NEUROKININ-1 RECEPTORS ON LUMBAR SPINOTHALAMIC NEURONS IN THE RAT

In order to determine whether spinothalamic neurons in the lumbar spin al cord of the rat possess neurokinin-1 (substance P) receptors, we in jected cholera toxin B subunit into the thalamus and carried out dual- labelling immunocytochemistry to search for neurons that were immunore active with antibodies to cholera toxin and neurokinin-1 receptor. We examined 356 spinothalamic neurons in transverse sections and found th at 35% of these were neurokinin-1 receptor-immunoreactive. Double-labe lled cells made up the majority of the spinothalamic population in lam ina I and the lateral spinal nucleus, and were also present in laminae III-V and the area around the central canal. On the side contralatera l to the injection site, 77% of spinothalamic neurons in lamina I also showed neurokinin-1 receptor immunoreactivity, while 33% of those in laminae III-V and 14% of the ventromedial group possessed the receptor . Several of the double-labelled neurons with cell bodies in laminae I II and IV had dendrites which could be followed dorsally into the supe rficial dorsal horn. These results indicate that substance P released from nociceptive primary afferents into the superficial dorsal horn is likely to act on spinothalamic tract neurons in lamina I, and also on those with cell bodies in laminae III-IV and long dorsal dendrites.