ULTRASTRUCTURAL-LOCALIZATION OF CHOLINERGIC MUSCARINIC RECEPTORS IN RAT-BRAIN CORTICAL CAPILLARIES

Cholinergic innervation of the cerebrovasculature is known to regulate vascular tone, perfusion rate and permeability of the microvascular w all. Notably the cholinergic innervation of cerebral capillaries is of interest since these capillaries form the blood-brain barrier. Althou gh there is a general consensus as to the presence of nicotinic and mu scarinic receptors in the domain of the capillary wall, their precise anatomical position is unknown. The subcellular localization of muscar inic receptors in rat cortical capillaries was approached by way of im munocytochemistry at the ultrastructural level using monoclonal antibo dy M35 against muscarinic receptor protein. Binding of this antibody i n the microvascular domain was found in 5% of the capillaries studied and was exclusively present in perivascular astroglia, and never in en dothelium or pericytes. Combined with reported data on presynaptic cho linergic innervation the results indicate a cholinergic innervation pa ttern of non-directed presynaptic terminal structures in apposition to cholinoceptive perivascular astroglia with muscarinic receptor positi ve endfeet embracing the capillary basement membrane. The possible fun ctional significance of such a cholinergic vascular innervation patter n is discussed with respect to capillary dynamics and barrier function .