Shu Hui Wu

GABAergic projections from the lateral lemniscus to the inferior colliculus of the rat

The objective of the present study was to provide direct evidence regarding GABAergic projections from the nuclei of the lateral lemniscus to the central nucleus of the inferior colliculus (ICC), and from the ICC to the opposite ICC. Projections of GABAergic neurons in the rat were investigated by a combination of fluorogold (FG) retrograde tracing and GABA immunocytochemistry. FG was first injected into a frequency-defined region (11-13 kHz) in the center of the ICC, and 1-2 weeks was allowed for retrograde transport. Vibratome sections were then cut through the brainstem and stained with GABA antibody. Double-labeling was taken as evidence of GABAergic neurons projecting to the ICC. The results from FG retrograde labeling alone showed that neurons in the dorsal nucleus of the lateral lemniscus (DNLL) bilaterally, in the intermediate and ventral nucleus of the lateral lemniscus (INLL and VNLL) ipsilaterally, and in the ICC contralaterally project to the ICC. GABA immunostaining alone showed substantial numbers of GABA positive neurons in the nuclei of the lateral lemniscus and the inferior colliculus. FG and GABA double-labeled neurons were present in all nuclei of the lateral lemniscus that project to the ICC. The greatest concentration of double-labeled neurons was found bilaterally in the DNLL, suggesting a prominent GABAergic projection from the DNLL to the ICC. The presence of many double-labeled neurons in the ipsilateral INLL and VNLL suggests that there are also GABAergic inputs from the INLL and VNLL to the ICC. No double-labeled neurons were found in the contralateral ICC, which suggests the possibility of a prominent non-GABAergic projection.

Contribution of AMPA, NMDA, and GABAA Receptors to Temporal Pattern of Postsynaptic Responses in the Inferior Colliculus of the Rat

The central nucleus of the inferior colliculus (ICC) is a major site of synaptic interaction in the central auditory system. To understand how ICC neurons integrate excitatory and inhibitory inputs for processing temporal information, we examined postsynaptic responses of ICC neurons to repetitive stimulation of the lateral lemniscus at 10-100 Hz in rat brain slices. The excitatory synaptic currents mediated by AMPA and NMDA receptors and the inhibitory current mediated by GABAA receptors were pharmacologically isolated and recorded by whole-cell patch-clamp techniques. The response kinetics of AMPA receptor-mediated EPSCs and GABAA receptor-mediated IPSCs were similar and much faster than those of NMDA receptor-mediated EPSCs. AMPA EPSCs could follow each pulse of stimulation at a rate of 10-100 Hz but showed response depression during the course of repetitive stimulation. GABAA IPSCs could also follow stimulus pulses over this frequency range but showed depression at low rates and facilitation at higher rates. NMDA EPSCs showed facilitation and temporal summation in response to repetitive stimulation, which was most pronounced at higher rates of stimulation. GABAA inhibition suppressed activation of NMDA receptors and reduced both the degree of AMPA EPSC depression and the extent of temporal summation of NMDA EPSCs. The results indicate that GABAA receptor-mediated inhibition plays a crucial role in maintaining the balance of excitation and inhibition and in allowing ICC neurons to process temporal information more precisely.

Long-term potentiation in the inferior colliculus studied in rat brain slice

The purpose of this study is to determine whether long-term potentiation (LTP) can be induced in the central nucleus of the inferior colliculus (ICC) by electrical stimulation of the lateral lemniscus. If LTP can be induced, is it mediated by N-methyl-D-aspartate (NMDA) and/or other receptors? Brain slices of the ICC were obtained from 14-35 day old Wistar rats. The field potentials were recorded from the ICC after GABAergic and glycinergic inhibition was suppressed. Following tetanic stimulation (50 Hz, 20 s), the amplitude of the response was increased to about 146% of control response for at least 30 min. LTP was observed in about 78% of the cases tested. Induction of LTP in the ICC required activation of both NMDA and gamma-aminobutyric acid (GABA)(B) receptors. GABAergic inhibitory postsynaptic potentials (IPSPs) were blocked by the GABA(A) receptor antagonist, but not by the GABA(B) receptor antagonist. The IPSPs were decreased by the GABA(B) receptor agonist, baclofen. The intrinsic postsynaptic membrane properties were not affected by baclofen. These results suggest that GABAergic inhibition in the ICC is mediated only by GABA(A) receptors, but that it is modulated by presynaptic GABA(B) receptors. The GABA(B) receptors in the ICC may suppress GABAergic inhibition and promote induction of LTP.

AMPA and NMDA receptors mediate synaptic excitation in the rat’s inferior colliculus

The synaptic mechanisms underlying excitation in the rats central nucleus of the inferior colliculus (ICC) were examined by making whole-cell patch clamp recordings in brain slice preparations of the auditory midbrain. Responses were elicited by current pulse stimulation of the lateral lemniscus and recordings were made in ICC using either current clamp or voltage clamp methods. The excitatory postsynaptic responses in either current or voltage clamp mode consisted of two distinct components, an early component that could be blocked by bath application of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), and a later component that could be blocked by application of the N-methyl-D-aspartate (NMDA) receptor antagonists, (+/-)-2-amino-5-phosphonovaleric acid (APV) or (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). Both AMPA and NMDA receptor-mediated responses were present at resting potential and could be isolated pharmacologically by application of receptor antagonists. Voltage clamp experiments revealed that the NMDA receptor-mediated current was voltage-dependent and increased in magnitude as the cell membrane was depolarized. This NMDA receptor-mediated response was enhanced at resting potential when Mg(2+) was eliminated from the bath solution. The ratio of response amplitudes associated with the late and early components, an estimate of the relative contribution of NMDA and AMPA receptor types, changed with age. There was a progressive decline in the ratio between 9 and 13 days of age, but no further reduction between days 13 and 16. The data show that both AMPA and NMDA receptors are important for determining excitatory responses in the ICC and that both receptor types probably play a role in auditory processing after the onset of hearing.

Physiological evidence for ipsilateral inhibition in the lateral superior olive: Synaptic responses in mouse brain slice

The incidence of ipsilateral inhibition in the lateral superior olive (LSO) was examined in a brain slice preparation of the mouse superior olivary complex. A 400 microns brain slice was taken in the frontal plane and maintained in a warm, oxygenated saline solution. Intracellular recordings were made from the LSO with micropipettes filled with 4 M potassium acetate. Synaptic responses were elicited by electrical stimulation of the trapezoid body in different slices at various locations between the cochlear nucleus and the ipsilateral superior olivary complex (SOC). The results show that ipsilateral stimulation can evoke inhibitory as well as excitatory postsynaptic potentials. The ipsilateral IPSPs have short latencies and are elicited by stimulation of the trapezoid body at any point along its course between cochlear nucleus and LSO. Short-latency IPSPs can also be produced by direct stimulation of the ventral cochlear nucleus itself. Ipsilateral IPSPs are blocked by low concentrations of the glycine antagonist, strychnine. In addition, bath application of sodium pentobarbital in one case eliminated ipsilateral IPSPs without eliminating EPSPs. The results suggest that there is a rapidly conducting, glycinergic pathway from cochlear nucleus through the trapezoid body to the LSO on the same side of the brain. This pathway is probably served by either a direct projection from the ventral cochlear nucleus to the LSO or an indirect one from cochlear nucleus to LSO through the lateral nucleus of the trapezoid body (LNTB).

Presynaptic modulation of gabaergic inhibition by GABAB receptors in the rat's inferior colliculus

Whole-cell patch clamp recordings were made from neurons in a brain slice preparation of the inferior colliculus in 11-15-day-old rat pups. Synaptic responses were elicited by applying a current pulse to the lateral lemniscus just below the central nucleus of the inferior colliculus. To examine GABAergic inhibition in the inferior colliculus all excitatory postsynaptic potentials and glycinergic inhibitory postsynaptic potentials were blocked by bath application of their respective antagonists and the contribution of GABA(B) receptors was determined for the remaining inhibitory postsynaptic potentials. For most cells the isolated inhibitory postsynaptic potential was completely blocked by the GABA(A) receptor antagonist, bicuculline, but was unaffected by the GABA(B) receptor antagonist, phaclofen. The GABA(B) receptor agonist, baclofen (10-20 microM), decreased the amplitude of the inhibitory postsynaptic potentials. This effect was completely blocked by phaclofen. Baclofen did not increase the cell membrane conductance or alter the rate of firing produced by depolarization of the cell membrane. In contrast, muscimol, a GABA(A) receptor agonist, greatly increased membrane conductance and lowered the firing rate produced by depolarization. Our results indicate that GABAergic inhibition in the auditory midbrain can be reduced by the activation of GABA(B) receptors and suggest that the effects are presynaptic.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus: whole-cell patch-clamp recordings from rat brain slice

The synaptic events underlying the excitation of neurons in the rats dorsal nucleus of the lateral lemniscus were studied by whole-cell patch-clamp recordings in a brain slice preparation of the auditory midbrain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 days of age at the time the recordings were made. Synaptic responses were evoked by a bipolar stimulating electrode placed on the lateral lemniscus just ventral to the dorsal nucleus. To eliminate glycinergic inhibitory responses, all physiological data were gathered with 0.5 microM strychnine added to the saline bath. Under current-clamp conditions, excitatory postsynaptic potentials could be subdivided into early and late components. The early component produced a single, highly reliable, short-latency spike and the later component produced a more variable, long-latency spike or train of spikes. The non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and its associated action potential. The N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory postsynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the same cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excitatory postsynaptic potential. Under voltage-clamp conditions, dorsal nucleus of the lateral lemniscus neurons responded to stimulation of the lateral lemniscus with excitatory postsynaptic currents. Outward excitatory postsynaptic currents were recorded with holding potentials that depolarized the cell membrane and inward currents were seen when the cell was hyperpolarized. The current-voltage (I-V) relation of the early peak portion of the excitatory postsynaptic current was nearly linear, whereas the I-V relation of the later excitatory postsynaptic current (12 ms after the peak) was non-linear over the range between -50 and - 100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later current that was blocked by D,L-2-amino-5-phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, but was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cerebrospinal fluid. however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later component was blocked by D,L-2-amino-5-phosphonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated synaptic responses are present in dorsal nucleus of the lateral lemniscus neurons of rats at 21-35 days of age. The N-methyl-D-aspartate component had a longer time-course and a higher threshold than the non-N-methyl-D-aspartate component, and was subject to a voltage-dependent Mg2+ block when the cells membrane was hyperpolarized. The long-duration N-methyl-D-aspartate component is probably responsible for the prolonged inhibitory effect of dorsal nucleus of the lateral lemniscus neurons on physiological responses in the rats inferior colliculus.

GABAB receptor-mediated presynaptic inhibition of glutamatergic transmission in the inferior colliculus

Whole-cell patch clamp recordings were made from ICC neurons in brain slices of 9-16 day old rats. Postsynaptic currents were evoked by electrical stimulation of the lemniscal inputs. Excitatory postsynaptic currents (EPSCs) were isolated pharmacologically by blocking GABA(A) and glycine receptors. EPSCs were further dissected into AMPA and NMDA receptor-mediated responses by adding the receptor antagonists, APV and CNQX, respectively. The internal solution in the recording electrodes contained CsF and TEA to block K(+) channels that might be activated by postsynaptic GABA(B) receptors. The modulatory effects of GABA(B) receptors on EPSCs in ICC neurons were examined by bath application of the GABA(B) receptor agonist, baclofen, and the antagonist, CGP 35348. The amplitudes of EPSCs in ICC neurons were reduced to 34.4+/-3.2% of the control by baclofen (5-10 microM). The suppressive effect by baclofen was concentration-dependent. The reduction of the EPSC amplitude was reversed by CGP35348. The ratio of the 2nd to 1st EPSCs evoked by paired-pulse stimulation was significantly increased after application of baclofen. These results suggest that glutamatergic excitation in the ICC can be modulated by presynaptic GABA(B) receptors. In addition, baclofen reduced NMDA EPSCs more than AMPA EPSCs. The GABA(B) receptor-mediated modulation of glutamatergic excitation in the ICC provides a likely mechanism for preventing overstimulation and/or regulating the balance of excitation and inhibition involved in processing auditory information.

The commissure of probst as a source of GABAergic inhibition.

Whole-cell patch-clamp recordings were made from neurons in the rats dorsal nucleus of the lateral lemniscus (DNLL) in a brain slice preparation. Planes of section were chosen to preserve the integrity of fibers in the commissure of Probst (CP) and postsynaptic responses were evoked by electrical stimulation along its length. Results showed that the crossed projection to the DNLL through the CP is mainly, if not exclusively, inhibitory in the rat. Inhibitory postsynaptic responses (IPSPs) evoked by stimulation of the CP were blocked by the gamma-aminobutyric acid (GABA)(A) receptor antagonist bicuculline, but were unaffected by the glycine receptor antagonist strychnine, supporting the conclusion that the crossed inhibitory projection to DNLL from the contralateral DNLL is GABAergic. Stimulation of the CP close to the DNLL frequently evoked excitatory postsynaptic responses as well as IPSPs, but stimulation near the midline evoked IPSPs only. Thus, the excitatory responses probably originated from a pathway other than the projection to the DNLL from the contralateral DNLL through the CP.

Physiological characteristics of postinhibitory rebound depolarization in neurons of the rat's dorsal cortex of the inferior colliculus studied in vitro

The inferior colliculus (IC) is a major center for neural integration in the auditory pathway. The IC processes inputs from the lower brainstem as well as from higher centers in the auditory system. To understand cellular mechanisms of IC neurons in auditory processing, we investigated physiological characteristics of the rebound depolarization (RD) following membrane hyperpolarization in neurons of the rats dorsal cortex of the inferior colliculus (ICD). Whole-cell patch clamp recordings were made from ICD neurons in brain slices. In more than half of the ICD neurons, there was a RD accompanied by one or two anode break action potentials (APs) following membrane hyperpolarization. The RD was Ca(2+) mediated and primarily due to activation of low-threshold T-type Ca(2+) channels. Generation of the RD and anode break APs depended on the magnitude and duration of the preceding hyperpolarization. Larger and longer hyperpolarization induced a larger, shorter and faster rebound, and therefore earlier anode break APs. However, with further hyperpolarization the RD became constant in amplitude and duration despite increases in the strength or duration of the preceding hyperpolarization. Usually, membrane hyperpolarization as small as -15 mV for 100-200 ms was enough to induce a pronounced rebound of 15-20 mV. The RD in IC neurons may provide a neuronal mechanism for integrating excitatory inputs arriving soon after a period of synaptic inhibition and therefore processing specific aspects of auditory information.