Visual efference in Limulus: in vitro temperature-dependent neuromodulation of photoreceptor potential timing by octopamine and substance P

Temperature-related Intraspecific Variability in the Behavioral Manipulation of Acanthocephalan Parasites on Their Gammarid Hosts

Understanding the effect of temperature on ecologically important species has become a major challenge in the context of global warming. However, the consequences of climate change cannot be accurately predicted without taking into consideration biotic interactions. Parasitic infection, in particular, constitutes a widespread biotic interaction, and parasites impact their hosts in multiple ways, eventually leading to consequences for communities and ecosystems. We explored the effect of temperature on the anti-predator behavior of a keystone freshwater invertebrate, the amphipod Gammarus fossarum. Gammarids regularly harbor manipulative acanthocephalan parasites that modify their anti-predator behavior in ways that potentially increase the probability of trophic transmission to their definitive hosts. We investigated the impact of temperature on gammarids infected by two acanthocephalan parasites, Pomphorhynchus tereticollis and Polymorphus minutus. Uninfected and naturally infected gammarids were acclimatized to different temperatures, and their behavior was measured. Our results showed that the effect of infection on the phototaxis of gammarids increased with increasing temperature, with a stronger effect induced by P. tereticollis. In contrast, temperature had no effect on the alteration of refuge use or geotaxis observed in infected gammarids. Our results provide the first direct evidence that temperature can affect the extent of behavioral alteration brought about by certain parasite species. However, the consequences of increased trophic transmission remain elusive; the supposedly key anti-predatory behavior was not significantly affected by exposure of gammarids to different temperatures.

Neuromodulation of olfactory sensitivity in the peripheral olfactory organs of the American cockroach, Periplaneta americana

Olfactory sensitivity exhibits daily fluctuations. Several studies have suggested that the olfactory system in insects is modulated by both biogenic amines and neuropeptides. However, molecular and neural mechanisms underlying olfactory modulation in the periphery remain unclear since neuronal circuits regulating olfactory sensitivity have not been identified. Here, we investigated the structure and function of these signaling pathways in the peripheral olfactory system of the American cockroach, Periplaneta americana, utilizing in situ hybridization, qRT-PCR, and electrophysiological approaches. We showed that tachykinin was co-localized with the octopamine receptor in antennal neurons located near the antennal nerves. In addition, the tachykinin receptor was found to be expressed in most of the olfactory receptor neurons in antennae. Functionally, the effects of direct injection of tachykinin peptides, dsRNAs of tachykinin, tachykinin receptors, and octopamine receptors provided further support for the view that both octopamine and tachykinin modulate olfactory sensitivity. Taken together, these findings demonstrated that octopamine and tachykinin in antennal neurons are olfactory regulators in the periphery. We propose here the hypothesis that octopamine released from neurons in the brain regulates the release of tachykinin from the octopamine receptor neurons in antennae, which in turn modulates the olfactory sensitivity of olfactory receptor neurons, which house tachykinin receptors.

Visual efference neuromodulates retinal timing: in vivo roles of octopamine, substance P, circadian phase, and efferent activation in Limulus

Efferent nerves coursing from the brain to the lateral eye of the horseshoe crab, Limulus polyphemus, increase its nighttime sensitivity to light. They release octopamine, which produces a categorical increase of photoreceptor response duration in vitro. Analogous in vivo timing effects on the electroretinogram (ERG) were demonstrated when octopamine was infiltrated into the eye of an otherwise intact animal; nighttime ERGs were longer than daytime ERGs. Related effects on the ERG were produced by daytime electrical stimulation of efferent fibers. Surprisingly, in a departure from effects predicted solely from in vitro octopamine data, nighttime ERG onsets were also accelerated relative to daytime ERG onsets. Drawing on earlier reports, these remarkable accelerations led to an examination of substance P as another candidate neuromodulator. It demonstrated that infiltrations of either modulator into the lateral eyes of otherwise intact crabs increased the amplitude of ERG responses but that each candidate modulator induced daytime responses that specifically mimicked one of the two particular aspects of the timing differences between day- and nighttime ERGs: octopamine increased the duration of daytime ERGs and substance P infiltrated during the day accelerated response onset. These results indicate that, in addition to octopamine's known role as an efferent neuromodulator that increases nighttime ERG amplitudes, octopamine clearly also affects the timing of photoreceptor responses. But these infiltration data go further and strongly suggest that substance P may also be released into the lateral eye at night, thereby accelerating the ERG's onset in addition to increasing its amplitude.

Osmotic properties of Limulus seawaters and organ cultures: an unrecognized issue

Excised eyes of the horseshoe crab (Limulus polyphemus) have long provided robust preparations for fundamental studies of the visual neuroscience of photoreceptors. Such preparations are particularly useful for immersion in perfusion chambers whose perfusate can be varied as needed. Little attention has been paid in the past to the osmotic properties of such perfusates because they have generally been virtually isotonic with natural seawater. We here report that (1) some recent Limulus studies have used organ culture perfusates that are severely hypotonic and (2) that that fact has not been adequately disclosed.