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Preface Mounting evidence suggests that acute and chronic stress, especially the stress-induced release of glucocorticoids, induces changes in glutamate neurotransmission in the prefrontal cortex and the hippocampus, thereby influencing some aspects of cognitive processing. In addition, dysfunction of glutamatergic neurotransmission is increasingly considered to be a core feature of stress-related mental illnesses. Recent studies have shed light on the mechanisms by which stress and glucocorticoids affect glutamate transmission, including effects on glutamate release, glutamate receptors and glutamate clearance and metabolism. This new understanding provides insights into normal brain functioning as well as the pathophysiology and potential new treatments of stress-related neuropsychiatric disorders.

The physiological state of the cell is controlled by signal transduction mechanisms which regulate the balance between protein kinase and protein phosphatase activities. Here we report that a single protein can, depending on which particular amino-acid residue is phosphorylated, function either as a kinase or phosphatase inhibitor. DARPP-32 (dopamine and cyclic AMP-regulated phospho-protein, relative molecular mass 32,000) is converted into an inhibitor of protein phosphatase 1 when it is phosphorylated by protein kinase A (PKA) at threonine 34. We find that DARPP-32 is converted into an inhibitor of PKA when phosphorylated at threonine 75 by cyclin-dependent kinase 5 (Cdk5). Cdk5 phosphorylates DARPP-32 in vitro and in intact brain cells. Phospho-Thr 75 DARPP-32 inhibits PKA in vitro by a competitive mechanism. Decreasing phospho-Thr 75 DARPP-32 in striatal slices, either by a Cdk5-specific inhibitor or by using genetically altered mice, results in increased dopamine-induced phosphorylation of PKA substrates and augmented peak voltage-gated calcium currents. Thus DARPP-32 is a bifunctional signal transduction molecule which, by distinct mechanisms, controls a serine/threonine kinase and a serine/threonine phosphatase.

SUMMARY Chronic stress could trigger maladaptive changes associated with stress-related mental disorders, however, the underlying mechanisms remain elusive. In this study, we found that exposing juvenile male rats to repeated stress significantly impaired the temporal order recognition memory, a cognitive process controlled by prefrontal cortex (PFC). Concomitantly, significantly reduced AMPAR- and NMDAR-mediated synaptic transmission and glutamate receptor expression were found in PFC pyramidal neurons from repeatedly stressed animals. All these effects relied on activation of glucocorticoid receptors and the subsequent enhancement of ubiquitin/proteasome-mediated degradation of GluR1 and NR1 subunits, which was controlled by the E3 ubiquitin ligase Nedd4-1 and Fbx2, respectively. Inhibition of proteasomes or knockdown of Nedd4-1 and Fbx2 in PFC prevented the loss of glutamatergic responses and recognition memory in stressed animals. Our results suggest that repeated stress dampens PFC glutamatergic transmission by facilitating glutamate receptor turnover, which causes the detrimental effect on PFC-dependent cognitive processes.

Synaptic spines are dynamic structures that regulate neuronal responsiveness and plasticity. Here we describe a role for the schizophrenia risk factor, Disrupted-in-Schizophrenia 1 (DISC1), in the maintenance of spine morphology and function. We show that DISC1 anchors Kalirin-7 (Kal-7) thereby regulating access of Kal-7 to Rac1 and so controlling the duration and intensity of Rac1 activation in response to NMDA receptor activation in cortical culture as well as in vivo brain. This offers explanation for why Rac1 and its activator (Kal-7) serve as key mediators of spine enlargement and that constitutive Rac1 activation decreases spine size. This novel mechanism likely underlies disturbances in glutamatergic neurotransmission frequently reported in schizophrenia that can lead to alteration of dendritic spines with consequential major pathological changes in brain function. Furthermore, the concept of a “signalosome” involving disease-associated factors, such as DISC1 and glutamate, may well contribute to the multifactorial and polygenetic characteristics of schizophrenia.

Cocaine enhances dopamine-mediated neurotransmission by blocking dopamine re-uptake at axon terminals. Most dopamine-containing nerve terminals innervate medium spiny neurons in the striatum of the brain. Cocaine addiction is thought to stem, in part, from neural adaptations that act to maintain equilibrium by countering the effects of repeated drug administration. Chronic exposure to cocaine upregulates several transcription factors that alter gene expression and which could mediate such compensatory neural and behavioural changes. One such transcription factor is DeltaFosB, a protein that persists in striatum long after the end of cocaine exposure. Here we identify cyclin-dependent kinase 5 (Cdk5) as a downstream target gene of DeltaFosB by use of DNA array analysis of striatal material from inducible transgenic mice. Overexpression of DeltaFosB, or chronic cocaine administration, raised levels of Cdk5 messenger RNA, protein, and activity in the striatum. Moreover, injection of Cdk5 inhibitors into the striatum potentiated behavioural effects of repeated cocaine administration. Our results suggest that changes in Cdk5 levels mediated by DeltaFosB, and resulting alterations in signalling involving D1 dopamine receptors, contribute to adaptive changes in the brain related to cocaine addiction.

In recent years, the distribution of dopamine receptor subtypes among the principal neurons of the neostriatum has been the subject of debate. Conventional anatomical and physiological approaches have yielded starkly different estimates of the extent to which D1 and D2 class dopamine receptors are colocalized. One plausible explanation for the discrepancy is that some dopamine receptors are present in physiologically significant numbers, but the mRNA for these receptors is not detectable with conventional techniques. To test this hypothesis, we examined the expression of DA receptors in individual neostriatal neurons by patch-clamp and RT-PCR techniques. Because of the strong correlation between peptide expression and projection site, medium spiny neurons were divided into three groups on the basis of expression of mRNA for enkephalin (ENK) and substance P (SP). Neurons expressing detectable levels of SP but not ENK had abundant mRNA for the D1a receptor. A subset of these cells (approximately 50%) coexpressed D3 or D4 receptor mRNA. Neurons expressing detectable levels of ENK but not SP had abundant mRNA for D2 receptor isoforms (short and long). A subset (10-25%) of these neurons coexpressed D1a or D1b mRNAs. Neurons coexpressing ENK and SP mRNAs consistently coexpressed D1a and D2 mRNAs in relatively high abundance. Functional analysis of neurons expressing lower abundance mRNAs revealed clear physiological consequences that could be attributed to these receptors. These results suggest that, although colocalization of D1a and D2 receptors is limited, functional D1 and D2 class receptors are colocalized in nearly one-half of all medium spiny projection neurons.