Search

The neurotrophin brain-derived neurotrophic factor (BDNF) is important in development and maintenance of neurons and their plasticity. Hippocampal BDNF has been implicated Alzheimer's disease (AD) because hippocampal levels in AD patients and AD animal models are consistently downregulated, suggesting that reduced BDNF contributes to AD. However, the location where hippocampal BDNF protein is most highly expressed, the mossy fiber (MF) axons of dentate gyrus (DG) granule cells (GCs), has been understudied, and never in controlled in vivo conditions. We examined MF BDNF protein in the Tg2576 mouse model of AD. Tg2576 and wild type (WT) mice of both sexes were examined at 2-3 months of age, when amyloid-β (Aβ) is present in neurons but plaques are absent, and 11-20 months of age, after plaque accumulation. As shown previously, WT mice exhibited high levels of MF BDNF protein. Interestingly, there was no significant decline with age in either genotype or sex. Notably, we found a correlation between MF BDNF protein and GC ΔFosB, a transcription factor that increases after 1-2 weeks of elevated neuronal activity. Remarkably, there was relatively little evidence of Aβ in GCs or the GC layer even at old ages. Results indicate MF BDNF is stable in the Tg2576 mouse, and MF BDNF may remain unchanged due to increased GC neuronal activity, since BDNF expression is well known to be activity-dependent. The resistance of GCs to long-term Aβ accumulation provides an opportunity to understand how to protect other vulnerable neurons from increased Aβ levels and therefore has translational implications., Competing Interests: Declarations of interest: none

The neurotrophin brain-derived neurotrophic factor (BDNF) is important in development and maintenance of neurons and their plasticity. Hippocampal BDNF has been implicated Alzheimer’s disease (AD) because hippocampal levels in AD patients and AD animal models are consistently downregulated, suggesting that reduced BDNF contributes to AD. However, the location where hippocampal BDNF protein is most highly expressed, the mossy fiber (MF) axons of dentate gyrus (DG) granule cells (GCs), has been understudied, and never in controlledin vivoconditions. We examined MF BDNF protein in the Tg2576 mouse model of AD. Tg2576 and wild type (WT) mice of both sexes were examined at 2-3 months of age, when amyloid-β (Aβ) is present in neurons but plaques are absent, and 11-20 months of age, after plaque accumulation. As shown previously, WT mice exhibited high levels of MF BDNF protein. Interestingly, there was no significant decline with age in either genotype or sex. Notably, we found a correlation between MF BDNF protein and GC ΔFosB, a transcription factor that increases after 1-2 weeks of elevated neuronal activity. Remarkably, there was relatively little evidence of Aβ in GCs or the GC layer even at old ages. Results indicate MF BDNF is stable in the Tg2576 mouse, and MF BDNF may remain unchanged due to increased GC neuronal activity, since BDNF expression is well known to be activity-dependent. The resistance of GCs to long-term Aβ accumulation provides an opportunity to understand how to protect other vulnerable neurons from increased Aβ levels and therefore has translational implications.SIGNIFICANCEDeclining hippocampal brain-derived neurotrophic factor (BDNF) has been implicated in the pathogenesis of Alzheimer’s disease (AD). However, few studies have examined where hippocampal BDNF protein has its highest concentration, and plays a critical role in memory, the dentate gyrus granule cell (GC) axons (mossy fibers; MFs). Using a well-established mouse model of cerebral amyloid overexpression, the Tg2576 mouse model of AD, we found that MF BDNF did not decline with age, suggesting a notable exception to the idea that reduced hippocampal BDNF contributes to AD pathobiology. We also identified that Tg2576 GC activity correlates with MF BDNF protein based on GC expression of the transcription factor ΔFosB. These data are consistent with the activity-dependence of MF BDNF. In addition, we found that Tg2576 GCs were relatively resistant to accumulation of amyloid-b, providing insight into AD resilience, which has strong therapeutic implications.

Specific cell populations may have unique contributions to schizophrenia but may be missed in studies of homogenate tissue. Here laser capture microdissection followed by RNA sequencing (LCM-seq) was used to transcriptomically profile the granule cell layer of the dentate gyrus (DG-GCL) in human hippocampus and contrast these data to those obtained from bulk hippocampal homogenate. We identified widespread cell-type-enriched aging and genetic effects in the DG-GCL that were either absent or directionally discordant in bulk hippocampus data. Of the ~9 million expression quantitative trait loci identified in the DG-GCL, 15% were not detected in bulk hippocampus, including 15 schizophrenia risk variants. We created transcriptome-wide association study genetic weights from the DG-GCL, which identified many schizophrenia-associated genetic signals not found in transcriptome-wide association studies from bulk hippocampus, including GRM3 and CACNA1C. These results highlight the improved biological resolution provided by targeted sampling strategies like LCM and complement homogenate and single-nucleus approaches in human brain. Jaffe et al. profile the granule cell layer of the human hippocampus and find unique molecular associations for aging and genetic variation, as well as diagnosis with schizophrenia and its genetic risk, that were previously undiscovered in homogenate tissue.

This corrects the article DOI: 10.1038/tp.2017.132.

Neuregulin-1 (NRG1) and ErbB receptors have been associated with psychopathology, and NRG1-ErbB3 signaling has been shown to increase hippocampal neurogenesis and induce antidepressant-like effects. In this study, we aimed to determine whether deficits in NRG1 or ErbBs might be present in the hippocampus of suicide completers. In well-characterized postmortem hippocampal samples from suicides and matched sudden-death controls, we assessed gene expression and methylation using qRT-PCR and EpiTYPER, respectively. Moreover, in hippocampal tissues stained with cresyl violet, stereology was used to quantify numbers of granule cells and of glia. Granule cell body size was examined with a nucleator probe, and granule cell layer volume with a Cavalieri probe. Unmedicated suicides showed sharply decreased hippocampal ErbB3 expression and decreased numbers of ErbB3-expressing granule cell neurons in the anterior dentate gyrus; a phenomenon seemingly reversed by antidepressant treatment. Furthermore, we found ErbB3 expression to be significantly decreased in the dentate gyrus of adult mice exposed to chronic social defeat stress. Taken together, these results reveal novel suicidal endophenotypes in the hippocampus, as well as a putative etiological mechanism underlying suicidality, and suggest that antidepressant or NRG1 treatment may reverse a potential deficit in anterior dentate gyrus granule cell neurons in individuals at risk of dying by suicide.

Toluene, a psychoactive volatile solvent found in adhesives and other products, is inhaled for its euphoric and intoxicating effects. Toluene inhalation additionally results in cognitive disturbances including impairments in select types of spatial and non-spatial memory, which converging evidence suggests may involve neurons of the dentate gyrus. In the present study we examined the effects of acute binge-like (~ 5000 ppm) toluene inhalation on dentate gyrus granule cell output and perforant path synaptic transmission, using extracellular field potential recordings in anesthetized adult rats in vivo. We found that toluene rapidly and reversibly increased or decreased the amplitudes of evoked population spikes from granule cells over a wide range of stimulation intensities. These changes in granule cell output could not be accounted for by changes in perforant path action potential discharge or presynaptic neurotransmitter release. A marked decrease in the power of the theta rhythm measured within the dentate gyrus was additionally noted. Overall our results suggest that inhalation of abuse-relevant concentrations of toluene changes the readout of perforant path inputs by dentate gyrus granule cells, putatively through a postsynaptic mechanism, and may contribute to explanations for specific learning and memory deficits associated with toluene inhalation.

Hippocampal granule cells transmit information about behaviourally-relevant stimuli to CA3 pyramidal cells via mossy fiber synapses. These synapses express a form of long-term potentiation (mfLTP) that is non-Hebbian and does not require NMDA receptors. mfLTP is thought to be induced and expressed presynaptically, hence, the major determinant of whether mfLTP occurs is activity in the granule cells. However, it remains unclear whether mfLTP can be induced by activity patterns that granule cells exhibit in vivo, and — if so — what context generates these patterns. To address these issues, we examined granule cell activity from in vivo recordings from rats during performance of a delayed nonmatch-to-sample (DNMS) task and found that granule cells exhibit a wide range of spike patterns. In vitro slice experiments in mice demonstrated that some, but not all, of these patterns of activity could induce mfLTP. By further defining the activity thresholds for mfLTP in hippocampal slices, we found that mfLTP can only be induced by spike patterns that fire in high frequency bursts with a low average firing frequency. Using this information, we then screened for supra-threshold bursts of activity during the DNMS task. In a subset of cells, supra-threshold bursts occurred preferentially during the sampling phase of the task. If supra-threshold bursting took place later, during the delay phase, task performance was disrupted. We conclude that mfLTP can be induced by granule cell spike patterns during a memory task, and that the timing of mfLTP induction can predict task performance.

Electroconvulsive shock (ECS) is the most effective treatment for depression, but the mechanism underlying the therapeutic action of this treatment is still unknown. To better understand the molecular changes that may be necessary for the clinical effectiveness of ECS we have combined the technologies of gene expression profiling using cDNA microarrays with T7-based RNA amplification and laser microdissection to identify regulated genes in the dentate gyrus granule cell layer of the hippocampus. We have identified genes previously reported to be up-regulated following ECS, including brain-derived neurotrophic factor, neuropeptide Y, and thyrotrophin releasing hormone, as well as several novel genes. Notably, we have identified additional genes that are known to be involved in neuroprotection, such as growth arrest DNA damage inducible beta (Gadd45beta), and the excitatory amino acid transporter-1 (EAAC1/Slc1A1). In addition, via in situ hybridization we show that EAAC1 is specifically up-regulated in the dentate gyrus, but not in other hippocampal subfields. This study demonstrates the utility of microarray analysis of microdissected subregions of limbic brain regions and identifies novel ECS-regulated genes.

Sex differences in spatial learning have been reported in both humans and rodents. Correspondingly, there have been reports of sexual dimorphism in the morphology of the hippocampal formation (HF), a brain structure implicated in spatial cognition. In Experiment 1, we confirmed earlier reports that the overall volume of the granule cell layer (GCL) of the dentate gyrus (DG) of A/J mice is larger in males than in females. We also found that male A/J mice have a larger GCL volume in the right hemisphere than the left. Female A/J mice displayed no such laterality. A similar pattern of laterality, favoring the right HF, had been reported previously in male, but not female, rats. In Experiment 2, we examined mice with a defective structural gene for androgen receptors (testicular feminization mutant, or tfm mice) on a C57/BL6J background. The C57/J strain had not previously been examined for hippocampal sexual dimorphism. We found no sexual dimorphism in the left, right, or total volume of the GCL in C57/BL6J mice whether they were wildtype or tfm. However, the right GCL volume was greater than the left in wildtype C57/BL6J mice of either sex. No lateralization of GCL volume was found in the androgen-insensitive tfm-affected males or the partially androgen-insensitive tfm-carrier females. These findings confirm earlier reports that sexual dimorphism in mouse HF is found in some inbred strains but not others, and indicate for the first time that mouse HF structures are lateralized. The absence of lateralization in partially or wholly androgen-insensitive mice suggests that androgen receptors may play a role in development of laterality in the GCL independently of any sexual dimorphism in this structure.

Recently developed stereological methods have been used in experiments to examine the effects of two levels of undernutrition during early postnatal life on the total number of rat dentate gyrus granule cells. This study has shown that previously undernourished rats have significant deficits in the total number of this particular type of neuron.

Aim We examined if tonic γ‐aminobutyric acid (GABA)‐activated currents in primary hippocampal neurons were modulated by insulin in wild‐type and tg‐APPSwe mice, an Alzheimer’s disease (AD) model. Methods GABA‐activated currents were recorded in dentate gyrus (DG) granule cells and CA3 pyramidal neurons in hippocampal brain slices, from 8‐10 weeks old (young) wild‐type mice and in dorsal DG granule cells in adult, 5‐6 and 10‐12 (aged) months old wild‐type and tg‐APPSwe mice, in the absence or presence of insulin, by whole‐cell patch‐clamp electrophysiology. Results In young mice, insulin (1 nM) enhanced the total spontaneous inhibitory postsynaptic current (sIPSCT) density in both dorsal and ventral DG granule cells. The extrasynaptic current density was only increased by insulin in dorsal CA3 pyramidal neurons. In absence of action potentials, insulin enhanced DG granule cells and dorsal CA3 pyramidal neurons miniature IPSCT (mIPSCT) frequency, consistent with insulin regulation of presynaptic GABA release. sIPSCT densities in DG granule cells were similar in wild‐type and tg‐APPSwe mice at 5‐6 months but significantly decreased in aged tg‐APPSwe mice where insulin normalized currents to wild‐type levels. The extrasynaptic current density was increased in tg‐APPSwe mice relative to wild‐type littermates but, only in aged tg‐APPSwe mice did insulin decrease and normalize the current. Conclusion Insulin effects on GABA signalling in hippocampal neurons are selective while multifaceted and context‐based. Not only is the response to insulin related to cell‐type, hippocampal axis‐location, age of animals and disease but also to the subtype of neuronal inhibition involved, synaptic or extrasynaptic GABAA receptors‐activated currents. Hayma Hammoud and Olga Netsyk are co‐first authors and contributed equally.Sergiy V. Korol and Zhe Jin contributed equally.

Highlights • Neonatal handling affects the developmental course of cannabinoid receptors in rat brain. • In adult handled rats, [3H]CP55,940 binding levels are reduced in CPu, NAc and BLA. • Dorsal hippocampal [3H]CP55,940 binding levels are reduced only in female handled rats. • Handling increases adolescent and decreases adult [3H]CP55,940 binding levels in mPFC., Neonatal handling is an experimental model of early life experience associated with resilience in later life challenges, altering the ability of animals to respond to stress. The endocannabinoid system of the brain modulates the neuroendocrine and behavioral effects of stress, while this system is also capable of being modulated by stress exposure itself. The present study has addressed the question of whether neonatal handling in rats could affect cannabinoid receptors, in an age- and sex-dependent manner, using in situ hybridization and receptor binding techniques. Different effects of neonatal handling were observed in adolescent and adult brain on CB1 receptor mRNA and [3H]CP55,940 binding levels, which in some cases were sexually dimorphic. Neonatal handling interfered in the developmental trajectories of CB1 receptor mRNA levels in striatum and amygdaloid nuclei, as well as of [3H]CP55,940 binding levels in almost all regions studied. Adult handled rats showed reduced [3H]CP55,940 binding levels in the prefrontal cortex, striatum, nucleus accumbens and basolateral amygdala, while binding levels in prefrontal cortex of adolescent handled rats were increased. Finally, handling resulted in decreases in female [3H]CP55,940 binding levels in the striatum, nucleus accumbens, CA3 and DG of dorsal hippocampus and basolateral amygdala. Our results suggest that a brief and repeated maternal separation during the neonatal period induces changes on cannabinoid receptors differently manifested between adolescence and adulthood, male and female brain, which could be correlated to their stress response.