Cervix Stimulation Evokes Predominantly Subthreshold Synaptic Responses in Mouse Thoracolumbar and Lumbosacral Superficial Dorsal Horn Neurons

Abstract

Sensory input from the female reproductive tract (FRT) plays a pivotal role in coordinating reproductive reflexes. Additionally, a number of disorders, especially chronic pelvic pain, may be due to disturbances in sensory processing of signals from the FRT. Our aim was to record synaptic responses in neurons from lumbar and sacral spinal cord segments during mechanical stimulation of the cervix. We developed an in vivo preparation of the mouse spinal cord to record synaptic potentials from superficial dorsal horn (SDH) neurons under whole-cell patch clamp recording conditions. We analyzed the strength and distribution of excitatory postsynaptic potentials in SDH neurons evoked during mechanical stimulation of the cervix and cutaneous sites. Resting membrane potential and neuronal input resistance was similar in thoracolumbar (TL, T13-L3) and lumbosacral (LS, L6-S2) segments. We elicited activity in 6/21 TL neurons and 15/39 LS neurons using mechanical stimulation of the cervix with a blunt probe. The majority of these neurons responded to cervix stimulation with bursts of subthreshold excitatory postsynaptic potentials (4/6 and 12/15 TL and LS neurons, respectively). The remainder responded with sufficient magnitude to generate action potentials (2/6 and 3/15 TL and LS neurons). Cutaneous synaptic inputs were also elicited in 11/21 TL neurons following stimulation of the flank/leg, 19/39 LS neurons by stimulation of the tail, and three LS neurons by perineal stimulation. Some neurons received convergent synaptic inputs from the cervix and cutaneous sites (4/6 TL and 4/15 LS). These data demonstrate that spinal projections of cervix afferents are widely dispersed in the SDH and considerable convergence exists between neurons innervating the cervix and cutaneous structures. Our results indicate that much of the synaptic activity evoked in SDH neurons following cervix stimulation is subthreshold.