Adolescents are more vulnerable to cocaine addiction: behavioral and electrophysiological evidence.

In humans, adolescence is a period of heightened propensity to develop cocaine addiction. It is unknown whether this is attributable to greater access and exposure to cocaine at this age, or whether the adolescent brain is particularly vulnerable to the addictive properties of cocaine. Here, we subjected male adolescent (P42) and adult (∼P88) rats to a wide range of cocaine self-administration procedures. In addition, to determine whether behavioral differences are associated with developmental differences in dopaminergic activity, we examined and manipulated the activity of dopamine neurons. Relative to adults, adolescent rats took cocaine more readily, were more sensitive to lower doses, showed greater escalation of cocaine intake, and were less susceptible to increases in price (i.e., were more "inelastic"). In parallel, adolescents also showed elevated activity of ventral tegmental area dopamine neurons, a feature known to be associated with increased self-administration behavior. Pharmacological manipulation of dopamine D2 receptor function with quinpirole (agonist) or eticlopride (antagonist), to alter dopamine neuron activity, eliminated age differences in cocaine self-administration. These data suggest a causal relationship between behavioral and electrophysiological determinants of cocaine addiction liability. In conclusion, adolescents show behavioral and electrophysiological traits of heightened addiction liability.

Different roles of BDNF in nucleus accumbens core versus shell during the incubation of cue-induced cocaine craving and its long-term maintenance.

Brain-derived neurotrophic factor (BDNF) contributes to diverse types of plasticity, including cocaine addiction. We investigated the role of BDNF in the rat nucleus accumbens (NAc) in the incubation of cocaine craving over 3 months of withdrawal from extended access cocaine self-administration. First, we confirmed by immunoblotting that BDNF levels are elevated after this cocaine regimen on withdrawal day 45 (WD45) and showed that BDNF mRNA levels are not altered. Next, we explored the time course of elevated BDNF expression using immunohistochemistry. Elevation of BDNF in the NAc core was detected on WD45 and further increased on WD90, whereas elevation in shell was not detected until WD90. Surface expression of activated tropomyosin receptor kinase B (TrkB) was also enhanced on WD90. Next, we used viral vectors to attenuate BDNF-TrkB signaling. Virus injection into the NAc core enhanced cue-induced cocaine seeking on WD1 compared with controls, whereas no effect was observed on WD30 or WD90. Attenuating BDNF-TrkB signaling in shell did not affect cocaine seeking on WD1 or WD45 but significantly decreased cocaine seeking on WD90. These results suggest that basal levels of BDNF transmission in the NAc core exert a suppressive effect on cocaine seeking in early withdrawal (WD1), whereas the late elevation of BDNF protein in NAc shell contributes to incubation in late withdrawal (WD90). Finally, BDNF protein levels in the NAc were significantly increased after ampakine treatment, supporting the novel hypothesis that the gradual increase of BDNF levels in NAc accompanying incubation could be caused by increased AMPAR transmission during withdrawal.

Group I mGluR activation reverses cocaine-induced accumulation of calcium-permeable AMPA receptors in nucleus accumbens synapses via a protein kinase C-dependent mechanism.

Following prolonged withdrawal from extended access cocaine self-administration in adult rats, high conductance Ca2+ -ermeable AMPA receptors (CP-AMPARs) accumulate in nucleus accumbens (NAc) synapses and mediate the expression of "incubated" cue-induced cocaine craving. Using patch-clamp recordings from NAc slices prepared after extended access cocaine self-administration and >45 d of withdrawal, we found that group I metabotropic glutamate receptor (mGluR) stimulation using 3,5-dihydroxyphenylglycine (DHPG; 50 µm) rapidly eliminates the postsynaptic CP-AMPAR contribution to NAc synaptic transmission. This is accompanied by facilitation of Ca2+ -impermeable AMPAR (CI-AMPAR)-mediated transmission, suggesting that DHPG may promote an exchange between CP-AMPARs and CI-AMPARs. In saline controls, DHPG also reduced excitatory transmission but this occurred through a CB1 receptor-dependent presynaptic mechanism rather than an effect on postsynaptic AMPARs. Blockade of CB1 receptors had no significant effect on the alterations in AMPAR transmission produced by DHPG in the cocaine group. Interestingly, the effect of DHPG in the cocaine group was mediated by mGluR1 whereas its effect in the saline group was mediated by mGluR5. These results indicate that regulation of synaptic transmission in the NAc is profoundly altered after extended access cocaine self-administration and prolonged withdrawal. Furthermore, they suggest that activation of mGluR1 may represent a potential strategy for reducing cue-induced cocaine craving in abstinent cocaine addicts.

Dopamine neurons in the ventral tegmental area fire faster in adolescent rats than in adults.

Adolescence may be a period of vulnerability to drug addiction. In rats, elevated firing activity of ventral tegmental area (VTA) dopamine neurons predicts enhanced addiction liability. Our aim was to determine if dopamine neurons are more active in adolescents than in adults and to examine mechanisms underlying any age-related difference. VTA dopamine neurons fired faster in adolescents than in adults as measured with in vivo extracellular recordings. Dopamine neuron firing can be divided into nonbursting (single spikes) and bursting activity (clusters of high-frequency spikes). Nonbursting activity was higher in adolescents compared with adults. Frequency of burst events did not differ between ages, but bursts were longer in adolescents than in adults. Elevated dopamine neuron firing in adolescent rats was also observed in cell-attached recordings in ex vivo brain slices. Using whole cell recordings, we found that passive and active membrane properties were similar across ages. Hyperpolarization-activated cation currents and small-conductance calcium-activated potassium channel currents were also comparable across ages. We found no difference in dopamine D2-class autoreceptor function across ages, although the high baseline firing in adolescents resulted in autoreceptor activation being less effective at silencing neurons. Finally, AMPA receptor-mediated spontaneous excitatory postsynaptic currents occurred at lower frequency in adolescents; GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents occurred at both lower frequency and smaller amplitude in adolescents. In conclusion, VTA dopamine neurons fire faster in adolescence, potentially because GABA tone increases as rats reach adulthood. This elevation of firing rate during adolescence is consistent with it representing a vulnerable period for developing drug addiction.

Repeated cocaine exposure decreases dopamine D2-like receptor modulation of Ca(2+) homeostasis in rat nucleus accumbens neurons.

The nucleus accumbens (NAc) is a limbic structure in the forebrain that plays a critical role in cognitive function and addiction. Dopamine modulates activity of medium spiny neurons (MSNs) in the NAc. Both dopamine D1-like and D2-like receptors (including D1R or D(1,5)R and D2R or D(2,3,4)R, respectively) are thought to play critical roles in cocaine addiction. Our previous studies demonstrated that repeated cocaine exposure (which alters dopamine transmission) decreases excitability of NAc MSNs in cocaine-sensitized, withdrawn rats. This decrease is characterized by a reduction in voltage-sensitive Na(+) currents and high voltage-activated Ca(2+) currents, along with increased voltage-gated K(+) currents. These changes are associated with enhanced activity in the D1R/cAMP/PKA/protein phosphatase 1 pathway and diminished calcineurin function. Although D1R-mediated signaling is enhanced by repeated cocaine exposure, little is known whether and how the D2R is implicated in the cocaine-induced NAc dysfunction. Here, we performed a combined electrophysiological, biochemical, and neuroimaging study that reveals the cocaine-induced dysregulation of Ca(2+) homeostasis with involvement of D2R. Our novel findings reveal that D2R stimulation reduced Ca(2+) influx preferentially via the L-type Ca(2+) channels and evoked intracellular Ca(2+) release, likely via inhibiting the cAMP/PKA cascade, in the NAc MSNs of drug-free rats. However, repeated cocaine exposure abolished the D2R effects on modulating Ca(2+) homeostasis with enhanced PKA activity and led to a decrease in whole-cell Ca(2+) influx. These adaptations, which persisted for 21 days during cocaine abstinence, may contribute to the mechanism of cocaine withdrawal.

Behavioral sensitization to amphetamine is not accompanied by changes in glutamate receptor surface expression in the rat nucleus accumbens.

We examined whether behavioral sensitization to amphetamine is associated with redistribution of glutamate receptors (GluR) in the rat nucleus accumbens (NAc) or dorsolateral striatum (DLSTR). Following repeated amphetamine treatment and 21 days of withdrawal, surface and intracellular levels of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) or NMDA receptor subunits were determined using a protein cross-linking assay. In contrast to our previous results in cocaine-sensitized rats, we did not observe redistribution of GluR1 or GluR2 to the cell surface in the NAc after amphetamine withdrawal, although a small increase in total GluR1 was found in the shell subregion. Nor did we observe activation of signaling pathways associated with cocaine-induced AMPA receptor trafficking or changes in NMDA receptor subunits. No significant changes were observed in the DLSTR. We also investigated the effect of administering a challenge injection of amphetamine to amphetamine-sensitized rats 24 h prior to biochemical analysis based on prior studies showing that cocaine challenge decreases AMPA receptor surface expression in the NAc of cocaine-sensitized rats. GluR1 and GluR2 were not significantly altered in either NAc or DLSTR, although a modest effect on GluR3 cannot be ruled out. Our results suggest that glutamate transmission in the NAc is dramatically different in rats sensitized to amphetamine versus cocaine.

Plasticity of L-type Ca2+ channels after cocaine withdrawal.

Increased reactivity of certain frontal cortical brain regions to cocaine re-exposure or drug-associated cues in cocaine-abstinent human addicts is linked to drug craving. Similarly, in rats tested after withdrawal from repeated cocaine exposure, cocaine or other strong excitatory stimuli produce greater activation of pyramidal neurons in the medial prefrontal cortex (mPFC). Our recent findings indicate that the increased mPFC neuronal activation depends primarily upon enhanced voltage-sensitive Ca(2+) influx, most likely through high-voltage activated (HVA) L-type Ca(2+) channels, but the mechanism underlying the enhanced Ca(2+) currents is unknown. In this study, we used a protein crosslinking assay to show that repeated cocaine injections, resulting in behavioral sensitization, increased total protein levels and cell surface expression of HVA-Ca(v)1.2 L-type channels in pyramidal neurons in deep layers of the mPFC. These changes in Ca(v)1.2 L-channels were time dependent and subtype specific (i.e., differed from those observed for Ca(v)1.3 L-channels). Furthermore, we found enhanced PKA activity in the mPFC of cocaine-sensitized rats that persisted for 21 days after withdrawal. PKA phosphorylation of L-channels increases their activity, so Ca(2+) currents after cocaine withdrawal could be enhanced as a result of both increased activity and number of HVA-Ca(v)1.2 L-channels on the cell surface. By increasing the suprafiring threshold excitability of mPFC pyramidal neurons, excessive upregulation of HVA L-channel activity and number may contribute to the cortical hyper-responsiveness that enhances vulnerability to cocaine craving and relapse. More generally, our results are the first to demonstrate that repeated cocaine exposure alters the membrane trafficking of a voltage-sensitive ion channel.

Estrogen restores cognition and cholinergic phenotype in an animal model of Down syndrome.

Estrogen maintains normal function of basal forebrain (BF) cholinergic neurons and estrogen replacement therapy (ERT) has therefore been proposed as a therapy for Alzheimer's disease (AD). We provide evidence to support this hypothesis in an animal model of Down syndrome (DS), a chromosome 16 segmental trisomy (Ts65Dn) mouse. These mice develop cholinergic degeneration similar to young adults with DS and AD patients. ERT has not been tested in women with DS, even though they are more likely than normosomic women to develop early menopause and AD. Female Ts65Dn and normosomic mice (11-15 months) received a subcutaneous estrogen pellet or a sham operation. After 60 days, estrogen treatment improved learning of a T-maze task and normalized behavior in the Ts65Dn mice in reversal learning of the task, a measure of cognitive flexibility. Stereological evaluation of choline acetyltransferase (ChAT) immunopositive BF neurons showed that estrogen increased cell size and total number of cholinergic neurons in the medial septum of Ts65Dn mice. In addition, estrogen increased NGF protein levels in the BF of trisomic mice. These findings support the emerging hypothesis that estrogen may play a protective role during neurodegeneration and cognitive decline, particularly in cholinergic BF neuronal systems underlying cognition. The findings also indicate that estrogen may act, at least partially, via endogenous growth factors. Collectively, the data suggest that ERT may be a viable therapeutic approach for women with DS coupled with dementia.

Synaptic depression via mGluR1 positive allosteric modulation suppresses cue-induced cocaine craving.

Cue-induced cocaine craving is a major cause of relapse in abstinent addicts. In rats, cue-induced craving progressively intensifies (incubates) during withdrawal from extended-access cocaine self-administration. After ~1 month of withdrawal, incubated craving is mediated by Ca(2+)-permeable AMPA receptors (CP-AMPARs) that accumulate in the nucleus accumbens (NAc). We found that decreased mGluR1 surface expression in the NAc preceded and enabled CP-AMPAR accumulation. Thus, restoring mGluR1 transmission by administering repeated injections of an mGluR1 positive allosteric modulator (PAM) prevented CP-AMPAR accumulation and incubation, whereas blocking mGluR1 transmission at even earlier withdrawal times accelerated CP-AMPAR accumulation. In studies conducted after prolonged withdrawal, when CP-AMPAR levels and cue-induced craving are high, we found that systemic administration of an mGluR1 PAM attenuated the expression of incubated craving by reducing CP-AMPAR transmission in the NAc to control levels. These results suggest a strategy in which recovering addicts could use a systemically active compound to protect against cue-induced relapse.

Different adaptations in AMPA receptor transmission in the nucleus accumbens after short vs long access cocaine self-administration regimens.

Ca(2+)-permeable AMPA receptors (CP-AMPARs) accumulate in the nucleus accumbens (NAc) after ∼1 month of withdrawal from a long-access cocaine self-administration regimen (6 h/d, 10d). This is functionally significant because CP-AMPARs mediate the 'incubated' cue-induced cocaine craving produced by this regimen. Our present goal was to determine if other commonly employed cocaine self-administration regimens also elicit CP-AMPAR accumulation. We compared four regimens, named according to whether sessions were short-access (ShA, 2 h) or long-access (LgA, 6 h) and the total number of sessions: LgA/10d (already shown to elicit CP-AMPAR accumulation), ShA/11d, ShA/20-24d, and LgA/20-24d. In the latter regimens, rats began with 10 days of ShA and then entered a differential phase (10-14 days) in which ShA sessions either continued or switched to LgA. Controls self-administered saline. After >40 days of withdrawal, whole-cell patch-clamp recordings were performed in NAc core medium spiny neurons to assess the contribution of CP-AMPAR transmission, based on the magnitude of synaptic suppression elicited by bath application of the selective CP-AMPAR antagonist naspm (100 µM). Naspm produced a non-significant (∼10%) attenuation of electrically evoked local excitatory postsynaptic current in the saline and ShA groups. By contrast, a significant naspm-induced synaptic attenuation (25-30%) was observed in both the LgA groups. Further analyses indicate that this emergence of CP-AMPAR transmission in the LgA groups is associated with increased baseline responsiveness of MSN to excitatory drive. Together with data on cocaine infusions in each group, our results show that CP-AMPAR accumulation and enhanced glutamate transmission is associated with longer sessions (6 h), rather than the number of sessions or cocaine infusions.

Alterations in AMPA receptor subunits and TARPs in the rat nucleus accumbens related to the formation of Ca²âº-permeable AMPA receptors during the incubation of cocaine craving.

Cue-induced cocaine seeking intensifies or incubates after withdrawal from extended access cocaine self-administration, a phenomenon termed incubation of cocaine craving. The expression of incubated craving is mediated by Ca²âº-permeable AMPA receptors (CP-AMPARs) in the nucleus accumbens (NAc). Thus, CP-AMPARs are a potential target for therapeutic intervention, making it important to understand mechanisms that govern their accumulation. Here we used subcellular fractionation and biotinylation of NAc tissue to examine the abundance and distribution of AMPAR subunits, and GluA1 phosphorylation, in the incubation model. We also studied two transmembrane AMPA receptor regulatory proteins (TARPs), γ-2 and γ-4. Our results, together with earlier findings, suggest that some of the new CP-AMPARs are synaptic. These are probably associated with γ-2, but they are loosely tethered to the PSD. Levels of GluA1 phosphorylated at serine 845 (pS845 GluA1) were significantly increased in biotinylated tissue and in an extrasynaptic membrane-enriched fraction. These results suggest that increased synaptic levels of CP-AMPARs may result in part from an increase in pS845 GluA1 in extrasynaptic membranes, given that S845 phosphorylation primes GluA1-containing AMPARs for synaptic insertion and extrasynaptic AMPARs supply the synapse. Some of the new extrasynaptic CP-AMPARs are likely associated with γ-4, rather than γ-2. The maintenance of CP-AMPARs in NAc synapses during withdrawal is accompanied by activation of CaMKII and ERK2 but not CaMKI. Overall, AMPAR plasticity in the incubation model shares some features with better described forms of synaptic plasticity, although the timing of the phenomenon and the persistence of related neuroadaptations are significantly different.

Persistent increases in cocaine-seeking behavior after acute exposure to cold swim stress.

BACKGROUND: Acute and chronic stress reinstates drug-seeking behavior. Current animal models show that these effects are contingent (temporally, contextually, or both) on the drug-conditioning environment. To date, no paradigm exists to model the common human situation in which stressors that are distinct from the experience of drugs can lead to relapse. METHODS: Rats were allowed to self-administer cocaine or saline over 8 days. They then underwent extinction training, during which responding was not reinforced with drug infusions. After 16 days of extinction, rats were submitted to a brief cold swim stress and then tested for seeking behavior (responding not reinforced with drug infusions) for 4 days. RESULTS: All rats developed self-administration behavior. Following extinction, cold swim stress induced reinstatement of drug-seeking behavior in cocaine-trained rats, an effect that was still present 3 days after stress exposure. CONCLUSIONS: This study indicates that cold swim stress can have long-term effects on drug-seeking behavior and may provide us with a suitable model to study the latent effects of stress on relapse to drug abuse.