Muneyuki Ito

Analysis of variance study of the rat cortical layer 4 barrel and layer 5b neurones

Unique formation of rodent cortical barrels by layer 4 neurones attracts study of the sensory function of cortical input stage neurones (layer 4) compared with that of output stage neurones (layer 5). We have recorded extracellular responses from rat somatosensory cortical neurones to deflections of contralateral vibrissae. Thirty‐two layer 4 barrel neurones and 29 layer 5b neurones were studied. Whisker stimulations were ramp‐and‐hold deflections with one of six different ramp velocities (100–2.5 mm s−1) and one of four different plateau amplitudes (2000–200 μm). Twenty‐four (6 × 4) different stimulus forms were applied to the tip of a whisker trimmed to 10 mm in a predetermined order in stimulus cycles of 20–50 repetitions. Spike counts for a period of 2560 ms in 10 ms bins were summed to construct a matrix of 24 peristimulus histograms for each neurone. Twenty‐four amplitude and 24 velocity values were computed from counts during the plateau and ramp phases, respectively. To determine the amplitude‐ and velocity dependence of a neurone, an amplitude F value (the ratio of variations among‐/within‐amplitude of the amplitude value) and a velocity F value (ratio of variations among‐/within‐velocity of the velocity value) were derived by analysis of variance. The amplitude F value of the layer 4 barrel neurones was greater than that of the layer 5b neurones (P < 0.0001). The velocity F value of the barrel neurones was smaller than that of the layer 5b neurones (P = 0.0226). The results suggests that barrel neurones and layer 5b neurones tend to detect amplitude and velocity components of whisker deflection, respectively.

Cortical somatosensory evoked potentials of seizure-sensitive and seizure-resistant gerbils

The Mongolian gerbil is known as an animal model that often exhibits spontaneous seizures that are characteristic of human epilepsy. Whereas there is much more information available relating this phenomenon to anatomical and electrophysiological characteristics of the hippocampal formation, the somatosensory cortex has rarely been the focus of attention. Given the existence of the fine grain cortical barrels developed in an orderly matrix, the vibrissa ascending system was thought the best sensory channel in which the gerbil neocortical excitability was to be tested. In the present study, we compared cortical evoked potentials to electric stimulation of vibrissa (whisker) follicle between the seizure-sensitive (SS) and seizure-resistant (SR) gerbils. Whereas our standard stimulation of the whisker follicle elicited a positive-negative biphasic somatosensory cortical potential in SR animals, it evoked only a positive monophasic potential in SS animals. Although the amplitude of positivity was generally larger in SR animals, the latency to reach this peak was significantly smaller than in the SS animals. Apparently there was no group difference in the laminar profile or the size of barrels that represent, in one-to-one fashion, the array of whisker follicles. It was suggested that an altered level of excitability in the absence of gross anatomy differentiates the seizure-sensitive gerbils from the seizure-resistant counterparts. Possible functional differences between the cortices of the two groups were discussed in light of the known synaptic electrophysiology.

Manipulation of the Somatosensory Cortex Modulates Stimulus-Induced Repetitive Ear Movements in a Seizure-Sensitive Strain of Gerbil

Abstract Some Mongolian gerbils (Meriones unguiculatus) respond to stimulation by seizures, the pattern of which changes progressively during development. We previously established a seizure-sensitive strain, MGS/Idr, in which all animals exhibit such stimulus-induced seizures. We have now noted that all adults of this strain also show repetitive backward movements of the ears at the beginning of stimulus-induced seizures, although the incidence varies with the individual. We examined whether the cerebral cortex was involved in these movements and found that electrical stimulation of an area of the somatosensory cortex elicited strong backward movement of the ear on the contralateral side, and that unilateral application of bicuculline, a GABAA receptor antagonist, induced spontaneous repetitive backward movements of the same ear. In this area, sharp waves appeared in the electrocortigram during the repetitive ear movements induced by seizure-inducing stimuli. Unilateral ablation of this area abolished stimulus-induced repetitive movements of the contralateral ear, but had no effects on those of the ipsilateral ear. These results suggest that, in certain types of seizure-susceptible subjects, it may be possible to modify stimulus-induced repetitive movements by manipulating a certain area of the somatosensory cortex which is related to these movements and that this gerbil strain may be useful in research on this subject.

Site of Cortical Utricular Representation with Special Reference to the Somatosensory Barrel Field in the Gerbil

In mammals, the osseous semicircular canals of the vestibular labyrinths are usually embedded in the pyramis of the temporal bone. Thus, the osseous semicircular canals are a cavity system that can only be visualized by injection molding. Exceptionally, the walls of the osseous semicircular canals of the Mongolian gerbil are not embedded, but exposed in the hollow space in the temporal bone. Under urethane anesthesia, a window was made in the periotic capsule of the gerbil to reach this hollow space (semicircular canal triangle), and a pair of wire electrodes were inserted through an opening made in the horizontal semicircular canal toward the utricular nerve endings. Repetitive electric stimuli at 80 Hz were applied, and the direction of eye movements was noted. Subsequently, the stimulation frequency was reduced to 0.3 Hz, and evoked potentials were recorded from the neocortex. Positive surface potentials and negative deep potentials were recorded in the somatosensory area and, more specifically, in the vibrissa “barrel field,” as judged by later histochemical staining of the cortical specimens. This unique anatomic feature of the gerbil labyrinth offers an opportunity whereby the vestibular organ can be reached without any heavy surgical insult, and the presence of fine-grain vibrissa barrels in this species (seven rows instead of five rows in most rodents) will help elucidate functional interactions between vestibular and somatosensory sensations.

Stimulus-induced behavior in F1 hybrids of seizure-sensitive and seizure-resistant gerbils.

Abstract We previously established two strains of Mongolian gerbil: a seizure-sensitive strain, established by selective inbreeding for motor seizures elicited by a stimulus called the S method and a seizure-resistant strain that does not exhibit inducible seizures. The behavior of the seizure-sensitive strain is characterized by a progressive increase in responsiveness to weekly application of the S method, from repetitive backward ear movements appearing after postnatal day 40, to a full-blown seizure, while the seizure-resistant strain is apparently unaffected by the stimulation. The difference between these two strains is presumably genetic. To determine the genetic factors underlying this difference, we first examined developmental changes in the stimulus-induced behavior of the F1 hybrids. When the S method was applied, most F1 hybrids had repetitive movements of the ears (and head) similar to the seizure-sensitive gerbils, but generalized seizures emerged considerably later than in seizure-sensitive gerbils. These results suggest that a half dose of the gene products involved renders most gerbils susceptible to the stimulus but is insufficient for the rapid accumulation of an as yet undefined change needed to spread the abnormal electrophysiologic activity to elicit generalized seizures.

Electrophysiological characteristic of the cerebral cortex of the gerbil

The Mongolian gerbil (Meriones unguiculatus) is a genetic animal model of epilepsy. In a seizure-sensitive strain that we have established, posture change, a potent seizure-inducer in adults, induces rhythmical backward movement of the pinnae in young animals. A major stimulus during posture change is vestibular; the vestibular cortex was localized in the somatosensory area and electrical stimulation of this site induced backward movement of the pinnae. Since this site may be involved in seizure, we studied its electrophysiological characteristics, using a telemetry machine. A puff of air was found to induce a sharp EEG wave in the region that includes the vestibular cortex; this effect and the movement of the pinnae were often repeated on stimulus cessation. Animals that had not undergone surgery also showed repetitive ear movement under similar conditions, suggesting that these electrophysiological features do no result from surgery, but are intrinsic to this strain.