Marina E. WolfChicago Medical School

Rosalind Franklin University of Medicine & Science

How do drugs of abuse rewire the motivational

circuitry?

Acute effects of cocaine and amphetamine: interference with monoamine reuptake

Addiction: long-lasting adaptations triggered by increased monoamine levels

These adaptations “rewire” the motivational circuitry, facilitating the formation of new habits that center

around drug-seeking, usually at the expense of more appropriate behaviors.

Hypothesis

Addiction is a form of neuronal plasticity. The adaptations leading to addiction involve the same

glutamate-dependent cellular mechanisms that enable learning and memory.

Glutamate-dependent plasticity and addiction

Animal model of addiction (behavioral sensitization)• Behavioral changes blocked by glutamate antagonists or

lesions of glutamate pathways• Behavioral changes are associated with changes in

glutamate receptor function and expression

Drugs of abuse and plasticity activate common signal transduction cascades (kinases & phosphatases, transcriptional regulators, neurotrophins, etc)

Drugs of abuse influence LTP and LTD in reward-related pathways

Glutamate and glutamate receptors

Glutamate: major excitatory transmitter in the brain, activates ionotropic receptors and GPCRs

Ionotropic receptors• AMPA receptors – bulk of fast excitatory transmission• NMDA receptors – slower synaptic potentials, enable

plasticity (and excitotoxicity)• Kainate receptors – less understood

Metabotropic receptors (GPCRs) • at least 8 subtypes (mGluR1-8)• important in normal synaptic transmission and plasticity

Long-term potentiation (LTP)

LTP Strong NMDAR activation Large postsynaptic Ca increase Preferential activation of kinases Potentiation of AMPAR currents

LTD More modest NMDAR activation Less robust postsynaptic Ca increase Preferential activation of

phosphatases Depression of AMPAR currents

LTP in NAc (Julie Kauer, Brown Univ)

stimulate record

Functional significance of LTP in addiction-related pathways?

Wolf, Molecular Interventions 2002

How do drugs that initially target monoamine transporters influence glutamate transmission and glutamate-

dependent processes such as LTP and LTD?

Drugs may modulate LTP via actions on neuronal circuits

Everitt and Wolf, J Neurosci 2002

Dendritic spine (postsynaptic)

Glutamate

Dopamine

Drugs may modulate LTP at the single cell level

Two inter-related mechanisms for regulating AMPA receptor transmission during LTP and LTD

a) GluR1 is phosphorylated during LTP, and dephosphorylated during LTD

b) GluR1-containing AMPA receptors are inserted into synapses during LTP and removed during LTD

Song & Huganir, TINS 2002

Bredt & Nicoll, Neuron 2003

AMPA receptor trafficking

Link to dopamine?

D1 receptors are coupled to PKA

PKA is involved in both mechanisms for regulating AMPA receptor function during LTP

Hypothesis: D1 receptors modulate AMPA receptor phosphorylation and trafficking through PKA-dependent mechanisms, and thereby influence LTP and LTD.

Song & Huganir, TINS 2002

Nucleus accumbens

Dopamine afferents

Glutamate afferents (cortex, hippocampus,

amygdala)

Medium spiny projection neuron

Do D1 receptors regulate AMPA receptor trafficking in NAc neurons?

~80% medium spiny neurons and 20% interneurons

Almost all neurons are GABAergic

GluR1 is expressed by all neurons

D1 receptors are expressed by ~80% of neurons

D2 receptors are expressed by ~80% of neurons

Postnatal NAc cultures reproduce many features of

the intact NAc

Chao et al, J Neurochem 83:704-12, 2002

D1 receptor stimulation increases GluR1 surface expression in NAc neurons

Control 1M SKF

Chao et al, J Neurochem 83:704-12, 2002

Pre-blocking protocol for selectively detecting newly externalized GluR1

Lu, Man, Ju, Trimble, MacDonald, & Wang, Neuron, 2001

Pre-block GluR1 on the surface of live cells with N-GluR1 antibody and unlabeled secondary antibody

Incubate at room temperature to allow GluR1 externalization

Second round of immunostaining, under non-permeant conditions, with N-GluR1 antibody and Cy3 secondary antibody

1o Ab

2o Ab

Cy3 2oAb

Media

SCH10uM SCH + SKF

1uM SKF

20um

Control

D1 receptor stimulation increases the rate of GluR1 externalization

Mangiavacchi & WolfJ Neurochem 88:1261-71, 2004

Requires PKA activation (Mangiavacchi & Wolf, J Neurochem 88:1261-71, 2004)

GluR1 may be the relevant substrate because D1 agonists also stimulate phosphorylation of GluR1 at the PKA site in NAc cultures (Chao et al, J Neurochem 81:984-992, 2002)

Consistent with results in hippocampus indicating that PKA phosphorylation of GluR1 promotes its trafficking to the cell surface (Ehlers, Neuron 28:511-25, 2000; Esteban et al, Nat Neurosci 6:136-43,2003)

Medium spiny neurons Interneurons

Control 50µM Glu

Control 50μM Glu

Glutamate agonists produce rapid internalization of GluR1

Mangiavacchi & Wolf, Eur J Neurosci, submitted

D1

NAc

Prefrontal cortex, BLA, HPC

VTA

Dopamine inputs increase rate of AMPAR externalization

Glutamate inputs increase rate of AMPAR internalization

Changes in afferent activity result in minute-by-minute modulation of AMPAR surface expression on NAc neurons?

DA

Glu

AMPA

Do D1 receptors drive GluR1 all the way into the synapse?

Modified from Bredt & Nicoll, Neuron 2003

D1R

PKA

???

Prefrontal Cortex

D1R

NMDAR

CB1R

Synaptic and extrasynaptic AMPA receptors in PFC cultures

Red = GluR1

Green = SB

Yellow = overlay

Sun & Wolf, unpublished

Summary of unpublished results presented at APA meeting but not provided on NIDA website

D1 receptor stimulation increases GluR1 externalization at extrasynaptic sites, facilitating synaptic insertion as a result of subsequent NMDA receptor stimulation (Sun et al., Soc Neurosci Abstr 30, in press)

D1R (PKA)

NMDAR

D1 receptor stimulation externalizes AMPAR at extrasynaptic sites, increasing the pool available for synaptic insertion during LTP

DA input

Glutamate input

Modified from Bredt & Nicoll, Neuron 2003

DA facilitates LTP induction in prefrontal cortex (Gurden et al, Neurosci 94:1019-1027, 1999; Blond et al, Eur J Pharmacol 438:115-116, 2002)

The facilitation is mediated by D1 receptors and PKA (Jay et al, Eur J Neurosci 10:3302-3306,1998; Gurden et al, J Neurosci 20:RC106, 2000)

Our results are consistent with studies showing that:

D1 receptor stimulation increases GluR1 surface expression through a PKA-dependent pathway at extrasynaptic sites.

By increasing extrasynaptic GluR1, D1 receptors increase the pool available for synaptic insertion as a result of NMDA receptor stimulation. This may facilitate the induction of LTP.

These findings suggest a mechanism by which drugs of abuse can directly “tap into” fundamental mechanisms for regulating synaptic strength.

Summary

D1

NAc

Prefrontal cortex, BLA, HPC

VTA

D1 and glutamate inputs normally regulate AMPA receptor trafficking

Chronic drug exposure: adaptations in D1 receptor and PKA signaling

Compensatory changes in AMPA receptor trafficking

Inappropriate synaptic plasticity, rewiring of motivational circuitry, and formation of new habits focused on drug seekingDA

Glu

AMPA

Cocaine Amphetamine

Relevance to addiction?

Proposed sequence of mechanisms for long-term plasticity

Luscher, Nicoll, Malenka & Muller (Nature Neurosci 2000)

Steven Chao Amy Bluestein

Marjorie Ariano Dan Peterson Kathy Steece-Collier

Robert Malenka Reed Carroll Richard Huganir Hey-Kyoung Lee

National Institute on Drug Abuse

Simona Mangiavacchi

Xiu Sun