5-HT-like immunoreactivity in the brains of plethodontid and salamandrid salamanders (Hydromantes italicus, Hydromantes genei, Plethodon jordani, Desmognathus ochrophaeus, Pleurodeles waltl): an immunohistochemical and biocytin double-labelling study

The distribution of 5-HT-like-immunoreactive cell bodies and fibres was studied in the brains of the salamanders Hydromantes italicus, H. genei, Plethodon jordani, Desmognathus ochrophaeus (family Plethodontidae), and Pleurodeles waltl (family Salamandridae). In addition, double-labelling experiments with biocytin were carried out to identify the relationship between serotonergic fibres and neurons involved in the processing of sensory and sensorimotor information. In all species, 5-HT-immunopositive somata are found in the ventral thalamus close to the ventricle forming the paraventricular organ. In the hypothalamus, cells are labelled in the ependymal layer around the infundibular recess and at the lateral edge of the periventricular grey. In the pretectum, a few immunoreactive cells are situated dorsolaterally in the grey matter. In the tegmentum and medulla oblongata, cells of the raphe nuclei are regularly distributed along the midline; labelled perikarya are occasionally found in the cervical spinal cord. 5-HT-like-immunoreactive fibres are widely distributed throughout the nervous system. Densely arborizing fibres are found in the olfactory bulb, striatum and amygdala. Distinct fibre projections extend in the ventral thalamus and tectum. Biocytin tracing of striatal and tectal projection neurons and ascending reticular neurons combined with the demonstration of 5-HT suggest that the striatum, the tectum and the ascending activating system are strongly influenced by 5-HT.