Muscarinic cholinergic receptor subtypes in the human brain. II. Quantitative autoradiographic studies

The distribution and characteristics of M1 and M2 muscarinic cholinergic receptors as defined by their affinity for the antagonist pirenzepine were studied in the human brain using in vitro quantitative autoradiographic techniques. The binding of N-[3H]methylscopolamine ([3H]NMS) to cortical and striatal microtome tissue sections was saturable and presented a Kd of 0.25 nM. The sensitivity of [3H]NMS-binding sites to 100 microM carbachol and 300 nM pirenzepine was analyzed in 30 brain areas. In selected brain regions, complete competition curves using carbachol, pirenzepine and atropine were analyzed. Finally, the regional distribution of M1 sites was studied using [3H]pirenzepine ([3H]PZ) as ligand. The binding of [3H]NMS to striatum, hippocampus and amygdala was very sensitive to pirenzepine but not to carbachol. The opposite situation was found in thalamus, hypothalamus, substantia innominata, pons and medulla, while intermediate sensitivity to both displacers was observed in different layers of the cortex and in the claustrum. Competition experiments showed that [3H]NMS binding was displaced with the same affinity by atropine in all the regions studied, while the IC50 of carbachol varied from 5 microM in the nucleus facialis to 830 microM in the caudate. Pirenzepine IC50 values for [3H]NMS sites varied from 66 nM to 1 microM. Results using [3H]PZ further confirm this pattern of distribution, with high densities of binding observed in the striatum, hippocampus and amygdala and very low in thalamic and brainstem areas. These results show that the putative M1 and M2 muscarinic receptor subtypes present a differential anatomical distribution in the human brain. This differential distribution is comparable to that observed in the rat brain. Some basal ganglia and limbic areas are enriched in M1 sites, while thalamus, brainstem, medulla and also the hypothalamus and substantia innominata contain predominantly receptors of the M2 type. The cerebral cortex is an example of a region containing a mixed population of M1 and M2 sites. These results provide an anatomical description of the distribution of subtypes of the muscarinic receptor in the human brain, which can be related to the known pharmacological effects of muscarinic agents in brain function.