Neurosecretion -- comparative and evolutionary aspects

Internal chemical communication within flatworms

An understanding of reproductive function is important for control of parasitic helminths. In cestodes and trematodes virtually nothing is known about regulatory and coordinating mechanisms that control maturation, gamete formation, egg production, and related processes. Neurosecretory neurons have been reported in various species but specific modes of action of neurohormones have yet to be demonstrated. The role of ecdysone is being investigated.

Neuropeptides in platyhelminths

The neuropeptide story began in 1928 with the description by Ernst Scharrer of gland-like nerve cells in the hypothalamus of the minnow,Phoxinus laevis.Because these nerve cells were overwhelmingly specialized for secretory activity, overshadowing other neuronal properties, Scharrer termed them ‘neurosecretory neurons’. What was even more remarkable about the cells was that their products were released into the bloodstream to act as hormones, specifically neurohormones. Neurosecretory cells were identified largely on morphological grounds. That is, they could be stained with special techniques, such as chrome-haematoxylin and paraldehyde-fuchsin, although the techniques are far from specific, staining non-neurosecretory cells as well. However, the basis for the ‘special’ neurosecretory techniques is the demonstration of sulphur-containing proteins – so they are indicative of peptide-producing neurones. An alternative characteristic of neurosecretory cells is the presence of large (> 100 nm), dense-cored vesicles at the electron microscope level; these are the so-called elementary granules of neurosecretion, or ENGs. However, implicit in the concept of neurosecretion is that the prime function of the neurosecretory cell is in endocrine regulation, exerting a hormone-like control over some aspect of the organism's metabolism, by controlling endocrine glands and other effector organs. To satisfy this criterion, evidence had to be obtained of cycles of secretory activity within the cell that could be correlated with a change in the physiological condition of the organism.

Immunoreactive neuropeptide systems in avian embryos (domestic mallard, domestic fowl, Japanese quail)

In embryos of the domestic mallard, domestic fowl, and Japanese quail vasotocin-, mesotocin-, luliberin (LHRH)-, met-enkephalin-, corticotropin-, and somatostatin-immunoreactive perikarya and fiber formations were visualized at different incubation stages by means of the PAP technique (Sternberger 1979). The most striking results were: (1) Vasotocin-, mesotocin-, and luliberin-immunoreactive systems display, up to the late embryonic period, morphological features most probably related to a neurohormonal function. (2) Met-enkephalin immunoreactivity appears very late during embryonic life; it is restricted to fiber networks and not found in perikarya. (3) Corticotropin immunoreactivity is observed in the tuberal region temporarily at the end of the second and the beginning of the last third of the incubation period. (4) Somatostatin-immunoreactive material is present (i) at the end of the first third of incubation, in association with the olfactory system; (ii) during the same period, adjacent to thin-layered portions of the roof of the brain; (iii) shortly thereafter, in cells of both pancreatic primordia and thyroid gland; and (iv) onward from the middle of the incubation period, in a mesencephalic cell group. The striking difference, in the early embryo, between the mature somatostatin plays a role in the development of the brain, as well as the pancreas, and the thyroid gland.