Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Abstract

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Fig. 1.

Nissl stained coronal section of mouse brain showing a representative electrolytic lesion site within the IC created by a carbon fiber voltammetry electrode.

Fig. 2.

Nafion®-coated electrodes selectively measure serotonin (5-HT) in vivo. Initial injections occurred at Time=0 (solid vertical line). The injection of L-5-HTP (N=7, black diamonds) increased the in vivo voltammetric signal above control levels (N=5, black crosses; ANCOVA, F_1,121=18.26, P<0.001). The injection of clorgyline prevented the synthesis of 5-hydroxy-indole-acetic acid (5-HIAA) and had no effect on the baseline voltammetric signal when given alone (N=7, gray triangles; ANCOVA, F_1,77=0.17, P=0.684) or when given following an L-5-HTP injection (N=7, gray squares, L-5-HTP injection at line, clorgyline injection at arrow; ANCOVA, F_1,179=0.97, P=0.325). Inset: representative voltammetric current traces taken 46 min after the injection of L-5-HTP (solid black line), clorgyline (solid gray line) or saline vehicle (broken black line). Values are means ± s.e.m.

Fig. 3.

Levels of inferior colliculus (IC) serotonin (5-HT) and 5-HT+5-HIAA in mice during waking. The voltammetric signal measured with Nafion®-coated electrodes increased as mice woke from anesthesia relative to a baseline of maintained anesthesia (black diamonds, Waking N=24, Anesthetized N=4; ANCOVA, F_1,316=44.81, P<0.001). When measured with non-coated (i.e. non-selective) electrodes, the voltammetric signal remained the same (gray squares, Waking N=23, Anesthetized N=6; ANCOVA, F_1,403=0.27, P=0.606). In the waking mice, the average waking time, determined by the recovery of motor function, was 33±3 min (broken vertical line). Values are means ± s.e.m.

Fig. 4.

Serotonin (5-HT) levels during the presentation of a variety of stimuli (boxed area). Mice exposed to noise (94 dB SPL, N=9, black triangles) demonstrated an increase in inferior colliculus (IC) 5-HT relative to control mice (N=10, black broken line, s.e.m. = shaded region; ANCOVA, F_1,57=12.44, P=0.053). Mice exposed to food (N=9, gray diamonds), light (N=7, gray squares) or TMT (N=11, gray crosses) showed no change in IC 5-HT relative to the no-stimulus control group. Values are means ± s.e.m.

Fig. 5.

Serotonin (5-HT) levels respond to restricted movement stress. Mice exposed to restricted movement (N=9, boxed area, black diamonds) displayed an increase in inferior colliculus (IC) 5-HT relative to control mice (N=10, black broken line, s.e.m. = shaded area; ANCOVA, F_1,51=20.27, P=0.001). Values are means ± s.e.m.

Fig. 6.

Summary of changes in 5-HT during different stimulus paradigms. Only the noise (paired t-test, t=−3.86, P=0.005) and restriction (paired t-test, t=−5.44, P=0.001) stimuli induced significant changes in IC 5-HT relative to pre-stimulus values. Values are means ± s.e.m.