Ageing reduces the number of vesicular glutamate transporter 2 containing immunoreactive inputs to identified rat pelvic motoneurons

The immunocytochemical localisation of vesicular glutamate transporters, VGLUT1 and VGLUT2, was employed to identify putative glutamatergic axon terminals innervating pelvic motoneurons. VGLUT1 terminals were sparsely distributed within lumbosacral spinal motoneuron pools, including the dorsolateral nucleus, retrodorsolateral nucleus and spinal nucleus of the bulbospongiosus. This was in marked contrast to VGLUT2 which was expressed in a robust innervation of these areas. Retrograde tracing was used to reveal motoneurons innervating the levator ani (LA) muscle. On these neurons, associations with VGLUT2 immunoreactive terminals were abundant while those with VGLUT1 were rare. Ultrastructural investigations revealed that VGLUT2 immunoreactive terminals made asymmetric synaptic contacts with dendrites of retrogradely labelled LA motoneurons. Quantification of VGLUT2 immunoreactive boutons in close association with these dendrites was carried out in young and aged animals using light microscopy. This revealed a significant decline in the numbers of VGLUT2 immunoreactive boutons on the more distal dendrites of motoneurons in aged rats. VGLUT2 boutons were reduced by approximately 21% from 11.25 ± 0.5 per 35-μm length of dendrite in young rats to 8.89 ± 0.5 in aged animals. This decline in glutamatergic input may reduce the excitability of LA motoneurons and consequently decrease the capacity of the rat to induce reflexive erections.