Crickets use sound to find mates (Alexander, 1960), to compete with rivals (Alexander, 1961) and to detect echolocating bats (Moiseff et al., 1978; Nolen and Hoy, 1986). Because of the importance of sound to the biology of crickets and the well-known advantages of insects for neurobiological studies, crickets have long served as subjects for the study of auditory processing (for a review, see Hoy et al., 1998). A prominent bilateral pair of auditory interneurons in crickets are the omega neurons 1 (ON1) (Casaday and Hoy, 1977; Popov et al., 1978; Wohlers and Huber, 1982). Each ON1 receives excitatory input mainly from one ear (the ear ipsilateral to its soma) and inhibits several auditory interneurons that receive their input from the opposite ear, including the contralateral ON1 and ascending neurons 1 and 2 (AN1, AN2) (Selverston et al., 1985; Faulkes and Pollack, 2000). The resulting inhibition enhances binaural contrast in the auditory pathway, thereby improving the ability to localize sounds (Atkins et al., 1984; Horseman and Huber, 1994; Schildberger and Hörner, 1988). The neuron ON1 responds to sound over a wide frequency range, but it is most sensitive to the carrier frequency of conspecific calling song (Popov et al., 1978; Wohlers and Huber, 1982; Atkins and Pollack, 1986), which is approximately 4.5 kHz in Teleogryllus oceanicus (Hill et al., 1972; Balakrishnan and Pollack, 1996). Previously, we showed that the latency of ON1 is longer, by up to 10 ms, for 4.5 kHz compared with other frequencies, including ultrasound (Pollack, 1994; Faulkes and Pollack, 1997; Faulkes and Pollack, 2000). We have reported on the functional consequences of this elsewhere (Faulkes and Pollack, 2000), and in the present paper we focus on the mechanisms that might be responsible for the delayed response to cricket-like frequencies. Fig. 1 illustrates several candidate mechanisms. Pollack (Pollack, 1994) suggested that the conduction velocities of auditory receptors might differ in a frequency-specific manner (Fig. 1A), but Pollack and Faulkes (Pollack and Faulkes, 1998) found no evidence for this. The remaining hypotheses are, 1295 The Journal of Experimental Biology 204, 1295–1305 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3201 In crickets (Teleogryllus oceanicus), the auditory interneuron omega neuron 1 (ON1) responds to sounds over a wide range of frequencies but is most sensitive to the frequency of conspecific songs (4.5 kHz). Response latency is longest for this same frequency. We investigate the mechanisms that might account for the longer latency of ON1 to cricket-like sounds. Intracellular recordings revealed no evidence for appropriately timed postsynaptic inhibition of ON1 that might increase its latency, nor was latency affected by picrotoxin. The onset of excitatory postsynaptic potentials (EPSPs) was delayed for 4.5 kHz stimuli compared with ultrasound stimuli, pointing to a presynaptic locus for the latency difference. When ON1 is stimulated with high frequencies, discrete, apparently unitary EPSPs can be recorded in its dendrite, and these are latency-locked to spikes recorded simultaneously in the auditory nerve. This suggests that input to ON1 from high-frequency-tuned auditory receptor neurons is monosynaptic. In agreement with this, brief ultrasound stimuli evoke a single, short-latency EPSP in ON1. In contrast, the EPSP evoked by a brief 4.5 kHz stimulus consists of an early component, similar in latency to that evoked by ultrasound and possibly evoked by ultrasound- tuned receptors, and a later, dominant component. We interpret the early peak as arising from a monosynaptic afferent pathway and the late peak from a polysynaptic afferent pathway. Multiple-peak EPSPs, with timing similar to those evoked by sound stimuli, were also evoked by electrical stimulation of the auditory nerve. Key words: cricket, Teleogryllus oceanicus, auditory interneuron, omega neuron 1 (ON1), audition, insect, song, excitatory postsynaptic potential, monosynaptic pathway, polysynaptic pathway. Summary Introduction MECHANISMS OF FREQUENCY-SPECIFIC RESPONSES OF OMEGA NEURON 1 IN CRICKETS (TELEOGRYLLUS OCEANICUS): A POLYSYNAPTIC PATHWAY FOR SONG? ZEN FAULKES* AND GERALD S. POLLACK‡ Department of Biology, McGill University, 1205 Avenue Docteur Penfield, Montréal, Québec, Canada H3A 1B1 *Present address: Department of Zoology, University of Melbourne, Royal Parade, Parkville, Victoria 3010, Australia ‡Author for correspondence (e-mail: gpollack@bio1.lan.mcgill.ca) Accepted 3 January; published on WWW 15 March 2001