Physiology and pharmacology of native glycine receptors in developing rat auditory brainstem neurons

Abrupt Change in Electrophysiological Properties Begins From Postnatal Day 7 Before Hearing Onset in the Developing Mice Auditory Cortical Layer II/III Neurons

Objectives

In the developing auditory cortex, maturation of electrophysiological properties and cell types before and after hearing onset has been reported previously. However, the exact timing of firing pattern change has not been reported. In this study, firing pattern change was investigated from postnatal day 3 (P3) to P12 in auditory cortical layer II/III neurons to investigate whether firing pattern changes dramatically after a specific point during development.

Methods

ICR mice pups aged from P3 to P12 were sacrificed to obtain 300-mm-thick brain slices containing the primary auditory cortex. From cortical layer II/III neurons, the patterns of action potential firing generated by current injection were examined using whole cell current clamp technique and the characteristics of Na⁺ currents involved in action potential firing were investigated using whole cell voltage clamp technique.

Results

From P3 to P6, most cells did not show action potential firing (29 of 46 cells), and some cells responding to current injection showed a single action potential at the initial depolarizing current step (17 of 46 cells). This firing pattern changes from P7. From P7 to P9, cells begin to show regular spiking to current injection. The spiking frequency increased after P10. In studying Na⁺ current with whole cell voltage clamp, Na⁺ current densities increased gradually (32.0±2.0 pA/pF [P3–P6, n=7], 51.2±2.0 pA/pF [P7–P9, n=13], and 69.5±3.7 pA/pF [P10–P12, n=13]) in low external [Na⁺] condition. Na⁺ current recovery was accelerated and inactivation curves shifted to hyperpolarization with age.

Conclusion

As regular spiking cells were observed from P7 but never from P3 to P6, P7 might be regarded as an important milestone in the development of auditory cortical layer II/III neurons. This change might mainly result from the increase in Na⁺ current density.

GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit

KEY POINTS

The lateral superior olive (LSO), a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABAB Rs modulate Ca2+ influx into medial nucleus of the trapezoid body axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABAA Rs. RNA sequencing revealed highly abundant δ subunits, which are characteristic of extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter.

ABSTRACT

GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole-cell patch-clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABAA receptors (GABAA Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABAA Rs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca2+ influx into MNTB axon terminals following activation of presynaptic GABAB Rs. GABAB R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABAA Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABAA R agonist. RNA sequencing showed high mRNA levels for the δ-subunit in the LSO. Moreover, GABA transporters GAT-1 and GAT-3 appear to control extracellular GABA. Finally, we show an age-dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABAA Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

Effects of hypotaurine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

To understand the action and mechanism of hypotaurine, an immediate precursor of taurine, on orofacial nociceptive processing, we examined the direct effects and receptor types involved in hypotaurine-induced responses using the whole-cell patch clamp technique in the substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc) of immature mice. Under the condition of high-chloride pipette solution, hypotaurine elicited inward currents or upward deflections of membrane potential, which increased in a concentration-dependent manner (30-3000 μM) with the EC₅₀ of 663.8 and 337.6 μM, respectively. The responses to 300 µM hypotaurine were reproducible and recovered upon washout. The 300 µM hypotaurine-induced currents were maintained in the presence of TTX, CNQX, and AP5, indicating direct postsynaptic action of hypotaurine on SG neurons. Responses to both low (300 µM) and high (1 or 3 mM) concentrations of hypotaurine were completely and reversibly blocked by the glycine receptor antagonist strychnine (2 µM), but unaffected by the GABA_A receptor antagonist gabazine (3 µM) which blocks synaptic GABA_A receptors at low concentration. Furthermore, responses to 300 µM hypotaurine and a maximal concentration of glycine (3 mM) were not additive, indicating that hypotaurine and glycine act on the same receptor. Hypotaurine-induced currents were partially antagonized by picrotoxin (50 µM) which blocks homomeric glycine receptors and by bicuculline (10 µM) which is an antagonist of α2 subunit-containing glycine receptors. These results suggest that hypotaurine-induced responses were mediated by glycine receptor activation in the SG neurons and hypotaurine might be used as an effective therapeutics for orofacial pain.

Activation of synaptic and extrasynaptic glycine receptors by taurine in preoptic hypothalamic neurons

Taurine is an essential amino-sulfonic acid having a fundamental function in the brain, participating in both cell volume regulation and neurotransmission. Using a whole cell voltage patch clamp technique, the taurine-activated neurotransmitter receptors in the preoptic hypothalamic area (PHA) neurons were investigated. In the first set of experiments, different concentrations of taurine were applied on PHA neurons. Taurine-induced responses were concentration-dependent. Taurine-induced currents were action potential-independent and sensitive to strychnine, suggesting the involvement of glycine receptors. In addition, taurine activated not only α-homomeric, but also αβ-heteromeric glycine receptors in PHA neurons. Interestingly, a low concentration of taurine (0.5mM) activated glycine receptors, whereas a higher concentration (3mM) activated both glycine and gamma-aminobutyric acid A (GABAA) receptors in PHA neurons. These results suggest that PHA neurons are influenced by taurine and respond via glycine and GABAA receptors.

Development of glycinergic innervation to the murine LSO and SPN in the presence and absence of the MNTB

Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1^CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1^CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1^CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2⁺ boutons located on LSO somata were seen at all ages in Egr2; En1^CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1^CKO mice. On the postsynaptic side, adult Egr2; En1^CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.

Differential distribution of glycine receptor subtypes at the rat calyx of Held synapse

The properties of glycine receptors (GlyRs) depend upon their subunit composition. While the prevalent adult forms of GlyRs are heteromers, previous reports suggested functional α homomeric receptors in mature nervous tissues. Here we show two functionally different GlyRs populations in the rat medial nucleus of trapezoid body (MNTB). Postsynaptic receptors formed α1/β-containing clusters on somatodendritic domains of MNTB principal neurons, colocalizing with glycinergic nerve endings to mediate fast, phasic IPSCs. In contrast, presynaptic receptors on glutamatergic calyx of Held terminals were composed of dispersed, homomeric α1 receptors. Interestingly, the parent cell bodies of the calyces of Held, the globular bushy cells of the cochlear nucleus, expressed somatodendritic receptors (α1/β heteromers) and showed similar clustering and pharmacological profile as GlyRs on MNTB principal cells. These results suggest that specific targeting of GlyR β-subunit produces segregation of GlyR subtypes involved in two different mechanisms of modulation of synaptic strength.

Developmental changes in intrinsic excitability of principal neurons in the rat medial nucleus of the trapezoid body

The calyx of Held synapse is a giant axosomatic synapse that has a fast relay function within the sound localization circuit of the brainstem. In the adult, each principal neuron of the medial nucleus of the trapezoid body (MNTB) is contacted by a single calyx terminal. In rodents, the calyx of Held synapse forms around the third postnatal day (P3). Here, we studied the developmental changes in the intrinsic excitability of the principal neurons during the first postnatal week by making whole-cell recordings from brainstem slices. In slices from P0-1 rats, about 20% of the principal neurons were spontaneously active, whereas after P3, no spontaneously active cells were observed. Already at P0, principal neurons received both glutamatergic and GABAergic/glycinergic inputs. The occurrence of spontaneous action potentials depended upon the presence of spontaneous glutamatergic inputs; summation of only a few quanta was enough to reach action potential threshold. The main cause for this high excitability was a high resting membrane resistance, which decreased at least four-fold during the first postnatal week. A relatively slow decay of synaptic currents and a relatively depolarized membrane potential may have contributed as well. We conclude that the decrease in the excitability of principal neurons in the MNTB matches the increase of the strength of the synaptic inputs resulting from the formation and maturation of the calyx of Held synapse during the first postnatal week. This decrease in excitability will make it progressively more difficult for non-calyceal inputs to trigger action potentials.

Stoichiometry of Recombinant Heteromeric Glycine Receptors Revealed by a Pore-Lining Region Point Mutation

Heteromeric glycine receptors mediate synaptic inhibition in the caudal areas of the adult mammalian central nervous system (CNS). These channels resemble other receptors in the nicotinic superfamily in that they are pentamers, but may differ in that they contain alpha and beta subunits in a 3:2 rather than a 2:3 ratio. Evidence in favor of a 3alpha:2beta stoichiometry of heteromeric glycine receptors comes from biochemical data and from the expression of chimeric subunits. We investigated this question using a potentially more direct approach and mutated the highly conserved hydrophobic residues in the middle (position 9') of the pore-lining domain. This mutation increases agonist potency in all channels in the nicotinic superfamily and its effects are in first approximation proportional to the number of mutant subunit incorporated into the receptor. We expressed in HEK 293 cells wild-type glycine alpha1beta receptors or receptors bearing the 9' mutation on either the alpha or the beta subunit, using an alpha:beta plasmid ratio of 1:40 in the transfection. This resulted in negligible levels of contamination by homomeric alpha1 receptors, as proven by low picrotoxin potency and by the extreme rarity of high conductances in single channel recording. Our data show that the effects of the 9' mutation on the receptor sensitivity to glycine were more marked when the alpha subunit bore the mutation. The magnitude of the leftward shift in the agonist dose-response curve for the two mutant combinations was in agreement with a subunit stoichiometry of 3alpha:2beta.

Chloride cotransporters, chloride homeostasis, and synaptic inhibition in the developing auditory system

The role of glycine and GABA as inhibitory neurotransmitters in the adult vertebrate nervous system has been well characterized in a variety of model systems, including the auditory, which is particularly well suited for analyzing inhibitory neurotransmission. However, a full understanding of glycinergic and GABAergic transmission requires profound knowledge of how the precise organization of such synapses emerges. Likewise, the role of glycinergic and GABAergic signaling during development, including the dynamic changes in regulation of cytosolic chloride via chloride cotransporters, needs to be thoroughly understood. Recent literature has elucidated the developmental expression of many of the molecular components that comprise the inhibitory synaptic phenotype. An equally important focus of research has revealed the critical role of glycinergic and GABAergic signaling in sculpting different developmental aspects in the auditory system. This review examines the current literature detailing the expression patterns and function (chapter 1), as well as the regulation and pharmacology of chloride cotransporters (chapter 2). Of particular importance is the ontogeny of glycinergic and GABAergic transmission (chapter 3). The review also surveys the recent work on the signaling role of these two major inhibitory neurotransmitters in the developing auditory system (chapter 4) and concludes with an overview of areas for further research (chapter 5).

Glycine-activated chloride currents of neurons freshly isolated from the prefrontal cortex of young rats

Strychnine-sensitive glycine receptors (GlyR) play a major role in the excitability of CNS neurons and are also a major target of many drugs including some general anesthetics and ethanol. The prefrontal cortex (PFC) is an important substrate responsible for cognitive function and for sedation, as well as hypnosis (unconsciousness) which is induced by general anesthetics and ethanol. However, the functions and the physiological and pharmacological properties of GlyRs in mature PFC neurons have not been well studied. In this study, whole-cell currents induced by glycine (I(Gly)) were recorded from freshly isolated PFC neurons of Sprague-Dawley rats aged 5 to 39 postnatal days (neonatal, P5-12; weanling, P17-21 and peri-adolescent, P30-39). We found that most of the neurons examined were responsive to glycine and the response was concentration dependent. With the increase of age, the sensitivity to glycine was significantly decreased and the sensitivity to picrotoxin was significantly increased. Conversely, the changes in sensitivity to strychnine were not significant. Interestingly, I(Gly) of all age groups was suppressed (to different scope) by low concentrations of picrotoxin (≤ 30 μM), which selectively blocked α homomeric GlyRs. Conversely, about 20-65% of I(Gly) remained in the presence of 300 μM picrotoxin, suggesting the picrotoxin-resistant subtype the αβ heteromeric GlyR, was also present. These data provide the first evidence that there are at least two subtypes of functional GlyRs in the PFC neurons of young rats, and their physiological and pharmacological properties change substantially during maturation.

Glycine-induced currents are insensitive to the glycine receptor α1 subunit-specific blocker, cyanotriphenylborate, in older circling mice

The pharmacologic characteristics of glycine receptors (GlyRs) in the lateral superior olive (LSO) of circling mice, animal model for inherited deafness, were investigated using a GlyR α1 subunit-specific receptor blocker (cyanotriphenylborate [CTB]). There was a statistically significant age-dependent increase in the antagonistic effect of CTB in heterozygous (+/cir) mice. In postnatal (P)0-P3 heterozygous (+/cir) mice, glycine currents evoked by glycine puffs were reduced to 20.4±2.6, 37.1±3.1, and 63.9±2.5% at 0.1, 1, and 10 μM CTB (n=13) compared to controls, while the glycine currents were reduced to 22.3±3.5, 52.9±4.1, and 78.3±3.5% at 0.1, 1, and 10 μM CTB (n=7) in P8-P12 heterozygous (+/cir) mice. In contrast, the antagonistic effect of CTB was not strong and even less than that of younger animals in older homozygous (cir/cir) mice. In P0-P3 homozygous (cir/cir) mice, the extent of inhibition was 20.2±3.7, 37.8±4.3, and 66.8±4.2% at 0.1, 1, and 10 μM CTB (n=6) compared to controls, while the extent of inhibition was 18.7±2.4, 28.1±3.9, and 39.1±8.2% (n=6) in P8-P12 homozygous (cir/cir) mice. The age-dependent decrease in the antagonistic effect of CTB indicates the abnormal development of the α1 subunit-containing GlyRs in homozygous (cir/cir) mice.

Native glycine receptor subtypes and their physiological roles

The glycine receptor chloride channel (GlyR), a member of the pentameric Cys-loop ion channel receptor family, mediates inhibitory neurotransmission in the spinal cord, brainstem and retina. They are also found presynaptically, where they modulate neurotransmitter release. Functional GlyRs are formed from a total of five subunits (alpha1-alpha4, beta). Although alpha subunits efficiently form homomeric GlyRs in recombinant expression systems, homomeric alpha1, alpha3 and alpha4 GlyRs are weakly expressed in adult neurons. In contrast, alpha2 homomeric GlyRs are abundantly expressed in embryonic neurons, although their numbers decline sharply by adulthood. Numerous lines of biochemical, biophysical, pharmacological and genetic evidence suggest the majority of glycinergic neurotransmission in adults is mediated by heteromeric alpha1beta GlyRs. Immunocytochemical co-localisation experiments suggest the presence of alpha2beta, alpha3beta and alpha4beta GlyRs at synapses in the adult mouse retina. Immunocytochemical and electrophysiological evidence also implicates alpha3beta GlyRs as important mediators of glycinergic inhibitory neurotransmission in nociceptive sensory neuronal circuits in peripheral laminae of the spinal cord dorsal horn. It is yet to be determined why multiple GlyR synaptic subtypes are differentially distributed in these and possibly other locations. The development of pharmacological agents that can discriminate strongly between different beta subunit-containing GlyR isoforms will help to address this issue, and thereby provide important insights into a variety of central nervous system functions including retinal signal processing and spinal pain mechanisms. Finally, agents that selectively potentiate different GlyR isoforms may be useful as therapeutic lead compounds for peripheral inflammatory pain and movement disorders such as spasticity.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.

Regulation of excitation by glycine receptors

Our knowledge of glycine receptor (GlyR) regulation of excitation has advanced significantly in recent years. GlyRs are widespread in the CNS, are heterogeneous, and undergo developmental changes. Activation of GlyRs of immature neurons induces outflow of Cl( - ), membrane depolarization, neuronal excitation, calcium influx, and transmitter release, in contrast to the inhibitory effects these receptors have in mature neurons. Thus, GlyRs are important for neuronal excitability in both the developing and the mature CNS. This chapter is an overview of selective studies on the newly discovered roles of GlyRs in regulating neuronal excitation, and inhibition, particularly in the upper brain areas.

GABA(B) receptor activation modulates GABA(A) receptor-mediated inhibition in chicken nucleus magnocellularis neurons

Neurons of nucleus magnocellularis (NM), a division of avian cochlear nucleus that performs precise temporal encoding, receive glutamatergic excitatory input solely from the eighth nerve and GABAergic inhibitory input primarily from the ipsilateral superior olivary nucleus. GABA activates both ligand-gated Cl- channels [GABA(A) receptors (GABA(A)Rs)] and G protein-coupled receptors (GABA(B) receptors). The net effect of GABA(A)R-mediated input to NM is inhibitory, although depolarizing. Several studies have shown that this shunting, inhibitory GABAergic input can evoke action potentials in postsynaptic NM neurons, which could interfere with their temporal encoding. While this GABA-mediated firing is limited by a low-voltage-activated K+ conductance, we have found evidence for a second mechanism. We investigated modulation of GABA(A)R-mediated responses by GABA(B)Rs using whole cell recording techniques. Bath-applied baclofen, a GABA(B)R agonist, produced dose-dependent suppression of evoked inhibitory postsynaptic currents (eIPSCs). This suppression was blocked by CGP52432, a potent and selective GABA(B)R antagonist. Baclofen reduced the frequency but not the amplitude of miniature IPSCs (mIPSCs) and did not affect postsynaptic currents elicited by puff application of a specific GABA(A)R agonist muscimol, suggesting a presynaptic mechanism for the GABA(B)R-mediated modulation. Firing of NM neurons by synaptic stimulation of GABAergic inputs to NM was eliminated by baclofen. However, endogenous GABA(B)R activity in the presynaptic inhibitory terminals was not observed. We propose that presynaptic GABA(B)Rs function as autoreceptors, regulating synaptic strength of GABA(A)R-mediated inhibition, and prevent NM neurons from generating firing during activation of the inhibitory inputs.

Staggered development of GABAergic and glycinergic transmission in the MNTB

Maturation of some brain stem and spinal inhibitory systems is characterized by a shift from GABAergic to glycinergic transmission. Little is known about how this transition is expressed in terms of individual axonal inputs and synaptic sites. We have explored this issue in the rat medial nucleus of the trapezoid body (MNTB). Synaptic responses at postnatal days 5-7 (P5-P7) were small, slow, and primarily mediated by GABA(A) receptors. By P8-P12, an additional, faster glycinergic component emerged. At these ages, GABA(A), glycine, or both types of receptors mediated transmission, even at single synaptic sites. Thereafter, glycinergic development greatly accelerated. By P25, evoked inhibitory postsynaptic currents (IPSCs) were 10 times briefer and 100 times larger than those measured in the youngest group, suggesting a proliferation of synaptic inputs activating fast-kinetic receptors. Glycinergic miniature IPSCs (mIPSCs) increased markedly in size and decay rate with age. GABAergic mIPSCs also accelerated, but declined slightly in amplitude. Overall, the efficacy of GABAergic inputs showed little maturation between P5 and P20. Although gramicidin perforated-patch recordings revealed that GABA or glycine depolarized P5-P7 cells but hyperpolarized P14-P15 cells, the young depolarizing inputs were not suprathreshold. In addition, vesicle-release properties of inhibitory axons also matured: GABAergic responses in immature rats were highly asynchronous, while in older rats, precise, phasic glycinergic IPSCs could transmit even with 500-Hz stimuli. Thus development of inhibition is characterized by coordinated modifications to transmitter systems, vesicle release kinetics, Cl- gradients, receptor properties, and numbers of synaptic inputs. The apparent switch in GABA/glycine transmission was predominantly due to enhanced glycinergic function.

Ginkgolide B preferentially blocks chloride channels formed by heteromeric glycine receptors in hippocampal pyramidal neurons of rat

It has been found recently that the platelet activating factor antagonist ginkgolide B is a selective use-dependent blocker of glycine-gated chloride channels. GABAA receptor antagonist picrotoxin is known to block alpha homomeric glycine (Gly) receptors, being less effective for heteromeric alpha1/beta glycine receptors. Studying pyramidal hippocampal neurons of rat, we have confirmed that the effect of picrotoxin depends on the age of the animals. Its blocking ability was characterised by IC50=140+/-12 microM and IC50=354+/-43 microM for 7 and 14 days old rats, respectively, indicating at a possibly increased contribution of heteromeric receptors with animals age. We have revealed that the blocking action of ginkgolide B is subjected to a more drastic change in the same range of ages: the IC50 value is decreased from 1.6+/-0.2 microM for 7 days old rats to 0.27+/-0.01 microM for 14 days old rats. When measured on the background of ginkgolide B (1 microM), IC50 for picrotoxin was 92+/-16 microM. Taken together, these findings indicate that ginkgolide B has higher affinity to heteromeric Gly receptor-gated channels than to the homomeric ones.

Homogenous glycine receptor expression in cortical plate neurons and cajal-retzius cells of neonatal rat cerebral cortex

Glycinergic membrane responses have been described in cortical plate neurons (CPn) and Cajal-Retzius cells (CRc) during early neocortical development. In order to elucidate the functional properties and molecular identity of glycine receptors in these two neuronal cell types, we performed whole-cell patch-clamp recordings and subsequent single-cell multiplex reverse transcriptase-polymerase chain reaction (RT-PCR) analyses on visually identified neurons in tangential and coronal slices as well as in situ hybridizations of coronal slices from neonatal rat cerebral cortex (postnatal days 0-4). In both CPn and CRc the glycinergic agonists glycine, beta-alanine and taurine induced inward currents with larger current densities in CRc. The functional properties of these currents were similar between CPn and CRc. In both cell types the glycine receptor showed a higher affinity for glycine than for the glycinergic agonists beta-alanine and taurine. The glycinergic responses of both cells were blocked by the glycinergic antagonist strychnine and were unaffected by the GABAergic antagonist bicuculline (100 microM), the N-methyl-D-aspartic acid receptor antagonist (+/-)-2-amino-5-phosphonopentatonic acid (60 microM) and by picrotoxin (30 microM), an antagonist of alpha homomeric glycine receptors. Single-cell multiplex RT-PCR revealed the expression of glycine receptor alpha(2) and beta subunits in CPn and CRc, while no alpha(1) and alpha(3) subunits were observed. In situ hybridization histochemistry showed the expression of mRNAs for alpha(2) and beta subunits within the cortical plate and in large neurons of the marginal zone, while there were no signals for alpha(1) and alpha(3) subunits. In summary, these results suggest that CPn and CRc express glycine receptors with similar functional and pharmacological properties. The correlation of pharmacological properties and mRNA expression suggests that the glycine receptors in both cell types may consist of alpha(2)/beta heteromeric receptors.

Glycine receptors and glycinergic synaptic transmission in the deep cerebellar nuclei of the rat: a patch-clamp study

To clarify possible glycinergic transmission in the cerebellum, principal neurons in deep cerebellar nuclei (DCN) of sliced cerebella (200 microm in thickness) from rats (aged 2-14 days) were studied using whole cell patch-clamp techniques. When glycine (100 microM) was applied to the DCN neurons from a "Y tube," large outward currents were induced (average peak amplitude of about 600 pA at -40 mV). The currents were blocked by strychnine (1 microM) and showed a reversal potential of -62 mV, which was approximately the estimated Cl- equilibrium potential. The dose-response relation of the currents showed an apparent dissociation constant of 170 microM for glycine and Hill coefficient of 1.6. In the presence of 6-cyano-7-nitroquinoziline-2, 3-dione (CNQX), d-(-)-2-amino-5-phosphonovaleric acid (APV) and bicuculline, which antagonize amino-3-hydroxy-5-methyl-isoxazol-propionate (APMA), N-methyl-d-aspartate (NMDA), and GABAA receptors, respectively, postsynaptic currents sensitive to strychnine (1 microM) were induced in DCN neurons by external perfusion of 20 mM K+ saline. Electrical stimulation of surrounding tissues in DCN evoked definite inhibitory postsynaptic currents (IPSCs) in these neurons. The IPSCs had a reversal potential of -62 mV and showed sensitivities to strychnine and tetrodotoxin. Thus this study has revealed that strychnine-sensitive glycine receptors are expressed in neurons of the DCN of rats and that glycinergic transmission mediated by these receptors is functional in these neurons from stages immediately after birth. The glycinergic innervations are presumably supplied by small interneurons located in the DCN.

Differences in glycinergic mIPSCs in the auditory brain stem of normal and congenitally deaf neonatal mice

We have investigated the fundamental properties of central auditory glycinergic synapses in early postnatal development in normal and congenitally deaf (dn/dn) mice. Glycinergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using patch-clamp methods in neurons from a brain slice preparation of the medial nucleus of the trapezoid body (MNTB), at 12-14 days postnatal age. Our results show a number of significant differences between normal and deaf mice. The frequency of mIPSCs is greater (50%) in deaf versus normal mice. Mean mIPSC amplitude is smaller in deaf mice than in normal mice (mean mIPSC amplitude: deaf, 64 pA; normal, 106 pA). Peak-scaled fluctuation analysis of mIPSCs showed that mean single channel conductance is greater in the deaf mice (deaf, 64 pS; normal, 45 pS). The mean decay time course of mIPSCs is slower in MNTB neurons from deaf mice (mean half-width: deaf, 2.9 ms; normal, 2.3 ms). Light- and electron-microscopic immunolabeling results showed that MNTB neurons from deaf mice have more (30%) inhibitory synaptic sites (postsynaptic gephyrin clusters) than MNTB neurons in normal mice. Our results demonstrate substantial differences in glycinergic transmission in normal and congenitally deaf mice, supporting a role for activity during development in regulating both synaptic structure (connectivity) and the fundamental (quantal) properties of mIPSCs at central glycinergic synapses.

Glycine facilitates transmitter release at developing synapses: a patch clamp study from Purkinje neurons of the newborn rat

Synaptic currents in immature Purkinje cells from rats on postnatal days 0-14 (P0-P14) were studied using whole-cell patch-electrodes applied to cerebellar slices (200 micro m in thickness). Purkinje cells (held at -40 mV) showed excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) spontaneously. From P2 to P12 the frequencies of miniature EPSCs and miniature IPSCs in the Purkinje cells increased by 10-fold or more, suggesting progressive formation of functional synapses during this period. Application of glycine (100 micro M) to an immature Purkinje cell at P3-10 immediately increased the frequencies of both EPSCs and IPSCs. The effects of glycine showed maximum at P5-6 for EPSCs and at P9-10 for IPSCs and decreased thereafter. Facilitatory effects of glycine were suppressed by strychnine (1 micro M), a specific blocker of the ionotropic glycine receptor, while the effects were also induced by other glycinergic agonists, including alpha-L-alanine (1 mM), L-serine (1 mM) and taurine (500 micro M). The site of glycinergic effects was studied by removing the action potential generation in cerebellar slices. Following the addition of tetrodotoxin (TTX, 1 micro M), the glycine-induced facilitation of EPSC almost disappeared, while that of IPSC remained (i.e. miniature IPSCs) and reached more than half of the value without TTX. These findings suggest that the ionotropic glycinergic receptors are expressed transiently but profoundly in the developing cerebellum, and that the distributions of these receptors causing excitation are different at excitatory and inhibitory presynaptic neurons. The glycine receptors may play distinct roles in the maturation and organization of cerebellar neural circuits.

Functional glycine receptor maturation in the absence of glycinergic input in dopaminergic neurones of the rat substantia nigra

The postnatal maturation pattern of glycine receptor channels (GlyRs) expressed by dopaminergic (DA) neurones of the rat substantia nigra pars compacta (SNc) was investigated using single-channel and whole-cell patch-clamp recordings in brain slices from rats aged 7-21 postnatal days (P). In neonatal rats (P7-P10), GlyRs exhibited a main conductance state of 100-110 pS with a mean open time of 16 ms. In juvenile rats (P19-P22), both the GlyR main conductance state (46-55 pS) and the mean open time (6.8 ms) were decreased. In neonatal rats, application of 30 microM picrotoxin, which is known to block homomeric GlyRs, strongly reduced glycine-evoked responses, while it was much less effective in juvenile rats. These results suggest that these GlyRs correspond functionally to alpha(2) homomeric GlyRs in neonatal rats and alpha(1)/beta heteromeric GlyRs in juvenile rats. A drastic but transient decrease in the glycine responsiveness of DA neurones occurred around P17 concomitant to the functional switch from the homomeric state to the heteromeric state. This age corresponds to a maturation phase for DA neurones. The application of 1 microM gabazine blocked spontaneous or evoked inhibitory synaptic current, while the addition of 1 microM strychnine had no effect, suggesting a lack of functional glycinergic synapses on DA neurones. Although it has been proposed that taurine is co-released with GABA at GABAergic synapses on DA neurones, in the present study the stimulation of GABAergic fibres failed to activate GlyRs. Blockade of taurine transporters and applications of high K(+) and hyposmotic solutions were also unable to induce any strychnine-sensitive current. We conclude that functional maturation of GlyRs can occur in the absence of any detectable GlyR activation in DA neurones of the SNc.

Pharmacological characterization of glycine-gated chloride currents recorded in rat hippocampal slices

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus has been overlooked, largely due to the misconception that GlyR expression ceases early during development and to few functional studies demonstrating their presence. As a result, little is known regarding the physiological and pharmacological properties of native GlyRs expressed by hippocampal neurons. In this study, we used pharmacological tools and whole cell patch-clamp recordings of CA1 pyramidal cells and interneurons in acutely prepared hippocampal slices from 3- to 4-wk old rats to characterize these understudied receptors. We show that glycine application to recorded pyramidal cells and interneurons elicited strychnine-sensitive chloride-mediated currents (I(gly)) that did not completely desensitize in the continued presence of agonist but reached a steady state at 45-60% of the peak amplitude. Additionally, the inhibitory amino acid, taurine, which has been shown to activate GlyRs in other systems, activated GlyRs expressed by both pyramidal cells and interneurons, although with much less potency than glycine, having an EC(50) 10-fold higher. To examine the potential subunit composition of hippocampal GlyRs, we tested the effect of the GABA(A) receptor antagonist, picrotoxin, on I(gly) recorded from both cell types. At low micromolar concentrations of picrotoxin (< or =100 microM), which selectively block alpha homomeric GlyRs, I(gly) was partially attenuated in both cell types, indicating that alpha homomeric receptors are expressed by pyramidal cells and interneurons. At picrotoxin concentrations < or =1 mM, approximately 10-20% of the whole cell current remained, suggesting that alphabeta heteromeric GlyRs are also expressed because this subtype of GlyR is relatively resistant to picrotoxin antagonism. Finally, we examined whether hippocampal GlyRs are modulated by zinc. Consistent with previous reports in other preparations, zinc elicited a bidirectional modulation of GlyRs, with physiological zinc concentrations (1-100 microM) increasing whole cell currents and concentrations >100 microM depressing them. Furthermore, the same concentration of zinc that potentiates I(gly) suppressed currents mediated by the N-methyl-D-aspartate subtype of the glutamate receptor. Thus we provide a pharmacological characterization of native GlyRs expressed by both major neuron types in hippocampus and show that these receptors can be activated by taurine, an amino acid that is highly concentrated in hippocampus. Furthermore, our data suggest that at least two GlyR subtypes are present in hippocampus and that GlyR-mediated currents can be potentiated by zinc at concentrations that suppress glutamate-mediated excitability.

Centrally administered corticotropin-releasing hormone and peripheral injections of strychnine hydrochloride potentiate the acoustic startle response in preweanling rats

Attempts to condition fear potentiation of startle (FPS) in rats younger than 23 days of age have not been successful, regardless of the type of aversively conditioned stimulus used (P. S. Hunt, R. Richardson, & B. A. Campbell, 1994; R. Richardson, G. Paxinos, & J. Lee, 2000; R. Richardson & A. Vishney, 2000). In the present study, the authors report that peripheral injections of strychnine hydrochloride, a glycine receptor antagonist, and intracerebroventricular infusions of corticotropin releasing hormone (CRH) both potentiated the acoustic startle response (ASR) in 16-18-day-old rats. Because strychnine and CRH have distinct sites of activation in the primary startle pathway, it can be concluded that this pathway is functional and modifiable in rats younger than 23 days of age. This finding suggests that the failure to observe conditioned FPS in preweanling rats is due to an immaturity of the secondary fear circuit responsible for enhancing the ASR during a fear state.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Characterisation of inhibitory and excitatory postsynaptic currents of the rat medial superior olive

The medial superior olive (MSO) is part of the binaural auditory pathway, receiving excitatory projections from both cochlear nuclei and an inhibitory input from the ipsilateral medial nucleus of the trapezoid body (MNTB). We characterised the excitatory and inhibitory synaptic currents of MSO neurones in 3- to 14-day-old rats using whole-cell patch-clamp methods in a brain slice preparation.A dual component EPSC was mediated by AMPA and NMDA receptors. The AMPA receptor-mediated EPSC decayed with a time constant of 1.99+/-0.16 ms (n = 8). Following blockade of glutamate receptors, a monosynaptic strychnine-sensitive response was evoked on stimulation of the MNTB, indicative of a glycine receptor-mediated IPSC. GABAA receptors contributed to IPSCs in rats under 6 days old (bicuculline blocked 30% of the IPSC). In older rats little or no bicuculline-sensitive component was detectable, except in the presence of flunitrazepam. These glycinergic IPSCs showed a reversal potential that varied with changes in [Cl-]i, as predicted by the Nernst equation. The IPSC exhibited two developmentally relevant changes. (i) At around postnatal day 6, the GABAA receptor-mediated component declined, leaving a predominant glycine-mediated IPSC. The isolated glycinergic IPSC decayed with time constants of 7.8+/-0.3 and 38.3+/-1.7 ms, with the slower component contributing 7.8+/-0.6% of the peak amplitude (n = 121, 3-11 days old, -70 mV, 25 deg C). (ii) After day 11 the IPSC fast decay accelerated to 3.9+/-0.3 ms (n = 12) and the magnitude of the slow component declined to less than 1%. Spontaneous miniature glycinergic IPSCs (mIPSCs) were variable in amplitude and were of large conductance (1.83+/-0.19 nS, n = 8). The amplitude was unchanged on lowering [Ca2+]o. The time course of evoked and spontaneous miniature glycinergic IPSCs were compared. The 10-90% rise times were 0.7 and 0.6 ms, respectively. The evoked IPSC decayed with a fast time constant of 7.2+/-0.7 ms, while the mIPSC decayed with a fast time constant of 5.3+/-0.4 ms in the same seven cells.The glycinergic IPSC decay was voltage dependent with an e-fold change over 118 mV. The temperature dependence of the IPSC decay indicated a Q10 value of 2. Picrotoxin and cyanotriphenylborate had little or no effect on IPSCs from 6- to 14-day-old animals, implying homomeric channels are rare. We conclude that the MSO receives excitatory inputs mediated by AMPA and NMDA receptors and a strong glycinergic IPSC which has a significant contribution from GABAA receptors in neonatal rats. Functionally, the IPSC could increase membrane conductance during the decay of binaural glutamatergic EPSCs, thus refining coincidence detection and interaural timing differences.

Direct inhibition of glycine receptors by genistein, a tyrosine kinase inhibitor

Genistein, a tyrosine kinase inhibitor, has been widely used to examine potential effects of protein tyrosine kinase (PTK)-mediated regulation of receptor/channel function. Alteration of ion channel function in the presence of genistein has typically led to the conclusion that PTK regulates the activity of the channel under investigation. In the present report, we have assessed the possibility that genistein directly inhibits the glycine receptor, independent of effects on protein tyrosine kinase. Coapplication of genistein with glycine reversibly inhibited the strychnine-sensitive, glycine-activated current recorded from hypothalamic neurons. The time course of genistein action was rapid (within ms). Equilibration of genistein in the intracellular solution did not affect the ability of extracellularly applied genistein to inhibit the glycine response. Glycine concentration-response profiles generated in the absence and presence of genistein indicated the block was due to non-competitive antagonism. The genistein effect also displayed voltage-dependence. Daidzein, an analog of genistein that does not block protein kinases, also inhibited glycine-activated current. Coapplication of lavendustin A, a specific inhibitor of PTK, had no effect on the glycine response. Our results demonstrate that the tyrosine kinase inhibitor genistein has a direct inhibitory effect on glycine receptors that is not mediated via inhibition of PTK.

Expression of glycine receptor subunit mRNAs in the rat cochlear nucleus

The distribution of glycine receptor subunit mRNAs in the cochlear nucleus of the adult rat was examined using radioactive in situ hybridization. Expression was compared among six cell types by counting silver grains over somata. Expression of the immature alpha2 subunit was not above the threshold for detection in any neurons. Levels of expression of mature subunits varied among different cell types. Spherical bushy, small cell cap/shell neurons and fusiform cells had high expression of glycine receptor mRNA for alpha1, alpha 3 and beta subunits. Octopus cells and corn cells had high expression for alpha 3 and beta subunits, and only moderate expression for alpha1 subunit. Granule cells located between the dorsal and ventral cochlear nucleus had moderate expression of alpha 3 and beta subunits and no detectable alpha1 expression. These patterns of expression predict differences in glycinergic pharmacological properties between the cochlear nucleus neurons.

Physiology and pharmacology of native glycine receptors in developing rat ventral tegmental area neurons

Properties of whole-cell glycine-activated currents (I(Gly)) of neurons freshly isolated from ventral tegmental area of rats between postnatal day (P) 0 and 40 (P0-P40) were examined using the gramicidin-perforated patch technique. I(Gly) was present throughout the period of investigation and displayed age-related alternations in its kinetics, reversal potentials (E(Gly)) and sensitivity to antagonists. During development, the time constant for desensitization of I(Gly) decreased about two-fold, and the E(Gly) shifted to more negative potentials. Specifically, E(Gly) was -29 mV in neonatal (P0-P7) and decreased to -50 mV in adult (P24-P40) neurons. While the neonatal glycine receptors (GlyRs) are resistant to strychnine, the adult GlyRs are sensitive to STR. The values for 50% inhibition (IC(50)) of I(Gly) was 110 and 12 nM in P0-P3 and P24-P40 neurons, respectively. Picrotoxin, on the other hand, was more effective in blocking neonatal neurons. The IC(50) values were 17 and 231 microM for the neonatal and adult neurons, respectively. These observations are discussed in terms of developmental changes of subunit composition of GlyRs and of intracellular Cl(-) concentrations.