Monoaminergic innervation of the caudal neurosecretory system of the brook trout Salvelinus fontinalis in relation to osmotic stimulation

Abstract

The distribution of monoamine fluorescence reaction (Falck and Hillarp technique) was investigated in the posterior spinal cord of the brook trout, Salvelinus fontinalis, in relation with the caudal neurosecretory system. A semiquantitative analysis of fluorescence was made following variations of external salinity, injections of salt solutions, and drugs aimed at modifying hydromineral balance. Under all experimental conditions, an extensive network of beaded fibers innervate a number of caudal neurosecretory cells whereas some others do not appear to be contacted by fluorescent varicosities. Three types of aminergic cells different from nonfluorescent caudal neurosecretory neurons were observed in the posterior spinal cord: (i) green fluorescent CSF-contacting neurons interspersed between ordinary ependymal cells; (ii) small perikarya with discrete spots of intense yellow fluorescence; (iii) small somata with diffuse green fluorescence. No fluorescence was observed in neurohemal region or urophysis. The action of nialamide and reserpine indicated that histofluorescent reaction was due to monoamines. In comparison to freshwater exposure to demineralized water for 3 days, but not 7 days, increased markedly the fluorescence of beaded fibers and varicosities contiguous to caudal neurosecretory cells while transfer to seawater for 1 and 7 days reduced them significantly. Injections of isotonic NaCl, hypertonic Na 2SO 4, and hypertonic choline chloride in freshwater fish, were less effective in producing fluorescence changes; however, these treatments showed a common tendency to enhance frequency of caudal neurosecretory cells with abutting varicosities in the most anterior part of the population. Acetazolamide and aldactone treatments also induced significant variations of aminergic innervations. These findings indicate a monoaminergic participation in the control of the function of caudal neurosecretory cells in relation to iono- and osmoregulation.