The Effect Of GABAergic Inhibition On Temporal Response Properties Of Inferior Colliculus Neurons In Mice

Encoding mechanism for sensory system in central nerve system is a research focus. Auditory system is one of important sensory systems, which is considered as an ideal research model for the information coding mechanism. Coding mechanism of sound information occupies an important position in studies of auditory function and information coding mechanism. As fundamental parameters of sound, the coding mechanism of frequency and amplitude are still unclear. Recently, the timing of spikes, especially the first spike latency (FSL) has received increasing attention as a more precise and reliable information carrier. In auditory system, FSL of a given neuron is often defined as the time from the onset of a stimulus to the time of occurrence of the first spike. Spike counts are generally used to evaluate the responses of neurons to amplitudes and frequencies of acoustic stimuli. In previous research in our lab, we found that FSL, with its precision and stability, appears to be a better parameter than spike counts in evaluation of the response of a neuron to frequency and amplitude in mouse inferior colliculus. Therefore, a further research should be carried out to clarify the mechanism underlying generation of the FSL and the factors which may influence it.The inferior colliculus is the principal midbrain nucleus of the auditory pathway of mammals and play a critical role in temporal and spatial information coding and integration. The inferior colliculus (IC) receives excitatory and inhibitory inputs. Most of the inhibitory inputs are GABAergic and glycinergic.Iontophoretic application of the GABAa receptor blocker bicuculline and of the glycine receptor blocker strychnine can change many acoustic response properties in IC neurons of mammals. Although the effects of bicuculline and strychnine on spike counts in bats have previously been studied, but research on FSL is rare. IC receives ascending inputs from many lower auditory nuclei as well as descending inputs from the corticofugal system. The corticofugal system means nerve fiber passing outward from the cerebral cortex to subcortical nucleus even cochlea. It is well known that corticofugal adjustment can evoke changes in the frequency, minimum threshold of IC neurons to sound stimulus, and subsequently affect the ascending acoustic signals. It has not been studied whether the corticofugal system can affect FSL and temporal properties or not.Our previous study indicates that FSL consists of two main components which were transmission delay and potency delay. Transmission delay is also called minimum first spike latency (MFSL) which is independent of acoustic amplitude. Transmission delay comprises the travel time of the sound from the source to the inner ear, delays due to the propagation of axon potentials along axons, and minimal synaptic delays. Potency delay changes with acoustic amplitude. BALB/c mouse were used in our study. In vivo extracellular recording method was used and comparison was made about the temporal response properties of IC neurons before and after the application of bicuculline methiodide (BMI) in IC and AC. Our study aims for exploring three problems. First, does GABAergic inhibition has effect on first spike latency in IC of mouse and, if has, in which part the effect may take place? Secondly, the significance of FSL change in representing information, and last, whether or not GABA could affect FSL of inferior colliculus through descending auditory pathways.Extracellular recording was performed in a soundproof and echo-attenuated room. The animal was held in a polyethylene foam body-mold with an elastic band hung over a steel pole, which was fixed on an anti-vibration table. The animal's head was immobilized by fixing the 1.5-cm-long nail which glued onto the dorsal surface of skull with dental cement to a small metal rod with screws. A 2×2 mm² area on one side of the IC was exposed under a surgical microscope by opening the skull and dura above the IC. Mouse with two holes of 2 mm in diameter drilled in the skull to expose the IC and AC, both of the right sides, which was prepared for corticofugal study. The neuronal activities were recorded with single-barrel glass electrode (tip:1μm) and Multi-barrel glass electrode (tip:10μm).The changes of temporal response properties of IC neurons were recorded and analyzed, which IC neurons were given an local iontophoretic application of BMI or AC neurons were applied BMI.During a recording session, the pinnae were maintained as in the normal awake animal. A loudspeaker was placed 30 cm away from the mouse sagittal plane. Sound signal contains three fundamental parameters: frequency, intensity and duration. In this study, we used pure tone stimulus, which would only activate a limited part of basilar membrane. Sound amplitude was controlled by envelope. FSLs in most auditory neurons are determined by the onset of the stimulus temporal envelope and shaped by the primary stimulus parameters, which determine the envelope onset status, such as the steady-state sound pressure (SP), rise time (RT) and rise function (RF). The duration of total tone burst is 75 ms each with a 20 ms rise time and a 5 ms fall time. Rise function was cosine square function. Pure tone bursts were used as acoustic stimuli and were generated and delivered using a workstation Tucker-Davis Technologies System 3 (TDT 3, Tucker-Davis Technologies, Alachua, FL, USA). Pure tone bursts were played back using a computer with BrainWare software which controlled the frequency and amplitude of pure tone bursts either manually or automatically.Spikes of neurons were induced by acoustic stimulus. Spike with its signal to noise ratio bigger than 4:1 was confirmed as an action potential. The spikes with similar shapes were considered as single unit neuron. The response properties were measured approximately by manually varying the frequency and amplitude of tone bursts after an IC neuron was isolated. Characteristic frequency (CF) and minimum threshold (MT) were measured by spike count (rate). Minimum threshold is defined as the minimum intensity at which the neuron has the 30% response possibility. CF is defined as the sound frequency at which the neuron may response to the minimum intensity. After local iontophoretic application of BMI of IC neuron or applied BMI to AC, we repeated the same frequency and amplitude (F-A) scan. After the data was acquired, the average FSL of action potential to the sound stimulus was calculated off line. The FSL and spike counts were used as analysis index respectively to clarify the effect on the temporal property of the response, which was from the application of BMI at IC and AC. The influence of the GABAergic inhibition and the corticofugal pathway to the temporal property of the response was then affirmed. Thus the possible property of the FSL which receive such influence can also be demonstrated to clarify the biological meaning of the FSL.A total of 88 IC neurons was obtained in our experiment. The characteristic frequencies (CF) of recorded neurons ranged from 7.5 to 38 kHz, and the minimum thresholds (MT) from 10 to 80 dB SPL. There was no special relationship between the CF and MT. Most of frequency tuning curves showed the letter V shape, followed by U shape, others band-pass shape and multi-peaked shape. The spontaneous activity was at low level under the light anesthesia condition. Based on the PSTH of spike firing, the neurons could be classified into four categories: tonic, quasi-tonic, phasic-tonic and phasic.The spike count-amplitude curve in recorded neurons could be divided into three types:monotonic, non-monotonic and saturated. The same neuron had different response properties to sound amplitude at different frequencies. The spike count-frequency curves at certain sound amplitude were mostly inverted-V type but had big variability. Nevertheless, the FSL-amplitude functions were in close registration, in which FSL decreased with the reinforcement of amplitude. The FSL-frequency curve at different sound amplitudes were mostly V or W type. We found that the FSLs of neurons responding to CF of sound stimulus were usually the shortest. Such response property indicated that FSL could represent sound amplitude and frequency more precisely and stably. The FSL-amplitude curve at different frequencies could be described with the Pieron's empirical Law in psychophysics: y = A_j×e^(-x/ti) + y_0i.In this equation, y means FSL, x means stimulus amplitude in dB SPL, yo, A, t are three fitment quantities,y₀ referred to the minimum value when the curve approached abscissa infinitely, i standed for different frequency. The experiment showed that y₀ and A were changed with the sound frequency, while the t value was not. But different neurons had different values.In the 88 IC neurons, 38 of them were studied with BMI.BMI increased spike count of 37(97.4%) neurons. Discharge pattern of neurons changed after disinhibition, most of neurons with phasic pattern changed to phasic-tonic and tonic pattern. Drug application evoked CF shifts in±0.5 kHz to 3 kHz of 8 neurons, CFs of most of neurons (30/38) remained changeless. There was no special relationship between the CF and CF shift. MT decreased in 22 (57.9%) neurons and increased in 4 (10.5%) neurons. The higher the minimum threshold of neuron, the larger changes in minimum threshold after application, and showed a linear trend (y=-0.3138x+ 5.8495 R²=0.3492). MT increased in neurons with small (?)(<20 dB SPL) after BMI application, MT decreased in neurons with bigger MT. The change of threshold at CF was smaller than at other frequency. Pre-drug MFSL ranged from 9.62 to 34.13 ms (Median =15.39 ms, SD = 5.62 ms). In 71% of the cells (27 of 38), BMI application resulted in shorter latency. In 21% of the neurons (8 of 38), BMI application resulted in longer latency. After BMI application, the FSL-amplitude functions could be fitted well by exponential first decay function. The FSL-amplitude curves before and after BMI application could overlap by shifting along coordinate. These demonstrated that BMI application could not change the FSL-amplitude property (i.e. slope of the curve) of IC neurons. Curve shifts along coordinate changed MFSL and minimum threshold of neural responses. Of 8 neurons with changed CF, 6 neurons showed vertical shift in curve and 25 of 30 neurons with unchanged CF showed horizontal shift in curve.Our results suggest GABA may participate in constituting FSL of inferior colliculus neurons of mouse. We could conclude that block of GABAergic receptors might affect minimum threshold and MFSL of neurons, that is, synaptic energy and synaptic groups, based on the above relationship of our FSL-amplitude curve. Furthermore, MFSL normally changed with CF shift, our results indicate that a further exploration of the change relationship of MFSL and CF of neurons in information representation not only illuminate the mechanism of timing coding of auditory center, but the mechanism of temporal coding of properties of sound frequency through auditory center.IC neurons receive general regulation from corticofugal system. The effects of blocking GABA_A receptors in auditory cortex neurons on response latency of mouse inferior colliculus neurons were observed. In the 88 IC neurons, 25 of them were recorded with BMI applied to AC. After BMI application on cortical neurons, the spike counts were decreased or unchanged, MT increased in 52% (13/25) neurons and remained invariable in 10.5% (11/25) neurons, the FSL of 16 neurons (64%) was prolonged and that of 3 neurons (12%) was shortened. A general response pattern was shown that FSL increased monotonically with stronger intensity of sound and FSL was the shortest at CF in a neuronal response. Therefore, it indicates that corticofugal fibers play an inhibitory role in inferior colliculus neurons; however, it does not change the response pattern of FSL to sound intensity and frequency.