Serotonin modulates the excitatory synaptic transmission in the dentate granule cells

Resilience to Early Life Adversity Effects on Stress Reactivity by Postnatal Knockdown of 5-HT 1A Autoreceptors

Early Life Adversity (ELA) predisposes to stress hypersensitivity in adulthood, but neurobiological mechanisms that can protect from long-lasting effects of ELA are poorly understood. Serotonin 1A (5HT 1A ) autoreceptors in the raphé nuclei regulate adult stress vulnerability, but if 5HT 1A could be targeted to prevent ELA effects on susceptibility to future stressors is unknown. Here, we exposed mice with postnatal knockdown of 5HT 1A autoreceptors to the limited bedding and nesting model of ELA from postnatal day (P)3-10. We then tested behavioral, neuroendocrine, neurogenic, and neuroinflammatory responses to an acute swim stress in male and female mice in adolescence (P35) and in adulthood (P56). In ELA-exposed females, adult swim stress exposure increased passive coping and despair-like behavior, corticosterone levels at baseline and after stress, and neuronal activity and corticotropin releasing hormone levels in the paraventricular nucleus of the hypothalamus. ELA also reduced neurogenesis and increased microglia activation in the ventral dentate gyrus (DG) of the hippocampus - an important mediator of individual differences in stress susceptibility. These effects of ELA were specific to females, but not males, and manifested predominantly in adulthood, but not earlier on in adolescence. Postnatal 5HT 1A autoreceptor knockdown prevented ELA effects on stress reactivity and on neurogenesis and neuroinflammation in the DG, indicating that reducing 5HT 1A autoreceptors confers resilience to ELA. Our findings demonstrate that ELA induces long-lasting and sex-specific impairments in stress reactivity and ventral DG function across development, and identify 5HT 1A autoreceptors as potential targets to prevent these persistent effects of ELA.

Serial electroconvulsive Seizure alters dendritic complexity and promotes cellular proliferation in the mouse dentate gyrus; a role for Egr3

BACKGROUND

Despite being one of the safest, most effective treatments for severe mood disorders, the therapeutic mechanisms of electroconvulsive therapy remain unknown. Electroconvulsive seizure (ECS) induces rapid, high-level expression of immediate early genes (IEGs) and brain-derived neurotrophic factor (BDNF), in addition to stimulation of neurogenesis and dendritic remodeling of dentate gyrus (DG) neurons. We have previously shown that this upregulation of BDNF fails to occur in the hippocampus of mice lacking the IEG Egr3. Since BDNF influences neurogenesis and dendritic remodeling, we hypothesized that Egr3-/- mice will exhibit deficits in neurogenesis and dendritic remodeling in response to ECS.

OBJECTIVE

To test this hypothesis, we examined dendritic remodeling and cellular proliferation in the DG of Egr3-/- and wild-type mice following repeated ECS.

METHODS

Mice received 10 daily ECSs. Dendritic morphology was examined in Golgi-Cox-stained tissue and cellular proliferation was analyzed through bromodeoxyuridine (BrdU) immunohistochemistry and confocal imaging.

RESULTS

Serial ECS in mice results in dendritic remodeling, increased spine density, and cellular proliferation in the DG. Loss of Egr3 alters the dendritic remodeling induced by serial ECS but does not change the number of dendritic spines or cellular proliferation consequences of ECS.

CONCLUSION

Egr3 influences the dendritic remodeling induced by ECS but is not required for ECS-induced proliferation of hippocampal DG cells.

cAMP-Dependent Synaptic Plasticity at the Hippocampal Mossy Fiber Terminal

Cyclic adenosine monophosphate (cAMP) is a crucial second messenger involved in both pre- and postsynaptic plasticity in many neuronal types across species. In the hippocampal mossy fiber (MF) synapse, cAMP mediates presynaptic long-term potentiation and depression. The main cAMP-dependent signaling pathway linked to MF synaptic plasticity acts via the activation of the protein kinase A (PKA) molecular cascade. Accordingly, various downstream putative synaptic PKA target proteins have been linked to cAMP-dependent MF synaptic plasticity, such as synapsin, rabphilin, synaptotagmin-12, RIM1a, tomosyn, and P/Q-type calcium channels. Regulating the expression of some of these proteins alters synaptic release probability and calcium channel clustering, resulting in short- and long-term changes to synaptic efficacy. However, despite decades of research, the exact molecular mechanisms by which cAMP and PKA exert their influences in MF terminals remain largely unknown. Here, we review current knowledge of different cAMP catalysts and potential downstream PKA-dependent molecular cascades, in addition to non-canonical cAMP-dependent but PKA-independent cascades, which might serve as alternative, compensatory or competing pathways to the canonical PKA cascade. Since several other central synapses share a similar form of presynaptic plasticity with the MF, a better description of the molecular mechanisms governing MF plasticity could be key to understanding the relationship between the transcriptional and computational levels across brain regions.

Evaluation of the hippocampal immunoreactivity of the serotonin 5-HT1A, 5-HT2 and 5-HT7 receptors in a pilocarpine temporal lobe epilepsy rat model with fast ripples

Fast ripples (FRs) are found in the hippocampus of epileptic brains, and this fast electrical activity has been described as a biomarker of the epileptogenic process itself. Results from our laboratory, such as the observation of decreased seizure rates and FR incidence at a specific citalopram dose, have suggested that serotonin (5-HT) may play a key role in the FR generation process. Therefore, to gather more details about the state of the serotoninergic system in the hippocampus under an epileptogenic process, we studied the immunoreactivity of three 5-HT receptors (5-HT1A, 5-HT2 and 5-HT7) as well as the extracellular levels of 5-HT in the hippocampal tissue of epileptic rats with FR. Wistar rats (210-300 g) were injected with a single dose of pilocarpine hydrochloride (2.4 mg/2 µl) in the right lateral ventricle and video-monitored 24 h/d to detect spontaneous and recurrent seizures; microelectrodes were implanted in the dentate gyrus (DG) and CA3 and CA1 regions of these rats ipsilateral to the pilocarpine injection site 1 day after the first spontaneous seizure was observed, and only rats who suffered FR events were used in this work. Thirty-three days after the first spontaneous seizure, an immunostaining procedure and high performance liquid chromatography were performed to measure the 5-HT levels. A general depletion of the 5-HT and 5-HIIA levels in hippocampal tissue from epileptic animals compared with those in controls was observed; in addition, a general decrease in immunoreactivity for the three receptors was found, especially in the DG, which may support the establishment of an excitatory/inhibitory imbalance in the trisynaptic circuit that underlies the FR generation process.

Spike-induced ordering: Stochastic neural spikes provide immediate adaptability to the sensorimotor system

The functional advantages of using a stochastically spiking neural network (sSNN) instead of a nonspiking neural network (NS-NN) have remained largely unknown. We developed an architecture which enabled the parametric adjustment of the spikiness (i.e., impulsive dynamics and stochasticity) of the sSNN output and observed that stochastic spikes instantaneously induced the ordered motion of a dynamical system. We demonstrated the benefits of sSNNs using a musculoskeletal bipedal walker and, moreover, showed that the decrease in the spikiness of motor neuron output leads to a reduction in adaptability. Stochastic spikes may aid the adaptation of a biological system to sudden perturbations or environmental changes. Our architecture can easily be connected to the conventional NS-NN and may superimpose the on-site adaptability.

Most biological neurons exhibit stochastic and spiking action potentials. However, the benefits of stochastic spikes versus continuous signals other than noise tolerance and energy efficiency remain largely unknown. In this study, we provide an insight into the potential roles of stochastic spikes, which may be beneficial for producing on-site adaptability in biological sensorimotor agents. We developed a platform that enables parametric modulation of the stochastic and discontinuous output of a stochastically spiking neural network (sSNN) to the rate-coded smooth output. This platform was applied to a complex musculoskeletal–neural system of a bipedal walker, and we demonstrated how stochastic spikes may help improve on-site adaptability of a bipedal walker to slippery surfaces or perturbation of random external forces. We further applied our sSNN platform to more general and simple sensorimotor agents and demonstrated four basic functions provided by an sSNN: 1) synchronization to a natural frequency, 2) amplification of the resonant motion in a natural frequency, 3) basin enlargement of the behavioral goal state, and 4) rapid complexity reduction and regular motion pattern formation. We propose that the benefits of sSNNs are not limited to musculoskeletal dynamics. Indeed, a wide range of the stability and adaptability of biological systems may arise from stochastic spiking dynamics.

CXCR7 regulates epileptic seizures by controlling the synaptic activity of hippocampal granule cells

C–X–C motif chemokine receptor 7 (CXCR7), which mediates the immune response in the brain, was recently reported to regulate neurological functions. However, the role of CXCR7 in epilepsy remains unclear. Here, we found that CXCR7 was upregulated in the hippocampal dentate gyrus (DG) of mice subjected to kainic acid (KA)-induced epilepsy and in the brain tissues of patients with temporal lobe epilepsy. Silencing CXCR7 in the hippocampal DG region exerted an antiepileptic effect on the KA-induced mouse model of epilepsy, whereas CXCR7 overexpression produced a seizure-aggravating effect. Mechanistically, CXCR7 selectively regulated N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic neurotransmission in hippocampal dentate granule cells by modulating the cell membrane expression of the NMDAR subunit2A, which requires the activation of extracellular signal-regulated kinase 1/2 (ERK1/2). Thus, CXCR7 may regulate epileptic seizures and represents a novel target for antiepileptic treatments.

Dentate Gyrus Mossy Cells Share a Role in Pattern Separation with Dentate Granule Cells and Proximal CA3 Pyramidal Cells

The complementary processes of pattern completion and pattern separation are thought to be essential for successful memory storage and recall. The dentate gyrus (DG) and proximal CA3 (pCA3) regions have been implicated in pattern separation, in part through extracellular recording studies of these areas. However, the DG contains two types of excitatory cells: granule cells of the granule layer and mossy cells of the hilus. Little is known about the firing properties of mossy cells in freely moving animals, and it is unclear how their activity may contribute to the mnemonic functions of the hippocampus. Furthermore, tetrodes in the dentate granule layer and pCA3 pyramidal layer can also record mossy cells, thus introducing ambiguity into the identification of cell types recorded. Using a random forests classifier, we classified cells recorded in DG (Neunuebel and Knierim, 2014) and pCA3 (Lee et al., 2015) of 16 male rats and separately examined the responses of granule cells, mossy cells, and pCA3 pyramidal cells in a local/global cue mismatch task. All three cell types displayed low correlations between the population representations of the rat's position in the standard and cue-mismatch sessions. These results suggest that all three excitatory cell types within the DG/pCA3 circuit may act as a single functional unit to support pattern separation.SIGNIFICANCE STATEMENT Mossy cells in the dentate gyrus (DG) are an integral component of the DG/pCA3 circuit. While the role of granule cells in the circuitry and computations of the hippocampus has been a focus of study for decades, the contributions of mossy cells have been largely overlooked. Recent studies have revealed the spatial firing properties of mossy cells in awake behaving animals, but how the activity of these highly active cells contributes to the mnemonic functions of the DG is uncertain. We separately analyzed mossy cells, granule cells, and pCA3 cells and found that all three cell types respond similarly to a local/global cue mismatch, suggesting that they form a single functional unit supporting pattern separation.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.