Distribution and morphology of tyrosine hydroxylase-immunoreactive neurons in the developing mouse retina

An antibody to tyrosine hydroxylase (TH), the rate-limiting enzyme in the production of catecholamines, was used to examine the morphology and distribution of catecholaminergic neurons in whole-mounted retinae of the developing and adult mouse. At adulthood TH-labeled cell bodies were located in the inner nuclear layer, stratifying mainly at the border to the inner plexiform layer (IPL). Few processes were found in the middle of the IPL. The majority of all TH-labeled cells also extended processes towards the outer plexiform layer and thus are interplexiform cells; the rest were considered to be amacrine cells. The TH-positive neurons were regularly distributed throughout the adult mouse retina. During development, the first TH-immunoreactive cells were observed by postnatal day 6 (P6) and most of them were present after the third postnatal week. The dendrites in the IPL only acquired varicosities after the eyes opened at P15. Biochemical measurements of the endogenous catecholamine content showed that at all developmental stages only dopamine was detectable, suggesting that the TH-labeled cells represent dopaminergic neurons. The content of dopamine was low before P6 and continuously increased during the following days. A strong increase in dopamine was observed during the time when varicosities formed.