Dopamine neurones form a discrete plexus with melanopsin cells in normal and degenerating retina

https://doi.org/10.1016/j.expneurol.2007.01.032Get rights and content

Abstract

In addition to rods and cones of the outer retina, a third class of photoreceptive cell has recently been described in the inner retina of mammals. These intrinsically photosensitive retinal ganglion cells (ipRGCs) have been shown to relay luminance information to the mammalian brain. In addition to their intrinsic photosensitivity, the function of ipRGCs may also be modulated by signals from within the retina itself. Such signals may emanate from classical photoreceptors in the outer retina or from the circadian activity of adjacent inner retinal neurones. Prime candidates for the latter are the retinal dopamine neurones which ramify at the border of the inner plexiform and inner nuclear layers. In order to investigate the nature of any interaction between dopamine and ipRGC populations in normal retina and to assess the impact of outer retinal degeneration on this interrelationship, we examined the retinae of normal and RCS dystrophic rats. We report a direct interaction between the dendrites of ipRGCs and dopaminergic neurones which is conserved across species. Triple immunolabelling using synaptic markers provides evidence for the unidirectionality of information transfer between the two cell types, with processes of ipRGCs being directly adjacent to sites of dopamine release. This fundamental architectural feature of the mammalian retina appears resistant to degeneration of classical photoreceptors and may provide the anatomical substrate by which dopamine cells influence the physiology of central circadian targets in the brain.

Introduction

A healthy retinal dopamine system underpins fundamental visual functions such as light adaptation, colour vision, contrast sensitivity and visual acuity. Deficits in this system (and associated visual function) are seen in the retina with increasing age and following disease (Djamgoz et al., 1997). In mammals, the production and release of dopamine from specialised amacrine (interplexiform) cells is stimulated by light exposure (Kramer, 1971, Iuvone et al., 1978). Once released, dopamine works both locally at synapses on adjacent neurones and by volume transmission, whereby it affects the functions of a multitude of cells across the retina (Bjelke et al., 1996). Retinal dopamine cells share several physiological characteristics with their counterparts in the brain, including responsiveness to visual stimuli (Dommett et al., 2005).

In the retina, release of dopamine in response to light has previously been considered to be entirely dependent upon photoreceptors in the outer retina (Frucht et al., 1982, Frucht and Melamed, 1984). However, a distinct population of intrinsically photosensitive retinal ganglion cells (ipRGCs) also exist in the inner retina of mammals. These cells signal irradiance information to circadian and non-image forming centres of the brain (Berson et al., 2002, Hattar et al., 2002) and colour information to image forming brain regions in primates (Dacey et al., 2005). The ipRGCs are thought to signal irradiance using melanopsin, a photopigment first described in photosensitive dermal melanophores of the frog (Provencio et al., 1998). In addition to their role in signalling luminance information to the brain, recent evidence strongly implicates ipRGCs in the regulation of local retinal function, specifically that of cone photoreceptors (Barnard et al., 2006).

The aims of the present study were two-fold. Firstly to provide baseline anatomical information concerning interactions between ipRGCs and dopaminergic amacrine cells in health and retinal disease. Secondly to determine the degree to which anatomical connectivity between these two important retinal cell types is conserved between species.

To achieve the first aim we examined the retinae of Royal College of Surgeons (RCS) dystrophic rats and their normal congenic (non-dystrophic) controls. This animal model is a major tool for studying retinal dystrophy and is characterised by a progressive loss of photoreceptors over the first 3 months of life due to defective retinal pigment epithelial (RPE) cells (Dowling and Sidman, 1962, D'Cruz et al., 2000). Despite the loss of classical photoreceptors in these animals, light is still able to modulate the retinal dopamine system (Doyle et al., 2002) and elicit non-image forming functions throughout life (LaVail et al., 1974). In order to achieve the second aim of the study we examined the interaction between ipRGCs and dopaminergic neurones in the human retina.

In order to visualise both retinal cell types together with a synaptic marker on the same section we employed triple immunolabelling techniques. This was achieved using antibodies to melanopsin, tyrosine hydroxylase and numerous components of the presynaptic machinery. In particular, we used vesicular monoamine transporter 2 (VMAT2) to localise sites of dopamine release. This protein packages dopamine into presynaptic vesicles and has been identified at sites of catecholamine production/release in brain (Hoffman et al., 1998) and retina (Witkovsky et al., 2004).

Section snippets

Tissue preparation

Human tissue was obtained from the Eye Bank at Moorfields Eye Hospital, with full local and national (COREC) ethical permission for research use and the protocol of our study adhered to the tenets of the Declaration of Helsinki regarding research involving human tissue. The retinae from two normal human donor eyes aged 38 and 46 years were used in this study.

Rat retinal tissue was obtained from 10 pigmented RCS dystrophic (rdy/p+) and 10 non-dystrophic (rdy+/p+) control rats (15 weeks of age)

Interaction between melanopsin and dopamine cells in normal and degenerate rat retina

The histological appearance of the RCS rat retina conformed well with previous descriptions of pathology in this animal (Villegas-Perez et al., 1998). There was a full complement of photoreceptors in the outer nuclear layer (ONL) of non-dystrophic rats but an almost complete absence of ONL in dystrophics which was instead replaced by an autofluorescent debris zone (compare Fig. 1A with E). We found melanopsin to be robustly expressed in a population of ganglion cells in the inner retina of both

Discussion

The close spatial proximity between retinal TH and melanopsin-positive processes in rat and human is strongly indicative of direct communication between the two systems, which at the present juncture we conjecture to be unidirectional. The notion of monosynaptic contact between dopaminergic neurones and ipRGCs in mammals is consistent with results from electron microscopy in the mouse retina which show synaptic contact between amacrine cells and ipRGCs (Belenky et al., 2003). However, Belenky

Acknowledgments

The authors would like to thank Dr. Ignacio Provencio for his gift of anti-human melanopsin antibody. This work was supported by grants from the Wellcome Trust awarded to JG, PR and PC.

References (38)

  • M. Puopolo et al.

    Extrasynaptic release of dopamine in a retinal neuron: activity dependence and transmitter modulation

    Neuron

    (2001)
  • J.R. Sparrow et al.

    RPE lipofuscin and its role in retinal pathobiology

    Exp. Eye Res.

    (2005)
  • J.C. Adams

    Heavy metal intensification of DAB-based HRP reaction product

    J. Histochem. Cytochem.

    (1981)
  • M.A. Belenky et al.

    Melanopsin retinal ganglion cells receive bipolar and amacrine cell synapses

    J. Comp. Neurol.

    (2003)
  • D.M. Berson et al.

    Phototransduction by retinal ganglion cells that set the circadian clock

    Science

    (2002)
  • M. Contini et al.

    GABAergic synapses made by a retinal dopaminergic neuron

    Proc. Natl. Acad. Sci. U. S. A.

    (2003)
  • P.M. D'Cruz et al.

    Mutation of the receptor tyrosine kinase gene Mertk in the retinal dystrophic RCS rat

    Hum. Mol. Genet.

    (2000)
  • D.M. Dacey et al.

    Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

    Nature

    (2005)
  • E. Dommett et al.

    How visual stimuli activate dopaminergic neurons at short latency

    Science

    (2005)
  • Cited by (73)

    • The intrinsically photosensitive retinal ganglion cell (ipRGC) mediated pupil response in young adult humans with refractive errors

      2022, Journal of Optometry
      Citation Excerpt :

      In addition to intrinsic activation, the ipRGCs also receive extrinsic synaptic input from rod and cone photoreceptors via bipolar cells.5,6 Studies have shown synaptic connections between dopaminergic amacrine cells and ipRGCs in the inner plexiform layer of the retina5,13 and evidence that these melanopsin cells may affect retinal dopamine release.14,15 Increased release of retinal dopamine through ON-bipolar cell activity inhibits experimental myopia in chicks16,17 and mice18 reared under high-intensity illumination.Furthermore, the protective effects of bright lighting on experimental myopia in primates19,20and guinea pigs21are believed to be mediated by light-induced increases in retinal dopamine release.

    • The lack of Irs2 induces changes in the immunocytochemical expression of aromatase in the mouse retina

      2022, Annals of Anatomy
      Citation Excerpt :

      In mammals, the synthesis and release of retinal dopamine is developed in the specialized interplexiform amacrine cells. In rodents, the amacrine cells are large and strongly TH-positive, and arranged at the border between INL and the inner plexiform layer (IPL), sending a thick plexus of prolongations along the scleral boundary of IPL, known as sub-lamina 1 of IPL (Savy et al., 1999; Vugler et al., 2007). In the WT mice of our study, TH-positivity was very similar to that described: large amacrine cells arranged between INL and IPL and a thick plexus in sub-lamina 1 of IPL.

    View all citing articles on Scopus
    View full text