Postnatal development of Homer1a in the rat hippocampus

Abstract

Homer1a (H1a) is an immediate early gene involved in multiple forms of synaptic plasticity. It exhibits a postnatal increase in the rat forebrain (Brakeman et al. (1997) Nature 386:284-288) and reduces the density and size of dendritic spines in hippocampal neurons (Sala et al. (2003) J Neurosci 23:6327-6337). We evaluated hippocampal H1a expression at different postnatal ages (P3, P5, P7, P9, P15, P19, P23, P35, and adult) using Fluorescence In Situ Hybridization (FISH) and qRT-PCR. Maximal electroconvulsive shock (MECS) was used to induce maximal expression relative to home cage (HC) controls. Large scale images and confocal z-stacks from dorsal subiculum (DS), CA1, CA3, and dentate gyrus (DG) were analyzed by both manual and automated methods. In DS, CA1, and CA3 a significant proportion of cells (40%) expressed small but detectable levels of H1a from P3; however, MECS did not up-regulate H1a during the first postnatal week. MECS induced H1a positive cells during the second postnatal week and induction reached adult levels at P9. H1a-Intra Nuclear Foci (INF) size and intensity varied with age, increasing at P19-23 in CA1 and CA3 and from P9 to P23 in DS. In DG, H1a expression exhibited a lamination pattern and an H1a-INF size and intensity gradient across the granule cell layer, consistent with the outside-in maturation of DG granule cells. The developmental progression of H1a corresponds to the synaptic refinement period supporting the conclusion that H1a could play an important role in this process.

Regulation and Function of Activity-Dependent Homer in Synaptic Plasticity

“…Postnatal forebrain expression of Homer1a increases from birth, peaking between 3 and 5 weeks [7]. Montes-Rodríguez et al [41] demonstrated that induction of Homer1a by neuronal activity was related to postnatal age. Maximal electroconvulsive shock treatment in rats only increased hippocampal Homer1a expression when applied in the second postnatal week (but not the first), which corresponds to the maturation of glutamatergic transmission required for synaptic plasticity.…”

“…Neuronal activity-induced expression of short Homer1 mRNA has been observed throughout brain structures known to be involved in learning and memory, including the cerebral cortex, hippocampal CA1, CA2, CA3, and dentate gyrus, striatum, and amygdala [6, 7, 16, 35-41]. The induction of Homer1a has been extensively investigated in vitro including neuronal depolarization with ionotropic glutamate receptor agonists, N-methyl-D-aspartate (NMDA), kainite, and potassium channel blockers [31], application of brain-derived neurotrophic factor (BDNF) [42], traumatic injury [43, 44], and evoking epileptiform activity with bicuculline and 4-aminopyridine [45].…”

“…Maximal electroconvulsive shock treatment in rats only increased hippocampal Homer1a expression when applied in the second postnatal week (but not the first), which corresponds to the maturation of glutamatergic transmission required for synaptic plasticity. The quantity of hippocampal activity-induced Homer1a induced by maximal electroconvulsive shock (measured by intra-nuclear foci intensity) peaked when applied at 3 weeks of age, in parallel with synaptic maturation, suggesting a role in the refinement of neuronal circuits [41, 70]. The balance of short to long Homer1 isoforms is also an important regulator of axonal path finding in developing neurons [71] and unsurprisingly, altered Homer1a expression causes developmental impairments in locomotor activity, motor coordination, and motor learning [72-74].…”

A methodological pipeline for serial-section imaging and tissue realignment for whole-brain functional and connectivity assessment

“…First, we conducted an experiment for the detection of the IEG Homer1a on a data set of serial images from a mouse brain. Homer1a and other IEGs (e.g., Arc, Narp, c-Fos) are transcribed rapidly following neuronal firing, and can therefore be used as markers of behaviorally driven ensemble activity following a specific event (Guzowski et al, 1999; Guzowski et al, 2005; Montes-Rodriguez et al, 2013). In the present study, we used environmental exploration to drive Homer1a expression.…”

“…Here, we describe an analysis platform written in a Matlab framework, which conveniently combines several open-source software platforms such that users may detect, display, and quantify input-output relationships in fluorescent images sampled across entire brains and in 3-D. The methods presented here include image capture and tissue collection procedures that have been previously published, however, these methods will be summarized briefly below (Montes-Rodriguez et al, 2013; Wilber et al, 2015). We demonstrate application of this approach to a variety of data sets including neuroanatomical and functional single cell activation studies in the rat brain, demonstrating that this software tool can be rapidly adapted for analysis of any nervous system marker or stain that has been serially-sectioned and block-face imaged.…”