Spinal and brain circuits to motoneurons of the bulbospongiosus muscle: Retrograde transneuronal tracing with rabies virus

Retrograde transneuronal tracing with rabies virus from the left bulbospong iosus muscle (BS) was used to identify the neural circuits underlying its p eripheral and central activation. Rats were killed at 2, 3, 4, and 5 days p ost-inoculation (p.i.). Rabies immunolabelling was combined with immunohist ochemical detection of choline acetyltransferase and oxytocin. Virus;uptake was restricted to ipsilateral BS motoneurons (2 days p.i.). The onset of t ransfer (3 days p.i.) visualized interneurons in the dorsal grey commissure (DGC), intermediate zone, and sacral parasympathetic nucleus (SPN), mainly in DGC at L5-S1, and revealed synaptic connections between BS and external urethral sphincter motoneurons. At 4 and 5 days p.i., higher-order interne urons were labelled in other spinal areas and segments. Supraspinal labelli ng initially involved only Barrington's nucleus, nucleus reticularis magnoc ellularis, and paragigantocellularis lateralis (4 days p.i.). Later, labell ing extended to other populations traditionally associated with control of sexual activity and micturition (periaqueductal grey, paraventricular nucle us, medial preoptic area, prefrontal cortex), but also indicated the interv ention of somatic descending motor pathways (vestibulospinal and reticulosp inal neurons, "hindlimb" regions of sensorimotor cortex and red nucleus) an d cerebellar nuclei in multisynaptic innervation of the labelled motoneuron s. Dual color immunofluorescence disclosed multisynaptic links between thes e motoneurons and thoracolumbar medial sympathetic (choline acetyltransfera se-immunoreactive) neurons. In contrast, preganglionic neurons in SPN and m ost oxytocinergic neurons in paraventricular hypothalamic nucleus remained unlabelled, suggesting that parasympathetic and somatic outflow to pelvic o rgans are probably controlled by separate interneuronal populations and tha t oxytocinergic spinal projections are more likely to influence sacral auto nomic rather than somatic outflow. (C) 1999 Wiley-Liss, Inc.