5-HT_3AReceptor-Bearing White Matter Interstitial GABAergic Interneurons Are Functionally Integrated into Cortical and Subcortical Networks

Abstract

In addition to axons and surrounding glial cells, the corpus callosum also contains interstitial neurons that constitute a heterogeneous cell population. There is growing anatomical evidence that white matter interstitial cells (WMICs) comprise GABAergic interneurons, but so far there is little functional evidence regarding their connectivity. The scarcity of these cells has hampered electrophysiological studies. We overcame this hindrance by taking recourse to transgenic mice in which distinct WMICs expressed enhanced green fluorescence protein (EGFP). The neuronal phenotype of the EGFP-labeled WMICs was confirmed by their NeuN positivity. The GABAergic phenotype could be established based on vasoactive intestinal peptide and calretinin expression and was further supported by a firing pattern typical for interneurons. Axons and dendrites of many EGFP-labeled WMICs extended to the cortex, hippocampus, and striatum. Patch-clamp recordings in acute slices showed that they receive excitatory and inhibitory input from cortical and subcortical structures. Moreover, paired recordings revealed that EGFP-labeled WMICs inhibit principal cells of the adjacent cortex, thus providing unequivocal functional evidence for their GABAergic phenotype and demonstrating that they are functionally integrated into neuronal networks.

Neurotransmitter signaling in white matter

“…However, caution must be used in extrapolating what glial cells are capable of doing in culture conditions from what they actually do in the CNS, and we have compiled a list of glutamatergic/purinergic (Table 1) and non-glutamatergic/purinergic WM receptor expression studies (Tables 2 and 3), restricted to reports that have confirmed expression in situ using established cell markers. In addition to distinguishing types of glia, a reliable test for cell identification is essential due to the presence of neuronal populations in some WM structures (von Engelhardt et al, 2011), although this is not true for the commonly used optic nerve preparations.…”

“…Notably, the optic nerve does not contain neuronal cell bodies or synapses, although this is not the case for all WM structures (von Engelhardt et al, 2011). It is surprising that neurotransmitter-mediated signaling is prominent in the optic nerve and other WM areas studied, since neurotransmitter signaling is generally considered to be an exclusive function of neurons confined to synapses.…”

Characterization of immature and mature 5-hydroxytryptamine 3A receptor-expressing cells within the adult SVZ–RMS–OB system

“…In the neonatal mice, the majority of 5-HT 3A R+ cells born in the SVZ migrate to the OB along the RMS. Some of these cells exit RMS and migrate to the corpus callosum and neocortex, where they express markers of GABAergic interneurons (Inta et al, 2008;Le Magueresse et al, 2011;von Engelhardt et al, 2011).…”

The 5-HT3A receptor is essential for fear extinction

“…Therefore, CCK transmission may also contribute to the 5-HT 3A receptor-mediated extinction of fear memory. It has been suggested that 5-HT 3 receptorcontaining interneurons are composed of a biochemically heterogeneous subpopulation of neurons that may be involved in different inhibitory circuits (Morales and Bloom 1997;Lee et al 2010;von Engelhardt et al 2011); however, the specific function of each subgroup in fear memory processes remains to be established. Therefore, detailed analysis based on the subgroups of 5-HT 3 receptor-expressing interneurons will provide further insight into their specific roles in the regulation of fear memory.…”