Neural connections of sparrow pineal: role in circadian control of activity.

Surgical and chemical interference with the neural connections of the house sparrow (Passer domesticus) pineal does not abolish the free-running rhythm in constant darkness, unlike pinealectomy. Pineals transplanted to the anterior chamber of the eye are capable of restoring rhythmicity to pinealectomized birds in constant darkness. The avian pineal does not appear to be neurally coupled to other components of the circadian system.

The pineal gland: a pacemaker within the circadian system of the house sparrow.

Transplantation of pineal tissue into the anterior chamber of the eye rapidly reestablishes rhythmicity in arhythmic pinealectomized sparrows and also transfers the phase of the donor bird's rhythm to the host. Thus, the transplanted pineal does not merely permit rhythmicity to be expressed but rather transfers an oscillation that controls the remainder of the circadian system and restores the capacity for self-sustained rhythmicity. Long-term recordings, during which sparrosw were exposed to various lighting conditions, demonstrate a remarkable similarity between the circadian system in normal birds and that in birds bearing pineal transplants.

Intrinsic neurons and neural connections of the pineal organ of the house sparrow, Passer domesticus, as revealed by anterograde and retrograde transport of horseradish peroxidase.

In Passer domesticus, intrapineal nerve cells were labeled by uptake of microiontophoretically administered horseradish peroxidase (HRP). Unipolar nerve cells with a dichotomously branching stem process are the main source of the dominant pinelaofugal component of the pineal tract, whereas multipolar and bipolar neurons appear to represent interneurons. HRP-Labeled nerve fibers are observed in the distal division (end-piece) of the pineal organ; they can be regarded either as processes of intrapineal neurons or projections of pinealopetal axons originating from central neurons. Furthermore, scattered labeled nerve fibers occur in different portions of the pineal stalk. Nerve fibers containing HRP were also demonstrated in the medial and lateral divisions of the habenular complex and in the periventricular layer of the hypothalamus; these axons apparently represent anterogradely labeled pinealofugal elements. On the other hand, retrogradely labeled neurons were found in the medial habenular complex and in the periventricular hypothalamic gray near the paraventricular nucleus, indicating that the pineal organ receives a pinealopetal innervation arising from the central nervous system. Ultrastructurally, the neuropil of the pineal organ of P. domesticus displays single basal processes of pinealocytes containing synaptic ribbons in association with clear synaptic vesicles. Occasionally, conventional synapses were observed the presynaptic terminals of which exhibit granular inclusions. The pineal tract consisting of four to six spatially separated fiber bundles comprises mainly unmyelinated elements accompanied by only few myelinated axons. The functional role of the neural apparatus revealed in the present study is discussed in context with the humoral (hormonal) control of circadian functions; the latter type of activity has been shown to exist in the pineal organ of P. domesticus (Zimmerman 1976).