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The existence of specific, high-affinity opioid binding sites was demonstrated in 21-week-old human fetuses with the use of tritiated etorphine, a non-selective opioid ligand. Binding capacities measured in the brain, the cerebellum and the spinal cord give values of 2.83, 3.71 and 5.42 pmol/g of tissue. Kd values do not vary from one region to the other and are respectively 0.29, 0.35 and 0.33 nM. These results are compared with the characteristics of opioid binding sites found in adults.

The existence of specific binding sites for the tachykinin Substance P was investigated in various regions of the central nervous system of a 28-week-old human foetus. Tritiated Substance P binds with high affinity to various amounts of binding sites in the limbic area of the brain, in the frontal, parietal and occipital cortices as well as in the cerebellum. The presence of these specific sites in the foetus is discussed in relation with the proposed neurotrophic role of Substance P.