Time- and voltage-dependent currents in locust ocellar L-neurones

Sparse but specific temporal coding by spikes in an insect sensory-motor ocellar pathway

We investigate coding in a locust brain neuron, DNI, which transforms graded synaptic input from ocellar L-neurons into axonal spikes that travel to excite particular thoracic flight neurons. Ocellar neurons are naturally stimulated by fluctuations in light collected from a wide field of view, for example when the visual horizon moves up and down. We used two types of stimuli: fluctuating light from a light-emitting diode (LED), and a visual horizon displayed on an electrostatic monitor. In response to randomly fluctuating light stimuli delivered from the LED, individual spikes in DNI occur sparsely but are timed to sub-millisecond precision, carrying substantial information: 4.5–7 bits per spike in our experiments. In response to these light stimuli, the graded potential signal in DNI carries considerably less information than in presynaptic L-neurons. DNI is excited in phase with either sinusoidal light from an LED or a visual horizon oscillating up and down at 20 Hz, and changes in mean light level or mean horizon level alter the timing of excitation for each cycle. DNI is a multimodal interneuron, but its ability to time spikes precisely in response to ocellar stimulation is not degraded by additional excitation. We suggest that DNI is part of an optical proprioceptor system, responding to the optical signal induced in the ocelli by nodding movements of the locust head during each wing-beat.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

Presynaptic depolarization rate controls transmission at an invertebrate synapse

Second-order neurons L1-3 of the locust ocellar pathway make inhibitory synapses with each other. Although the synapses transmit graded potentials, transmission depresses rapidly and completely so that a synapse only transmits when the presynaptic terminal depolarizes rapidly. The rate at which a presynaptic neuron depolarizes determines the rate at which a postsynaptic neuron hyperpolarizes, and neurotransmitter is only released during a fixed 2 ms long period. Consequently, the amplitude of a postsynaptic potential depends on the rate rather than the amplitude of a presynaptic depolarization. Following a postsynaptic potential, a synapse recovers from depression over about a second. The synapse recovers from depression even if the presynaptic terminal is held depolarized.

Signal processing in a simple visual system: the locust ocellar system and its synapses

The neurons with the widest axons that carry information into a locust brain belong to L-neurons, the large, second-order neurons of the ocelli. L-neurons play roles in flight control and boosting visual sensitivity. Their morphology is simple, and their axons convey graded potentials from the ocellus with little decrement to the brain, which makes them good subjects in which to study transmission of graded potentials. L-neurons are very sensitive to changes in light, due to an abnormally high gain in the sign inverting synapses they receive from photoreceptors. Adaptation ensures that L-neurons signal contrast in a light signal when average light intensity changes, and that their responses depend on the speed of change in light. Neurons L1-3 make excitatory output synapses with third-order neurons and with L4-5. These synapses transmit tonically, but are unable to convey hyperpolarising signals about large increases in light. Graded rebound spikes enhance depolarising responses. L1-3 also make reciprocal inhibitory synapses with each other and transmission at these decrements so rapidly that it normally requires a presynaptic spike. The resolution with which graded potentials can be transferred has been studied at the inhibitory synapses, and is limited by intrinsic variability in the mechanism that determines neurotransmitter release. Electron microscopy has shown that each excitatory connection made from an L-neuron to a postsynaptic partner consists of thousands of discrete synaptic contacts, in which individual dense-staining bars in the presynaptic neuron are associated with clouds of vesicles. Acetylcholine is likely to be a neurotransmitter released by L-neurons.

Sensitivity of ocellar interneurons of the honeybee to constant and temporally modulated light

Sinusoidally modulated and discrete light pulses, the parameters of which approximated natural light conditions, were used to determine the response characteristics of ocellar first-order interneurons of the worker honeybee (Apis mellifera carnica). Large ocellar interneurons which terminate within the brain (LB neurons) were recorded from intracellularly and were identified visually after dye injection. Absolute sensitivity of LB neurons to light flashes ranges from 4 X 10(9) quanta/cm2s (Q) for MOC1,7 neurons to 1 X 10(12) Q for MOC3,4. The slope of the response-intensity (R/I) functions, which were calculated for intensities between 2 X 10(9) and 4 X 10(13) Q, varies in different types of LB neurons. The strongest response is given by one group of median ocellar neurons. With constant light around 10(13) Q, most LB neurons exhibit oscillatory hyperpolarizations which, upon increasing the stimulus to even higher intensities (10(14)-10(15) Q), gradually evolve to a hyperpolarized plateau. The frequency of these oscillatory voltage fluctuations increases with the rate of modulation of the stimulating light and reaches maximum values at 5-15 Hz modulation frequency. Two groups of MOC neurons follow sinusoidally modulated light up to 32 +/- 8 Hz (n = 5) and 29 +/- 6 Hz (n = 3), respectively, whereas lateral ocellar neurons cut off at 17 +/- 5 Hz (n = 4). The possible role of LB neurons is discussed. They may be inactivated when the bee is flying in bright sunlight.

Immunohistochemical and electrophysiological evidence that locust ocellar photoreceptors contain and release histamine

Immunohistochemistry with an antiserum directed against histamine revealed histamine-like immunoreactivity in ocellar photoreceptors of Locusta migratoria. Prefixative incubation of the tissue in Ringer solution containing 10 microM histamine increased number and intensity of labeled photoreceptors. Additionally, glial cells became immunoreactive. High extracellular potassium concentration abolished labeling in retinula cells but not in glial cells. These results suggest that locust ocellar photoreceptors contain histamine and release it upon depolarization. Histamine itself or/and a metabolite is taken up by photoreceptors and glial cells. In high-potassium/calcium-free Ringer solution labeling in photoreceptors persisted, indicating that release is calcium-dependent. Electrophysiological recordings confirm the hyperpolarizing and localized action of histamine onto dendrites of second-order neurons in the ocellar cup. These results support the hypothesis that histamine is a neurotransmitter in insect photoreceptor cells.