Prostaglandin E2 Receptors in the Chicken Spinal Cord: 2. Radioautographic Localization

High and low affinity binding sites for PGE2 were localized at the cellular level by means of radioautography. Two types of radioautographic reactions were observed in cryostat sections incubated with 0.6 nM [3H]PGE2: (i) a diffuse reaction scattered over the neuropil; (ii) a stronger reaction over the perikarya of some motoneurons. However, not all the motoneurons were equally labelled; the density of silver grains increased 12 times from poorly labelled to highly labelled cells. After incubation with an excess of unlabelled PGE2 the labelling was drastically reduced over motoneurons and the neuropil, so that the specific binding corresponded to 90 and 85% respectively of the total binding. Two facts indicated that the motoneurons possessed mainly high affinity binding sites which could be saturated by PGE2 concentrations in the nanomolar range: (i) a five-fold increase of [3H]PGE2 concentration (from 0.6 to 3 nM) produced only a 1.5 times increase in the density of silver grains over the perikarya; (ii) incubation in presence of 14 nM unlabelled PGE2 induced a drastic reduction of 0.6 nM [3H]PGE2 binding to the motoneurons. The presence of high affinity binding sites in motoneurons suggests that PGE2 could act as a modulator of spinal reflexes.