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

We have attempted to identify which subpopulations of rat sensory neur ons possess functional neurotransmitter receptors which elevate the fr ee concentration of intracellular calcium. Subpopulations of sensory n eurons were identified using three accepted criteria: (i) the distribu tion and proportion of neurons with differing somatic diameters; (ii) the expression of substance P-like immunoreactivity; and (iii) the res ponsiveness of each neuron to capsaicin. The total neuronal population was primarily grouped into three classes according to somatic diamete r and defined as small- (< 17 mu m), intermediate- (17-25 mu m) and la rge- (> 25 mu m) sized neurons. It was not possible to distinguish bet ween small and intermediate-sized neurons since a similar percentage o f each class expressed substance P-like immunoreactivity or sensitivit y to capsaicin. Large-sized neurons did not possess these characterist ics and, therefore, represented a distinct neuronal population. In sin gle, 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 substan ce P elevated the resting level of the free concentration of intracell ular calcium in small and intermediate-sized neurons. The large-sized neurons were unresponsive to these receptor agonists with the exceptio n of ATP. The response to ATP was relatively transient in nature and d id not differ between neurons of differing somatic diameter. Two types of calcium response were evoked by either bradykinin or adenosine whi ch were observed equally in the small- and intermediate-sized neurons. Capsaicin evoked a biphasic response which rose to a peak and was fol lowed by a long-lasting plateau component. The peak was completely dep endent 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 n eurons 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 hope d that by identifying the subpopulations of sensory neurons which poss ess neurotransmitter receptors responsible for elevating the free conc entration of intracellular calcium, the physiological role fulfilled b y these receptors in vivo may be elucidated.