Mu- and delta-opioid receptors are downregulated in the largest diameter primary sensory neurons during postnatal development in rats

The aim of this study was to investigate the postnatal development of mu-(M OR) and delta -opioid receptor (DOR) immunoreactivity in rat dorsal root ga nglia. Lumbar dorsal root ganglia (DRG) from postnatal day (P) 0, 3, 7 and 21 rat pups were immunostained for MOR and DOR. Proportions of MOR +ve and DOR +ve cells were calculated from profile counts. Diameters of MOR + ve an d DOR +ve cells were measured and compared to -ve cells. The coexpression o f MOR and neuro;filament (NF200) in DRG over this postnatal period was also investigated. A greater proportion of cells were immunoreactive for MOR an d DOR in neonatal rat DRG at PO, P3 and P7 compared to P21. At P3, 39.5 +/- 1.7% of cells were MOR +ve and 30.3 +/- 1.5% were DOR +ve, whereas at P21, the values were 30.1 +/- 1.7% and 21.8 +/- 1.6% (mean + SEM), respectively . During the first postnatal week both opioid receptors were expressed in c ells across the whole diameter range but by 3 weeks of age, expression was restricted to small and medium diameter cells. Furthermore, a significantly higher proportion of NF200 +ve cells expressed MOR in new-born compared to P21 rats. The results show that MOR and DOR expression is downregulated in the largest diameter, NF200 +ve primary sensory neurons postnatally. Since these neurons are mainly non-nociceptive, this may explain previous report s of opioid agonists affecting reflex responses to both innocuous and noxio us stimuli in rat pups. The results highlight an important difference betwe en opioid function in the immature and adult nervous system. (C) 2001 Inter national Association for the Study of Pain. Published by Elsevier Science B .V. All rights reserved.