Dendrite development and target innervation of displaced retinal ganglion cells of the chick (Gallus gallus)

Displaced retinal ganglion cells in albino and pigmented rats

We have studied in parallel the population of displaced retinal ganglion cells (dRGCs) and normally placed (orthotopic RGCs, oRGCs) in albino and pigmented rats. Using retrograde tracing from the optic nerve, from both superior colliculi (SC) or from the ipsilateral SC in conjunction with Brn3 and melanopsin immunodetection, we report for the first time their total number and topography as well as the number and distribution of those dRGCs and oRGCs that project ipsi- or contralaterally and/or that express any of the three Brn3 isoforms or melanopsin. The total number of RGCs (oRGCs+dRGCs) is 84,706 ± 1249 in albino and 90,440 ± 2236 in pigmented, out of which 2383 and 2428 are melanopsin positive (m-RGCs), respectively. Regarding dRGCs: i/ albino rats have a significantly lower number of dRGCs than pigmented animals (0.5% of the total number of RGCs vs. 2.5%, respectively), ii/ dRGCs project massively to the contralateral SC, iii/ the percentage of ipsilaterality is higher for dRGCs than for oRGCs, iv/ a higher proportion of ipsilateral dRGCs is observed in albino than pigmented animals, v/ dRGC topography is very specific, they predominate in the equatorial temporal retina, being densest where the oRGCs are densest, vi/ Brn3a detects all dRGCs except half of the ipsilateral ones and those that express melanopsin, vii/ the proportion of dRGCs that express Brn3b or Brn3c is slightly lower than in the oRGC population, viii/ a higher percentage of dRGCs (13% albino, 9% pigmented) than oRGCs (2.6%) express melanopsin, ix/ few m-RGCs (displaced and orthotopic) project to the ipsilateral SC, x/ the topography of m-dRGCs does not resemble the general distribution of dRGCs, xi/ The soma size in m-oRGCs ranges from 10 to 21 μm and in m-dRGCs from 8 to 15 μm, xii/ oRGCs and dRGCs have the same susceptibility to axonal injury and ocular hypertension. Although the role of mammalian dRGCs remains to be determined, our data suggest that they are not misplaced by an ontogenic mistake.

Growth hormone and retinal ganglion cell function: QNR/D cells as an experimental model

Retinal ganglion cells (RGCs) have been shown to be sites of growth hormone (GH) production and GH action in the embryonic (embryo day 7, ED7) chick neural retina. Primary RGC cell cultures were previously used to determine autocrine or paracrine actions of GH in the retina, but the antibody used in their immunopanning (anti-Thy-1) is no longer available. We have therefore characterized an immortalized neural retina (QNR/D) cell line derived from ED7 embryonic quail as a replacement experimental model. These cells express the GH gene and have GH receptor (GHR)-immunoreactivity. They are also immunoreactive for RGC markers (islet-1, calretinin, RA4) and neural fibers (neurofilament, GAP 43, vimentin) and they express the genes for Thy-1, neurotrophin 3 (NTF3), neuritin 1 (NRN1) and brn3 (POU4F). These cells are also electrically active and therefore resemble the RGCs in the neural retina. They are also similarly responsive to exogenous GH, which induces overexpression of the neurotrophin 3 and insulin-like growth factor (IGF) 1 genes and stimulates cell survival, as in the chick embryo neural retina. QNR/D cells are therefore a useful experimental model to assess the actions of GH in retinal function.

Chicken suprachiasmatic nuclei: I. Efferent and afferent connections

The avian circadian system is composed of multiple inputs, oscillators, and outputs. Among its oscillators are the pineal gland, retinae, and a hypothalamic structure assumed to be homologous to the mammalian suprachiasmatic nucleus (SCN). Two structures have been suggested as this homolog -- the medial SCN (mSCN) and the visual SCN (vSCN). The present study employed biotin dextran amine (BDA) and cholera toxin B subunit (CTB) as anterograde and retrograde tracers to investigate the connectivity of the mSCN and vSCN in order to address this issue. Intravitreal injections of CTB were used to determine whether one or both of these structures receives afferent input from retinal ganglion cells. Both the vSCN and mSCN receive terminal retinal input, with the strongest input terminating in the vSCN. Precise iontophoretic injections of BDA and CTB in the mSCN and vSCN were used to identify efferents and afferents. The avian mSCN and vSCN collectively express more efferents and afferents than does the mammalian SCN. A subset of these connections matches the connections that have been established in rodent species. Individually, both the mSCN and vSCN are similar to the mammalian SCN in terms of their connections. Based on these data and other studies, we present a working model of the avian SCN that includes both the mSCN and vSCN as hypothalamic oscillators. We contend that both structures are involved in a suprachiasmatic complex that, as a functional group, may be homologous to the mammalian SCN.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.

Distribution of heat shock protein 108 mRNA during the development of the chicken brain

The developmental expression of heat shock protein 108 (HSP108) mRNA was mapped in chicken brain using in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR showed that HSP108 mRNA increased from embryonic day 5 (E5) to 13 (E13), significantly decreased from E17 to E21 and then increased again at the adult stage. In situ hybridization showed that while intense HSP108 positive (HSP108+) signals were localized in the cerebellum from E7 to E14, the intensities of these signals were significantly decreased at E17. However, at the adult stage, HSP108 expression increased in a cell type dependent manner. A decrease in HSP108 mRNA expression during differentiation was also observed in an in vitro study of brain cells treated with nerve growth factor (NGF).

Optical detection of neural function in the chick visual pathway in the early stages of embryogenesis

We investigated the developmental pattern of functional synaptogenesis in the chick visual pathway using a multiple-site optical recording method. Responses to optic nerve stimulation were recorded from the diencephalon and mesencephalon of the chick embryo. The first excitatory postsynaptic responses to optic nerve stimulation appeared in the contralateral diencephalon at Hamburger-Hamilton stage 27, which corresponds to an incubation day 5.5 (E5.5). At more developed stages, the optical signals evoked by optic nerve stimulation spread to several different regions, including the tectum and extra-tectal visual nuclei. We constructed maps of neural activity in the diencephalon and mesencephalon at different stages to investigate the spatio-temporal patterns of functional development in the chick visual system. The maps revealed that distinct postsynaptic response areas in the extra-tectal regions showed different onsets of activity, suggesting that the corresponding visual nuclei exhibit different time courses of functional synaptogenesis. We also identified the onset and location of the first functional synaptic connection in the optic tectum, which had been a point of controversy in earlier studies. In the tectal region, the action potential and the excitatory postsynaptic potential first appeared at E8, although these signals were recognized in the tecto/tegmental region at E7. The response area expanded with retinotectal fibre elongation, and reached the area centralis at E9. These results show that the onset of synaptic function in the tectum occurs 2-3 days earlier than was previously reported.

Neuronal MAP2 mRNA: Species-dependent Differential Dendritic Targeting Competence

Providing the basis for local protein synthesis in dendritic microdomains, RNA transport in dendrites is thought to be underlying long-term neuronal plasticity. Dendritic RNA targeting mechanisms can therefore be expected to confer selective advantages in the evolution of complex neural systems. The question thus arises as to when and how dendritically targeted transcripts first acquired their targeting competence. To address this question, the dendritic targeting competence of MAP2 transcripts was examined in chicken, mouse and rat. In one approach, we established the somato-dendritic distribution of MAP2 transcripts in vivo. We found that in contrast to rodent MAP2 mRNAs, which are highly enriched in dendritic regions of the retina, chicken MAP2 transcripts are virtually absent from such areas and are rather confined to neuronal somata. In an independent line of investigation, we determined that a dendritic targeting element (DTE) corresponding to the mammalian MAP2 DTE is not contained in the 3' untranslated region (UTR) of avian MAP2 mRNA. The combined results indicate that in contrast to mammalian MAP2 transcripts, avian MAP2 mRNA is lacking dendritic targeting competence. The data thus suggest that the acquisition of such competence has likely been a relatively recent event in evolution.

Discharge patterns evoked by depolarizing current injection in basal optic nucleus neurons of the pigeon

The nucleus of the basal optic root of the accessory optic system in birds is involved in optokinetic nystagmus, which stabilizes images on the retina by compensatory movements of the eyes. The present paper studies the physiological and morphological properties of basal optic neurons in the pigeon by using a brain slice preparation and intracellular recordings. Sixty-one cells examined could be categorized into six types based on their firing patterns in response to depolarizing current injection. Type I cells (54%) fire spontaneously and more spikes as current intensity is increased. Type II cells (15%) discharge regular spikes with similar interspike intervals. Type III cells (5%) show an early burst followed by tonic firing. Type IV cells (5%) fire regular bursts with similar interburst intervals. Type V cells (16%) fire a few spikes in a cluster only at onset of current application. Type VI cells (5%) produce a hump-like depolarization or a single spike depending on current intensities. Seventeen cells stained with Lucifer yellow have multipolar or piriform perikarya (15-28 microm) with two to eight primary dendrites. In some cases, an axon is observed to originate from the cell body, traveling dorsolaterally or dorsally. The physiological significance of these findings is discussed.