Reconstruction of cone-system contributions to responses of colour-opponent neurones in monkey lateral geniculate

Responses of colour-opponent X-cells to intensity-modulation at various wavelengths were obtained in the lateral geniculate nucleus (LGN) of the anaesthetized (N₂O/O₂) rhesus monkey. The gaussian white noise (GWN) analysis method was used to describe the stimulus-response relationship. Two different methods were used to estimate sign and relative strength of the response contribution of each of the three known cone systems as a function of time. Both methods revealed that, in contrast to the wellknown variability in gain and sign, the time course of the cone-type contributions was remarkably stereotyped in all cells. Surround-mediated cone-type contributions appeared to have a consistently longer delay than centre-mediated inputs. Response contributions from different types of cone appeared to add linearly in LGN neurones. Apart from rectification, it was possible to predict the response of the same neurone to step-modulation of intensity at various wavelengths successfully with the first-order Wiener kernel. This demonstrates that the cells behaved linearly under our stimulus conditions, which justifies the use of the first-order kernel as a means to characterize the system we wished to study.     

Cholesterol 7alpha-hydroxylase. 2. Biochemical properties and participation of endogenous cholesterol in the assay in vitro.

70% of the microsome-bound cholesterol is directly accessible to cholesterol 7alpha-hydroxylase in an assay in vitro. After 5 min of incubation this endogenous cholesterol makes a single pool with the exogenously added substrate and modifies its specific radioactivity. Thus, an accurate estimation of the enzymic activity should take the participation of endogenous cholesterol into account. Cholesterol 7alpha-hydroxylase activity is enhanced in vitro by thiol-containing substances like mercaptoethanol, dithiothreitol, or cysteamine. On the contrary, the enzymic activity is inhibited by heavy cations (Hg2+, Pb2+, Cu2+, Zn2+), or --SH-blocking agents (mersalic acid p-chloro-mercuribenzoic acid). Several steroids are potent inhibitors (Ki less than Km) of the enzyme, among them pregnenolone and its acetate derivative and the cholesterol closely related 7-oxocholesterol and 7-dehydrocholesterol. The cholesterol esters are neither substrates nor inhibitors of cholesterol 7alpha-hydroxylase. Only a high concentration (1 mM) of biliary acids or of their glyco or tauro derivatives inhibits cholesterol 7alpha-hydroxylase. The quantitatively less important lithocholic acid and deoxycholic acid are the strongest inhibitors; the most common cholic acid does not affect the enzymic activity even at 1 mM.     

Long-term maintenance of monoxygenase activities in cultured fetal rat hepatocytes

Fetal hepatocytes cultured in the presence of dexamethasone even in low concentration were maintained alive for several weeks. The expression of monoxygenase in these cells is switched from fetal to adult type. Their aldrin epoxidase and ethoxycoumarin-o-de-ethylase activities were maintained at a high level. Cytochrome P-450 concentration remains stable in these cells throughout the culture period. Cell-cell and cell-biomatrix interactions seem to play an important role in the control of growth, maturation and enzymatic activity expression of the cells in culture. This model may constitute an interesting approach for the study of drug metabolism and drug toxicity in vitro.