The hippocampus contains more than 20 types of inhibitory interneurons that express different proteins and impinge on different regions of pyramidal cells to regulate spatiotemporal integration of EPSPs and define temporal windows for spiking. Neurogliaform cells (NGFCs) form synapses on the distal tufts of pyramidal cell apical dendrites alongside excitatory inputs from the entorhinal cortex. NGFCs express neuronal nitric oxide synthase (nNOS), are often synaptically coupled, and fire during theta oscillations in vivo. Li et al. published a “featured article” in the Journal of Neuroscience (34(4):1280-1292, 2014) reporting a novel physiological action mediated by this interneuron type. They found that when theta-associated activity patterns were evoked in NGFCs in hippocampal slices of rat or mouse, the cells showed a transient reduction in unitary IPSP amplitude. This “firing-induced suppression of inhibition” (FSI) required back-propagation of action potentials, calcium influx through L-type calcium channels, nNOS activity, and activation of NO-sensitive guanylyl cyclase (NO-sGC) receptors, which are present on presynaptic terminals. FSI also indirectly increased the amplitude of EPSPs. Thus FSI may enhance spatial and temporal summation of excitatory inputs to NGFCs, regulating their inhibition of pyramidal cells. More in general, this work demonstrates: 1) retrograde signaling initiated by “in vivo firing pattern”, 2) interneuron back-propagation detected with fast time resolution voltage imaging, and 3) physiological role for nNOS expressed by specific interneuron types.
Dr Marco Canepari visits Marco Capogna’s lab to optically record back-propagating action potential with calcium and voltage-imaging in interneurons of hippocampal slices. This ongoing collaboration joins visitor’s expertise in high-resolution imaging techniques with the expertise of Marco Capogna on synaptic inhibition and interneuron physiology. Marco Canepari is a senior scientist at the Grenoble Institute of Neuroscience, Inserm, France. His visit to Oxford is supported by a short-term travel grant by the Physiological Society UK.
Miroslawa Manko, Thomas Bienvenu, Yannis Dalezios and Marco Capogna report in the last issue of the Journal of Physiology (590.22:5611-5627, 2012) a novel interneuron type of the amygdala, termed neurogliaform cell, and study its function by a combination of in vivo and in vitro techniques. They used a mouse line expressing a green fluorescent protein (GFP) under the neuropeptide Y (NPY) promoter. Paired recordings between presynapticNPY-GFP-expressing (+) cells and postsynaptic principal neurons (PNs) of the basolateral amygdala (BLA) were performed.