Distribution of dopamine immunoreactivity in the forebrain and midbrain of the snake Python regius: a study with antibodies against dopamine

The distribution of dopamine (DA) immunoreactivity in the forebrain and midbrain of the ball python, Python regius, was studied by using recently developed antibodies against DA. In order to determine general and species-specific features of the DA system in reptiles, we have selected the ball python as a representative of a reptilian radiation that hitherto has not been the subject of (immuno)histochemical studies. Dopamine-containing cell bodies were found around the glomeruli and in the external plexiform layer of both the main and accessory olfactory bulb, but not in the telencephalon proper. In the diencephalon, DA cells were observed in several parts of the periventricular hypothalamic nucleus, in the periventricular organ, the ependymal wall of the infundibular recess, the lateral hypothalamic area, the magnocellular ventrolateral thalamic nucleus, and the pretectal posterodorsal nucleus. In the midbrain, DA cells were found in the ventral tegmental area, the substantia nigra, and the presumed reptilian homologue of the mammalian A8 cell group. Dopaminergic fibers and varicosities were observed throughout the whole brain, particularly in the telencephalon and diencephalon. The nucleus accumbens, striatum, olfactory tubercle, and nucleus of the accessory olfactory tract appear to have the most dense innervation, but the lateral septal nucleus, the dorsal ventricular ridge, and the nucleus sphericus also show numerous DA-containing fibers and varicosities. Except for the lateral cortex, cortical areas are not densely innervated by DA fibers. The DA system of the snake Python regius shares many features with that of lizards and turtles as determined with the same antibodies. The taxonomically close relationship between lizards and snakes, which together constitute the Squamata, is reflected in a similar distribution of DA fibers and varicosities to the dorsal ventricular ridge and the lateral cortex, and in the limited number of CSF-contacting DA neurons in the hypothalamus.