Michinori Kubota

Effects of amino acids on cat red nucleus neurons in vitro

SummaryIntracellular records were obtained from neurons in the region of the red nucleus (RN) of cat brain slices. Both EPSPs and IPSPs were recorded in response to local electrical stimulation and these resembled similar electrophysiological responses observed in experiments conducted in vivo. Monosynaptic and polysynaptic IPSPs were observed, suggesting the existence of inhibitory interneurons near or within the RN region.When added to the bathing solution, L-glutamate and L-aspartate depolarized RN neurons with a decrease in input resistance. γ-Aminobutyric acid (GABA) and glycine hyperpolarized the cells with a decrease in input resistance. GABA also elicited a depolarizing response. These amino acid actions had direct postsynaptic effects, since the experiments were conducted in a low Ca2+/high Mg2+ medium which blocked synaptic transmission.

Release of endogenous GABA from the cat red nucleus slices.

The release of endogenous gamma-aminobutyric acid (GABA) from the cat red nucleus was studied in a slice preparation. Potassium-induced depolarization caused a release of GABA in a calcium-dependent manner. The release of GABA was also induced by veratrine and blocked by tetrodotoxin. The demonstration of release satisfies one of the important criteria for identification of GABA as the inhibitory neurotransmitter.

Layer-specific horizontal propagation of excitation in the auditory cortex.

Optical imaging was conducted in rat auditory cortex slice preparations to study how horizontal excitation propagates in the cerebral cortex. When a horizontal cut was made between supragranular and infragranular layers, electrical stimulation of layer II/III elicited horizontally propagating excitation in supragranular layers with the same propagating velocities as those seen before a cut was made. Electrical stimulation of layer VI or the border between the white matter and layer VI also elicited horizontally propagating excitation in infragranular layers with the same propagating velocities as those seen before a cut was made. These results suggest that the horizontal propagation of excitation in supragranular and infragranular layers can occur independently in the auditory cortex.

Spatiotemporal dynamics of neural activity related to auditory induction in the core and belt fields of guinea-pig auditory cortex.

Auditory induction is a continuity illusion in which missing sounds are perceived under appropriate conditions, for example, when noise is inserted during silent gaps in the sound. To elucidate the neural mechanisms underlying auditory induction, neural responses to tones interrupted by a silent gap or noise were examined in the core and belt fields of the auditory cortex using real-time optical imaging with a voltage-sensitive dye. Tone stimuli interrupted by a silent gap elicited responses to the second tone following the gap as well as early phasic responses to the first tone. Tone stimuli interrupted by broad-band noise (BN), considered to cause auditory induction, considerably reduced or eliminated responses to the tone following the noise. This reduction was stronger in the dorsocaudal field (field DC) and belt fields compared with the anterior field (the primary auditory cortex of guinea pig). Tone stimuli interrupted by notched (band-stopped) noise centered at the tone frequency, considered to decrease the strength of auditory induction, partially restored the second responses from the suppression caused by BN. These results suggest that substantial changes between responses to silent gap-inserted tones and those to BN-inserted tones emerged in field DC and belt fields. Moreover, the findings indicate that field DC is the first area in which these changes emerge, suggesting that it may be an important region for auditory induction of simple sounds.

Dynamic spatiotemporal inhibition in the guinea pig auditory cortex

Real-time optical imaging was conducted in the guinea pig auditory cortex to study spatiotemporal interrelations of excitation and inhibition in response to tone stimulation. Tone stimulation elicited responses consisting of three phases in the anterior field (the primary auditory cortex of guinea pig) and in the dorsocaudal field of the auditory cortex. An early depolarization was followed by a late hyperpolarization and an even later depolarization both in the maximum excitatory regions and in the lateral regions beside and/or between them. The late hyperpolarization began significantly earlier and was stronger in the lateral regions than in the maximum excitatory regions. These results show that inhibition is dynamic, both in time and in space, in the auditory cortex.

Calcium-dependent potentials in mammalian red nucleus neurons in vitro

Intracellular recordings were made from red nucleus (RN) neurons in guinea-pig slice preparations. The slow afterhyperpolarization (AHP) following an action potential was reversibly abolished by Co2+ or Mn2+. Its amplitude was dependent on the extracellular K+ concentration. When tetraethylammonium was added to the perfusing solution, a tetrodotoxin-resistant regenerative depolarization was evoked which was blocked by Co2+ or Mn2+. There results suggest that the slow AHP is produced by an increase in Ca2+-dependent K+ conductance and that RN neurons have a voltage-dependent Ca2+ conductance.

Optical imaging of neural activity to vocalized sounds in the guinea‐pig auditory cortex

Spatiotemporal neural activity in response to vocalized sounds (calls) was investigated in the auditory cortex of anesthetized guinea pigs using an optical imaging technique with a voltage‐sensitive dye (RH795). Guinea‐pig calls were recorded digitally and presented to the ear from a loudspeaker located contralaterally to the recording cortex. Pure tone (PT), click (CLK), and white noise (WN) stimuli were used for comparison. The response in the primary (AI) and dorsocaudal (DC) fields of the auditory cortex to a PT appeared at the stimulus onset and in the corresponding frequency band, followed by nonsimultaneous lateral inhibition in the adjacent frequency bands. The responses to CLK and WN stimuli appeared in a wide area of the fields corresponding to the wide frequency components of the stimuli. The response to calls changed spatially and temporally because the components’ frequencies changed over time. Strong responses occurred in the frequency bands corresponding to prominent instantaneous frequency...

Extraction of sounds’ time‐frequency features in the auditory cortex

Although ‘‘time‐frequency feature extraction’’ is a fundamental cue that is used to recognize speech sounds, its physiological mechanisms remain largely unknown. This research is aimed at understanding what time‐frequency features of sounds are extracted by auditory cortical neurons. We investigated spatiotemporal response patterns in the guinea‐pig auditory cortex, using an optical recording method (MiCAM Ultima; Brain Vision) with a voltage‐sensitive dye (RH795). Experiments were performed under anesthesia (ketamine, 100 mg/kg, and xylazine, 25 mg/kg). Pure tones (PTs), frequency‐modulated (FM) sounds with various modulation rates, and their combinations were presented to the animals. Cortical activation was induced at the onset and offset of PTs along the isofrequency contours. The onset response was followed by strong inhibition. In contrast, FM sounds evoked an additional activation moving across the isofrequency contours in the cortex, with locations corresponding to the instantaneous frequency inpu...

Optical recordings of layer-specific excitation propagation in the rat auditory cortex in slice preparations

Corticofugal modulation of the responses to acoustic stimulus of the medial geniculate (MGB) neurons was examined in I2 anesthetized guinea pigs. After the tonotopicity of the auditory cortex was determined, three stimulation electrodes were implanted into the tonotopic fields of the cortex at a depth of OSI .O mm. The cortex was then stimulated regionally by applying a train of electrical pulses to one of the electrodes. Toneand noise-bursts were applied to the ear contralateral to the recording thalamus. Responses of 120 thalamic neurons to acoustic stimuli were recorded extracellularly with and without cortical stimulation. Stimulation of a small region in the tonotopic auditory fields caused a facilitatory effect on the neurons of the ventral division of the MGB with the same best-frequency as that of the stimulating site in the cortex, and a small inhibitory effect on the neurons in the surrounding area. Mainly inhibitory effects were obtained from the neurons in the medial division and the shell nucleus of the MGB by cortical stimulation, which was larger for the medial division than the shell nucleus. The present study adds evidence to the hypothesis that corticofugal projection selectively gates the ascending auditory information.

1707 Optical recordings of spatio-temporal patterns of excitation in the rat auditory cortex in slice prepatations

HE, JUFANG The present study examined neurons in the dorsal zone (DZ) of cat auditory cortex whose responses depended on the duration of noise bursts. Of 150 long-latency neurons, 78 were classified as selective for long durations; 41 responded selectively to noise bursts of short duration; 13 showed maximal responses to noise bursts of a particular duration. We suggest that the duration-selectivity results from integration along the time domain of a stimulus during the latent period. Wheat germ agglutinin-horseradish peroxidase was injected to the DZ after physiological study. The main projection to the target region was the dorsal medial geniculate body (MGB) and posterior complex of the thalamus. Together with previous physiological and anatomical results from monkey, cat, and bat, the present results may lead to a hypothesis that the auditory pathway via the dorsal MGB is related to temporal information processing while the ventral MGB processes frequency information.