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The neurochemical modulation of the dorsal raphe nucleus performed by the medial amygala

Abstract

Studies aimed at the neurobiology of anxiety have, during the last years, dedicated considerable attention to the role played by different neurochemical systems on the modulation of defensive responses to aversive stimuli. In this sense, the important role played by serotonin (5-HT) has been emphasized by the clinical efficacy of serotonergic drugs for the treatment of different anxiety disorders. Aside from 5-HT, corticotrophin releasing factor (CRF) has also been pointed out as a promissor target for the treatment of anxiety-related disorders. Evidences suggest that both systems, 5-HT and CRF, interact. The dorsal raphe nucleus (DRN) is the region that gives rise to most of the forebrain 5-HT projections. Its activity is modulated by several brain structures that regulate the release of 5_HT in innervated regions. It is known, for instance, that the activation of DRN CRF type 2 receptors stimulates 5-HT neurons and increases the release of 5-HT in prosencephalic regions. One of the main amygdaloid nucleus that projects to the DRN is the medial amygdala. Nevertheless, until the present moment, the neurochemical identity of the projections medial amygdala - DRN is unknown. Interestingly, it has been previously demonstrated by our research group that the activation of both CRF type 1 and 2 receptors of the medial amygdala facilitates anxiety-related responses. The present study has two main purposes: 1) to verify through the use of retrograde tracers, the neurochemical identity of the projections that connect the medial amygdala to the DRN. Since both GABAergic and glutamatergic cells have been identified in the medial amygdala, this first experiment will be performed through the use of a double immunohistochemistry methodology with the retrograde tracer cholera toxin subunit B (Ctb), injected into the DRN, and in situ hybridization for the mRNA of the type 2 vesicular transporter of glutamate (VGLUT-2) or the enzime glutamate decarboxilase (GAD), the enzyme that limits GABA synthesis in the medial amygdala. The second aim of the study will be to verify if CRF type 1 and 2 receptors of the medial amygdala are localized in neurons that project to the DRN. For that we will perform a double immunohistochemistry study for the retrograde tracer CTb and the method of in situ hybridization for the mRNA of CRF type 1 and 2 receptors. (AU)