A mechanism of polarized light sensitivity in cone photoreceptors of the goldfish Carassius auratus

Abstract

An integrated laser tweezer and microphotometry device has been used to characterize in detail how individual, axially orientated goldfish photoreceptors absorb linearly polarized light. This work demonstrates that the mid-wavelength sensitive members of double cone photoreceptors display axial differential polarization sensitivity. The polarization contrast was measured to be 9.2 +/- 0.4%. By comparison, rod photoreceptors only exhibit isotropic absorbance. These data, combined with the square cone mosaic of double cones in the retina, suggest that intrinsic axial dichroism forms part of the underlying biophysical detection mechanism for polarization vision in this species.

FIGURE 1

Schematic diagram of the laser tweezer microphotometer (Measurement optics: AL, absorbance 532-nm laser; IC, intensity controller; SH, shutter; and ND, neutral density filter. Viewing optics: DL, 980-nm diode illumination laser; FH, fiber holder; IL, collimating lens; CM, 45° cold mirror; CO, 50× ULWD Olympus MPlan objective; SA, sample; TS, temperature-controlled stage; XY, x-y stage; MIC, microscope body; CAM, video camera; OB, 100× Zeiss Neoplan oil immersion objective; and HM, 45° hot mirror. Detector system: LP, lowpass filter; LL, 532-nm laser line filter; PMT; photomultiplier tube; AMP, amplifier; and DI, discriminator. Optical tweezers: TL, 1064-nm trapping laser; L1–L4, beam steering lenses; and GV, galvanometer-controlled mirrors.)

FIGURE 2

(a–d) A time series of video images illustrating a 180° rotation of a rod photoreceptor in the plane of the sample using a dual beam optical trap. (e and f) The controlled rotation of a rod photoreceptor using a single beam trap into its physiological end on orientation. Scale bar, 10 μm.

FIGURE 3

A typical example of an experimental linearly polarized transverse absorbance data set obtained from a rod (52) photoreceptor as it is rotated through 360° in the plane of the sample. The solid symbols clearly demonstrate the dichroic transverse absorbance of the outer segment. The open symbols illustrate the baseline post-bleach measurements. Error bars represent mean ±1 SD.

FIGURE 4

(a) A time series of video images illustrating a 180° rotation of an axially orientated double cone photoreceptor. The rotation is centered on the mid-wavelength sensitive (MWS) outer segment. Scale bar, 10 μm. (b) A typical set of axial absorbance measurements from an MWS outer segment indicating the axial dichroism of the cell type. (c) A time series of video images illustrating a 360° rotation of an axially orientated rod photoreceptor. Scale bar, 5 μm. (d) The corresponding constant axial absorbance measurements from a rotating rod photoreceptor. In panels b and d, the solid symbols represent the absorbance and the open symbols show the post-bleach baseline. (e) The mean axial dichroic ratios from all measured rods and MWS cones. The mean values are significantly different between cell types (n = 9; p < 0.05; one-way ANOVA). Error bars represent mean ±1 SD.