ABSTRACT
The medial frontal cortex (MFC) in rodents emits rhythmic activity that is entrained to the animal’s licking cycle during consumption and encodes the value of consumed fluids (Horst and Laubach, 2013; Amarante et al., 2017; Amarante and Laubach, 2021). These signals are especially prominent in the rostral half of the MFC. This region is located above an orbitofrontal region where mu opioid receptors regulate intake (Mena et al., 2011; Castro and Berridge, 2017) and reversible inactivation reduces behavioral measures associated with the incentive value and palatability of liquid sucrose (Parent et al., 2015a). Here, we examined the effects of reversible inactivation and stimulation of mu opioid receptors in rostral MFC on behavior in an incentive contrast licking task. Adult male rats licked to receive access to liquid sucrose, which alternated between high (16%) and low (4%) values over 30 sec periods. Bilateral infusion of muscimol reduced the total number of licks emitted over the 30 min test sessions, the time spent actively consuming sucrose, and the ratio of licks for the higher and lower value fluids. Inactivation did not alter licking frequency or variability or microstructural measures such as the duration of licking bouts that are classically associated with the palatability of a liquid reward. Infusions of DAMGO (1μg/μL) at the same sites had inconsistent behavioral effects across different subjects. Our findings suggest that the rostral MFC has a distinct role in the control of consummatory behavior and contributes to peristent consumption and not to the expression of palatability.
SIGNIFICANCE STATEMENT The medial frontal cortex (MFC) of rodents has received attention in recent years and is considered as a singular cortical region with a potential unitary function. Increasing evidence suggests that MFC is composed of distinct subregions, with unique roles in the control of behavior. The present study adds to this literature by showing unique effects of reversibly inactivating the most rostral part of the medial frontal cortex and a lack of consistent effects of stimulating mu opioid receptors in the subregion. Findings are in contrast to previous reports on the more ventral orbitofrontal cortex and caudal medial frontal cortex and are important for understanding the general role of the rodent frontal cortex and how opioids may act to control behavior.
Competing Interest Statement
The authors have declared no competing interest.
