Circadian regulation of insect olfactory learning

Abstract

Olfactory learning in insects has been used extensively for studies on the neurobiology, genetics, and molecular biology of learning and memory. We show here that the ability of the cockroach Leucophaea maderae to acquire olfactory memories is regulated by the circadian system. We investigated the effect of training and testing at different circadian phases on performance in an odordiscrimination test administered 30 min after training (short-term memory) or 48 h after training (long-term memory). When odor preference was tested by allowing animals to choose between two odors (peppermint and vanilla), untrained cockroaches showed a clear preference for vanilla at all circadian phases, indicating that there was no circadian modulation of initial odor preference or ability to discriminate between odors. After differential conditioning, in which peppermint odor was associated with a positive unconditioned stimulus of sucrose solution and vanilla odor was associated with a negative unconditioned stimulus of saline solution, cockroaches conditioned in the early subjective night showed a strong preference for peppermint and retained the memory for at least 2 days. Animals trained and tested at other circadian phases showed significant deficits in performance for both short-and long-term memory. Performance depended on the circadian time (CT) of training, not the CT of testing, and results indicate that memory acquisition rather than retention or recall is modulated by the circadian system. The data suggest that the circadian system can have profound effects on olfactory learning in insects.biological rhythms ͉ cockroach ͉ differential conditioning ͉ memory I n view of the widespread effects of circadian regulation on behavior and physiology, the possibility that learning and memory processes may be subject to modulation by the circadian system has long been recognized (1). Studies on this problem have suggested that the influence of the circadian system may take several forms. In some instances, the circadian phase may function as a context for learning (time stamping) so that recall and performance are better at 24-h intervals after learning (1-4). In other cases, performance may be modulated by the circadian phase independent of the phase of learning (5), and there have been several recent reports that memory acquisition or consolidation may depend on the circadian phase of training in mollusks (6-8), rodents (9, 10), and humans (11). Finally, there have been reports that disruption of the circadian system by phase shifting (jet lag) can impair memory (12, 13). Although these studies suggest that the circadian system may have widespread effects on various aspects of learning and memory, including acquisition, retention, and recall, there is virtually no information on the mechanisms by which the circadian system regulates these processes.Olfactory learning is an excellent model for exploration of the neural basis of learning and memory because of the remarkable similarity in the organization of olfactory systems acr...

Circadian regulation of olfaction and an evolutionarily conserved, nontranscriptional marker in Caenorhabditis elegans

“…As has been reported for other animals, we found daily oscillations in olfaction (19,29,40,48,49), suggesting that this sensory function has an important status in the circadian system. Regulation of olfaction by the circadian clock in nematodes has farreaching and ecologically important functions and is an indication of the adaptive nature of the clock in a dark-living animal, as they use olfaction to find appropriate food sources (34) and thus to orient themselves spatially in their environment.…”

Circadian modulation of learning abilities in a disease vector insect,Rhodnius prolixus

“…In the cockroach R. maderae, a similar temporal modulation of learning was observed when the animals were subjected to classical conditioning (Decker et al, 2007) but not when operant conditioning methods were employed (Garren et al, 2013). In the present work, the conditioning procedure that was used is close to operant conditioning as bugs learned the relationship between their own behaviour (i.e.…”

“…Regarding the adaptive value of the temporal modulation of learning ability, as formulated by Decker et al (2007), memories are only beneficial when formed in the environmental (including temporal) context in which they will be used. A non-insect example of this is the case of the diurnal Aplysia californica, which performed significantly better when trained and tested during the subjective day, compared with animals trained and tested in the subjective night, in contrast to the nocturnal species, Aplysia fasciata, that demonstrated significant long-term memory when trained and tested during the night (Lyons et al, 2005).…”

“…In insects, circadian regulation of learning ability has been demonstrated thus far in three different contexts: (i) olfactory conditioning in the honeybee Apis mellifera (Lehmann et al, 2011) is modulated in a circadian way; (ii) the ability of the cockroach Rhyparobia (Leucophaea) maderae (Decker et al, 2007) to form new olfactory memories is under the control of a circadian clock for classical but not for operant conditioning (Garren et al, 2013); and (iii) short-term memory formation in Drosophila melanogaster is subject to circadian modulation (Lyons and Roman, 2009). …”

What Role do Circadian Rhythms Play in Learning and Memory?

“…They also showed that mice displayed the same periodicity of peak memory when housed under constant darkness condition and this peak memory is independent of the time of training. In contrast, other studies suggest that memory formation is dependent upon time of training [46][47][48][49]. Rats demonstrate better acquisition and performance on an operant task when trained during the dark phase.…”