Central oxotremorine antagonist properties of pirenzepine

Pirenzepine, the prototype M1 muscarinic receptor antagonist, is an important compound for investigating the functional significance of M1 receptors at the integrated level of behavior but may have limitations imposed by its physical chemistry. Like the nonselective antagonist methylatropine, pirenzepine is highly hydrophilic and crosses the blood-brain-barrier with difficulty. We compared methylatropine with pirenzepine, given intraperitonealy, as antagonists of the behavioral effects of peripheral or central muscarinic activation. Lever-press responses of male Sprague-Dawley rats were maintained under a schedule requiring 10 responses for each food delivery. Administration of oxotremorine or the quaternary analog oxotremorine-M decreased rates of responding by at least 90%. Both methylatropine and pirenzepine antagonized the behavioral effects of oxotremorine-M; maximum reversal was 70%. Although methylatropine was about 30 times more potent than pirenzepine as an antagonist of the peripheral muscarinic activity of oxotremorine-M, it was inactive as an antagonist of oxotremorine when given in doses up to 153 mumol/kg. Pirenzepine, however, reversed oxotremorine-induced behavioral effects, with a maximum antagonism of 50%. These results suggest that pirenzepine interacts with central muscarinic receptors when administered systemically without producing marked behavioral effects of its own. Systemically administered pirenzepine may thus be a useful tool in further investigations of the relevance of M1 receptors to behavioral function.