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

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Abstract
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).