title: SP-SR interneurones: a novel class of neurones of the CA2 region of the hippocampus.
abstract: The CA2 region of the hippocampus has distinctive properties and inputs and may be linked with the pathology of specific psychiatric and neurological disorders. It is, therefore, important to understand CA2 circuitry and its involvement in the circuitry of the hippocampus. Properties of CA2 basket cells have been reported. However, other classes of CA2 interneurones with cell bodies located in stratum pyramidale remained to be described. In this study, the unusual axonal arbors of a novel subclass of dendrite-preferring CA2 interneurones whose somata are located in the pyramidal cell layer was revealed following intracellular recordings and biocytin labeling. One to four apical dendrites emerged from the soma, branched in stratum radiatum (SR) forming a tuft, but rarely penetrated stratum lacunosum-moleculare (SLM). One or two basal dendrites branched close to the soma, the branches extended through stratum oriens (SO) and often reached the alveus. Unlike CA2 bistratified cells, the axons of these cells arborized almost exclusively in SR with few, if any, branches extending to stratum pyramidale (SP), SO, or SLM. These interneurones again, unlike bistratified cells, were immunonegative for parvalbumin and cholecystokinin. Electrophysiologically, they were similar to some CA2 basket and bistratified cells in that they presented a "sag" in response to hyperpolarizing current injections and displayed spike frequency adaptation. They targeted the apical dendrites of neighboring CA2 pyramidal cells and received inputs from them.
authors.affiliation: Department of Pharmacology, UCL School of Pharmacy, London, United Kingdom. audrey.mercer@pharmacy.ac.uk
authors.forename: Audrey
authors.initials: A
authors.lastname: Mercer
authors.forename: Nicola A
authors.initials: NA
authors.lastname: Botcher
authors.forename: Karen
authors.initials: K
authors.lastname: Eastlake
authors.forename: Alex M
authors.initials: AM
authors.lastname: Thomson
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001369:Axons / Q000378:metabolism; D056651:CA2 Region, Hippocampal / Q000166:cytology* / Q000378:metabolism; D002766:Cholecystokinin / Q000276:immunology / Q000378:metabolism; D003712:Dendrites / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D007395:Interneurons / Q000166:cytology* / Q000378:metabolism; D008297:Male; D010320:Parvalbumins / Q000276:immunology / Q000378:metabolism; D018408:Patch-Clamp Techniques / Q000379:methods; D017966:Pyramidal Cells / Q000166:cytology* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013569:Synapses
chemical_list: D010320:Parvalbumins; D002766:Cholecystokinin