Direct projections from the spinal cord to the trigeminal sympathetic ganglion of the puffer fish, Takifugu niphobles

Confocal imaging of autonomic preganglionic neurons in the spinal cord of the caecilian Typhlonectes natans (Amphibia: Gymnophiona)

Little is known about the spinal sympathetic organization in the caecilian amphibians. We examined for the first time the location of sympathetic preganglionic neurons (SPNs) in the spinal cord using a panel of specific markers expressed in SPNs. The SPNs of anuran amphibians form two cell columns segregated mainly in the lateral and medial marginal areas of the central gray matter. In the caecilian Typhlonectes natans immunoreactivity for galanin and ChAT is found in most laterally arranged neurons lying in spinal segments 2-7. They are encircled by TH- and nNOS-immunoreactive nerve fibers. These neurons might project specifically to a population of adrenergic sympathetic postganglionic neurons in paravertebral ganglia and/or non-adrenergic sympathetic postganglionic neurons in the celiac ganglia. However the segmental restriction and target specificity of the neurons of the species studied are not known. As mucous and granular glands in the dermis may represent one of the peripheral targets of the adrenergic ganglion cells and reflect the prominent preganglionic cell columns, an immunohistochemical study was done also on these glands. Retrograde-tracing studies are, however, needed to study the segmental localization of the preganglionic neurons and their projections to the postganglionic neurons in sympathetic ganglia.

The Sympathetic Nervous System of Anamniotes

The sympathetic nervous system develops as an evolutionary trait with gnathostomes (jawed vertebrates), but not with agnathan fishes (i.e., hagfishes and lampreys). Organization of the sympathetic preganglionic neuronal columns is different in teleosts and anurans. In the teleosts so far examined, the majority of sympathetic preganglionic neurons (SPNs) are located in the dorsal part of the spinal central gray matter. In Tetraodontiformes, the cell column occupies only two rostral spinal segments, which are distinct in their cytoarchitecture and projections. On the other hand, the SPNs of anurans form two cell columns segregated mediolaterally. The lateral and medial columns are also distinct in their cytoarchitecture and projections. The neuroactive substances expressed in the SPNs both in teleosts and anurans are coded to the projections. In anurans, the SPNs containing gonadotrophin-releasing hormone and those containing calcitonin gene-related peptide are involved in the regulation of blood vessels and cutaneous glands, respectively. In the filefish, the SPNs containing galanin project specifically to non-adrenergic non-cholinergic postganglionic neurons in the cranial sympathetic ganglia. Therefore, both anuran and teleost systems have different morphological and chemical-coded patterns for functional variation, although the anuran sympathetic nervous system has more organizational similarity with that of amniotes.

Distinct localization and target specificity of galanin-immunoreactive sympathetic preganglionic neurons of a teleost, the filefish Stephanolepis cirrhifer

Immunoreactivity for galanin was examined in the sympathetic preganglionic neurons in the spinal cord, adrenal glands, sympathetic ganglia, and some sensory ganglia of the filefish Stephanolepis cirrhifer. Galanin-immunoreactive neurons were found only in the rostral part, but not in the caudal part of the central autonomic nucleus (a column of sympathetic preganglionic neurons of teleosts). Many galanin-immunoreactive nerve terminals were found in contact with neurons in the celiac ganglia and the cranial sympathetic ganglia on both sides of the body. Most neurons encircled by galanin-immunoreactive nerve fibers were negative for tyrosine hydroxylase. Galanin-immunoreactive nerve fibers were very sparse in the spinal sympathetic paravertebral ganglia. No galanin-immunoreactive nerve fibers were found in the adrenal glands. No sensory neurons of the trigeminal, vagal, or spinal dorsal root ganglia were positive for galanin-immunoreactivity. These results suggest that galanin-immunoreactive sympathetic preganglionic neurons have distinct segmental localization and might project specifically to a population of non-adrenergic sympathetic postganglionic neurons in the celiac and cranial sympathetic ganglia.

Spinal and vagal projections to the sympathetic trunk of the wrasse, Halichoeres poecilopterus

The sympathetic preganglionic neurons of the teleost, Halichoeres poecilopterus, were identified by retrograde axonal tracing. After horseradish peroxidase was applied to the sympathetic trunk, labeled neurons were found at the caudalmost level of the medulla, in the spinal cord near the fourth spinal nerve root (rostral spinal group), and in the spinal cord from rostral to the sixth spinal nerve root to caudal to the tenth spinal nerve root (caudal spinal group). The rostral spinal group has three cell columns segregated mediolaterally from the central gray zone to the lateral funiculus. Labeled neurons were found predominantly on the side ipsilateral to the application. In the caudal spinal group, labeled neurons were found bilaterally in the central gray zone. This condition is different from that previously reported in the puffer fish and filefish. The labeling in the medulla suggests that the preganglionic neurons in the brainstem may send fibers to the sympathetic trunk of this fish, although their peripheral targets are unknown.

Contralateral projections by preganglionic neurons to the sympathetic trunk of the puffer fish, Takifugu niphobles

Previous studies have shown that the sympathetic preganglionic neurons of teleosts send axons to the sympathetic trunk on the contralateral side. After severing the spinal nerve roots at a level proximal to the sympathetic ganglia (i.e., nerve roots containing the preganglionic axons) on one side of puffer fish, Takifugu niphobles, horseradish peroxidase was applied to the other side of the sympathetic trunk. Retrogradely labeled sympathetic preganglionic neurons were found bilaterally in the central autonomic nucleus (a distinct cell column in the rostral part of the spinal cord). The contralaterally labeled neurons were located almost exclusively in the caudal part of the nucleus. These results suggest that some sympathetic preganglionic neurons in teleosts, unlike those in other vertebrates, send their axons across the midline to the contralateral nerve roots.