A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function

Morphine-induced alterations in gene expression of calbindin immunopositive neurons in nucleus accumbens shell and core

Chronic opiate administration induces a number of biochemical alterations within the mesolimbic dopamine system that may mediate various aspects of the addictive process. In the present study, rats were administered morphine (1.0 mg/infusion) for 20 days (17.6+/-3.0 infusions/day) based on infusion histories of self-administering rats. Calbindin-D28K immunoreactive neurons were microdissected from the nucleus accumbens (NAc) shell and core subregions and gene expression was assessed using cDNA macroarrays. Comparison of gene expression between the shell and core subregions of vehicle-treated rats revealed significantly higher relative abundance of GABA-A alpha1, Galphai2 and post-synaptic density protein 95 transcript (PSD-95) mRNA levels in the shell, whereas Ggamma2 and synuclein 1 were more abundant in the core of the NAc. In the NAc shell, morphine administration resulted in upregulation of caspace 9, NF-kappaB, NF-H, tau, GABA-A delta subunit, FGFR1, Ggamma2, synuclein 1, syntaxin 5 and 13, GRK5, and c-fos mRNAs. Caspace 1, D2 dopamine receptor, GABA-A alpha1 subunit, GRIA 1/3/4, Galphai2, PSD-95 and CREB were down-regulated in the NAc shell with morphine administration. In the core, neuronal apoptotic inhibitory protein (NAIP), GABA-A alpha1 subunit, GRIN2C, GRIA1, mGluR1, D4 dopamine receptor and PSD-95 were upregulated by morphine administration whereas bax, bcl-x, cox-1 and MAP2 were decreased. These data demonstrate that morphine administration alters gene expression differentially in NAc subregions. Specifically, GABA-A alpha1 subunit, GRIA1 subunit and PSD-95 mRNAs were decreased in the shell but increased in the core following morphine administration. In addition, these results provide potential targets for further evaluation in models of morphine reinforcement as well as novel mechanisms of action in morphine-induced pathophysiology.

Chronic prenatal exposure to cocaine alters cerebrovascular responses in newborn pigs

Maternal cocaine abuse may increase the incidence of perinatal asphyxia. In nonexposed asphyxiated neonates, decreased cerebrospinal fluid (CSF) cAMP concentrations are associated with poor neurological outcome. On the other hand, cocaine increases central nervous system (CNS) cAMP. Therefore, we hypothesized that in utero cocaine exposure may increase brain cAMP and thereby preserve cerebrovascular responses to cAMP-dependent stimuli following asphyxia. Pregnant pigs received either cocaine (1 mg/kg, i.v.) twice weekly during the last trimester or normal saline vehicle (sham-control) and were allowed to deliver vaginally at term. Cranial windows were implanted in the newborn pigs within the first week of life and used to collect CSF for cAMP determinations and to assess changes in pial arteriolar diameters (PAD). In the first part of the study, pial arteriolar responses to different vasodilator and vasoconstrictor stimuli were evaluated in piglets prior to asphyxia (n = 20). In newborn pigs exposed to cocaine, cerebrovascular responses to hypercapnia and norepinephrine were significantly exaggerated compared to controls. Then, piglets were randomly selected for the second part of the study that involved prolonged asphyxia (n = 12). In cocaine-exposed but not sham-control piglets, CSF cAMP increased markedly during asphyxia. In the sham piglets, but not the cocaine-exposed piglets, CSF cAMP fell progressively below the baseline during recovery. Cerebrovascular reactivity to cAMP-dependent stimuli (hypercapnia and isoproterenol) was preserved during recovery from asphyxia in the cocaine-exposed piglets but significantly attenuated in the sham controls. We conclude that piglets with chronic prenatal exposure to cocaine show exaggerated cerebrovascular responses to vasogenic stimuli and preserved cAMP-dependent cerebral vasoreactivity following asphyxia.

A progressive ratio schedule of self-stimulation testing in rats reveals profound augmentation of d-amphetamine reward by food restriction but no effect of a "sensitizing" regimen of d-amphetamine

RATIONALE

Prior research indicates that psychostimulant-induced sensitization is not expressed in lateral hypothalamic electrical self-stimulation (LHSS)-based measures of drug reward, although the augmenting effect of chronic food restriction is. Neuroadaptations within the brain dopamine system have been identified in both psychostimulant-sensitized and food-restricted animals. Consequently, a variant of the LHSS paradigm in which responding is particularly sensitive to changes in dopaminergic tone may be best suited to detect and compare effects of chronic d-amphetamine and food restriction. Instrumental responding on a progressive ratio (PR) schedule is more sensitive to dopaminergic manipulations than is responding on a continuous reinforcement (CRF) schedule, but has not previously been used to examine chronic psychostimulant and food restriction effects on LHSS-based measures of drug reward.

OBJECTIVE

The first aim of this study was to determine whether a regimen of d-amphetamine treatment, that produces locomotor sensitization (5 mg/kg per day x5 days), increases the reward-potentiating effect of d-amphetamine in a PR LHSS protocol. The second aim, was to determine whether chronic food restriction produces a marked increase in the reward-potentiating effect of d-amphetamine in the PR LHSS protocol and, if so, whether it is reversible in parallel with body weight recovery when free feeding is restored.

METHOD

Reward-potentiating effects of a challenge dose of d-amphetamine (0.25 mg/kg, IP) were measured in terms of the break point of LHSS responding on a PR schedule of reinforcement, in ad libitum fed and food-restricted rats.

RESULTS

A regimen of d-amphetamine treatment that produced locomotor sensitization did not increase the break point for LHSS in the presence or absence of d-amphetamine. Chronic food restriction produced a marked increase in the break point-increasing effect of d-amphetamine (3-fold), which returned to baseline in parallel with body weight recovery over a 4-week period of restored free-feeding.

CONCLUSIONS

A locomotor-sensitizing regimen of d-amphetamine treatment does not increase the rewarding effect of LH electrical stimulation or the reward-potentiating effect of d-amphetamine in a PR LHSS protocol. The augmenting effect of chronic food restriction on drug reward is mechanistically and functionally different from psychostimulant sensitization and may be controlled by signals associated with adipose depletion and repletion.

Repeated intracerebroventricular forskolin administration enhances behavioral sensitization to cocaine

Repeated cocaine exposure produces behavioral sensitization expressed as an increased locomotor response to subsequent drug administration. Chronic cocaine administration also results in increased activity of adenylyl cyclase and cyclic-AMP (cAMP) dependent protein kinase (PKA) in the nucleus accumbens. To investigate the relationship between cocaine-induced behavioral sensitization and cAMP signaling, the present study examined the effect of forskolin, a direct adenylyl cyclase activator, on cocaine-induced hyperlocomotion and behavioral sensitization to cocaine. Rats were given intracerebroventricular (i.c.v.) injections of a water soluble form of forskolin (7DMB-forskolin) or vehicle 10 min prior to intraperitoneal (i.p.) cocaine or saline administration on 7 consecutive days. Acute or chronic forskolin alone had no effect on locomotor activity at the doses tested. On days 1 and 2, the activity of rats that received i.c.v. forskolin paired with cocaine was not significantly different from rats that received i.c.v. injections of vehicle co-administered with cocaine. By the third day of forskolin/cocaine co-administration, rats displayed enhanced cocaine-induced hyperlocomotor activity compared to rats that received cocaine alone, an effect that persisted through day 7. When challenged with cocaine on day 14, animals that had previously received forskolin paired with cocaine on days 1-7 displayed similar locomotor activity to animals that received cocaine only. These results suggest that alterations in adenylyl cyclase activity and/or cAMP levels may underlie the hyperlocomotor response to cocaine and may play a role in behavioral sensitization.

Sexual experience alters D1 receptor-mediated cyclic AMP production in the nucleus accumbens of female Syrian hamsters

Drugs of abuse produce long-term changes in dopamine neurotransmission and receptor-effected intracellular signaling. Similar changes in neuronal activity are mediated by motivated behaviors. To explore cellular mechanisms underlying these neuroadaptations following sexual experience, cyclic AMP accumulation following stimulation of D1 dopamine receptors, G-proteins, and adenylate cyclase was compared in the nucleus accumbens and caudate nucleus of sexually naive and experienced female hamsters following sexual behavior. Direct stimulation of adenylate cyclase with forskolin or indirectly by activation of G-proteins with Gpp(NH)p produced dose-dependent increases in the formation of cyclic AMP in the nucleus accumbens and caudate nucleus, with no effects of sexual experience on these measures. Specific D1 receptor stimulation increased Gpp(NH)p-induced adenylate cyclase activity in the nucleus accumbens and caudate nucleus of all animals. Interestingly, this stimulation was further enhanced only in membranes from the nucleus accumbens, but not from the caudate nucleus, of sexually experienced hamsters compared to the response of naive females. These results demonstrate that sexual behavior experience can sensitize mesolimbic dopamine pathways and that this sensitization occurs through an increase in D1 receptor-mediated signaling.

Molecular mechanisms in dizocilpine-induced attenuation of development of morphine dependence: an association with cortical Ca2+/calmodulin-dependent signal cascade

We investigated how dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, affects the development of morphine dependence in mice. Co-administration of dizocilpine (0.25 mg/kg) and morphine (10 mg/kg) for 5 days attenuated the development of tolerance to the antinociceptive effects of morphine. The withdrawal manifestation induced by the naloxone-challenge (5 mg/kg) was significantly reduced in mice that were treated with a combination of dizocilpine and morphine, compared to the mice treated with morphine and saline. The present study revealed a significant increase in c-Fos protein expression in the cortex and thalamus of mice showing naloxone-precipitated withdrawal syndrome. The combination of dizocilpine and morphine prevented the increase of c-Fos protein expression in the cortex and thalamus. Interestingly, repeated co-administration of dizocilpine and morphine prevented the withdrawal-induced phosphorylation of Ca2+/calmodulin kinase II (p-CaMK II) in the cortex, but not in the thalamus. Acute dizocilpine treatment prior to the naloxone-challenge and repeated treatment with dizocilpine alone had no effect on analgesia, withdrawal manifestations, p-CaMK II levels or c-Fos protein levels. These results showed that co-administration of dizocilpine and morphine prevented the development of morphine tolerance and dependence and suggested that the preventive effect of dizocilpine results from the regulation of c-Fos protein expression, which is possibly involved in the activation of the Ca2+/calmodulin-dependent signal cascade in the cortex.

Neurotrophic mechanisms in drug addiction

The involvement of neurotrophic factors in neuronal survival and differentiation is well established. The more recent realization that these factors also play pivotal roles in the maintenance and activity-dependent remodeling of neuronal functioning in the adult brain has generated excitement in the neurosciences. Neurotrophic factors have been implicated in the modulation of synaptic transmission and in the mechanisms underlying learning and memory, mood disorders, and drug addiction. Here the evidence for the role of neurotrophins and other neurotrophic factors-and the signaling pathways they activate-in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction is reviewed.

Alterations in ionotropic glutamate receptor subunits during binge cocaine self-administration and withdrawal in rats

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in glutamatergic function in brain regions associated with reinforcement. Previous studies have examined ionotropic glutamate receptor (iGluR) subunit protein level changes following acute and chronic experimenter-administered cocaine or after withdrawal periods from experimenter-administered cocaine. To evaluate whether alterations in expression of iGluRs are associated with cocaine reinforcement, protein levels were assessed after binge (8 h/day, 15 days; 24-h access, days 16-21) cocaine self-administration and following 2 weeks of abstinence from this binge. Western blotting was used to compare levels of iGluR protein expression (NR1-3B, GluR1-7, KA2) in the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NAc), striatum and prefrontal cortex (PFC) of rats. iGluR subunits were altered in a time-dependent manner in all brain regions studied; however, selective alterations in certain iGluR subtypes appeared to be associated with binge cocaine self-administration and withdrawal in a region-specific manner. In the SN and VTA, alterations in iGluR protein levels compared with controls occurred only following binge access, whereas in the striatum and PFC, iGluR alterations occurred with binge access and following withdrawal. In the NAc, GluR2/3 levels were increased following withdrawal compared with binge access, and were the only changes observed in this region. Because subunit composition determines the functional properties of iGluRs, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine.

Cocaine dependence and d2 receptor availability in the functional subdivisions of the striatum: relationship with cocaine-seeking behavior

Striatal dopamine D2 receptors have been implicated in the neurobiology of cocaine addiction. Previous imaging studies showed reduced striatal D2 receptor availability in chronic cocaine abusers, and animal studies suggested that low D2 receptor availability promotes cocaine self-administration. Here, D2 receptor availability was assessed with positron emission tomography (PET) and [11C]raclopride in the limbic, associative, and sensori-motor subdivisions of the striatum in 17 recently detoxified chronic cocaine-dependent (CCD) subjects and 17 matched healthy control (HC) subjects. In addition, the relationship between regional D2 receptor availability and behavioral measures obtained in cocaine self-administration sessions was investigated in CCD subjects. [11C]Raclopride binding potential was significantly reduced by 15.2% in the limbic striatum, 15.0% in the associative striatum, and 17.1% in the sensori-motor striatum in CCD subjects compared to HC subjects. In CCD subjects, no relationship was detected between D2 availability in striatal regions and either the positive effects of smoked cocaine or the choice of cocaine over an alternative reinforcer (money) following a priming dose of cocaine (a laboratory model of relapse). Thus, this study confirms previous reports of a modest decrease in D2 receptor availability in CCD subjects, and establishes that this decrease is generalized throughout the striatum. However, this study failed to demonstrate a relationship between D2 receptor availability and cocaine-induced cocaine-taking behavior. Additional research is warranted to unravel potential neurobiological traits that might confer vulnerability to relapse in detoxified CCD subjects.

Molecular neurobiology of drug addiction

Addiction can be viewed as a form of drug-induced neural plasticity. One of the best-established molecular mechanisms of addiction is upregulation of the cAMP second messenger pathway, which occurs in many neuronal cell types in response to chronic administration of opiates or other drugs of abuse. This upregulation and the resulting activation of the transcription factor CREB appear to mediate aspects of tolerance and dependence. In contrast, induction of another transcription factor, termed DeltaFosB, exerts the opposite effect and may contribute to sensitized responses to drug exposure. Knowledge of these mechanisms could lead to more effective treatments for addictive disorders.

Historical review: Molecular and cellular mechanisms of opiate and cocaine addiction

The National Institute on Drug Abuse was founded in 1974, and since that time there have been significant advances in understanding the processes by which drugs of abuse cause addiction. The initial protein targets for almost all drugs of abuse are now known. Animal models that replicate key features of addiction are available, and these models have made it possible to characterize the brain regions that are important for addiction and other drug effects, such as physical dependence. A large number of drug-induced changes at the molecular and cellular levels have been identified in these brain areas and rapid progress is being made in relating individual changes to specific behavioral abnormalities in animal models of addiction. The current challenges are to translate this increasingly impressive knowledge of the basic neurobiology of addiction to human addicts, and to identify the specific genes that make some individuals either particularly vulnerable or resistant to addiction. In this article, I present a historical review of basic research on opiate and cocaine addiction.

In vivo regulation of extracellular signal-regulated protein kinase (ERK) and protein kinase B (Akt) phosphorylation by acute and chronic morphine

In vitro evidence suggests that extracellular signal-regulated protein kinases (ERKs) and Akt (also referred to as protein kinase B) are among the myriad of intracellular signaling molecules regulated by opioid receptors. The present study examined the regulation of ERK and Akt activation in the nucleus accumbens and caudate putamen following acute and chronic morphine administration in the rat. ERK and Akt are activated by phosphorylation, hence the levels of phosphorylated ERK (pERK) and Akt (pAkt) as well as total levels of ERK and Akt protein were measured by Western blot analysis. Male Sprague-Dawley rats received either a single injection of morphine or twice daily injections of morphine for 6 or 10 days. Following acute morphine, pERK levels were significantly decreased in the nucleus accumbens but not in the caudate putamen. Phosphorylated Akt levels in the nucleus accumbens were significantly increased after a single morphine injection. Naltrexone pretreatment prevented both the morphine-induced pERK down-regulation and pAkt up-regulation. Although reductions in pERK levels were evident after 6 days of morphine administration, no differences were observed in pERK levels after 10 days. In contrast to the up-regulation seen after acute morphine, pAkt levels in the nucleus accumbens were significantly decreased after chronic morphine administration. Thus, the differential activation patterns of both ERK and Akt after acute and chronic morphine administration could have important implications for understanding additional pathways mediating opioid signaling in vivo.

Endogenous regulator of G protein signaling proteins suppress Galphao-dependent, mu-opioid agonist-mediated adenylyl cyclase supersensitization

Chronic mu-opioid agonist treatment leads to dependence with withdrawal on removal of agonist. At the cellular level withdrawal is accompanied by a supersensitization of adenylyl cyclase, an effect that requires inhibitory Galpha proteins. Inhibitory Galpha protein action is modulated by regulator of G protein signaling (RGS) proteins that act as GTPase activating proteins and reduce the lifetime of Galpha-GTP. In this article, we use C6 glioma cells expressing the rat mu-opioid receptor (C6mu) to examine the hypothesis that Galphao alone can mediate mu-opioid agonist induced adenylyl cyclase supersensitivity and that endogenous RGS proteins serve to limit the extent of this supersensitization. C6mu cells were stably transfected with pertussis toxin (PTX)-insensitive Galphao that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin or morphine. Supersensitization was observed in cells expressing wild-type Galpha, but this was lost on PTX treatment. In cells expressing PTX-insensitive Galphao supersensitization was recovered, confirming that Galphao alone can support supersensitization. In cells expressing the RGS-insensitive mutant Galphao, there was a greater degree of supersensitization and the concentration of micro-agonist needed to achieve half-maximal supersensitization was reduced by 10-fold. The amount of supersensitization seen did not directly relate to the degree of acute inhibition of adenylyl cyclase. These results demonstrate a role for Galphao in adenylyl cyclase supersensitization after mu-agonist exposure and show that this action is modulated by endogenous RGS proteins.

Changes in sodium, potassium-ATPase induced by repeated fencamfamine: the roles of cyclic AMP-dependent protein kinase and the nitric oxide-cyclic GMP pathway

Fencamfamine (FCF) is an indirect dopamine agent with effects similar to amphetamine and cocaine. In the present study, we investigate changes in Na,K-ATPase, cyclic AMP-dependent protein kinase (PKA) and nitric oxide synthase (NOS) activity and cyclic GMP levels in the nucleus accumbens (NAc) and striatum (ST) of animals acutely or repeatedly treated with FCF (3.5 mg/kg). Na,K-ATPase had a similar activity in control and repeatedly treated animals, but was reduced in the NAc of the acute group. This enzyme was reduced in the ST in acute and repeatedly treated animals, compared to the control group. Expression of the alpha(1,2,3)-Na,K-ATPase isoforms in the NAc and the ST was not altered in all groups studied. Acute FCF induced a significant increase in PKA activity in both the ST and the NAc. Repeatedly treated animals showed a higher increase in PKA activity in the NAc, but not in the ST, when compared to the acute group. There was also an increase in both NOS activity and cyclic GMP levels only in the NAc of FCF repeatedly treated animals compared to the acute and control groups. We suggest that chronic FCF treatment is linked to a modification in Na,K-ATPase activity through the PKA and NO-cyclic GMP pathway.

Changes of the level of G protein alpha-subunit mRNA by tolerance to and withdrawal from butorphanol

Butorphanol was infused continuously into cerebral ventricle at a constant rate of 26 nmol/microl/h for 3 days, and the withdrawal from opioid was rendered 7 h after the cessation of infusion. The G-protein alpha-subunit has been implicated in opioid tolerance and withdrawal. The effects of continuous infusion of butorphanol on the modulation of G protein alpha-subunit mRNA were investigated by using in situ hybridization techniques. In situ hybridization showed marked changes in the levels of Galpha s during butorphanol tolerance and withdrawal. Specifically, the level of Galpha s mRNA was significantly decreased in almost all areas of brain except hippocampus during the butorphanol withdrawal. It was also decreased in the septum and cerebellar granule layer in butorphanol tolerant rats. The level of Galpha i mRNA was significantly decreased only in the cerebral cortex of butorphanol tolerant rat. However, no such change was noted during the withdrawal from butorphanol. The level of Galpha o mRNA was not changed either in butorphanol tolerant or in the butorphanol withdrawal rats. No alterations were noted in the level of [3H]forskolin binding to adenylyl cyclase in butorphanol tolerant as well as withdrawing rats. The levels of pCREB were significantly elevated in the hippocampus in the butorphanol withdrawal rats. These results suggest that region-specific changes of G protein alpha-subunit mRNA and pCREB without marked changes in the level of adenylyl cyclase may underlie the tolerance to and withdrawal from butorphanol.

Incubation of cocaine craving after withdrawal: a review of preclinical data

Using a rat model of drug craving and relapse, we recently found that cocaine seeking induced by re-exposure to drug-associated cues progressively increases over the first 2 months after withdrawal from cocaine self-administration, suggesting that drug craving incubates over time [Nature 412 (2001) 141]. Here, we summarize data from studies that further characterized this incubation phenomenon and briefly discuss its implications for drug addiction. The main findings of our ongoing research are: 1. Incubation of cocaine craving is long-lasting, but not permanent: cocaine seeking induced by exposure to cocaine cues remains elevated for up to 3 months of withdrawal, but decreases after 6 months. 2. Incubation of reward craving is not drug specific: sucrose seeking induced by re-exposure to the reward cues also increases after withdrawal, but for a time period that is shorter than that of cocaine. 3. Incubation of cocaine craving is not evident after acute re-exposure to cocaine itself: cocaine seeking induced by cocaine priming injections remains essentially unchanged over the first 6 months of withdrawal. 4. Incubation of cocaine craving after withdrawal is associated with increases in the levels of brain-derived neurotrophic factor (BDNF) in mesolimbic dopamine areas.

The brain stem but not forebrain independently supports morphine tolerance and withdrawal effects in cats

We employed polygraphic recordings and behavioral measures to study the effects of chronic morphine use upon the isolated forebrain and the decerebrate animal in cats with a midbrain transection. Cats received morphine for 12 days, and 24 h recording sessions were conducted on days 1 and 11. For the decerebrate cat, the percent time of rapid eye movement (REM) sleep was reduced during the 24 h period on both days 1 and 11. However, the values on day 11 were consistently higher than the values on day 1. Other tolerance indicators were decreases in the number of early behavioral signs and in the onset delay for REM sleep, together with an increase in onset time for motor activation. After naloxone (day 12) all cats displayed "wet shakes," tachypnea and eye squinting, as well as either pyloerection, elevated tail, salivation, licking, micturition, and yawning. In the isolated forebrain, the percent time for waking increased through the first 18 h post-morphine on both days 1 and 11. Conversely, the duration of non-REM (NREM) sleep and of drowsiness decreased. But importantly, the duration of sleep-waking states did not vary between days 11 and 1, indicating absence of tolerance. Additionally, after naloxone, the isolated forebrain entered NREM sleep, contrasting with opposite findings in intact cats. Therefore, while we could not demonstrate chronic use effects in the isolated forebrain, the decerebrate cat still displayed typical tolerance/withdrawal manifestations. This suggests that the effects of chronic opiate use are deeply seated in the brain stem, which might help understanding the ingrained nature of physical dependence.

Prenatal exposure to cocaine decreases adenylyl cyclase activity in embryonic mouse striatum

Adenylyl cyclase activity was measured in the striatum of naive mice as a function of age and in mice exposed in utero to cocaine. In naive Swiss-Webster mice, basal and forskolin-stimulated adenylyl cyclase activity increased gradually from embryonic day 13 (E13) until 2-3 weeks of age when activity peaked before decreasing slightly to adult levels. The ability of the dopamine D1 receptor agonist, SKF 82958, to stimulate adenylyl cyclase activity also increased in magnitude until P15. In a separate study, pregnant Swiss-Webster mice were injected twice daily with cocaine (15 mg/kg, s.c.) or an equal volume of saline from E10 to E17. Adenylyl cyclase activity was measured in the striatum of E18 embryos. Basal adenylyl cyclase activity was significantly reduced following prenatal exposure to cocaine. Likewise, the ability of forskolin or SKF 82958 to stimulate adenylyl cyclase was attenuated following cocaine exposure. DeltaFosB was not induced, contrary to what is seen in adult mice. These results demonstrate a functional change in a critical signal transduction pathway following chronic in utero exposure to cocaine that might have profound effects of the development of the brain. Alterations in the cAMP system may underlie some of the deficits seen in humans exposed in utero to cocaine.

The pineal gland is critical for circadian Period1 expression in the striatum and for circadian cocaine sensitization in mice

Sensitization to psychostimulants can be influenced by circadian rhythms. The pineal gland, the main source of circadian melatonin synthesis, may influence behavioral sensitization to cocaine; mice with normal melatonin rhythms do not get sensitized at night. Clock genes such as Period1 (Per1) show rhythmic region- and strain-dependent expression in the mouse brain, and mice mutant for the Per1 gene lack cocaine sensitization. Here, for the first time we show circadian changes of PER1 protein levels in the mouse striatum, a brain region crucial for the development of locomotor sensitization to cocaine. In male C3H/HeJ mice, we found peak striatal PER1 protein levels during the day; this was preceded by a Per1 mRNA peak 16 h earlier. Pinealectomized mice did not show this circadian pattern. We analyzed circadian cocaine sensitization at times when striatal PER1 protein levels in control mice (naive and sham-pinealectomized) were high and low, respectively. Only mice with circadian changes in striatal Per1 expression showed the night-time absence of cocaine sensitization, whereas pinealectomized mice were without circadian changes in striatal Per1 and were sensitized to cocaine regardless of diurnal rhythm. Our results indicate that both the striatal circadian Per1 expression and diurnal locomotor cocaine sensitization are strongly influenced by pineal products. Since we found evidence for the expression of melatonin receptor mRNA in the striatum, we suggest that further studies on pineal-driven mechanisms will help us better understand the mechanisms of drug abuse and identify novel targets for the prevention and/or treatment of addictions.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Discrete cell gene profiling of ventral tegmental dopamine neurons after acute and chronic cocaine self-administration

Chronic cocaine administration induces a number of biochemical alterations within the mesolimbic dopamine system that may mediate various aspects of the addictive process such as sensitization, craving, withdrawal, and relapse. In the present study, rats were allowed to self-administer cocaine (0.5 mg/infusion) for 1 or 20 days. Tyrosine hydroxylase immunopositive cells were microdissected from the ventral tegmental area (VTA) using laser capture microdissection, and changes in the abundances of 95 mRNAs were assessed using cDNA macroarrays. Five GABA-A receptor subunit mRNAs (alpha4, alpha6, beta2, gamma2, and delta) were down-regulated at both 1 and 20 days of cocaine self-administration. In contrast, the catalytic subunit of protein phosphatase 2A (PP2alpha), GABA-A alpha1, and Galphai2 were significantly increased at both time points. Additionally, calcium/calmodulin-dependent protein kinase IIalpha mRNA levels were increased initially followed by a slight decrease after 20 days, whereas neuronal nitric-oxide synthase mRNA levels were initially decreased but returned to near control levels by day 20. These results indicate that alterations of specific GABA-A receptor subtypes and other signal transduction transcripts seem to be specific neuroadaptations associated with cocaine self-administration. Moreover, as subunit composition determines the functional properties of GABA-A receptors, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine.

Importance of ERK activation in behavioral and biochemical effects induced by MDMA in mice

Little is known about the cellular effects induced by 3,4-methylenedioxymethamphetamine (MDMA, ecstasy), although changes in gene expression have been observed following treatments with other psychostimulants. Thus, the aim of this study was to investigate in mice, the relationships between the ras-dependent protein kinase ERK and MDMA-induced reinforcement using the conditioned place preference (CPP) and locomotor activity measurements. This was completed using real-time quantitative PCR method by a study of immediate early-genes (IEGs) transcription known to be involved in neuronal plasticity. A significant CPP was observed after repeated MDMA treatment in CD-1 mice at a dose of 9 mg kg-1 i.p. but not at 3 and 6 mg kg-1. This rewarding effect was abolished by the selective inhibitor of ERK activation, SL327 (50 mg kg-1; i.p.). Similar results were obtained on MDMA-induced locomotor activity, clearly suggesting a role of ERK pathway in these behavioral responses. Following acute i.p. injection, MDMA induced a strong c-fos transcription in brain structures, such as caudate putamen, nucleus accumbens and hippocampus, whereas egr-1 and egr-3 transcripts were only increased in the caudate putamen. MDMA-induced IEGs transcription was selectively suppressed by SL327 in the caudate putamen, suggesting a role for other signaling pathways in regulation of IEGs transcription in the other brain structures. In agreement with these results, MDMA-induced c-fos protein expression was blocked by SL327 in the caudate putamen. This study confirms and extends to mice the reported role of ERK pathway in the development of addiction-like properties of MDMA. This could facilitate studies about the molecular mechanism of this process by using mutant mice.

Regulation of gene expression and cocaine reward by CREB and ΔFosB

DeltaFosB (a truncated form of FosB) and CREB (cAMP response element binding protein) are transcription factors induced in the brain's reward pathways after chronic exposure to drugs of abuse. However, their mechanisms of action and the genes they regulate remain unclear. Using microarray analysis in the nucleus accumbens of inducible transgenic mice, we found that CREB and a dominant-negative CREB have opposite effects on gene expression, as do prolonged expression of DeltaFosB and the activator protein-1 (AP-1) antagonist DeltacJun. However, unlike CREB, short-term and prolonged DeltaFosB induction had opposing effects on gene expression. Gene expression induced by short-term DeltaFosB and by CREB was strikingly similar, and both reduced the rewarding effects of cocaine, whereas prolonged DeltaFosB expression increased drug reward. Gene expression after a short cocaine treatment was more dependent on CREB, whereas gene expression after a longer cocaine treatment became increasingly DeltaFosB dependent. These findings help define the molecular functions of CREB and DeltaFosB and identify clusters of genes that contribute to cocaine addiction.

Differential distribution of CREB in the mesolimbic dopamine reward pathway

The transcription factor cAMP response element binding protein (CREB) has been implicated in the long-term neuronal plasticity associated with addiction. While CREB is expressed in many cells throughout the brain, very little is known about the relative concentrations of CREB protein in various brain regions. Studies in which CREB levels have been altered, either constitutively throughout the brain via gene targeting or transiently in specific brain regions, demonstrate variable roles for this protein in mediating reinforcing properties of drugs of abuse. To investigate the complex nature of CREB function in addiction, we examined the distribution of CREB protein in the nucleus accumbens (NAc) and ventral tegmental area (VTA), two brain regions that are part of the well-defined mesolimbic dopamine pathway involved in reward processing. Our data demonstrate significantly more CRE binding activity and CREB protein in the NAc compared to levels present in the VTA of wild-type mice. Phospho-CREB levels are increased in the NAc of both wild-type and CREBalphaDelta mutant animals after cocaine. However, morphine-induced increases of phospho-CREB levels are seen in the VTA of wild-type mice but not CREBalphaDelta mutant mice. Consequently, the 90% reduction of CREB in CREBalphaDelta mutant mice differentially affects CREB phosphorylation and induction of downstream targets of CREB in the NAc and VTA.

Dopamine-dependent increases in phosphorylation of cAMP response element binding protein (CREB) during precipitated morphine withdrawal in primary cultures of rat striatum

Chronic morphine leads to compensatory up-regulation of cAMP signaling pathways in numerous brain regions. One potential consequence of up-regulated cAMP signaling is increased phosphorylation of cAMP response element binding protein (CREB), a transcription factor that may regulate neuroadaptations related to morphine dependence. Altered gene expression within the nucleus accumbens (NAc), a ventral component of the striatum that receives substantial dopaminergic input, may play a role in some of the motivational aspects of opiate withdrawal. To determine if morphine withdrawal leads to increased CREB phosphorylation in striatal tissues, we examined the effects of naloxone-precipitated morphine withdrawal on CREB phosphorylation in primary cultures of rat striatal neurons. Precipitated morphine withdrawal was associated with enhanced dopamine-, SKF 82958 (D1 receptor agonist)-, and forskolin-induced CREB phosphorylation. During precipitated withdrawal, D1 receptor-mediated CREB phosphorylation was dependent on cAMP-dependent protein kinase (PKA). Precipitated withdrawal also led to up-regulation of c-fos mRNA in response to SKF 82958. CREB protein levels were not altered by acute or chronic morphine. These results suggest that D1 receptor-mediated signal transduction is enhanced during morphine withdrawal. Furthermore, they are consistent with in vivo evidence suggesting that increased CREB activation in portions of the striatum (e.g. the NAc) is related to dysphoric states associated with drug withdrawal.

The expression of a high level of morphine antinociceptive tolerance in mice involves both PKC and PKA

We have previously reported that intracerebroventricular (i.c.v.) injection of either a PKC or PKA inhibitor completely reversed the expression of 5- to 8-fold morphine antinociceptive tolerance. We developed a model of 45-fold morphine tolerance that included a 75-mg morphine pellet and twice daily morphine injections. PKC inhibitor doses of bisindolylmaleimide I and Gö-7874 that completely reversed 8-fold tolerance only partly reversed the 45-fold level of antinociceptive tolerance. A component of tolerance was resistant to PKC inhibition, since even higher inhibitor doses failed to further reverse the high level of morphine tolerance. In addition, the 45-fold tolerance was only partly reversed by the PKA inhibitor KT-5720 at a dose previously cited by others to reverse 5-fold tolerance. Another PKA inhibitor 4-cyano-3-methylisoquinoline only partly reversed the morphine tolerance as well. In other experiments PKC and PKA inhibitors were co-administered together to determine their effectiveness for completely reversing the 45-fold level of morphine tolerance. Co-administering either bisindolylmaleimide I with KT-5720, or Gö-7874 with KT-5720, completely reversed the high level of tolerance. The high level of morphine tolerance was also completely reversed by co-administering Gö-7874 with 4-cyano-3-methylisoquinoline. Thus, high levels of morphine tolerance may reflect increases in protein phosphorylation by the terminal kinases of both the adenylyl cyclase and phosphatidylinositol cascades in brain and spinal cord areas critical to the expression of antinociception.

Altering behavior with gene transfer in the limbic system

There is now sufficient knowledge of the workings of the limbic system to allow experimental manipulation of behaviors anchored in limbic function. While such manipulations have traditionally involved lesions, stimulation or pharmacological approaches, it has become plausible to use gene transfer technology to alter patterns of gene expression in the nervous system. In this review, I consider ways in which gene transfer has been used to alter limbic function. These involve altering (a) cognition, (b) the rewarding properties of addictive substances, (c) patterns of social affiliation, and (d) responses to stress.

Estrogen-dependent alterations in D2/D3-induced G protein activation in cocaine-sensitized female rats

Estrogen potentiates behavioral sensitization to cocaine in the female rat by mechanisms that remain undetermined. In this study, functional receptor autoradiography was used to investigate estrogen modulation of D2/D3 receptor-induced G protein activation in components of the reward pathway of female rats treated acutely and repeatedly with cocaine. Rats were ovariectomized and given an empty (OVX group) or estradiol benzoate-filled (OVX-EB group) implant. After a week, animals received a daily saline or cocaine injection (15 mg/kg, i.p.) for 5 days, and again on day 13. Animals were killed, and brains were removed and cryosectioned. D2/D3-stimulated [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding was assessed in the cingulate cortex area 2 (Cg2), striatum (STR), nucleus accumbens (NAc) and ventral tegmental area (VTA). OVX-EB rats showed more [35S]GTPgammaS binding in the Cg2 and lower binding in the VTA than OVX rats; in the VTA this effect was reversed by a single cocaine injection. Repeated cocaine administration had opposite effects in OVX and OVX-EB rats. [35S]GTPgammaS binding was decreased in the Cg2, NAc and STR of OVX-EB rats, and increased in OVX rats. The present results support the hypothesis that cocaine-induced changes in D2/D3 receptor activation are regulated by estrogen. These data suggest that changes in D2/D3 receptor function represent one mechanism by which estrogen regulates behavioral sensitization to cocaine.

Molecular neuroadaptations in the accumbens and ventral tegmental area during the first 90 days of forced abstinence from cocaine self-administration in rats

Cocaine self-administration is associated with a propensity to relapse in humans and reinstatement of drug seeking in rats after prolonged withdrawal periods. These behaviors are hypothesized to be mediated by molecular neuroadaptations within the mesolimbic dopamine system. However, in most studies of drug-induced neuroadaptations, cocaine was experimenter-delivered and molecular measurements were performed after short withdrawal periods. In the present study, rats were trained to self-administer intravenous cocaine or oral sucrose (a control non-drug reward) for 10 days (6-h/day) and were killed following 1, 30, or 90 days of reward withdrawal. Tissues from the accumbens and ventral tegmental area (VTA) were assayed for candidate molecular neuroadaptations, including enzyme activities of cAMP-dependent protein kinase (PKA) and adenylate cyclase (AC), and protein expression of cyclin-dependent kinase 5 (cdk5), tyrosine hydroxylase (TH) and glutamate receptor subunits (GluR1, GluR2 and NMDAR1). In the accumbens of cocaine-trained rats, GluR1 and NMDAR1 levels were increased on days 1 and 90, while GluR2 levels were increased on days 1 and 30, but not day 90; PKA activity levels were increased on days 1 and 30, but not day 90, while AC activity, TH and cdk5 levels were unaltered. In the VTA of cocaine-trained rats, NMDAR1 levels were increased for up to 90 days, while GluR2 levels were increased only on day 1; TH and Cdk5 levels were increased only on day 1, while PKA and AC activity levels were unaltered. Cocaine self-administration produces long-lasting molecular neuroadaptations in the VTA and accumbens that may underlie cocaine relapse during periods of abstinence.

Methamphetamine exposure during the preweanling period causes prolonged changes in dorsal striatal protein kinase A activity, dopamine D2-like binding sites, and dopamine content

Exposure to methamphetamine (METH) during the preweanling period produces few, if any, neurotoxic effects (using criteria established in adult rats), yet it has substantial long-term effects on a variety of behavioral measures (e.g., locomotor activity, acoustic startle response, and spatial learning). The purpose of the present study was to examine the long-term changes in dopaminergic functioning brought about by early METH exposure. Rats were injected with METH (10 mg/kg) or saline four times daily on postnatal days (PD) 11-20 and housed undisturbed until PD 90, at which time they were killed and their dorsal striata (i.e., caudate-putamen) were removed and frozen for assay. The ability of early METH exposure to alter protein kinase A (PKA) activity and dopamine (DA) D(2)-like binding sites, as well as DA and DOPAC content, were assessed. Results showed that METH exposure on PD 11-20 caused long-term reductions in all of the dopaminergic markers assayed. METH-induced reductions in DA content and D(2)-like receptors were observed. Some sex differences were apparent, as the METH-induced decreases in PKA activity and DOPAC content were more evident in male rats. In conclusion, preweanling METH exposure caused changes in DA markers that were still detectable at PD 90; however the magnitude of many of these effects (e.g., the reductions in DA and DOPAC levels) was substantially less than typically reported for rats treated with METH in adulthood. The ability of METH to cause long-term reductions in PKA activity may partially account for some of behavioral deficits exhibited by rats exposed to METH prior to weaning.

Gene transfer of constitutively active protein kinase C into striatal neurons accelerates onset of levodopa-induced motor response alterations in parkinsonian rats

Alterations in motor response that complicate levodopa treatment of Parkinson's disease appear to involve sensitization of striatal ionotropic glutamate receptors. Since protein kinase C (PKC)-mediated phosphorylation regulates glutamatergic receptors of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) subtype and has been linked to several forms of behavioral plasticity, activation of PKC signaling in striatal spiny neurons may also contribute to the motor plasticity changes associated with chronic levodopa therapy. To evaluate this possibility, we sought to augment PKC signaling by using Herpes Simplex Virus type 1 vectors (pHSVpkcDelta) to directly transfer the catalytic domain of the PKCbetaII gene into striatal neurons of parkinsonian rats. Microinjection of pHSVpkcDelta vectors lead to the persistent expression of PkcDelta (35% loss over 21 days) in medium spiny neurons together with an increase in serine 831 phosphorylation on AMPA receptor GluR1 subunits and hastened the appearance of the shortened response duration produced by chronic levodopa treatment (P<0.05). In pHSVpkcDelta-infected animals, intrastriatal injection of the PKC inhibitor NPC-15437 (1.0 microg) attenuated both the increased GluR1 phosphorylation (P<0.01) and the accelerated onset of the levodopa-induced response modifications (P<0.01). However, in rats that received levodopa treatment for 21 days without the gene transfer, intrastriatal NPC-15437 had no effect on the response shortening or on GluR1 S831 phosphorylation. The results suggest that an increase in PKC-mediated signaling, including, in part, phosphorylation of AMPA receptors, on striatal spiny neurons may be sufficient to promote the initial appearance, but not necessary the ultimate expression, of the levodopa-induced motor response changes occurring in a rodent model of the human motor complication syndrome.

Molecular basis of opioid dependence: role of signal regulation by G-proteins

1. Morphine and opiate narcotics are potent analgesics that have a high propensity to induce tolerance and physical dependence following their repeated administration. 2. The molecular basis of opiate dependence has not been completely elucidated, although the participation of opioid receptors is a prerequisite. Cellular dependence on opioids is believed to result from the chronic stimulation of opioid-regulated signalling networks. 3. As G-protein-coupled receptors, the opioid receptors must rely on heterotrimeric G-proteins for signal transduction. Recent advances in our understanding of G-protein signalling have unveiled novel signalling molecules and mechanisms, some of which may be intricately involved in the manifestation of opiate dependence. 4. In the present review, we will attempt to trace chronic opioid signals along elaborate G-protein-regulated pathways.

Molecular profiling of midbrain dopamine regions in cocaine overdose victims

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in neuronal function in brain regions associated with drug reinforcement. To evaluate whether the alterations in gene expression in cocaine overdose victims are associated with specific dopamine populations in the midbrain, cDNA arrays and western blotting were used to compare gene and protein expression patterns between cocaine overdose victims and age-matched controls in the ventral tegmental area (VTA) and lateral substantia nigra (l-SN). Array analysis revealed significant up-regulation of numerous transcripts in the VTA, but not in the l-SN, of cocaine overdose victims including NMDAR1, GluR2, GluR5 and KA2 receptor mRNA (p < 0.05). No significant alterations between overdose victims and controls were observed for GluR1, R3 or R4 mRNA levels. Correspondingly, western blot analysis revealed VTA-selective up-regulation of CREB (p < 0.01), NMDAR1 (p < 0.01), GluR2 (p < 0.05), GluR5 (p < 0.01) and KA2 (p < 0.05) protein levels of cocaine overdose victims. The present results indicate that selective alterations of CREB and certain ionotropic glutamate receptor (iGluR) subtypes appear to be associated with chronic cocaine use in humans in a region-specific manner. Moreover, as subunit composition determines the functional properties of iGluRs, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine in humans.

Vocalizations during withdrawal from opiates and cocaine: possible expressions of affective distress

Intense anxiety has been postulated to trigger relapse to abuse of opiates and psychomotor stimulants. Preclinical research methodologies need to be developed to adequately characterize the affective or emotional component of withdrawal. Classically, withdrawal from psychomotor stimulants and opiates focuses on somatic and autonomic indices, foremost based on observational assessments and, additionally, on measures of disrupted conditioned behavior. These measures depict the intensity and time course of withdrawal from specific doses of opiates and psychomotor stimulants, but require large numbers of subjects due to single use of each individual. Behavioral disruptions have been attributed to anhedonia, a core symptom of drug withdrawal, as well as major depressive and psychotic disorders. In spite of some pharmacological validation, inferences about anxiety-like disturbances, based on observed somatic and autonomic signs or on changes in conditioned responses, have to remain tentative. High-pitched vocalizations may communicate affective expressions and, in rodents, different kinds of ultrasonic vocalizations communicate maternal separation distress in infants, accompany the intensely arousing phases of agonistic confrontations, signal submission and distress in defensive responses to threats and painful events, and are part of the excitatory and inhibitory phases of sexual behavior. While acute treatment with opiates, psychomotor stimulants, alcohol and benzodiazepines suppresses ultrasonic vocalizations in the 22-25-kHz range, rats emit high rates of ultrasonic vocalizations upon withdrawal from prolonged exposure to these drugs, particularly if they have been startled. Peak rates of ultrasonic distress calls occur ca. 1-3 days after cessation of cocaine or opiate treatment and decline within 5-7 days. Ultrasonic vocalizations during withdrawal from cocaine, alcohol or benzodiazepines can be attenuated by renewed access to the drug. It will be informative to learn how the neural circuit mediating vocalizations interacts with the ones subserving self-administration of alcohol, opiates and psychomotor stimulants.

Striatal cell type-specific overexpression of DeltaFosB enhances incentive for cocaine

The transcription factor DeltaFosB accumulates in substance P-dynorphin-containing striatal neurons with repeated cocaine use. Here, we show that inducible transgenic DeltaFosB overexpression in this same striatal cell type facilitates acquisition of cocaine self-administration at low-threshold doses, consistent with increased sensitivity to the pharmacological effects of the drug. Importantly, DeltaFosB also enhances the degree of effort mice will exert to maintain self-administration of higher doses on a progressive ratio schedule of reinforcement, whereas levels of cocaine intake are not altered on less demanding fixed-ratio schedules. Acquisition and extinction of behavior reinforced by food pellets is not altered in DeltaFosB-overexpressing mice, indicating that DeltaFosB does not alter the capacity to learn an instrumental response or cause response perseveration in the absence of reinforcement. These data suggest that accumulation of DeltaFosB contributes to drug addiction by increasing the incentive properties of cocaine, an effect that could increase the risk for relapse long after cocaine use ceases.

Regulation of CRE-mediated transcription in mouse brain by amphetamine

Previous work has demonstrated that acute and chronic administration of amphetamine causes phosphorylation of the transcription factor CREB, the cAMP response element (CRE) binding protein, in striatum, a brain region important for the behavioral actions of the drug. To determine whether such phosphorylation is associated with changes in CREB transcriptional activity, we mapped beta-galactosidase (beta-gal) expression in a CRE-LacZ transgenic mouse, in which the beta-gal reporter is downstream of CRE sequences, following acute or chronic amphetamine administration. We found that acute amphetamine induced beta-gal expression in a relatively small number of brain regions, including nucleus accumbens (ventral striatum), amygdala, ventral tegmental area, and locus coeruleus. Chronic amphetamine generally produced greater changes in CRE-mediated transcription in most brain regions and induced CRE-transcription in several regions unaffected by acute drug exposure. Interestingly, amphetamine regulation of CRE activity differed dramatically between males and females. In nucleus accumbens, beta-gal expression colocalized predominantly with dynorphinergic neurons after acute amphetamine administration, while chronic administration induced beta-gal expression in both dynorphinergic and enkephalinergic neurons. In ventral tegmental area, acute and chronic amphetamine induced beta-gal expression mainly in dopaminergic neurons, while induction in the locus coeruleus occurred mainly in nonnoradrenergic neurons. This study identifies several brain regions where CRE-mediated transcription may play a key role in the development of neuronal plasticity associated with amphetamine administration.

Intra-accumbens pertussis toxin sensitizes rats to the locomotor activating effects of a single cocaine challenge

Drugs of abuse share common neurochemical signaling substrates, many of which are components of the cAMP cascade. Interestingly, a number of these substrates have been linked to drug-influenced behaviors. This study sought to understand the role of one signaling substrate, inhibitory G-proteins, in a drug-induced phenomenon known as behavioral sensitization. Specifically, we used pertussis toxin (PTX) as a tool to investigate the relationship between cocaine-induced alterations in cAMP signaling and behavior. Vehicle (1 micro l/side) or PTX (0.15 or 0.25 micro g/1 micro l/side) was bilaterally infused into the nucleus accumbens of rats. Locomotor activity was assessed on days 7, 14 and 21 post-infusion. Intra-accumbal PTX produced a dose-dependent increase in locomotor activity. On day 21 following behavioral monitoring for 1 h, rats were acutely challenged with cocaine (15 mg/kg, i.p.) and behavioral data were accumulated for an additional 2 h. Intra-accumbal PTX sensitized rats to the locomotor-activating effects of a single cocaine challenge which was dose-dependent. After behavioral testing, brains were removed and processed for in vitro receptor autoradiography using the D(1) receptor ligand [3H] SCH 23390. No changes in D(1) dopamine receptor binding were observed. These findings suggest a role for inhibitory proteins (G(i)/G(o)) within the nucleus acumbens in locomotor activity and also cocaine-induced behavioral sensitization.

Role of G(i)alpha2-protein in opioid tolerance and mu-opioid receptor downregulation in vivo

Although opioid receptors are G-protein coupled, the role that specific G-protein subunits play in the development of opioid tolerance and the regulation of opioid receptor number is not well understood. In the present study, we used a G((i)alpha2) antisense oligodeoxynucleotide (ODN) to examine the contribution of G((i)alpha2) proteins to mu-opioid tolerance and receptor downregulation in the mouse. Mice were injected intracerebroventricularly (ICV) and into the spinal intrathecal space (IT) for 4-5 consecutive days (30 microg/site/day), with an antisense ODN or a mismatch ODN directed at mRNA for the G((i)alpha2) subunit of G-proteins. Controls were treated with dH(2)O. On the second day of ODN treatment continuous subcutaneous (SC) infusion of etorphine (200 microg/kg/day) or morphine (40 mg/kg/day + 25 mg pellet) was begun. Control mice were implanted with inert placebo pellets. Three days later, pumps and pellets were removed and mice were tested for morphine analgesia or mu-opioid receptor density was determined in whole brain. Etorphine produced significant tolerance (ED(50) shift = approximately 11-fold) and downregulation of mu-opioid receptors (approximately 25%). Morphine treatment produced significant tolerance (ED(50) shift approximately 9-fold), but no mu-opioid receptor downregulation. Antisense treatment reduced G((i)alpha2) protein levels in striatum and spinal cord by approximately 25%. G((i)alpha2) antisense reduced the acute potency of morphine. G((i)alpha2) antisense blocked the development of tolerance to morphine treatment and reduced the development of tolerance to etorphine treatment. Antisense did not have any effect on etorphine-induced mu-opioid receptor downregulation. In another experiment, 7-day treatment with morphine or etorphine similarly increased G((i)alpha2) mRNA and protein abundance in spinal cord. Overall, these results support an important role for G((i)alpha2)-protein in the acute effects of opioids and opioid tolerance. However, G((i)alpha2) is not required for agonist-induced mu-opioid receptor density regulation in vivo.

Enhanced morphine preference following prolonged abstinence: association with increased Fos expression in the extended amygdala

We previously found that chronically morphine-pretreated, abstinent rats show stronger preferences for morphine-associated environments than placebo-pretreated rats. Here we show that this increased preference persisted for at least 5 weeks after withdrawal of chronic morphine. To determine brain regions involved in this behavior, we examined neural activation (as indexed by Fos-like proteins) induced by a morphine-conditioned place preference test. Placebo-pretreated (P) morphine-conditioned rats showed significantly elevated Fos in the anterior cingulate cortex (Cg), nucleus accumbens core (Ac-C) and shell (Ac-S), ventral lateral and dorsal lateral bed nucleus of the stria terminialis (BNST-VL and -DL), and central and basolateral amygdala nuclei (ACE, ABL) when compared to nonconditioned P rats. Chronically morphine-pretreated (M) rats that exhibited enhanced morphine preference 5 weeks after morphine withdrawal showed significantly greater Fos in all the same areas except the BNST-DL relative to conditioned P or nonconditioned M rats. Place preference measures and Fos expression were positively correlated in the Cg and ABL, for conditioned P animals, and in the Cg, ABL and BNST-VL for conditioned M animals. These results indicate a relationship between place preference behavior and neural indices of activation in the forebrain in response to morphine-conditioned cues that may be chronically modulated by prior morphine exposure.

Changes in PKA activity and Gs? and Golf? levels after amphetamine- and cocaine-induced behavioral sensitization

Neuroadaptations in the cAMP signal transduction system have been proposed to play a critical role in psychostimulant-induced behavioral sensitization. Studies relevant to this hypothesis have provided conflicting evidence, however, because repeated cocaine and amphetamine treatment has been alternately reported to either increase or decrease protein kinase A (PKA) activity in the nucleus accumbens and dorsal striatum. In an attempt to reconcile these disparate findings, PKA activity and the stimulatory G-protein subunits, G(salpha) and G(olfalpha), were measured after rats had received a sensitization-inducing regimen of amphetamine or cocaine. Results showed that seven consecutive daily injections of amphetamine or cocaine produced locomotor sensitization and caused a reduction in accumbal PKA activity. A challenge injection of amphetamine on the test day also reduced dorsal striatal PKA activity. G(salpha) levels were unaffected by psychostimulant exposure, while G(olfalpha) levels in the nucleus accumbens declined significantly after amphetamine or cocaine pretreatment. Because repeated amphetamine and cocaine treatment both induced behavioral sensitization and decreased accumbal PKA activity, it appears that psychostimulant-induced increases in PKA activity are not necessary for the ultimate expression of behavioral sensitization.

Cocaine treatment increases expression of a 40 kDa catecholamine-regulated protein in discrete brain regions

Previous reports from our laboratory have described brain-specific catecholamine-regulated proteins, which bind dopamine and related catecholamines. Evidence from the molecular cloning of a 40 kDa catecholamine-regulated protein (CRP40) revealed that CRP40 is dopamine-inducible and has properties similar to those of the 70 kDa heat shock protein (HSP70) family. The present study investigates the effects of acute and chronic cocaine treatment on CRP40 expression in the striatum, nucleus accumbens, prefrontal cortex, and medulla. Acute treatment with cocaine increased CRP40 expression in the nucleus accumbens and striatum, whereas chronic treatment with cocaine increased CRP40 expression in the nucleus accumbens only. Neither of these treatments affected CRP40 levels in the prefrontal cortex or medulla. In addition, pretreatment with the spin-trapping agent alpha-phenyl-tert-butylnitrone did not attenuate cocaine-induced expression of CRP40, suggesting that the observed increases in CRP40 levels were not caused by free radicals. On the other hand, pretreatment with anisomycin, a protein synthesis inhibitor, blocked the cocaine-induced expression of CRP40. Thus, protein synthesis may be involved in the observed CRP40 level increases. Furthermore, neither acute nor chronic cocaine treatment affected levels of inducible or constitutively expressed HSP70, which indicates a specificity of cocaine's effects on CRP40. Since cocaine has been shown to increase extracellular dopamine levels, these findings suggest that increased expression of CRP40 is associated with high extracellular levels of dopamine (or its metabolites). Elevated levels of CRP40 could play a protective role for dopamine neurons in response to increased oxidative stress that has been shown to be induced by cocaine and that can lead to apoptosis and neurodegeneration.

Cocaine alters mu but not delta or kappa opioid receptor-stimulated in situ [35S]GTPgammaS binding in rat brain

Chronic cocaine administration produces alterations in mu and kappa opioid receptor density as well as striatal and accumbens opioid-regulated adenylyl cyclase activity, suggesting a psychostimulant responsive interaction between opioidergic and dopaminergic systems. Stimulation of G-protein-coupled opioid receptors inhibits adenylyl cyclase production of cyclic AMP. The present study employed in situ [(35)S]GTPgammaS binding to measure opioid receptor-stimulated activation of G-proteins in response to acute and chronic cocaine exposure. Male Fischer rats received acute (1 or 3 days) or chronic (14 days) binge pattern cocaine administration. Three and 14 days of cocaine injections resulted in greater increases in the ability of the mu receptor agonist DAMGO to stimulate [(35)S]GTPgammaS binding in both the core and the shell of the nucleus accumbens, all regions of the caudate putamen and the cingulate cortex compared with saline-matched controls. The greatest increases in DAMGO-stimulated [(35)S]GTPgammaS binding were observed in the dorsal areas of the caudate putamen in animals that received 14 days of cocaine. No significant changes in delta (DPDPE), or kappa (dynorphin A(1-17)) receptor-stimulated [(35)S]GTPgammaS binding were found in any brain region in response to cocaine administration. These results demonstrate that binge pattern cocaine administration induce changes in mu but not delta or kappa opioid receptor-mediated G-protein activity. This study provides support for the hypothesis that the addictive properties of both psychostimulants and opiates may share common neurochemical signaling substrates.

Melatonin alters behavior and cAMP levels in nucleus accumbens induced by cocaine treatment

This study describes the effects of melatonin on cocaine-induced anxiety-like behavior and nucleus accumbens (NAc) cAMP levels in rats. Animals drinking a solution of melatonin (200 ng/ml) at night, either during repeated cocaine administration (15 mg/kg i.p., twice a day for 9 days) or during its withdrawal, showed less anxiety-like behavior in a defensive withdrawal paradigm 48 h after the last injection of cocaine. Melatonin did not alter behavior in control rats treated with saline. Animals exposed for 1 week to unrestricted free-choice oral melatonin self-administration (200 ng/ml) did not show preference for the drinking solution containing melatonin. Pretreatment with melatonin (200 ng/kg i.p. or 200 ng/ml orally) significantly attenuated the augmentation of cAMP levels in NAc following acute cocaine administration (15 mg/kg i.p.). Taken together, these results suggest that a low-dose night-time melatonin treatment results in anxiolytic-like effects in rats withdrawn from repeated cocaine administration, can antagonize cocaine-induced activation of NAc cAMP levels and has low dependence liability.

Locomotor sensitization to cocaine is associated with increased Fos expression in the accumbens, but not in the caudate

Behavioral sensitization following repeated intermittent cocaine administrations is thought to involve alterations in cocaine regulation of neural activity within the accumbens and caudate brain regions. Although Fos immunohistochemistry and c-fos in situ hybridization have frequently been used to assess changes in cocaine-induced neural activity following prior cocaine exposure, these techniques have rarely been used to examine neural activity in the accumbens of behaviorally sensitized animals. In the present experiment, we compared the ability of increasing doses of cocaine to induce Fos in the accumbens and caudate of rats following a treatment procedure (7 once daily injections of 15 mg/kg of cocaine or the saline vehicle) shown to produce robust and persistent (1 week) locomotor sensitization. In sensitized animals, there was a leftward shift in the dose-response curve for cocaine induction of Fos in the accumbens, but not in the caudate. These results provide the first parametric evidence for sensitization of cocaine-induced Fos expression in the accumbens.

Loss of antinociceptive efficacy in rat pups infused with morphine from osmotic minipumps

We determined the susceptibility of two ages of rat pups to become tolerant to and dependent on morphine infusions from osmotic minipumps. Neonatal rats (postnatal day 6; P6) were infused for 72 h with morphine at 0.175 or 0.7 mg/kg/h. On P9, morphine's antinociceptive efficacy was reduced in both groups. P14 infant rats were also infused at 0.7 mg/kg/h. Unlike P9 neonates, morphine's potency was reduced in P17 infant rats, without a loss in efficacy. Yet raising the infusion rate to 1.1 mg/kg/h reduced morphine's efficacy. (3)H-DAMGO D-Ala2, N-MePhe4, Gly5-ol-enkephalin) binding revealed no change in the affinity or density of mu-opioid receptors at any age or in any treatment group. P9 and P17 pups were physically dependent on each infusion dose. Thus, chronic infusion of morphine affected both ages to such an extent that acutely administered morphine doses of even 1,000 mg/kg failed to restore antinociception. However, this effect cannot be attributed to changes in mu-opioid receptor number or affinity.

CREB activity in the nucleus accumbens shell controls gating of behavioral responses to emotional stimuli

The transcription factor cAMP response element (CRE)-binding protein (CREB) has been shown to regulate neural plasticity. Drugs of abuse activate CREB in the nucleus accumbens, an important part of the brain's reward pathways, and local manipulations of CREB activity have been shown to affect cocaine reward, suggesting an active role of CREB in adaptive processes that follow exposure to drugs of abuse. Using CRE-LacZ reporter mice, we show that not only rewarding stimuli such as morphine, but also aversive stimuli such as stress, activate CRE-mediated transcription in the nucleus accumbens shell. Using viral-mediated gene transfer to locally alter the activity of CREB, we show that this manipulation affects morphine reward, as well as the preference for sucrose, a more natural reward. We then show that local changes in CREB activity induce a more general syndrome, by altering reactions to anxiogenic, aversive, and nociceptive stimuli as well. Increased CREB activity in the nucleus accumbens shell decreases an animal's responses to each of these stimuli, whereas decreased CREB activity induces an opposite phenotype. These results show that environmental stimuli regulate CRE-mediated transcription within the nucleus accumbens shell, and that changes in CREB activity within this brain area subsequently alter gating between emotional stimuli and their behavioral responses. This control appears to be independent of the intrinsic appetitive or aversive value of the stimulus. The potential relevance of these data to addiction and mood disorders is discussed.

Stimulation of protein kinase a activity in the rat amygdala enhances reward-related learning

BACKGROUND

Drug addiction in humans is associated with abnormal metabolic activity within the amygdala and heightened control of behavior by drugs and drug-related (conditioned) stimuli. Drug-induced neuroadaptations, including activation of cAMP (cyclic adenosine monophosphate)-dependent protein kinase A (PKA), within the amygdala may contribute to the synaptic plasticity and reward-related learning that underlies pathologic behavior in addicted individuals.

METHODS

In this study, we tested the hypothesis that stimulation of PKA activity within the rat amygdala would facilitate the acquisition of Pavlovian approach behavior, a measure of reward-related learning.

RESULTS

Intraamygdala infusions of Sp-cAMPS (which activates PKA) produced concentration-dependent enhancements of the acquisition of approach to a conditioned stimulus that predicted water availability; intraamygdala infusions of cholera toxin (which elevates cAMP levels) produced a similar effect. Conversely, intraamygdala infusions of Rp-cAMPS, an inhibitor of PKA, impaired acquisition of approach behavior.

CONCLUSIONS

Together, these data demonstrate that stimulation of PKA activity in the amygdala can facilitate reward-related learning and suggest that neuroadaptative changes in the PKA pathway within this brain region may be a mechanism by which chronic drug abuse alters the control of behavior by drug-associated stimuli.

Differential modulation of nucleus accumbens synapses

The nucleus accumbens (NAcc) is a brain region involved in functions ranging from motivation and reward to feeding and drug addiction. The NAcc is typically divided into two major subdivisions, the shell and the core. The primary output neurons of both of these areas are medium spiny neurons (MSNs), which are quiescent at rest and depend on the relative input of excitatory and inhibitory synapses to determine when they fire action potentials. These synaptic inputs are, in turn, regulated by a number of neurochemical signaling agents that can ultimately influence information processing in the NAcc. The present study characterized the ability of three major signaling pathways to modulate synaptic transmission in NAcc MSNs and compared this modulation across different synapses within the NAcc. The opioid [Met](5)enkephalin (ME) inhibited excitatory postsynaptic currents (EPSCs) in shell MSNs, an effect mediated primarily by micro-opioid receptors. Forskolin, an activator of adenylyl cyclase, potentiated shell EPSCs. An analysis of miniature EPSCs indicated a primarily presynaptic site of action, although a smaller postsynaptic effect may have also contributed to the potentiation. Adenosine and an adenosine A(1)-receptor agonist inhibited shell EPSCs, although no significant tonic inhibition by endogenous adenosine was detected. The effects of these signaling agents were then compared across four different synapses in the NAcc: glutamatergic EPSCs and GABAergic inhibitory postsynaptic currents (IPSCs) in both the core and shell subregions. ME inhibited all four of these synapses but produced a significantly greater inhibition of shell IPSCs than the other synapses. Forskolin produced an increase in transmission at each of the synapses tested. However, analysis of miniature IPSCs in the shell showed no sign of a postsynaptic contribution to this potentiation, in contrast to the shell miniature EPSCs. Tonic inhibition of synaptic currents by endogenous adenosine, which was not observed in shell EPSCs, was clearly present at the other three synapses tested. These results indicate that neuromodulation can vary between the different subregions of the NAcc and between the different synapses within each subregion. This may reflect differences in neuronal interconnections and functional roles between subregions and may contribute to the effects of drugs acting on these systems.

Inducible and brain region-specific CREB transgenic mice

To investigate the role of cAMP response element-binding protein (CREB) in the adaptive responses to psychotropic drugs, we have developed inducible, brain region-specific CREB transgenic mice using the tetracycline-regulated gene expression system. The tetracycline transactivator (tTA) was placed under the control of 1.8-kilobase neuron-specific enolase (NSE) promoter for this purpose. Different patterns of CREB overexpression were found in striatum, nucleus accumbens, and cingulate cortex in different lines of bitransgenic mice, and CREB expression was blocked by addition of doxycycline, an analog of tetracycline. Overexpression of CREB influenced the expression of other members of the CREB/ATF family of transcription factors, consistent with previous reports. In addition, psychostimulant induction of dynorphin, a neuropeptide regulated by drugs of abuse, was up-regulated in striatum. Finally, there was a significant reduction in cocaine-induced locomotor activity in the CREB bitransgenic mice. These results are consistent with a role for CREB in mediating adaptive changes that occur in response to drugs of abuse.