Reversal of Cocaine-Associated Synaptic Plasticity in Medial Prefrontal Cortex Parallels Elimination of Memory Retrieval

Abstract

Addiction is characterized by abnormalities in prefrontal cortex that are thought to allow drug-associated cues to drive compulsive drug seeking and taking. Identification and reversal of these pathologic neuroadaptations are therefore critical for treatment of addiction. Previous studies using rodents reveal that drugs of abuse cause dendritic spine plasticity in prelimbic medial prefrontal cortex (PL-mPFC) pyramidal neurons, a phenomenon that correlates with the strength of drug-associated memories in vivo. Thus, we hypothesized that cocaine-evoked plasticity in PL-mPFC may underlie cocaine-associated memory retrieval, and therefore disruption of this plasticity would prevent retrieval. Indeed, using patch clamp electrophysiology we find that cocaine place conditioning increases excitatory presynaptic and postsynaptic transmission in rat PL-mPFC pyramidal neurons. This was accounted for by increases in excitatory presynaptic release, paired-pulse facilitation, and increased AMPA receptor transmission. Noradrenergic signaling is known to maintain glutamatergic plasticity upon reactivation of modified circuits, and we therefore next determined whether inhibition of noradrenergic signaling during memory reactivation would reverse the cocaine-evoked plasticity and/or disrupt the cocaine-associated memory. We find that administration of the β-adrenergic receptor antagonist propranolol before memory retrieval, but not after (during memory reconsolidation), reverses the cocaine-evoked presynaptic and postsynaptic modifications in PL-mPFC and causes long-lasting memory impairments. Taken together, these data reveal that cocaine-evoked synaptic plasticity in PL-mPFC is reversible in vivo, and suggest a novel strategy that would allow normalization of prefrontal circuitry in addiction.

Figure 1

Cocaine conditioning increases spontaneous excitatory synaptic transmission in PL-mPFC pyramidal neurons: reversal by β-AR blockade during memory retrieval. (a) Schematic illustrating behavioral design of the experiment, wherein saline (red) or propranolol (black) was injected before the first CPP test. (b) Dot plots revealing that propranolol injections, but not vehicle injections, before the first CPP test prevented CPP expression during that test (test 1) and during a subsequent propranolol-free test (test 2). (c) Coronal section illustrating patch-clamp recordings in PL-mPFC. (d) Representative example of a biocytin-filled PL-mPFC pyramidal neuron. (e) Representative waveforms for rheobase recordings. (f) Dot plots revealing that no differences in rheobase were found between groups. (g) Representative waveforms for excitability sweep recordings. (h) Line graphs revealing that no differences in excitability sweep recordings were found between groups. (i) Representative waveforms for sEPSC recordings. (j) Cumulative frequency distribution and dot plots (inset) revealing that neurons from vehicle-treated rats had significantly higher sEPSC amplitudes as compared with neurons from naive and propranolol-treated rats. (k) Cumulative frequency distribution and dot plots (inset) revealing that neurons from vehicle-treated rats had significantly higher sEPSC frequencies as compared with neurons from naive and propranolol-treated rats. *p<0.05 vs control. Lines in dot plots represent the mean±SE.

Figure 2

Cocaine conditioning increases evoked AMPA receptor transmission in PL-mPFC pyramidal neurons: reversal by β-AR blockade during memory retrieval. (a) Coronal section illustrating patch-clamp recordings of stimulation-evoked synaptic currents in PL-mPFC. (b) Representative waveforms for paired-pulse ratio recordings. (c) Dot plots revealing that neurons from vehicle-treated rats had significantly higher paired-pulse ratios as compared with neurons from naive and propranolol-treated rats. (d) Representative waveforms for AMPA and NMDA receptor-mediated currents. (e) Dot plots revealing that neurons from vehicle-treated rats had significantly higher AMPA receptor-mediated currents as compared with neurons from naive rats and propranolol-treated rats. (f) Dot plots revealing no group differences for NMDA receptor-mediated currents. (g) Dot plots revealing that neurons from vehicle-treated rats had significantly higher AMPA/NMDA^Volt as compared with neurons from naive rats and propranolol-treated rats. (h) Dot plots revealing that neurons from vehicle-treated rats had significantly higher AMPA/NMDA^Pharm as compared with neurons from naive and propranolol-treated rats. (i) Correlation between AMPA/NMDA^Volt and AMPA/NMDA^Pharm reveals a significant relationship between these two AMPA/NMDA ratios. *p<0.05 vs control; **p<0.01 vs control; ***p<0.001 vs control. Lines in dot plots represent the mean±SE.

Figure 3

β-AR blockade after memory retrieval (during reconsolidation) has no effect on cocaine-evoked synaptic plasticity in PL-mPFC pyramidal neurons. (a) Schematic illustrating behavioral design of the experiment, wherein saline (red) or propranolol (black) was injected after the first CPP test. (b) Dot plots revealing no differences in CPP scores when propranolol was injected after the first CPP test (test 1). (c) Coronal section illustrating patch-clamp recordings in PL-mPFC. (d) Representative waveforms for rheobase recordings. (e) Dot plots revealing that no difference in rheobase was found between groups. (f) Representative waveforms for excitability sweep recordings. (g) Line graph revealing that no differences in excitability sweeps were found between groups. (h) Representative waveforms for sEPSC recordings. (i) Cumulative frequency distribution and dot plots (inset) revealing no differences in sEPSC amplitudes between groups. (j) Cumulative frequency distribution and dot plots (inset) revealing no differences in sEPSC frequencies between groups. (k) Dot plots revealing that no difference in paired-pulse ratio was found between groups. (l) Dot plots revealing that no difference in AMPA receptor-mediated current was found between groups. (m) Dot plots revealing that no difference in NMDA receptor-mediated current was found between groups. (n) Dot plots revealing that no difference in AMPA/NMDA^Volt was found between groups. Lines in dot plots represent the mean±SE.