Much is known of the structure and immunological properties of nerves in rat molar teeth and of the changes in these properties that are associated with pulpal inflammation; however, mainly because of the small size of the teeth and the difficulty in gaining access to a sufficient length of peripheral nerve for single fibre recordings, less is known of the physiological properties of the sensory receptors in these teeth. We have recently developed a method that enables multi-fibre recordings to be made from the larger diameter nerve fibres in rat molars. The method is a miniaturised version of that which was developed to record from intradental nerves in cat canine teeth (Horiuchi & Matthews, 1974). A rat is anaesthetized with sodium pentobarbitone (42 mg kg^-1 I.P. then 3 mg kg^-1 I.V. as required) and killed at the end of the experiment with an overdose. It is placed on its back and its skull fixed to an external support with its mouth wide open. The upper first and second molar teeth on one side are cleaned and the exposed surfaces of the crowns etched with acid for 30 s. An Ag/AgCl reference electrode (diameter 0.5 mm) is formed around the end of a silver wire lead (diameter 0.125 mm) in a small occlusal cavity in the distal fissure of the first molar or in the second molar. A Perspex cap for the first molar is made from a piece of tube (o.d. 4.0 mm, i.d. 2.8 mm) that is cut off at an angle of 60 deg to its long axis to give an oval cross-section. The short side of the tube is 2 mm long. An Ag/AgCl recording electrode is formed in a cavity in the inner surface of the cap. The crown of the molar is dried with ethanol and then the oval end of the cap is placed over it with the tube aligned so that the operator can look down it with a dissecting microscope and see the tip of the mesial cusp of the tooth. The edge of the cap is sealed to the tooth with a drop of bis-GMA resin (tooth fissure sealant) so that the mesial cusp is isolated electrically from the reference electrode and from the surrounding tissues. Approximately 0.5 mm of dentine is removed from the tip of this cusp with a small bur under Ringer solution and the dentine so exposed is etched with acid or EDTA to remove debris from the exposed ends of the dentinal tubules. The cap is filled with Ringer solution and recordings are made between the electrode in the cap and the reference electrode. With this arrangement, the dentinal tubules act as biological micro-electrodes and action potentials are recorded from nerve terminals in the dentine and pulp under the exposed dentine surface. Responses have been recorded with mechanical, chemical (2.5 mol l^-1 NaCl), and thermal stimulation of the exposed dentine that are similar to those obtained in the cat. However, unlike the cat, changes in hydrostatic pressure between 2000 mmHg above atmospheric and 500 mmHg below atmospheric evoked no response. This is probably due to dentine having a much lower hydraulic conductance in the rat than the cat. The resolution of the recording system appears to be as good in the rat as in the cat, or even better.

This work was supported by the MRC.

Horiuchi, H. & Matthews, B. (1974). J. Physiol. 243, 797-829. abstract