Antiserum to prolactin decreases rapid eye movement sleep (REM sleep) in the male rat

Sleep and local field potential effect of the D2 receptor agonist bromocriptine during the estrus cycle and postpartum period in female rats

BACKGROUND

Pituitary lactotrophs are under tonic dopaminergic inhibitory control and bromocriptine treatment blocks prolactin secretion.

METHODS

Sleep and local field potential were addressed for 72 h after bromocriptine treatments applied during the different stages of the estrus cycle and for 24 h in the early- and middle postpartum period characterized by spontaneously different dynamics of prolactin release in female rats.

RESULTS

Sleep changes showed strong dependency on the estrus cycle phase of the drug application. Strongest increase of wakefulness and reduction of slow wave sleep- and rapid eye movements sleep appeared during diestrus-proestrus and middle postpartum treatments. Stronger sleep-wake effects appeared in the dark phase in case of the estrus cycle treatments, but in the light phase in postpartum treatments. Slow wave sleep and REM sleep loss in case of estrus cycle treatments was not compensated at all and sleep loss seen in the first day post-injection was gained further later. In opposition, slow wave sleep loss in the light phase after bromocriptine injections showed compensation in the postpartum period treatments. Bromocriptine treatments resulted in a depression of local field potential delta power during slow wave sleep while an enhancement in beta and gamma power during wakefulness regardless of the treatment timing.

CONCLUSIONS

These results can be explained by the interplay of dopamine D2 receptor agonism, lack of prolactin release and the spontaneous homeostatic sleep drive being altered in the different stages of the estrus cycle and the postpartum period.

Brain prolactin is involved in stress-induced REM sleep rebound

REM sleep rebound is a common behavioural response to some stressors and represents an adaptive coping strategy. Animals submitted to multiple, intermittent, footshock stress (FS) sessions during 96h of REM sleep deprivation (REMSD) display increased REM sleep rebound (when compared to the only REMSD ones, without FS), which is correlated to high plasma prolactin levels. To investigate whether brain prolactin plays a role in stress-induced REM sleep rebound two experiments were carried out. In experiment 1, rats were either not sleep-deprived (NSD) or submitted to 96h of REMSD associated or not to FS and brains were evaluated for PRL immunoreactivity (PRL-ir) and determination of PRL concentrations in the lateral hypothalamus and dorsal raphe nucleus. In experiment 2, rats were implanted with cannulas in the dorsal raphe nucleus for prolactin infusion and were sleep-recorded. REMSD associated with FS increased PRL-ir and content in the lateral hypothalamus and all manipulations increased prolactin content in the dorsal raphe nucleus compared to the NSD group. Prolactin infusion in the dorsal raphe nucleus increased the time and length of REM sleep episodes 3h after the infusion until the end of the light phase of the day cycle. Based on these results we concluded that brain prolactin is a major mediator of stress-induced REMS. The effect of PRL infusion in the dorsal raphe nucleus is discussed in light of the existence of a bidirectional relationship between this hormone and serotonin as regulators of stress-induced REM sleep rebound.

Neuroendocrine and Peptidergic Regulation of Stress-Induced REM Sleep Rebound

Sleep homeostasis depends on the length and quality (occurrence of stressful events, for instance) of the preceding waking time. Forced wakefulness (sleep deprivation or sleep restriction) is one of the main tools used for the understanding of mechanisms that play a role in homeostatic processes involved in sleep regulation and their interrelations. Interestingly, forced wakefulness for periods longer than 24 h activates stress response systems, whereas stressful events impact on sleep pattern. Hypothalamic peptides (corticotropin-releasing hormone, prolactin, and the CLIP/ACTH18-39) play an important role in the expression of stress-induced sleep effects, essentially by modulating rapid eye movement sleep, which has been claimed to affect the organism resilience to the deleterious effects of stress. Some of the mechanisms involved in the generation and regulation of sleep and the main peptides/hypothalamic hormones involved in these responses will be discussed in this review.

Stress, arousal, and sleep

Stress is considered to be an important cause of disrupted sleep and insomnia. However, controlled and experimental studies in rodents indicate that effects of stress on sleep-wake regulation are complex and may strongly depend on the nature of the stressor. While most stressors are associated with at least a brief period of arousal and wakefulness, the subsequent amount and architecture of recovery sleep can vary dramatically across conditions even though classical markers of acute stress such as corticosterone are virtually the same. Sleep after stress appears to be highly influenced by situational variables including whether the stressor was controllable and/or predictable, whether the individual had the possibility to learn and adapt, and by the relative resilience and vulnerability of the individual experiencing stress. There are multiple brain regions and neurochemical systems linking stress and sleep, and the specific balance and interactions between these systems may ultimately determine the alterations in sleep-wake architecture. Factors that appear to play an important role in stress-induced wakefulness and sleep changes include various monominergic neurotransmitters, hypocretins, corticotropin releasing factor, and prolactin. In addition to the brain regions directly involved in stress responses such as the hypothalamus, the locus coeruleus, and the amygdala, differential effects of stressor controllability on behavior and sleep may be mediated by the medial prefrontal cortex. These various brain regions interact and influence each other and in turn affect the activity of sleep-wake controlling centers in the brain. Also, these regions likely play significant roles in memory processes and participate in the way stressful memories may affect arousal and sleep. Finally, stress-induced changes in sleep-architecture may affect sleep-related neuronal plasticity processes and thereby contribute to cognitive dysfunction and psychiatric disorders.

A dopamine receptor d2-type agonist attenuates the ability of stress to alter sleep in mice

Although sleep disruptions that accompany stress reduce quality of life and deteriorate health, the mechanisms through which stress alters sleep remain obscure. Psychological stress can alter sleep in a variety of ways, but it has been shown to be particularly influential on rapid eye movement (REM) sleep. Prolactin (PRL), a sexually dimorphic, stress-sensitive hormone whose basal levels are higher in females, has somnogenic effects on REM sleep. In the current study, we examined the relationship between PRL secretion and REM sleep after restraint stress to determine whether: 1) the ability of stress to increase REM sleep is PRL-dependent, and 2) fluctuating PRL levels underlie sex differences in sleep responses to stress. Because dopamine D2 receptors in the pituitary gland are the primary regulator of PRL secretion, D2 receptor agonist, 1-[(6-allylergolin-8β-yl)-carbonyl]-1-[3-(dimethylamino) propyl]-3-ethylurea (cabergoline), was used to attenuate PRL levels in mice before 1 hour of restraint stress. Mice were implanted with electroencephalographic/electromyographic recording electrodes and received an ip injection of either 0.3-mg/kg cabergoline or vehicle before a control procedure of 1 hour of sleep deprivation by gentle handling during the light phase. Six days after the control procedure, mice received cabergoline or vehicle 15 minutes before 1 hour of restraint stress. Cabergoline blocked the ability of restraint stress to increase REM sleep amount in males but did not alter REM sleep amount after stress in females even though it reduced basal REM sleep amount in female controls. These data provide evidence that the ability for restraint stress to increase REM sleep is dependent on PRL and that sex differences in REM sleep amount may be driven by PRL.

REM Sleep Rebound as an Adaptive Response to Stressful Situations

Stress and sleep are related to each other in a bidirectional way. If on one hand poor or inadequate sleep exacerbates emotional, behavioral, and stress-related responses, on the other hand acute stress induces sleep rebound, most likely as a way to cope with the adverse stimuli. Chronic, as opposed to acute, stress impairs sleep and has been claimed to be one of the triggering factors of emotional-related sleep disorders, such as insomnia, depressive- and anxiety-disorders. These outcomes are dependent on individual psychobiological characteristics, conferring even more complexity to the stress-sleep relationship. Its neurobiology has only recently begun to be explored, through animal models, which are also valuable for the development of potential therapeutic agents and preventive actions. This review seeks to present data on the effects of stress on sleep and the different approaches used to study this relationship as well as possible neurobiological underpinnings and mechanisms involved. The results of numerous studies in humans and animals indicate that increased sleep, especially the rapid eye movement phase, following a stressful situation is an important adaptive behavior for recovery. However, this endogenous advantage appears to be impaired in human beings and rodent strains that exhibit high levels of anxiety and anxiety-like behavior.

Decreased REM sleep and altered circadian sleep regulation in mice lacking vasoactive intestinal polypeptide

OBJECTIVES

Vasoactive intestinal polypeptide (VIP) has been implicated in sleep regulation as a promoter of rapid eye movement (REM) sleep. Previous work has shown that the amount of time spent in REM sleep is increased by intracerebroventricular administration of VIP, and reduced by treatment with VIP antagonists or antibodies against VIP. A variety of evidence suggests that VIP is critical for normal expression of circadian rhythmicity of diverse physiological and behavioral parameters. In the present study, we investigated the role of this peptide in sleep regulation using VIP-deficient (VIP-/-) mice.

METHODS

EEG/EMG sleep-wake patterns were recorded in VIP-/- mice and their wild-type littermate controls under normal light-dark (LD), constant darkness (DD) and sleep deprivation conditions.

RESULTS

VIP-/- mice exhibited reduced REM sleep time over the 24-h cycle while total daily amounts of NREM sleep and wakefulness were not altered significantly. The reduced REM sleep time in VIP-/- mice occurred entirely during the day due to a reduction in the duration, but not the frequency, of REM sleep bouts. In response to sleep deprivation, compensatory rebounds in NREM sleep and REM sleep were also attenuated in VIP-/- mice. Finally, the loss of VIP altered the temporal distribution of sleep in that the VIP -/- mice exhibited smaller amplitude rhythms in total sleep, NREM sleep, and REM sleep under both LD and DD.

CONCLUSIONS

These results indicate that VIP regulates the duration of REM sleep, sleep homeostatic mechanisms as well as the temporal patterning of sleep.

Neurochemical regulation of sleep

This review summarizes recent developments in the field of sleep regulation, particularly in the role of hormones, and of synthetic GABA(A) receptor agonists. Certain hormones play a specific role in sleep regulation. A reciprocal interaction of the neuropeptides growth hormone (GH)-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. At least in males GHRH is a common stimulus of non-rapid-eye-movement sleep (NREMS) and GH and inhibits the hypothalamo-pituitary adrenocortical (HPA) hormones, whereas CRH exerts opposite effects. Furthermore CRH may enhance rapid-eye-movement sleep (REMS). Changes in the GHRH:CRH ratio in favor of CRH appear to contribute to sleep EEG and endocrine changes during depression and normal ageing. In women, however, CRH-like effects of GHRH were found. Besides CRH somatostatin impairs sleep, whereas ghrelin, galanin and neuropeptide Y promote sleep. Vasoactive intestinal polypeptide appears to be involved in the temporal organization of human sleep. Beside of peptides, steroids participate in sleep regulation. Cortisol appears to promote REMS. Various neuroactive steroids exert specific effects on sleep. The beneficial effect of estrogen replacement therapy in menopausal women suggests a role of estrogen in sleep regulation. The GABA(A) receptor or GABAergic neurons are involved in the action of many of these hormones. Recently synthetic GABA(A) agonists, particularly gaboxadol and the GABA reuptake inhibitor tiagabine were shown to differ distinctly in their action from allosteric modulators of the GABA(A) receptor like benzodiazepines as they promote slow-wave sleep, decrease wakefulness and do not affect REMS.

Neuropeptides as possible targets in sleep disorders

Insomnia and hypersomnia are frequent sleep disorders, and they are most often treated pharmacologically with hypnotics and wake-promoting compounds. These compounds act on classical neurotransmitter systems, such as benzodiazepines on GABA-A receptors, and amfetamine-like stimulants on monoaminergic terminals to modulate neurotransmission. In addition, acetylcholine, amino acids, lipids and proteins (cytokines) and peptides, are known to significantly modulate sleep and are, therefore, possibly involved in the pathophysiology of some sleep disorders. Due to the recent developments of molecular biological techniques, many neuropeptides have been newly identified, and some are found to significantly modulate sleep. It was also discovered that the impairment of the hypocretin/orexin neurotransmission (a recently isolated hypothalamic neuropeptide system) is the major pathophysiology of narcolepsy, and hypocretin replacement therapy is anticipated to treat the disease in humans. In this article, the authors briefly review the history of neuropeptide research, followed by the sleep modulatory effects of various neuropeptides. Finally, general strategies for the pharmacological therapeutics targeting the peptidergic systems for sleep disorders are discussed.

Roles of peptides and steroids in sleep disorders

A bidirectional interaction exists between the electrophysiological and neuroendocrine components of sleep. The first is represented by the nonrapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) cycles, the latter by distinct patterns of the secretion of various hormones. Certain hormones (neuropeptides and steroids) play a specific role in sleep regulation. Changes in their activity contribute to the pathophysiology of sleep disorders. A reciprocal interaction of the peptides growth hormone-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. GHRH promotes growth hormone secretion and, at least in males, NREMS, whereas CRH impairs NREMS, promotes REMS and stimulates the secretion of adrenocorticotropic hormone and cortisol. Changes in the CRH:GHRH ratio in favor of CRH contribute to impaired sleep, elevated cortisol secretion and blunted GH levels during depression and normal aging. However, in women, GHRH exerts CRH-like effects. Galanin, ghrelin and neuropeptide Y are other sleep-promoting peptides, whereas somatostatin impairs sleep. A decline of orexin activity causes narcolepsy. In addition to CRH overactivity, hypercortisolism appears to be involved in the pathophysiology of sleep- electroencephalogram (EEG) changes in depression. Various neuroactive steroids exert specific effects on sleep. The changes of sleep EEG in women after the menopause are related to the decline of estrogen and progesterone. Furthermore, sleep-EEG changes in dwarfism, acromegaly, Addison's disease, Cushing's disease, brain injury, sleep apnea syndrome, primary insomnia, prolactinoma and dementia appear to be related to changes in the activity of peptides and steroids.

Rapid eye movement sleep is reduced in prolactin-deficient mice

Prolactin (PRL) is implicated in the modulation of spontaneous rapid eye movement sleep (REMS). Previous models of hypoprolactinemic animals were characterized by changes in REMS, although associated deficits made it difficult to ascribe changes in REMS to reduced PRL. In the current studies, male PRL knock-out (KO) mice were used; these mice lack functional PRL but have no known additional deficits. Spontaneous REMS was reduced in the PRL KO mice compared with wild-type or heterozygous littermates. Infusion of PRL for 11-12 d into PRL KO mice restored their REMS to that occurring in wild-type or heterozygous controls. Six hours of sleep deprivation induced a non-REMS and a REMS rebound in both PRL KO mice and heterozygous littermates, although the REMS rebound in the KOs was substantially less. Vasoactive intestinal peptide (VIP) induced REMS responses in heterozygous mice but not in KO mice. Similarly, an ether stressor failed to enhance REMS in the PRL KOs but did in heterozygous littermates. Finally, hypothalamic mRNA levels for PRL, VIP, neural nitric oxide synthase (NOS), inducible NOS, and the interferon type I receptor were similar in KO and heterozygous mice. In contrast, tyrosine hydroxylase mRNA was lower in the PRL KO mice than in heterozygous controls and was restored to control values by infusion of PRL, suggesting a functioning short-loop negative feedback regulation in PRL KO mice. Data support the notion that PRL is involved in REMS regulation.

The effect of restraint stress on paradoxical sleep is influenced by the circadian cycle

It is well known that the physiological impact imposed by events or behaviors displayed during the waking period determines the way organisms sleep. Among the situations known to affect sleep both in its duration and quality, stress has been widely studied and it is now admitted that its effects on sleep architecture depend on several factors specific to the stressor or the individual itself. Although numerous reports have highlighted the prominent role of the circadian cycle in the physiological, endocrine and behavioral consequences of restraint stress, a possible circadian influence in the effects of stress on the sleep-wake cycle has never been studied. Thus the present study was designed to compare the effects on sleep of a 1 h-lasting restraint stress applied at light onset to those observed after the same stressor was applied at light offset. We report that in both conditions stress induced a marked paradoxical sleep increase, whereas wakefulness displayed a moderate decrease and slow wave sleep a moderate augmentation. Although the effects of stress at lights on were of similar magnitude than those of stress at lights off, important differences in the sleep rebound latencies were observed: whatever the time of day the stress was applied, its effects on sleep always occurred during the dark period. This result thus shows that restraint stress could be efficiently used to study the interaction between the circadian and homeostatic components of sleep regulation.

An ether stressor increases REM sleep in rats: possible role of prolactin

Sleep alterations after a 1-min exposure to ether vapor were studied in rats to determine if this stressor increases rapid eye-movement (REM) sleep as does an immobilization stressor. Ether exposure before light onset or dark onset was followed by significant increases in REM sleep starting approximately 3-4 h later and lasting for several hours. Non-REM (NREM) sleep and electroencephalographic slow-wave activity during NREM sleep were not altered. Exposure to ether vapor elicited prolactin (Prl) secretion. REM sleep was not promoted after ether exposure in hypophysectomized rats. If the hypophysectomy was partial and the rats secreted Prl after ether exposure, then increases in REM sleep were observed. Intracerebroventricular administration of an antiserum to Prl decreased spontaneous REM sleep and inhibited ether exposure-induced REM sleep. The results indicate that a brief exposure to ether vapor is followed by increases in REM sleep if the Prl response associated with stress is unimpaired. This suggests that Prl, which is a previously documented REM sleep-promoting hormone, may contribute to the stimulation of REM sleep after ether exposure.

Hypoprolactinemic rats under conditions of constant darkness or constant light. Effects on the sleep-wake cycle, cerebral temperature and sulfatoxymelatonin levels

In genetic hypoprolactinemic rats under light-dark (LD) conditions, the circadian rhythms of slow-wave (SWS) and paradoxical (PS) sleep display an alteration of their phase relationship. The aim of our study was to investigate the effects of constant darkness (DD) or constant light (LL) on the daily distribution and amounts of sleep-wake stages, cerebral temperature and concentrations of the urinary melatonin metabolite, 6-sulfatoxymelatonin, in prolactin-deficient rats. After 3 weeks of DD, the SWS period was 24 h 8+/-6 min and the acrophase occurred at 15:44+/-1:35, while for PS, the period was more stable than during LD (24 h 10+/-8 min vs. 24 h 55+/-43 min) and the acrophase occurred at 16:44+/-1:54. Under LL conditions, circadian sleep rhythms persisted during the first 3 days, then completely disappeared during the third week, to be replaced by ultradian rhythms (period of 4-6 h). Time-series analysis showed that the two sleep states became synchronous as early as the second day under constant conditions. The total amount of PS was increased under both conditions (LL and DD) at the expense of duration of waking. Under LD and constant conditions, the pattern of changes in cerebral temperature was similar to that for wakefulness (W). Sulfatoxymelatonin was rhythmically secreted under both LD and DD conditions, whereas, under LL conditions, its rhythm was abolished. The results show that, in IPL rats in the absence of a zeitgeber, the PS and SWS rhythms recover a synchronous phase relationship and PS amounts are increased.

Endogenous and exogenous factors on sleep-wake cycle regulation

A number of theories have proposed the involvement of different brain structures and neurotransmitters in order to explain the regulation of the sleep wake cycle. However, there is no clear consensus as to the mechanisms through which the brain structures and their various neurotransmitters interact to produce theses phases. Perhaps the problem is related to the fact sleep is a very fragile state, easily modified or influenced by a variety of substances or experimental manipulations. In this paper, we describe the evidence of two different groups of factors that induce important changes on the sleep wake cycle. The endogenous factors: neurotransmitters; hormone; peptides; and some substances of lipidic nature and exogenous factors: stress, food intake, learning, sleep deprivation, sensorial stimulation, exercise and temperature on the regulation the sleep-wake cycle. Likewise, we propose a hypothesis which attempts to reconcile the fact that endogenous and exogenous factors have similar effects.

Enhanced slow wave sleep in patients with prolactinoma

Bidirectional interactions between nocturnal hormone secretion and sleep regulation are well established. In particular, a link between PRL and rapid eye movement (REM) sleep has been hypothesized. Short-term administration of PRL and even long-term hyperprolactinemia in animals increases REM sleep. Furthermore, sleep disorders are frequent symptoms in patients with endocrine diseases. We compared the sleep electroencephalogram of seven drug-free patients with prolactinoma (mean PRL levels 1450 +/- 1810 ng/mL; range between 146 and 5106 ng/mL) with that of matched controls. The patients had secondary hypogonadism but no other endocrine abnormalities. They spent more time in slow wave sleep than the controls (79.4 +/- 54.4 min in patients vs. 36.6 +/- 23.5 min in controls, P < 0.05). REM sleep variables did not differ between the samples. Our data suggest that chronic excessive enhancement of PRL levels exerts influences on the sleep electroencephalogram in humans. Our result, which seems to be in contrast to the enhanced REM sleep under hyperprolactinemia in rats, leads to the hypothesis that both slow wave sleep and REM sleep can be stimulated by PRL. These findings are in accordance with reports of good sleep quality in patients with prolactinoma, which is in contrast to that of patients with other endocrine diseases.

Sleep in rats rendered chronically hyperprolactinemic with anterior pituitary grafts

A hyperprolactinemic rat model [rats bearing anterior pituitary grafts under the capsule of the kidney (AP-grafted rats)] was used to study sleep-wake activity and cortical brain temperature (T(crt)). Fisher 344 male rats (n = 24) were implanted with anterior pituitaries from rat pups; the control rats (n = 12) were sham-operated. Sleep-wake activity and T(crt) were recorded for 2 days between weeks 3 and 7 after surgery. The hyperprolactinemic state of the rats was confirmed by plasma prolactin (PRL) assays on week 7 and by determination of PRL mRNA levels in the anterior pituitary of the AP-grafted rats. Neither growth hormone plasma concentration nor pituitary mRNA levels were affected by the pituitary grafts. Duration of non-rapid eye movement sleep (NREMS) was slightly enhanced in the AP-grafted rats. A large increase in rapid eye movement sleep (REMS) during the 12-h light period was the major effect of the implantation of the extra pituitaries. Both the duration and the frequency of the REMS episodes increased and persisted for weeks 4-7 post-implantation. The nocturnal states of vigilance, T(crt), and intensity of NREMS (EEG slow wave activity) were not altered. The results clearly indicate that the enhancements in REMS persist during hyperprolactinemia, and support the hypothesis that PRL possesses REMS-promoting activity.

Pituitary adenylate cyclase activating polypeptide enhances rapid eye movement sleep in rats

Pituitary adenylate cyclase activating polypeptide (PACAP), a member of the vasoactive intestinal polypeptide family, was tested for its effects on sleep in adult male Sprague-Dawley rats. PACAP was injected via intracerebro-ventricular cannula at light or dark onset; sleep and brain temperature (Tbr) were recorded for 12 h after injection. Rapid eye movement sleep (REMS) was significantly enhanced by 30 pmol, but not 3 or 300 pmol PACAP injected at dark onset. Non-REMS was not influenced by 3, 30, or 300 pmol PACAP Sleep and Tbr were not influenced by 3 or 30 pmol PACAP injected at light onset.

Neurobiological correlates of masculine sexual behavior

The experimental analysis of the neuroendocrine interactions regulating sexual behavior has traditionally relied on studying the effects of CNS lesions and pharmacological treatments with hormones or drugs purportedly acting through specific neurotransmitter systems. New methodological developments have allowed the assessment of several indices of neural function in experimental animals, particularly the rat, as they relate to behavioral changes. In the field of sexual behavior, ex vivo analyses have been used to measure markers of energy metabolism, such as 2-deoxyglucose uptake and Na,K-ATPase activity, the tissue content of neurotransmitters and metabolites, the levels of steroid receptors and neurosteroids, and immediate-early gene expression products in different areas of the CNS. In vivo studies have monitored brain electrical activity and temperature, as well as the extracellular levels of neurotransmitters and metabolites by cerebrospinal fluid sampling, push-pull perfusion and, especially, electrochemical recordings and microdialysis, in the course of mating and exposure to various relevant stimuli. The findings with the different methodologies are generally consistent and agree with those of previous surgical and pharmacological manipulations. They provide data on temporal relationships between neurobiological and behavioral events and suggest new interpretations for different aspects of the male copulatory pattern.

Increase of prolactin mRNA in the rat hypothalamus after intracerebroventricular injection of VIP or PACAP

Vasoactive intestinal peptide (VIP), the structurally homologous pituitary adenylate cyclase-activating peptide (PACAP) and the pituitary hormone, prolactin (PRL) enhance rapid eye movement sleep (REMS). VIP and PACAP are both inducers of PRL gene expression and release in the pituitary gland. Little is known about PRL regulation in the brain although it is hypothesized that the REMS-promoting activity of i.c.v. administered VIP may be mediated via the activation of cerebral PRL. To test whether VIP or PACAP in fact increase intracerebral mRNA, the peptides (VIP: 30 or 300 pmol; PACAP: 220 pmol) were injected i.c.v. into rats at dark onset. 1 h later, cDNA was synthesized from purified hypothalamic mRNA. Standardized amounts were analysed for PRL using the polymerase chain reaction followed by Southern blotting and hybridization. Compared with beta-actin mRNA levels, both VIP and PACAP increased PRL mRNA levels in a dose-dependent fashion though VIP was more effective on a molar basis. The previously reported alternatively spliced PRL mRNA (lacking exon 4) was not detected. The data support the hypothesis that the REMS-promoting activity of central VIP and PACAP might be mediated by cerebral PRL.

Involvement of prolactin in the REM sleep-promoting activity of systemic vasoactive intestinal peptide (VIP)

The involvement of pituitary prolactin (PRL) in systemic vasoactive intestinal peptide (VIP)-induced sleep was studied. Male rats were implanted with electrodes for EEG-recording, with brain thermistors to record cortical temperature (Tcrt) and with chronic intracardial catheters to obtain blood samples and to deliver substances. One group of rats (n = 8) received normal rabbit serum (NS)+physiological saline (SAL) on the baseline day and was injected with NS+VIP on the experimental day. In the other group of rats (n = 6), the baseline day was followed by administration of PRL-antiserum (PRL-AS) + VIP on the experimental day. The sera and VIP or SAL were injected 30 min before and at light onset, respectively. Sleep-wake activity was then recorded for the next 12-h light period. Systemic VIP-stimulated PRL secretion as measured by RIA in serial samples obtained hour 1 postinjection. VIP also elicited selective increases in REM sleep (REMS) in the rats pretreated with NS. Tcrt was not affected by VIP. Administration of PRL-AS blocked the increase in circulating levels of free (non-IgG-bound) PRL and prevented VIP-enhanced REMS. Comparisons of the sleep effects of PRL-AS+VIP with the previously reported changes in sleep after PRL-AS alone indicate that PRL has a major role in the mediation of the REMS-promoting activity of systemic VIP. The results suggest that an increased release of endogenous pituitary PRL modulates REMS.