Exploratory behaviour and 50-kHz ultrasonic vocalizations in rats: behavioural and neurochemical profiles of persistent inter-individual differences
Affective disorders, including depression and anxiety disorders are a major cause of disability in the world. The biological basis of affective disorders as well as the relationship between the pharmacological profiles and therapeutic effects of antidepressants still lack a coherent theory. One reason for this is the relative lack of adequate animal models for studies of neurobiological basis of these disorders. The main aim of the present studies was development of two potential models of affective states that were based on exploratory behaviour and ultrasonic vocalizations in rats. Exploratory behaviour is influenced by fear and curiosity towards novel objects and environments, and many behavioural tests hence use it as a measure for anxiety. The 50-kHz ultrasonic vocalizations emitted by rats have been associated with positively valenced conditions. Significant inter-individual variations have been previously found in both behaviours. In the present studies exploratory behaviour and 50-kHz ultrasonic vocalizations were investigated as stable individual dispositions. Methods were developed for adequate measurement of individually characteristic levels of both behaviours and behavioural and neurobiological differences between the resulting low-exploring versus high-exploring and low-chirping versus high-chirping groups were studied. It was found that low-exploring animals are more anxious and use more passive coping strategies. The differences between high- and low-exploring animals are at least in part influenced by differences in midbrain dopamine and serotonin neurotransmission in prefrontal cortex and hippocampus (with possible influences from molecules regulating transcriptional processes and neural development). Male rats who emit low levels of 50-kHz ultrasonic vocalizations show greater susceptibility to stressful stimuli which is especially expressed in chronic stress-induced changes in brain metabolic activity. In females, the animals who emit high levels of 50-kHz ultrasonic vocalizations are more sensitive to stress.