Distribution of choline acetyltransferase-immunoreactive neurons in the brain of a cyprinid teleost (Phoxinus phoxinus L.).

The distribution of putative cholinergic neurons in the brain of a cyprinid teleost was investigated by immunocytochemistry, with well-characterized polyclonal antibodies to porcine choline acetyltransferase (ChAT), correlated with acetylcholinesterase (AChE) histochemistry. AChE-positive neurons were more numerous than ChAT-immunoreactive (ChAT-IR) neurons. Regions with ChAT-IR neurons generally also contained AChE-positive ones, but regions with AChE-positive neurons often did not contain (or contained only small numbers of) ChAT-IR neurons. ChAT-IR neurons were located in the brainstem cranial nerve motor nuclei, in the brainstem reticular formation, in the nucleus lateralis valvulae and an adjacent subnucleus "a," in the nucleus isthmi, and in the stratum griseum periventriculare of the tectum opticum. All neurons in these areas were AChE positive. ChAT-IR neurons were also observed within the boundaries of the nucleus sensibilis nervi trigemini and the n. descendens nervi trigemini. The periventricular hypothalamus and the paraventricular organ, the pineal organ, and (possibly) the nucleus suprachiasmaticus also contained ChAT-IR neurons. In these areas, AChE activity was either low or located mainly in neurons other than the ChAT-IR ones. A small population of ChAT-IR neurons was observed in area ventralis telencephali pars lateralis. This was the only telencephalic ChAT-IR cell group. Furthermore, some previously unrecognized cell groups were observed. A small number of ChAT-IR neurons, located on the dorsal aspect of the fasciculus longitudinalis medialis (caudal to n. raphe dorsalis), emitted axons that passed caudally along the raphe midline and innervated some of the large reticular neurons. Another group of ChAT-IR neurons was observed caudal to the thalamic nucleus centralis posterior and was tentatively designed n. tractus rotundus, on the basis of the neuronal morphology. The almost Golgilike staining of some of the ChAT-IR cell groups permitted the identification of their efferent connections and the areas covered by their dendrites.