Somatic and neuritic spines on tyrosine hydroxylase–immunopositive cells of rat retina
Previous studies have hypothesized that TH cells release dopamine from varicose axons after excitatory synapses depolarize TH cell dendrites in the inner plexiform layer and these depolarizations spread radially to the varicosities (Dacey, 1990; Witkovsky, Gábriel, & Krizaj, 2008). Although anatomical, electrophysiological, and imaging observations are consistent with radially spreading depolarizations in some medium‐ and wide‐field amacrine cells (Famiglietti, 1991; Taylor, 1996; Euler, Detwiler, & Denk, 2002; Davenport, Detwiler, & Dacey, 2007), it is less clear how the structural compartments of TH cells function. For example, although varicosities in TH cell neurites form synapses onto other neurons (Pourcho, 1982; Holmgren, 1982; Voigt & Wässle, 1987; Kolb, Cuenca, Wang, & Dekorver, 1990; Kolb, Cuenca, & Dekorver, 1991; van Haesendonck, Marc, & Missotten, 1993; Contini & Raviola, 2003), dopamine release has been detected only from TH cell somata (Puopolo, Hochstetler, Gustincich, Wightman, & Raviola, 2001). Similarly, although dopamine release can be blocked by a voltage‐gated Na+ channel toxin (Puopolo et al., 2001; see also Piccolino, Witkovsky, & Trimarchi, 1987) and can be reduced by Ca2+ channel toxins and heavy metals (Sarthy & Lam, 1979; Frederick, Rayborn, Laties, Lam, & Hollyfield, 1982; Kolbinger & Weiler, 1993; Tamura et al., 1995; Boelen, Boelen, & Marshak, 1998), antibodies directed against voltage‐gated Na+ and Ca2+ channels bind to neurites in the inner plexiform layer but not in the outer plexiform layer (Xu, Zhao, & Yang, 2003; Witkovsky et al., 2004; Witkovsky, Shen, & McRory, 2006; Witkovsky et al., 2008). For that matter, TH has been found to colocalize with synaptic vesicle‐ and release‐related proteins in the inner plexiform layer, but in only some varicosities within the inner nuclear layer and not in the outer plexiform layer (Witkovsky et al., 2004; Witkovsky, Arango‐Gonzalez, Haycock, & Kohler, 2005). Additionally, TH cells have been reported to differ at the level of their postsynaptic architecture. In particular, dendritic spine‐like appendages have been found on TH cell dendrites in some species, but not on dendrites in other species, and also not on somata that release dopamine when exposed to glutamate receptor agonists (Dacey, 1990; Teakle, Wildsoet, & Vaney, 1984; Kolb et al., 1990; Tauchi, Madigan, & Masland, 1990; Gábriel, Zhu, & Straznicky, 1992; Guimarães & Hokoç, 1997; Puopolo et al., 2001).
It is possible that TH cells release dopamine by different mechanisms in the inner and outer plexiform layers, and that TH cell shape and ion channel distribution vary with species. However, most studies to date have examined TH cells in retinae that were incubated in aldehyde‐based fixatives that we have found to deform, and reduce the detectability of, retinal ganglion cell axons and dendrites (Stradleigh, Greenberg, Partida, Pham, & Ishida, 2015). The present study examines the shape, contour, and immunostaining of TH cells by use of protocols we developed to preserve the dendritic and axonal morphology of proximal retinal neurons and to localize transmembrane and membrane‐associated proteins (Stradleigh et al., 2011).