DISTRIBUTION OF PHOSPHATASE INHIBITOR-1-IMMUNOREACTIVE NEURONS IN THE SUPRACHIASMATIC NUCLEUS OF THE SYRIAN-HAMSTER

The protein phosphatase inhibitor-1 (I-1) is phosphorylated by a cyclic AMP-dependent protein kinase, and is itself involved in the regulation of phosphorylation of other proteins. The enzyme has been shown to be present in skeletal muscles and in distinct neuronal systems of the brain. The suprachiasmatic nucleus is essential in generation of circadian rhythms, but the cellular mechanisms by which the oscillator is entrained are not understood. Since cyclic AMP is known to phase shift the rhythm of electrical activity in SCN neurons in vitro, we aimed by an avidin-biotin immunohistochemical technique to localize I-1-containing neurons in the hamster suprachiasmatic nucleus and thereby identify potential target neurons for cyclic AMP effects. Numerous densely stained neurons were observed in the hamster SCN. The I-1-immunoreactive cell bodies were intermingled with non-immunoreactive neurons and occupied mostly the ventral half of the nucleus, but cell bodies were found in all compartments of the nucleus. The I-1-immunoreactive neurons located in the ventral SCN sent dendrite-like processes into the underlying optic chiasm, indicating that they are directly innervated from the retina, the intergeniculate leaflet of the thalamus, and/or the dorsal raphe. A few I-1-immunoreactive neurons were observed immediately outside the borders of the SCN, but their pronounced staining intensity and their similar morphology to those found inside the SCN indicate that they belong to the same type of neurons as found in the SCN. Delicate I-1-immunoreactive nerve fibers possessing boutons were found throughout the SCN. Furthermore, axonal fibers were followed dorsally into the subparaventricular area. These data demonstrate that the I-1 is highly concentrated in a large portion of neurons within the hamster SCN. The functional significance of the I-1 in SCN neurons remains to be established.