Distribution of thyrotropin‐releasing hormone immunoreactivity in the brain of the dogfish Scyliorhinus canicula

Abstract

To improve knowledge of the peptidergic systems of elasmobranch brains, the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers was studied in the brain of the small-spotted dogfish (Scyliorhinus canicula L.). In the olfactory bulbs, small granule neurons richly innervated the olfactory glomeruli. In the telencephalic hemispheres, small TRHir neurons were observed in the superficial dorsal pallium, whereas TRHir fibers were widely distributed in pallial and subpallial regions. In the preoptic region, TRHir neurons formed a caudal ventrolateral group in the preoptic nucleus. In the hypothalamus, the most conspicuous TRHir populations were associated with the lateral hypothalamic recess, but small TRHir populations were found in the posterior tubercle and ventral wall of the posterior recess. The preoptic region and hypothalamus exhibited rich innervation by TRHir fibers. TRHir fibers were observed coursing to the neurohypophysis and the neuroepithelium of the saccus vasculosus, but not to the neurohemal region of the median eminence. Some stellate-like TRHir cells were observed in a few cell cords of the neurointermediate lobe of the hypophysis. The thalamus, pretectum, and midbrain lacked TRHir neurons. Further TRHir neuronal populations were observed in the central gray and superior raphe nucleus of the isthmus, and a few TRHir cells were located in the nucleus of the trigeminal descending tract at the level of the rostral spinal cord. In the brainstem, the central gray, interpeduncular nucleus, secondary visceral region of the isthmus, rhombencephalic raphe, inferior olive, vagal lobe, and Cajal's commissural nucleus were all richly TRHir-innervated. Comparison of the distribution of TRHir neurons observed in the dogfish brain with that observed in teleosts and tetrapods reveals strong resemblance but also interesting differences, indicating the presence of both a conserved basic vertebrate pattern and a number of derived characters.

Distribution of Thyrotropin-Releasing Hormone (TRH) Immunoreactivity in the Brain of Urodele Amphibians

“…In the following section the general organization and variations of the TRH systems in amphibians (anurans and urodeles) is discussed and compared with those of other vertebrates. In particular, the data of recent studies in different anamniotes obtained with techniques similar to the one used in our study will be discussed [Díaz et al, 2001De Andrés et al, 2002;Teijido et al, 2002].…”

“…Lampreys, however, lack TRHir structures in the olfactory bulbs and it was proposed that this would be the primitive character [De Andrés et al, 2002]. In addition, TRHir cells and fibers have been reported in the olfactory bulbs of an elasmobranch fish [Teijido et al, 2002]. Therefore, the situation found in amphibians in which anurans seem to lack TRHir structures in the bulbs [Mimnagh et al, 1987;Andersen et al, 1992;Miranda and Affanni, 2000] which is also the case in the urodele P. waltl (present results) would support the contention that the importance of a TRH system in the olfactory system varies across anamniotes.…”

“…Thus, in untreated animals TRHir material was detectable in nerve fibers and terminals, but the cell bodies were only weakly stained or did not stain at all [Tsuruo et al, 1987[Tsuruo et al, , 1988aJózsa et al, 1988]. However, in recent studies in different anamniotes wide distribution patterns for TRH were described that included nicely stained cell bodies and in none of these studies colchicines was used [Lamacz et al, 1989;Miranda and Affanni, 2000;Díaz et al, 2001Díaz et al, , 2002De Andrés et al, 2002;Teijido et al, 2002]. In our study, we have used one animal of each species with colchicine pretreatment as controls and have observed that, at least in urodeles, the same immunoreactive cell groups are observed for each species with only a slight improvement of the intensity of the reaction; a result that contrasts with the situation found in amniotes.…”

“…In general, TRH was primarily detected in neurons of the preoptic area and hypothalamus, although the presence of TRH in neurons of other brain regions has also been reported [Matz and Takahashi, 1994;Díaz et al, 2001Díaz et al, , 2002Teijido et al, 2002].…”

New Insights on Saccus vasculosus Evolution: A Developmental and Immunohistochemical Study in Elasmobranchs

“…On the basis of neuron counts in the SV and axon counts in the SV tract, Zimmermann and Altner [1970] estimated that at least half of the SV tract axons are saccopetal but no clear evidence of their existence was obtained with tract-tracing methods [Yáñez et al, 1997]. On the other hand, saccopetal fibers related to the hypothalamus-hypophyseal system were reported in some elasmobranchs with classical methods for neurosecretion [ Torpedo: Bargmann, 1954] and with immunocytochemistry to neurophysin [ Squalus acanthias : Meurling et al, 1996] and thyrotropin-releasing hormone [ Scyliorhinus canicula : Teijido et al, 2002]. In bony fishes, saccopetal fibers have only been reported in Acipenser with classical neurosecretion methods [Sathyanesan and Chavin, 1967], although the possibility that neurosecretory tuberal nuclei contribute to the innervation of the teleost SV was suggested by Jansen and Flight [1969] on the basis of ultrastructural observations.…”