Trends in Neurosciences
Volume 26, Issue 10, October 2003, Pages 564-571
Journal home page for Trends in Neurosciences

Interneuron Diversity series: Hippocampal interneuron classifications – making things as simple as possible, not simpler

https://doi.org/10.1016/j.tins.2003.08.002Get rights and content

Abstract

The nervous system is made up of many specific types of neuron intricately intertwined to form complex networks. Identifying and defining the characteristic features of the many different neuronal types is essential for achieving a cellular understanding of complex activity from perception to cognition. So far, cortical GABAergic interneurons have represented the epitome of cellular diversity in the CNS. Despite the desperate need for effective classification criteria allowing a common language among neuroscientists, interneurons still evoke memories of Babel. Several approaches are now available to overcome the challenges and problems associated with the various classification systems used so far.

Section snippets

Morphological

The morphological appearance of interneurons is a source of important information regarding their specific role in a neuronal circuit. Indeed, their anatomy alone can provide intuitive insights into cell-type-specific contributions in an active network, by relating the somatodendritic location to the layer specificity of synaptic input and the axonal projections to the postsynaptic target domain. The earliest studies, based on Golgi impregnations 1, 7, distinguished ∼20 different types of

The role of interneurons in cortical circuits and the problem with a unified ‘all encompassing’ classification

Combined morphological and physiological approaches have been used with paired intracellular recordings in slices to include the characteristics of interneuron target synapses in their classification (e.g. axo–axonic cells targeting axon initial segments; basket cells targeting primary dendrites and somatic regions 8, 11; Schaffer-collateral-associated interneurons targeting dendritic regions of pyramidal cells [15]; and O–LM cells targeting distal dendrites [11]). The physiological relevance

Concluding remarks

Compared with simpler categorizations, a complex classification based on the simultaneous evaluation of multiple parameters seems to offer a clear advantage for the consistent definition of a specific interneuronal subtype. In the hippocampus, an excellent example of such a well-defined interneuron subtype might be the O–LM cell (Figure 2). This interneuron, included previously in the larger group of oriens-alveus ‘horizontal cells’, has a consistent anatomy with horizontally oriented dendrites

Acknowledgements

We thank Chris J. McBain, Tamas F. Freund and an anonymous reviewer for their constructive comments and insights on the manuscript during the reviewing process. Our research is supported by the National Institute of Mental Health (grant MH067561 to G.M.), the Canadian Institutes of Health Research and the Fonds de la Recherche en Santé du Québec (J.C.L.). J.C.L. is the recipient of a Canada Research Chair in Cellular and Molecular Neurophysiology.

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