Time-of-day effect on a food-induced conditioned place preference task in monkeys.

Time can be an important contextual cue for cognitive performance, with implications for reward-associated learned behaviors such as (drug and food) addiction. So, we analyzed: (1) if marmoset monkeys develop a place preference that is conditioned to previous pairings with a highly-palatable food reward; (2) if the response is strongest when training and testing times match - time stamp effect; and (3) if there is an optimal time of the day (morning vs. afternoon) when this preference occurs - time-of-day effect. Subjects were first habituated to a two-compartment conditioned-place-preference (CPP) box. Then, during six training sessions held either in the morning or afternoon, a mixture of jellybeans and live mealworms was made available in a specific compartment. Marmosets were subsequently tested for preferring the food-paired context at the circadian time that either matched or was different from that of training. Compared to baseline levels, only subjects trained and tested in the afternoon made significantly longer and more frequent visits to the food-paired context and with a shorter latency to first entry. Thus, highly-palatable food rewards induced a CPP response. This behavior was exhibited only when training and testing times overlapped and during a restricted circadian timeframe (afternoon), consistent with a time-stamp and time-of-day effect, respectively. In this case, time may have been an internal circadian contextual cue. Whether due to circadian-mediated oscillations in memory and/or reward processes, such findings may be applied to addiction and other learned behaviors.