Distribution of somatostatin‐like immunoreactivity in the brain of the caecilian Dermophis mexicanus (amphibia: Gymnophiona): Comparative aspects in amphibians

Abstract

The organization of the somatostatin-like-immunoreactive (SOM-ir) structures in the brain of anuran and urodele amphibians has been well documented, and significant differences were noted between the two amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study, we analyzed the anatomical distribution of SOM-ir cells and fibers in the brain of the gymnophionan Dermophis mexicanus. In addition, because of its known relationship with catecholamines in other vertebrates, double immunostaining for SOM and tyrosine hydroxylase was used to investigate this situation in the gymnophionan. Abundant SOM-ir cell bodies and fibers were widely distributed throughout the brain. In the telencephalon, pallial and subpallial cells were labeled, being most numerous in the medial pallium and amygdaloid region. Most of the SOM-ir neurons were found in the preoptic area and hypothalamus and showed a clear projection to the median eminence. Less conspicuously, SOM-ir structures were found in the thalamus, tectum, tegmentum, and reticular formation. Both SOM-ir cells and fibers were demonstrated in the spinal cord. The double-immunohistofluorescence technique revealed that catecholaminergic neurons and SOM-ir cells are largely intermingled in many brain regions but form totally separated populations. Many differences were found between the distribution of SOM-ir structures in Dermophis and that in anurans or urodeles. Some features were shared only with anurans, such as the abundant pallial SOM-ir cells, whereas others were common only to urodeles, such as the organization of the hypothalamohypophysial SOM-ir system. In addition, some characteristics were found only in Dermophis, such as the localization of the SOM-ir spinal cells and the lack of colocalization of catecholamines and SOM throughout the brain. Therefore, any conclusions concerning the SOM system in amphibians are incomplete without considering evidence for gymnophionans.

Localization of Calbindin-D28k and Calretinin in the Brain of Dermophis Mexicanus (Amphibia: Gymnophiona) and Its Bearing on the Interpretation of Newly Recognized Neuroanatomical Regions

“…However, research in the last years about the telencephalic organization in anurans and urodeles has proved that at least 3 main amygdaloid subdivisions can be considered that were named for facilitating their comparison with their counterparts in amniotes: lateral, central, and medial González, 2006, 2007a, b]. Several immunohistochemical studies in the brain of gymnophionans support the presence of the same subdivisions of the amygdaloid complex, and we have tentatively identified them in this study González et al, 2002a, b;López et al, 2006López et al, , 2007.…”

“…Only the rostralmost part of the septum contains a distinct CBir cell population that by its position and in comparison with results in anurans [Morona and González, 2008] may represent the counterpart of the anuran postolfactory eminence, as proposed earlier [Northcutt and Kicliter, 1980]. The actual boundary between the medial pallium and the septal region located just below is hard to distinguish and becomes evident only when immunohistochemistry for the detection of several peptides is used [González et al, 2002b;López et al, , 2007. The largest septal region of gymnophionans was considered homologous to the lateral septal nucleus of anurans on the basis of its abundant catecholaminergic innervation and lack of cholinergic and GnRH immunoreactive cells Pinelli et al, 1997;Rastogi et al, 1998;Ebersole and Boyd, 2000;González et al, 2002a].…”

“…By means of immunohistochemistry to unravel the distribution of catecholamines, acetylcholine, and several peptides, a nucleus accumbens was proposed rostrally in the hemispheres, just medial to the ventral tip of the lateral ventricles González et al, 2002a, b;López et al, 2006López et al, , 2007. However, tract-tracing studies and specific stainings to unravel possible pallidal zones in the basal telencephalon have not been conducted in gymnophionans.…”

“…This subdivision was prompted by the distinct distribution of diverse markers in the telencephalon of gymnophionans González et al, 2002a, b;López et al, 2006López et al, , 2007 and by comparison with anurans and urodeles González, 2006, 2007a]. The lateral amygdala was defined as a multimodal area in the ventral pallium and the actual ventral pallial origin of this region was confirmed by the expression of territorial genetic markers [SmithFernandez et al, 1998;Brox et al, 2004;.…”

“…In fact, the available experimental studies conducted in the brain of gymnophionans have shown many complex features of its organization that could not be detected on the basis of cytoarchitecture alone and that were distinct from those of anurans and urodeles. These features of gymnophionans include, among others, the presence of particular cholinergic and somatostatinergic cell groups in the thalamus, the presence of a well-developed mesencephalic group of dopaminergic cells, the existence of complex cholinergic and nitrergic cell populations in the basal ganglia, or the particular arrangement of the motor nuclei in the brainstem Smeets, 1994, 1997;Pinelli et al, 1997;Hilscher-Conklin et al, 1998;Ebersole and Boyd, 2000;Ebersole et al, 2001;Sánchez-Camacho et al, 2001;González et al, 2002a, b;López et al, 2006López et al, , 2007. Therefore, any study that attempts to clarify the amphibian condition of a given system must include results not only of anurans and urodeles but also of gymnophionans.…”