The mesencephalic tectum plays a prominent role in integrating
both visual and multimodal sensory information essential for
normal behavior in amphibians. Activity in the mesencephalic
tectum is thought to be modulated by the influence of distinct
neurochemical inputs, including the catecholaminergic and the
cholinergic systems. In the present study, we have investigated
the distribution and the origin of the catecholaminergic
innervation of the mesencephalic tectum in two amphibian species,
the anuran Rana perezi and the urodele Pleurodeles
waltl. Immunohistochemistry for dopamine and two enzymes
required for the synthesis of catecholamines, tyrosine hydroxylase
(TH) and dopamine β-hydroxylase (DBH), revealed a complex
pattern of catecholaminergic (CA) innervation in the anuran
and urodele mesencephalic tectum. Dopaminergic fibers were
primarily present in deep tectal layers, whereas noradrenergic
(DBH immunoreactive) fibers predominated in superficial layers.
Catecholaminergic cell bodies were never observed within the
tectum. To determine the origin of this innervation, applications
of retrograde tracers into the optic tectum were combined with
immunohistochemistry for TH. Results from these experiments
demonstrate that dopaminergic neurons in the suprachiasmatic
and juxtacommissural nuclei (in Rana) or in the nucleus
pretectalis (in Pleurodeles), together with noradrenergic
cells of the locus coeruleus, are the sources of CA input to
the amphibian mesencephalic tectum. The present results suggest
that similar CA modulatory inputs are present in the
mesecencephalic tectum of both anurans and urodeles.