Lotte Houtepen

Abstract: BACKGROUND: The amygdala is involved in the coordination of stress but is also an important gatekeeper involved in the regulation of vigilance. The amygdala is structurally complex, consisting of several nuclei with specific functions in the affective response to environmental stimuli. There are indications that the medial amygdaloid nucleus may be a pivotal player in acute responses to emotional environmental stimuli. METHODS: The present study therefore aimed to study the effects of bilateral electrolytic lesions of the medial amygdala on unconditioned anxiety-related behavior as well as a sensorimotor gating parameter (prepulse inhibition, PPI) in rats. Anxiety-related behavior was assessed with the use of stress-induced hyperthermia (SIH), light-enhanced startle (LES) and open field behavior. RESULTS: Bilateral electrolytic lesions of the medial amygdala decreased the SIH response and anxiety-related open field behavior. In contrast, lesioned animals displayed augmented LES and disrupted PPI. No changes in basal locomotor activity, body temperature and acoustic startle were found between lesioned and sham animals. CONCLUSIONS: The present study suggests that the medial amygdala is an important player in response to acute environmental stimuli. Decreased unconditioned psychological stress responses were found, whereas LES was enhanced and sensorimotor processing was disrupted. However, considering the existing data on basolateral amygdala involvement in PPI and bed nucleus of the stria terminalis involvement in LES, local infusion studies into the MeA should be performed to further substantiate these findings.

Abstract: While anxiety models are often based on locomotor activity responses, the stress-induced hyperthermia (SIH) paradigm uses the autonomic stress response by measuring body temperature. The effects of putative anxiogenic compounds in the SIH paradigm are inconclusive in mice and have not been examined in rats. Furthermore, it has been suggested that drug-induced effects on body temperature could be dependent on locomotor activity levels. Therefore, the effects of three anxiogenic substances, yohimbine (an alpha(2) receptor antagonist), mCPP (a 5HT(2C) receptor agonist) and FG-7142 (a GABA(A) receptor inverse agonist acting at the benzodiazepine site) on the stress-induced body temperature and locomotor activity response were studied in rats using novel cage stress. All anxiogenic compounds resulted in hypothermia. In contrast, FG-7142 and yohimbine increased locomotor activity levels, whereas mCPP reduced locomotor activity levels. The lack of an increased body temperature response of anxiogenic compounds indicates that the anxiogenic capacity of a drug does not necessarily yield increased autonomic stress responsivity. Moreover, the present study shows that a drug-induced decreased body temperature can be accompanied by increased locomotor activity, suggesting that both parameters represent independent parameters of the stress response.