To identify ultraviolet (UV) and middle- (M) wavelength-sensitive cone
and rod signals in murine retinal ganglion cells, single ganglion cell
responses were studied in anesthetized, light-adapted C57/BL6 mice
with tungsten microelectrodes driven through the sclera and vitreous to
the neural retina. One hundred fifty-four ganglion cells were examined in
43 retinas of 34 mice. The retina was stimulated with diffuse flashes
and/or pulses of ultraviolet (360 nm) or green (520 nm) light in the
presence and absence of a strong steady orange adapting light. Twelve
ganglion cells were studied in the dark-adapted retina in order to
identify the signals of rods. Three functionally different types of
ganglion cells were found: (1) phasic responding cells (31%) with no
spontaneous activity and large impulse amplitudes; (2) tonic responding
cells (60%) with irregular, low frequency (5–10 Hz) spontaneous
activity and smaller impulse amplitudes; and (3) metronome-like cells (9%)
with regular, relatively high-frequency (20–40 Hz) spontaneous
activity. A few cells (1%) had habituating responses. Every cell
encountered was affected by diffuse stimulation. The more common two types
were excited at either the ON or OFF or at both the ON and OFF phases of
stimulation. Type III cells had weaker responses, sometimes only inhibited
by turning off a light. In the light-adapted state, most cells received
signals of the same polarity from UV- and M-cones but UV-cone inputs were
usually more dominant, especially in ventral retina. A fraction of cells
received signals from only UV- (18%) or only M- (3%) cones. In rare cases
(2%) these cone inputs had an opposite polarity on the same cell. In the
dark-adapted state, all cells were at least four or five logarithmic units
more sensitive and more to green than ultraviolet light. The results
indicate that co-expression of both UV-and M-cone opsins cannot be
ubiquitous in murine retina. Some cones, especially UV cones, exist
without the presence of any functional M-cone opsin. This must be the case
to explain the presence of ganglion cells that receive inputs only from
UV-cones and others that receive inputs of opposite polarity from UV- and
M-cones. The results support the hypothesis that murine retina has the
physiological capacity to relay signals to the brain that allow the
sensing of chromatic contrast and color vision.