Nicotinic acetylcholine receptors (nAChRs) are ionotropic receptors comprised of <FONT FACE="Symbol">a</font> and ß subunits. These receptors are widely distributed in the central nervous system, and previous studies have revealed specific patterns of localization for some nAChR subunits in the vertebrate brain. In the present study we used immunohistochemical methods and monoclonal antibodies to localize the <FONT FACE="Symbol">a</font>2, <FONT FACE="Symbol">a</font>3, and <FONT FACE="Symbol">a</font>5 nAChR subunits in the chick mesencephalon and diencephalon. We observed a differential distribution of these three subunits in the chick brain, and showed that the somata and neuropil of many central structures contain the <FONT FACE="Symbol">a</font>5 nAChR subunit. The <FONT FACE="Symbol">a</font>2 and <FONT FACE="Symbol">a</font>3 subunits, on the other hand, exhibited a more restricted distribution than <FONT FACE="Symbol">a</font>5 and other subunits previously studied, namely <FONT FACE="Symbol">a</font>7, <FONT FACE="Symbol">a</font>8 and ß2. The patterns of distribution of the different nAChR subunits suggest that neurons in many brain structures may contain several subtypes of nAChRs and that in a few regions one particular subtype may determine the cholinergic nicotinic responses
cholinergic system; neurotransmitters; receptors; receptor subunits
