Current Neuropharmacology - Volume 9, Issue 1, 2011

To explore the functional consequences of cannabinoid withdrawal in the rat mesolimbic dopamine system, we investigated the anatomical morphology of the mesencephalic, presumed dopaminergic, neurons and their main post-synaptic target in the Nucleus Accumbens. We found that TH-positive neurons shrink and Golgi-stained medium spiny neurons loose dendritic spines in withdrawal rats after chronic cannabinoids administration. Similar results were observed after administration of the cannabinoid antagonist rimonabant to drug-naive rats supporting a role for endocannabinoids in neurogenesis, axonal growth and synaptogenesis. This evidence supports the tenet that withdrawal from addictive compounds alters functioning of the mesolimbic system. The data add to a growing body of work which indicates a hypodopaminergic state as a distinctive feature of the “addicted brain”.

Cannabinoids are the constituents of the marijuana plant (Cannabis sativa). There are numerous cannabinoids and other natural compounds that have been reported in the cannabis plant. The recent progress in marijuana-cannabinoid research include the discovery of an endocannabinoid system with specific genes coding for cannabinoid receptors (CBRs) that are activated by smoking marijuana, and that the human body and brain makes its own marijuana-like substances called endocannabinoids that also activate CBRs. This new knowledge and progress about cannabinoids and endocannabinoids indicate that a balanced level of endocannabinoids is important for pregnancy and that the breast milk in animals and humans has endocannabinoids for the growth and development of the new born. There are two well characterized cannabinoid receptors termed CB1-Rs and CB2-Rs and these CBRs are perhaps the most abundant Gprotein coupled receptors that are expressed at high levels in many regions of the mammalian brain. The expression of CB1-Rs in the brain and periphery and the identification of CB2-Rs in immune cells and during inflammation has been extensively studied and characterized. However, the expression of functional neuronal CB2-Rs in the CNS has been much less well established and characterized in comparison to the expression of abundant brain CB1-Rs and functional neuronal CB2-Rs has ignited debate and controversy. While the issue of the specificity of CB2-R antibodies remains, many recent studies have reported the discovery and functional characterization of functional neuronal CB2-Rs in the CNS beyond neuro-immuno cannabinoid activity.

Autism spectrum disorders (ASDs) are heterogenous neurodevelopmental disorders characterized by impairment in social, communication skills and stereotype behaviors. While autism may be uniquely human, there are behavioral characteristics in ASDs that can be mimicked using animal models. We used the BTBR T+tf/J mice that have been shown to exhibit autism-like behavioral phenotypes to 1). Evaluate cannabinoid-induced behavioral changes using forced swim test (FST) and spontaneous wheel running (SWR) activity and 2). Determine the behavioral and neurochemical changes after the administration of MDMA (20 mg/kg), methamphetamine (10 mg/kg) or MPTP (20 mg/kg). We found that the BTBR mice exhibited an enhanced basal spontaneous locomotor behavior in the SWR test and a reduced depressogenic profile. These responses appeared to be enhanced by the prototypic cannabinoid, Δ9-THC. MDMA and MPTP at the doses used did not modify SWR behavior in the BTBR mice whereas MPTP reduced SWR activity in the control CB57BL/6J mice. In the hippocampus, striatum and frontal cortex, the levels of DA and 5-HT and their metabolites were differentially altered in the BTBR and C57BL/6J mice. Our data provides a basis for further studies in evaluating the role of the cannabinoid and monoaminergic systems in the etiology of ASDs.

Methamphetamine is a potent addictive stimulant drug that activates certain systems in the brain. It is a member of the amphetamine family, but the effects of methamphetamine are much more potent, longer lasting, and more harmful to the central nervous system. Repeated administration of methamphetamine induces behavioral sensitization, which is considered to be related to compulsive drug-seeking behavior. Although the mechanism responsible for methamphetamine- induced behavioral sensitization remains unclear, it is believed that the mesolimbic dopaminergic system in the central nervous system plays a critical role in the development of behavioral sensitization. Our previous studies indicate that the involvement of the μ-opioid receptor system underlies the development of methamphetamine-induced behavioral sensitization. Understanding the mechanisms of behavioral sensitization that are regulated by the μ-opioid receptor system would be helpful in developing therapeutic programs against methamphetamine addiction. This review briefly discusses the neural circuitry and cellular mechanisms that are known to play a central role in methamphetamine-induced behavioral sensitization and outlines the role of the μ-opioid receptor system in the development of methamphetamine-induced sensitization.

Increasing evidence suggests that μ opioid receptor (MOP) expression is altered during the development of and withdrawal from substance dependence. Although anti-MOP antibodies have been hypothesized to be useful for estimating MOP expression levels, inconsistent MOP molecular weights (MWs) have been reported in studies using anti-MOP antibodies. In the present study, we generated a new anti-MOP antibody (N38) against the 1-38 amino acid sequence of the mouse MOP N-terminus and conducted Western blot analysis with wildtype and MOP knockout brain lysates to determine the MWs of intrinsic MOP. The N38 antibody detected migrating bands with relative MWs of 60-67 kDa in the plasma membrane fraction isolated from wildtype brain, but not from the MOP knockout brain. These migrating bands exhibited semi-linear density in the range of 3-30 μg membrane proteins/lane. The N38 antibody may be useful for quantitatively detecting MOP.

The possibility that pain perception and processing in the CNS results in cellular stress and may influence heat shock protein (HSP) expression was examined in a rat model of morphine dependence and withdrawal. Since activation of pain pathways result in exhaustion of growth factors, we examined the influence of cerebrolysin, a mixture of potent growth factors (BDNF, GDNF, NGF, CNTF etc,) on morphine induced HSP expression. Rats were administered morphine (10 mg/kg, s.c. /day) for 12 days and the spontaneous withdrawal symptoms were developed by cessation of the drug administration on day 13th that were prominent on day 14th and continued up to day 15th (24 to 72 h periods). In a separate group of rats, cerebrolysin was infused intravenously (5 ml/kg) once daily from day one until day 15th. In these animals, morphine dependence and withdrawal along with HSP immunoreactivity was examined using standard protocol. In untreated group mild HSP immunoreaction was observed during morphine tolerance, whereas massive upregulation of HSP was seen in CNS during withdrawal phase that correlated well with the withdrawal symptoms and neuronal damage. Pretreatment with cerebrolysin did not affect morphine tolerance but reduced the HSP expression during this phase. Furthermore, cerebrolysin reduced the withdrawal symptoms on day 14th to 15th. Taken together these observations suggest that cellular stress plays an important role in morphine induced pain pathology and exogenous supplement of growth factors, i.e. cerebrolysin attenuates HSP expression in the CNS and induce neuroprotection. This indicates a new therapeutic role of cerebrolysin in the pathophysiology of drugs of abuse, not reported earlier.

Advances in computer technology have allowed quantification of the electroencephalogram (EEG) and expansion of quantitative EEG (qEEG) analysis in neurophysiology, as well as clinical neurology, with great success. Among the variety of techniques in this field, frequency (spectral) analysis using Fast Fourier Transforms (FFT) provides a sensitive tool for time-course studies of different compounds acting on particular neurotransmitter systems. Studies presented here include Electrocorticogram (ECoG) analysis following exposure to a glutamic acid analogue - domoic acid (DOM), psychoactive indole alkaloid - ibogaine, as well as cocaine and gamma-hydroxybutyrate (GHB). The ECoG was recorded in conscious rats via a tether and swivel system. The EEG signal frequency analysis revealed an association between slow-wave EEG activity delta and theta and the type of behavioral seizures following DOM administration. Analyses of power spectra obtained in rats exposed to cocaine alone or after pretreatment with ibogaine indicated the contribution of the serotonergic system in ibogaine mediated response to cocaine (increased power in alpha1 band). Ibogaine also lowered the threshold for cocaine-induced electrographic seizures (increased power in the low-frequency bands, delta and theta). Daily intraperitoneal administration of cocaine for two weeks was associated with a reduction in slow-wave ECoG activity 24 hrs following the last injection when compared with controls. Similar decreased cortical activity in low-frequency bands observed in chronic cocaine users has been associated with reduced metabolic activity in the frontal cortex. The FFT analyses of power spectra relative to baseline indicated a significant energy increase over all except beta2 frequency bands following exposure to 400 and 800 mg/kg GHB. The EEG alterations detected in rats following exposure to GHB resemble absence seizures observed in human petit mal epilepsy. Spectral analysis of the EEG signals combined with behavioral observations may prove to be a useful approach in studying chronic exposure to drugs of abuse and treatment of drug dependence.

We have earlier reported that γ-hydroxybutyric acid (GHB) disrupts the acquisition of spatial learning and memory in adolescent rats. GHB is known to interact with several neurotransmitter systems that have been implicated in cognitive functioning. The N-methyl-D-aspartate receptor (NR) -type of glutamate receptor is considered to be an important target for spatial learning and memory. Molecular mechanisms governing the neuroadptations following repeated GHB treatment in adolecent rats remain unknown. We examined the role of NMDA receptor in adolescent GHB-induced cognitive deficit. Adolescent rats were administered with GHB on 6 consecutive days, and surface-expressed NMDA receptor subunits levels were measured. GHB significantly decreased NR1 levels in the frontal cortex. Adolescent GHB also significantly reduced cortical NR2A subunit levels. Our findings support the hypothesis that adolescent GHB-induced cogntive deficits are associated with neuroadaptations in glutamatergic transmission, particulaly NR functioning in the frontal cortex.

Addictive drugs, such as opioids, ethanol, cocaine, amphetamine, and phencyclidine (PCP), affect many functions of the nervous system and peripheral organs, resulting in severe health problems. G protein-activated inwardly rectifying K+ (GIRK, Kir3) channels play an important role in regulating neuronal excitability through activation of various Gi/o protein-coupled receptors including opioid and CB1 cannabinoid receptors. Furthermore, the channels are directly activated by ethanol and inhibited by cocaine at toxic levels, but not affected by methylphenidate, methamphetamine, and 3,4-methylenedioxymethamphetamine (MDMA) at toxic levels. The primary pharmacological action of PCP is blockade of N-methyl-D-aspartate (NMDA) receptor channels that are associated with its psychotomimetic effects. PCP also interacts with several receptors and channels at relatively high concentrations. However, the molecular mechanisms underlying the various effects of PCP remain to be clarified. Here, we investigated the effects of PCP on GIRK channels using the Xenopus oocyte expression system. PCP weakly but significantly inhibited GIRK channels at micromolar concentrations, but not Kir1.1 and Kir2.1 channels. The PCP concentrations effective in inhibiting GIRK channels overlap clinically relevant brain concentrations in severe intoxication. The results suggest that partial inhibition of GIRK channels by PCP may contribute to some of the toxic effects after overdose.

It has been demonstrated that 5-HT1A receptors play an important role in the pathophysiology of schizophrenia. Because Gastrodia elata Bl (GE) modulates the serotonergic system, we examined whether GE could affect phencyclidine (PCP)-induced abnormal behavior in mice. Repeated treatment with PCP increased immobility time, while it decreased social interaction time and recognition memory. PCP-induced abnormal behaviors were significantly attenuated by GE, and these effects were comparable to those of 8-OH-DPAT, a 5-HT1A receptor agonist. Furthermore, GE-mediated effects were counteracted by WAY 100635, a 5-HT1A receptor antagonist. Our results suggest that the antipsychotic effects of GE are, at least in part, mediated via activation of 5-HT1A in mice.

Ketamine is a noncompetitive antagonist of the NMDA-receptors, used as a dissociative anesthetic, presently included in the category of the psychoactive substances known as “club drugs”. Ketamine administration was associated with impaired working memory and increased psychopathological symptoms, but there is a lack of information regarding the effects of chronic sub-anesthetic doses. Adult Wistar rats were administered ketamine, 5 and 10 mg/kg twice daily, subcutaneously for 14 days. One week later, rats were tested in an object recognition/object location task and in the open field arena. There was altered performance in both the object recognition/location and in the open field tests by the group chronically exposed to the lower dose of ketamine. These animals displayed a decreased discrimination index (p<0.05) in the object recognition task, were unable to recognize the displacement of a familiar object and displayed decreased activity across open filed sessions. Importantly, these alterations were not observed in animals administered a higher dose of ketamine. Collectively, these results consistently show that chronic administration of ketamine in sub-anesthetic doses may lead to decreased habituation and inability to update spatial representations.

Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, is widely used for analgesia and anesthesia in obstetric and pediatric practice. Recent reports indicate that ketamine causes neuronal cell death in developing rodents and nonhuman primates. The present study assessed the potential dose- and time-dependent neurotoxic effects and associated changes in gene expression after ketamine administration to postnatal day 7 (PND-7) rat pups. Pups were exposed to ketamine subcutaneously at doses of 5, 10, or 20 mg/kg, in one, three or six injections respectively. Control animals received the same volume of saline at the same time points. The animals were sacrificed 6 h after the last ketamine or saline administration and brain tissues were collected for RNA isolation and histochemical examination. Six injections of 20 mg/kg ketamine significantly increased neuronal cell death in frontal cortex, while lower doses and fewer injections did not show significant effects. The ketamine induced cell death seemed to be apoptotic in nature. In situ hybridization demonstrated that NMDA receptor NR1 subunit expression was dramatically increased in the frontal cortex of ketamine treated rats. Microarray analysis revealed altered expression of apoptotic relevant genes and increased NMDA receptor gene expression in brains from ketamine treated animals. Quantitative RT-PCR confirmed the microarray results. These data suggest that repeated exposures to high doses of ketamine can cause compensatory up-regulation of NMDA receptors and subsequently trigger apoptosis in developing neurons.