VESICULAR GLUTAMATE TRANSPORTER 1-POSITIVE PRIMARY AFFERENT TERMINALS CONTACT NK1-POSITIVE NEURONS IN THE RAT SACRAL PARASYMPATHETIC NUCLEUS

Abstract

Multiple lines of evidence suggest that central terminals of primary afferents conveying sensory information to the spinal cord release glutamate as neurotransmitter. This has been questioned recently by the microscopic lack of evidence, in peptidergic afferents, for any of the known vesicular glutamate transporters (VGLUTs), which are considered specific markers for glutamatergic transmission. Since the fraction of peptidergic fibers is higher in visceral than in somatic afferents, we wanted to test whether afferents from the urinary bladder express VGLUTs in their cell bodies in dorsal root ganglia (DRG) and in their central terminals. Most DRG neurons labeled with a tracer injected in the bladder wall were VGLUT1-positive. In the spinal cord, VGLUT1-positive terminals were dense in the sacral parasympathetic nucleus (SPN), a territory of termination of bladder afferents, and distinct from the peptidergic terminals, as identified by immunostaining for CGRP. Staining for VGLUT1 in SPN was abolished by L6-S1 dorsal rhizotomy, suggesting that VGLUT1-positive terminals are of primary afferent origin. VGLUT2-positive terminals in SPN were also distinct from the CGRP-positive terminals but are unlikely to be of primary afferent origin because the staining for VGLUT2 in SPN remained unchanged after dorsal rhizotomy. To characterize the postsynaptic targets of VGLUT1-positive terminals in SPN, we combined immunostaining for VGLUT1 with markers for projecting neurons (NK1), interneurons (nNOS), or preganglionic parasympathetic neurons (ChAT). VGLUT1-positive terminals contacted NK1-positive neurons but not nNOS- or ChAT-positive neurons. These results suggest termination of VGLUT1-positive, non-peptidergic, afferents onto SPN neurons, likely to mediate the central transmission of visceral sense but not the reflex activity of pelvic viscera.