Biomedical subjects

M A Webster

Publications and source records attributed to M A Webster.

39 records · Page 3Linked to original sources

PubMed1985

Relationship between spatial-frequency and orientation tuning of striate-cortex cells.

If striate cells had the receptive-field (RF) shapes classically attributed to them, their preferred spatial frequencies would vary considerably with orientation. Other models of RF shape would predict a greater independence between orientation and spatial-frequency tuning. We have examined this by recording the responses of cat striate-cortex cells to a wide range of different spatial-frequency and orientation combinations. In almost all cells studied, peak orientation did not consistently vary with spatial frequency, but the majority of cells showed some change in peak spatial-frequency tuning with orientation. The amount of change in peak spatial frequency tended to be greater for cells that were narrowly tuned for orientation. However, cells narrowly (and also very broadly) tuned for spatial frequency tended to show considerable independence of spatial-frequency and orientation tuning, and in all but a few cells the degree of change was less than predicted by the classic RF model. Such cells were found to fire only to patterns whose local spatial spectra fell within a compact, restricted, roughly circular two-dimensional spatial-frequency region. We conclude that the two-dimensional RF shape of striate cells more closely approximates that predicted by a two-dimensional Gabor model or by a Gaussian-derivative model than it does the classic shape based on the output of geniculate cells with aligned RF's.

Animals↗

PubMed1985

Effect of inhibition of prostaglandin synthesis on cervical softening and uterine activity during ovine parturition resulting from progesterone withdrawal induced by epostane.

The effects of an inhibitor of 3 beta-hydroxysteroid dehydrogenase (epostane) on uterine activity and cervical softening have been studied in eight sheep during late pregnancy. Treatment with epostane led to a rapid decline in the concentration of progesterone measured in utero-ovarian venous plasma, to less than 10% of the pretreatment value within 30 min of bolus injection. This was followed by a significant (P less than 0.02) increase in the concentrations of metabolites of prostaglandins E and F in utero-ovarian venous plasma and uterine activity similar to that seen in the final stages of normal labour. Measurements of cervical tissue extensibility made ex vivo showed the cervix to have softened considerably. These changes occurred without any significant change in the concentration of oestradiol-17 beta in utero-ovarian venous plasma. Infusion of mefenamic acid, an inhibitor of prostaglandin synthesis, prevented the changes in uterine activity and cervical softening that occurred after injection of epostane alone. Mefenamic acid also reduced the increase in concentrations of metabolites of prostaglandins E and F in plasma, although the concentration of progesterone in these animals showed the same abrupt fall which occurred in sheep after injection of epostane alone. These results suggest that progesterone withdrawal, in the absence of any subsequent rise in circulating oestrogen concentrations, is sufficient stimulus to induce cervical softening in the ewe. Cervical softening following progesterone withdrawal can be prevented by inhibition of prostaglandin synthesis.

3-Hydroxysteroid Dehydrogenases↗

PubMed1984

Inhibition of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity in first- and second-trimester human pregnancy and the luteal phase using Epostane.

The effects of a competitive inhibitor of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) (Epostane, Sterling Winthrop, Guildford, England) on serum progesterone (P), estradiol (E2), and cortisol have been studied in three groups of pregnant women awaiting termination of pregnancy (5 to 8 weeks, 8 to 12 weeks, and 12 to 18 weeks of pregnancy) and 15 women in the luteal phase of the menstrual cycle. A single-dose randomized double-blind study was performed, each woman receiving a placebo, 50 mg of Epostane, or 100 mg of Epostane. In the pregnant group, there was a significant decline in the serum P concentration after both 50 mg and 100 mg of Epostane. The percentage fall increased with both drug dosage and advancing gestation. A similar fall in serum E2 was observed. Both of these effects were temporary. In the luteal phase group, a significant decline in serum P was observed after 100 mg of Epostane, but the serum E2 was not significantly different from the pretreatment concentration. Serum cortisol did not differ significantly from control values. These findings suggest that Epostane is an effective inhibitor of placental and ovarian 3 beta-HSD, which may have a role as an interceptive agent.

3-Hydroxysteroid Dehydrogenases↗