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

Abstract

Retrograde transneuronal tracing with rabies virus from the left bulbospongiosus muscle (BS) was used to identify the neural circuits underlying its peripheral and central activation. Rats were killed at 2, 3, 4, and 5 days post-inoculation (p.i.). Rabies immunolabelling was combined with immunohistochemical detection of choline acetyltransferase and oxytocin. Virus uptake was restricted to ipsilateral BS motoneurons (2 days p.i.). The onset of transfer (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 interneurons were labelled in other spinal areas and segments. Supraspinal labelling initially involved only Barrington's nucleus, nucleus reticularis magnocellularis, and paragigantocellularis lateralis (4 days p.i.). Later, labelling extended to other populations traditionally associated with control of sexual activity and micturition (periaqueductal grey, paraventricular nucleus, medial preoptic area, prefrontal cortex), but also indicated the intervention of somatic descending motor pathways (vestibulospinal and reticulospinal neurons, "hindlimb" regions of sensorimotor cortex and red nucleus) and cerebellar nuclei in multisynaptic innervation of the labelled motoneurons. Dual color immunofluorescence disclosed multisynaptic links between these motoneurons and thoracolumbar medial sympathetic (choline acetyltransferase-immunoreactive) neurons. In contrast, preganglionic neurons in SPN and most oxytocinergic neurons in paraventricular hypothalamic nucleus remained unlabelled, suggesting that parasympathetic and somatic outflow to pelvic organs are probably controlled by separate interneuronal populations and that oxytocinergic spinal projections are more likely to influence sacral autonomic rather than somatic outflow.

Neurotransmitters: central and peripheral mechanisms

“…2 The oxytocinergic spinal projections are more likely to in¯uence sacral autonomic rather than somatic out¯ow. 24 Plasma oxytocin concentrations are known to be elevated in humans following sexual stimulation.…”

The dynein light chain 8 binding motif of rabies virus phosphoprotein promotes efficient viral transcription

“…Yet, the molecular basis for the transport of the RV particle remains unclear and controversial. Most transneuronal studies of the CNS in nonhuman primates (5,6) and mice (7,8) reveal that fixed (laboratory-adapted) strains of RV spread exclusively in the retrograde direction; however, past in vitro and in vivo studies suggest that anterograde transport might also be involved (7,(9)(10)(11). The discovery that the dynein light chain, LC8 (DNLC1), interacts with the RV phosphoprotein (RVP) provides a putative molecular link between the virus and host cell transport system and, thus, a potential mechanism for the neuroinvasive property of RV.…”

Perspectives in Diagnosis and Treatment of Rabies Viral Encephalitis: Insights from Pathogenesis

“…Rabies virus propagates to the CNS from its site of inoculation via axonal transport in a retrograde fashion [41][42][43]. Travel to the CNS via the peripheral axons occurs at a fairly constant rate of 12 to 24 mm per day [44][45][46].…”