Demonstration of an orexinergic central innervation of the pineal gland of the pig

Abstract

Orexins/hypocretins, two isoforms of the same prepropeptide, are widely distributed throughout the brain and are involved in several physiological and neuroendocrine regulatory patterns, mostly related to feeding, sleep, arousal, and cyclic sleep-wake behaviors. Orexin-A and orexin-B bind with different affinities to two G-protein-coupled transmembrane receptors, orexin-1 and orexin-2 receptors (OR-R1 and OR-R2, respectively). Because of the similarities between the human and the swine brain, we have studied the pig to investigate the orexinergic system in the diencephalon, with special emphasis on the neuroanatomical projections to the epithalamic region. By using antibodies against orexin-A and orexin-B, immunoreactive large multipolar perikarya were detected in the hypothalamic periventricular and perifornical areas at the light and electron microscopic levels. In the region of the paraventricular nucleus, the orexinergic neurons extended all the way to the lateral hypothalamic area. Immunoreactive nerve fibers, often endowed with large varicosities, were found throughout the hypothalamus and the epithalamus. Some periventricular immunoreactive nerve fibers entered the epithalamic region and continued into the pineal stalk and parenchyma to disperse among the pinealocytes. Immunoelectron microscopy confirmed the presence of orexinergic nerve fibers in the pig pineal gland. After extraction of total mRNA from the hypothalamus and pineal gland, we performed RT-PCR and nested PCR using primers specific for porcine orexin receptors. PCR products were sequenced, verifying the presence of both OR-R1 and OR-R2 in the tissues investigated. These findings, supported by previous studies on rodents, suggest a hypothalamic regulation of the pineal gland via central orexinergic nervous inputs.

Sleep–wake regulation and hypocretin–melatonin interaction in zebrafish

“…In sheep (28), rat (29), and pig (30), HCRT fibers and/or HCRT receptors were indeed also found in the pineal gland, demonstrating conservation across diurnal and nocturnal vertebrates. In all vertebrates, diurnal or nocturnal, melatonin is secreted in the dark by the pineal gland and controls daily and seasonal changes in physiology (18,19).…”

Hypocretins: The Timing of Sleep and Waking

“…Thus, the hypocretin and histamine systems seem to play specific roles in the expression of circadian rhythms. Hypocretins exert dose-dependent effects on the discharge rate of SCN neurons in vitro (Farkas et al, 2002) and mediate signals to the pineal gland (Mikkelsen et al, 2001), which contains high densities of hypocretin fibers and receptors (Fabris et al, 2004). The levels of hypocretin-1 in the brain vary according to behavioral state, and lesions of the SCN eliminate the daily rhythm of hypocretin-1 release (Mignot et al, 2002;Siegel, 2004).…”

Altered Circadian Rhythm of Melatonin Concentrations in Hypocretin-Deficient Men

“…Given the recent indications for a hypocretinergic innervation of the pineal gland, we hypothesized that melatonin secretion and/or its circadian rhythmicity might be altered in hypocretin-deficient narcolepsy patients, and that SXB might reverse these alterations (Appelbaum et al, 2009(Appelbaum et al, , 2010Fabris et al, 2004;Mikkelsen et al, 2001;Zhang et al, 2005). To evaluate this hypothesis, we performed 24-h blood sampling studies in combination with polysomnography on a group of hypocretin-deficient narcolepsy patients and rigorously matched healthy controls, both before and after SXB administration.…”

“…Specialized melanopsin-containing cells in the retina detect a narrow band of blue wavelengths and subsequently signal the SCN, resulting in melatonin suppression during exposure to (day)light (Brainard et al, 2001). Furthermore, accumulating evidence derived from experiments on various animal models indicates that the hypocretinergic innervation of the pineal gland could also be important for the regulation of diurnal melatonin synthesis and secretion (Appelbaum et al, 2009(Appelbaum et al, , 2010Fabris et al, 2004;Mikkelsen et al, 2001;Zhang et al, 2005).…”