David Ehrlich

Asymmetry in the chicken forebrain during development and a possible involvement of the supraoptic decussation

During the first week of post-hatch life, the chicken forebrain is asymmetrically susceptible to the action of cycloheximide. Between days 2 to 8 treatment of the left hemisphere, but not the right, causes long-lasting changes in behaviour. The right hemisphere is susceptible on days 10 and 11. These developmental events may be related to a significant loss of fibres in the supraoptic decussation, which occurs around the same time.

A quantitative analysis of the effects of excitatory neurotoxins on retinal ganglion cells in the chick

The present study examines the differential effects of three excitotoxins, kainic acid (KA), N-methyl-D-aspartate (NMDA), and alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) on neurons within the ganglion cell layer (GCL) of the chick retina. Two-day-old chicks were given a single, 5 microliters, intravitreal injection of KA, NMDA, or AMPA at a range of doses. Following treatment with 40 nmol KA, there was a 21% loss of neurons in the GCL. At 200 nmol KA, the loss increased to 46%. Exposure to KA eliminated mainly small neurons of soma area 5-15 microns2, and medium-sized ganglion cells of soma area 15-25 microns2. Large ganglion cells (greater than 25 microns2) remained unaffected. The vast majority of small cells were probably displaced amacrine cells. Exposure to 400 nmol NMDA resulted in a 14% loss of neurons, predominantly involving the large ganglion cells. At a dose of 3000 nmol NMDA, no further loss of cells was evident. Exposure to 200 nmol AMPA resulted in a 30% loss of large and some medium-sized ganglion cells. In a further series of experiments, exposure to excitotoxin was followed by a retinal scratch, which eliminated retinal ganglion cells within the axotomized region. The results indicate that only a small proportion of displaced amacrine cells are destroyed by NMDA and AMPA, whereas virtually all displaced amacrine cells are sensitive to KA. The findings of this study indicate the existence of subclasses of ganglion cells with specificity towards different types of excitatory amino acids (EAA).

Sex-dependent structural asymmetry of the medial habenular nucleus of the chicken brain.

SummaryAn investigation of structural asymmetry in the avian brain was conducted on the epithalamic medial habenular nucleus of the chicken. Twelve male and ten female two-day-old chickens were used for a morphometric evaluation of asymmetry. The medial habenular nucleus was measured from paraffin-wax-embedded, 8 μm-thick sections by use of a semiautomatic image analyser. The volumes of the right and left medial habenula of each animal were statistically analysed (‘within animal experimental design’). The right medial habenula in males showed significant group asymmetry. In contrast, females failed to demonstrate group bias in favour of either hemisphere. However, individual females were lateralised, with either a larger right or left medial habenula. Although individuals of both sexes were lateralised, there was no significant sex difference in volume in either the right or left medial habenula.We propose that sex-linked structural asymmetry may be influenced by steroid hormonal effects in the central nervous system, and that such asymmetry could be more prevalent in the non-mammalian vertebrate brain than previously considered.

Excitatory amino acids interfere with normal eye growth in posthatch chick

This study examines the effects of excitotoxic amino acids on eye growth and retinal morphology. Day old chicks received a single intraocular injection of either 200 nmoles kainic acid (KA), 200 nmoles quisqualic acid (QUIS) or 400 nmoles N-methyl-D,L-aspartate (NMDA). Following survival periods of 7, 14 and 21 days, eyeballs were removed and weighed. Measurements of axial length, equatorial length, anterior chamber depth and corneal diameter were taken. Treatment with KA increased eye weight and equatorial length. Treatment with QUIS increased the anterior chamber depth but decreased the equatorial length. Treatment with NMDA increased anterior chamber depth, but to a lesser extent than QUIS. The effects of QUIS and NMDA could be distinguished from those of KA since the former excitotoxins resulted in a marked increase in anterior chamber depth with no enlargement of vitreal chamber. Changes in eye size were evident by day 7 and were sustained throughout the duration of the experiment. Examination of retinae revealed that KA lesions amacrine cells, bipolar cells, some ganglion cells and photoreceptors. Exposure to QUIS lesions amacrine cells, horizontal cells and causes mild disruption of photoreceptor outer segments. In contrast, NMDA predominantly lesions amacrine cells. The results demonstrate that these neurotoxins have different effects on eye growth, which may be associated with differences in retinal pathology. It is proposed that photoreceptors are ideally suited to play a role in the control of eye growth.

Sequential treatment with low doses of kainic acid alters sensitivity of retinal cell types

Examination of chick retinae soon after treatment with a single intraocular injection of 5 nmol kainic acid revealed degenerative changes in a small population of neurons located in the outer aspect of the inner nuclear layer, consistent with either bipolar and/or horizontal cells. Damage to the outer plexiform layer was also present. After one week, affected retinae were clear of neuronal debris. Following a second, identical injection, one week after the first, there was degeneration of a different population of neurons confined to the inner aspect of the inner nuclear layer indicative of amacrine cells. As amacrine cells were the only cell type affected by the second injection, our results suggest that they are directly affected by the second dose of kainic acid. The pattern of neuronal damage following successive doses of kainic acid appears to be potentially useful technique in elucidating retinal circuitry.

Specific ganglion cell death induced by intravitreal kainic acid in the chicken retina

In young chickens, intravitreal kainic acid (60 nmol) causes axonal and terminal degeneration in some retinorecipient areas of the chicken brain. The accessory optic nuclei are markedly affected, suggesting that the large displaced ganglion cells are destroyed by kainic acid, and a specific pattern of degeneration is caused in the optic tectum, which suggests that other minor ganglion cell groups may also be sensitive to intravitreal kainic acid.

Composition of the supraoptic decussation of the chick (Gallus gallus)

SummaryIn the chick the supraoptic decussation consists of dorsal, ventral, and subventral subregions. The dorsal region contains about 580000 axons of which 24% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.31 μm and 1.0 μm, respectively. The ventral region contains about 520000 axons of which only 0.53% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.35 μm and 1.09 μm, respectively. A small proportion of unmyelinated fibers has regions of localized expansions. The subventral region, the smallest subregion, contains about 31000 fibers of which 20% are myelinated. The mean diameters of unmyelinated and myelinated axons are 0.31 μm and 1.15 μm, respectively. On the basis of the interhemispheric channels available to the visual system it is argued that visual information must be modified to some extent prior to its transfer.

Neurotoxic Effects of Kainic Acid on Developing Chick Retina

The neurotoxic effects of kainic acid (KA) on developing neurons in the chick retina was investigated in an in vitro preparation. Eyecups from chick embroyos at 6 (E-6), 8, 10, 12, 14, 16, and 20 days of incubation and from chicks on day 1 posthatch (D-1) were exposed to different doses of KA for 30 min and then processed for light microscopy. Neurotoxic damage was evaluated by the presence of swollen cell bodies, containing pale cytoplasm and pyknotic nuclei. At E-8, amacrine cells first became sensitive to KA and displayed neurotoxic damage at a threshold concentration of 20 microM. Their sensitivity to KA increased over the following 4 days, so that by E-12 they attained a threshold sensitivity of 5.0 microM KA. At E-14, one third of the retinae showed amacrine cell damage at 0.5-2.0 microM KA, less than the threshold dose of 5.0 microM KA required at D-1. Bipolar cells first become sensitive to KA at E-12, at a threshold concentration of 5.0 microM. The threshold concentration decreased over the following 10 days: 2.0 microM at E-16, 1.0 microM at E-20, and 0.5 microM at D-1. At E-8 and E-10, horizontal cells were susceptible to a relatively high concentration of 80 microM KA. The sensitivity to KA is evident prior to the formation of photoreceptor input. These results indicate that amacrine and horizontal cells are susceptible to KA at an earlier age than bipolar cells. Both amacrine and bipolar cells exhibit an age-dependent relationship with the threshold concentration of KA required to cause neurotoxicity; in general, the older the embryo, the lower the dose of KA. However, the increased susceptibility of amacrine cells at E-14 suggests a transient hypersensitivity to KA during this period which may reflect an overproduction of the receptor-ionic channel complex necessary for KA to exert its effect.

Composition of the tectal and posterior commissures of the chick (Gallus domesticus)

In chick, the tectal and posterior commissures form a continuous band of axons lying in the dorsal aspect of the meso-diencephalon. In midline sagittal sections, two zones can be clearly defined. The rostral zone (RZ) contains about 290,000 axons of which 32% are myelinated. The caudal zone (CZ) contains about 911,000 fibers of which 12% are myelinated. The majority of unmyelinated fibers in RZ and CZ are between 0.15 and 0.40 micrometers in diameter. The majority of myelinated fibers in both RZ and CZ are less than 1 micrometer in diameter. It is likely that RZ corresponds to the posterior commissure while CZ corresponds to the tectal commissure.

Gelatin embedding of central nervous system tissue improves the quality of vibratome sections.

We have developed a procedure for Vibratome (Oxford Laboratories) sections that is particularly valuable for providing uniformly thick, well preserved CNS tissue sections for morphometric applications.