Targeting Corticotropin-Releasing Factor Projections from the Oval Nucleus of the Bed Nucleus of the Stria Terminalis Using Cell-Type Specific Neuronal Tracing Studies in Mouse and Rat Brain

The bed nucleus of the stria terminalis (BNST) is known to play a critical role in mediating the behavioural and autonomic responses to stressors. The oval nucleus of the BNST (BNSTov) contains cell bodies that synthesise the stress hormone corticotropin‐releasing factor (CRF). Although afferent fibres originating from the BNSTov have been shown to innervate several key structures of the neuroendocrine and central autonomic system, the question remains as to whether some of these fibres are CRF‐positive. To directly address this question, we injected a ‘floxed’ anterograde tracer (rAAV5/EF1a‐DIO‐mCherry) into the BNSTov of CRFp3.0CreGFP transgenic mice, which express a green fluorescent protein (GFP) under the control of the CRF promoter. Serial sections were then analysed for the presence of double‐labelled fibres in potential projection sites. To determine whether CRF neurons in the rat BNSTov send comparable projections, we infused rat BNSTov with an adeno‐associated viral vector (AAV) in which the human synapsin promoter drives enhanced GFP expression. We then used CRF immunoreactivity to examine double‐labelled fluorescent fibres and axon terminals in projection sites from brain sections of the AAV‐infused rats. We have observed several terminal fields in the mouse and rat brain with double‐labelled fibres in the Dorsal raphe nucleus (DRD), the paraventricular nucleus of the hypothalamus and, to a lesser extent, in the ventral tegmental area. We found double‐labelled terminal boutons in the nucleus accumbens shell, prelimbic cortex and posterior basolateral nucleus of the amygdala. The most intense double‐labelling was found in midbrain, including substantia nigra pars compacta, red nucleus, periaqueductal grey and pontine nuclei, as well as DRD. The results of the present study indicate that CRF neurons are the output neurons of the BNSTov and they send projections not only to the centres of neuroendocrine and autonomic regulation, but also regions modulating reward and motivation, vigilance and motor function, as well as affective behaviour.