Glycinergic Synaptic Transmission Research Articles

Glycinergic synaptic transmission to bullfrog retinal bipolar cells is input-specific

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites ( ipc-IPSCs) had a much slower decay time constant (25.2±7.8 ms) than IPSCs driven by input from amacrine cells at bipolar cell axon terminals ( ac-IPSCs) (14.7±5.5 ms). Furthermore, peak-scaled non-stationary noise analysis revealed that the weighted mean single-channel conductance of the glycine receptors underlying bipolar cell dendritic ipc-IPSCs (20.8±6.6 pS) was significantly larger than that of those underlying bipolar cell axon terminal ac-IPSCs (12.9±2.9 pS). These results demonstrate that glycinergic synaptic transmission with different properties at bipolar cell dendrites and axon terminals differentially mediates intraretinal centrofugal signal transfer from the inner retina to the outer retina provided by interplexiform cells and lateral inhibition offered by amacrine cells in the inner retina.

PGE2 selectively blocks inhibitory glycinergic neurotransmission onto rat superficial dorsal horn neurons

Despite the crucial role that prostaglandins (PGs) have in the sensitization of the central nervous system to pain, their cellular and molecular targets leading to increased pain perception have remained elusive. Here we investigated the effects of PGE(2) on fast synaptic transmission onto neurons in the rat spinal cord dorsal horn, the first site of synaptic integration in the pain pathway. We identified the inhibitory (strychnine-sensitive) glycine receptor as a specific target of PGE(2). PGE(2), but not PGF(2 alpha), PGD(2) or PGI(2), reduced inhibitory glycinergic synaptic transmission in low nanomolar concentrations, whereas GABAA, AMPA and NMDA receptor-mediated transmission remained unaffected. Inhibition of glycine receptors occurred via a postsynaptic mechanism involving the activation of EP2 receptors, cholera-toxin-sensitive G-proteins and cAMP-dependent protein kinase. Via this mechanism, PGE(2) may facilitate the transmission of nociceptive input through the spinal cord dorsal horn to higher brain areas where pain becomes conscious.

Transition from GABAergic to glycinergic synaptic transmission in newly formed spinal networks.

The role of glycinergic and GABAergic systems in mediating spontaneous synaptic transmission in newly formed neural networks was examined in motoneurons in the developing rat spinal cord. Properties of action potential-independent miniature inhibitory postsynaptic currents (mIPSCs) mediated by glycine and GABA(A) receptors (GlyR and GABA(A)R) were studied in spinal cord slices of 17- to 18-day-old embryos (E17-18) and 1- to 3-day-old postnatal rats (P1-3). mIPSC frequency and amplitude significantly increased after birth, while their decay time decreased. To determine the contribution of glycinergic and GABAergic synapses to those changes, GlyR- and GABA(A)R-mediated mIPSCs were isolated based on their pharmacological properties. Two populations of pharmacologically distinct mIPSCs were recorded in the presence of glycine or GABA(A) receptors antagonists: bicuculline-resistant, fast-decaying GlyR-mediated mIPSCs, and strychnine-resistant, slow-decaying GABA(A)R-mediated mIPSCs. The frequency of GABA(A)R-mediated mIPSCs was fourfold higher than that of GlyR-mediated mIPSCs at E17-18, indicating that GABAergic synaptic sites were functionally dominant at early stages of neural network formation. Properties of GABA(A)R-mediated mIPSC amplitude fluctuations changed from primarily unimodal skewed distribution at E17-18 to Gaussian mixtures with two to three discrete components at P1-3. A developmental shift from primarily long-duration GABAergic mIPSCs to short-duration glycinergic mIPSCs was evident after birth, when the frequency of GlyR-mediated mIPSCs increased 10-fold. This finding suggested that either the number of glycinergic synapses or the probability of vesicular glycine release increased during the period studied. The increased frequency of GlyR-mediated mIPSCs was associated with more than a twofold increase in their mean amplitude, and in the number of motoneurons in which mIPSC amplitude fluctuations were best fitted by multi-component Gaussian curves. A third subpopulation of mIPSCs was apparent in the absence of glycine and GABA(A) receptor antagonists: mIPSCs with both fast and slow decaying components. Based on their dual-component decay time and their suppression by either strychnine or bicuculline, we assumed that these were generated by the activation of co-localized postsynaptic glycine and GABA(A) receptors. The contribution of mixed glycine-GABA synaptic sites to the generation of mIPSCs did not change after birth. The developmental switch from predominantly long-duration GABAergic inhibitory synaptic currents to short-duration glycinergic currents might serve as a mechanism regulating neuronal excitation in the developing spinal networks.

Central mechanisms controlling upper airway patency

The neurones of the ponto-medullary respiratory network drive two functionally and anatomically distinct pools of motoneurones. One set is located within the spinal cord that innervates the diaphragm and intercostal muscles. A second group of motoneurones is located within the nucleus ambiguus imbedded in the ventral respiratory group that project via cranial motor outflows to co-ordinate the activity of laryngeal and bronchial muscles to control airway resistance and airflow. These spinal and cranial motor activities have to be precisely co-ordinated to ensure efficient ventilation. We recently included this behaviour as imperative for a definition of eupnoea. Thus, beside a rhythmic eupnoeic (ramp) discharge pattern of pump motoneurones, a phasic respiratory modulation of glottal resistance should be observed and expressed as glottal dilation during inspiration and transient glottal constriction during post-inspiration [1]. During the last decade mechanisms underlying respiratory rhythm generation were studied primarily in reduced in vitro preparations. The work concluded that the respiratory rhythm is generated by pacemaker neurones located in the Pre-Botzinger complex and is independent of inhibitory glycinergic synaptic transmission (for review see [2,3]). However, a potential role for glycinergic transmission for the eupneic co-ordination of circuitry controlling the upper airway was largely disregarded. To determine a role for glycinergic inhibition within the pon-tomedullary network, we used the arterially perfused working heart-brainstem preparation (WHBP) of neonatal and mature rat. This preparation allows both kinesiological and cellular studies of central and peripheral mechanisms controlling upper airway resistance [1]. Recording of the recurrent laryngeal nerve activity as an index of motor output to the glottis revealed post-inspiratory activity that shifted towards the inspiratory phase after strychnine antagonism of glycine receptors (0.5–1.5 μM). This shift of post-inspiratory activity was also obtained at the cellular level: intracellular recordings of post-inspiratory neurones revealed that the hyperpo-larisation during the inspiratory phase was converted to a depolarisation with spike discharge after exposure to strychnine. This lead to a massive disturbance of the eupnoeic modulation of glottal resistance by converting the inspiratory glottic dilatation (seen during control) to a paradoxical constriction, as demonstrated by measuring changes in laryngeal resistance. Similar results were obtained in both neonatal and juvenile rats suggesting that glycinergic mechanisms co-ordinating ventilatory movements with upper airway resistance are functional at birth. The effects of glycinergic inhibition were mimicked during exposure of neonatal preparations to prolonged hypoxia. During the secondary hypoxic depression of respiration post-inspiratory activity was shifted towards inspiration causing a paradoxical glottic constriction during neural inspiration. We conclude that integrity of glycinergic neurotransmission within the ponto-medullary respiratory network is essential for co-ordinating the neuronal activities which control upper airway resistance and ventilatory movements and consequently the eupnoeic breathing pattern in rats from birth.

Room 224-226, 10/18/2000 10: 30 AM - 12: 00 PM (PD) Enflurane Enhances Glycinergic Synaptic Transmission by Both Presynaptic and Postsynaptic Mechanisms in Rat Spinal Cord

Room 224-226, 10/18/2000 10: 30 AM - 12: 00 PM (PD) Enflurane Enhances Glycinergic Synaptic Transmission by Both Presynaptic and Postsynaptic Mechanisms in Rat Spinal Cord

Naloxone reduces the amplitude of IPSPs evoked in lumbar motoneurons by reticular stimulation during carbachol-induced motor inhibition

During active sleep or carbachol-induced motor inhibition, electrical stimulation of the medullary nucleus reticularis gigantocellularis (NRGc) evoked large amplitude, glycinergic inhibitory postsynaptic potentials (IPSPs) in cat motoneurons. The present study was directed to determine whether these IPSPs, that are specific to the state of active sleep, are modulated by opioid peptides. Accordingly, intracellular recordings were obtained from lumbar motoneurons of acute decerebrate cats during carbachol-induced motor inhibition while an opiate receptor antagonist, naloxone, was microiontophoretically released next to the recorded cells. Naloxone reversibly reduced by 26% the mean amplitude of NRGc-evoked IPSPs (1.9±0.2 mV (S.E.M.) vs. 1.4±0.2 mV; n=11, control and naloxone, respectively, p<0.05), but had no effect on the other waveform parameters of these IPSPs (e.g., latency-to-onset, latency-to-peak, duration, etc.). The mean resting membrane potential, input resistance and membrane time constant of motoneurons following naloxone ejection were not statistically different from those of the control. These data indicate that opioid peptides have a modulatory effect on NRGc-evoked IPSPs during carbachol-induced motor inhibition. We therefore suggest that endogenous opioid peptides may act as neuromodulators to regulate inhibitory glycinergic synaptic transmission at motoneurons during active sleep.

Development of glycinergic synaptic transmission to rat brain stem motoneurons.

Using an in vitro rat brain stem slice preparation, we examined the postnatal changes in glycinergic inhibitory postsynaptic currents (IPSCs) and passive membrane properties that underlie a developmental change in inhibitory postsynaptic potentials (IPSPs) recorded in hypoglossal motoneurons (HMs). Motoneurons were placed in three age groups: neonate (P0-3), intermediate (P5-8), and juvenile (P10-18). During the first two postnatal weeks, the decay time course of both unitary evoked IPSCs [mean decay time constant, taudecay = 17.0 +/- 1.6 (SE) ms in neonates and 5.5 +/- 0.4 ms in juveniles] and spontaneous miniature IPSCs (taudecay = 14.2 +/- 2.4 ms in neonates and 6.3 +/- 0.7 ms in juveniles) became faster. As glycine uptake does not influence IPSC time course at any postnatal age, this change most likely results from a developmental alteration in glycine receptor (GlyR) subunit composition. We found that expression of fetal (alpha2) GlyR subunit mRNA decreased, whereas expression of adult (alpha1) GlyR subunit mRNA increased postnatally. Single GlyR-channels recorded in outside-out patches excised from neonate motoneurons had longer mean burst durations than those from juveniles (18.3 vs. 11.1 ms). Concurrently, HM input resistance (RN) and membrane time constant (taum) decreased (RN from 153 +/- 12 MOmega to 63 +/- 7 MOmega and taum from 21.5 +/- 2.7 ms to 9.1 +/- 1.0 ms, neonates and juveniles, respectively), and the time course of unitary evoked IPSPs also became faster (taudecay = 22.4 +/- 1.8 and 7.7 +/- 0.9 ms, neonates vs. juveniles, respectively). Simulated synaptic currents were used to probe more closely the interaction between IPSC time course and taum, and these simulations demonstrated that IPSP duration was reduced as a consequence of postnatal changes in both the kinetics of the underlying GlyR channel and the membrane properties that transform the IPSC into a postsynaptic potential. Additionally, gramicidin perforated-patch recordings of glycine-evoked currents reveal a postnatal change in reversal potential, which is shifted from -37 to -73 mV during this same period. Glycinergic PSPs are therefore depolarizing and prolonged in neonate HMs and become faster and hyperpolarizing during the first two postnatal weeks.

Presynaptic inhibition by serotonin of glycinergic inhibitory synaptic currents in the rat brain stem.

1. With the use of a thin brain stem slice preparation, we recorded in visualized neonatal rat hypoglossal motoneurons unitary glycinergic inhibitory postsynaptic currents (IPSCs) that were evoked by extracellular stimulation of nearby interneurons. We found that 10 microM serotonin (5-HT) presynaptically inhibited this glycinergic synaptic transmission by 85.5%. 2. In the somata of presynaptic interneurons, 5-HT1A receptor activation potentiated inwardly rectifying K+ channels and inhibited voltage-activated calcium channels. 3. In contrast, the 5-HT1B receptor was primarily responsible for inhibition of evoked glycinergic IPSCs; a selective 5-HT1B receptor agonist, N-(3-trifluoromethylphenyl)piperazine (TFMPP, 10 microM), inhibited synaptic transmission by 97.3%. On the other hand, 5-HT1A receptor activation by (+)-8-OH-dipropylaminotetralin (8-OHDPAT, 1 microM) inhibited IPSCs by only 24.1%. A 5-HT1A antagonist, 1-(2-methyoxyphenyl)-4-[4-(2-phthalimido)-butyl]piperazine hydrobromide (NAN-190, 1 microM), had no effect on synaptic inhibition by 5-HT. 4. In the presence of tetrodotoxin (TTX) as well as TTX with cadmium (50 microM), we found that 5-HT1B receptor activation by TFMPP reduced the frequency of spontaneous miniature IPSCs (mIPSCs) without changing their mean amplitude. The results suggested that the 5-HT1B receptors activated at the presynaptic terminal inhibited synaptic transmission independent of inhibiting calcium influx through voltage-activated calcium channels. 5. These results indicate that activation of inwardly rectifying K+ channels and inhibition of voltage-activated calcium channels by 5-HT1A receptor activation do not constitute a main pathway for presynaptic inhibition by 5-HT of glycinergic synaptic transmission.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of adenosine A1 and A2 receptors differentially modulates calcium channels and glycinergic synaptic transmission in rat brainstem

Multiple types of calcium channels are responsible for calcium influx that triggers transmitter release in the mammalian CNS. To test the contribution of each calcium channel type on synaptic modulation, we recorded calcium currents from somata of presynaptic interneurons and unitary glycinergic postsynaptic currents in the rat brainstem. In interneuron somata, A1 receptor activation inhibited predominantly N-type (omega-conotoxin GVIA-sensitive) and, to a lesser extent, P-type (omega-agatoxin IVA-sensitive) channels. At the presynaptic terminal, N- and P-type channels mediated synaptic transmission. omega-CgTx occluded synaptic inhibition by A1 receptor activation, suggesting that synaptic inhibition was mediated predominantly by N-type channel inhibition. A2 receptor activation facilitated synaptic transmission, probably through potentiation of P-type channels at the presynaptic terminal.

Quantal analysis of synaptic potentials in neurons of the central nervous system.

This review concentrates on the results of quantal analysis at central synapses, particularly where the morphology and the electrophysiology of the same synaptic connection has been investigated. Results from quantal analysis at peripheral synapses are drawn on to help resolve some issues. The major topics covered in this review are the properties of the quantal synaptic potential and the junctional mechanisms involved in its generation

Inhibitory of glycine on spinal neurons in the cat.

Inhibitory of glycine on spinal neurons in the cat.