The muscarinic receptor blocker quinuclidinyl benzilate induces acetylcholinesterase positive fiber increases within undamaged adult rat hippocampus

Abstract

Prolonged blockade of nicotinic receptors induces sprouting of axon collaterals at the neuromuscular junction (Holland, R.L. and Brown, M.C., 1980). The present study examined the role of muscarinic receptors in the control of collateral sprouting in the adult hippocampus. It was hypothesised that the state of muscarinic receptor activation could promote plastic change at hippocampal cholinergic synapses. In experiment one, quinuclidinyl benzilate (QNB; 20 μg/2 μl), an irreversible muscarinic antagonist was injected unilaterally into the hippocampus of female Sprague Dawley rats. Beta hydroxypropylcyclodextrin (36%) served as a vehicle which was injected into the opposite hemisphere. Subjects were sacrificed 1, 4, 14 or 21 days after treatment. Brain sections (40 μm) were reacted foracetylcholinesterase (AChE). Increased AChE staining in the area of the injection site was visible at the 4 day interval in the QNB treated hippocampi. No increase in AChE staining was seen at the 4 day interval in vehicle treated hippocampi. Brains examined after a 1 day interval showed increased staining limited to the needle track in both vehicle and drug injected hemispheres which appeared to be a function of blood accumulation. At the 14 day interval staining increases were still present at the QNB injection site but were reduced compared to the 4 day interval. An increase in AChE staining was no longer evident at the QNB injection site at the 21 day interval. These data suggest that irreversible blockade of muscarinic receptors can induce collateral sprouting of AChE reactive fibers in the hippocampus. It is hypothesised that sprouting is rapidly induced and retraction occurs following renewed receptor activation. -- Experiment two assessed the effect of hemisphere, sex and drug dosage on QNB-induced collateral sprouting in Sprague Dawley rats. Quinuclidinyl benzilate (20 μg/2 μl or 40 μg/2 μl) was intrahippocampally injected into either right or left hemispheres of male and female rats. Rats were sacrificed four days following treatment and brains were reacted for AChE. Acetylcholinesterase positive fiber outgrowth was demonstrated using both QNB doses, in right and left hemispheres and in both male and female rats. Neither variable of experiment two effected the occurrence of AChE positive collateral sprouting in the hippocampus.