Neuronal mechanisms of reward signaling in the prefrontal cortex - This AREA award will investigate neuronal mechanisms of reward signaling by the prefrontal cortex. It will
support the training of PhD students and undergraduates majoring in neuroscience working in the Laubach
Laboratory at American University in Washington, DC. Trainees will develop skills in animal behavior, multi-
electrode recording, optical imaging, optogenetics, pharmacology, and neuroanatomy. Facilities provided
through AU's Center for Behavioral Neuroscience, the lab's management of OpenBehavior (a website for
sharing open-source designs for behavioral devices and software), and interactions with collaborating
researchers at the nearby NIH campus support a unique training environment.
Aim 1: Determine if the medial prefrontal cortex (mPFC) exerts top-down control over reward signaling by
the orbitofrontal cortex (OFC). Unpublished data from my laboratory suggest that a 4-8 Hz “theta” rhythm
generated by mPFC neurons drives reward signaling in the OFC. This hypothesis will be tested in behaving rats
using an incentive contrast licking task, in which rewards of different magnitudes are presented over extended
blocks of trials or in a randomly interleaved manner. Top-down control depends on the animal’s ability to estimate
the expected value of reward. mPFC should influence OFC processing when rewards vary over blocks of trials,
but not when reward values vary randomly over trials. Two experiments will test this hypothesis. (1) Optogenetic
inactivation (ArchT-3.0) will be used to disrupt processing in one cortical area while neuronal activity is recorded in
the other. If mPFC has top-down influences over OFC, then inactivation of mPFC should reduce reward-encoding
in OFC in sessions with blocked reward values compared to sessions with interleaved reward values. By contrast,
if OFC has bottom-up influences on mPFC, then inactivation of OFC should reduce reward signaling by mPFC
independent of whether rewards vary over blocks or are randomly interleaved. (2) GCaMP-based fiber photometry
will be used to measure if identified mPFC→OFC and OFC→mPFC projection neurons encode reward value and
vary with the blocked or randomly interleaved schedules of reward delivery.
Aim 2: Measure effects of opioid drugs on reward signaling by the mPFC. Mu opioid receptors mediate
the primary effects of opioid drugs and are prominent in the mPFC. Stimulation of these receptors increases
behavioral measures of reward value, possibly by enhancing mPFC reward signals. We will examine how
opioid drugs alter prefrontal activity in awake, behaving animals. Opioid drugs will be infused directly into the
mPFC, or into the lateral ventricles, while neuronal activity is locally recorded in the mPFC with recently
developed “pharmatrodes”. Fiber photometry will also be used to determine if identified mPFC→OFC and
OFC→mPFC projection neurons are sensitive to opioid drugs. These studies will be the first in vivo studies of the
actions of opioid drugs on the mPFC during a reward-guided task that depend on mPFC function.
