Hippocampal Synapse Density Research Articles

Differential Effects of Sevoflurane Exposure on Long-Term Fear Memory in Neonatal and Adult Rats.

It remains unclear whether exposure to sevoflurane produces different effects on long-term cognitive function in developing and mature brains. In the present study, Sprague-Dawley neonatal rats at postnatal day (PND) 7 and adult rats (PND 56) were used in all experiments. We performed fear conditioning testing to examine long-term fear memory following 4-h sevoflurane exposure. We assessed hippocampal synapse ultrastructure with a transmission electron microscope. Moreover, we investigated the effect of sevoflurane exposure on the expression of postsynaptic protein 95 (PSD-95) and its binding protein kalirin-7 in the hippocampus. We observed that early exposure to sevoflurane in neonatal rats impairs hippocampus-dependent fear memory, reduces hippocampal synapse density, and dramatically decreases the expressions of PSD-95 and kalirin-7 in the hippocampus of the developing brain. However, sevoflurane exposure in adult rats has no effects on hippocampus-dependent fear memory and hippocampal synapse density, and the expressions of PSD-95 and kalirin-7 in the adult hippocampus are not significantly altered following sevoflurane treatment. Our results indicate that sevoflurane exposure produces differential effects on long-term fear memory in neonatal and adult rats and that PSD-95 signaling may be involved in the molecular mechanism for early sevoflurane exposure-caused long-term fear memory impairment.

Genetic Ablation of All Cerebellins Reveals Synapse Organizer Functions in Multiple Regions Throughout the Brain.

Cerebellins are synaptic organizer molecules that bind to presynaptic neurexins and postsynaptic receptors. They are well studied in the cerebellum, but three of the four cerebellins (Cbln1, Cbln2, and Cbln4) are also broadly expressed outside of the cerebellum, suggesting that they perform general functions throughout the brain. Here, we generated male and female constitutive single (KO), double KO (dKO), and triple KO (tKO) mice of Cbln1, Cbln2, and Cbln4. We found that all constitutive cerebellin-deficient mice were viable and fertile, suggesting that cerebellins are not essential for survival. Cbln1/2 dKO mice exhibited salience-induced seizures that were aggravated in Cbln1/2/4 tKO mice, suggesting that all cerebellins contribute to brain function. As described previously, Cbln1 KO mice displayed major motor impairments that were aggravated by additional KO of Cbln2. Strikingly, the Cbln1/2 dKO did not cause alterations in synapse density in the hippocampus of young adult (1- and 2-month-old) mice, but produced a selective ∼50% decrease in hippocampal synapse density in the stratum lacunosum moleculare of the CA1 region and in the dentate gyrus of aging, 6-month-old mice. A similar decrease in excitatory synapse density was observed in the striatum and retrosplenial cortex. Behaviorally, the Cbln1 KO produced dramatic changes in motor behaviors that were partly aggravated by additional deletion of Cbln2 and/or Cbln4. Our results show that cerebellins are not essential for survival and do not contribute to initial synapse formation, but perform multiple functions throughout the brain; as a consequence, their ablation results in a delayed loss of synapses and in behavioral impairments.SIGNIFICANCE STATEMENT Cerebellins (Cbln1-4) are trans-synaptic cell adhesion molecules. In the cerebellum, Cbln1 functions as a bidirectional organizer of parallel fiber-Purkinje cell synapses by binding to presynaptic neurexins and postsynaptic GluRδ2. Little is known about the function of cerebellins outside of the cerebellum; therefore, the present study used single, double, and triple constitutive KO mice of Cbln1, Cbln2, and Cbln4 to analyze the overall function of cerebellins. We show that cerebellins act as important synaptic organizers in specific subsets of neurons and likely contribute to many different brain functions. We also show that cerebellins are not initially required for synapse formation, but rather for specification and long-term synapse maintenance and demonstrate that all cerebellins, not just Cbln1, contribute to brain function.

Erythrocyte intracellular Mg(2+) concentration as an index of recognition and memory.

Magnesium (Mg2+) plays an important role in the neural system, and yet scarcely any research has quantitatively analyzed the link between endogenous Mg2+ level and memory. Using our original technique, we measured erythrocyte intracellular ionized Mg2+ concentration (RBC [Mg2+]i), which linearly correlated to recognition and spatial memory in normal aging rats. In the brain, RBC [Mg2+]i significantly correlated to hippocampus extracellular fluid Mg2+ concentration, and further correlated to hippocampal synapse density. Elevation of Mg2+ intake in aged rats demonstrated an association between RBC [Mg2+]i increase and memory recovery. The therapeutic effect of Mg2+ administration was inversely correlated to individual basal RBC [Mg2+]i. In summary, we provide a method to measure RBC [Mg2+]i, an ideal indicator of body Mg2+ level. RBC [Mg2+]i represents rodent memory performance in our study, and might further serve as a potential biomarker for clinical differential diagnosis and precise treatment of Mg2+-deficiency-associated memory decline during aging.

MHC class I limits hippocampal synapse density by inhibiting neuronal insulin receptor signaling.

Proteins of the major histocompatibility complex class I (MHCI) negatively regulate synapse density in the developing vertebrate brain (Glynn et al., 2011; Elmer et al., 2013; Lee et al., 2014), but the underlying mechanisms remain largely unknown. Here we identify a novel MHCI signaling pathway that involves the inhibition of a known synapse-promoting factor, the insulin receptor. Dominant-negative insulin receptor constructs decrease synapse density in the developing Xenopus visual system (Chiu et al., 2008), and insulin receptor activation increases dendritic spine density in mouse hippocampal neurons in vitro (Lee et al., 2011). We find that genetically reducing cell surface MHCI levels increases synapse density selectively in regions of the hippocampus where insulin receptors are expressed, and occludes the neuronal insulin response by de-repressing insulin receptor signaling. Pharmacologically inhibiting insulin receptor signaling in MHCI-deficient animals rescues synapse density, identifying insulin receptor signaling as a critical mediator of the tonic inhibitory effects of endogenous MHCI on synapse number. Insulin receptors co-immunoprecipitate MHCI from hippocampal lysates, and MHCI unmasks a cytoplasmic epitope of the insulin receptor that mediates downstream signaling. These results identify an important role for an MHCI-insulin receptor signaling pathway in circuit patterning in the developing brain, and suggest that changes in MHCI expression could unexpectedly regulate neuronal insulin sensitivity in the aging and diseased brain.

Increased estrogen receptor β expression correlates with decreased spine formation in the rat hippocampus

Estrogens play an important role in the brain function acting through two receptor types, ERalpha and ERbeta, both well-recognized as transcription factors. In this study, we investigated the ERbeta mRNA and protein levels in the rat hippocampus by using two in vivo models that are known to affect synapse formation. Natural estrous-proestrous cycle was used as a model in which a marked decrease in the density of hippocampal synapses was previously observed between proestrus and estrus. We have found that ERbeta mRNA and protein were displayed in high levels in the estrus and in low levels in the proestrous phase. By applying kainic acid (KA) to adult rats, we demonstrated that up-regulation of ERbeta mRNA and protein in hippocampal CA regions was vulnerable to KA-induced excitotoxicity. Furthermore, we note a concomitant decrease of ERbeta in the excitotoxicity-resistant denate gyrus that undergoes intense plastic changes, including synaptogenesis. These data suggested that decreases in ERbeta expression correlated with increase in synapse formation. This notion has been tested in vitro in hippocampal cultures, in which overexpression of ERbeta by means of gene transfection resulted in the lowering of the dendritic spine density that was elevated by estrogen. In summary, our results suggest that ERbeta inhibits synapse formation in hippocampal neurons.

Erratum for Woolley et al., Estradiol Mediates Fluctuation in Hippocampal Synapse Density during the Estrous Cycle in the Adult Rat

Due to a publisher's error, the article entitled “Estradiol Mediates Fluctuation in Hippocampal Synapse Density during the Estrous Cycle in the Adult Rat” (C.S. Woolley and B.S. McEwen;J Neurosci12:2549–2554, July 1992) was printed with an incorrect formula for a “conventional” method for calculation of synapse density. The formula should read as follows:est Nv = N/(corr A x d)

Estradiol mediates fluctuation in hippocampal synapse density during the estrous cycle in the adult rat [published erratum appears in J Neurosci 1992 Oct;12(10):following table of contents

We have found that the density of synapses in the stratum radiatum of the hippocampal CA1 region in the adult female rat is sensitive to estradiol manipulation and fluctuates naturally as the levels of ovarian steroids vary during the 5 d estrous cycle. In both cases, low levels of estradiol are correlated with lower synapse density, while high estradiol levels are correlated with a higher density of synapses. These synaptic changes occur very rapidly in that within approximately 24 hr between the proestrus and estrus stages of the estrous cycle, we observe a 32% decrease in the density of hippocampal synapses. Synapse density then appears to cycle back to proestrus values over a period of several days. To our knowledge, this is the first demonstration of such short-term steroid-mediated synaptic plasticity occurring naturally in the adult mammalian brain.