Organization of the orexin/hypocretin system in the brain of two basal actinopterygian fishes, the cladistians Polypterus senegalus and Erpetoichthys calabaricus

Hypothalamic orexinergic neuron changes during the hibernation of the Syrian hamster

Hibernation in small mammals is a highly regulated process with periods of torpor involving drops in body temperature and metabolic rate, as well as a general decrease in neural activity, all of which proceed alongside complex brain adaptive changes that appear to protect the brain from extreme hypoxia and low temperatures. All these changes are rapidly reversed, with no apparent brain damage occurring, during the short periods of arousal, interspersed during torpor—characterized by transitory and partial rewarming and activity, including sleep activation, and feeding in some species. The orexins are neuropeptides synthesized in hypothalamic neurons that project to multiple brain regions and are known to participate in the regulation of a variety of processes including feeding behavior, the sleep-wake cycle, and autonomic functions such as brown adipose tissue thermogenesis. Using multiple immunohistochemical techniques and quantitative analysis, we have characterized the orexinergic system in the brain of the Syrian hamster—a facultative hibernator. Our results revealed that orexinergic neurons in this species consisted of a neuronal population restricted to the lateral hypothalamic area, whereas orexinergic fibers distribute throughout the rostrocaudal extent of the brain, particularly innervating catecholaminergic and serotonergic neuronal populations. We characterized the changes of orexinergic cells in the different phases of hibernation based on the intensity of immunostaining for the neuronal activity marker C-Fos and orexin A (OXA). During torpor, we found an increase in C-Fos immunostaining intensity in orexinergic neurons, accompanied by a decrease in OXA immunostaining. These changes were accompanied by a volume reduction and a fragmentation of the Golgi apparatus (GA) as well as a decrease in the colocalization of OXA and the GA marker GM-130. Importantly, during arousal, C-Fos and OXA expression in orexinergic neurons was highest and the structural appearance and the volume of the GA along with the colocalization of OXA/GM-130 reverted to euthermic levels. We discuss the involvement of orexinergic cells in the regulation of mammalian hibernation and, in particular, the possibility that the high activation of orexinergic cells during the arousal stage guides the rewarming as well as the feeding and sleep behaviors characteristic of this phase.

Morphological analysis of the cerebellum and its efferent system in a basal actinopterygian fish, Polypterus senegalus

Although all vertebrate cerebella contain granule cells, Purkinje cells, and efferent neurons, the cellular arrangement and neural circuitry are highly diverse. In amniotes, cerebellar efferent neurons form clusters, deep cerebellar nuclei, lie deep in the cerebellum, and receive synaptic inputs from Purkinje cells but not granule cells. However, in the cerebellum of teleosts, the efferent neurons, called eurydendroid cells, lie near the cell bodies of Purkinje cells and receive inputs both from axons of Purkinje cells and granule cell parallel fibers. It is largely unknown how the cerebellar structure evolved in ray-finned fish (actinopterygians). To address this issue, we analyzed the cerebellum of a bichir Polypterus senegalus, one of the most basal actinopterygians. We found that the cell bodies of Purkinje cells are not aligned in a layer; incoming climbing fibers terminate mainly on the basal portion of Purkinje cells, revealing that the Polypterus cerebellum has unique features among vertebrate cerebella. Retrograde labeling and marker analyses of the efferent neurons revealed that their cell bodies lie in restricted granular areas but not as deep cerebellar nuclei in the cerebellar white matter. The efferent neurons have long dendrites like eurydendroid cells, although they do not reach the molecular layer. Our findings suggest that the efferent system of the bichir cerebellum has intermediate features between teleosts and amniote vertebrates, and provides a model to understand the basis generating diversity in actinopterygian cerebella.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.

Spatiotemporal Development of the Orexinergic (Hypocretinergic) System in the Central Nervous System of Xenopus laevis

The present immunohistochemical study represents a detailed spatiotemporal analysis of the localization of orexin-immunoreactive (OX-ir) cells and fibers throughout development in the brain of the anuran amphibian Xenopus laevis, a model frequently used in developmental studies. Anurans undergo remarkable physiological changes during the early life stages, and very little is known about the ontogeny and the localization of the centers that control functions such as appetite and feed ingestion in the developing brain. We examined the onset of the orexinergic system, demonstrated to be involved in appetite regulation, using antibodies against mammalian orexin-A and orexin-B peptides. Simultaneous detection of orexins with other territorial markers was used to assess the precise location of the orexinergic cells in the hypothalamus, analyzed within a segmental paradigm. Double staining of orexins and tyrosine hydroxylase served to evaluate possible interactions with the catecholaminergic systems. At early embryonic stages, the first OX-ir cells were detected in the hypothalamus and, soon after, long descending projections were observed through the brainstem to the spinal cord. As brain development proceeded, the double-staining techniques demonstrated that this OX-ir cell group was located in the suprachiasmatic nucleus within the alar hypothalamus. Throughout larval development, the number of OX-ir cells increased notably and a widespread fiber network that innervated the main areas of the forebrain and brainstem was progressively formed, including innervation in the posterior tubercle and mesencephalon, the locus coeruleus, and the nucleus of the solitary tract where catecholaminergic cells are present. In addition, orexinergic cells were detected in the preoptic area and the tuberal hypothalamus only at late prometamorphic stages. The final distribution pattern, largely similar to that of the adult, was achieved through metamorphic climax. The early expression of orexins in Xenopus suggests important roles in brain development in the embryonic period before feeding, and the progression of the temporal and spatial complexity of the orexinergic system might be correlated to the maturation of appetite control regulation, among other functions.

Organization of the nitrergic neuronal system in the primitive bony fishes Polypterus senegalus and Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Cladistians are a group of basal actinopterygian fishes that constitute a good model for studying primitive brain features, most likely present in the ancestral bony fishes. The analysis of the nitrergic neurons (with the enzyme nitric oxide synthase; NOS) has helped in understanding important aspects of brain organization in all vertebrates studied. We investigated the nitrergic system of two cladistian species by means of specific antibodies against NOS and NADPH-diaphorase (NADPH-d) histochemistry, which, with the exception of the primary olfactory and terminal nerve fibers, labeled only for NADPH-d, yielded identical results. Double immunohistochemistry was conducted for simultaneous detection of NOS with tyrosine hydroxylase, choline acetyltransferase, calbindin, calretinin, and serotonin, to establish accurately the localization of the nitrergic neurons and fibers and to assess possible interactions between these neuroactive substances. The pattern of distribution in both species showed only subtle differences in the density of labeled cells. Distinct groups of NOS-immunoreactive cells were observed in pallial and subpallial areas, paraventricular region, tuberal and retromammillary hypothalamic areas, posterior tubercle, prethalamic and thalamic areas, optic tectum, torus semicircularis, mesencephalic tegmentum, interpeduncular nucleus, superior and middle reticular nuclei, magnocellular vestibular nucleus, solitary tract nucleus, nucleus medianus magnocellularis, the spinal cord and amacrine cells in the retina. Large neurons in cranial nerve sensory ganglia were also labeled. The comparison of these results with those from other vertebrates, using a neuromeric analysis, reveals a conserved pattern of organization of the nitrergic system from this primitive fish group to amniotes, including mammals.