A high density of muscarinic receptors in the rostral ventrolateral medulla of the rat is revealed by correction for autoradiographic efficiency.

Previous reports of a low density of muscarinic receptors in the vicinity of the rostral ventrolateral reticular nucleus (RVL) conflict with the presence of a muscarinic cholinergic mechanism regulating arterial pressure, and may reflect low autoradiographic efficiency due to quenching of tritium emissions by white matter. Regional autoradiographic efficiency was determined using brains uniformly labeled with O-[3H]methylglucose. Within the RVL, autoradiographic efficiency was only 50 +/- 2% relative to gray matter. The uncorrected density of muscarinic receptors (M2) labeled by [3H]quinuclidinyl benzilate in the RVL appeared to be low relative to cranial nerve nuclei, but the corrected muscarinic receptor density approximated that within the nucleus tractus solitarii. We conclude that a high density of M2 muscarinic acetylcholine receptors is present in the RVL.