Effects of norepinephrine on [3H]dopamine release and horizontal cell receptive-field size in the goldfish retina

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Modulation of ganglion cell activity in the pineal gland of the rainbow trout: effects of cholinergic, catecholaminergic, and GABAergic receptor agonists

Second order neurons within intact isolated pineal glands of the rainbow trout were explored by extracellular recordings to investigate modulatory effects of putative intrapineal neurotransmitters. Acetylcholine, dopamine, and norepinephrine were found to increase ganglion cell activity in a majority of cells tested. The excitatory effects of acetylcholine, dopamine, and norepinephrine were mimicked by muscarinic, dopamine D2, and beta-adrenergic receptor agonists and significantly increased with the applied light intensity, resulting in an attenuation of the ganglion cell response to light. GABA decreased discharge activity in most cells tested. This effect, which could be mimicked with the GABAA receptor agonist piperidine, was independent from the adaptive status. Acetylcholine and GABA were still active if applied during synaptic blockade with low Ca++ high Mg(++)-perfusion medium, whereas dopamine and norepinephrine exhibited no effects if applied during synaptic blockade, suggesting a differential cellular distribution of neurotransmitter receptors in the trout pineal gland. These data demonstrate that ganglion cell activity in the trout pineal gland is under the influence of several neurotransmitters, including acetylcholine, dopamine, norepinephrine, and GABA, which is in contrast to the originally proposed simple bineuronal transduction pathway from photoreceptors onto ganglion cells. Since the above-mentioned neurotransmitters are believed to be released from pineal interneurons, we may conclude that ganglion cell activity in the teleost pineal gland is, similarly to the retina, the product of photoreceptor signals and a modulatory active interneuronal system.

Volume transmission of dopamine may modulate light-adaptive plasticity of horizontal cell dendrites in the recovery phase following dopamine depletion in goldfish retina

We investigated the recovery of light-adaptive spinule formation following dopamine depletion with intraocular injection of 6-hydroxydopamine (6-OHDA) and subsequent neogeneration of dopamine interplexiform cells (DA-IPC) at the marginal zone. DA-IPCs were gone by 2 weeks postinjection and appeared at the marginal zone by 6 weeks postinjection, at which time DA-IPC neurites grew toward the central retina, reaching within 0.5 mm of the central retina by 1 year. Retinas from day time, light-adapted fish at 2 weeks, 4 weeks, 3 months, and 1 year postinjection with 6-OHDA were processed for pre-embedding tyrosine hydroxylase immunoreactivity (TOH-IR) and compared to sham-injected and control retinas at the electron-microscopical (EM) level. Only 6-OHDA fish that tilted markedly toward the injected eye were used for these experiments. The tilt mimics the dorsal light reaction, indicating a 2-2.5 log unit increase in the photopic sensitivity of the 6-OHDA eye. Spinule formation was reduced by about 60% in the 2- and 4-week 6-OHDA retinas, but returned to control levels throughout the entire retina of 3-month and 1 year 6-OHDA retinas even though the central region of these retinas contained no detectable TOH-IR. Intraocular injection with 10 microM SCH 23390 (a D1 antagonist) reduced light-adaptive spinule formation by 50% both in control eyes as well as those eyes that were 3 months post 6-OHDA injected. The full return of spinule formation with only partial reinnervation of the retina with DA-IPC processes and their subsequent inhibition by SCH 23390 indicates that dopamine diffused large distances within the retina to regulate this synaptic plasticity (i.e. displayed volume transmission). Also, since all 6-OHDA injected fish displayed an increased photopic sensitivity in the injected eye when sacrificed, we suggest that horizontal cell spinules are not required for photopic luminosity coding in the outer retina.