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Abstract
Inhibitory interneurons make up around 10–20% of the total neuron population in the cerebral cortex. A hallmark of inhibitory interneurons is their remarkable diversity in terms of morphology, synaptic connectivity, electrophysiological and neurochemical properties. It is generally understood that there are three distinct and non-overlapping interneuron classes in the mouse neocortex, namely, parvalbumin-expressing, 5-HT3A receptor-expressing and somatostatin-expressing interneuron classes. Each class is, in turn, composed of a multitude of subclasses, resulting in a growing number of interneuron classes and subclasses. In this review, I will focus on the diversity of somatostatin-expressing interneurons (SOM+ INs) in the cerebral cortex and elucidate their function in cortical circuits. I will then discuss pathological consequences of a malfunctioning of SOM+ INs in neurological disorders such as major depressive disorder, and present future avenues in SOM research and brain pathologies.
Highlights
Brief Overview on the Diversity of GABAergic Interneurons in the BrainTwo main neuron populations are found within the rodent and human cerebral cortex: excitatory projection neurons and inhibitory interneurons
Their relative numbers differ from brain region to brain region but generally, excitatory projection neurons make up around 70–80% of the overall neuron population, whereas inhibitory interneurons constitute the remaining 20–30%
In contrast to pyramidal neurons that represent a rather homogenous group of cells, GABAergic interneurons are characterized by a stand-alone diversity of cellular properties ranging from the expression of different neurochemical marker proteins and a great variety of morphological phenotypes to highly varying electrophysiological properties [1,2,3]
Summary
Two main neuron populations are found within the rodent and human cerebral cortex: excitatory projection neurons and inhibitory interneurons. Performing an intersect analysis from camera lucida drawings, it could be shown that the axons of neocortical SOM+ INs constitute at least three quarters (76%) of GABAergic axons in layer 1 [79] Given their dense axonal morphology, Martinotti cells can modulate the activity of a large number of pyramidal neurons that represent their preferential postsynaptic Itnat.rgJ.eMtso.l. SIcni. Given their dense axonal morphology, Martinotti cells can modulate the activity of a large number of pyramidal neurons that represent their preferential postsynaptic targets. Given the fact that synapses between a presynaptic pyramidal neuron and a postsynaptic SOM+ IN are strongly facilitating, feedback inhibition onto the same or neighboring pyramidal cell is dependent on the firing frequency in the presynaptic pyramidal neuron, i.e., SOM+ INs act as rate detectors providing a frequency filter to the postsynaptic pyramidal neuron
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