Journal of Neuroscience, Vol 15, 2733-2747, Copyright © 1995 by Society for Neuroscience
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
X Zhang, ZQ Xu, L Bao, A Dagerlind and T Hokfelt
Department of Neuroscience, Karolinska Institute, Stockholm, Sweden.
Neurotensin (NT) has been reported to have antinociceptive effects at the
spinal level. In situ hybridization, electrophysiology,
immunohistochemistry, and electronmicroscopy were used to investigate the
distribution of NT receptors, possible effects of NT on primary sensory
neurons, 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 small
dorsal root ganglion (DRG) neurons, which lacked neuropeptide Y NPY-R mRNA
and essentially other neuropeptide mRNAs. Intracellular recording using
voltage-clamp mode showed that NT evokes an outward current in
NPY-insensitive small neurons, and NPY an outward current in NT-insensitive
small neurons. Both peptides lacked effect on several small DRG neurons. In
the superficial dorsal horn NT immunoreactive (IR) terminals directly
contacted primary afferent terminals without synaptic specializations. This
new category (> 25%) of the small DRG neurons expressing NT-R mRNA was
complementary to the around 60% of small neurons expressing NPY-R mRNA (and
also substance P and calcitonin gene-related peptide mRNAs) and to the rest
exhibiting somatostatin mRNA expression. The electrophysiological results
support this classification, showing that NT and NPY have inhibitory
effects on separate subpopulations of small DRG neurons. After sciatic
nerve transection, a marked decrease was observed in (1) the number of NT-R
mRNA-positive neurons in DRGs, (2) NT mRNA-positive neurons in the dorsal
horn, and (3) NT-IR cell bodies and fibers in laminae I-II. Thus, axotomy
causes downregulation of several NT systems at the spinal level, suggesting
that the possible effects of NT on primary sensory neurons is attenuated
after peripheral axotomy.