The development of monoamine‐containing neurons in the brain and spinal cord of the salamander, Ambystoma mexicanum

The distribution of monoamine‐containing neurons in the CNS of the developing and adult axolotl, Ambystoma mexicanum , has been investigated using the histochemical fluorescence technique of Falck and Hillarp combined with microspectrofluorimetry. The earliest catecholamine‐containing neurons to be detected are located in the ventral ependymal zone of the spinal cord at the time of hatching (Stage 41). Between stages 43 and 46, catecholamine fluorescence can be detected in neurons in the following regions: nucleus preopticus, the hypothalamic‐infundibular region, and the brain stem reticular fomation. 5‐HT‐containing neurons are only observed in the midbrain raphe region and are first detected at stage 44. In contrast to these early monoamine fluorescing groups, catecholamine‐containing neurons are not routinely detectable in the nucleus interpeduncularis until six months of age. All monoamine‐containing neuronal groups detected in developing axolotls are also present in both sexes of the adult. However, the fluorescence intensity is less in monoamine‐containing neurons observed in adults than in early developing subjects. All catecholamine‐containing neuronal groups, with the exception of those located in the midbrain region (nucleus interpeduncularis, reticular zone) have fluorescent processes that contact the cerebrospinal fluid (CSF). The presence of CSF‐containing processes in the hypothalamic and spinal cord regions suggest that the CSF may act as a medium through which bioactive substances are transported from one brain region to another. Intense catecholamine fluorescence is observed in cells of the notochord prior to the detection of monoamine‐containing neurons in the CNS. A possible involvement of catecholamines in the inductive effects of the notochord during development is discussed.