Tyrosine hydroxylase-immunoreactive neurons in the mouse cerebral cortex during the postnatal period.

Using specific antiserum against tyrosine hydroxylase, the key enzyme in catecholamine synthesis, we found tyrosine hydroxylase immunoreactive neurons (TH-IN) in the cerebral cortex of Swiss Webster mice. They were found in the deeper layers of the cerebral isocortex in all the mice examined up to 60 days postnatal (P60). However, they appeared to be more numerous in early postnatal days. On P3 and P5, a few neurons with faintly stained perikarya were detected in the layer V and VI. The number and intensity of TH-IN increased gradually after P7, reaching to the maximum between P10 and P14 and then gradually decreased. Many of these neurons were multipolar but some were bipolar. Significance of TH-IN in the cortex and their apparent increase in immunoreactivity during the second postnatal week is not certain. Such an increase may be a transient increase in activity or phenotypic expression of tyrosine hydroxylase (TH) by young neurons in response to as yet unknown intrinsic environmental changes during this particular period of the brain development.