Preoptic projections to Barrington's nucleus and the pericoerulear region: architecture and terminal organization.

The medial preoptic area (MPO), a sexually dimorphic region, plays a pivotal role in neuroendocrine function and reproductive behavior. We recently reported that MPO projects heavily to the midbrain periaqueductal gray (PAG). We also noted that MPO projects to the dorsolateral pontine tegmentum. Here we identified the cells of origin of the MPO-->tegmental projection and delineated the terminal organization of MPO projections to Barrington's nucleus, locus coeruleus (LC), and the rostromedial pericoerulear region (pLCrm). Correlative cyto- and chemoarchitectonic studies were done to define better the nuclear groups of the dorsolateral pontine tegmentum. Retrograde tracing revealed that MPO neurons projecting to the dorsolateral pontine tegmentum are preferentially distributed in distinct subregions of MPO, including the sexually dimorphic medial preoptic nucleus (MPN). Anterograde tracing with wheat germ agglutinin-horseradish peroxidase or Phaseolus vulgaris leucoagglutinin demonstrated considerable target specificity in projections from MPO to the dorsolateral pontine tegmentum. Barrington's nucleus receives a dense focal input along its entire rostrocaudal axis. In addition, pLCrm is heavily targeted by MPO inputs; pLCrm contains a concentrated plexus of extranuclear dendrites of LC neurons. The lateral dorsal tegmental (LDT) nucleus and LC proper receive only sparse input from MPO. MPO projections to Barrington's nucleus could regulate micturition reflexes during reproductive behavior. The MPO-->pLCrm projection could influence noradrenergic LC neurons in relation to reproductive and/or gonadal steroid function. Given the strong established connections from olfactory structures to MPO, it is possible that the MPO-->LC pathway provides an anatomical substrate for olfactory modulation of arousal.