RAPID COMMUNICATION Dorsal Column But Not Lateral Column Transection Prevents Down- Conditioning of H Reflex in Rats

Chen, Xiang Yang and Jonathan R. Wolpaw. Dorsal column Wolpaw 1995a,b, 1996). Motivated by a paradigm in which but not lateral column transection prevents down-conditioning of reward depends on reflex amplitude, both primates and rats H reflex in rats. J. Neurophysiol. 78: 1730–1734, 1997. Operant can gradually increase or decrease the SSR or the H reflex. conditioning of the H reflex, the electrical analogue of the spinal The conditioning paradigm appears to induce a change in stretch reflex, in freely moving rats is a relatively simple model for descending influence that modifies the spinal cord and studying long-term supraspinal control over spinal cord function. changes the reflex (Carp and Wolpaw 1994; Feng-Chen and Motivated by food reward, rats can gradually increase or decrease Wolpaw 1996; Wolpaw and Lee 1989). the soleus H reflex. This study is the first effort to determine Recent studies in rats show that contusion injuries to thowhich spinal cord pathways convey the descending influence from racic spinal cord impair operant conditioning of the soleus supraspinal structures that changes the H reflex. In anesthetized Sprague-Dawley rats, the entire dorsal column (DC), which inH reflex and that the degree of impairment is correlated with cludes the main corticospinal tract, or the right lateral column (LC) the size of the lesion (Chen et al. 1996). These results was transected by electrocautery. Animals recovered quickly and confirm the essential role of spinal cord pathways in H reflex the minimal transient effects of transection on the right soleus H conditioning. However, because contusions cause diffuse reflex disappeared within 16 days. Beginning at least 18 days after damage, they do not indicate which pathways are essential transection, 12 rats were exposed to the HRdown-conditioning for conditioning. mode, in which reward was given when the H reflex of the right This study is the first effort to define the roles of specific soleus muscle was below a criterion value. In seven LC rats exspinal cord pathways in H reflex conditioning. We investiposed to the HRdown mode, the H reflex fell to 71 { 8% (mean { gate the effects on down-conditioning of transecting the dorSE) of its initial value. In six of the seven, conditioning was sal column (DC), which in rats contains the main corticospisuccessful ( i.e., decrease to °80%). These results were comparable with those previously obtained from normal rats. In contrast, nal tract, or the lateral column (LC), which contains the in five DC rats exposed to the HRdown mode, the H reflex at the rubrospinal, vestibulospinal, and reticulospinal tracts (Holsend of exposure was 106 { 12% of its initial value. In none of tege and Kuypers 1987; Kennedy 1990; Kuypers 1981; Trathese rats was HRdown-conditioning successful. DC rats differed cey 1995). The results are quite clear, and with further exsignificantly from normal and LC rats in both final H reflex values ploration they should lead to greater understanding of longand number successful. In five DC and three LC rats that continued term supraspinal control over spinal cord function and of under control conditions over 30–78 days, the H reflex at the end the spinal reflex abnormalities that occur when injury impairs of the period was 98 { 4% and 100 { 8%, respectively, of its that control. initial value, indicating that DC or LC transection itself did not