Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

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Abstract
Monoclonal antibodies against two α‐bungarotoxin‐binding subunits (α7 and α8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the α7 and α8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the β2 structural subunit of the nAChRs.Structures that contained high numbers of α7‐like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio‐pretecto‐subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for α7‐LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of α8‐LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio‐pretecto‐subpretectalis, and intense neuropil staining for α8‐LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum tecti, nucleus lentiformis mesencephali, nucleus interpeduncularis, and stratum griseum et fibrosum superficiale of the tectum. High numbers of β2‐LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for β2‐LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis‐mesencephali, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum.These results indicate that there are major disparities in the localization of the α‐bungarotoxin‐binding α7 and α8 nAChR subunits and the β2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.