Three Paradigms of Emotional Learning Differentially Affect Brainstem, Hypothalamic, and Limbic Circuits in the Rat

Three Paradigms of Emotional Learning Differentially Affect Brainstem, Hypothalamic, and Limbic Circuits in the RatElizabeth A. Myers, M.S.University of Pittsburgh, 2004Noradrenergic (NA) signaling in limbic forebrain regions, such as the central nucleus of the amygdala (CeA), shapes the encoding and expression of emotional learning, and modulates responses to stress and anxiety. The present study examined whether categorically different emotional stress models, [cholecystokinin (CCK), trimethylthiazoline (TMT), and yohimbine (YO)] support behaviorally aversive conditioning and differentially activate CRH-positive neurons in the hypothalamus, medullary and pontine NA neurons, and ascending inputs to the CeA. A conditioned flavor avoidance (CFA) paradigm using a flavor preference test was implemented as a measure of aversive conditioning for each stressor. In a terminal experiment, rats received either an injection of CCK (10 µg/kg, i.p.), YO (5 mg/kg. i.p.), or 15 min exposure to an aversive odor, TMT, and were perfused 60-120 min later. In a subset of rats, retrograde neural tracer was microinjected into the CeA prior to stressor treatment and perfusion. Brainstem and forebrain sections were processed for immunocytochemical localization of cFos and either dopamine beta hydroxylase (DbH) to identify NA neurons, corticotropin-releasing hormone (CRH), or neural tracer to identify hindbrain CeA-projecting neurons. All stressors activated hypothalamic CRH neurons, produced a relatively strong CFA in a 2-bottle choice test, and recruited similar proportions of CeA-projecting neurons arising from the parabrachial nucleus, a projection path critical for this behavioral paradigm. All stressors recruited NA neurons within the medullary A2 cell group to a similar extent, whereas those in the medullary A1 cell group and pontine A6 cell group were recruited more selectively by TMT and YO compared to CCK. Afferent inputs to the CeA arising in these hindbrain cell groups were activated in a parallel manner, with TMT and YO recruiting a much greater proportion of CeA-projecting neurons in the A1 and A6 cell groups. These findings lend support to the working hypothesis that different emotional stimuli may potentially influence emotional learning via stressor-specific ascending NA projection pathways to the CeA. In general, elucidating stressor-specific neural circuitry may provide new insight into how to design effective therapeutic measures for a wide range of human disorders and conditions involving the NA system