The medullary raphe nuclei are regarded as the final common neural relay connecting higher brain centres to the spinal cord and sympathetic outflow to brown adipose tissue (BAT), an observation which is based on functional (electrpophysiological) data. In order to define the trajectory of populations of neurons in the hypothalamus, synaptically linked to the medullary raphe nuclei and ultimately BAT, a novel approach was utilised which involves the injection of a modified form of pseudorabies virus (PRV) into the BAT. This changes the colour of its fluorescent reporter when it comes into contact with Cre recombinase. We hypothesise that contrary to the dogma stated at the outset there will be two populations of neurons projecting to BAT, one which passes through the midline raphe and the other that involves alternate premotor pathways in the brainstem.
An (AAV)-Cre recombinase construct was injected stereotaxically into the raphe nuclei of male Sprague Dawley rats weighing between 230 and 250 grams. This injection was followed 2 weeks later by injection of PRV Brainbow virus (PRV-263) into the interscapular BAT. After 4 days survival, rats were killed and their brains prepared for histological analyses.
PRV-263 which is replication competent was transported retrogradely from the BAT through chains of synaptically-connected neurons in the spinal cord, brainstem and hypothalamus including the midline raphe. After transport through neurons expressing Cre recombinase in the raphe there was recombination of the viral genome at either paired lox2272 or loxP sites, resulting in the loss of the red reporter and expression of either cyan (mCerulean) or yellow (eYFP)., Importantly there were distinct groups of neurons in the rostroventrolateral medulla, lateral hypothalamus and paraventricular nucleus that retained their red fluorescent reporter consistent with a trajectory other than through the raphe nuclei.
These data define the nature of descending neural projections to BAT.