Comparison of single cell sensitivities to acetone, 1-octen-3-ol and 3-methylphenol in the riverine tsetse species Glossina fuscipes fuscipes and G. palpalis palpalis.

Action potentials from individual cells were recorded from antennae (funiculi) of living tsetse flies, Glossina p. palpalis and Glossina f. fuscipes using a "surface-contact" recording technique. Stimuli were vapours of 1-octen-3-ol, acetone and 3-methylphenol. Of the 101 and 128 olfactory cells tested for their sensitivity to odour stimuli in G. p. palpalis and G. f. fuscipes, respectively, the majority (83 and 77%) were activated by more than one chemical. The numbers of these "generalist" cells were 20 and 15% higher in females than in males. Response intensity increased with increasing odour dose. Temporal patterns of excitation were phasic-tonic and showed cells with relatively rapid cessation of spike activity after the end of stimulation and cells which continued firing for several seconds or even minutes after stimulation. Inhibition by odours only occurred in a minority of cells and was dose-dependent. For each of the three substances the excitatory response was significantly higher in G. f. fuscipes than in G. p. palpalis, whereas no significant differences between inhibitory responses were found. In G. f. fuscipes each stimulus evoked equal excitatory responses. In G. p. palpalis, however, acetone induced significantly higher responses than 1-octen-3-ol and 3-methylphenol. Response intensities to each of the three chemicals did not differ between male and female G. p. palpalis, whereas in G. f. fuscipes 1-octen-3-ol evoked significantly higher responses in males. Possible mechanisms of receptor cell odour coding and behavioural effects of the various cell type activities are discussed.

Background odour induces adaptation and sensitization of olfactory receptors in the antennae of houseflies.

The presence of background odour was found to have a small but significant effect on the sensitivity of the antennal olfactory system of houseflies, Musca domestica Linnaeus (Diptera: Muscidae), to new pulses of odour. We show that cross-adaptation and cross-sensitization between a background odour of (+/-)-1-octen-3-ol and pulses of (+/-)-1-octen-3-ol, 2-pentanone and R-(+)-limonene can occur, confirming that olfactory receptor cells are sensitive to different odours. Background odour can increase the responses to low concentration odour pulses and decrease the responses to higher concentration odour pulses. It is suggested that background odour has a larger effect on olfactory receptor cells that respond with a tonic increase of spike frequency, giving information about the level of odour concentration, i.e. the 'static' environment. Cells that respond in a phasic way only provide information on the dynamics of the olfactory environment.

Electrophysiological characterization of olfactory cell types in the antennae and palps of the housefly.

A set of odours was presented to the housefly Musca domestica and the electrophysiological responses of single olfactory receptor cells in the antennae and palps were recorded. The olfactory cells in the antennae of the housefly showed a large variability of response profiles, but multidimensional cluster analysis suggested a moderate clustering in olfactory response types. Receptor cells with similar or with different odour response profiles can reside in one and the same sensillum. No fixed spatial distribution of olfactory response types over the antennal of palpal surface was found. The odours of 1-octen-3-ol, amyl acetate, 3-methylphenol, 2-pentanone and R(+)limonene elicited the largest responses in antennal cells. Most odours elicited responses in cells of only a few of the clusters, but 1-octen-3-ol was detected by cells of almost all clusters of the antenna. Surprisingly, rather low responses were found to acetic acid, skatole, indole and muscalure, odours that are known to attract flies. Response profiles of palpal cells differed considerably from those of antennal cells. Palpal cells mostly responded to 3-methylphenol and 2-pentanone. In the palps, the clusters of cells responding to 3-methylphenol and 2-pentanone are clearly separated from the other olfactory cells.

Olfactory responses to attractants and repellents in tsetse.

The aims of this study were to investigate how antennal olfactory cells of tsetse (Diptera: Glossinidae) code odour quality and how they are able to discriminate between attractive and repellent odours. For Glossina pallidipes Austen, a survey is presented of the cells' responses to attractive (1-octen-3-ol, acetone, 3-methylphenol, carbon dioxide) and repellent stimuli (2-methoxyphenol, acetophenone, lactic acid, naphthalene). In addition, the responses of these cells to binary mixtures and the dose-response curves of 1-octen-3-ol, 3-methylphenol, 2-methoxyphenol and acetophenone are presented. A minority of the cells responded to one attractant or repellent only, whereas the vast majority were excited by more than one of the attractive and/or repellent stimuli. It is proposed that the peripheral olfactory cells of tsetse discriminate between different compounds via an across-fibre pattern coding, in which the cells that specifically code for attractants or repellents may play a substantial role in composing a unique excitation pattern that informs the central nervous system about the specificity of odours.

Olfactory sensitivities of mosquitoes with different host preferences (Anopheles gambiae s.s., An. arabiensis, An. quadriannulatus, An. m. atroparvus) to synthetic host odours.

Responses of antennal olfactory cells associated with sensilla trichodea were recorded in females of four Anopheles species (Diptera, Culicidae) with different host preferences: the anthropophilic An. gambiae s.s., the opportunistic An. arabiensis, and the zoophilic An. quadriannulatus and An. maculipennis atroparvus. Stimuli were vapours of synthetic host-odours: ethanoic, propanoic, butanoic, 3-methyl propanoic, 4-methyl butanoic acid, 1-octen-3-ol, and 3- and 4-methyl phenol. On stimulation with fatty acids and phenols either excitation or inhibition of spike activity was found, whereas responses to 1-octen-3-ol were invariably excitatory. The odour spectra of the cells could include activating as well as inhibiting substances. Differences in host preferences may be reflected in the numbers of olfactory cells responding to different odours and/or in the sensitivities of these cells. In An. gambiae more cells were excited by fatty acids than in An. arabiensis and An. m. atroparvus, whereas inhibition occurred more often in the latter two species. In addition, the fatty acid-excited cells in An. gambiae were more sensitive to these substances than in An. m. atroparvus and An. quadriannulatus. On the contrary, in the latter two species cells were more responsive to 1-octen-3-ol. In An. arabiensis, responses of stimulus-excited cells were intermediate between those in the anthropophilic and zoophilic species.

Neural coding in antennal olfactory cells of tsetse flies (Glossina spp.).

Spike trains from individual antennal olfactory cells of tsetse flies (Glossina spp.) obtained during steady-state conditions (spontaneous as well as during stimulation with 1-octen-3-ol) and dynamic stimulation with repetitive pulses of 1-octen-3-ol were investigated by studying the spike frequency and the temporal structure of the trains. In general, stimulation changes the intensity of the spike activity but leaves the underlying stochastic structure unaffected. This structure turns out to be a renewal process. The only independently varying parameter in this process is the mean interspike interval length, suggesting that olfactory cells of tsetse flies may transmit information via a frequency coding. In spike records with high firing rates, however, the stationary records had significant negative first-order serial correlation coefficients and were non-renewal. Some cells in this study were capable of precisely encoding the onset of the odour pulses at frequencies up to at least 3 Hz. Cells with a rapid return to pre-stimulus activity at the end of stimulation responded more adequately to pulsed stimuli than cells with a long increased spike frequency. While short-firing cells process information via a frequency code, long-firing cells responded with two distinctive phases: a phasic, non-renewal response and a tonic, renewal response which may function as a memory of previous stimulations.

Olfactory sensitivity in tsetse flies: a daily rhythm.

The diurnal tsetse Glossina morsitans morsitans bites especially in early morning and late afternoon; around midday feeding is at a low. In laboratory apparatus that measures the amount of locomotion under constant conditions over the photophase, the flies display a similar patterning of activity levels. The profile of daily rhythms for G. morsitans reported in the literature includes a number of motor and sensory motor systems that fluctuate cophasically. Lacking is a study on the patterning of the senses' response levels. In this paper we present the first instance of a daily modulation in the sense of smell. We stimulated the antennae with concentration series of host-derived odours and measured the spiking rate of cells at different times during the photophase. The concentration-response curves suggest that the sensitivity of antennal olfactory cells flows in parallel with the other daily rhythms. This was also reflected in electroantennograms (EAGs). The electroantennography was extended to G. fuscipes fuscipes, whose level of spontaneous locomotor activity--instead of following a U-shaped pattern--rises gradually over the photophase. Again, the EAGs appeared to parallel the species' locomotor activity. What we believe happens is that the organism tones down the sensitivity of its odour receptors during periods of anticipated inactivity for reasons of economy.

Search for tsetse attractants: A structure-activity study on 1-octen-3-ol inGlossina fuscipes fuscipes (Diptera: Glossinidae).

Trapping tsetse flies belonging to thepalpalis group still relies totally upon luring by visual cues even though odor-baited trapping is used effectively against themorsitans-group species. Forty-three percent of the antennal olfactory cells ofGlossina f. fuscipes, a member of thepalpalis group, respond to 1-octen-3-ol. For this species we report a structure-activity relationship between 1-octen-3-ol analogs, in which carbon chain length and the configuration of the hydroxyl and π-bond moieties are varied, and biological activity. Although the optimum chain length for all cells sensitive to 1-octen-3-ol is eight and most cells give lower responses when the hydroxyl function is omitted, there is a clear division into two groups. One group is diverse and represents cells that appear indifferent to the presence or position of the π bond; many will respond to such disparate structures as acetone and 3-methylphenol as well as to 1-octen-3-ol. In the other group, the structural requirements for the stimulus are more stringent; the cells appear to be specifically tuned to 1-octen-3-ol. Their thresholds are three orders of magnitude lower than those of the former group. The existence of two clusters points to a functional division in the olfactory sense. We suggest that the latter low-threshold group is involved in host detection from a distance while the former diverse group is involved in host discrimination at close range. Trap harvests with 1-octen-3-ol as a bait may have been disappointing because the appropriate mixture for generating a landing response on the traps is still lacking.