Electrophysiological evidence that a set of interfascicular cells of the rat anterior commissure are neurons

Chandelier and interfascicular neurons in the adult mouse piriform cortex

The structure of two neuron types native to the adult mouse piriform cortex (PC) is described. The first cell, termed an interfascicular neuron (IFN), lies between the axon fascicles of layer I. The IFN axon divides dichotomously and daughter fibrils run horizontally in the domain of layer Ia. The frequent apposition of the IFN axon to distal dendrites of the underlying pyramidal cells suggests an en passage synaptic interaction with them. A second neuron observed in layer II, or less frequently in layer III, matched in most respects the structure of the chandelier cell (CC) described elsewhere in the neo- and archi-cortex. In the PC, chandelier cells (PC-CC) display the following peculiarities. First, the PC-CC axonal field distributes in the neuropil of layers II and III and candlesticks are in close apposition to the initial axonal segment of the pyramidal cell, although somatic interactions cannot be rule out. Second, the PC-CC ascending dendrites pierce layer I, receiving short collaterals and boutons en passage from the olfactory axons therein. The possible role of IFN's and PC-CC and their interactions with the adjacent cells is discussed in the broad context of the cellular organization of the PC.

Reelin-expressing neurons in the anterior commissure and corpus callosum of the rat

Reelin is an extracellular matrix protein, which plays a crucial role for the formation of laminated and nonlaminated structures in the central nervous system. To elucidate its roles in the postnatal brain, in the present study, we raised a polyclonal antibody specific for rat Reelin, and investigated Reelin-expressing neurons in the rat brain during the postnatal periods in detail. We found that some Reelin-expressing cells existed in the anterior commissure and corpus callosum. These Reelin-expressing cells were also immunostained with the antibody specific for neurons, but not immunostained with the antibodies specific for astrocytes nor oligodendrocytes, suggesting that these Reelin-expressing cells in the white matter are neurons. They are also immunostained with anti-GAD67 antibody, indicating that Reelin-expressing cells in the commissure systems are GABAergic neurons. Reelin-expressing neurons in the anterior commissure had many conspicuous varicosities on their dendritic arbors and mimic to the interfascicular neurons. These results suggest that Reelin may participate in the regulatory mechanism of neuronal activities through the commissure structure during the postnatal periods.

Electrophysiological responses of interfascicular neurons of the rat anterior commissure to activation from the anterior olfactory nucleus, medial frontal cortex, and posterior nucleus of the amygdala

Interfascicular neurons (IFNs) of the anterior commissure (AC) include short-axon and projection types which receive inputs from commissural collaterals. Therefore, it was proposed that IFNs may play a role in processing nerve impulses arising from the forebrain and delivered by these collaterals [Brain Res. 931 (2002) 81-91]. To determine possible inputs from the forebrain to IFNs we performed extracellular recordings of 25 neurons from anesthetized adult rats. Short-latency evoked potentials in IFNs were elicited by electrical stimulation of the anterior olfactory, posterior amygdaloid nuclei (PA), and medial frontal cortex. The IFN responses showed three distinct patterns, namely, a single action potential (AP) followed by what appear to be spontaneous discharge; a burst of high-frequency APs, and a single AP followed by a period devoid of APs. The latter response which was elicited by stimulation of the PA, may be explained by an intervening inhibitory interneuron, perhaps GABAergic in nature. Finally, IFNs seem not to project back to any of these three forebrain areas, as we failed to demonstrate antidromic activation.