Reversal of desipramine-induced suppression of paradoxical sleep by a long-acting somatostatin analogue (octreotide) in rats

Cross-talk between orexins (hypocretins) and the neuroendocrine axes (hypothalamic-pituitary axes)

Lesioning and electrical stimulation experiments carried out during the first half of the twentieth century showed that the lateral hypothalamic area (LHA) is involved in the neuroendocrine control of hormone secretion. However, the molecular basis of this phenomenon remained unclear until fifty years later when in 1998, two different laboratories discovered a new family of hypothalamic neuropeptides, the orexins or hypocretins (OX-A/Hcrt1 and OX-B/Hcrt2). Since then, remarkable evidence has revealed that orexins/hypocretins play a prominent role in regulating virtually all the neuroendocrine axes, acting as pivotal signals in the coordination of endocrine responses with regards to sleep, arousal and energy homeostasis. The clinical relevance of these actions is supported by human data showing impairment of virtually all the neuroendocrine axes in orexin/hypocretin-deficient narcoleptic patients. Here, we summarize more than ten years of knowledge about the orexins/hypocretins with particular focus on their role as neuroendocrine regulators. Understanding this aspect of orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and endocrine pathologies.

Orexins (hypocretins) actions on the GHRH/somatostatin-GH axis

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system that includes two major hypothalamic regulators, namely GH-releasing hormone (GHRH) and somatostatin (SST) that stimulate and inhibit, respectively, GH release. Classical experiments involving damage and electrical stimulation suggested that the lateral hypothalamic area (LHA) modulated the somatotropic axis, but the responsible molecular mechanisms were unclear. Evidence obtained during the last decade has demonstrated that orexins/hypocretins, a family of peptides expressed in the LHA controlling feeding and sleep, play an important regulatory role on GH, by inhibiting its secretion modulating GHRH and SST neurones. Considering that GH release is closely linked to the sleep-wake cycle and feeding state, understanding orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and GH-related pathologies, such as GH deficiency.

The effects of serotoninergic, noradrenergic, cholinergic and dopaminergic drugs on vigilance states in MPTP-treated mice

Sleep/wakefulness disorders are frequent in Parkinson's disease. Although the causes have yet to be established, it is known that dopaminergic neuronal lesions modulate paradoxical sleep (PS) regulation structures containing serotonin, noradrenaline and acetylcholine. Our previous vigilance state studies have revealed an increase in the amount of PS over the nyctohemeral period in the MPTP-treated mouse model of Parkinson's disease. The aim of the present work was to compare the effect of drugs modulating serotonin (citalopram), noradrenaline (desipramine), acetylcholine (arecoline) and dopamine (GBR 12909) neurotransmission on sleep/wakefulness patterns in MPTP mice and control mice. Citalopram reduced the amount of PS in MPTP and control mice to the same extent. Desipramine also induced a PS reduction, which was less pronounced in MPTP mice than in control mice. Arecoline increased the amount of PS in MPTP mice but not in controls. GBR 12909 induced a PS reduction (for the highest dose) more pronounced in MPTP mice than in control animals. Given that the responsiveness of MPTP mice differs markedly from that of controls, our study suggests that MPTP can alter sleep/wakefulness neurotransmission systems. Dysfunction of the latter may be responsible for PS disorders in MPTP mice.

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.

Orexin-A regulates growth hormone-releasing hormone mRNA content in a nucleus-specific manner and somatostatin mRNA content in a growth hormone-dependent fashion in the rat hypothalamus

The orexins or hypocretins are two neuropeptides involved in the regulation of diverse biological processes such as feeding, sleep and neuroendocrine function. Recent findings suggest a possible functional interaction between orexins, somatostatin and growth hormone-releasing hormone (GHRH) in the rat hypothalamus. In order to understand the possible functional linkage between orexins and these neuropeptides, we determined the effects of intracerebroventricular orexin-A administration on hypothalamic somatostatin and GHRH mRNA levels. Furthermore, we examined whether growth hormone (GH) mediates these interactions by using two animal models that showed GH deficiency: hypophysectomized rats and dwarf Lewis rats. Using in situ hybridization, our data showed that GHRH mRNA levels in the paraventricular nucleus of the hypothalamus are decreased after orexin-A treatment, without changes in the arcuate nucleus of the hypothalamus. On the other hand, orexin-A treatment induces a GH-dependent stimulatory effect on somatostatin mRNA content in the periventricular nucleus of the hypothalamus. Finally, we demonstrated, for the first time, that hypophysectomized rats and dwarf Lewis rats, two classical models of GH deficiency with alterations in sleep patterns, showed a marked reduction in the GHRH mRNA levels in the paraventricular nucleus of the hypothalamus. These data improve our understanding of the interactions among the different systems involved in the control and pathophysiology of food intake, sleep and GH secretion.

The somatostatin analogue octreotide impairs sleep and decreases EEG sigma power in young male subjects

The long-acting somatostatin (SRIF) analogue octreotide decreased nonrapid eye movement sleep (NREMS) in the rat. This effect is opposite to the promotion of sleep after growth hormone (GH)-releasing hormone (GHRH) in various species including humans. Therefore, it appears likely that GHRH and SRIF, besides their opposite action on pituitary GH release, interact reciprocally in sleep regulation. In previous studies, SRIF impaired sleep in elderly subjects, although sleep in young men remained unchanged. We hypothesized that octreotide is a useful tool to study the role of SRIF in human sleep regulation. We examined the effect of subcutaneous administration of 0.1 mg octreotide at 2245 on the sleep EEG of seven young male controls (age, mean+/-SD, 22.3+/-3.0 years). In comparison to placebo, octreotide administration prompted decreases of sleep stage 4 during the total night and of rapid eye movement sleep (REMS) density during the first half of the night. Intermittent wakefulness increased during the second half of the night. The spectral analysis of total night NREMS revealed a significant decrease of sigma power. Similar to the effect of the short-acting SRIF in the elderly, the long-acting SRIF analogue octreotide impaired sleep in young healthy subjects. Obviously, the influence of octreotide on sleep is superior to that of short-acting SRIF, which did not affect sleep in young men. We suggest a reciprocal interaction of GHRH and SRIF in sleep regulation.

Intracerebroventricular and locus coeruleus microinjections of somatostatin antagonist decrease REM sleep in rats

In order to study the role of endogenous somatostatin in the physiologic modulation of REM sleep (REMS), we measured the effect of intracerebroventricular (ICV) injection of somatostatin antagonist (SA) cyclo-(7-aminoheptanoyl-phe-d-trp-lys-thr(bzl)) on sleep in rats. The effect of ICV SA was also tested after 24-h REMS deprivation with the platform method. To study the role of locus coeruleus (LC) as a site of the sleep inducing action for somatostatin and galanin we microinjected SA, somatostatin, and galanin locally into LC. In all experiments, vigilance state was analyzed visually from 6 h post-injection EEG/EMG recording. Injection of 0.5 and 2 nmol of SA ICV reduced spontaneous REMS and 2 nmol dose reduced also rebound REMS after REMS deprivation when compared with controls (artificial cerebrospinal fluid vehicle). Microinjection of 0.25 nmol of SA into LC reduced REMS, whereas microinjection of somatostatin, galanin, and a combined injection of them were not effective to induce REMS. The results suggest that endogenous somatostatin may contribute to facilitation of REMS. Somatostatin receptors in the LC may be one possible mediator of this effect.

Central administration of the somatostatin analog octreotide induces captopril-insensitive sleep responses

The effects of intracerebroventricular injections of the long-lasting somatostatin analog octreotide (Oct) were studied on sleep and behavior in rats. Pyrogen-free physiological saline and Oct (0.001, 0.01, 0.1 microgram) or vehicle were administered at light onset, and the electroencephalogram (EEG), motor activity, and cortical brain temperature were recorded during the 12-h light period. Plasma growth hormone (GH) concentrations were measured in samples taken at 30-min intervals after Oct. Oct (0.01 and 0.1 microgram) suppressed non-rapid eye movement sleep (NREMS) for 1-2 h. NREMS intensity (delta EEG activity during NREMS) dose dependently increased in hour 3 postinjection and thereafter (0.1 microgram). Plasma GH concentrations were suppressed after Oct (0.01 and 0.1 microgram), but pulses of GH secretions occurred 90-120 min postinjection in each rat. Oct (0.1 microgram) enhanced behavioral activity, including prompt drinking followed by grooming, scratching, and feeding. Intracerebroventricular injection of the angiotensin-converting enzyme inhibitor captopril (30 microgram, 10 min before Oct), blocked these behavioral responses but not the Oct-induced sleep alterations. The changes in sleep after intracerebroventricular Oct suggest an intracerebral action site and might result from Oct-induced variations in the sleep-promoting activity of GH-releasing hormone.

Changes in rat sleep after single and repeated injections of the long-acting somatostatin analog octreotide

Somnogenic activity is attributed to both growth hormone (GH) and GH-releasing hormone (GHRH). The aim of our experiments was to study sleep after suppression of the somatotropic axis by means of administration of a long-lasting somatostatin analog, octreotide. Rats received subcutaneous injections of physiological saline (baseline), octreotide (1, 10, and 200 microg/kg), or a control solution just before light onset, and sleep-wake activity and cortical brain temperature were recorded for 23 h. Each dose of octreotide slightly promoted rapid eye movement sleep (REMS) during the 12-h light period. Non-REM sleep (NREMS) was strongly suppressed for 1 h in response to 10 and 200 microg/kg octreotide. This was followed by slight increases in NREMS time and significant enhancements in electroencephalogram slow-wave activity during NREMS after 200 microg/kg octreotide. The octreotide-induced inhibition of the somatotropic axis, as determined by plasma GH levels, vanished by the time sleep increased. Another group of rats received 10 microg/kg octreotide twice a day for 5 days. This treatment elicited persistent decreases in both NREMS time and NREMS intensity. The results support the previously reported REMS-promoting activity of somatostatin in rats. The decreases in sleep after repeated injections of octreotide are attributed to a withdrawal of the normal sleep-promoting activity of GH. The role of GHRH-GH in octreotide-induced acute suppression of NREMS is currently not clear. Other mechanisms, such as mimicking central transmitter functions of somatostatin by octreotide, should also be considered.

Non-endocrine applications of somatostatin and octreotide acetate: facts and flights of fancy

Somatostatin, originally detected by Krulich and ultimately isolated by Brazeau, was initially described as a growth hormone release-inhibiting factor. Subsequent investigation into the use of native somatostatin and the development of long-acting somatostatin analogues, especially octreotide acetate, have fostered increasing uses of these compounds. Though the clinical and investigational uses of somatostatin and its analogues are varied, one central theme remains constant: the ability of these agents to suppress circulating peptide levels. This article, a review of the current non-endocrine applications of somatostatin and its analogues, covers a wide range of potential applications for somatostatin-like compounds. These include use in cirrhosis and variceal bleeding, peptic ulcer disease, pancreatic fistulas, acute and chronic pancreatitis, dumping syndrome, cancer therapy, small bowel fistulas, psoriasis, pain control, and autonomic hypotension. Somatostatin may also play a role in the development and potential treatment of neurologic disease and may have profound found influence on behavior.

Preclinical and clinical studies with somatostatin related to the central nervous system

1. The tetradecapeptide somatostatin (SS) has a widespread, uneven distribution within several organs including the central nervous system (CNS), with particularly high concentration in the hypothalamus. 2. The SS-related peptides (SS28, SS28(1-12), SS28(15-28)) are originated from the precursor pre-prosomatostatin. 3. SS is suggested to be involved in a large number of CNS functions, locomotion, sedation, excitation, catatonia, body temperature, feeding, nociception, paradoxical sleep, self-stimulation, seizure, learning and memory. 4. SS influences central neurochemical processes. 5. It is possible that SS is related to various neurological and psychiatric illnesses, like Huntington's disease, multiple sclerosis, Parkinson's disease, epilepsy, eating disorders, Alzheimer's disease, schizophrenia and major depressive illness.