The distribution of muscarinic M1 receptors in the human hippocampus

• A muscarinic M1 receptor antibody is validated for specificity using Western blotting and immunohistochemistry using CNS from wild type and muscarinic M1 receptor knockout mice.
• This antibody is used to visualise the distribution of muscarinic M1 receptors in the human hippocampus.
• Muscarinic M1 receptor positive cells were most apparent in the deep polymorphic layer of the dentate gyrus, the pyramidal cell layer of cornu ammonis region 3, the cellular layers of the subiculum, layer II of the presubiculum and layer III and V of the parahippocampal gyrus.
• Immunostaining suggested some muscarinic M1 receptors were not localised to the cellular membrane. These receptors may be the muscarinic M1 receptor reserve or internalised receptors being degraded.

The muscarinic M1 receptor plays a significant role in cognition, probably by modulating information processing in key regions such as the hippocampus. To understand how the muscarinic M1 receptor achieves these functions in the hippocampus, it is critical to know the distribution of the receptor within this complex brain region. To date, there are limited data on the distribution of muscarinic M1 receptors in the human hippocampus which may also be confounded because some anti-muscarinic receptor antibodies have been shown to lack specificity.Initially, using Western blotting and immunohistochemistry, we showed the anti-muscarinic M1 receptor antibody to be used in our study bound to a single 62 kDa protein that was absent in mice lacking the muscarinic M1 receptor gene. Then, using immunohistochemistry, we determined the distribution of muscarinic M1 receptors in human hippocampus from 10 subjects with no discernible history of a neurological or psychiatric disorder.Our data shows the muscarinic M1 receptor to be predominantly on pyramidal cells in the hippocampus. Muscarinic M1 receptor positive cells were most apparent in the deep polymorphic layer of the dentate gyrus, the pyramidal cell layer of cornu ammonis region 3, the cellular layers of the subiculum, layer II of the presubiculum and layer III and V of the parahippocampal gyrus. Positive cells were less numerous and less intensely stained in the pyramidal layer of cornu ammonis region 2 and were sparse in the molecular layer of the dentate gyrus as well as cornu ammonis region 1. Although immunoreactivity was present in the granular layer of the dentate gyrus, it was difficult to identity individual immunopositive cells, possibly due to the density of cells.This distribution of the muscarinic M1 receptors in human hippocampus, and its localisation on glutamatergic cells, would suggest the receptor has a significant role in modulating excitatory hippocampal neurotransmission.