Anatomical and functional characterisation of a dopaminergic system in the suprachiasmatic nucleus of the neonatal Siberian hamster.

In altricial rodents, maternal influences entrain the developing circadian system in the perinatal period before the capacity to respond directly to photic cues develops. The aim of these studies was to investigate the potential role of dopamine in this process in the Siberian hamster. An initial study investigated the ontogeny of retinal innervation of the suprachiasmatic nuclei (SCN) by using cholera toxin B subunit as a tracer. This revealed that retinal fibres first innervate the SCN on postnatal day 3 (PD3), and ingrowth of fibres is extensive by PD6. In situ hybridisation studies revealed the presence of D1-dopamine receptor (D1-R) mRNA in the SCN on PD2, and levels of expression were similar in PD6 pups and adult hamsters. Immunocytochemical staining for tyrosine hydroxylase revealed abundant catecholaminergic fibres within the ventromedial zone of the SCN from the day of birth through PD20; however, in contrast, few fibres were present in adult SCN. Dopamine-beta-hydroxylase-immunoreactive fibres were absent from the neonatal and adult SCN, suggesting that the fibres in the SCN are dopaminergic. The function of this dopaminergic system was investigated by determining the effects of D1-R agonists on the expression of the immediate-early gene c-fos in the SCN. This was assessed in pups ages PD1- PD5 by in situ hybridisation and immunocytochemical localisation of its protein product. No induction was seen in the SCN, in marked contrast to studies in the developing rat. A final series of studies investigated dopaminergic function by determining whether a D1-agonist could induce phosphorylation of Ca2+/cyclic AMP response element-binding protein (CREB) on Ser133. Hypothalamic slices containing SCN taken from PD1 and PD2 hamsters were treated with D1-R agonists, and levels of phosphorylated CREB were assayed by Western blots. Phosphorylation of CREB was stimulated by D1-R agonists in both Syrian and Siberian hamster hypothalamus, but the response was far greater in Syrian hamster tissue (+138%+/-28%) than in Siberian hamster tissue (+43%+/-11%). Although the anatomical studies demonstrate the existence of a dopaminergic system in the SCN of the early postnatal Siberian hamster, the unresponsiveness of c-fos expression and the relative lack of phosphorylation of CREB after D1-R activation suggests a diminished role for dopamine in the regulation of circadian events during the postnatal period in this species.