RAPID COMMUNICATION Nucleus Gracilis: An Integrator for Visceral and Somatic Information

Al-Chaer, Elie D., Karin N. Westlund, and William D. Willis. ipsilateral fasciculus gracilis of the spinal cord, the ipsilateral Nucleus gracilis: an integrator for visceral and somatic information. NG, and the region of decussation of the ML. Moreover, J. Neurophysiol. 78: 521–527, 1997. The nucleus gracilis (NG) our group has shown that visceral as well as cutaneous input receives an abundance of visceral input from various abdominal into the NG can be abolished by a lesion of the DC at the organs and is proposed to play an important role in visceral pain level of T10 (Al-Chaer et al. 1996b). The T10 DC lesion also processing. The purpose of this study was to investigate the necesdramatically reduced the responses of VPL cells to visceral sity of the NG for colorectal input into the ventral posterolateral and innocuous cutaneous stimuli (Al-Chaer et al. 1996a). (VPL) nucleus of the thalamus. Single-cell recordings were made Although it is clear that visceral responses can be recorded from nine VPL cells isolated in nine different male Sprague Dawley from neurons of the NG that project to the VPL nucleus, rats anesthetized with pentobarbital sodium. Responses of the VPL cells to colorectal distension (CRD) and to cutaneous stimuli were this does not define the NG as a relay for visceral information obtained before and after lesioning of the NG. Electrolytic (n Å carried in the DC into the VPL nucleus, nor does it rule out 5) and chemical (n Å 4) lesions of the NG were made in different relays for visceral information in nuclei other than the NG. preparations. The chemical lesions were made by injecting a soluFor instance, visceral information carried by DC axons could tion of kainic acid into the NG. Kainic acid presumably kills neube relayed via DC collaterals onto spinothalamic tract neuronal cell bodies and spares axons of passage. The results indicate rons located in the upper cervical spinal cord (Burstein et that a lesion of the NG, regardless of its type, reduces dramatically al. 1990; Kemplay and Webster 1989). Axons of cervical the responses of VPL neurons to innocuous cutaneous stimuli, and, spinothalamic tract neurons could then convey the visceral to a lesser extent, the responses to CRD. Attenuation of VPL information to the VPL nucleus. The purpose of this study neuronal responses to CRD as well as to innocuous cutaneous was to investigate how essential the NG is for colorectal stimuli by the NG lesions emphasizes the role of the dorsal column in visceral nociception and suggests that the NG is an integration input into the VPL nucleus of the thalamus. Therefore recenter for visceral and cutaneous information flowing into the VPL cordings were made from single cells in the VPL nucleus nucleus. in response to graded colorectal distension (CRD) and to cutaneous stimuli before and after a lesion of the NG. The hypothesis was that lesioning of the NG would reduce the I N T R O D U C T I O N responses of VPL cells to CRD and innocuous cutaneous stimuli as effectively as a DC lesion, indicating that visceral The nucleus gracilis (NG) plays an important role in processing pelvic visceral input and relaying it to the ventral input carried by DC axons into the VPL nucleus is largely relayed in the NG. The lesions were made either by passing posterolateral (VPL) nucleus of the thalamus. Single cells in the NG that can be antidromically activated from the VPL current through an electrode inserted into the NG or by an injection of kainic acid into the NG. A preliminary report nucleus or the medial lemniscus (ML) respond to mechanical and chemical stimulation of the descending colon and of this work has been made (Westlund et al. 1996). rectum as well as to cutaneous stimuli (Al-Chaer et al. M E T H O D S 1996b). Although cutaneous input into the NG is mostly Experiments were performed on nine male Sprague-Dawley rats mediated by primary afferent projections, the visceral input weighing between 280 and 350 g. The rats were initially anestheis believed largely to involve a synaptic relay between pritized with an intraperitoneal injection of pentobarbital sodium (40 mary afferents and postsynaptic dorsal column (DC) neumg/kg). The trachea was intubated and a catheter was inserted rons (Al-Chaer et al. 1996b). Earlier studies have shown into one of the jugular veins to allow a continuous infusion of the that field potentials and single-unit responses can be recorded anesthetic (5 mgrkgrh) . Body temperature was monitored from the DC nuclei (DCN), mainly the NG, in response to and kept around 377C by a servo-controlled heated blanket. The splanchnic nerve stimulation in the cat (Aidar et al. 1952; head of the rat was fixed in a stereotaxic instrument. An incision was made in the skin over the head and the cervical vertebral Rigamonti and Hancock 1974, 1978). Recently, Berkley and column. The underlying muscles were retracted. A craniotomy was Hubscher (1995) reported that 50% of their sample of neumade to expose the area of cortex above the thalamus. Part of the rons in the NG that responded to gentle skin stimulation also occipital bone above the cerebellum was removed and a small responded to uterine and vaginal distension. Anatomically, laminectomy was made to expose C1. The procedure allowed easy the NG has been shown to receive primary afferents from the access to the NG while enabling recordings from the thalamus. splanchnic nerve (Kuo and De Groat 1985) and nonprimary The dura mater was cut and exposed brain tissues were covered afferents from the lumbar and sacral cord (Cliffer and with warm mineral oil. Giesler 1989; Hirshberg et al. 1996; Rustioni 1973). Input Stimulation into the NG is carried mainly by fibers that ascend in the DC. Aidar et al. (1952) recorded fast responses to splanchnic The visceral stimulus used was CRD. CRD was applied with the use of an inflatable balloon inserted rectally into the descending nerve stimulation, ‘‘in logical time relationships,’’ in the