The Rat Medial Prefrontal Cortex Exhibits Flexible Neural Activity States during the Performance of an Odor Span Task

Medial prefrontal cortex (mPFC) activity is fundamental for working memory (WM), attention, and behavioral inhibition; however, a comprehensive understanding of the neural computations underlying these processes is still forthcoming. Toward this goal, neural recordings were obtained from the mPFC of awake, behaving rats performing an odor span task of WM capacity. Neural populations were observed to encode distinct task epochs and the transitions between epochs were accompanied by abrupt shifts in neural activity patterns. Putative pyramidal neuron activity increased earlier in the delay for sessions where rats achieved higher spans. Furthermore, increased putative interneuron activity was only observed at the termination of the delay thus indicating that local processing in inhibitory networks was a unique feature to initiate foraging. During foraging, changes in neural activity patterns associated with the approach to a novel odor, but not familiar odors, were robust. Collectively, these data suggest that distinct mPFC activity states underlie the delay, foraging, and reward epochs of the odor span task. Transitions between these states likely enables adaptive behavior in dynamic environments that place strong demands on the substrates of working memory

Intra-mPFC microwire recording locations, waveform analysis and effects of intra-BLA ANI on neuronal encoding of opiate reward memory.

<p><b><i>A</i></b>, Microphotograph showing a representative intra-mPFC recording wire placement (black arrows). <b><i>B</i></b>, Schematic representation of sample intra-mPFC microwire recording location, demonstrating rostral-caudal distribution within the mPFC. <b><i>C</i></b>, Sample mPFC neuronal unit waveforms from single channels, recorded over 3 days (baseline, conditioning day, CPP recall test) from four experimental groups receiving either intra-BLA vehicle ANI or vehicle at 0 or 12 hrs post-conditioning <b><i>D</i></b>.</p

Analysis of pERK and ERK 1/2 protein expression levels in BLA and mPFC tissue during 0 and 12 hr post-conditioning memory consolidation time-points.

<p><b><i>A</i></b><i>,</i> Analysis of BLA tissue samples at 0 hr post-conditioning reveals significantly elevated pERK levels relative to total ERK levels, but not in vehicle control or 12 hr post-conditioning experimental groups. <b><i>B</i></b><i>,</i> In contrast, no significant differences across groups are observed in the pERK/ERK ratio analysed in mPFC tissue samples. <i>C</i>, Representative Western Blot samples showing fluorescent imaging of pERK and ERK/GADPH bands across experimental groups at 0 and 12 hr post-conditioning time-points.</p

Effects of acute (0 hr) intra-BLA protein synthesis inhibition on mPFC neuronal associative morphine responses during CPP testing.

<p><i>A</i>, When administered at 0 hrs post-conditioning, intra-BLA ANI blocks mPFC neuronal encoding of associative opiate reward memory, demonstrated by a lack of associative firing activity increases during exposure to morphine environments during CPP testing or during a morphine cued recall CPP test. In contrast, vehicle controls at 0 or 12 hrs or intra-BLA ANI administration at 12 hrs, does not block mPFC neuronal encoding of opiate reward memory, demonstrated by robust mPFC neuronal firing activity during CPP exposure to morphine-paired environments. B, Cumulative mPFC neuronal firing frequency rates are presented for the same experimental groups shown in panel A. Cumulative neuronal firing frequency demonstrates morphine-environment specific associative increases in firing rates relative to saline-paired environments in groups receiving intra-BLA vehicle at 0 or 12 hrs post-conditioning or in rats receiving intra-BLA anisomycin at 12 hrs post-conditioning. However, this associative increase in firing activity is absent in rats receiving intra-BLA anisomycin at 0 hrs post-conditioning.</p

Effects of intra-BLA or mPFC anisomycin on temporal consolidation of acute opiate reward memory.

<p><b><b><i>A</i></b></b>, Post-conditioning intra-BLA microinfusions of ANI completely block consolidation of opiate reward memory when administered at 0 and 3 hrs, but has no effect at 6 or 12 hrs. A Morphine cue-recall control test administered 24 hrs after the initial CPP test in the 0 hr group does not re-store the memory. <b><i>B</i></b>, Post-conditioning intra-mPFC microinfusions of ANI completely block consolidation of opiate reward memory when administered at 12 hrs, but has no effect at time-points before this. A morphine cue-recall control test administered 24 hrs after the initial CPP test in the 12 hr group does not re-store the memory. <b><i>C</i></b><b>,</b> Vehicle control groups for the effective time points of 0 hrs (intra-BLA) or 12 hrs (intra-mPFC) caused no block of morphine reward CPP memory consolidation. <b><i>D,</i></b> Temporally divergent effects of opiate reward memory consolidation interference presented as CPP difference scores (calculated as times in drug minus vehicle-paired environments).</p

Roles of ERK and CaMKII signalling during recent and remote opiate memory consolidation.

<p><b><i>A</i></b><b>,</b> Relative to vehicle controls, both intra-BLA U0126 (0.1–1.0 µg/0.5 µl) and KN-62 (0.05–0.5 µg/0.5 µl), dose-dependently block opiate reward memory consolidation when administered at the 0 hr time point. <b><i>B</i></b><b>,</b> Relative to vehicle controls, intra-mPFC KN-62, dose-dependently blocked opiate reward memory consolidation when administered at the 12 hr time point, while U0126 has no effect.</p

Effects of later (12 hr) intra-BLA protein synthesis inhibition on behavioral and mPFC neuronal opiate memory consolidation.

<p>Group CPP data showing that intra-BLA vehicle <b><i>A</i></b><i>,</i> or anisomycin administration (<b><i>C</i></b>) at 12 hrs post-conditioning does not block subsequent behavioral recall of consolidated opiate reward memory. In addition, mPFC neuronal populations show robust, associative firing rate increases in response to morphine environment exposures during real-time CPP test recordings (panels <b><i>B</i></b><i>, </i><b><i>D</i></b>; raster samples taken from single rats showing mPFC neuronal firing recorded in real-time across saline or morphine-environmental exposure).</p

Effects of acute (0 hr) intra-BLA protein synthesis inhibition on behavioral CPP expression and mPFC neuronal activity.

<p><b><i>A</i></b><i>,</i> Group data showing robust behavioral morphine CPP from rats receiving intra-BLA vehicle at 0 hrs post-conditioning. <b><i>B</i></b><i>,</i> A sample rastergram from a single rat, showing typical mPFC neuronal associative responding across saline vs. morphine-paired environments during real-time CPP testing. <b><i>C</i></b><i>,</i> Group data showing blockade of behavioral morphine CPP expression from rats receiving intra-BLA anisomycin at 0 hrs post-conditioning. <b><i>D</i></b><i>,</i> Sample rastergram from a single rat, showing mPFC neuronal responding during real-time CPP testing, across saline or morphine-environmental exposure. Associative neuronal increases during morphine environmental exposure is absent, consistent with a lack of behavioral CPP expression.</p