Cholinergic enzyme activity in neurons of the developing anuran spinal cord

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

Principal neurons of the lumbar sympathetic ganglia increase in number with body size

Neuron number appears to be matched to body size during early development by the modulation of the processes of proliferation and naturally occurring cell death. However, body size increases rapidly as the juvenile becomes an adult, long after these processes cease to operate. The present study shows that principal neurons of lumbar sympathetic ganglia increase in number four- to fivefold during postmetamorphic life of the bullfrog. Rana catesbeiana. This increase in neuron number cannot be attributed to either counting error or selection bias and was associated with greater innervation of particular hindlimb targets, as demonstrated by retrograde labeling with horseradish peroxidase. Injection of [3H]thymidine (a marker of DNA synthesis) every third day for 20-22 weeks failed to provide evidence of neuron proliferation, although, on the basis of changes in body length during this period, substantial numbers of neurons likely were added. These results combined with previous studies of hindlimb motor and sensory neuron addition are consistent with the hypothesis that the population of sympathetic neurons is augmented by late differentiation of existing precursor cells.

Early expression of acetylcholinesterase activity in functionally distinct neurons of the zebrafish

The first expression and distribution of acetylcholinesterase (AChE) activity was studied among a distinct population of early neurons in embryonic zebrafish by using histochemical and retrograde labeling techniques. AChE first appeared in the nervous system in the primary motoneurons of the rostral spinal cord when the embryo had nine somites, approximately 14 hours postfertilization. Subsequent expression of AChE activity in the spinal cord proceeded in a rostral-to-caudal sequence. Cranial neurons expressed AChE activity shortly after it appeared in the rostral spinal cord. Several hours later, near the end of the first day, primary neurons in the hind-brain and spinal cord all contained AChE, including sensory neurons, reticulospinal interneurons, and primary motoneurons. AChE activity was also detected in the nucleus of the medial longitudinal fasciculus. Presumptive cranial ganglia transiently expressed AChE activity between 14 and 24 hours of development. These results, combined with previous observations that examined the time of origin and axogenesis of primary neurons, suggest that primary neurons in the embryonic zebrafish contain AChE before they sprout axons. The primary neurons appear to follow a common sequence of development consisting of a withdrawal from the cell division cycle, the expression of AChE, and axogenesis. Although this sequence is followed by all primary neurons, lack of a rostral-to-caudal sequence in the time of birth and variability in the time of axon outgrowth demonstrate that the relative timing of these three events is not rigidly programmed in individual neurons. Moreover, the very early expression of AChE in such diverse cell types suggests that it may have a developmental role in addition to its function in transmitter metabolism.

The development of acetylcholinesterase activity in the embryonic nervous system of the frog, Xenopus laevis

Histochemical detection of acetylcholinesterase (AChE) activity in Xenopus embryos was found first in primary motoneurons, Rohon-Beard neurons and somitic myotubes at early tail bud stages. At late tail bud stages all primary neurons, including primary interneurons, cranial ganglion cells and ventral brainstem cells expressed this enzyme. The onset of detectable AChE activity in some primary neurons occurred near the time of initial axon outgrowth, whereas in others it occurred at much later stages. At early independent-feeding and continuous-swimming stages nearly all seemingly postmitotic neurons began to express AChE activity, and by the beginning of limb bud stages, when many secondary neuronal populations were going through their final rounds of mitosis, nearly all CNS cells outside the ventricular zone were intensely stained. Thus, the onset of detectable AChE activity in secondary neurons occurred near the time of their final mitoses. In trunk somites the enzyme activity initially was located diffusely throughout the myotube, and with progressing development it became localized to the myocommata. From the onset of AChE activity both head somites and head muscles had discrete patches of reaction product all over their surfaces. The onset of detectable AChE activity occurred in muscles near the time that they were contacted by motor axons. These data demonstrate that the primary neurons are the first to express AChE activity, and that as the secondary neurons begin to proliferate, AChE is expressed by nearly all embryonic neuronal populations.