The use of ultrasonic vocalizations as an experimental tool for studying emotional states in rodents has led to an increased understanding of the basic science of affect as well as the development of novel diagnostics and therapeutics for the treatment of affective disorders. At the behavioral level, the rules that govern the generation of affective ‘feeling’ states are similar to those of the psychophysics of sensory perception. Emotions are elicited primarily in response to active social stimuli. A linear increase in affective response requires a logarithmic increase in stimulation and habituation of a given affective response allows for transition across the cycle of emotional/affective states (approach → consummatory phase → avoidance). At the neuronal level, the coordinated expression of affective responses in the medial prefrontal cortex is orchestrated by rhythmic activity, which is initiated and maintained by a variety of short-term and long-term synaptic plasticity processes. An objective measure of affective states may emerge from these psychophysical and neuronal properties of emotion. Enhancing synaptic plasticity with pharmacological agents that modulate NMDA receptor activity as well as IGFI receptor activity may have therapeutic potential for the treatment of affective disorders.
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