Inhibition of either the sympathetic or parasympathetic nervous system increases tonic immobility duration in laying hens.

Tonic immobility (TI) is a temporary state where a chicken remains immobile after restraint. It is a commonly used fear test in research, where a longer latency to right is equated with more fear. Despite its commonality, there is limited research done on the physiological systems that underlie TI. The objective of our study was to examine the roles of the sympathetic (SNS) and parasympathetic nervous systems (PNS) on TI. In experiment 1, sixteen Dekalb White hens (43 weeks of age) were given 0.1 mL/kg of either saline as a control (CON), labetalol (5 mg/mL) to inhibit the SNS (LAB), or glycopyrrolate (0.2 mg/mL) to inhibit the PNS (GLY). In experiment 2, eight Hy-Line Silver hens (20 weeks of age) were given either 0.2 mL/kg of saline as a control (CON2) or 0.2 mg/kg of a 1:1 glycopyrrolate and labetalol mixture (DRUG). For both experiments, latency to first leg movement, first head movement, and to right were measured, as well as thermal temperatures. In experiment 1, heart rate (HR) and respiratory rate (RR) were also measured. The MIXED procedure was used to analyze latencies for experiment 1. Experiment 2 utilized a Welch's T-test. An ANOVA was used for all RR, HR, and thermal temperatures. Data were considered significant when P < 0.05 and trending when 0.05 < P < 0.10. For experiment 1, after injection and before induction, GLY had a lower respiratory rate (p = 0.019) compared to LAB and CON. This trend continued through the first minute (p = 0.020) and the average time under TI (p = 0.010). The CON group was quicker to right than the GLY (p = 0.007) and LAB (p = 0.0726). For experiment 2, the CON2 group was also quicker to right than DRUG (p = 0.052). These results indicate that both the PNS and SNS play a role in TI recovery, however further studies need to be undertaken to better understand the neurobiology of tonic immobility.