Central neuronal circuit innervating the lordosis-producing muscles defined by transneuronal transport of pseudorabies virus

The lordosis reflex is a hormone-dependent behavior displayed by female rat s during mating. This study used the transneuronal tracer pseudorabies viru s (PRV) to investigate the CNS network that controls the lumbar epaxial mus cles that produce this posture. After PRV was injected into lumbar epaxial muscles, the time course analysis of CNS viral infection showed progressive ly more PRV-labeled neurons in higher brain structures after longer surviva l times. In particular, the medullary reticular formation, periaqueductal g ray (PAG), and ventromedial nucleus of the hypothalamus (VMN) were sequenti ally labeled with PRV, which supports the proposed hierarchical network of lordosis control. Closer inspection of the PRV-immunoreactive neurons in th e PAG revealed a marked preponderance of spheroid neurons, rather than fusi form or triangular morphologies. Furthermore, PRV-immunoreactive neurons we re concentrated in the ventrolateral column, rather than the dorsal, dorsol ateral, or lateral columns of the PAG. Localization of the PRV-labeled neur ons in the VMN indicated that the majority were located in the ventrolatera l subdivision, although some were also in other subdivisions of the VMN. As expected, labeled cells also were found in areas traditionally associated with sympathetic outflow to blood vessels and motor pathways, including the intermediolateral nucleus of the spinal cord, the paraventricular hypothal amic nucleus, the red nucleus, and the motor cortex. These results suggest that the various brain regions along the neuraxis previously implicated in the lordosis reflex are indeed serially connected.