Pelvic and pudendal reflexes in the in vitro neonatal rat preparation

Differential vulnerabilities of urethral afferents in diabetes and discovery of a novel urethra-to-urethra reflex

Urethral reflexes are important regulators of micturition, and impairment of urethral afferent neuronal function may disrupt coordinated bladder and urethral activity, thereby contributing to voiding dysfunction in lower urinary tract disorders. Chemical stimulation by intraurethral irritant solution perfusion was used to determine whether urethral afferent neuronal function is altered in diabetes mellitus (DM). Sprague-Dawley rats were studied 10 wk after streptozotocin injection to induce DM or vehicle alone. Escalating doses of capsaicin (0.1-30 microM) or acetic acid (0.01-1%; AA) were perfused intraurethrally while recording isovolumetric bladder activity, urethral perfusion pressure, and electromyography of the external urethral sphincter (EUS-EMG). Some rats were additionally treated with alpha-bungarotoxin, hexamethonium, or bilateral transection of the sensory branches of the pudendal nerves (PudSNx). Intraurethral capsaicin inhibited bladder contractions in six out of seven control rats but not in any of six DM rats. Low-frequency oscillations (LFOs) of intraurethral pressure were observed in five out of six control rats with capsaicin-induced bladder inhibition. In contrast, intraurethral AA inhibited bladder contractions and enhanced tonic EUS-EMG activity in six out of six control and five out of six DM rats. LFOs occurred in four out of six control and three of five DM rats with AA-induced bladder inhibition. Chemically induced bladder inhibition and LFOs were not prevented by alpha-bungarotoxin but were eliminated by PudSNx and hexamethonium. Finally, LFOs were followed by phasic EUS activity. These findings show that DM affects urethral afferent neurons differentially, compromising those expressing TRPV1 receptors. Urethral smooth muscle LFOs are neurogenically mediated and induce EUS activity, revealing the existence of a hitherto undescribed reflex pathway: a smooth-to-striated muscle urethra-to-urethra reflex.

VGLUT1 and GLYT2 labeling of sacrocaudal motoneurons in the spinal cord injured spastic rat

Spasticity of the midline (axial) musculature may hinder (1) performing transfers, (2) efficient extremity and head movements, and (3) efficient respiration. Currently, gaps exist in our knowledge of the pathophysiology involved in spasticity development within the axial musculature. The goals of this study were (1) to study the effects of S(2) transection on the number and distribution of glutamatergic inputs, arising from primary afferents, and glycinergic inputs to sacrocaudal motoneurons; and (2) to correlate changes in these synaptic inputs with the development of spasticity within the tail musculature, which are the caudal counterparts to the trunk axial musculature. Animals with S(2) spinal transection were tested behaviorally using our established system. At 1, 2, 4, and 12 weeks post-injury, sacrocaudal motoneurons were retrogradely labeled with cholera toxin beta-subunit (CTB), and temporal changes in vesicular glutamate transporter 1 (VGLUT1) and glycine transporter 2 (GlyT2) inputs to CTB-labeled motoneurons were visualized using antibodies specific for each synaptic type and confocal microscopy. These time points correspond to each of 4 stages of spasticity development. There was no significant change in either VGLUT1 or GlyT2 labeling of sacrocaudal motoneurons at any of the time points examined. Spinal cord injury-induced spasticity, in the tail musculature, does not appear to involve either an increase in monosynaptic glutamatergic inputs from myelinated afferents or a decrease in glycinergic inputs to sacrocaudal motoneurons.

Changes in vesicular glutamate transporter 2, vesicular GABA transporter and vesicular acetylcholine transporter labeling of sacrocaudal motoneurons in the spastic rat

Spasticity of the midline musculature can significantly hinder performing transfers and lead to development of pressure sores. Currently, significant gaps exist in our knowledge of the pathophysiology involved in spasticity development following SCI, especially regarding the axial musculature. The goals of this study were: (1) to determine the effects of S(2) transection on the number and distribution of glutamatergic, GABAergic and cholinergic inputs on more caudal motoneurons, (2) to correlate these changes with the development of spasticity within the tail musculature, which are the caudal counterparts to the axial musculature. Animals with S(2) spinal transection were tested behaviorally for the progression of spasticity within the tail musculature. At 1, 2, 4, or 12 weeks post-injury, the animals were sacrificed and temporal changes in glutamatergic, GABAergic, and cholinergic inputs to sacrocaudal motoneurons were assessed using antibodies for the specific vesicular transporter of each neurotransmitter and confocal microscopy. At 1 week post-injury, when the tail musculature demonstrated decreased responsiveness, an overall increase in the ratio of excitatory to inhibitory input to sacrocaudal motoneurons was observed. From 2 to 12 weeks post-injury, when the tail musculature demonstrated increased reflex behavior, an overall decrease in the ratio of excitatory to inhibitory inputs was observed. Additionally, from 2 to 12 weeks following spinal transection, a progressive loss of cholinergic labeling of sacrocaudal motoneurons was observed. The increase in the overall level of excitation with a concomitant loss of cholinergic influence following spinal transection could, in part, explain the development of spasticity within the tail musculature.

Alteration in axial motoneuronal morphology in the spinal cord injured spastic rat

Following spinal cord injury (SCI), exaggerated reflexes and muscle tone emerge that contribute to a general spastic syndrome in humans. At present, the underlying mechanisms involved with the development of spasticity following traumatic spinal cord injury, especially with regard to axial musculature, remains unclear. The purpose of the present study was to examine the temporal changes in sacrocaudal motoneuronal morphology following complete transection of the sacral spinal cord and to correlate these changes with the onset and progression of spasticity within the tail musculature. The spinal cords of rats were transected at the upper sacral (S(2)) level. Animals were behaviorally tested for the onset and progression of spasticity in the tail and at 1, 2, 4, or 12 weeks postinjury were sacrificed. At these time points, the animals demonstrated stage 1, 2, 3, or 4 spastic behavior, respectively. Sacrocaudal motoneurons innervating selected flexor muscles within the tail were retrogradely labeled with cholera toxin beta-subunit and neuronal morphology was analyzed using a combination of immunocytochemistry and standard microscopy. Initially over the first 2 weeks postinjury, a transient increase in the lengths of primary and secondary dendrites occurred. However, a progressive decrease in the overall number of dendritic branches was observed between 2 and 12 weeks postinjury, which parallels the time frame for the progressive increase in spastic behavior in the tail musculature. Following spinal cord injury, there is an alteration in the morphology of tail flexor motoneurons, which may be relevant to the development of spasticity within the tail.

Coupling between lumbar and sacral motor networks in the neonatal rat spinal cord

We have studied the rhythm-generating capabilities of the lumbar, sacral and coccygeal (Co) areas using an isolated spinal cord preparation of the newborn rat. The bath-application of a mixture of N-methyl-D-L-aspartate (NMA) and serotonin (5-HT) on the whole spinal cord induced a coordinated rhythmic activity that could be recorded from the lumbar to the coccygeal ventral roots. The phase relationships and mean burst duration between the activity in the rostral lumbar segments and the activity in the sacral segments was analysed. The direct activation of the sacral network, by using sections or by selective pharmacological activation, showed that these caudal segments possess their own rhythmogenic capability. By combining section experiments and compartmentation of the spinal cord, we demonstrated that a strong coupling exists between the lumbar and sacral motor networks. In addition, we found that in an intact spinal cord the activity of the sacral networks is driven by the lumbar networks. We have found that different modes of coordination between the lumbar and the sacral activity may occur. Finally, we have shown that the coupling between the lumbar and sacral networks can be modified by sensory inputs, suggesting that the spinal machinery could modulate and adapt the coupling of these two spinal networks.

Spasticity in rats with sacral spinal cord injury

We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.