The postnatal development of benzodiazepine receptor sites in the chick optic lobe is modulated by environmental lighting

Abstract

The present paper describes the ability of benzodiazepin receptor sites to undergo light mediated-plastic changes during the early postnatal development of the chick optic lobe. The postnatal development pattern of these receptors was studied under different levels of light stimulation, i.e. normal-, light- and dark-rearing. At hatching the specific binding of [ 3H]Flunitrazepam was 0.23 ± 0.01 pmol/mg protein. The developmental profile shows a sharp and transient peak of receptor overexpression between the 1st and the 2nd postnatal day in the three experimental groups. Between the 2nd and the 6th day significant differences were found between the three groups, being this difference maximal during the peak of over-expression. In fact, on the 2nd day the specific [ 3H]Flunitrazepam binding showed an increase of 17% ( P < 0.0005) and a decrease of 34% ( P < 0.0005) for light- and dark-reared animals as compared with normally-reared ones. The changes in receptor density were transient since from the 6th day onward they gradually disappeared, being almost identical in the three groups by the day 15. At this moment the number of benzodiazepine receptor sites stabilized at the adult level. Scatchard analysis at the 2nd postnatal day revealed that the differences observed in the high affinity benzodiazepine binding sites between the three groups were due to modifications in the total number of binding sites while the affinity remained unchanged. The maximal number of binding sites were: 2.76 ± 0.03, 3.40 ± 0.01 and 1.46 ± 0.11 pmol/mg protein in normally-, light- and dark-reared chicks, respectively; while the apparent dissociation constants were unaffected. The present work shows that variations in the light intensity during the early postnatal life affect the postnatal developmental pattern of benzodiazepine receptor sites. This effect is maximal at the time of appearance of the peak of receptor overexpression (1–2 postnatal day) suggesting that the peak corresponds to a brief period of high plasticity for the receptor development.