Development of adenosine deaminase-immunoreactive neurons in the rat brain

It has previously been demonstrated that neurons immunoreactive for the enzyme adenosine deaminase (ADA) have a highly restricted distribution pattern in the adult rat brain. In order to determine whether the pattern of ADA expression is equally limited during the period of brain development, the localization of ADA was investigated immunohistochemically in brains of embryonic, early postnatal and young adult rats. No immunostaining for ADA was detected on the 12th embryonic day. On embryonic day 15, ADA-immunoreactive cells were first observed in the hypoglossal motor nucleus, and on day 18 in cingulate, retrosplenial and visual cortex, in the posterior basal hypothalamus, and in the facial motor nucleus. On the 20th embryonic day ADA-immunoreactive neurons appeared in various olfactory and related systems and in the superior colliculus. On the 1st postnatal day, immunoreactivity was intensified in all structures in which it was observed at preceeding ages and, in addition, appeared in several brainstem regions. On postnatal day 10 and 15, immunostained neurons appeared in several subcortical structures whereas the number of these decreased in the anterior olfactory nucleus and some related cortical areas. In animals 25 days of age the intensity of immunostaining continued to increase, essentially producing the adult pattern in all except olfactory areas where there was a dramatic loss of ADA-immunoreactive cells. These results show that the restricted pattern of ADA-immunostaining observed in adult rat brain is generated over a protracted period of development, various stages of which are characterized predominantly by the expression of ADA in greater abundance, at least to the extent this can be gleaned immunohistochemically, in greater numbers of neurons and to a minor degree by a decreased capacity to express this enzyme.