Prefrontal Dopaminergic Regulation of Cue-Guided Risky Decision-Making Performance in Rats

Ramelteon coordinates theta and gamma oscillations in the hippocampus for novel object recognition memory in mice

Object recognition memory is an animal's ability to discriminate between novel and familiar items and is supported by neural activities in not only the perirhinal cortex but also the hippocampus and prefrontal cortex. Since we previously demonstrated that ramelteon enhanced object recognition memory in mice, we sought neural correlates of the memory improvement. We recorded neural activity in the hippocampus and prefrontal cortex of mice while they performed a novel object recognition task. We found that theta oscillations in the hippocampus were enhanced when ramelteon-treated mice explored both novel and familiar objects. Moreover, we showed high coherence in phases at low gamma frequencies between the hippocampus and prefrontal cortex. We assume that theta enhancement is indicative of increased cholinergic activity by melatonin receptor activation. High coherence of low gamma oscillations between the hippocampal and prefrontal network in ramelteon-treated mice sampling novel objects suggests better cognitive operations for discrimination between novelty and familiarity. The current study sheds light upon physiological consequences of melatonin receptor activation, further contributing improved cognitive functions.

Enhanced Risky Choice in Male Rats Elicited by the Acute Pharmacological Stressor Yohimbine Involves Prefrontal Dopamine D1 Receptor Activation

BACKGROUND

Acute stress alters risk-based decision-making; however, the underlying neural and neurochemical substrates are underexplored. Given their well-documented stress-inducing effects in humans and laboratory animals, glucocorticoids such as cortisol and corticosterone and the α2-adrenoceptor antagonist yohimbine represent potent pharmacological tools to mimic some characteristics of acute stress.

METHODS

Here, we analyzed the effects of the pharmacological stressors corticosterone and yohimbine given systemically on risk-based decision-making in male rats. Moreover, we investigated whether pharmacological stressor effects on risk-based decision-making involve dopamine D1 receptor stimulation in the dorsal prelimbic cortex (PL). We used a risk discounting task that requires choosing between a certain/small reward lever that always delivered 1 pellet and a risky/large reward lever that delivered 4 pellets with a decreasing probability across subsequent trials.

RESULTS

Systemic administration of yohimbine increased the preference for the risky/large reward lever. By contrast, systemic single administration of corticosterone did not significantly promote risky choice. Moreover, co-administration of corticosterone did not enhance the effects of yohimbine on risky choice. The data further show that the increased preference for the risky/large reward lever under systemic yohimbine was lowered by a concurrent pharmacological blockade of dopamine D1 receptors in the PL.

CONCLUSIONS

Our rodent data provide causal evidence that stimulation of PL D1 receptors may represent a neurochemical mechanism by which the acute pharmacological stressor yohimbine, and possibly nonpharmacological stressors as well, promote risky choice.