Phosphatidylinositol 3-kinase (PI3K) has been implicated in synaptic plasticity and other neural functions in the brain. However, the role of individual PI3K isoforms in the brain is unclear. We investigated the role of PI3Kγ in hippocampal-dependent synaptic plasticity and cognitive functions. We found that PI3Kγ has a crucial and specific role in NMDA receptor (NMDAR)-mediated synaptic plasticity at mouse Schaffer collateral-commissural synapses. Both genetic deletion and pharmacological inhibition of PI3Kγ disrupted NMDAR long-term depression (LTD) while leaving other forms of synaptic plasticity intact. Accompanying this physiological deficit, the impairment of NMDAR LTD by PI3Kγ blockade was specifically correlated with deficits in behavioral flexibility. These findings suggest that a specific PI3K isoform, PI3Kγ, is critical for NMDAR LTD and some forms of cognitive function. Thus, individual isoforms of PI3Ks may have distinct roles in different types of synaptic plasticity and may therefore influence various kinds of behavior.
Newborn neurons in the subgranular zone (SGZ) of the hippocampus incorporate into the dentate gyrus and mature. Numerous studies have focused on hippocampal neurogenesis because of its importance in learning and memory. However, it is largely unknown whether hippocampal neurogenesis is involved in memory extinction per se. Here, we sought to examine the possibility that hippocampal neurogenesis may play a critical role in the formation and extinction of hippocampus-dependent contextual fear memory. By methylazoxymethanol acetate (MAM) or gamma-ray irradiation, hippocampal neurogenesis was impaired in adult mice. Under our experimental conditions, only a severe impairment of hippocampal neurogenesis inhibited the formation of contextual fear memory. However, the extinction of contextual fear memory was not affected. These results suggest that although adult newborn neurons contribute to contextual fear memory, they may not be involved in the extinction or erasure of hippocampus-dependent contextual fear memory.
MicroRNAs (miRNAs) are small, noncoding RNAs that posttranscriptionally regulate gene expression in many tissues. Although a number of brain-enriched miRNAs have been identified, only a few specific miRNAs have been revealed as critical regulators of synaptic plasticity, learning, and memory. miR-9-5p/3p are brain-enriched miRNAs known to regulate development and their changes have been implicated in several neurological disorders, yet their role in mature neurons in mice is largely unknown. Here, we report that inhibition of miR-9-3p, but not miR-9-5p, impaired hippocampal long-term potentiation (LTP) without affecting basal synaptic transmission. Moreover, inhibition of miR-9-3p in the hippocampus resulted in learning and memory deficits. Furthermore, miR-9-3p inhibition increased the expression of the LTP-related genes Dmd and SAP97, the expression levels of which are negatively correlated with LTP. These results suggest that miR-9-3p-mediated gene regulation plays important roles in synaptic plasticity and hippocampus-dependent memory.
Serotonin (5-HT) plays a critical role in modulating synaptic plasticity in the marine mollusc Aplysia and in the mammalian nervous system. In Aplysia sensory neurons, 5-HT can activate several signal cascades, including PKA and PKC, presumably via distinct types of G proteincoupled receptors. However, the molecular identities of these receptors have not yet been identified. We here report the cloning and functional characterization of a 5-HT receptor that is positively coupled to adenylyl cyclase in Aplysia neurons. The cloned receptor, 5-HT apAC1, stimulates the production of cAMP in HEK293T cells and in Xenopus oocytes. Moreover, the knockdown of 5-HT apAC1 expression by RNA interference blocked 5-HT-induced cAMP production in Aplysia sensory neurons and blocked synaptic facilitation in nondepressed or partially depressed sensory-to-motor neuron synapses. These data implicate 5-HT apAC1 as a major modulator of learning related synaptic facilitation in the direct sensory to motor neuron pathway of the gill withdrawal reflex.5-HT receptor ͉ memory ͉ cAMP ͉ protein kinase A 5-Hydroxytryptamine (5-HT), or serotonin, is a key neurotransmitter that modulates a variety of behaviors in both invertebrate and vertebrate animals and is involved in the regulation of mood and mood disorders in humans (1). Serotonin also modulates synaptic plasticity in the marine mollusc Aplysia (2, 3). Synaptic facilitation of the connections between sensory and motor neurons of the gill-withdrawal reflex is mediated by 5-HT, and this form of synaptic plasticity has been found to be a critical cellular mechanism of behavioral sensitization (4-6). A number of pharmacological studies have found that, depending on the behavioral history and pattern of sensory stimulation, 5-HT stimulates several downstream signaling pathways, including protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK), suggesting that serotonin acts on more than one receptor type (2,3,7,8). Of these signaling cascades, the adenylyl cyclase-cAMP-PKA cascade has been most extensively investigated because of its important roles in both behavioral sensitization and synaptic facilitation (3, 4, 9, 10). Historically, this was the initially identified second-messenger system involved in the regulation of synaptic plasticity, behavior, and memory storage (4).A single pulse of 5-HT activates PKA, which phosphorylates and inactivates potassium channels (11) and subsequently increases synaptic strength at nondepressed synapses. At depressed synapses, however, PKC becomes the major downstream kinase to be activated by a single pulse of 5-HT (8). In addition, repetitive exposures to 5-HT that induce long-term facilitation result in the activation of additional kinases, including MAPK (12), that translocate to the nucleus to induce gene expression. However, the molecular mechanism for this dynamic coupling specificity of downstream signaling pathways is not known.In vertebrates, seven families of 5-HT receptors have been characterized; six of the...
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