Modifications of plastic properties and metaplasticity of glutamatergic synapses in the rat cortex and hippocampus under conditions of reserpine-induced behavioral depression

Intraperitoneal injection of 1 mg/kg reserpine into rats caused the development of behavioral depression that was especially clearly pronounced 24 h after injection. Under such conditions, induction of long-term potentiation of synaptic transmission was suppressed, the development of long-term depression in glutamatergic synapses of pyramidal neurons of the hippocampal CA1 area and layers II/III of the parietal cortex was facilitated, and metaplasticity threshold (θ_M) was shifted to the right. Such modifications of plasticity and metaplasticity of glutamatergic synapses were determined by changes in the functional state of postsynaptic NMDA receptors, which was confirmed by a decrease in the duration of NMDA component of field EPSPs generated in the studied neurons and by an increase in the sensitivity of this component to the action of a nonselective blocker of NMDA receptors, ketamine. Simultaneously, the sensitivity to zinc and haloperidol, which are selective with respect to NMDA receptors with the subunit composition NR1/NR2B, decreased. It is hypothesized that, under conditions of depression, either replacement of a part of NR2B subunits in the structure of NMDA receptors by NR2A subunits or biochemical inactivation of NMDA receptors containing NR2B subunit, as well as a decrease in the clearance of transmitter in glutamatergic synapses, occur; these events determine the impairment of plastic properties of the latter contacts. Neirofiziologiya/Neurophysiology, Vol. 39, No. 3, pp. 214–221, May–June, 2007.     

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A–D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA_A receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.     

Cholinergic cells in the nucleus basalis of mice express the N-methyl-D-aspartate-receptor subunit NR2C and its replacement by the NR2B subunit enhances frontal and amygdaloid acetylcholine levels

It is known that glutamatergic and cholinergic systems interact functionally at the level of the cholinergic basal forebrain. The N-methyl-d-aspartate receptor (NMDA-R) is a multiprotein complex composed of NR1, NR2 and/or NR3 subunits. The subunit composition of NMDA-R of cholinergic cells in the nucleus basalis has not yet been investigated. Here, by means of choline acetyl transferase and NR2B or NR2C double staining, we demonstrate that mice express both the NR2C and NR2B subunits in nucleus basalis cholinergic cells. We generated NR2C-2B mutant mice in which an insertion of NR2B cDNA into the gene locus of the NR2C gene replaced NR2C by NR2B expression throughout the brain. This NR2C-2B mutant was used to examine whether a subunit exchange in cholinergic neurons would affect acetylcholine (ACh) content in several brain structures. We found increased ACh levels in the frontal cortex and amygdala in the brains of NR2C-2B mutant mice. Brain ACh has been implicated in neuroplasticity, novelty-induced arousal and encoding of novel stimuli. We therefore assessed behavioral habituation to novel environments and objects as well as object recognition in NR2C-2B subunit exchange mice. The behavioral analysis did not indicate any gross behavioral alteration in the mutant mice compared with the wildtype mice. Our results show that the NR2C by NR2B subunit exchange in mice affects ACh content in two target areas of the nucleus basalis.     

Grin1 Receptor Deletion within CRF Neurons Enhances Fear Memory

Corticotropin releasing factor (CRF) dysregulation is implicated in mood and anxiety disorders such as posttraumatic stress disorder (PTSD). CRF is expressed in areas engaged in fear and anxiety processing including the central amygdala (CeA). Complicating our ability to study the contribution of CRF-containing neurons to fear and anxiety behavior is the wide variety of cell types in which CRF is expressed. To manipulate specific subpopulations of CRF containing neurons, our lab has developed a mouse with a Cre recombinase gene driven by a CRF promoter (CRFp3.0Cre) (Martin et al., 2010). In these studies, mice that have the gene that encodes NR1 (Grin1) flanked by loxP sites (floxed) were crossed with our previously developed CRFp3.0Cre mouse to selectively disrupt Grin1 within CRF containing neurons (Cre+/^fGrin1+). We find that disruption of Grin1 in CRF neurons did not affect baseline levels of anxiety, locomotion, pain sensitivity or exploration of a novel object. However, baseline expression of Grin1 was decreased in Cre+/^fGrin1+ mice as measured by RTPCR. Cre+/^fGrin1+ mice showed enhanced auditory fear acquisition and retention without showing any significant effect on fear extinction. We measured Gria1, the gene that encodes AMPAR1 and the CREB activator Creb1 in the amygdala of Cre+/^fGrin1+ mice after fear conditioning. Both Gria1 and Creb1 were enhanced in the amygdala after training. To determine if the Grin1-expressing CRF neurons within the CeA are responsible for the enhancement of fear memory in adults, we infused a lentivirus with Cre driven by a CRF promoter (LV pCRF-Cre/^fGrin1+) into the CeA of floxed Grin1 mice. Cre driven deletion of Grin1 specifically within CRF expressing cells in the CeA also resulted in enhanced fear memory acquisition and retention. Altogether, these findings suggest that selective disruption of Grin1 within CeA CRF neurons strongly enhances fear memory.     

L-type voltage-dependent calcium channels facilitate acetylation of histone H3 through PKCγ phosphorylation in mice with methamphetamine-induced place preference

The present study investigated regulation of histone acetylation by L-type voltage-dependent calcium channels (VDCCs), one of the machineries to provide Ca(2+) signals. Acetylation of histone through the phosphorylation of protein kinase Cγ (PKCγ) in the development of methamphetamine (METH)-induced place preference was demonstrated in the limbic forebrain predominantly but also in the nucleus accumbens of α1C subunit knockout mice. Chronic administration of METH produced a significant place preference in mice, which was dose-dependently inhibited by both chelerythrine (a PKC inhibitor) and nifedipine (an L-type VDCC blocker). Protein levels of acetylated histone H3 and p-PKCγ significantly increased in the limbic forebrain of mice showing METH-induced place preference, and it was also significantly attenuated by pre-treatment with chelerythrine or nifedipine. METH-induced place preference was also significantly attenuated by deletion of half the α1C gene, which is one of the subunits forming Ca(2+) channels. Furthermore, increased acetylation of histone H3 was found in specific gene-promoter regions related to synaptic plasticity, such as Nrxn, Syp, Dlg4, Gria1, Grin2a, Grin2b, Camk2a, Creb, and cyclin-dependent kinase 5, in wild-type mice showing METH-induced place preference, while such enhancement of multiple synaptic plasticity genes was significantly attenuated by a deletion of half the α1C gene. These findings suggest that L-type VDCCs play an important role in the development of METH-induced place preference by facilitating acetylation of histone H3 in association with enhanced expression of synaptic plasticity genes via PKCγ phosphorylation following an increase in the intracellular Ca(2+) concentration.