Centre and surround influences on the proximal negative response of the pigeon retina

Biochemical and physiological aspects of endogenous androgens

This review attempts to give a synopsis of the major aspects concerning the biochemistry of endogenous androgens, supplemented with several facets of physiology, particularly with respect to testosterone. Testosterone continues to be the most common adverse finding declared by World Anti-Doping Agency accredited laboratories, such samples having an augmented testosterone to epitestosterone ratio. Knowledge regarding the precursors and metabolism of endogenous testosterone is therefore fundamental to understanding many of the issues concerning doping with testosterone and its prohormones, including the detection of their administration. Further, adverse findings for nandrolone are frequent, but this steroid and 19-norandrostenedione are also produced endogenously, an appealing hypothesis being that they are minor by-products of the aromatization of androgens. At sports tribunals pertaining to adverse analytical findings of natural androgen administration, experts often raise issues that concern some aspect of steroid biochemistry and physiology. Salient topics included within this review are the origins and interconversion of endogenous androgens, the biosynthesis of testosterone and epitestosterone, the mechanism of aromatization, the molecular biology of the androgen receptor, the hypothalamic-pituitary-testicular axis, disturbances to this axis by anabolic steroid administration, the transport (binding) of androgens in blood, and briefly the metabolism and excretion of androgens.

A centrifugally controlled circuit in the avian retina and its possible role in visual attention switching

The isthmo-optic nucleus (ION) is the main source of efferents to the retina in birds. Isthmo-optic neurons project in topographical order on amacrine cells in the ventral parts of the retina, and a subclass of these known as proprioretinal neurons project onto the dorsal retina. We propose that, through the intermediary of the amacrine target cells, activity in the isthmo-optic pathway excites ganglion cells locally in the ventral retina but inhibits those in dorsal regions. This circuit would thereby mediate centrifugally controlled switches in attention between the dorsal retina, involved in feeding, and the more ventral parts, involved in scanning for predators. This hypothesis accounts for a wide range of disparate data from behavior, comparative anatomy, endocrinology, hodology, and neurophysiology.

Depth-marking the proximal negative response in the pigeon retina

The depth distribution of proximal negativity in the pigeon retina was investigated with a dye-marking technique. Proximal negativity is recorded in three stages as the retina is penetrated. At and near the surface (level 1) the response is small and phasic: next, during "level 2" the one response exceeds the off response in amplitude: then, at "level 3" the off response exceeds the on response in amplitude. The dye marks were located in thin light microscope sections; they were about 20 micrometer in diameter, and were associated with larger areas of tissue damage. Level 3 dye marks were centred at the middle of the inner plexiform layer, and level 2 dye-marks were distributed more proximally. The amplitude maxima of the PNR are close to the middle of the inner plexiform layer, and are separated by approximately 15 micrometer.

GABA-mediated control of transient signals in the inner retina

The effects of chemicals on visual activity was investigated by recording the proximal negative response (PNR) in the perfused eyecup of frogs. Various amino acids reversibly decrease or abolish the PNR over an equivalent concentration range but differ in this depressive action in the presence of convulsant alkaloids. Depression produced by GABA was antagonized by picrotoxin or bicuculline; while glycine-produced depression was selectively antagonized by strychnine. In addition to selective antagonism of amino acids, the convulsants produced differential enhancement of sustained and transient phases of the PNR. In this respect, the effects of picrotoxin and bicuculline are similar to those of semicarbazide and chloride-free solutions which may also inactivate GABA pathways. The effects of these chemicals on the time course and input-output relation of the PNR suggest that processes involving GABA may regulate the transformation from sustained to transient signals which occurs in the inner retina.

A comparison of the proximal negative response and ganglion cell responses to sinusoidal flicker

In the mudpuppy retina, sinusoidal light stimulation of the proximal negative response (PNR) demonstrates two main components whose phases are essentially the same as those of the spike discharges of ON/OFF ganglion cells. Oscillations superimposed upon these components are synchronized with the spike discharges of ON/OFF cells, but not of ON or OFF cells. Intracellular recordings from ON/OFF ganglion cells reveal slow potentials with nearly identical waveforms and phases to those of the PNR. These results, together with previously published comparisons between the flesh-evoked PNR and amacrine cell responses, support the suggestion that the PNR arises from activity in the on/off system (amacrine and/or ganglion cells).

Ganglion cell discharge and proximal negativity in the pigeon retina

1. Proximal negativity has been compared with the spike responses and membrane responses of pigeon ganglion cells. 2. Spike discharge peaks develop during the rising phase of proximal negativity. 3. Proximal negativity is never directional, even when recorded concurrently with directional ganglion cells. 4. To moving stimuli, on-centre cells respond in association with the leading edge of the stimulus, off-centre cells with the trailing edge, and on-off cells with the leading and trailing edges. Proximal negativity is produced in association with the leading and trailing edges. 5. Quasi-intracellular records show that discharge peaks occur in association with depolarizing potentials. The synaptic responses of on-off cells bear a closer resemblance to the proximal negative response than do the responses of on-centre cells, off-centre cells, or directional cells.