Expression of substance P and of a Ca2+-activated Cl- current in quail sensory trigeminal neurons

A chloride current activated by an increase in intracellular calcium concentration is not present in all neurons of the trigeminal ganglion. It is not known whether the trigeminal neurons expressing calcium-activated chloride current belong to a defined class of neurons or whether they could belong to any class of sensory neurons. An answer to this question would be of importance because the physiological role of calcium-activated chloride current in neurons has not yet been completely established, nonetheless it is clear that this current, when activated, would act to modulate neuronal excitability. The goal of this study was to determine whether there was a difference in the expression of calcium-activated chloride current between neurons with and without substance P. The rationale was that the use of this morphological marker, which is present in a substantial fraction of embryonic trigeminal neurons, may give a first estimate of a possible inhomogeneity in the expression of calcium-activated chloride current among different classes of sensory neurons. The study was done on freshly dissociated neurons in order to minimize the influence of the culture conditions on the expression of the current or of substance P. By recording from large samples of neurons in cultures either enriched or depleted in substance P-containing neurons, we found that neurons with substance P expressed calcium-activated chloride current three times less frequently than neurons without substance P. This observation was confirmed by performing the immunocytochemical labelling for substance P immediately after the electrophysiological assessment of the presence or absence of calcium-activated chloride current. This result indicates that calcium-activated chloride current may not be randomly distributed in neurons of a sensory ganglion. It raises the possibility that neurons belonging to certain sensory modalities may need calcium-activated chloride current for their physiological functioning.