Somatic and neuritic spines on tyrosine hydroxylase–immunopositive cells of rat retina

Abstract

Dopamine- and tyrosine hydroxylase–immunopositive cells (TH cells) modulate visually driven signals as they flow through retinal photoreceptor, bipolar, and ganglion cells. Previous studies suggested that TH cells release dopamine from varicose axons arborizing in the inner and outer plexiform layers after glutamatergic synapses depolarize TH cell dendrites in the inner plexiform layer and these depolarizations propagate to the varicosities. Although it has been proposed that these excitatory synapses are formed onto appendages resembling dendritic spines, spines have not been found on TH cells of most species examined to date or on TH cell somata that release dopamine when exposed to glutamate receptor agonists. By use of protocols that preserve proximal retinal neuron morphology, we have examined the shape, distribution, and synapse-related immunoreactivity of adult rat TH cells. We report here that TH cell somata, tapering and varicose inner plexiform layer neurites, and varicose outer plexiform layer neurites all bear spines, that some of these spines are immunopositive for glutamate receptor and postsynaptic density proteins (viz., GluR1, GluR4, NR1, PSD-95, and PSD-93), that TH cell somata and tapering neurites are also immunopositive for a γ-aminobutyric acid (GABA) receptor subunit (GABAARα1), and that a synaptic ribbon-specific protein (RIBEYE) is found adjacent to some colocalizations of GluR1 and TH in the inner plexiform layer. These results identify previously undescribed sites at which glutamatergic and GABAergic inputs may stimulate and inhibit dopamine release, especially at somata and along varicose neurites that emerge from these somata and arborize in various levels of the retina.

Dopaminergic Retinal Cell Loss and Visual Dysfunction in Parkinson Disease

“…The sheep polyclonal anti-tyrosine hydroxylase (AB1542, Merck Millipore) was generated against the native tyrosine hydroxylase protein from rat pheochromocytoma, and it has been previously characterized by immunohistochemistry in the rat and mouse retina. 26,27 Its staining pattern in the human retina is comparable to that of other species, such as squirrel, cat, or rat, 20,26,28 and the density and distribution of control human dopaminergic cells when stained with this antibody is comparable to previous studies done in other primates. 29,30 Also, the dopaminergic staining pattern observed is the same as that obtained with other antityrosine hydroxylase antibodies (ab112; Abcam, Cambridge, UK; data not shown).…”

NMDA Receptors Contribute to Retrograde Synaptic Transmission from Ganglion Cell Photoreceptors to Dopaminergic Amacrine Cells

“…Our data reveal that both the NR2A and NR2B subunits are expressed in TH-positive somata and processes. Combined with a previous study in the mouse retina showing that DACs express NR1 immunoreactivity (Fasoli et al, 2017 ), it appears that a functional DAC NMDA receptor contains NR1/NR2A, NR1/NR2B or NR1/NR2A/NR2B. The greater expression of NR2B subunits (compared to NR2A subunits) on DAC somata and dendrites (Figures 5 , 6 ) further supports the hypothesis that NR2B subunits may contribute to functional DAC NMDA receptors more than NR2A subunits.…”

“…A functional NMDA receptor consists of two obligatory NR1 subunits and two NR2 subunits, of which there are four subtypes, NR2A, NR2B, NR2C and NR2D (Ishii et al, 1993 ; Monyer et al, 1994 ; Laube et al, 1998 ; Dingledine et al, 1999 ). DACs were recently found to express NR1 subunits (Fasoli et al, 2017 ), suggesting that NMDA receptors could play a role in signal transmission to DACs. The NR2 subunits, which have not yet been confirmed to be expressed on DACs, are thought to confer distinct physiological and pharmacological properties on the NMDA receptor (Monyer et al, 1992 ; Flint et al, 1997 ; Chen et al, 1999 ; Tovar and Westbrook, 1999 ; Sagdullaev et al, 2006 ).…”

Rod Photoreceptor Activation Alone Defines the Release of Dopamine in the Retina

“…The dense plexus of processes emanating from DACs stratifies in the middle of the retina, mostly in the outermost stratum of the inner plexiform layer [12,13]. Dopamine release is thought to occur by volume transmission, diffusing throughout the entire depth of the retina to activate dopamine receptors on all known classes of neuron [14,15].…”