Malaria infection does not affect the sensitivity of peripheral receptor neurons in Anopheles stephensi.

BACKGROUND: Mosquitoes transmit many important diseases including malaria, dengue and yellow fever. Disease transmission from one vertebrate host to another depends on repeated blood feedings by single mosquitoes. In order for the mosquito to acquire the blood that it needs to complete oogenesis, the insect must locate a suitable host. Olfactory cues (including carbon dioxide) released by the host and detected by the mosquito are the primary signals that vector insects use for host location. Previous studies have suggested that the physiological status - including bacterial, fungal, viral and Plasmodium infections - can modulate aspects of behavior in haematophagous insects. METHODS: Standard electrophysiological techniques were used to record extracellular responses from the receptor neurons located in sensilla found on the maxillary palps of the insects. The recording microelectrode was inserted through the cuticle at the base of an individual sensillum and the extracellular electrical signals obtained from the three neurons within the sensillum were recorded. Stimulations consisted of 2 s pulses of the desired concentrations of CO(2) or dosages of 1-octen-3-ol. RESULTS: Accordingly, we were interested in determining whether Plasmodium infection affects the sensitivity of those peripheral olfactory sensors that are involved in host-seeking in mosquitoes. Our studies indicate that infection of female Anopheles stephensi with Plasmodium berghei does not alter the response characteristics of the neurons innervating the maxillary palp sensilla that respond to the attractants carbon dioxide and 1-octen-3-ol. Although the response characteristics of the peripheral sensory neurons are not affected by infection status, we found that the age of the mosquito alone does affect the threshold of sensitivity of these neurons to carbon dioxide. The proportion of older insects (21-30 d post-emergence) that responds to 150 ppm carbon dioxide is higher than the proportion that responds among younger insects (1-10 d post-emergence). CONCLUSIONS: Anopheles stephensi infected with Plasmodium berghei exhibit sensitivities to stimulation with carbon dioxide and 1-octen-3-ol similar to those of uninfected mosquitoes. However, the age of the infected or uninfected mosquito does affect the threshold of sensitivity of these neurons to carbon dioxide.

Functional characterization of the octenol receptor neuron on the maxillary palps of the yellow fever mosquito, Aedes aegypti.

BACKGROUND: 1-Octen-3-ol (octenol) is a common attractant released by vertebrates which in combination with carbon dioxide (CO(2)) attracts hematophagous arthropods including mosquitoes. A receptor neuron contained within basiconic sensilla on the maxillary palps of adult mosquitoes responds selectively to 1-octen-3-ol. Recently, an odorant receptor (AaegOR8) known to occur on the maxillary palps was expressed in a heterologous system and demonstrated to be selectively sensitive to (R)-(-)-1-octen-3-ol, one of two enantiomeric forms. Lesser responses were elicited by stimulation with the (S)-enantiomer and various structural analogs. METHODOLOGY/PRINCIPAL FINDINGS: Here we characterize the specificity of the octenol receptor neuron in the yellow fever mosquito, Aedes aegypti (L.), in vivo using single cell recordings. The octenol neuron is exquisitely sensitive to (R)-(-)-1-octen-3-ol; comparable responses to (S)-(+)-1-octen-3-ol were elicited only at stimulus doses over 100× that required for the (R)-enantiomer. An intermediate response closer to that elicited by the (R)-(-)-enantiomer was elicited by racemic 1-octen-3-ol. Small structural changes in (R)-(-)-1-octen-3-ol resulted in large decreases in responses. Increases in spike activity were also elicited in the octenol neuron by 2-undecanone, a known repellent; other repellents (DEET, IR3535 and picaridin) were inactive. CONCLUSIONS/SIGNIFICANCE: The results of our electrophysiological studies of the octenol receptor neuron in vivo approximates results of a previous study of the octenol receptor (AaegOR8 with its obligate partner Aaeg\ORco) expressed heterologously in Xenopus oocytes. By comparison of our current results with those of the heterologous expression study, we conclude that specificity of the octenol receptor neuron can be explained largely by characteristics of the OR alone without other associated proteins present in vivo. Our findings show that repellents may have specific stimulatory effects on receptor neurons and support the notion of repellents as modulators of mosquito odorant receptor activity.

Age-related changes in female mosquito carbon dioxide detection.

Mosquitoes rely on carbon dioxide (CO2) as a primary component in host-seeking behavior. CO2 is detected by specialized receptor neurons in basiconic sensilla located on the maxillary palps of the mosquito. The sensitivity and specificity of these sensors can be studied using single-cell electrophysiological methods. Such electrophysiological data reveal that certain aspects of the sensitivity of these sensors change during the maturation of adult female Aedes aegypti (L.) (Diptera: Culicidae). Although the mean threshold of response is similar between the ages examined, the overall sensitivity and temporal pattern of discharge of the neurons vary with age. Older females, which are likely to engage in host-seeking behavior, are more responsive to CO2 than very young females that are unlikely to seek hosts. Male mosquitoes did not show a similar pronounced pattern of sensitivity. The implications of such differences are discussed with respect to behavior.

Electrophysiological responses from Culicoides (Diptera: Ceratopogonidae) to stimulation with carbon dioxide.

Because of their impact on human and veterinary health, there is considerable interest in understanding how Culicoides use olfactory cues in host location. The adequate chemical stimulus for sensilla located on the maxillary palps was determined for several species of female Culicoides. Electrophysiological studies identified and characterized the sensory neurons on Culicoides maxillary palps that responded to stimulation with low concentrations of CO2. The concentration response function in different background concentrations of CO2 was established for C. furens (Poey), C. stellifer (Coquillet) and C. mississippiensis Hoffman. Comparisons were made to previously studied CO2-sensitive neurons in mosquitoes. Understanding what sensory signals the host releases and how they are detected may lead to the development of strategies aimed at controlling these insects.