Differential hypothalamic tyrosine hydroxylase distribution and activation by light in adult mice reared under different light conditions during the suckling period

In mammals, early light experience during a critical period within the first 3 weeks of postnatal development has long-lasting effects on circadian locomotor activity behaviour and neuropeptide expression in the suprachiasmatic nucleus (SCN) of the hypothalamus, site of the principal pacemaker. Dopamine is thought to be involved in the modulation of photic input within the SCN and in tadpoles, the expression of tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of dopamine, in the SCN is altered by previous light history. We thus hypothesised that dopaminergic neurons may be important for the development of the adapted responses to light that we have previously observed. To test this, we raised mice in either constant darkness, 12: 12 h light-dark cycles or constant light during the first 3 weeks after birth, and later examined the expression of TH and FOS in the hypothalamus of these mice as adults, both in the dark and after exposure to a light pulse. We found that early light experience affects TH and FOS expression, both baseline levels and in response to a light pulse, in brain areas which are directly connected to the SCN, and are associated with the circadian control of neuroendocrine function. Therefore, our results suggest that the long-lasting alterations induced by early light environment on several hypothalamic nuclei may be relayed through the SCN, and that TH-expressing cells may play a role in conveying/establishing these alterations. These data suggest a role of early light experience in the regulation of future hormonal homeostasis and circadian behaviour.