Tyrosine hydroxylase-immunoreactive cell groups in the brain of the teleost fish Gnathonemus petersii.

Different antibodies against tyrosine hydroxylase (TH) were used to obtain detailed information about the distribution, morphology and chemical differentiation of catecholaminergic neurons in the highly differentiated brain of the electric mormyrid fish Gnathonemus petersii. The results show that the distribution of catecholaminergic neurons is much more widespread than was previously thought on the basis of dopamine and noradrenaline immunohistochemistry. Tyrosine hydroxylase-immunoreactive neurons were observed not only in clearly dopaminergic regions (the suprachiasmatic nucleus, the magnocellular hypothalamic nucleus and the area postrema) and noradrenergic cell groups (the locus coeruleus and inferior reticular cell group), but also in regions that do not, or only fragmentarily, display dopamine or noradrenaline immunoreactivity, including the ventral and intermediate telencephalon, the anterior and posterior preoptic cell group, the ventromedial thalamus, the pretectal region and the nucleus of the solitary tract, suggesting that they either represent depleted dopaminergic cell groups or L-dihydroxy phenylalanine-producing nuclei. Most TH-immunoreactive neurons are rather small (< 10 microns) and have only a few slender processes, but neurons in the magnocellular hypothalamic nucleus and the inferior reticular formation are multipolar and larger (10-20 microns), while those of the locus coeruleus are even more than 20 microns in diameter. The hypothalamic paraventricular organ, which is strongly dopamine and noradrenaline immunoreactive, displays minimal TH immunoreactivity, suggesting that its cerebrospinal fluid-contacting neurons do not synthesize catecholamines, but acquire them from external sources. Comparison with other teleosts shows that the catecholaminergic system in the brain of Gnathonemus is similarly organized as in Carassius, Gasterosteus, Anguilla and Aperonotus, with some variations that may partly be due to technical reasons, and partly reflect true species differences. However, TH-immunoreactive neurons in the midbrain tegmentum were not observed, confirming previous conclusions that a major difference between teleosts and mammals concerns the absence of dopaminergic midbrain groups and correlated mesencephalo-telencephalic projections in teleosts.