Expression of TRPM8 in the distal cerebrospinal fluid-contacting neurons in the brain mesencephalon of rats

BACKGROUND: It has been shown that distal cerebrospinal fluid-contacting neurons (dCSF-CNs) exist near the ventral midline of the midbrain aqueduct and also in the grey matter of the inferior third ventricle and the fourth ventricle floor in the superior segment of the pons. The dCSF-CNs communicate between the cerebrospinal fluid (CSF) and the brain parenchyma and may participate in the transduction and regulation of pain signals. The cold sensation receptor channel, TRPM8 is involved in analgesia for neuropathic pain, but whether the TRPM8 receptor exists on dCSF-CNs remains unknown. However, there is preliminary evidence that TRPM8 is expressed in dCSF-CNs and may participate in the transmission and regulation of sensory information between brain parenchyma and cerebrospinal fluid (CSF) in rats. METHODS: Retrograde tracing of the cholera toxin subunit B labeled with horseradish peroxidase (CB-HRP) injected into the lateral ventricle was used to identify dCSF-CNs. A double-labeled immunofluorescent technique and laser scanning confocal microscopy were used to identify the expression of TRPM8 in dCSF-CNs. Software Image-Pro Plus was used to count the number of neurons in three sections where CB-HRP positive neurons were located in the mesencephalon of six rats. RESULTS: The cell bodies of CB-HRP-positive dCSF-CNs were found in the brain parenchyma near the midline of the ventral Aq, also in the grey of the 3V, and the 4V floor in the superior segment of the pons. In the mesencephalon their processes extended into the CSF. TRPM8 labeled neurons were also found in the same area as were CB-HRP/TRPM8 double-labeled neurons. CB-HRP/TRPM8 double-labeled neurons were found in 42.9 ± 2.3% of neurons labeled by TRPM8, and all CB-HRP-labeled neurons were also labeled with TPRM8. CONCLUSION: This study has demonstrated that the cold sensation receptor channel, TRPM8, is localised within the dCSF-CNs of the mesencephalon. TRPM8 acts as receptor of dCSF-CNs for sensation transmission and pain regulation.