Structural and functional abnormalities of the hippocampal formation in rats with environmentally induced reductions in prepulse inhibition of acoustic startle

The effects of social isolation on prepulse inhibition of acoustic startle (PPI), electrophysiology and morphology of subicular pyramidal neurons and the densities of interneuronal sub-types in the hippocampal formation were examined. Wistar rats (male weanlings) were housed socially (socials, n = 8 ) or individually (isolates, n = 7). When tested eight weeks later, PPI was lower in isolates. Rats then received terminal anaesthesia before slices o f hippocampal formation were made in which the electrophysiological propert ies of a total of 108 subicular neurons were characterized. There were no d ifferences in neuronal sub-types recorded in socials compared with isolates . Intrinsically burst-firing and regular spiking pyramidal neurons were exa mined in detail. There were no differences in resting membrane potential or input resistance in isolates compared with socials but action potential he ight was reduced and action potential threshold raised in isolates. A limit ed morphological examination of Neurobiotin-filled intrinsically burst-firi ng neurons did not reveal differences in cell-body area or in number of pri mary dendrites. Sections from the contralateral hemispheres of the same rat s were stained with antibodies to calretinin, parvalbumin and the neuronal isoform of nitric oxide synthase (nNOS). In isolates, the density of calret inin positive neurons was increased in the dentate gyrus but unchanged in a reas CA3, CA1 and subiculum. Parvalbumin and nNOS positive neuronal densiti es were unchanged. Hence in rats with environmentally induced reductions in PPI there are stru ctural and functional abnormalities in the hippocampal formation. If the re duction in PPI stems from these abnormalities, and reduced PPI in rats is r elevant to schizophrenia, then drugs that correct the reported electrophysi ological changes might have antipsychotic effects. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.