Characterization of the head-twitch response induced by hallucinogens in mice: detection of the behavior based on the dynamics of head movement

Abstract

Rationale: The head-twitch response (HTR) is a rapid side-to-side rotational head movement that occurs in rats and mice after administration of serotonergic hallucinogens and other 5-HT2A agonists. The HTR is widely used as a behavioral assay for 5-HT2A activation and to probe for interactions between the 5-HT2A receptor and other transmitter systems.

Objective: High-speed video recordings were used to analyze the head movement that occurs during head twitches in C57BL/6J mice. Experiments were also conducted in C57BL/6J mice to determine whether a head-mounted magnet and a magnetometer coil could be used to detect the HTR induced by serotonergic hallucinations based on the dynamics of the response.

Results: Head movement during the HTR was highly rhythmic and occurred within a specific frequency range (mean head movement frequency of 90.3 Hz). Head twitches produced wave-like oscillations of magnetometer coil voltage that matched the frequency of head movement during the response. The magnetometer coil detected the HTR induced by the serotonergic hallucinogens 2,5-dimethoxy-4-iodoamphetamine (DOI; 0.25, 0.5, and 1.0 mg/kg, i.p.) and lysergic acid diethylamide (LSD; 0.05, 0.1, 0.2, and 0.4 mg/kg, i.p.) with extremely high sensitivity and specificity. Magnetometer coil recordings demonstrated that the non-hallucinogenic compounds (+)-amphetamine (2.5 and 5.0 mg/kg, i.p.) and lisuride (0.8, 1.6, and 3.2 mg/kg, i.p.) did not induce the HTR.

Conclusions: These studies confirm that a magnetometer coil can be used to detect the HTR induced by hallucinogens. The use of magnetometer-based HTR detection provides a high-throughput, semi-automated assay for this behavior, and offers several advantages over traditional assessment methods.

Figure 1

Reconstructions of head movement sequences during head-twitches in mice after administration of 2 mg/kg DOI. Two representative plots are shown. Head position was sampled every 2.38 ms. The circles show the position of the center of the head in x,y coordinates over successive video frames. Open and filled circles represent rotational head movements toward the left or right side of the mouse, respectively; grey circles represent movements immediately preceding or subsequent to the HTR. The arrow shows the direction the head was facing during the responses.

Figure 2

Effect of filtering on magnetometer coil responses. The left panels show the voltage response of the magnetometer coil, and the right panels are periodograms showing the spectral density of the responses. (A) Unfiltered magnetometer coil responses. (B) Output of the magnetometer coil after band-pass filtering (40–200 Hz).

Figure 3

Characteristic magnetometer coil responses induced by head-twitches. (A) Response of the magnetometer coil to head-twitches. (B) Periodograms showing the spectral density of the responses. The magnetometer recordings were band-pass filtered (40–200 Hz) to attenuate the response to extraneous head movements and high-frequency noise.

Figure 4

Effect of DOI on the head-twitch response. (A) Manually scored head-twitch counts from video recordings. Data are presented as group means±S.E.M. **p<0.01, significant difference from vehicle control group. (B) Correlation between head-twitch counts in the videos and the magnetometer coil recordings.

Figure 5

Plots of magnetometer coil voltage responses showing the temporal distribution of head twitches induced by DOI. A single C57BL/6J mouse was treated with vehicle and DOI (0.25, 0.5, and 1.0 mg/kg, i.p.), with 1-week between test sessions. At this time resolution, head twitches appear as high-amplitude bipolar deflections. The numbers of head twitches detected were 2 (vehicle), 13 (0.25 mg/kg), 22 (0.5 mg/kg), and 30 (1.0 mg/kg).

Figure 6

Effects of SKF38393 (10 mg/kg) on (A) the duration of grooming, and (B) the head-twitch response. Data are presented as group means±S.E.M. Five C57BL/6J mice were used per group. *p<0.05, significant difference from vehicle control group.

Figure 7

Effect of LSD on the head-twitch response. (A) Total counts over the 30-min test session. Data are based on the magnetometer recordings, and are presented as group means±S.E.M. (B) Time-course of the HTR induced by LSD, in 2-min blocks. Data are based on the magnetometer recordings, and are presented as group means. Five C57BL/6J mice were used per group. *p<0.05, **p<0.01, significant difference from vehicle control group.

Figure 8

Effect of lisuride on the head-twitch response. Data are based on the magnetometer recordings over the 30-min test session. Five C57BL/6J mice were used per group.