Abstract
In this study we characterize the presence of muscarinic acetylcholine receptors (mAChR) in the isthmo-optic nucleus (ION) of chicks by immunohistochemistry with the M35 antibody. Some M35-immunoreactive fibers were observed emerging from the retinal optic nerve insertion, suggesting that they could be centrifugal fibers. Indeed, intraocular injections of cholera toxin B (CTb), a retrograde tracer, and double-labeling with M35 and CTb in the ION confirmed this hypothesis. The presence of M35-immunoreactive cells and the possible mAChR expression in ION and ectopic neuron cells in the chick brain strongly suggest the existence of such a cholinergic system in this nucleus and that acetylcholine release from amacrine cells may mediate interactions between retinal cells and ION terminals.
Highlights
In the retina of many vertebrates there is an afferent component formed by a centrifugal projection from the brain [1]
In our preparations some fibers located close to the amacrine cell layer of the retina and running parallel to the inner plexiform layer (IPL) (Figure 1c) were observed. These transverse fibers were similar to those described as centrifugal fibers from the isthmo-optic nucleus (ION) in chick retina [22]
The pathway comprises a closed loop from ganglion cells in the dorsal retina to the tectum, to ION, to amacrine cells in the ventral retina, and back to ganglion cells in the dorsal retina [1,2,3,4,5,6,26,27]
Summary
In the retina of many vertebrates there is an afferent component formed by a centrifugal projection from the brain [1]. To the axons of ganglion cells which project to the contralateral optic tectum, the majority of centrifugal fibers project to the contralateral retina in chicks [2,3]. These retinopetal fibers originate from the caudal mesencephalic nucleus denominated isthmo-optic nucleus (ION) and from ectopic neurons (Ecn) which are present near the ION [1]. Nickla and co-workers [7] demonstrated immunoreactivity against the alpha-7 subunit of the nicotinic acetylcholine receptor (nAChR) in centrifugal fibers and in ION neurons in chick species [7]. Immunohistochemistry has been used to study cellular expression of mAChR in chick retina [17,18]
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