No abstract

Locomotor sensitization is a common and robust behavioral alteration in rodents whereby following exposure to abused drugs such as cocaine, the animal becomes significantly more hyperactive in response to an acute drug challenge. Here, we further analyzed the role of cocaine-induced silent synapses in the nucleus accumbens (NAc) shell and their contribution to the development of locomotor sensitization. Using a combination of viral vector-mediated genetic manipulations, biochemistry and electrophysiology in a locomotor sensitization paradigm with repeated, daily noncontingent cocaine (15 mg/kg) injections, we show that dominant negative cAMP-element binding protein (CREB) prevents cocaine-induced generation of silent synapses of young (30 d) rats, whereas constitutively active CREB is sufficient to increase the number of NR2B-containing NMDA receptors (NMDAR) at synapses and to generate silent synapses. We further show that occupancy of CREB at the NR2B promoter increases and is causally related to the increase in synaptic NR2B levels. Blockade of NR2B-containing NMDARs by administration of the NR2B-selective antagonist Ro256981 directly into the NAc, under conditions that inhibit cocaine-induced silent synapses, prevents the development of cocaine-elicited locomotor sensitization. Our data are consistent with a cellular cascade whereby cocaine-induced activation of CREB promotes CREB-dependent transcription of NR2B and synaptic incorporation of NR2B-containing NMDARs, which generates new silent synapses within the NAc. We propose that cocaine-induced activation of CREB and generation of new silent synapses may serve as key cellular events mediating cocaine-induced locomotor sensitization. These findings provide a novel cellular mechanism that may contribute to cocaine-induced behavioral alterations.

HIV infection of the central nervous system can result in neurologic dysfunction with devastating consequences in AIDS patients. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence that HIV can infect neurons. Here we show that the HIV-encoded protein tat triggers formation of a macromolecular complex involving the low-density lipoprotein receptor-related protein (LRP), postsynaptic density protein-95 (PSD-95), N-methyl-D-aspartic acid (NMDA) receptors, and neuronal nitric oxide synthase (nNOS) at the neuronal plasma membrane, and that this complex leads to apoptosis in neurons negative as well as positive for NMDA receptors and also in astrocytes. Blockade of LRPmediated tat uptake, NMDA receptor activation, or neuronal nitric oxide synthase significantly reduces ensuing neuronal apoptosis, suggesting that formation of this complex is an early step in tat toxicity. We also show that the inflammatory chemokine, CCL2, protects against tat toxicity and inhibits formation of the complex. These findings implicate the complex in HIV-induced neuronal apoptosis and suggest therapeutic targets for intervention in the pathogenesis of NeuroAIDS.glutamate ͉ dementia ͉ HIV-1 ͉ NeuroAIDS ͉ excitotoxicity H IV enters the CNS early after infection. Viral persistence within the CNS can produce cognitive impairment, HIV encephalitis, and, in some cases, dementia. NeuroAIDS is characterized by neuronal damage and loss and cognitive and motor deficits and can have devastating consequences in a significant number of individuals with AIDS. As HIV-infected individuals live longer on antiretroviral therapy, the prevalence of cognitive impairment is increasing, and the study of the pathogenesis of NeuroAIDS becomes even more critical (1, 2).Although HIV infection of the CNS causes neuronal cell damage and loss, the virus cannot directly infect neurons. Rather, HIV-associated damage is thought to be due to an indirect mechanism whereby virally infected, as well as uninfected, cells elaborate neurotoxins. Candidate toxins include cytokines, glutamate, and virally encoded proteins such as the HIV transactivator protein, tat (3). Tat potentiates glutamateinduced excitotoxicity (4, 5) and promotes neuronal apoptosis (6-8). Antagonists of the N-methyl-D-aspartic acid (NMDA) receptor (NMDAR) protect against tat-induced apoptosis (5, 7), implicating NMDARs in this process.The low-density lipoprotein receptor-related protein (LRP) is expressed by many cells in the CNS, including neurons and astrocytes (9, 10). LRP is a receptor for at least 16 endogenous ligands and also for the viral protein tat, and mediates uptake of these ligands into endosomes in various cells, including neurons (9). Tat and some other LRP ligands can activate NMDARs and mediate calcium signaling in neurons (4,11,12). Tat, in contrast to other LRP ligands, escapes from the endosomal/lysosomal compartment (9) and, by mechanisms still poorly known, induces apoptosis in both neurons and astrocytes.This study was undertaken to examine mechanisms by...

Highly specific radioligands and quantitative autoradiography reveal strikingly different neuroanatomical patterns for the ,u, 8, and K opioid receptors of rat brain. The ,I receptors are most densely localized in patches in the striatum, layers I and III of the cortex, the pyramidal cell layer of the hippocampal formation, specific nuclei of the thalamus, the pars reticulata of the substantia nigra, the interpeduncular nucleus, and the locus coeruleus. In contrast, 8 receptors are highly confined, exhibiting selective localization in layers I, H, and VIa of the neocortex, a diffuse pattern in the striatum, and moderate concentration in the pars reticulata of the substantia nigra and in the interpeduncular nucleus. 8 receptors are absent in most other brain structures. This distribution is unexpected in that the enkephalins, the putative endogenous ligands of the 8 receptor, occur essentially throughout the brain. The K receptors of rat brain exhibit a third pattern distinct from that of the ,u and 8 receptors. K receptors occur at low density in patches in the striatum and at particularly high density in the nucleus accumbens, along the pyramidal and molecular layers of the hippocampus, in the granular cell layer of the dentate gyrus, specific midline nuclei of the thalamus, and hindbrain regions. K receptors appear to be uniformly distributed across regions in the neocortex with the exception of layer m, which revealed only trace levels of binding. An important conclusion of the present study is that 8 receptors occur at high density only in the forebrain and in two midbrain structures, whereas ,u and K receptors exhibit discrete patterns in most major brain regions.

K opioid receptors (K receptors) have been characterized in homogenates of guinea pig and rat brain under in vitro binding conditions. K receptors were labeled by using the tritiated prototypic K opioid ethylketocyclazocine under conditions in which ,u and 8 opioid binding was suppressed. In the case of guinea pig brain membranes, a single population of high-affinity K opioid receptor sites (K sites; Kd = 0.66 nM, BMMX = 80 fmol/mg of protein) was observed. In contrast, in the case ofrat brain, two populations of K sites were observedhigh-affinity sites at low density (Kd = 1.0 nM, Bm. = 16 fmol/mg of protein) and low-affinity sites at high density (Kd = 13 nM, Bin" = 111 fmol/mg of protein). To test the hypothesis that the high-and low-affinity K sites represent two distinct K receptor subtypes, a series of opioids were tested for their abilities to compete for binding to the two sites. U-69,593 and Cambridge 20 selectively displaced the high-affinity K site in both guinea pig and rat tissue, but were inactive at the rat-brain low-affinity site. Other K opioid drugs, including U-50,488, ethylketocyclazocine, bremazocine, cyclazocine, and dynorphin (1-17), competed for binding to both sites, but with different rank orders of potency. Quantitative light microscopy in vitro autoradiography was used to visualize the neuroanatomical pattern of K receptors in rat and guinea pig brain. The distribution patterns of the two K receptor subtypes of rat brain were clearly different. The pattern of rat high-affmiity K sites paralleled that of guinea pig in the caudate-putamen, midbrain, central gray substance of cerebrum, and substantia nigra; interspecies differences were apparent throughout most of the rest of the brain. Collectively, these data provide direct evidence for the presence of two K receptor subtypes; the U-69,593-sensitive, high-affinity K, site predominates in guinea pig brain, and the U-69,593-insensitive, low-affinity K2 site predominates in rat brain. Pharmacological studies have established that ketocyclazocine-like opioids produce their antinociceptive and unique sedative actions through an interaction with K receptors (2). These drugs effect a more pronounced sedation than do other opioids and have been evaluated as anesthetic agents. K opioid drugs neither suppress morphine abstinence nor induce abstinence in morphine-dependent monkeys (3). The endogenous opioid peptide dynorphin also interacts with high selectivity at K receptors.Evidence for a separate K receptor distinct from the morphine (A) and enkephalin (Enk; 8) receptors has been provided by pharmacological (2, 4), electrophysiological (5, 6), binding (4,7,8), and solubilization and purification (9)(10)(11) studies. In vitro autoradiography was used to visualize K receptors in rat (12) and guinea pig brain (13)