Dorsomedial prefrontal cortex neurons encode nicotine-cue associations

Abstract

The role of medial prefrontal cortex (mPFC) in regulating nicotine taking and seeking remains largely unexplored. In this study we took advantage of the high time-resolution of optogenetic intervention by decreasing (Arch3.0) or increasing (ChR2) the activity of neurons in the dorsal and ventral mPFC during 5-s nicotine cue presentations in order to evaluate their contribution to cued nicotine seeking and taking. Wistar rats were trained to self-administer intravenous nicotine in 1 h self-administration sessions twice a day for a minimum of 10 days. Subsequently, dmPFC or vmPFC neuronal activity was modulated during or following presentation of the 5-s nicotine cue, both under extinction and self-administration conditions. We also used in vivo electrophysiology to record the activity of dmPFC neurons during nicotine self-administration and extinction tests. We show that optogenetic inhibition of dmPFC neurons during, but not following, response-contingent presentations of the nicotine cue increased nicotine seeking. We found no effect on nicotine self-administration or on food seeking in an extinction test. We also show that this effect is specific to dmPFC, because optogenetic inhibition of vmPFC had no effect on nicotine seeking and taking. In vivo recordings revealed that dmPFC network neuronal activity was modulated more strongly following nicotine cue presentation in extinction, compared to following nicotine self-administration. Our results strongly suggest that a population of neurons within the dmPFC is involved in encoding the incentive value of nicotine-associated cues.

Fig. 1

Optogenetic inhibition of dmPFC neurons during cue exposure increases nicotine seeking. a Outline of experimental procedure. b Mean ± sem number of nose-pokes during the nicotine self-administration sessions prior to the extinction tests (Arch on cue: n = 8; eYFP on cue: n = 9; Arch off cue: n = 10; eYFP off cue: n = 8). c Schematic of the two different test conditions of optogenetic inhibition of dmPFC neurons. d Mean ± sem number of nose-pokes during test during the on cue stimulation of Arch3.0 test (left) and on the extinction test the following day (right) (eYFP: n = 9; Arch3.0: n = 8; ** = p < 0.01; *** = p < 0.005). e Mean ± sem number of nose-pokes during test during the off cue stimulation of Arch3.0 test (left) and on the extinction test the following day (right) (eYFP: n = 8; Arch3.0: n = 10)

Fig. 2

Optogenetic excitation of dmPFC neurons during cue exposure has no effect on nicotine seeking. a Outline of experimental procedure. b Mean ± sem number of nose-pokes during the nicotine self-administration sessions prior to the extinction tests (ChR2 on cue: n = 6; eYFP on cue: n = 7; ChR2 off cue: n = 7; eYFP off cue: n = 7). c Schematic of the two different test conditions of optogenetic excitation of dmPFC neurons. d Mean ± sem number of nose-pokes during test during the on cue stimulation of ChR2 test (left) and on the extinction test the following day (right) (eYFP: n = 7; ChR2: n = 6). e Mean ± sem number of nose-pokes during test during the off cue stimulation of ChR2 test (left) and on the extinction test the following day (right) (eYFP: n = 7; ChR2: n = 7)

Fig. 3

Optogenetic inhibition of vmPFC neurons during cue exposure has no effect on nicotine seeking. a Outline of experimental procedure. b Mean ± sem number of nose-pokes during the nicotine self-administration sessions prior to the extinction tests (eYFP: n = 10; Arch: n = 9). c Schematic of the two different test conditions of optogenetic inhibition of dmPFC neurons. d Mean ± sem number of nose-pokes during the on cue stimulation of Arch3.0 test (left) and on the extinction test the following day (right) (eYFP: n = 10; Arch3.0: n = 9). e Mean ± sem number of nose-pokes during the off cue stimulation of Arch3.0 test (left) and on the extinction test the following day (right) (eYFP: n = 10; Arch3.0: n = 9)

Fig. 4

Electrophysiological correlates of cued nicotine self-administration and extinction in dmPFC. a Example location of recording electrodes in the dmPFC. Arrow indicates recording location. b Raster plots and waveforms of two example units recorded during a nicotine self-administration (red) and extinction (blue) session aligned to nose-poke (dashed line). Waveforms are from a single lead and are represented as mean ± std. c, d Trial median firing rates per neuron 0–3 s after nose-poke in c Δ Firing rates (Hz) from baseline and d Z-scored firing rates. Boxplot shows median ± 1st and 3rd quartile