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Brain Res
2003 Apr 18;9691-2:36-43. doi: 10.1016/s0006-8993(03)02273-x.
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Co-localization of mesotocin and opsin immunoreactivity in the hypothalamic preoptic nucleus of Xenopus laevis.
Alvarez-Viejo M
,
Cernuda-Cernuda R
,
DeGrip WJ
,
Alvarez-López C
,
García-Fernández JM
.
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The purpose of the present investigation was to provide a detailed description of the encephalic photoreceptors of Xenopus laevis at the light microscopic level and to determine their relationship with the neurosecretory cells of the hypothalamus in order to further our understanding of photoperiodic regulation of seasonal rhythms. Numerous opsin-positive neurons were found in the hypothalamic magnocellular preoptic nucleus and their axonal processes were seen to run laterally towards the basal regions of the brain, reaching the neural lobe of the hypophysis. Analysis of labelling with different antisera in adjacent sections, as well as double-immunolabelling carried out on the same section, revealed that mesotocin immunoreactivity was present in most of the opsin-positive neurons; however, no evidence for opsin and vasotocin coexpression was found in any of the sections analysed. The close localization of LHRH and opsin/mesotocin fibers in some regions of the brain, such as the median eminence, suggests that some interaction between these two systems might exist. In conclusion, in this study we provide the first strong evidence that the hypothalamic mesotocinergic neurons, which have been proved to be connected to the GnRH system in other species, are directly involved in photoreception in Xenopus laevis. These findings represent a novel contribution to our understanding of how light influences the seasonal reproductive cycles in lower vertebrates.
Fig. 1.
CERN-874 (cone-opsin) labelling in the retina of X laevis. Some cone cells (arrowheads) and also the rod cells are labelled in both their outer segments (OS) and inner segments (IS). RPE, retinal pigment epithelium; ONL, outer nuclear layer. Calibration bar: 50 μm.
Fig. 2.
CERN-874 labelling in the pineal gland. Pinealocytes form rossette-like aggregates (arrows) showing immunopositive outer segments facing the inside. Bar: 50 μm.
Fig. 3.
Cone-opsin IR in the hypothalamic MPON (magnocellular preoptic nucleus). Many immunopositive cell somata (some of them indicated by arrows) can be seen sparsely within this nucleus (V, third ventricle). Bar: 100 μm.
Fig. 4.
CERN-874-immunopositive fibers in a section of the brain which is more caudal than the MPON. Bundles of fibers running towards the basal areas are observed (V, third ventricle). Bar: 100 μm.
Fig. 5.
Cone-opsin-immunoreactive fibers in the median eminence (ME). Note that these positive fibers occupy the inner region of the ME (close to the third ventricle, V). Bar: 100 μm.
Fig. 6.
Cone-opsin labelling of the hypophysis. The neural lobe shows many cone-opsin positive fibers and axonal terminals. A higher density of fibers can be seen at the periphery. Note that the intermediate lobe (IL) and the anterior lobe (AL) of the hypophysis do not exhibit any IR (arrows point to some of the numerous Herring bodies). Bar: 100 μm.
Fig. 7.
Cone-opsin/AVT double labelling in the MPON (indicated at top right corner as opsin+AVT). Dark brown stained neurons are opsin-positive, whereas vasotocinergic neurons are red. No co-localization of both antigens at any neuron (which should be intensely stained) can be observed. Bar: 50 μm.
Fig. 8.
(A) and (B) represent two adjacent sections showing opsin and mesotocin (MT) IR, respectively. Note that the majority of the opsin-positive cells are also MT-labelled in the consecutive section (the black arrows point to these cells). Some exceptions (white arrows) show opsin, but not MT, immunostaining. Calibration bars: 50 μm.
