Koichi Fujimura

Autogenous oscillatory potentials in neurons of the guinea pig substantia nigra pars compacta in vitro

In spontaneously firing neurons of the guinea pig substantia nigra pars compacta (SNC) maintained in slices, blockade of the fast spikes by tetrodotoxin (TTX) revealed a slow oscillatory potential change, which was depressed by Cd2+, a Ca2+-channel blocker. The spontaneous firing was suppressed by the application of Ca2+-free saline, Cd2+ or Co2+, but not by nifedipine. These findings lend support to the view that the spontaneous firing of SNC neurons is produced by an intrinsic, Ca2+-dependent pacemaking process affected by Cd2+ but not by nifedipine.

Acute Corneal Epithelial Change after Instillation of Benzalkonium Chloride Evaluated Using a Newly Developed in vivo Corneal Transepithelial Electric Resistance Measurement Method

Objective: Acute corneal permeability change after instillation of benzalkonium chloride (BAC) was evaluated using a newly developed in vivo corneal transepithelial electric resistance (TER) measurement method. Method: Corneal TER was measured by Ag/AgCl electrodes placed in the anterior aqueous chamber and on the cornea of live rabbit eyes. TER was measured and TER change after instillation of 0.05% BAC solution was monitored. After TER measurement, cornea was excised and fixed for transmission and scanning electron microscopy. For the control study, physiologic saline was used instead of BAC. Results: The TER of normal rabbit cornea was 602.3 ± 195.0 Ωcm2. TER decreased instantly after instillation of 0.05% BAC. In 5 s, TER decreased to 58.3 ± 5.2%. In 60 s, TER decreased to 18.5 ± 3.2%. At all time points, TER after instillation of 0.05% BAC was significantly lower than that of the control (p < 0.0001). Dissociation of tight junctions and the destruction of superficial cell membranes were observed under electron microscopy. Conclusion: Corneal epithelial change with increased permeability is rapid and intense after the instillation of highly concentrated BAC solution, accompanied by disorder of tight junctions and cell membranes of superficial cells. The newly developed in vivo corneal TER measurement method is suitable for assessing acute corneal change after drug instillation.

Two types of neurons in the substantia nigra pars compacta studied in a slice preparation.

Two types of neurons were differentiated, on the basis of electrophysiological properties, in the substantia nigra pars compacta of the guinea pig maintained in slices. While the majority of neurons, presumably dopaminergic neurons, produced a broad spike accompanied by a relatively long-lasting after-hyperpolarization, a small number of neurons were characterized by the generation of a depolarizing after-potential (DAP) following a fast spike. The DAP was depressed by a slight depolarizing shift of the resting membrane potential and by the removal of Ca2+ from the bathing medium, which suggested that it was a low-threshold Ca2+-dependent response. Intracellular staining revealed that the neurons producing DAP were smaller than the major neurons.

Action of habenular efferents on ventral tegmental area neurons studied in vitro

Action of habenular efferents on neurons of the ventral tegmental area (VTA) was studied with a slice preparation that preserved the habenula (Hb) and the VTA together with the interconnecting fiber bundle, the fasciculus retroflexus (FR). In the VTA, two types of neurons, presumably corresponding to the dopaminergic and nondopaminergic neurons, were discerned on the basis of the electrophysiological properties. Of 52 VTA neurons sampled, 42 [with the mean resting membrane potential of 56 +/- 7 mV (mean +/- SD)] responded with excitatory postsynaptic potentials (EPSPs) to FR stimulation. The EPSPs were monosynaptic in nature and rather weak in effect in the sense that they rarely triggered spikes. No significant differences in latency, duration, and time to peak were noted between the EPSPs generated in different types of neurons. FR stimulation evoked inhibitory postsynaptic potentials (IPSPs) in only six neurons, their resting membrane potential being 51 +/- 4 mV. The IPSPs frequently showed a fluctuation in latency. FR stimulation also produced antidromic responses in a few VTA neurons, but their long latencies precluded the possibility that the VTA-Hb projections contributed to the FR-evoked orthodromic responses in the VTA. EPSPs evoked by FR stimulation could be suppressed by kynurenic acid (1 mM). The findings indicate that the efferents of the Hb primarily have an excitatory effect on VTA neurons of any type and that the excitation may be mediated by amino acid receptors.

Light response of the neuronal firing activity in the suprachiasmatic nucleus of mice.

To investigate the neural mechanisms underlying the mammalian photic entrainment of circadian rhythms, the response of neuronal extracellular firing activity to retinal light stimulation was investigated in the suprachiasmatic nucleus (SCN) of anesthetized mice during nighttime and daytime. In nighttime, most recorded SCN cells (83%) increased their firing frequency in response to retinal illumination. Some SCN cells (11%) responded by decreasing their firing rate. In daytime, the retinal illumination increased the firing rate in only 26% of the SCN cells, and no response was observed in the remaining cells. The light intensity threshold for the activation of SCN cells at zeitgeber time (ZT) 16 was approximately 3 x 10(11) photons cm(-2)s(-1) and the maximum response was observed at approximately 1 x 10(14) photons cm(-2)s(-1). Therefore, photic response in the firing of mouse SCN cells may be phase-dependent and have a higher threshold, which corresponds to properties of the photic entrainment in locomotor activity of mice.

Suppressive effect of callosotomy on epileptic seizures is due to the blockade of enhancement of cortical reactivity by transcallosal volleys

The present work demonstrates that the cortical reactivity of the rat, monitored by thalamocortical responses, can be enhanced by repetitive transcallosal volleys (5-20 Hz). This effect can be completely inhibited by callosotomy. We believe that the interhemispheric epileptogenesis and the suppressive effect of callosotomy for intractable epilepsy can be explained by this facilitatory effect of the corpus callosum on the cerebral cortex.

Effects of quinidine and quinine on the excitability of pyramidal neurons in guinea-pig hippocampal slices

Effects of quinidine (25μM–1mM) and its stereoisomer, quinine (1–5 mM), on the excitability of CA3 pyramidal neurons were investigated in guinea-pig hippocampal slices using intracellular recording techniques. At concentrations of quinidine higher than 100 μM (and higher than 1 mM for quinine), 1) the resting potential shifted to the depolarizing direction with an increase of the input resistance, 2) the spike duration was prolonged, 3) the spike amplitude was decreased, 4) the late component of the afterhyperpolarization (AHP) (caused by the activity of the Ca2+-mediated K conductance) were suppressed, and 5) finally, neurons became inexcitable. The results indicate that the blocking action of quinidine and quinine is not specific to the Ca2+-mediated K conductance in mammalian hippocampal neurons, and that this conductance is much less sensitive to the drugs in comparison with other preparations.

A new-generation apparatus for studying memory-related performance in mice.

Abstract1. We developed a new kind of food search test that can measure murine nocturnal memory without handling hard work for setting up.2. This apparatus has four food stations, but only one station had accessible food at any time. The one station with accessible food was changed at 4-h intervals.3. We compared the performance of transient forebrain global Ischemic mice, which are a hippocampal lesion model, with the performance of control C57BL/6J mice.4. The correct visit ratio, i.e., the ratio of the number of visits to the correct food station to the number of visits to all stations, gradually increased in the control mice, but did not change in the Ischemic mice.5. This new system was demonstrated to be an additional and useful tool for studying memory-related performance in mice.

Responses of the medial prefrontal cortex to stimulation of the amygdala in the rat: a study with laminar field potential recording

Responses generated in the prelimbic-infralimbic cortices (areas 32 and 25) by stimulation of the basolateral amygdala were studied by means of laminar field potential recordings. Based on the analysis of wave forms and depth potential profiles of the responses, three constituent responses were identified: (1) a surface-positive potential with a short onset latency (mean, 4.5 ms) and a long duration (ca. 70 ms) that reversed the polarity at a depth of 400 microns; (2) a surface-positive potential with a mean onset latency of 15 ms and a duration of 20 ms that reversed to a negative potential at a depth of 300 microns; and (3) a long-latency (ca. 20 ms) superficial-negative potential that reversed to a positive potential at depths below 400 microns. Lesion experiments ruled out the possibility that impulse traffic via the thalamic mediodorsal nucleus (MD) contributed to the responses. Amygdala stimulation also produced responses in the insular cortex, but they did not contaminate the responses in the mesial cortex. It is proposed that impulses originating from the amygdala provide excitation for cortical pyramidal cells at a short latency at deeper layers, but with considerable delays at upper and superficial layers, in the medial prefrontal cortex (mPFC).

Neonatal frontal lesion in unilateral hemisphere enhances the development of the intact higher motor cortex in the rat

The influence of the neonatal frontal lesion in unilateral cerebral hemisphere for the organization of intact forelimb motor cortex in the rat was investigated by intracortical microstimulation (ICMS). The relative size of the rostral forelimb area (RFL) compared to the caudal forelimb area (CFL) in the ipsilateral motor field of lesioned rat was significantly greater than those of contralateral in normal and lesioned rats. The optimal sites of the stimulation for ipsilateral responses in lesioned rats were located in the RFL, while the optimal sites for contralateral were located caudolaterally, as for those of normal rats. At the ipsilateral optimal sites within the RFL in the lesioned animals, the threshold for the ipsilateral responses was lower than that for the contralateral responses. That is, the intact hemisphere of the animal preferentially developed the RFL rather than the CFL, for the ipsilateral forelimb. This may suggest a critical role for the RFL in individual forelimb motor control within the normal hemisphere.