Modulation of calcium by inhibitory systems in the developing auditory midbrain

Postnatal Maturation of Glutamatergic Inputs onto Rat Jaw-closing and Jaw-opening Motoneurons

Motoneurons that innervate the jaw-closing and jaw-opening muscles play a critical role in oro-facial behaviors, including mastication, suckling, and swallowing. These motoneurons can alter their physiological properties through the postnatal period during which feeding behavior shifts from suckling to mastication; however, the functional synaptic properties of developmental changes in these neurons remain unknown. Thus, we explored the postnatal changes in glutamatergic synaptic transmission onto the motoneurons that innervate the jaw-closing and jaw-opening musculatures during early postnatal development in rats. We measured miniature excitatory postsynaptic currents (mEPSCs) mediated by non-NMDA receptors (non-NMDA mEPSCs) and NMDA receptors in the masseter and digastric motoneurons. The amplitude, frequency, and rise time of non-NMDA mEPSCs remained unchanged among postnatal day (P)2-5, P9-12, and P14-17 age groups in masseter motoneurons, whereas the decay time dramatically decreased with age. The properties of the NMDA mEPSCs were more predominant at P2-5 masseter motoneurons, followed by reduction as neurons matured. The decay time of NMDA mEPSCs of masseter motoneurons also shortened remarkably across development. Furthermore, the proportion of NMDA/non-NMDA EPSCs induced in response to the electrical stimulation of the supratrigeminal region was quite high in P2-5 masseter motoneurons, and then decreased toward P14-17. In contrast to masseter motoneurons, digastric motoneurons showed unchanged properties in non-NMDA and NMDA EPSCs throughout postnatal development. Our results suggest that the developmental patterns of non-NMDA and NMDA receptor-mediated inputs vary among jaw-closing and jaw-opening motoneurons, possibly related to distinct roles of respective motoneurons in postnatal development of feeding behavior.

Developmental changes in GABAergic and glycinergic synaptic transmission to rat motoneurons innervating jaw-closing and jaw-opening muscles

Trigeminal motoneurons (MNs) innervating the jaw-closing and jaw-opening muscles receive numerous inhibitory synaptic inputs from GABAergic and glycinergic neurons, which are essential for oromotor functions, such as the orofacial reflex, suckling, and mastication. The properties of the GABAergic and glycinergic inputs of these MNs undergo developmental alterations during the period in which their feeding behavior proceeds from suckling to mastication; however, the detailed characteristics of the developmental patterns of GABAergic and glycinergic transmission in these neurons remain to be elucidated. This study was conducted to investigate developmental changes in miniature inhibitory postsynaptic currents (mIPSCs) in masseter (jaw-closing) and digastric (jaw-opening) MNs using brainstem slice preparations obtained from Wistar rats on postnatal day (P)2-5, P9-12, and P14-17. The frequency and amplitude of glycinergic mIPSCs substantially increased with age in both the masseter and digastric MNs. The rise time and decay time of glycinergic mIPSCs in both MNs decreased during development. In contrast, the frequency of GABAergic components in masseter MNs was higher at P2-5 than at P14-17, whereas that in the digastric MNs remained unchanged throughout the postnatal period. The proportion of currents mediated by GABA-glycine co-transmission was higher at P2-5, and then it decreased with age in both MNs. These results suggest that characteristics related to the development of inhibitory synaptic inputs differ between jaw-closing and jaw-opening MNs and between GABAergic and glycinergic currents. These distinct developmental characteristics may contribute to the development of feeding behaviors.

Impact of cochlear ablation on calretinin and synaptophysin in the gerbil anteroventral cochlear nucleus before the hearing onset

Mammalian auditory system undergoes many structural and functional modifications during postnatal development, which are dependent on the relationship between auditory nerve fibers and their nuclei. In the present study, the cochlea of Meriones unguiculatus was ablated unilaterally on postnatal day 5 or 9 (P5 or P9), before the onset of hearing. Histochemical analysis of synaptophysin (SYN) and calretinin (CR) in anterior anteroventral cochlear nucleus (AVCN-A) was performed to analyze whether unilateral cochlea ablation induces changes in the auditory terminal endings and somata of spherical bushy cells (SBCs). During the period of postnatal development, CR-labeling was evident in somata of SBCs and in auditory nerve terminals. SYN was most apparent in puncta encircled cell bodies, progressing with age. Cochlear removal at P5 induced a decrease in CR-labeling in SBCs somata 6 h and 48 h post-lesion; whereas, ablation at P9 increased the somatic CR-labeling in the lesioned AVCN-A after 24 and 48 h post-lesion. The SYN-labeled synaptic puncta were remarkably reduced in the AVCN-A of P5- and P9-cochlea-ablated gerbils with stronger effects in P5 animals (a 50% reduction after 48 h). Interestingly, a significant increase in the SYN-immunolabeled puncta was found after 48 h compared to 24 h in the lesioned AVCN-A of P9 gerbils, indicating reactive synaptogenesis. Our study shows, that following the destruction of the cochlea at different postnatal periods, the CR- and SYN-labeling are differentially influenced in the AVCN-A, which in turn coincides with different critical developmental periods before the onset of hearing.

Sodium salicylate reduces the level of GABAB receptors in the rat's inferior colliculus

Previous studies have indicated that sodium salicylate (SS) can cause hearing abnormalities through affecting the central auditory system. In order to understand central effects of the drug, we examined how a single intraperitoneal injection of the drug changed the level of subunits of the type-B γ-aminobutyric acid receptor (GABAB receptor) in the rat's inferior colliculus (IC). Immunohistochemical and western blotting experiments were conducted three hours following a drug injection, as previous studies indicated that a tinnitus-like behavior could be reliably induced in rats within this time period. Results revealed that both subunits of the receptor, GABABR1 and GABABR2, reduced their level over the entire area of the IC. Such a reduction was observed in both cell body and neuropil regions. In contrast, no changes were observed in other brain structures such as the cerebellum. Thus, a coincidence existed between a structure-specific reduction in the level of GABAB receptor subunits in the IC and the presence of a tinnitus-like behavior. This coincidence likely suggests that a reduction in the level of GABAB receptor subunits was involved in the generation of a tinnitus-like behavior and/or used by the nervous system to restore normal hearing following application of SS.

The level and distribution of the GABA(B)R1 and GABA(B)R2 receptor subunits in the rat's inferior colliculus

The type B γ-aminobutyric acid receptor (GABA(B) receptor) is an important neurotransmitter receptor in the midbrain auditory structure, the inferior colliculus (IC). A functional GABA(B) receptor is a heterodimer consisting of two subunits, GABA(B)R1 and GABA(B)R2. Western blotting and immunohistochemical experiments were conducted to examine the expression of the two subunits over the IC including its central nucleus, dorsal cortex, and external cortex (ICc, ICd, and ICx). Results revealed that the two subunits existed in both cell bodies and the neuropil throughout the IC. The two subunits had similar regional distributions over the IC. The combined level of cell body and neuropil labeling was higher in the ICd than the other two subdivisions. Labeling in the ICc and ICx was stronger in the dorsal than the ventral regions. In spite of regional differences, no defined boundaries were formed between different areas. For both subunits, the regional distribution of immunoreactivity in the neuropil was parallel to that of combined immunoreactivity in the neuropil and cell bodies. The density of labeled cell bodies tended to be higher but sizes of cell bodies tended to be smaller in the ICd than in the other subdivisions. No systematic regional changes were found in the level of cell body immunoreactivity, except that GABA(B)R2-immunoreactive cell bodies in the ICd had slightly higher optic density (OD) than in other regions. Elongated cell bodies existed throughout the IC. Many labeled cell bodies along the outline of the IC were oriented in parallel to the outline. No strong tendency of orientation was found in labeled cell bodies in ICc. Regional distributions of the subunits in ICc correlated well with inputs to this subdivision. Our finding regarding the contrast in the level of neuropil immunoreactivity among different subdivisions is consistent with the fact that the GABA(B) receptor has different pre- and postsynaptic functions in different IC regions.

Synaptic activity-induced Ca(2+) signaling in avian cochlear nucleus magnocellularis neurons

Neurons of the avian cochlear nucleus magnocellularis (NM) receive glutamatergic inputs from the spiral ganglion cells via the auditory nerve and feedback GABAergic inputs primarily from the superior olivary nucleus. We investigated regulation of Ca(2+) signaling in NM neurons with ratiometric Ca(2+) imaging in chicken brain slices. Application of exogenous glutamate or GABA increased the intracellular Ca(2+) concentration ([Ca(2+)](i)) in NM neurons. Interestingly, GABA-induced Ca(2+) responses persisted into neuronal maturation, in both standard and energy substrate enriched artificial cerebrospinal fluid. More importantly, we found that electrical stimulation applied to the glutamatergic and GABAergic afferent fibers innervating the NM was able to elicit transient [Ca(2+)](i) increases in NM neurons, and the amplitude of the Ca(2+) responses increased with increasing frequency and duration of the electrical stimulation. Antagonists for ionotropic glutamate receptors significantly blocked these [Ca(2+)](i) increases, whereas blocking GABA(A) receptors did not affect the Ca(2+) responses, suggesting that synaptically released glutamate but not GABA induced the Ca(2+) signaling in vitro. Furthermore, activation of GABA(A) receptors with exogenous agonists inhibited synaptic activity-induced [Ca(2+)](i) increases in NM neurons, suggesting a role of GABA(A) receptors in the regulation of Ca(2+) homeostasis in the avian cochlear nucleus neurons.

The physiological role of pre- and postsynaptic GABA(B) 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 alpha-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.

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1–P4 neonatal rats and inhibitory in P9–P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

Developmental changes in distribution of γ-aminobutyric acid- and glycine-immunoreactive boutons on rat trigeminal motoneurons. I. Jaw-closing motoneurons

We have previously described the distribution pattern of inhibitory synapses on rat jaw-closing (JC) alpha- and gamma-motoneurons. In the present study, we investigated developmental changes in inhibitory synapses on JC motoneurons. We performed a quantitative ultrastructural analysis of putative inhibitory synaptic boutons on JC motoneuron somata by using postembedding immunogold labeling for GABA and glycine. In total, 206, 350, and 497 boutons contacting JC motoneuron somata were analyzed at postnatal days 2 (P2), 11 (P11) and 31 (P31), respectively. The size of the somata increased significantly during postnatal development. The size distribution was bimodal at P31. Mean length of the boutons and percentage of synaptic covering also increased during postnatal development, whereas bouton density did not differ significantly among the three age groups. Synaptic boutons on the somata of JC alpha-motoneurons could be classified into four types: boutons immunoreactive for 1) GABA only, 2) glycine only, 3) both GABA and glycine, and 4) neither GABA nor glycine. There was no developmental change in the proportion of putative inhibitory boutons to the total number of studied boutons. However, the glycine-only boutons increased significantly (15.1% to 27.3%), and the GABA-only boutons decreased significantly (17.7% to 2.6%) during the period from P11 to P31. Our ultrastructural data indicate that the inhibitory synaptic input to JC motoneurons is developmentally regulated and that there is a postnatal switch from GABA to glycine. The postnatal changes revealed in the present study could play an important role in the maturation of the oral motor system.

Unilateral cochlear ablation produces greater loss of inhibition in the contralateral inferior colliculus

Bilateral cochlear ablation leads to a profound weakening of synaptic inhibition within the inferior colliculus (IC) of gerbils [Vale & Sanes (2000) J. Neurosci., 20, 1912-1921]. To examine whether unilateral deafening leads to similar functional alterations, we studied the effect of unilateral cochlear ablation on inhibitory synaptic properties both ipsilateral and contralateral to the deafened ear. Lateral lemniscal and commissure of the IC-evoked inhibitory postsynaptic currents (IPSCs) were recorded in an IC brain slice preparation using whole-cell and gramicidin perforated-patch electrodes in the presence of kynurenic acid. Unilateral cochlear ablation led to a 23 mV depolarizing shift in the IPSC equilibrium potential for IC neurons contralateral to the deafened ear, but only a 10 mV depolarization in the ipsilateral IC. Lateral lemniscal-evoked inhibitory synaptic conductance declined significantly in the ipsilateral and contralateral IC, whereas commissural-evoked inhibitory synaptic conductance declined only contralateral to the ablated cochlea. An analysis of paired-pulse facilitation showed that inhibitory transmitter release was more affected ipsilateral to the ablated cochlea. Thus, unilateral cochlear ablation modifies inhibitory synapses in the inferior colliculus, but these changes appear to be dominated by postsynaptic alterations in the contralateral IC, and by presynaptic changes in the ipsilateral IC.

Activity-dependent organization of inhibitory circuits: lessons from the auditory system

In contrast to our detailed knowledge about the development and plasticity of excitatory neuronal circuits, little is known about the development of inhibitory circuits. Recent studies from the developing mammalian auditory system have revealed the presence of substantial activity-dependent synaptic reorganization in several inhibitory pathways. These studies importantly shed some new light on the general rules and cellular mechanisms that manage the organization of precise inhibitory circuits in the developing brain.

Glycine mediated alterations in intracellular pH

Glycinergic transmission shapes the coding properties of the lateral superior olivary nucleus (LSO). We investigated intracellular pH responses in the LSO to glycine using BCECF-AM in brain slices. With extracellular bicarbonate, glycine produced an alkalinization followed by an acidification while, in the nominal absence of bicarbonate, glycine produced acidifications. Separately, in whole-cell recordings from LSO neurons, glycine caused hyperpolarization followed by long-lasting depolarization. While the bicarbonate-dependent intracellular alkalinization could be related to chloride/bicarbonate exchange, bicarbonate-independent acidification may be triggered by depolarization.

The effect of bilateral deafness on excitatory and inhibitory synaptic strength in the inferior colliculus

The consequences of deafness on the central auditory nervous system have been examined at many levels, from molecular to functional. However, there has never been a direct and selective measurement of excitatory synaptic function following total hearing loss. In the present study, gerbils were deafened at postnatal day 9, an age at which there is no deafferentation-induced cell death of ventral cochlear nucleus neurons. One to five days after bilateral cochlear ablation, the amplitude of evoked excitatory postsynaptic currents (EPSC) was measured with whole-cell voltage-clamp recordings in an inferior colliculus (IC) brain slice preparation in response to electrical stimulation of the ipsilateral lateral lemniscus (LL) or the commissure of the inferior colliculus (CIC). Deafness resulted in larger LL- and CIC-evoked EPSC amplitudes and durations. This result was observed at a depolarized holding potential. In addition, deafness caused a decrease in excitatory neurotransmitter release at the LL pathway, as assessed with a paired-pulse stimulation protocol. In contrast to its effect on excitatory synapses, bilateral cochlear ablation reduced inhibitory synaptic strength in IC neurons. The effects included a postsynaptic decrease in IPSC conductance, a 25-mV depolarization in the IPSC equilibrium potential and a decrease of neurotransmitter release. Thus normal innervation differentially affects excitatory and inhibitory synaptic strength in IC neurons, and these changes may contribute to alterations in auditory coding properties following sensory deprivation.

Contribution of AMPA and NMDA receptors to excitatory responses in the inferior colliculus

Brain slice studies of neurons in the central nucleus of the inferior colliculus (ICC) indicate that excitatory responses evoked by electrical stimulation of the lateral lemniscus consist of two components, an early, rapid response mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and a later, a slower one mediated by N-methyl-D-aspartate (NMDA) receptors. The early response can be selectively blocked by AMPA receptor antagonists (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX]; or 6-cyano-7-nitroquinoxaline-2,3-dione) [CNQX], and the later one by NMDA receptor antagonists ((+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CPP]; or (+/-)-2-amino-5-phosphonovaleric acid) [APV]. Both AMPA and NMDA receptor-mediated responses can be elicited at resting potential, although the NMDA response is voltage dependent and makes a greater contribution when the cell membrane is depolarized. In vivo studies indicate that both AMPA and NMDA receptors contribute to sound-evoked responses. Both AMPA and NMDA receptor antagonists reduce the firing rate of single neurons in the ICC to contralaterally presented tones. Both classes of antagonist lower evoked activity over a wide range of sound intensities from threshold to maximum sound pressure levels. Thus, both NMDA and AMPA receptors contribute to responses over the full dynamic range of auditory sensitivity. The AMPA receptor antagonist, NBQX, is more effective than the NMDA receptor antagonist, CPP, in blocking responses of onset cells. Furthermore, NBQX and CPP have preferential effects in blocking the early or late responses of neurons that exhibited sustain activity to a 100 ms tone. Excitatory responses to sinusoidally amplitude-modulated stimuli are also reduced by application of either AMPA or NMDA antagonists. However, the synchrony of firing of action potentials to the modulation period (vector strength) is largely unaffected. The data suggest that the synchrony of firing of neurons in the inferior colliculus is determined primarily by the pattern of activity at lower levels of the auditory pathway and/or the local intrinsic properties of the cells.

Glycinergic and GABAergic calcium responses in the developing lateral superior olive

The lateral superior olive (LSO), a binaural nucleus involved in sound localization, receives tonotopically organized inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). During development, the tonotopic organization of this glycinergic/GABAergic MNTB-LSO pathway is established by activity-dependent axonal reorganization. However, the underlying mechanisms by which this reorganization takes place have remained largely unknown. As cytosolic calcium is one of the most important second messengers responsible for inducing synaptic plasticity and reorganization, we examined whether and how activity in the MNTB-LSO pathway changes the intracellular calcium concentration ([Ca2+]i) in developing LSO neurons. By applying calcium imaging techniques to Fura-2-labelled slices from neonatal rats and mice, we found that glycine and GABA (gamma-aminobutyric acid) affect [Ca2+]i in LSO neurons in an age-dependent manner; during the first postnatal week, the period at which glycine and GABA are depolarizing in the LSO, glycine and GABA always increased [Ca2+]i. However, in 2-week-old animals, the time around hearing onset when glycine and GABA are hyperpolarizing, glycine and GABA slightly decreased [Ca2+]i. Calcium responses could also be elicited by stimulation of afferent fibres from the MNTB, and these synaptic responses were mediated by glycine and GABA(A) receptors. Furthermore, GABA, which is a neurotransmitter only in the immature MNTB-LSO pathway, played a major role in generating MNTB-elicited Ca2+ responses. The direct link of glycinergic/GABAergic synaptic activity to intracellular calcium signalling during the period of inhibitory synaptic plasticity could be one of the mechanisms by which tonotopic MNTB-LSO connections become established.

Glycinergic/GABAergic synapses in the lateral superior olive are excitatory in neonatal C57Bl/6J mice

The lateral superior olive (LSO), a nucleus involved in sound localization, receives tonotopically organized, inhibitory input from the medial nucleus of the trapezoid body (MNTB). To better understand the development of this glycinergic/GABAergic pathway, we used Gramicidin-perforated patch clamp recordings to characterize MNTB-evoked postsynaptic potentials in LSO neurons of neonatal C57Bl/6J mice. We found that during the first postnatal week, MNTB-evoked responses change from being depolarizing to being hyperpolarizing. Most interestingly, depolarizing glycinergic/GABAergic synaptic potentials were able to trigger action potentials, demonstrating that the MNTB-LSO pathway can act as a true excitatory pathway. This transient excitatory action of immature MNTB-LSO synapses might play an important role in activity-dependent sharpening of the tonotopic organization of inhibitory connections in the LSO.

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.

Characteristics of glycine receptors expressed by embryonic rat brain mRNAs

A study was made of glycine (Gly) and gamma-aminobutyric acid (GABA) receptors expressed in Xenopus oocytes injected with rat mRNAs isolated from the encephalon, midbrain, and brainstem of 18-day-old rat embryos. In oocytes injected with encephalon, midbrain, or brainstem mRNAs, the Gly-current amplitudes (membrane current elicited by Gly; 1 mM Gly) were respectively 115 +/- 35, 346 +/- 28, and 389 +/- 22 nA, whereas the GABA-currents (1 mM GABA) were all < or =40 nA. Moreover, the Gly-currents desensitized faster in oocytes injected with encephalon or brainstem mRNAs. The EC(50) for Gly was 611 +/- 77 microM for encephalon, 661 +/- 28 microM for midbrain, and 506 +/- 18 microM for brainstem mRNA-injected oocytes, and the corresponding Hill coefficients were all approximately 2. Strychnine inhibited all of the Gly-currents, with an IC(50) of 56 +/- 3 nM for encephalon, 97 +/- 4 nM for midbrain, and 72 +/- 4 nM for brainstem mRNAs. During repetitive Gly applications, the Gly-currents were potentiated by 1.6-fold for encephalon, 2.1-fold for midbrain, and 1.3-fold for brainstem RNA-injected oocytes. Raising the extracellular Ca(2+) concentration significantly increased the Gly-currents in oocytes injected with midbrain and brainstem mRNAs. Reverse transcription-PCR studies showed differences in the Gly receptor (GlyR) alpha-subunits expressed, whereas the beta-subunit was present in all three types of mRNA. These results indicate differential expression of GlyR mRNAs in the brain areas examined, and these mRNAs lead to the expression of GlyRs that have different properties. The modulation of GlyRs by Ca(2+) could play important functions during brain development.

GABAergic inhibition suppresses paroxysmal network activity in the neonatal rodent hippocampus and neocortex

In the adult cerebral cortex, the neurotransmitter GABA is strongly inhibitory, as it profoundly decreases neuronal excitability and suppresses the network propensity for synchronous activity. When fast, GABA(A) receptor (GABA(A)R)-mediated neurotransmission is blocked in the mature cortex, neuronal firing is synchronized via recurrent excitatory (glutamatergic) synaptic connections, generating population discharges manifested extracellularly as spontaneous paroxysmal field potentials (sPFPs). This epileptogenic effect of GABA(A)R antagonists has rarely been observed in the neonatal cortex, and indeed, GABA in the neonate has been proposed to have an excitatory, rather than inhibitory, action. In contrast, we show here that when fast GABAergic neurotransmission was blocked in slices of neonatal mouse and rat hippocampus and neocortex, sPFPs occurred in nearly half the slices from postnatal day 4 (P4) to P7 neocortex and in most slices from P2 to P7 hippocampus. In Mg(2+)-free solution, GABA(A)R antagonists elicited sPFPs in nearly all slices of P2 and older neocortex and P0 and older hippocampus. Mg(2+)-free solution alone induced spontaneous events in the majority of P2 and older slices from both regions; addition of GABA(A)R antagonists caused a dramatic increase in the mean amplitude, but not frequency, of these events in the hippocampus and in their mean frequency, but not amplitude, in the neocortex. In the hippocampus, GABA(A)R agonists suppressed amplitudes, but not frequency, of sPFPs, whereas glutamate antagonists suppressed frequency but not amplitudes. We conclude that neonatal rodent cerebral cortex possesses glutamatergic circuits capable of generating synchronous network activity and that, as in the adult, tonic GABA(A)R-mediated inhibition prevents this activity from becoming paroxysmal.

Development of inhibitory synaptic transmission to motoneurons

The inhibitory effects of the neurotransmitters glycine and gamma-aminobutyric acid (GABA) on motoneurons and their role in mediating the timing of motor output have been understood for some years. Recent work, however, has revealed that these neurotransmitters function very differently in developing motor circuits. Most strikingly, both GABA and glycine depolarize neonatal motoneurons, and, in many instances, provide excitatory drive to developing motor networks. Additionally, the relative contributions of GABA and glycine to inhibitory synaptic transmission in a circuit or, indeed, within the same synapse, change with postnatal development. Here, we review three fundamental properties of inhibitory neurotransmission that are altered postnatally and may be important in shaping the unique behaviors of these synapses early in development.

Developmental changes in the modulation of synaptic glycine receptors by ethanol

During postnatal motoneuron development, the glycine receptor (GlyR) alpha subunit changes from alpha2 (fetal) to alpha1 (adult). To study the effect this change has on ethanol potentiation of GlyR currents in hypoglossal motoneurons (HMs), we placed neurons into two groups: neonate [postnatal day 1 to 3 (P1-3)], primarily expressing alpha2, and juvenile (P9-13), primarily expressing alpha1. We found that glycinergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in neonate HMs are less sensitive to ethanol than in juveniles. Thirty millimolar ethanol increased the amplitude of juvenile mIPSCs but did not significantly change neonatal mIPSCs. However, 100 mM ethanol increased the amplitudes of both neonate and juvenile mIPSCs. There was a significant difference between age groups in the average ethanol-induced increase in mIPSC amplitude for 10, 30, 50, and 100 mM ethanol. In both age groups ethanol increased the frequency of glycinergic mIPSCs, but there was no difference in the amount of frequency increase between age groups. Ethanol (100 mM) also potentiated evoked IPSCs (eIPSCs) in both neonate and juvenile HMs. As we observed for mIPSCs, 30 mM ethanol increased the amplitude of juvenile eIPSCs, but had no significant effect on eIPSCs in neonate HMs. Ethanol also potentiated currents induced by exogenously applied glycine in both neonate and juvenile HMs. These results suggest that ethanol directly modulates the GlyR. To investigate possible mechanisms for this, we analyzed the time course of mIPSCs and single-channel conductance of the GlyR in the presence and absence of ethanol. We found that ethanol did not significantly change the time course of mIPSCs. We also determined that ethanol did not significantly change the single-channel conductance of synaptic GlyRs, as estimated by nonstationary noise analysis of mIPSCs. We conclude that the adult form of the native GlyR is more sensitive to ethanol than the fetal form. Further, enhancement of GlyR currents involves mechanisms other than an increase in the single-channel conductance or factors that alter the decay kinetics.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Afferent regulation of inhibitory synaptic transmission in the developing auditory midbrain

To determine whether afferent innervation regulates the strength of inhibitory connections in the gerbil auditory midbrain, both cochleas were surgically removed in postnatal day 7 animals, before sound-driven activity is first observed. Inhibitory synaptic currents were measured in a brain slice preparation 1-7 d after the ablations. Whole-cell and gramicidin-perforated patch recordings were obtained from inferior colliculus neurons, and IPSCs were evoked by stimulation of the commissure of the inferior colliculus (CIC) or the ipsilateral lateral lemniscus (LL) in the presence of kynurenic acid. Deafferentation led to a 24 mV depolarizing shift in the IPSC equilibrium potential within 1 d of deafferentation. As a consequence, there was a large reduction of IPSC amplitude at a holding potential of -20 mV in neurons from bilaterally ablated animals. Furthermore, both afferent pathways displayed a 50% reduction of the inhibitory synaptic conductance after deafferentation, indicating that driving force was not solely responsible for the decline in IPSC amplitude. When paired pulses were delivered to the LL or CIC pathway in control neurons, the evoked IPSCs exhibited facilitation. However, paired pulse facilitation was nearly eliminated after deafferentation. Thus, normal innervation affects inhibitory synaptic strength by regulating postsynaptic chloride homeostasis and presynaptic transmitter release properties.

Commissural and lemniscal synaptic input to the gerbil inferior colliculus

The central nucleus of the inferior colliculus (ICC) receives direct inputs, bilaterally, from all auditory brain stem nuclear groups. To evaluate the contribution made to gerbil ICC neuron physiology by two major afferent pathways, we examined the synaptic responses evoked by direct stimulation of the commissure of the inferior colliculus (CIC) and the ipsilateral lateral lemniscus (LL). Frontal midbrain slices were obtained from postnatal day (P) 9-P19 gerbils, and whole cell recordings were made under current- (n = 22) or voltage-clamp (n = 52) conditions. Excitatory and inhibitory synaptic responses were characterized by sequentially exposing the slice to ionotropic glutamate receptor antagonists [6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + aminophosphonpentanoic acid (AP-5), or kynurenic acid)], a gamma-aminobutryic acid type A receptor antagonist (bicuculline), and a glycine receptor antagonist (strychnine). In current clamp, LL stimulation typically produced a short latency depolarization followed by a longer duration hyperpolarization. The depolarization was abolished by AP-5 + CNQX, and the remaining inhibitory potential displayed either bicuculline or strychnine sensitivity. In voltage clamp, 79% of ICC neurons displayed synaptic currents after stimulation of each pathway. The synaptic currents were typically complex waveforms, and ionotropic glutamate receptor antagonists reduced inward currents at a holding potential of -80 mV in the majority of neurons. In addition, this treatment reduced outward synaptic currents at a holding potential of -20 mV, indicating that inhibitory interneuronal input was often activated by LL or CIC afferents. A minority of neurons had synaptic currents that were unaffected by glutamate receptor antagonists, but it was more common for CIC-evoked currents to be unaffected (38%) rather than LL-evoked currents (22%). The CIC provided a strong inhibitory input that was almost exclusively GABAergic, whereas the LL inhibition often included a glycinergic component. These experiments have shown that the CIC provides a major glutamatergic and GABAergic input to most ICC neurons. However, much of the inhibitory input from both the CIC and the LL appears to be mediated by interneuronal connections.

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.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

Role of synaptic inhibition in processing of dynamic binaural level stimuli

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the inferior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD), a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ipsilateral stimulation. Sound pressure level was modulated trapezoidally at the ipsilateral "inhibitory" ear, whereas the contralateral "excitatory" level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression under static conditions, an unexpected discharge was often elicited, apparently because of an aftereffect of synaptic inhibition. In contrast, when the inhibitory stimulus was increased within a range of sound levels that produced only modest suppression under static conditions, neuronal discharge was often profoundly suppressed. In many cases the "conditioned enhancement" or "conditioned suppression" persisted for several seconds after the modulation of ILD, and such conditioned responses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory stimulus. The acoustically elicited discharge rate was potentiated markedly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad implications for processing ensuing auditory stimuli.