COMPLEMENTARY DISTRIBUTION OF RECEPTORS FOR NEUROTENSIN AND NPY IN SMALL NEURONS IN RAT LUMBAR DRGS AND REGULATION OF THE RECEPTORS AND PEPTIDES AFTER PERIPHERAL AXOTOMY

Neurotensin (NT) has been reported to have antinociceptive effects at the spinal level, In situ hybridization, electrophysiology, immunohist ochemistry, and electronmicroscopy were used to investigate the distri bution of NT receptors, possible effects of NT on primary sensory neur ons, and the effect of nerve injury on the expression of NT receptors and NT, NT receptor (R) mRNA was observed in more than 25% of the smal l dorsal root ganglion (DRG) neurons, which lacked neuropeptide Y NPY- R mRNA and essentially other neuropeptide mRNAs, Intracellular recordi ng using voltage-clamp mode showed that NT evokes an outward current i n NPY-insensitive small neurons, and NPY an outward current in NT-inse nsitive small neurons. Both peptides lacked effect on several small DR G neurons, In the superficial dorsal horn NT immunoreactive (IR) termi nals directly contacted primary afferent terminals without synaptic sp ecializations. This new category (> 25%) of the small DRG neurons expr essing NT-R mRNA was complementary to the around 60% of small neurons expressing NPY-R mRNA (and also substance P and calcitonin gene-relate d peptide mRNAs) and to the rest exhibiting somatostatin mRNA expressi on. The electrophysiological results support this classification, show ing that NT and NPY have inhibitory effects on separate subpopulations of small DRG neurons. After sciatic nerve transection, a marked decre ase was observed in (1) the number of NT-R mRNA-positive neurons in DR Gs, (2) NT mRNA-positive neurons in the dorsal horn, and (3) NT-IR cel l bodies and fibers in laminae I-II, Thus, axotomy causes downregulati on of several NT systems at the spinal level, suggesting that the poss ible effects of NT on primary sensory neurons is attenuated after peri pheral axotomy.