Abstract
Purpose: :
Serotonin is a neurotransmitter that is known to be synthesized by one type of amacrine neuron in the retina. Bipolar neurons in the retina of many vertebrate classes are thought to accumulate serotonin. However the mechanisms, functions and purpose underlying this accumulation remain unknown. In this study, we investigate the serotonin-accumulating bipolar cells in the chicken retina.
Methods: :
We injected exogenous serotonin with or without transport inhibitors into the eyes of postnatal day 14 chickens. Treated retinas were harvested and labeled for various well-known markers of bipolar cells. To determine whether these cells accumulate serotonin in the absence of amacrine cells, we destroyed the amacrines using NMDA or Quisqualate followed by injections of exogenous serotonin. KCl was injected to depolarize retina neurons, and monoamine oxidase (MAO) inhibitors to block degradation of serotonin.
Results: :
We found that exogenous serotonin transiently accumulates in bipolar neurons which are positive for Lim3 and Prox1. Serotonin is specifically taken up by bipolar neurons even when serotonin-synthesizing amacrine neurons are destroyed; excluding the possibility that serotonin diffuses through gap junctions from amacrine into bipolar neurons. In the presence of the depolarizing agent KCl, serotonin is depleted in amacrine neurons and, subsequently, taken up by bipolar neurons. The accumulation of endogenous or exogenous serotonin by bipolar neurons can be blocked by selective reuptake inhibitors Zimelidine and 6-Nitroquipazine maleate. Further, the monoamine oxidase (A) inhibitor clorgyline blocks the degradation of serotonin in bipolar cells, suggesting that MAO(A) is present in these neurons.
Conclusions: :
The morphological characteristics and immunohistochemical profile of the novel serotonin-accumulating bipolar neuron is distinctly different from other types of bipolar cells in the chicken retina. Our findings suggest that these neurons perform a glial function in the retina by scavenging and degrading serotonin that is actively transported into these cells.
Keywords: bipolar cells • retina: proximal (bipolar, amacrine, and ganglion cells) • neurotransmitters/neurotransmitter systems