Aromatic L-amino acid decarboxylase immunohistochemistry in the suprachiasmatic nucleus of the sheep. Comparison with tyrosine hydroxylase immunohistochemistry

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

The Suprachiasmatic Nucleus of the Dromedary Camel (Camelus dromedarius): Cytoarchitecture and Neurochemical Anatomy

In mammals, biological rhythms are driven by a master circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Recently, we have demonstrated that in the camel, the daily cycle of environmental temperature is able to entrain the master clock. This raises several questions about the structure and function of the SCN in this species. The current work is the first neuroanatomical investigation of the camel SCN. We carried out a cartography and cytoarchitectural study of the nucleus and then studied its cell types and chemical neuroanatomy. Relevant neuropeptides involved in the circadian system were investigated, including arginine-vasopressin (AVP), vasoactive intestinal polypeptide (VIP), met-enkephalin (Met-Enk), neuropeptide Y (NPY), as well as oxytocin (OT). The neurotransmitter serotonin (5-HT) and the enzymes tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) were also studied. The camel SCN is a large and elongated nucleus, extending rostrocaudally for 9.55 ± 0.10 mm. Based on histological and immunofluorescence findings, we subdivided the camel SCN into rostral/preoptic (rSCN), middle/main body (mSCN) and caudal/retrochiasmatic (cSCN) divisions. Among mammals, the rSCN is unusual and appears as an assembly of neurons that protrudes from the main mass of the hypothalamus. The mSCN exhibits the triangular shape described in rodents, while the cSCN is located in the retrochiasmatic area. As expected, VIP-immunoreactive (ir) neurons were observed in the ventral part of mSCN. AVP-ir neurons were located in the rSCN and mSCN. Results also showed the presence of OT-ir and TH-ir neurons which seem to be a peculiarity of the camel SCN. OT-ir neurons were either scattered or gathered in one isolated cluster, while TH-ir neurons constituted two defined populations, dorsal parvicellular and ventral magnocellular neurons, respectively. TH colocalized with VIP in some rSCN neurons. Moreover, a high density of Met-Enk-ir, 5-HT-ir and NPY-ir fibers were observed within the SCN. Both the cytoarchitecture and the distribution of neuropeptides are unusual in the camel SCN as compared to other mammals. The presence of OT and TH in the camel SCN suggests their role in the modulation of circadian rhythms and the adaptation to photic and non-photic cues under desert conditions.

The substructure of the suprachiasmatic nucleus: Similarities between nocturnal and diurnal spiny mice

Evolutionary transitions between nocturnal and diurnal patterns of adaptation to the day-night cycle must have involved fundamental changes in the neural mechanisms that coordinate the daily patterning of activity, but little is known about how these mechanisms differ. One reason is that information on these systems in very closely related diurnal and nocturnal species is lacking. In this study, we characterize the suprachiasmatic nucleus (SCN), the primary brain structure involved in the generation and coordination of circadian rhythms, in two members of the genus Acomys with very different activity patterns, Acomys russatus (the golden spiny mouse, diurnal) and Acomys cahirinus (the common spiny mouse, nocturnal). Immunohistochemical techniques were used to label cell bodies containing vasoactive intestinal polypeptide (VIP), vasopressin (VP), gastrin-releasing peptide (GRP) and calbindin (CalB) in the SCN, as well as two sets of inputs to it, those containing serotonin (5-HT) and neuropeptide Y (NPY), respectively. All were present in the SCN of both species and no differences between them were seen. On the basis of neuronal phenotype, the SCN was organized into three basic regions that contained VIP-immunoreactive (-ir), CalB-ir and VP-ir cells, in the ventral, middle and dorsal SCN, respectively. In the rostral SCN, GRP-ir cells were in both the VIP and the CalB cell regions, and in the caudal area they were distributed across a portion of each of the other three regions. Fibers containing NPY-ir and serotonin (5-HT)-ir were most concentrated in the areas containing VIP-ir and CalB-ir cells, respectively. The details of the spatial relationships among the labeled cells and fibers seen here are discussed in relation to different approaches investigators have taken to characterize the SCN more generally.

Hydrogen peroxide generation by monoamine oxidases in rat white adipocytes: role on cAMP production

In rat, white adipocytes monoamine oxidases (EC 1.4.3.4.) generate hydrogen peroxide (H(2)O(2)). Recent studies suggested that, in addition to its toxic features, H(2)O(2) may behave as a cell second messenger. In the present study, using fluorimetric and chemiluminescence (CL) assays, we showed that tyramine degradation by monoamine oxidases in intact adipocytes resulted in the concentration-dependent generation of H(2)O(2). In addition, we found that, in the presence of low tyramine concentrations, forskolin-dependent cAMP production was significantly increased as compared to that of the control and this increase was prevented by the monoamine oxidase inhibitor pargyline or by the H(2)O(2) trapping system homovanillic acid-peroxidase. Finally, we demonstrated that tyramine degradation by monoamine oxidases increased the ability of isoproterenol to induce cell lipolysis. Taken together, these data suggest that H(2)O(2) produced during substrate degradation by monoamine oxidases may participate in the regulation of adipocyte metabolism.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-containing cells in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus)

Catecholamines, including dopamine (DA), affect the activity of cells in the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. This study examined the distribution of dopaminergic cells in the SCN of the male Syrian hamster, using both single- and double-label immunocytochemistry for tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis and for aromatic L-amino acid decarboxylase (AADC), the second enzyme needed to produce DA. Some neurons immunopositive for TH (TH + ) were found in the SCN, but most of the TH + cells of the region were located just outside the borders of the nucleus, as defined by pyronin Y staining. In the SCN, 91% of these cells were also immunopositive for AADC and thus, likely to be dopaminergic. Cells positive for AADC, many of which were not TH +, were found throughout the SCN, with the highest concentration seen in the ventral aspects of the nucleus. Cells containing AADC, but lacking TH may synthesize products other than DA, such as trace amines. These anatomical observations suggest that local neurons that produce DA and perhaps trace amines, may play a role in SCN function and in the neural control of circadian rhythms.

Progesterone receptor immunoreactivity in aromatic L-amino acid decarboxylase-containing neurons of the guinea pig hypothalamus and preoptic area

A double-labeling immunofluorescence procedure was used to determine whether progesterone receptor (PR)-immunoreactive (IR) neurons in the preoptic area and hypothalamus of female guinea pigs also contained aromatic L-amino acid decarboxylase (AADC), an enzyme involved in the synthesis of both catecholamines and serotonin. Immunostaining was performed on cryostat sections prepared from ovariectomized guinea pigs primed by estradiol to induce PR. The nuclear presence of PR was visualized by a red fluorescence while the AADC-containing perikarya showed a yellow-green fluorescence. The topographic distribution of AADC-IR neurons was investigated by using a specific antiserum obtained by immunization of rabbits with a recombinant protein beta-galactosidase-AADC in the two regions known to contain the densest populations of estradiol-induced PR-IR cells: the preoptic area and the mediobasal hypothalamus. The localization of PR-IR and AADC-IR cell populations showed considerable overlap in these areas, mainly in the medial and periventricular preoptic nuclei and in the arcuate nucleus. A quantitative analysis of double-labeled cells estimated that about 15% to 23% of AADC-IR cells in the preoptic area and about 11% to 21% of AADC-IR cells in the arcuate nucleus possessed PR. This colocalization persisted throughout the rostrocaudal extent of these areas and represented 3% to 9% of the population of PR-IR cells. These findings provide neuroanatomical evidence that a subset of AADC neurons is directly regulated by progesterone. The exact physiological role of this enzyme in target cells for progesterone is not understood. AADC may be involved in functions other than that for the synthesis of the classical neurotransmitters.