Rapid changes in cerebral blood flow and initial visual experience in the developing chick

Microsphere determination of cochlear blood flow in chickens

Chickens were injected with 9-micron-diameter radioactive microspheres. Cochleas were removed through the external auditory meatus, and the positions of all embedded microspheres were drawn under camera-lucida. Constant measurements of arterial pressures and postinjection blood-gas determinations confirmed that injections were made into normal circulatory systems. The averaged estimate of cochlear blood flow in chickens is 0.75 microliter/min. Variability in these data from chickens is similar to that reported from mammals. A potentially important but puzzling observation is an inverse relationship between blood flow to the cochlea and to the brain. The ease of cochlear extraction makes chickens ideal models for study of cochlear blood flow.

The nutrient supply of the chick visual system: responses to form and light deprivation

Blood flow and uptake of two nutrient analogues have been studied in the neural retina, pigment epithelium and optic lobes of 5-7-day-old chicks after systemic administration of isotopically labeled compounds. The pigment epithelium accumulated the inert amino acid, alpha-aminoisobutyric acid, 6 times as effectively on a wet weight basis than did the neural retina or optic lobes. However, the neural retina was over 3 times as active in transport of deoxyglucose as was either the pigment epithelium or optic lobe. These differences could not be attributed to variations in blood flow since the penetrance of the freely diffusible antipyrine was low in the neural retina and roughly equal in the vascularized epithelium and optic lobes. Two days of unilateral eyelid suture significantly depressed blood flow and deoxyglucose uptake in the optic lobe innervated by the occluded eye. The neural retina of the eye receiving reduced light input also had a reduced level of deoxyglucose accumulation but the capacity to concentrate alpha-aminoisobutyrate was increased. The corresponding dark-adapted pigment epithelium had a major (48%) increase in the uptake of this inert amino acid. The data show that regional glucose utilization may not be directly proportional to blood flow or glucose diffusion rates. The non-vascularized neural retina possesses an unusually powerful glucose transport mechanism while the pigment epithelium has a high capacity for amino acid accumulation.

Brain blood flow: alteration by prior exposure to a learned task

Chicks with the eyelids of one eye sutured were trained to discriminate between grains and pebbles. The learned experience was completely recognizable by the naive eye that had been occluded during training. When both eyes were opened after monocular training, the velocity of blood flow through paired left and right brain regions was identical. However, when chicks were reexposed to the discrimination situation, blood flow through the cerebral hemisphere associated with the naive eye was greater than that through the hemisphere associated with the trained eye.

Putative neurotransmitters of the avian visual pathway

The ability of homogenates of the chick optic lobe to accumulate a series of possible neurotransmitters has been studied. High affinity uptake of several possible neurotransmitters was examined in optic lobes of 21-day-old embryos that had a single eye removed on the third day of incubation and in 23-day-old chicks that had an eye removed at hatch. Embryonic enucleation resulted in severe reduction of development of the ability of the contralateral optic lobe to take up tritiated GABA, dopamine, choline, serotonin and glutamate from solutions around 10(-8)M. Unilateral eye removal of new-hatched chicks caused failure of the denervated optic lobe to grow, but only the uptake capacity for glutamate was significantly recuced. This deficit was apparent as early as 4 days after enucleation. The transport of other compounds was unimpaired. The uptake of glutamate by homogenates of the optic tract was 43% of that or the optic lobe. This was a much greater fraction than the corresponding value for other postulated neurotransmitters. These data suggest that glutamate may be the primary neurotransmitter of the fibers of the optic tract originating in the retinal ganglion cells.