Synaptic circuitry of serotonin-synthesizing and serotonin-accumulating amacrine cells in the retina of the cane toad, Bufo marinus

Abstract

The synaptic connections of amacrine cells synthesizing or accumulating serotonin in the retina of the cane toad, Bufo marinus, were studied by using preembedding double-labeling electron-microscopic immunocytochemistry. The binding sites of an anti-serotonin antibody were revealed by the diaminobenzidine reaction, whilst a colloidal gold-conjugated secondary antibody was used to detect an antibody to phenylalanine hydroxylase. Since the latter antibody recognizes tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin, as well as tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis, the double labeling of the present study enabled us to identify three groups of labeled profiles at the ultrastructural level. The profiles of serotonin-synthesizing amacrine cells contained both diaminobenzidine reaction product and colloidal gold particles, whilst those of serotonin-accumulating and dopaminergic amacrine cells contained only diaminobenzidine reaction product or colloidal gold particles, respectively. The synapses of serotoninsynthesizing or serotonin-accumulating amacrine cells were distributed all through the inner plexiform layer of the retina. The profiles of serotonin-synthesizing amacrine cells predominantly received synapses from, and made synapses onto, unlabeled amacrine cell dendrites. They also received synapses from, and made synapses onto, bipolar cell terminals. They also made synapses onto presumed ganglion cell dendrites. However, the profiles of serotonin-accumulating cells made synapses only with unlabeled amacrine cell processes. There were close contacts between the profiles of serotonin-synthesizing and serotonin-accumulating amacrine cells. No synaptic relationships were observed between dopaminergic and serotonin-synthesizing or serotonin-accumulating amacrine cells. These observations indicate that serotonin-synthesizing amacrine cells and serotonin-accumulating amacrine cells in the retina of Bufo marinus are involved in different microcircuits in visual information processing.

Serotonin released from amacrine neurons is scavenged and degraded in bipolar neurons in the retina

“…Our findings are consistent with the notion that some types of bipolar cells accumulate serotonin in different vertebrate retinas (Witkovsky et al. 1984; Zhu et al. 1992, 1995; Gabriel et al.…”

The number and distribution of bipolar to ganglion cell synapses in the inner plexiform layer of the anuran retina

“…Possible postsynaptic elements are the serotonin immunoreactive amacrines of Xenopus which receive input from bipolars mostly in SLs 2 and 5 of the IPL. Serotonin synthesizing amacrines in Bufo were also found to receive input from bipolar cells but the peak of the distribution is in SL3 Zhu et al, 1995) where the direct bipolar to ganglion cell contacts are few. GABA immunoreactivity appears in about 50% of the amacrine cells in Bufo and they are postsynaptic to bipolar cells.…”

Localization of p75 neurotrophin receptor in the retina of the adult SD rat: An immunocytochemical study at light and electron microscopic levels

“…The strips were rinsed in PBS containing 0.1% BSA, followed by a rinse in 50% ethanol in distilled water for 10 min to facilitate antibody penetration (Llewellyn-Smith et al, 1992). The combined treatment of 1% sodium borohydride with washing in 50% ethanol significantly enhanced the penetration of antibody (Zhu et al, 1995), but either treatment alone had less effect. Retinal strips were initially incubated in 0.6% hydrogen peroxide in PBS for 30 min; blocked in 10% HS, 10% RS, and 5% BSA in PBS without Triton X-100 for 1 h; and subsequently incubated with 192-IgG (1:50, Oncogene) for about 72 h at 4°C.…”