Autoradiographic analysis of muscarinic receptors and choline-uptake mechanisms within foetal basal forebrain transplants

The pharmacological characteristics of both muscarinic receptors and high-affinity choline uptake sites were examined within intracerebral implants of foetal basal forebrain cell suspensions. Approximately 12 weeks after implantation, the transplants were identified by acetylcholinesterase histochemistry. Muscarinic receptors were labelled by [³H]quinuclidinyl benzylate (QNB) autoradiography. The M₁ and M₂ receptor components of QNB binding were differentiated by pirenzepine competition. The distribution of the high-affinity choline uptake site was examined using [³H]hemicholinium-3 (HC3) autoradiography. Unilateral lesion of the nucleus basalis reduced [³H]QNB (8–25%) and [³H]HC3 (19–43%) binding throughout host frontoparietal cortex ipsilateral to the lesion but did not significantly alter these cholinergic markers within cingulate cortex, subcortical white matter, striatum or septum. Saturation analysis of the implanted tissue revealed the presence of a single population of [³H]QNB and [³H]HC3 binding sites with affinities similar to those of the host tissue (K_D = 0.43 nM and 20.4 nM respectively). However, the receptor profile which developed appeared to be intrinsic to the implant; it was unaffected by the site of implantation and was dissimilar to that which ultimately developed in the donor tissue when left in situ.