Chronic antidepressant treatment impairs the acquisition of fear extinction

BACKGROUND

Like fear conditioning, the acquisition phase of extinction involves new learning that is mediated by the amygdala. During extinction training, the conditioned stimulus is repeatedly presented in the absence of the unconditioned stimulus, and the expression of previously learned fear gradually becomes suppressed. Our previous study revealed that chronic treatment with a selective serotonin reuptake inhibitor (SSRI) impairs the acquisition of auditory fear conditioning. To gain further insight into how SSRIs affect fear learning, we tested the effects of chronic SSRI treatment on the acquisition of extinction.

METHODS

Rats were treated chronically (22 days) or subchronically (9 days) with the SSRI citalopram (10 mg/kg/day) before extinction training. The results were compared with those after chronic and subchronic treatment with tianeptine (10 mg/kg/day), an antidepressant with a different method of action. The expression of the NR2B subunit of the N-methyl-D-aspartate receptor in the amygdala was examined after behavioral testing.

RESULTS

Chronic but not subchronic administration of citalopram impaired the acquisition of extinction and downregulated the NR2B subunit of the N-methyl-D-aspartate receptor in the lateral and basal nuclei of the amygdala. Similar behavioral and molecular changes were found with tianeptine treatment.

CONCLUSIONS

These results provide further evidence that chronic antidepressant treatment can impair amygdala-dependent learning. Our findings are consistent with a role for glutamatergic neurotransmission in the final common pathway of antidepressant treatment.

β-Estradiol unmasks metabotropic receptor-mediated metaplasticity of NMDA receptor transmission in the female rat dentate gyrus

Loss of estrogen in women following menopause is associated with increased risk for cognitive decline, dementia and depression, all of which can be prevented by estradiol replacement. The dentate gyrus plays an important role in cognition, learning and memory. The gatekeeping function of the dentate gyrus to filter incoming activity into the hippocampus is modulated by estradiol in a frequency-dependent manner and involves activation of metabotropic glutamate receptors (mGluR). In the present study, we investigated whether estradiol (EB) modulates the metaplastic effect of inducing synaptic long-term potentiation (LTP) on subsequent propensity for expression of LTP in the dentate gyrus. At medial perforant path-dentate granule cell synapses in hippocampal slices of ovariectomized female rats, EB replacement was critical for an initial induction of LTP to enhance the magnitude of subsequent LTP elicited by a second high-frequency stimulation, metaplasticity, which was not present in slices from oil-treated control animals. EB enhanced expression of group I mGluRs, and the metaplastic effect of EB on LTP required activation of group I mGluRs that led to Src-family tyrosine kinase-mediated phosphorylation of NR2B subunits of N-methyl-d-aspartate receptors (NMDAR) that enhanced the magnitude of NMDAR-dependent LTP. Our data show that EB effects on LTP in the hippocampal dentate gyrus require activation of group I mGluRs, which in turn leads to functional metaplastic regulation of NR2B subunit-containing NMDARs, as opposed to direct effects of EB on NMDARs.

Targeting the NMDA receptor subunit NR2B for the treatment of neuropathic pain

Neuropathic pain is generally defined as a chronic pain state resulting from peripheral or central nerve injury, or both. An effective treatment for neuropathic pain is still lacking. The NMDA receptor, one type of the ionotropic glutamate receptors, is known to be important for triggering long-lasting changes in synapses. NMDA receptor-dependent synaptic plasticity plays roles not only in physiological functions such as learning and memory, but also in unwanted pathological conditions such as chronic pain. This review addresses recent progress on NMDA receptors in neuropathic pain, with particular emphasis on the NR2B-subunit-containing receptors. The expression and function of NMDA receptors in synaptic plasticity in the pain transmission pathway from dorsal root ganglia to the anterior cingulate cortex is reviewed, and preclinical and clinical investigations of selective NMDA receptor in neuropathic pain are discussed. The NMDA receptors, in particular NR2B-containing NMDA receptors, serve as promising targets for treatment of neuropathic pain.

NR2B-selective conantokin peptide inhibitors of the NMDA receptor display enhanced antinociceptive properties compared to non-selective conantokins

NR2B-selective inhibitors show lower side-effects in preclinical pain models than non-selective NMDA receptor (NMDAR) antagonists, but it is unclear whether the improved safety of NR2B-selective inhibitors is due to their subtype selectivity or to a unique mode of inhibition of the receptor. In this study, the analgesic effects of intracerebral bolus injections of conantokin peptides with different NMDAR subunit selectivity were determined in mice by the standard hot-plate test, and following stimuli with acetic acid, formalin and complete Freund's adjuvant (CFA). In the standard hot-plate model, con-G[S16Y], a NR2B-selective inhibitor, showed the highest analgesic activity among conantokin peptides tested. In the acetic acid- and CFA-induced pain models, con-G[S16Y] and, to a lesser extent, con-G exhibited higher analgesic activity compared to non-selective inhibitors, such as con-R[1-17]. In the formalin test, while all conantokin peptides could partially suppress the first phase response, only con-G[S16Y] and con-G significantly inhibited the second phase response and suppressed paw edema. Our results suggest that the antinociceptive action of the conantokins may be related to their NR2B-selectivity and that these peptides may be useful as both neurobiological tools for probing mechanisms of nociception and as therapeutic agents for pain relief.

Contribution of NR2B subunits to synaptic transmission in amygdaloid interneurons

Synaptic responses of interneurons in the rat lateral amygdala (LA) to electrical microstimulation of putative cortical and thalamic afferents were studied in slice preparations in situ. The EPSPs at both thalamic and cortical inputs were composed of two major components that were sensitive to 6,7-dinitroxaline-2,3-dione and DL-2-amino-5-phosphonovaleric acid (APV), indicating mediation through AMPA and NMDA receptors. NMDA receptor activation contributed to basal synaptic transmission, as evidenced through a reduction of EPSP amplitudes and integrals by APV. NMDA receptor-mediated postsynaptic currents showed magnesium-regulated voltage dependence, and current-voltage relationships displayed a region of negative slope conductance negative to resting potential. Deactivation of NMDA receptor-mediated currents followed a two exponential time course, with both components being significantly reduced by ifenprodil (10 microm), an antagonist of the NR2B subunit of NMDA receptors. Significant differences were not observed between NMDA currents or ifenprodil effects at thalamic and cortical inputs. Furthermore, recordings from a sample of projection neurons in the LA provided additional evidence for the existence of ifenprodil-sensitive components of thalamically and cortically evoked NMDA receptor-mediated responses. Immunohistochemical double-labeling and combined in situ hybridization/immunohistochemistry demonstrated that GABA-immunoreactive as well as GABA-negative cells express the NR2B subunit. Overall, these results show that GABAergic interneurons in the LA express functional NMDA receptors, which participate in basal synaptic transmission at both thalamic and cortical inputs. The finding that NR2B subunits are critically involved in NMDA receptor-mediated signaling at the two major input pathways to interneurons and projection cells in the LA is particularly interesting in the light of previous observations that NR2B antagonists interfere with plastic changes in the LA related to associative fear conditioning.

Intra-amygdala blockade of the NR2B subunit of the NMDA receptor disrupts the acquisition but not the expression of fear conditioning

The lateral nucleus of the amygdala (LA) is an essential component of the neural circuitry underlying Pavlovian fear conditioning. Although blockade of NMDA receptors in LA and adjacent areas before training disrupts the acquisition of fear conditioning, blockade before testing also often disrupts the expression of fear responses. With this pattern of results, it is not possible to distinguish a contribution of NMDA receptors to plasticity from a role in synaptic transmission. In past studies, NMDA blockade has been achieved using the antagonist d,l-2-amino-5-phosphovalerate, which blocks the entire heteromeric receptor complex. The present experiments examined the effects of selective blockade of the NR2B subunit of the NMDA receptor in LA using the selective antagonist ifenprodil. Systemic injections of ifenprodil before training led to a dose-dependent impairment in the acquisition of auditory and contextual fear conditioning, whereas injections before testing had no effect. Intra-amygdala infusions of ifenprodil mirrored these results and, in addition, showed that the effects are attributable to a disruption of fear learning rather than a disruption of memory consolidation. NMDA receptors in LA are thus involved in fear conditioning, and the NR2B subunit appears to make unique contributions to the underlying plasticity.

NMDA and glutamate evoke excitotoxicity at distinct cellular locations in rat cortical neurons in vitro

The development of cortical neurons in vivo and in vitro is accompanied by alterations in NMDA receptor subunit expression and concomitant modifications in the pharmacological profile of NMDA-activated ionic currents. For example, we observed that with decreasing NR2B/NR2A subunit expression ratio, the block of NMDA receptor-mediated whole-cell responses by the NR2B-selective antagonist haloperidol was also decreased. In mature cultures (>22 d in vitro), however, NMDA responses obtained from excised nucleated macropatches, which comprised a large portion of the soma, remained strongly antagonized by haloperidol. These results suggest that in more mature neurons NR1/NR2B receptors appear to be preferentially expressed in the cell body. As predicted from the whole-cell recording pharmacological profile, NMDA-induced toxicity was largely unaffected by haloperidol in mature cultures. However, haloperidol effectively blocked glutamate toxicity in the same cultures, suggesting that the neurotoxic actions of this amino acid were mostly due to the activation of somatic NMDA receptors. In experiments in which the potency of glutamate toxicity was increased by the transport inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid, the neuroprotective effects of haloperidol were significantly diminished. This was likely because of the fact that glutamate, now toxic at much lower concentrations, was able to reach and activate dendritic receptors under these conditions. These results strongly argue that exogenous glutamate and NMDA normally induce excitotoxicity at distinct cellular locations in mature mixed neuronal cultures and that NR1/NR2B receptors remain an important component in the expression of glutamate, but not NMDA-induced excitotoxicity.