The sensitivity of retinal cells is altered in background light to optimize the detection of contrast. For scotopic (rod) vision, substantial adaptation occurs in the first two cells, the rods and rod bipolar cells (RBCs), through sensitivity adjustments in rods and postsynaptic modulation of the transduction cascade in RBCs. To study the mechanisms mediating these components of adaptation, we made whole-cell, voltage-clamp recordings from retinal slices of mice from both sexes. Adaptation was assessed by fitting the Hill equation to response-intensity relationships with the parameters of half-maximal response (I_1/2), Hill coefficient (n), and maximum response amplitude (R_max). We show that rod sensitivity decreases in backgrounds according to the Weber–Fechner relation with an I_1/2 of ~50 R* s^–1. The sensitivity of RBCs follows a near-identical function, indicating that changes in RBC sensitivity in backgrounds bright enough to adapt the rods are mostly derived from the rods themselves. Backgrounds too dim to adapt the rods can however alter n, relieving a synaptic nonlinearity likely through entry of Ca²⁺ into the RBCs. There is also a surprising decrease of R_max, indicating that a step in RBC synaptic transduction is desensitized or that the transduction channels became reluctant to open. This effect is greatly reduced after dialysis of BAPTA at a membrane potential of +50 mV to impede Ca²⁺ entry. Thus the effects of background illumination in RBCs are in part the result of processes intrinsic to the photoreceptors and in part derive from additional Ca²⁺-dependent processes at the first synapse of vision.

SIGNIFICANCE STATEMENT Light adaptation adjusts the sensitivity of vision as ambient illumination changes. Adaptation for scotopic (rod) vision is known to occur partly in the rods and partly in the rest of the retina from presynaptic and postsynaptic mechanisms. We recorded light responses of rods and rod bipolar cells to identify different components of adaptation and study their mechanisms. We show that bipolar-cell sensitivity largely follows adaptation of the rods but that light too dim to adapt the rods produces a linearization of the bipolar-cell response and a surprising decrease in maximum response amplitude, both mediated by a change in intracellular Ca²⁺. These findings provide a new understanding of how the retina responds to changing illumination.

视网膜细胞的敏感性在背景光下发生改变，以优化对比度的检测。对于暗视觉（视杆细胞），通过视杆细胞的敏感性调整和红细胞中转导级联的突触后调节，前两个细胞（视杆细胞和视杆细胞双极细胞（RBC））发生了显着的适应。为了研究调节这些适应成分的机制，我们对两性小鼠的视网膜切片进行了全细胞电压钳记录。通过使用半最大响应 ( I _1/2 )、希尔系数 ( n ) 和最大响应幅度 ( R _max )参数将希尔方程拟合到响应强度关系来评估适应性。我们表明，根据 Weber-Fechner 关系，视杆细胞的敏感性在背景中降低，I _1/2约为 50 R* s ^–1。红细胞的敏感性遵循几乎相同的函数，表明在足够亮以适应视杆的背景中红细胞敏感性的变化主要源自视杆本身。然而，太暗而无法适应视杆的背景可以改变n ，可能通过 Ca ²⁺进入红细胞来缓解突触非线性。R _max也出现令人惊讶的下降，表明红细胞突触转导中的某个步骤脱敏或转导通道变得不愿打开。在 +50 mV 膜电位下透析 BAPTA 以阻止 Ca ²⁺进入后，这种效应大大降低。因此，红细胞中背景照明的影响部分是光感受器固有过程的结果，部分源自视觉第一个突触处的附加 Ca ^{2+依赖性过程。}

意义声明光适应随着环境照明的变化调整视觉的敏感度。已知暗视（视杆细胞）视觉的适应部分发生在视杆细胞中，部分发生在突触前和突触后机制的视网膜其余部分。我们记录了视杆细胞和视杆双极细胞的光反应，以识别适应的不同组成部分并研究其机制。我们表明，双极细胞敏感性很大程度上遵循视杆细胞的适应，但光线太暗而无法适应视杆细胞会产生双极细胞反应的线性化和最大反应幅度的惊人下降，这两者都是由细胞内 Ca 2+ 的变化介导^的。这些发现为视网膜如何响应变化的光照提供了新的理解。