Striatal proenkephalin gene induction: coordinated regulation by cyclic AMP and calcium pathways

Autocrine activation of neuronal NMDA receptors by aspartate mediates dopamine- and cAMP-induced CREB-dependent gene transcription

cAMP can stimulate the transcription of many activity-dependent genes via activation of the transcription factor, cAMP response element-binding protein (CREB). However, in mouse cortical neuron cultures, prior to synaptogenesis, neither cAMP nor dopamine, which acts via cAMP, stimulated CREB-dependent gene transcription when NR2B-containing NMDA receptors (NMDARs) were blocked. Stimulation of transcription by cAMP was potentiated by inhibitors of excitatory amino acid uptake, suggesting a role for extracellular glutamate or aspartate in cAMP-induced transcription. Aspartate was identified as the extracellular messenger: enzymatic scavenging of l-aspartate, but not glutamate, blocked stimulation of CREB-dependent gene transcription by cAMP; moreover, cAMP induced aspartate but not glutamate release. Together, these results suggest that cAMP acts via an autocrine or paracrine pathway to release aspartate, which activates NR2B-containing NMDARs, leading to Ca(2+) entry and activation of transcription. This cAMP/aspartate/NMDAR signaling pathway may mediate the effects of transmitters such as dopamine on axon growth and synaptogenesis in developing neurons or on synaptic plasticity in mature neural networks.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

In Vivo Regulation of Homer1a Expression in the Striatum by Cocaine

The glutamate receptor adaptor protein Homer is concentrated in the postsynaptic density of excitatory synapses and is critical for normal operation of synaptic transmission. In this study, we investigated the responsiveness of Homer family proteins to dopamine stimulation with the psychostimulant cocaine in rat striatal neurons both in vivo and in vitro. We found that a single dose of cocaine specifically induced a rapid and transient increase in protein levels of the Homer1a, but not Homer1b/c and Homer2a/b, isoforms in the striatum. This selective Homer1a induction was mediated primarily through activation of dopamine D1, but not D2, receptors. Both protein kinase A and Ca(2+)/calmodulin-dependent protein kinases are important for mediating the cocaine stimulation of Homer1a expression. At the transcriptional level, cAMP response element-binding protein serves as a prime transcription factor transmitting the signals derived from D1 receptors and associative pathways to the CaCRE sites within the Homer1a promoter. From a functional perspective, non-cross-linking Homer1a, once induced, competed with the cross-linking isoforms of Homer proteins (Homer1b/c and Homer2a/b) to uncouple the connection of group I metabotropic glutamate receptors (mGluRs) with inositol-1,4,5-triphosphate receptors. These results indicate that cocaine possesses the ability to stimulate Homer1a expression in striatal neurons through a specific synapse-to-nucleus pathway. Moreover, inducible Homer1a expression may represent a transcription-dependent mechanism underlying the dynamic regulation of submembranous macromolecular complex formation between group I mGluRs and their anchoring proteins.

Chronic estrogenic drug treatment increases preproenkephalin mRNA levels in the rat striatum and nucleus accumbens

Estrogens modulate the expression of preproenkephalin (PPE) in the hypothalamus but little is known for other brain regions. The present study investigated the effect of hormonal withdrawal and replacement therapy on PPE expression in the striatum, nucleus accumbens and cortex. Ovariectomized Sprague-Dawley rats were treated for 2 weeks with estradiol, a specific ligand for estrogen receptor alpha (ERalpha), 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) and estrogen receptor beta (ERbeta) 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), or the selective estrogen receptor modulators (SERMs) tamoxifen and raloxifene. Brain PPE mRNA levels, measured by in situ hybridization, were high in the striatum and nucleus accumbens compared to the low expression in the cortex. Ovariectomy decreased uterine weights compared to intact uterus, which was corrected by estradiol and PPT. Tamoxifen and raloxifene partially stimulated uterine weights while DPN left it unchanged. In the anterior, median and posterior striatum and in the core and shell of the nucleus accumbens, ovariectomy decreased PPE mRNA levels compared to intact rats, this was corrected by estradiol treatment except for the posterior striatum. PPT, DPN, tamoxifen and raloxifene reproduced the estradiol effect. In the prefrontal and cingulate cortices, neither ovariectomy nor treatments changed PPE mRNA levels. These results show for the first time that estradiol increases PPE mRNA in the striatum and nucleus accumbens. This effect is observed also with estrogen receptor agonists for the ERalpha and ERbeta as well as with SERMs.