Selective effect of a new antituberculous drug, rifampicin, on the c-wave of the sheep electroretinogram

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. I. Intensity-amplitude relations of the D.C. recorded ERG with special reference to the c-wave

The main positive component of the c-wave of the ERG is generated by the pigment epithelium-receptor complex, which is supplied from the choroidal circulation. Occlusion of the central retinal artery (OCRA) causes serious morphological damage only to the inner retina. An effect on the c-wave is therefore not primarily to be expected. In this study on five Cynomolgus monkeys with unilateral, laser-induced OCRA the c-wave was decreased, slightly in the early stage and markedly in the late stage. A hypothetical explanation of the reduced c-wave amplitude, involving changes in potassium ion concentration, is discussed. The conflicting results in other reports may be explained by uncontrolled influence of slow c-wave amplitude oscillations, by differences between acute and late stages, and by possible surgical damage to the choroidal circulation in some cases.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. II. Influence on the cyclic changes in the amplitude of the c-wave of the ERG and in the standing potential of the eye

The slow (2-3/h) oscillations of the c-wave amplitude and of the standing potential of the eye (SP) were studied in the Cynomolgus monkey during the acute and late stages after experimentally induced (laser photocoagulation) occlusion of the central retinal artery (OCRA). Whereas the healthy control eyes showed large cyclic variations in both the c-wave amplitude and the SP, no oscillations in the c-wave amplitude were observed in the OCRA eyes at any stage, and the SP oscillations were barely detectable in these eyes. OCRA causes morphological damage to the inner retina but not to the pigment epithelium-photoreceptor complex, which generates the positive component of the c-wave, and where the SP is also believed to originate. The findings of the present study strongly indicate that the SP and c-wave oscillations are related, and that they are either dependent on an intact inner retina or that the pigment epithelium-photoreceptor complex is functionally affected by OCRA. If not taken into consideration, the marked difference in oscillations between the OCRA eye and the healthy eye may be a major source of error when comparing c-waves from the two eyes, and it seems that some of the conflicting results reported by others can be explained in this way.

Changes in ultrastructure and function of the sheep pigment epithelium and retina induced by sodium iodate. II. Early effects

The present investigation shows that the membrane properties of the sheep pigment epithelial cells were very rapidly and severely affected by sodium iodate, whereas the effects concerning the neuroretina were delayed. The c-wave of the ERG was immediately abolished and replaced by a cornea-negative potential, but the a- and b-waves were preserved for about 80-100 min. Ultrastructurally the plasma membranes (particularly the basal plasma membrane) of the pigment epithelial cells were destroyed or less distinct than normally. The cell organelles were swollen and ruptured. There were indications that the pigment epithelium could no longer participate in the receptor outer segment turnover. The photoreceptor cells were morphologically undamaged, and few or no signs of injury were observed in the inner layers of the retina. The effects upon the neuroretinal functions seen after 80-100 min, consisting of a reduction of alpha- and beta-wave amplitudes, were most likely caused by an inability of the pigment epithelium to maintain in the long run its metabolic and barrier properties. It appears that at an early stage after sodium iodate injection, the present preparation may be useful for the study of the effects on the neuroretina proper of drugs and other agents.

Melanotropic drugs and retinal functions. II. Effects of phenothiazine and rifampicin on the sheep ERG

The acute effects of chlorpromazine, promethazine and rifampicin on the a-, b- and c-waves of the conventional electroretinogram (ERG) were studied in sheep. Iv administration of chlorpromazine and promethazine resulted in a b-wave amplitude decrease and an initial c-wave amplitude decrease, followed by cyclic amplitude changes resembling damped oscillations. Iv injections of rifampicin, however, resulted in cyclic changes of the c-wave amplitude without initial concomitant b-wave changes. The results of the present study indicate that rifampicin has a selective influence on the pigment epithelial cells, while chlorpromazine and promethazine seem to have more generalized retinal effects on both the neuroretina and the pigment epithelial cells.

Effects of ethyl alcohol on the directly recorded standing potential of the human eye

The effects of ethanol on the human standing potential (SP) were studied with a recently developed method, which allows direct SP recordings by means of a suction contact lens, temperature stabilized calomel electrodes and d.c. amplification. It is well known that the human SP oscillates with a frequency of about 2/hour in response to a sudden change in illumination. In the present paper marked cyclic variations of the SP, resembling damped oscillations, were provoked by a small oral dose of ethyl alcohol. A first maximum was reached after about 10 min. The difference in amplitude between the peak and the trough of the first oscillation was of the order of 4 mV. The oscillatory frequency was about 2/hour. The length of a cycle varied between 25 and 34 min in different volunteers, being fairly constant in the same subject on different occasions. The SP response to ethanol was similar both under scotopic and photopic conditions. The results correlate well with earlier findings of 2/hour oscillations in c-wave amplitude in response to ethanol, as may be expected considering the partly common origin of the c-wave and the SP.