SUBPOPULATIONS OF NEONATAL RAT SENSORY NEURONS EXPRESS FUNCTIONAL NEUROTRANSMITTER RECEPTORS WHICH ELEVATE INTRACELLULAR CALCIUM

We have attempted to identify which subpopulations of rat sensory neurons possess functional neurotransmitter receptors which elevate the free concentration of intracellular calcium. Subpopulations of sensory neurons were identified using three accepted criteria: (i) the distribution and proportion of neurons with differing somatic diameters; (ii) the expression of substance P-like immunoreactivity; and (iii) the responsiveness of each neuron to capsaicin. The total neuronal population was primarily grouped into three classes according to somatic diameter and defined as small- (< 17 mu m), intermediate- (17-25 mu m) and large- (> 25 mu m) sized neurons. It was not possible to distinguish between small and intermediate-sized neurons since a similar percentage of each class expressed substance P-like immunoreactivity or sensitivity to capsaicin. Large-sized neurons did not possess these characteristics and, therefore, represented a distinct neuronal population. In single, intact neurons of differing diameter, the ability of a variety of receptor agonists to elevate the free concentration of intracellular calcium was determined using the calcium-sensitive indicator, Fura-2. Local application of capsaicin, adenosine, bradykinin, ATP and substance P elevated the resting level of the free concentration of intracellular calcium in small and intermediate-sized neurons. The large-sized neurons were unresponsive to these receptor agonists with the exception of ATP. The response to ATP was relatively transient in nature and did not differ between neurons of differing somatic diameter. Two types of calcium response were evoked by either bradykinin or adenosine which were observed equally in the small- and intermediate-sized neurons. Capsaicin evoked a biphasic response which rose to a peak and was followed by a long-lasting plateau component. The peak was completely dependent on extracellular calcium whereas the plateau was maintained by an intracellular calcium source. The rise in the free concentration of intracellular calcium evoked by substance P was monophasic in nature. Our results show that although the majority of dorsal root ganglion neurons express functional ATP receptors, small- and intermediate-sized neurons clearly differ from large-sized neurons according to: (i) the presence of substance P-like immunoreactivity; (ii) responsiveness to bradykinin and capsaicin; (iii) presence of functional adenosine and substance P receptors which have not been described before. It is hoped that by identifying the subpopulations of sensory neurons which possess neurotransmitter receptors responsible for elevating the free concentration of intracellular calcium, the physiological role fulfilled by these receptors in vivo may be elucidated.