Acute Ramelteon Treatment Maintains the Cardiac Rhythms of Rats during Non-REM Sleep

Top-down brain circuits for operant bradycardia

Heart rate (HR) can be voluntarily regulated when individuals receive real-time feedback. In a rat model of HR biofeedback, the neocortex and medial forebrain bundle were stimulated as feedback and reward, respectively. The rats reduced their HR within 30 minutes, achieving a reduction of approximately 50% after 5 days of 3-hour feedback. The reduced HR persisted for at least 10 days after training while the rats exhibited anxiolytic behavior and an elevation in blood erythrocyte count. This bradycardia was prevented by inactivating anterior cingulate cortical (ACC) neurons projecting to the ventromedial thalamic nucleus (VMT). Theta-rhythm stimulation of the ACC-to-VMT pathway replicated the bradycardia. VMT neurons projected to the dorsomedial hypothalamus (DMH) and DMH neurons projected to the nucleus ambiguus, which innervates parasympathetic neurons in the heart.

Short-Term Preexposure to Novel Enriched Environment Augments Hippocampal Ripples in Urethane-Anesthetized Mice

Learning and memory are affected by novel enriched environment, a condition where animals play and interact with a variety of toys and conspecifics. Exposure of animals to the novel enriched environments improves memory by altering neural plasticity during natural sleep, a process called memory consolidation. The hippocampus, a pivotal brain region for learning and memory, generates high-frequency oscillations called ripples during sleep, which is required for memory consolidation. Naturally occurring sleep shares characteristics in common with general anesthesia in terms of extracellular oscillations, guaranteeing anesthetized animals suitable to examine neural activity in a sleep-like state. However, it is poorly understood whether the preexposure of animals to the novel enriched environment modulates neural activity in the hippocampus under subsequent anesthesia. To ask this question, we allowed mice to freely explore the novel enriched environment or their standard environment, anesthetized them, and recorded local field potentials in the hippocampal CA1 area. We then compared the characteristics of hippocampal ripples between the two groups and found that the amplitude of ripples and the number of successive ripples were larger in the novel enriched environment group than in the standard environment group, suggesting that the afferent synaptic input from the CA3 area to the CA1 area was higher when the animals underwent the novel enriched environment. These results underscore the importance of prior experience that surpasses subsequent physical states from the neurophysiological point of view.

Low Atmospheric Oxygen Attenuates Alpha Oscillations in the Primary Motor Cortex of Awake Rats

Oxygen is pivotal for survival of animals. Their cellular activity and cognitive behavior are impaired when atmospheric oxygen is insufficient, called hypoxia. However, concurrent effects of hypoxia on physiological signals are poorly understood. To address this question, we simultaneously recorded local field potentials in the primary motor cortex, primary somatosensory, and anterior cingulate cortex, electrocardiograms, electroolfactograms, and electromyograms of rats under acute hypoxic conditions (i.e., 5.0% O2). Exposure to acute hypoxia significantly attenuated alpha oscillations alone in the primary motor cortex, while we failed to find any effects of acute hypoxia on the oscillatory power in the somatosensory cortex or anterior cingulate cortex. These area- and frequency-specific effects by hypoxia may be accounted for by neural innervation from the brainstem to each cortical area via thalamic relay nuclei. Moreover, we found that heart rate and respiratory rate were increased during acute hypoxia and high heart rate was maintained even after the oxygen level returned to the baseline. Altogether, our study characterizes a systemic effect of atmospheric hypoxia on neural and peripheral signals from physiological viewpoints, leading to bridging a gap between cellular and behavioral levels.

Ramelteon administration enhances novel object recognition and spatial working memory in mice

Ramelteon is used to ameliorate sleep disorders that negatively affect memory performance; however, it remains unknown whether ramelteon strengthens neutral memories, which do not involve reward or punishment. To address this, we monitored behavior of mice treated with vehicle/ramelteon while they performed a novel object recognition task and a spontaneous alternation task. Object memory performance in the novel object recognition task was improved only if ramelteon was injected before training, suggesting that ramelteon specifically enhances the acquisition of object recognition memory. Ramelteon also enhanced spatial working memory in the spontaneous alternation task. Altogether, acute ramelteon treatment enhances memory in quasi-natural contexts.

Delayed reinforcement hinders subsequent extinction

In operant conditioning, animals associate their own behavior with a reinforcer, and the probability of the behavioral responses is increased. This form of learning is called reinforcement. In contrast, when the previously reinforced responses are no longer paired with a reinforcer, these responses are eventually extinguished. The effectiveness of reinforcement depends primarily on time intervals between reinforcers and responses, but it is not fully understood how the intervals affect subsequent extinction. To address this question, we performed electrical stimulation of the rat medial forebrain bundle (MFB), a part of the brain reward system, and an operant task in which the MFB was electrically stimulated 0.1 s (immediate condition) or 1 s (delayed condition) after the rat's nose was poked. During the first half of the task period (a reinforcement period), nose pokes were associated with MFB stimulation. In contrast, during the second half (an extinction period), we did not stimulate the MFB irrespective of nose pokes. We found that rats exhibited increased nose-poke behaviors during the reinforcement period under both conditions, whereas during the extinction period, nose pokes were more persistent in the delayed condition than in the immediate condition. The persistent responses in the extinction period were independent of responses in the reinforcement period. Therefore, reinforcement and extinction are driven by independent neural mechanisms.