Douglas D. Colwell

Parasites and Behaviour:: An Ethopharmacological Perspective

Ethopharmacology combines an ethological approach to the understanding of the causes and functions of behaviour with pharmacological analysis of the underlying neuromodulatory mechanisms. Recently, this approach has been applied to the analysis of the responses of animals to parasitized individuals and to the effects of parasites on various host behaviours (eg. host defences, mate responses) and their neurobiological correlates. Martin Kavaliers, Douglas Colwell and Elena Choleris explain here how ethopharmacology can be used to address the mechanisms that underlie the often subtle effects of parasites on host behaviour.

Altered responses to female odors in parasitized male mice : neuromodulatory mechanisms and relations to female choice

Abstract There is accumulating evidence that females may preferentially select parasite-free or -resistant males. Minimal attention has, however, been paid to the mate preferences and responses of the parasitized male hosts themselves. Here, we considered the effects of parasitic infection on male host mate responses, the neuromodulatory correlates of these responses, and the relations of these responses to female mate choice. Using an odor “preference” test, we examined the effects of different stages of an acute, sub-clinical infection with the naturally occurring, enteric, single host, protozoan parasite, Eimeria vermiformis, on the responses of male mice, Mus musculus domesticus, to the odors of estrous females along with the responses of uninfected females to the parasitized males. At 4 days post-infection (non-infective, pre-patent stage) E. vermiformis-infected male mice showed a significantly decreased preference for the odors of estrous females, whereas at 10 days post-infection (infective, patent stage) infected males showed a significantly increased preference for the odors of estrous females. Parasitized males displayed no significant changes in their responses to the odors of non-estrous females, supporting effects on the reproductively related responses of the host. In parallel, estrous females displayed a reduced interest in the odors of infected males. Least interest was expressed in the odors of the patent, infective males, consistent with the avoidance of contagion. Using selective opioid peptide receptor agonists and antagonists we found evidence that enhanced kappa opioid peptide (e.g., dynorphin) activity was related to the decreased sexual interest of the pre-infective males, while augmented delta opioid peptide (e.g., enkephalin) activity was associated with the enhanced responses of the infective males to females. We further showed that acute kappa opiate administration reduced the responses of uninfected males to females and that uninfected females displayed modified responses to the odors of uninfected males subject to acute modifications of opioid activity. We suggest that these differential shifts in endogenous opioid activity in the parasitized males are associated with and, or related to alterations in neuro-immune and endocrine functions. These findings show that parasitic infection can have, depending on the stage of infection and associated neuromodulatory changes, either significant facilitatory or inhibitory effects on male host preferences for and responses to females.

Parasitic infection impairs spatial learning in mice

Abstract Although parasite modification of host behaviour is well established, little is known about the effects of parasites on the spatial learning abilities of the host. The effects of an acute, sub-clinical infection with the naturally occurring, single host, enteric, protozoan parasite, Eimeria vermiformis , on spatial water-maze learning by male laboratory mice, Mus musculus , were examined. Individual mice had to learn the spatial location of a submerged hidden platform using extra-maze visual cues. Determinations of spatial acquisition were made with different groups of mice on days 4 (pre-nfective), 7 (onset of infectivity or patency) and 10 (infective) post-infection. Task retention was examined in a probe trial 1 day later. The infected mice displayed significantly poorer acquisition and retention of the water-maze task over 1 day of testing (six blocks of four trials) than either sham-infected or control mice, with mice at days 4 and 7 post-infection displaying significantly poorer spatial performance than mice on day 10 post-infection. This attenuation of spatial learning occurred in the absence of either any symptoms of malaise or illness, or any evident motor, visual and motivational impairments. These parasitic-infection-induced decreases in spatial learning are likely to arise as side-effects of the immunological and neuromodulatory responses of the host and may be considered as a fitness cost associated with response to infection.

Sex differences in opioid and N-methyl-D-aspartate mediated non-opioid biting fly exposure induced analgesia in deer mice.

Abstract There is evidence for sex differences in responses to noxious stimuli and in the expression and mediation of analgesia. In particular, results of investigations with swim stress and the more ethologically appropriate stress of predator odor exposure have suggested sex differences in N‐methyl‐d‐aspartate (NMDA) receptor system involvement in the mediation of analgesia. Whether or not this sex difference generalizes to other environmental stressors is, however, not clear. Biting flies are a natural aversive stimuli commonly encountered by wild and domestic animals and humans. The present study examined the opioid and non‐opioid mediated nociceptive (50°C hot plate) responses of reproductive male and female deer mice, Peromyscus maniculatus, exposed to biting fly attack. A 30 min exposure to biting flies (stable flies, Stomoxys calcitrans (L.)) elicited a naloxone sensitive, opioid‐mediated analgesia that was of a greater magnitude in males than in female deer mice. In contrast, a 5 min exposure to biting flies elicited a ‘on‐opioid’ analgesia that was of similar magnitude in both sexes and insensitive to both naloxone and the specific kappa opiate antagonist, nor‐binaltorphimine. In male mice this non‐opioid analgesia was antagonised by the competitive NMDA antagonist, NPC 1262, while in reproductive females the biting fly‐induced analgesia was insensitive to NPC 12626. These results show that there are sex differences in NMDA involvement in the mediation of the non‐opioid analgesia arising from brief exposure to the stress of biting fly attack. These data from a common, natural environmental challenge support the presence of basic sex difference in NMDA involvement in the mediation of stress‐induced analgesia.

NMDA-mediated social learning of fear-induced conditioned analgesia to biting flies.

Although fear conditioning has received extensive neurobiological attention little is known about social learning whereby one individual may learn and acquire the fear responses of another. A 30 min exposure to intact biting flies (stable fly, Stomoxys calcitrans L.) elicits in individual fly-naive mice analgesia and active self burying responses to avoid the flies. Fly-naive observer mice that witnessed other demonstrator mice being attacked by biting flies exhibited analgesia and self-burying to avoid flies when exposed 24 h later to altered flies whose biting mouth parts were removed. The opiate antagonist naloxone, while reducing the analgesic responses elicited by exposure to a fly-stressed demonstrator, did not affect either the subsequent conditioned analgesia or self-burying. However, the specific NMDA receptor antagonist NPC 12626, given to observers prior to, but not after, presentation of fly attacked demonstrators blocked the socially determined conditioned analgesia and self burying avoidance. This supports NMDA involvement in the mediation of the social transmission and long-term (24 h) retention of conditioned analgesia and fear.

Learning to cope with biting flies : Rapid NMDA-mediated acquisition of conditioned analgesia

A 30-min exposure to intact biting flies (stable flies) induced an opioid-mediated analgesia in fly-naive male deer mice, whereas exposure to either altered biting flies whose biting mouthparts were removed or nonbiting house flies had no significant effects. However, mice that were previously exposed to intact stable flies for 30 min exhibited significant analgesia when exposed 24-168 hr later to stable flies whose biting parts were removed, but not to nonbiting house flies. Administration of the specific N-methyl-D-aspartate (NMDA) antagonist NPC 12626 to fly-naive mice before exposure to intact flies, although not significantly reducing the analgesic response, blocked the subsequent conditioned analgesia. Naloxone, which blocked the intact biting fly-induced analgesia, did not alter the acquisition of the conditioned analgesic response to the altered stable flies. This demonstrates an NMDA-mediated acquisition of conditioned analgesia to a natural aversive stimulus.

Detection of circulating hypodermin C: an antigen capture ELISA for diagnosis of cattle grub (Diptera: Oestridae) infestations.

An antigen capture assay for the detection of circulating hypodermin C was developed for diagnosis of hypodermosis. A murine monoclonal antibody to recombinant hypodermin C was raised using rapid immunization and a one-step hybridization-cloning technique. A highly reactive, specific monoclonal antibody was tested using sera spiked with known quantities of purified, native hypodermin C or with recombinant hypodermin C. Sensitivity of 96.4% and specificity of 95.6% for the antigen capture assay was assessed using a panel of sera from animals unexposed to cattle grubs and from cattle with palpation proven cattle grub infestations. Data from this panel of sera was used to establish the cut-off OD for further testing. The kinetics of circulating hypodermin C was assessed using the assay in three groups of cattle artificially infested with 50, 100 or 200 first instar Hypoderma lineatum. Antigen was first detected approximately 6 weeks after infestation. The amount of antigen detected increased in each group of animals reaching peaks at different times in each group. Levels of antigen fell quickly following arrival of grubs at the back and completion of the molt to second instar.

Impact of previous infestation on dynamics of circulating hypodermin C in cattle artificially infested with Hypoderma lineatum (Diptera: Oestridae)

Four groups of cattle were artificially infested with 50 first instar Hypoderma lineatum after either a primary natural infestation or in the absence of a primary infestation. In two groups the primary infestation had been terminated by the application of either an organophosphate insecticide or a macrocyclic lactone parasiticide. Circulating hypodermin C and specific antibodies were measured for 40 weeks after the artificial infestation. Stage specific mortality of the larvae was also monitored. Previously uninfested cattle exhibited typical antibody and antigen profiles during the infestations. Antibodies were first detected on Week 7p.i., they rose to maximum values between Weeks 24 and 25, then declined as larvae reached the back. A second peak occurred at Week 32 and antibody remained more less constant thereafter. Previously infested groups exhibited a dramatic anamnestic response by Week 3p.i. with antibody levels peaking at Week 8. A second peak was noted between Weeks 24 and 26p.i. after which antibody levels declined and then remained relatively stable. The dynamics of circulating hypodermin C in the previously infested cattle resembled those in the previously uninfested cattle. Mortality of first instars did not differ among the four groups. Similarly mortality of second and third instars, in the warble, did not differ although there was a tendency for higher mortality in the previously infested, untreated group. These results reinforce previous work demonstrating the development of a significant immune response during the primary infestation that is reflected in the rapid and substantial production of antibodies upon re-infestation. It is significant that a challenge model using subcutaneous injection of newly hatched first instars avoids host immune responses in the skin that result in substantial larval mortality. The current data also support the concept that migrating first instars induce significant reduction in host immune response. There is a peak of antibody production as antigen levels peak during first instar migration to the back. As larvae molt to the second instar and antigen production ceases there is a persistence of antibody which suggests release of the immune response from the suppression induced by the first instar secretory antigens.