Use of the jaw opening reflex for assessing the effects of local anaesthetics in freely moving rats

Introduction: In order to characterize a nonbehavioral model for assessing local anaesthetic (LA) activity, the effects of different LA agents (articaine, bupivacaine, procaine, and tetracaine) were measured in the conscious rat using the jaw-opening reflex (JOR). Methods: One hundred sixty rats were chronically implanted with stimulating electrodes in the dental pulp of the low incisor. While animals were conscious and unrestrained, the JOR threshold was measured electrophysiologically via electrodes wrapped around the digastric muscle. Each LA was administered in the infratemporal area. The increase of the JOR threshold was assessed during a 3-h period following injection. Results: Statistical analysis of the data showed a dose-dependent response to the four drugs tested. When the highest dose of each drug (articaine and procaine: 24 mg kg(-1), bupivacaine: 6 mg kg(-1), tetracaine: 3 mg kg(-1)) was administered (i) an immediate effect was observed for tetracaine and bupivacaine, whereas a 5-min delay was needed for articaine and procaine to act on the JOR threshold and (ii) an increase (>60%) of the JOR threshold was observed. The effects lasted 90 min for articaine, 45 min for procaine and bupivacaine, and 15 min for tetracaine before a return to baseline values. Discussion: The rat JOR response combined with infratemporal injection of test drugs can be used for the pharmacological evaluation of LAs. (C) 2003 Elsevier Science Inc. All rights reserved.