Characterization of an oro-facial route of injection in the rat

Objectives: In order to mimic regional mandibular anaesthesia, an experimental model, the jaw opening reflex (JOR) in the rat was used to evaluate a new route of a administration, namely the infratemporal injection. Methods: In unanesthetized, chronic preparations, electrical stimulation of the lower incisor pulp elicits an oro-facial reflex, the JOR, characterized by a long (12-18ms) latency (JOR-LL) involving C-pulpal incisor fibers. In order to target the inferior alveolar nerve in its pathway from the oval to the mandibular foramen, the infratemporal injection was performed by vertically inserting the tip of the needle at the ear upper cutaneous insertion. Pharmacological, radiological, and anatomical approaches were considered. Results: In the pharmacological approach, the local anaesthetic bupivacaine, injected infratemporally (9 mg.kg-1), suppressed the JOR-LL (ANOVA, F(2,24), p<0.001, n=28) during 60 minutes. In the radiological approach, head rat standardized X-ray pictures were shot when bupivacaine was injected to locate the site of administration (tip of needle). In bupivacaine JOR-LL-suppressed group (n=13), the tip of the needle was closer to the targeted mandibular foramen both in the saggital section (Student's t-test, t=2.42, p=0.03), and in the frontal section (Student's t-test , t=4.77, p<0.001) than for animals in which bupivacaine injection failed (n=17). In the anatomical approach, dissection of the temporo-mandibular area was performed in rats with JOR-LL suppression, 15 minutes after bupivacaine (mixed with a dye) injection. As expected, the presence of the dye near to the mandibular foramen was observed. Conclusion: Taking together, these results suggest that concerning local anesthesics, the infratemporal targeted site of injection can be hit. Nevertheless, if the suppression of the JOR by bupivacaine without any apparent systemic consequence evokes a local effect, additional investigations are required to further characterize this route of administration