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drug addiction

drug addiction

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drug addiction

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    1. Drug Addiction

    3. Too simple! No consistent effects on opioid receptors, transporters, etc. Try intracellular mechanisms: cAMP in locus coeruleus (LC) Moderate activity of LC ? arousal (via NE) High activity of LC ? anxiety Morphine initially decreases cAMP. Repeated morphine ? cAMP normalizes Opioid antagonist ? great increase in cAMP

    5. Locus coeruleus Similar effects on LC neuron firing Clonidine (a2 agonist at autoreceptors): Can be used as a non-opiate treatment of addiction Increased expression of isoforms of adenylyl cyclase (AC I, AC VIII) and PKA subunits (C-catalytic, RII-regulatory) in LC.

    7. Locus coeruleus Antisense to CREB, or viral vector-induced increases or decreases in CREB, alters electrophysiology of LC neurons. CREB-deficient mice: less physical dependence on opioids & less withdrawal.

    8. Does addiction entail physical dependence? DSM definition: Addiction = drug dependence. BUT: Cocaine doesnt ? physical dependence Two mechanisms for reward/motivation: All abused drugs increase DA in NAc. Some drugs also act on NAc via DA-indep. mechanisms Separate mechanism for physical depend.

    10. Role of DA DA neurons in flies and worms regulate responses to food and drugs. Abnormalities may ? compulsive gambling, eating, sex, depression. BUT: Drug addiction is not primarily a disorder of acute drug reward, but compulsive seeking and taking the drug. Kent Berridge: decreased liking, increased wanting KC Berridge, 2003, Pleasures of the brain, Brain & Cognition, 52:106-128. (See figure below)

    11. An analysis of facial expressions in the rat. H. Davis, S. Simmons, 1979

    13. Role of Dopamine Hyper-dopaminergic (DAT-knockdown) mice showed fewer liking responses to sucrose (fig. below). However, they ate more food, drank more water, ran faster for food in a runway, & gained more weight than wild-type. i.e., they showed more wanting. S. Pecina et al., 2003, J. Neurosci. 23:9395-9402.

    14. Hyper-dopaminergic mice show fewer liking responses to sucrose.

    15. Role of Dopamine Back to Nestler & Malenka (04) One possible mechanism of tolerance is increased dynorphin, which inhibits VTA cell firing and NAcc DA release via kappa opioid receptors. (next slide)

    17. Role of DA (cont.) Direct pathway: D1?cAMP?PKA?CREB CREB ? prodynorphin ? dynorphin ? kappa receptors ? inhibit DA release in NAc & inhibit DA cell firing in VTA (neg. feedback) Glutamate ? NMDA ? Ca2+ ? CaM K or MAPK ? CREB ? prodynorphin, etc. Possible drug treatment: kappa treat negative emotional effects of withdrawal.

    18. Role of ?FosB Repeated cocaine ? decreases in c-Fos and other Fos proteins BUT: INCREASED levels of activator protein 1 (AP-1) complexes (dimers of Fos+Jun), which are transcriptionally active. Due to increased levels of ?FosB (splice variant of FosB: long-lasting: weeks-months). Overexpression of ?FosB

    19. Role of ?FosB Overexpression of ?FosB in NAc & CPu: Increased locomotor & rewarding effects of cocaine and morphine (Nestler, 04). Increased responding on progressive ratio tests Increased wheel-running, sucrose intake. Dominant negative: opposite effects ?FosB = necessary and sufficient for sensitization to morphine and cocaine

    20. Role of ?FosB Experiment: cocaine for 4 da or 5 wks DNA microarray 5-week cocaine increased the number of genes activated by cocaine via ?FosB & decreased those activated via CREB

    21. Blue: increased by cocaine Red: mediated by ?FosB Yellow: mediated by CREB

    22. More permanent effects Dependence lasts more than weeks or months. Morphine ? decreased DA neuron size in VTA & decreased spine density of medium spiny neurons in NAc (main GABA output) Cocaine ? increased spine density on MSN in Nac (Note opp. effects of morph.) Therefore, temporary biochemical changes MAY ? morphology changes

    23. ~ Learning & Memory Both use BDNF & other neurotrophins Both use CREB Similar changes in dendritic spine density (But note differences btwn coc and morph!!) Similar changes in synaptic plasticity at glutamate synapses VTA ? NAc path and hipp/amyg/cortex are interconnected

    25. Role of Environment Home cage vs. novel cage administration Unilateral 6-OHDA lesion ? circling Intravenous catheter: no cues to onset of drug Low doses ? sensitization only in novel cage High doses ? sensitization in both cages Similar results with locomotor activity in rats w/o lesions (Badiani & Robinson, 2004)

    26. Role of Environment Home cage vs. novel cage (Badiani & Robinson, 2004) Drug discrimination: discrimination of low dose amphet. or cocaine only in novel cage BUT: inconsistent effects with morphine Novelty increased: Motor activity, sensitization Did not increase: Pain suppression, analgesic tolerance Small effect on stimulation of eating

    27. Role of Environment Not due only to conditioning a response to a cue in novel environment. (Badiani & Robinson, 04) Add cues to home cage?condition motor R to cues; but this does not affect amphet. sensitization. Stress response may contribute BUT: sensitization occurs even in adrenalec-tomized rats. HOWEVER: CRH in VTA, LC, hipp., or amyg. potentiate some effects.

    28. Role of Dopamine? No simple correlation between DA levels in NAc and locomotor increase by novelty. Home cage lo-dose amphet. (measure Fos): ? only direct pathway:D1, dynorphin, subst. P Novel cage lo-dose amphet.: Both direct & indirect (D2, enkephalin) paths May work by stress activation of cortical glutamate projections (via NMDA R). (Badiani & Robinson, 04)

    29. Structural changes Repeated low-dose cocaine increased spine density on MSN in NAc shell in both home and novel cages. (Home cage ? NO sensitization) Repeated low-dose cocaine increased spine density in NAc core only in novel cages (sensit.) Repeated high-dose cocaine increased spine density in shell & core of NAc. (sensitization) Therefore, only core is involved in physical changes that ? sensitization. Change in shell reflects only drug history, not sensitization. (Badiani & Robinson, 04)

    31. Effects of Stress Cocaine can ? anxiety via CRH release. May seem odd that CRH could mediate the reward, as well as anxiety. But, self-administration is a CONTROLLABLE stressor, which often has better effects than no stress at all. Analogous to thrill-seeking (Goeders, 2002)

    33. Effects of Stress 3 yoked animals: controllable shock + food (same lever), uncontrollable shock + noncontingent food, no shock but yoked food. Cocaine dose-response curve: inverted U Lower doses on ascending curve ? reward Uncontrollable shock ? increased sensitivity to low doses of cocaine All self-administered doses increased corticosterone above a certain level. Further increase above that level ? no further effect. (Goeders, 2002)

    34. Effects of Stress Exogenous corticosterone (no shock) mimicked effects of uncontrollable stress. Also sensitized NAc DA response to cocaine Adrenalectomy abolished acquisition of self-admin. of cocaine, but not of food. Also decreased DA receptor binding and transporter binding in SHELL of NAc. Neither stress nor corticosterone affected MAINTENANCE of self-administration (Goeders, 2002)

    35. Effects of Stress Reinstatement after extinction Inhibited by benzodiazepine, CRH antagonist, or glucocorticoid synthesis antagonist (ketoconazole) Humans: ketoconazole decreased anxiety, depression, and craving. 3 of 5 did continue to use cocaine and still got high 2 remained drug-free Note: this concerned maintenance, wh. Goeders said was more resistant to GC effects. (Goeders, 2002)

    36. E & Sex Differences Estrogen (E) rapidly increases DA release in NAc & CPu Test OVX females, OVX + E, CAST males, intact males (all w/ unilat. 6-OHDA) E or oil 30 before cocaine for 4 da Nothing for 3 da; continue pattern for 3 wks. Test for rotation behav (Hu & Becker, 2003)

    37. No group differences on Day 1

    38. OVX+E? greater sensitization to 20 mg/kg cocaine

    39. OVX+E? greater sensitization to 10 mg/kg cocaine

    40. OVX+E? greater sensitization to 5 mg/kg cocaine

    41. Challenge R to 10 mg/kg cocaine, w/o E (10 da after last doses)

    42. E & Sex Differences Summary No group difference on first day OVX+E sensitized more to each dose of cocaine. Even 10 da later, after no more E or cocaine, OVX females previously treated with E+cocaine showed greater remaining sensitization to cocaine. May explain why women are more easily addicted to various substances. (Hu & Becker, 2003)

    43. Role of Glutamate Receptors(Dong et al., 2004) Cocaine increases AMPA receptors on DA neurons in VTA. Acute stress + saline ? increased AMPA/NMDA ratio in electrophysiological responses. Cocaine + stress ? similar increase, but not additive. Therefore, separate mechanisms GC antagonist blocked effect of stress, not cocaine

    45. D1 (not D2) antagonist blocked the effect of cocaine---but not the effect of stress.

    46. GluR1-/- mice: No increase in AMPA/NMDA ratio by cocaine or stress

    47. GluR1-/- mice: Behavioral effects Inject 15 mg/kg cocaine or saline every other day for 5 injections. Both WT and KO mice show increased loco-motion after first injection of 15 mg/kg cocaine (Fig 4A, below). Then, 2 weeks w/o injections: both grps also show sensitization to lower doses (Fig. 4B). Therefore, GluR1 not necessary for motor sensitization to cocaine. (see below for 4C, D, E, F)

    48. Locomotion in response to coc or sal: day 1 & 2 wk after drug injections (A,B); Response to sal/coc (Day 1) or sal only (Day 2) in novel environment (4C,D,E); Time in prev. non-preferred compartment (4F)

    49. GluR1-/- mice: Behavioral effects (cont.) Both WT & KO mice increased activity in response to cocaine on day 1 of next exp. (Fig. 4C, above) Day 2: saline-only in the same test cage: Only WT increased activity (response to cue, Fig. 4D). Repeat experiment w/ WT, but give D1 antagonist (SCH23390) before the day 2 saline test: Blocked cue-induced sensitization (Fig 4E).

    50. GluR1-/- mice: Behavioral effects (cont.) KO mice did not develop conditioned place preference to cocaine (Fig. 4F). Therefore, both D1 and AMPA receptors are necess. for cue-induced sensitization. (Because both the D1 antagonist and the AMPA-1 R KO blocked cue-induced sensitization) However, sensitization to cocaine itself was NOT blocked in AMPA-1-KO mice.

    51. Summary and Common Threads 2 main elements: tolerance to the rewarding effects (liking) & sensitization to motivational effects (wanting). Disagreement re. mechanisms of tolerance: Receptor down-regulation/internalization? Dynorphin inhibiting DA release? (Nestler, 04) LC: decreased inhibitory effects of morphine on cAMP/CREB? (Nestler, 04) Maybe liking effects are mediated in NAcc shell.

    52. Summary and Common Threads Sensitization to cocaine: Increased ?FosB in NAc & CPu (Nestler, 04) Increased VTA DA cell size (Nestler, 04) Increased dendritic spines on NAc CORE (not shell) MSN (Nestler, 04; Badiani & Robinson, 04) Core has previously been implicated in learning, whereas shell is more important for affective Rs.

    53. Summary and Common Threads Sensitization to cocaine (cont.): Increased Fos-ir in both direct & indirect pathways w/ low-dose cocaine in NOVEL cage (Badiani & Robinson, 04) Home cage cocaine (low-dose) ? Fos-ir only in direct path (D1-mediated, no sensitiz.) D2 DECREASES cAMP; so GLUTAMATE ?Fos in indirect path via NMDA Rs to induce sensitization.

    54. Summary and Common Threads Sensitization to morphine: DECREASE in VTA DA cell size (Nestler, 04) DECREASE in MSN dendritic spines (The above 2 findings were cited in Nestler, 04, and are OPPOSITE the findings with cocaine.) So its not clear what mediates the sensitization to morphine!!

    55. Some Remaining Questions Badiani & Robinson, 04, note that reinforcing properties of addictive drugs are sensitized; yet Nestler (04) and others say reinforcing properties are decreased. Maybe there is a linguistic problem with reinforcing properties: i.e., maybe liking is decreased, but wanting is increased.

    56. Some Remaining Questions The figure in Nestler & Malenka (Sci Am, 04) shows CREB?Dynorphin?Kappa R? inhibition of DA release ? tolerance. But the same figure shows ?FosB INHIBITING production of dynorphin, which should DECREASE tolerance, as it increases sensitization. Its not clear whether different neurons show up-vs. down-reg. of dynorphin. Core vs. shell??? Indirect vs. direct path???

    57. Some Remaining Questions Badiani & Robinson (04) say adrenalectomy did not affect the novelty-induced sensitization to amphetamine. But Goeders (02) said that it did abolish sensitization. Its not clear whether differences in procedures or drug doses account for these differences. Its also not clear whether CRH in the brain may mediate the sensitization that was observed by B&R after adrenalectomy.

    58. Final thoughts Kent Berridge (03): Liking is mediated by opioid receptors in medial caudal shell of NAcc, as well as GABA/benzodiazepine receptors in parabrachial nucleus of the pons and neurons in the ventral pallidum. Recall that ventral pallidum gets output from the NAcc as part of the direct and indirect pathways. Therefore, tolerance probably results from decreased activation in these areas.

    59. Really Final Thoughts Craving (wanting), then, probably results from increased activation of the indirect pathway of the NAcc core. Dependent on glutamate via NMDA receptors to activate ?FosB, since D2 receptors inhibit adenylyl cyclase (and therefore cant increase ?FosB).