Distribution of tyrosine hydroxylase-immunoreactive neurones in the diencephalon and mesencephalon of the coypu (Myocastor coypus)

The aim of the present study was to examine the catecholaminergic neurones located within the midbrain of the coypu, a South American hystricomorph ro dent. The neuronal distribution of the catecholaminergic systems and morpho logical parameters of the immunostained cell bodies and fibres were investi gated, using an immunohistochemical method. The brains of five coypu were f ixed, immersed in gelatine-glycerol and cut in 40-mum slices using a freezi ng microtome. Samples were processed with ultrasound-based antigen retrieva l and stained with labelled antityrosine hydroxylase monoclonal antibody. A n image analyser was used to measure the neuronal bodies. The catecholamine rgic neurones of the tuberoinfundibular system were mainly observed in the arcuate and periventricular nuclei with their axons projecting towards to t he median eminence; they represented 28% of the global population of tyrosi ne hydroxylase-immunoreactive cells observed. Significant morphological dif ferences were observed in comparison with the other two studied systems. Fi fty per cent of total catecholaminergic neurones were detected in the nigro striatal system distributed in the reticular and compact substance nigra. M ost neuronal bodies had a fusiform aspect. The immunoreactive neurones of t he mesolimbic system represented 22% of the total population. They were dis tributed around the interpeduncular nucleus. Two types of morphologically d ifferent catecholaminergic systems of the brain were established: hypothala mic neurones located in the periventricular and arcuate nuclei and mesencep halic neurones located in the substance nigra and interpeduncular nuclei. T hese systems showed morphological and probably physiological-pharmacologica l differences.