Prolactin-Releasing Peptide Contributes to Stress-Related Mood Disorders and Inhibits Sleep/Mood Regulatory Melanin-Concentrating Hormone Neurons in Rats.

Stress disorders impair sleep and quality of life; however, their pathomechanisms are unknown. Prolactin-releasing peptide (PrRP) is a stress mediator; we therefore hypothesized that PrRP may be involved in the development of stress disorders. PrRP is produced by the medullary A1/A2 noradrenaline (NA) cells, which transmit stress signals to forebrain centers, and by non-NA cells in the hypothalamic dorsomedial nucleus. We found in male rats that both PrRP and PrRP-NA cells innervate melanin-concentrating hormone (MCH) producing neurons in the dorsolateral hypothalamus (DLH). These cells serve as a key hub for regulating sleep and affective states. Ex vivo, PrRP hyperpolarized MCH neurons and further increased the hyperpolarization caused by NA. Following sleep deprivation, intracerebroventricular PrRP injection reduced the number of REM sleep-active MCH cells. PrRP expression in the dorsomedial nucleus was upregulated by sleep deprivation, while downregulated by REM sleep rebound. Both in learned helplessness paradigm and after peripheral inflammation, impaired coping with sustained stress was associated with (1) overactivation of PrRP cells, (2) PrRP protein and receptor depletion in the DLH, and (3) dysregulation of MCH expression. Exposure to stress in the PrRP-insensitive period led to increased passive coping with stress. Normal PrRP signaling, therefore, seems to protect animals against stress-related disorders. PrRP signaling in the DLH is an important component of the PrRP's action, which may be mediated by MCH neurons. Moreover, PrRP receptors were downregulated in the DLH of human suicidal victims. As stress-related mental disorders are the leading cause of suicide, our findings may have particular translational relevance.SIGNIFICANCE STATEMENT Treatment resistance to monoaminergic antidepressants is a major problem. Neuropeptides that modulate the central monoaminergic signaling are promising targets for developing alternative therapeutic strategies. We found that stress-responsive prolactin-releasing peptide (PrRP) cells innervated melanin-concentrating hormone (MCH) neurons that are crucial in the regulation of sleep and mood. PrRP inhibited MCH cell activity and enhanced the inhibitory effect evoked by noradrenaline, a classic monoamine, on MCH neurons. We observed that impaired PrRP signaling led to failure in coping with chronic/repeated stress and was associated with altered MCH expression. We found alterations of the PrRP system also in suicidal human subjects. PrRP dysfunction may underlie stress disorders, and fine-tuning MCH activity by PrRP may be an important part of the mechanism.

AM-251, A Cannabinoid Antagonist, Modifies the Dynamics of Sleep-Wake Cycles in Rats.

Study Objectives: (a) To describe the microarchitecture of wakefulness and sleep following administrations of 5- and 10-mg/kg AM-251 in rats. (b) To develop a new statistical method to follow bout-to-bout dynamics. Method: Wistar rats (n = 6) had been equipped with electroencephalography (EEG) and electromyography (EMG) electrodes. Following their recovery and habituation after the surgery, the animals were injected with vehicle and 5- and 10-mg/kg AM-251 intraperitoneally and EEG, EMG, and motor activity were analyzed for the subsequent 3 h. Results: AM-251 induced a dose- and time-dependent increase in the number of bouts in active wake (AW), and it decreased this number in all other vigilance states except in passive wake (PW). In contrast, the bout duration in PW compensatory decreased. The effect of AM-251 on the sleep transition dynamics was monitored with a new tool we call "transition heatmap." The analysis of bout trajectories with transition heatmaps reveals a highly organized pattern. Conclusion: AM-251 selectively influences the frequency of vigilance state transitions, but it has no direct impact on the state lengths. AM-251 markedly changed the state transition dynamics, which was visualized with the help of state transition heatmaps.

Additive effect of 5-HT2C and CB1 receptor blockade on the regulation of sleep-wake cycle.

BACKGROUND: Previous data show that serotonin 2C (5-HT2C) and cannabinoid 1 (CB1) receptors have a role in the modulation of sleep-wake cycle. Namely, antagonists on these receptors promoted wakefulness and inhibited rapid eye movement sleep (REMS) in rodents. The interaction of these receptors are also present in other physiological functions, such as the regulation of appetite. Blockade of 5-HT2C receptors modulat the effect of CB1 receptor antagonist, presumably in consecutive or interdependent steps. Here we investigate, whether previous blockade of 5-HT2C receptors can affect CB1 receptor functions in the sleep-wake regulation. RESULTS: Wistar rats were equipped with electroencephalography (EEG) and electromyography (EMG) electrodes. Following the recovery and habituation after surgery, animals were injected intraperitoneally (ip.) with SB-242084, a 5-HT2C receptor antagonist (1.0 mg/kg) at light onset (beginning of passive phase) followed by an injection with AM-251, a CB1 receptor antagonist (5.0 or 10.0 mg/kg, ip.) 10 min later. EEG, EMG and motor activity were analyzed for the subsequent 2 h. Both SB-242084 and AM-251 increased the time spent in active wakefulness, while decreased the time spent in non-REMS and REMS stages in the first 2 h of passive phase. In combination, the effect of the agents were additive, furthermore, statistical analysis did not show any interaction between the effects of these drugs in the modulation of vigilance stages. CONCLUSIONS: Our results suggest that 5-HT2C receptor blockade followed by blockade of CB1 receptors evoked additive effect on the regulation of sleep-wake pattern.

Blockade of Serotonin 2C Receptors with SB-242084 Moderates Reduced Locomotor Activity and Rearing by Cannabinoid 1 Receptor Antagonist AM-251.

The endocannabinoid and serotonin (5-HT) systems have key roles in the regulation of several physiological functions such as motor activity and food intake but also in the development of psychiatric disorders. Here we tested the hypothesis, whether blockade of serotonin 2C (5-HT2C) receptors prevents the reduced locomotor activity and other behavioral effects caused by a cannabinoid 1 (CB1) receptor antagonist. As a pretreatment, we administered SB-242084 (1 mg/kg, ip.), a 5-HT2C receptor antagonist or vehicle (VEH) followed by the treatment with AM-251 (5 or 10 mg/kg, ip.), a CB1 receptor antagonist or VEH. The effects of the two drugs alone or in co-administration were investigated in social interaction (SI) and elevated plus maze (EPM) tests in male Wistar rats. Our results show that AM-251 decreased the time spent with rearing in the SI test and decreased locomotor activity in EPM test. In contrast, SB-242084 produced increased locomotor activity in SI test and evoked anxiolytic-like effect in both SI and EPM tests. When applied the drugs in combination, these behavioral effects of AM-251 were moderated by SB-242084. Based on these findings, we conclude that certain unwanted behavioral effects of CB1 receptor antagonists could be prevented by pretreatment with 5-HT2C receptor antagonists.

Acute and chronic escitalopram alter EEG gamma oscillations differently: relevance to therapeutic effects.

Brain oscillations in the gamma frequency band of the electroencephalogram (EEG) have been implicated in several sensory and cognitive processes, and have also been associated with numerous neuropsychiatric disorders, including depression. The widely prescribed selective serotonin reuptake inhibitors (SSRIs), similarly to other antidepressants, are known to produce markedly different effects on sleep and behavioral measures with acute and chronic administration. Although there are studies examining the acute effect of escitalopram on slower (<30 Hz) oscillations, we hardly could find any data about the effect of the drug on higher-frequency EEG oscillations (>30 Hz) in different sleep-wake stages, particularly comparing the acute and chronic effects of the drug concerning gamma oscillations. Our aim was to investigate, how escitalopram affects gamma power in different sleep-wake stages, and to discover possible differential effects between acute and chronic treatment. EEG-equipped Wistar rats were treated with escitalopram or vehicle acutely (10 mg/kg, i.p.) or chronically (10 mg/kg/day for 21 days, osmotic minipumps) and frontoparietal EEG, electromyogram and motor activity were recorded during the first 3 h of passive phase. We found that acute and chronic escitalopram treatment affected gamma oscillations differently. While acute escitalopram caused a reduction in gamma power during rapid eye movement sleep (REMS) and intermediate stage of sleep (IS), chronic treatment caused an elevation in gamma power during non-REMS stages, namely in light and deep slow-wave sleep (SWS-1 and SWS-2, respectively) and in IS. However, gamma activity during active and passive wakefulness (AW and PW, respectively) was not influenced by either acute or chronic dosing of escitalopram. Furthermore, we found that in drug-free (vehicle-treated) rats, a relatively high gamma power was present during wakefulness and REMS, while a much lower power was measured during non-REMS stages. These findings indicate that acute and chronic administration of escitalopram alter gamma activity differently, moreover, in a sleep-wake stage dependent manner that may be related to differential therapeutic and/or side effects.