Audible and ultrasonic vocalization elicited by single electrical nociceptive stimuli to the tail in the rat

We describe audible and ultrasonic vocalization elicited in rats by a short electrical pulse applied to the tail. Three types of vocal emissions were recorded: (1) 'peep', characterized by a repartition of energy over a wide range (0-50 kHz) of frequencies without any clear structure; (2) 'chatters', characterized by an audible (frequencies in hearing range of humans) fundamental frequency (2.47 +/- 0.03 kHz) and harmonics; and (3) 'ultrasonic emissions', characterized by a succession of slightly modulated pulses with frequencies in the 20-35 kHz range. Peeps and chatters were never recorded before the application of the stimuli. Several different vocalization patterns were described in terms of these types of responses. Just after the stimulation, all the animals emitted a 1st peep, which was generally (61%) followed by a 2nd one. They appeared with reproducible latencies, durations and envelopes. The envelopes of the audible (peeps and chatters) responses were intensity-dependent. Experimental data (moving the stimulation site, lidocaine injection) indicated that the 1st and 2nd peeps were triggered by two different groups of peripheral fibres with mean conduction velocities of 7.3 +/- 0.8 and 0.7 +/- 0.1 m/sec, respectively. This suggested an involvement of A delta and C fibres. Morphine showed a naloxone-reversible and dose-dependent antinociceptive effect by decreasing the 1st and 2nd peep envelopes. It is concluded that a short stimulus applied to the tail triggers a complex behavioural repertoire. It is proposed that this model will be a useful tool for physiological and pharmacological studies of nociception.