SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE SACRAL SPINAL-CORD OF RATS .2. RESPONSES TO CUTANEOUS AND VISCERAL STIMULI

1. A goal of this study was to determine whether neurons in the sacral spinal cord that project to the diencephalon are involved in the proc essing and transmission of sensory information that arises in the peri neum and pelvis. Therefore, 58 neurons in segments L(6)-S-2 were activ ated antidromically with currents less than or equal to 30 mu A from p oints in the contralateral diencephalon in rats that were anesthetized with urethan. 2. Responses to mechanical stimuli applied to the cutan eous receptive fields of these neurons were used to classify them as l ow-threshold (LT), wide dynamic range (WDR) or high-threshold (HT)neur ons. Twenty-two neurons (38%) responded preferentially to brushing (LT neurons). Eighteen neurons (31%)re, sponded to brushing but responded with higher firing frequencies to noxious mechanical stimuli (WDR neu rons). Eighteen neurons (31%) responded only to noxious intensities of mechanical stimulation (HT neurons). LT neurons were recorded predomi nantly in nucleus proprius of the dorsal hem. Nociceptive neurons (WDR and HT) were recorded throughout the dorsal horn. 3. Cutaneous recept ive fields were mapped for 56 neurons. Forty-five (80%) had receptive fields that included at least two of the following regions ipsilateral ly: the rump, perineum, or tail. Eleven neurons (20%) had receptive fi elds that were restricted to one of these areas or- to the ipsilateral hind limb. Thirty-eight neurons (68%) had cutaneous receptive fields that also included regions of the contralateral tail or perineum. On t he perineum, receptive fields usually encompassed perianal and perivag inal areas including the clitoral sheath. There were no statistically significant differences in the locations or sizes of receptive fields for LT neurons compared with nociceptive (WDR and HT) neurons. 4. Thir ty-seven LT, WDR, and HT neurons were tested for their responsiveness to heat stimuli. Five (14%) responded to increasing intensities of hea t with graded increases in their firing requencies. Thirty-two LT, WDR , and HT neurons also were tested with cold stimuli. None responded wi th graded increases in their firing Frequencies to increasingly colder stimuli. There were no statistically significant differences among th e responses of LT, WDR, and HT neurons to either heat or cold stimuli. 5. Forty LT, WDR, and HT neurons were tested for their responsiveness to visceral stimuli by distending a balloon placed into the rectum an d colon with a series of increasing pressures. Seventeen (43%) exhibit ed graded increases in their firing frequencies in response to increas ing pressures of colorectal distention (CrD). None of the responsive n eurons responded reproducibly to CrD at an intensity of 20 mmHg, and a ll responded at intensities of greater than or equal to 80 mmHg. More than 90% responded abruptly al stimulus onset, responded continuously throughout the stimulus period, and stopped responding immediately aft er termination of the stimulus. 6. Thirty-one neurons were tested for their responsiveness to distention of a balloon placed inside the vagi na. Eleven (35%) exhibited graded increases in their firing frequencie s in response to increasing pressures of vaginal distention (VaD). The thresholds and temporal profiles of the responses to VaD were similar to those for CrD. Twenty-nine neurons were tested with both CrD and V aD. Thirteen (45%) were excited by both stimuli, four (14%) responded to CrD but not VaD, and one (3%) was excited by VaD but not CrD. Neuro ns excited by CrD, VaD, or both were recorded throughout the dorsal ho rn. 7. As a population, WDR neurons, bur not LT dr HT neurons, encoded increasing pressures of CrD and VaD with graded increases in their fi ring frequencies. The responses of WDR neurons to CrD differed signifi cantly from those of either LT or HT neurons. Repression analyses of t he stimulus-response functions of responsive WDR neurons to CrD and Va D were described by power functions with exponents of 1.6 and 2.4, res pectively. Extrapolated response thresholds were 26 mmHg for CrD and 2 8 mmHg for VaD. 8. Eight neurons that were excited by CrD also were re aped for their responsiveness to squeeze of a small area of the rectal wall. Seven (88%) were excited by this stimulus. Of the seven neurons excited by VaD that were tested, all seven (100%) also were excited b y squeeze of a small area of the vaginal wall. 9. Fifty-four LT, WDR, and WT neurons were recorded in female rats at different stages of the ir estrus cycles. Four WDR neurons were recorded in females that were in estrus. The mean responses of these WDR neurons to cutaneous stimul i were com pared with those of 14 WDR neurons recorded in rats that we re not in estrus. Although the sample size was small,the WDR neurons r ecorded in estrus rats were significantly more responsive to pinch tha n those recorded in rats that were not in estrus. However, the respons es of these two groups to brush were not significantly different. Resp onses of WDR neurons in estrus and nonestrus female rats to CrD and Va D also were not significantly different. IO. The conduction velocities to the contralateral posterior diencephalon of LT, WDR, and HT neuron s were compared. The mean +/- SE conduction velocity of LT neurons was 30.0 +/- 1.5 m/s, of WDR neurons was 16.6 +/- 1.2 m/s, and of HT neur ons was 14.7 +/- 0.9 m/s. LT neurons conducted significantly faster th an either WDR or HT neurons; The conduction velocities of WDR and HT n eurons were not significantly different. Il. These findings demonstrat e that neurons in the dorsal horn of spinal segments L(6)-S-2 In rats that project to the contralateral diencephalon respond to cutaneous me chanical stimuli and that similar to 50% also respond to stimulation o f pelvic visceral structures including the rectum and colon and the va gina. As a class, WDR neurons respond to visceral stimuli and encode t he intensity of visceral stimulation in their mean firing frequencies.