Fos expression in the ferret trigeminal nuclear complex following tooth pulp stimulation

Abstract

The aim of this study was to establish which regions of the trigeminal nucleus are activated by tooth pulp stimulation in the normal ferret. The distribution of Fos-like immunoreactivity was examined following electrical stimulation of the tooth pulp in the awake and anaesthetized ferret. Stimulus-specific labelling was found in subnuclei caudalis and oralis of the trigeminal spinal nucleus. Three groups of chronically prepared animals; conscious, anaesthetized (alphaxolone/alphadolone) and anaesthetized-paralysed (alphaxolone/alphadolone with gallamine triethiodide), received electrical stimuli to both the upper and lower left canine teeth (1 Hz train of 3 x 0.5 ms at 200 Hz) at an amplitude of 10 times the threshold of the jaw opening reflex. Three control groups were treated identically except no stimulus was given. In stimulated anaesthetized and anaesthetized-paralysed animals, Fos-positive profiles were seen in laminae I and II of subnucleus caudalis and in the medial part of subnucleus oralis. There was no labelling evident in subnucleus interpolaris or the main sensory nucleus, or contralaterally in any of the subnuclei. In all conscious stimulated animals there was additional bilateral Fos-positive labelling, mainly in the deeper laminae of subnucleus caudalis. This bilateral labelling was not stimulus-specific as it was also seen in conscious non-stimulated animals. After correction for this bilateral labelling no significant difference wasfound between conscious, anaesthetized and anaesthetized-paralysed groups of stimulated animals or between the different groups of control animals. These results support the concept that the rostral parts of the trigeminal spinal nucleus are involved in processing of nociceptive input. They also demonstrate that light alphaxolone/alphadolone anaesthesia has no effect on stimulus-specific Fos expression following tooth pulp stimulation. The second aim of this study was to develop a clearly defined model for future studies in which Fos expression is no different to that seen in the conscious state. As in the conscious animal, labelling not associated with the stimulus is difficult to distinguish from stimulus specific labelling, further studies using this model of trigeminal nociceptive pathways would be best carried out in lightly anaesthetized animals.