glycine- and GABA-immunoreactive neurons that were also immunostained for the other amino acid were remarkably constant during development. The characteristic features of the development of the investigated putative inhibitory spinal interneurons are discussed and correlated with previous neuroanatomical and physiological studies.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Glycinergic inhibitory synaptic currents and related receptor channels in the zebrafish brain

To extend our study of the inhibitory synaptic network we have developed an isolated whole-brain preparation of the 52-h-old zebrafish (Brachydanio rerio) in which the structural and functional integrity of the brain is preserved. We report the characterization of quantal inhibitory events and the correlation of their properties with those of the underlying activated channels. During whole-cell recordings of the Mauthner cells, applications of 10(-6) M tetrodotoxin greatly reduced the frequency and amplitude of the spontaneously occurring synaptic events, which were dominated by Cl--dependent inhibitory postsynaptic currents (IPSCs). Lowering Ca2+ and adding Mg2+ to tetrodotoxin-containing solutions resulted in a further decrease in amplitude of the recorded synaptic currents, the remaining ones being considered as miniature IPSCs (mIPSCs). Applications of 0.5-1 microM strychnine in the presence of tetrodotoxin eliminated > 90% of the inhibitory currents in the preparation. The amplitude histograms of these mIPSCs exhibited two initial equally spaced peaks, followed by a skewed distribution for higher values. The first two components were well fitted by the sum of two Gaussian curves, giving a mean quantal amplitude of 35.7 pA (at a holding potential of -50 mV) and a coefficient of variation of 0.25 for the first peak. Outside-out recordings showed at least two classes of glycine receptor channels, one having multiple conductance levels with a main state of 81-86 pS and another displaying only one opening level of 41-43 pS. These two mean conductance states had similar mean open times, of 0.6-1 and 4.5-6 ms respectively. In addition, three mean closed times were observed for the 41-43 pS level. The shortest group (0.6-1 ms) was considered as representing gaps within bursts. Burst analysis revealed three mean burst durations, of 0.6, 4 and 35 ms. Comparisons of the amplitude of the first class of mIPSCs and of the open channel conductances indicated that one quantum opens 14-22 channels. Moreover, the correspondence between the mean decay time of mIPSCs and the mean open time or medium burst duration (4-5 ms) suggests that glycine-activated channels open only once in response to a single exocytosis. The pre- and postsynaptic origins of mIPSCs amplitude fluctuations are discussed in the context of multivesicular release versus the hypothesis of postsynaptic receptor saturation.

Development of spontaneous synaptic activity in culture of the chick spinal cord neurons

Spontaneous postsynaptic currents in chick spinal cord neurons cultured for up to three weeks were recorded by using standard whole-cell patch-clamp techniques. Beginning with approximately the 7th to 14th day in vitro, giant postsynaptic currents, mediated presumably by glycine, were single synaptic events (inhibitory postsynaptic currents). After the 14th day in vitro excitatory postsynaptic currents appeared. Both types of currents were predominantly arranged in bursts. This pattern of synaptic activity did not change appreciably during further cultivation. Characteristics of inhibitory postsynaptic currents were studied. Decay of the majority of giant inhibitory postsynaptic currents was two-exponential. Time-constants of the decay (fast and slow) increased with depolarization and decreased with increasing temperature. Decay of miniature inhibitory postsynaptic currents was single exponential and did not depend on the membrane potential. Strychnine at concentrations of 1-2 microM was found not only to reduce the amplitude of giant inhibitory postsynaptic currents but also to prolong their decay. The time-constant of the slow component of the decay was mostly affected during the inhibitor action. Repeated binding of glycine to postsynaptic receptors due to a large presynaptic release is proposed as an explanation for the properties of giant inhibitory postsynaptic currents decay. Correlations between the development of synaptic networks under in vivo and in vitro conditions are discussed.