The responses of the hypothalamic-pituitary-adrenal (HPA) system are characterized by sensitivity to both the initial condition and the recent history of the subject. This observation suggests that a memory of the original stimulus is retained that can modify responses to future stimuli. Although feedback inhibition of HPA responses by corticosteroids has been widely studied, potentiated responses of the HPA to a repeated stimulus are not well characterized. The objective of this proposal is to identify the site(s) and mechanism(s) of memory of the HPA and to better understand how organisms balance facilitator and inhibitory input to produce a given response to a stimulus. The limbic system is known to modulate the responses of the HPA system based upon anatomic connections, lesion experiments, and a high concentration of corticosteroid receptors. The limbic system is also the site where long- term potentiation (LTP) of synaptic transmission has been best characterized. The hypothesis of this proposal is that facilitatory memory of the HPA system is stored in the limbic system by mechanisms similar to those of LTP. We will test this hypothesis through the following specific goals: (1) Identify specific anatomic sites within the limbic system where memory may be stored. Immunohistochemistry and in situ hybridization will detect limbic areas where the immediate-early transcription factor (IETs), fos and zif/268, and their gene expression increase after hemorrhage. (2) Define the role of excitatory and inhibitory amino acid transmitter in the limbic system in the control of paraventricular function. ACTH release and CRH expression (In situ and soluble hybridization) will be studied after microinjection of glutamate agonists and antagonists into areas of the limbic system with increased IETF expression after hemorrhage. (3) Determine whether hemorrhage induces changes in the activity of receptors (NMDA receptor) or in the expression of synthetic enzymes (NMDA receptor & NO synthase) associated with LTP within these limbic areas. We will block LTP in specific limbic sites using localized lesions or local administration of specific antagonists to attempt to block enhanced HPA responses to a second stimulus without altering those to the first. Finally, we will attempt to induce LTP in the specific sites with injection of glutamate, NMDA and NO donors and see if this treatment can lead to increased HPA responsiveness. Unique aspects of this project include: (1) a focus on potentiation in the HPA system rather than inhibition, and (2) an attempt to link changes in synaptic efficiency (LTP) at specific sites to modulation of a well define autonomic reflex.
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NIH ExPorter