Dr. Skingsley et al., PROPERTIES OF HISTAMINE-ACTIVATED CHLORIDE CHANNELS IN THE LARGE MONOPOLAR CELLS OF THE DIPTERAN COMPOUND EYE - A COMPARATIVE-STUDY, Journal of comparative physiology. A, Sensory, neural, and behavioral physiology, 176(5), 1995, pp. 611-623
The large monopolar cells (LMCs) of the first optic neuropil (lamina)
in insects respond to the photoreceptor neurotransmitter histamine wit
h an increase in chloride conductance. We have compared the properties
of this conductance from a range of diptera from different visual env
ironments: Tipula paludosa (slow flying, crepuscular), Drosophila mela
nogaster (slow-flying diurnal), and 3 fast-flying diurnal species Musc
a domestica, Calliphora vicina and Lucilia sericata. In whole-cell rec
ordings of dissociated LMCs, histamine-induced currents were elicited
using a multichannel parallel perfusion device, allowing rapid determi
nation of the dose-response function, characterised by affinity (K-d)
and Hill coefficient (n). Calliphora, Lucilia and Musca had the steepe
st dose response curves (n = 2.8) and the lowest affinity for histamin
e (K-d 35-50 mu M); the crepuscular Tipula had a significantly higher
affinity (K-d = 16 mu M) and lower Hill coefficient (n = 1.8). Drosoph
ila had a high affinity (K-d 24 mu M), and a high Hill coefficient (n
= 2.5). In excised inside-out patch recordings all species showed simi
lar single channel properties (conductance 40-60 pS, mean open time <
1 ms). The low Hill coefficient in Tipula would be expected to result
in lower synaptic gain. We suggest this may be an adaptation to preven
t the LMC's response bandwidth being filled with the high levels of ph
oton noise typical of photoreceptors adapted for low light levels. The
lower affinity for histamine found in the more photopic species sugge
sts that the concentration of histamine (and therefore presumably numb
er of synaptic vesicles released from the photoreceptors) should be hi
gher. This might improve signal-to-noise ratio by decreasing the contr
ibution of the shot event noise introduced by stochastic release of sy
naptic vesicles.