Fumiaki Motokizawa

Odor representation and discrimination in mitral/tufted cells of the rat olfactory bulb

Extracellular single-unit responses to odorants with various properties were recorded from mitral/tufted cells over large areas of the olfactory bulb of anesthetized rats. Each cell was exposed to one stimulus set consisting of five different odorants each at five concentrations. The resulting concentration-response profiles were compared. All mitral/tufted cells examined responded to two or more odorants, and the largest proportion of the cells were sensitive to all five odorants. Cells unresponsive to all five odorants regardless of concentration were not observed. Mitral/tufted cells sensitive to all three of the odorants that are known to evoke maximal electro-olfactograms in different regions of the olfactory epithelium were distributed widely throughout the olfactory bulb. There were no significant differences in latencies of odor responses either across recording sites or across odorants. A comparison of the concentration-response profiles suggested that all of the mitral/tufted cells were equally capable of responding to any odorant with their own distinctive pattern, but that the cells tended to show an identical pattern rather than variable pattern of response to different odorants. Five mitral/tufted cells isolated within 800 μm of one electrode track showed different concentration-response profiles. Of 18 simultaneously recorded spike pairs with different amplitudes and discharge patterns recorded incidentally through one electrode at different sites, 10 had different and 8 had identical response patterns to odorants. These results suggest that: (1) mitral/tufted cells are sensitive to a broad spectrum of odorants, but respond with their own patterns to odorants; (2) odor discrimination is not uniform in neighboring cells, and a discrimination unit is comprised of a single cell.

A non-thalamic olfactory pathway to the orbital gyrus in the cat

Extracellular unit responses were recorded from the cortical layer of the orbital gyrus following stimulation of the piriform cortex fronting on that gyrus. The responses were obtained only from the dorsal bank of the rhinal sulcus. The responses were presynaptic axon and postsynaptic soma spikes with latencies of about 4 and 4.8 msec, respectively. When the dorsal bank of the rhinal sulcus was stimulated, antidromic responses with a latency of about 4.4 msec were recorded from the superficial and deep soma layers of the piriform cortex. Following injections of horseradish peroxidase into the dorsal bank of the rhinal sulcus, labeled cells were found in the piriform cortex, the lateral, basolateral and central amygdaloid nuclei, and the prelimbic area. These results indicate that the piriform cortex projects directly to part of the orbital gyrus by way of association fiber pathways.

Discharge properties of mitral/tufted cells in the olfactory bulb of cats

The spontaneous activity of mitral/tufted cells of the cat olfactory bulb was subjected to time-series analysis. Ninety percent of 284 cells displayed either bimodal or skewed distribution under both tracheal and nasal respiration, indicating that the spontaneous activity is characterized by burst discharge. We conclude that the source of the burst discharge originates in the olfactory bulb, rather than being induced by nasal airflow or by centrifugal inputs to the olfactory bulb.

The piriform cortex is not a direct olfactory relay to the mediodorsal thalamic nucleus in cats and rabbits.

Following injections of horseradish peroxidase into the mediodorsal thalamic nucleus (MD), retrogradely labeled cells were found in various areas in the cat and rabbit. Among these, olfactory-related areas to which the olfactory bulb projects directly or indirectly via the piriform cortex were the olfactory tubercle, amygdala and insular cortex, while no labeled cells were detected in the piriform cortex and endopiriform nucleus in both species. These results indicate that the piriform cortex and endopiriform nucleus do not send their axons directly to the MD.

The principal pathway from the piriform cortex to the deep amygdaloid nuclei in the cat

Pathways relaying olfactory information to the deep amygdaloid nuclei (AMYGd) were analyzed with electrophysiological techniques in anesthetized cats. Stimulation of the piriform cortex (PC) produced orthodromic spikes in some AMYGd neurons with a mean latency of 12.2 ms and antidromic responses in other neurons with a mean latency of 9.3 ms. Stimulation of the AMYGd produced antidromic spikes in some PC neurons with a mean latency of 11.5 ms. Some neurons in the entorhinal area (EA) were activated orthodromically from the PC with a mean latency of 22 ms, and a proportion of these cells was also activated antidromically from the AMYGd. Some neurons in the agranular insular cortex were activated orthodromically from the PC, but none of them responded antidromically to AMYGd stimulation. From these observations, it is suggested that olfactory information reaches the AMYGd directly from the PC or indirectly via the EA, and that the direct path conveys the major olfactory input from the PC to the AMYGd.

2004 Correlation of olfactory bulb responses with various properties of odorants

The localization of voltage-gated ion channels was investigated on mouse taste bud cells in the peeled tongue epithelium under voltage clamp conditions. Since the peeled tongue epithelium separated perfusing solutions on the receptor and basolateral membrane of the taste bud cells, we could apply channel blockers on either of these membranes. The perfusion of the basolateral membrane with 1 PM TTX blocked voltage-gated Na current. The perfusion with 10 mM TEA or 10 mM 4AP blocked delayed-rectifier K current, inactivating outward K current and saturating outward K currents. On the other hand, the blockers applied on the receptor membranes had no effect. These results indicate that the majority of these channels exists on basolateral membrane. Inward rectifier K channels and L and T type Ca channels were also suggested to exist on the basolateral membrane. Since ionic environments on the receptor membranes are unstable under physiological conditions, such localization seems to have biological significance.