Effect Of Stimulating Saphenous Nerve On Intracellular Potentials Of Neurons In Anterior Cingulate Gyrus Of Cats

Visceral and somatic pain have the differences in the perceptual qualities. Visceral pain is poorly localized and is often referred to somatic regions. The perception of pain intensity and affect in visceral and somatic pain syndromes is often different, with visceral pain reported as more unpleasant. Visceral pain results in quiescence whereas somatic pain results in a flight and fight response. The precise reasons for the differences in perception between visceral and somatic pain remain unclear up to now. It has very important theoretical value for exploitation of new pain-killer and analgesia, to elucidate the mechanism of the differences in perception between visceral and somatic pain.The anterior cingulate gyrus (ACG) plays an important role in pain sensation. Neurons are basic structural and functional units of nervous system. The analysis function of sensory cortex mostly depends on the properties of different cortical neurons. The integration of afferent messages in a single sensory neuron is a basic for sophisticated sense analysis function of the cerebrum. We discovered that both visceronociceptive neurons (VNNs) and non-visceral nociceptive neurons (NVNNs) had different electrophysiological properties with intracellular recording techniques (in vivo). The findings indicated that neurons with various function possess different electrophysiological properties. Therefore, we presumed VNNs and somatic nociceptive neurons (SNNs) had different electrophysiological properties because the both have different functions,which might be one of reasons for the differences in perception between visceral and somatic pain. However, little is available with respect to research on difference in electrophysiological properties between VNNs and SNNs in ACG. Saphenous nerve (SN) conducts impulses of somatic pain from lower limb. No studies have been done to examine whether ACG is one of the representative areas of the SN afferent pathway. Therefore, in the present study, the intracellular recording methods were used to investigate effect of electrically stimulating SN on intracellular potentials of ACG neurons, and to examine whether ACG is one of the representative areas of the SN afferent pathway. We explored electrophysiological properties of ACG SNNs, which could provide valuable electrophysiological data for the differences in perception between visceral and somatic pain.A total of 557 neurons in ACG were recorded. Among 557 neurons, 98 were stimulus-relative and 459 stimulus-irrelative. The former had the evoked responses to noxious stimulation of SN. The latter did not. According to latency of the evoked responses induced by stimulating SN, stimulus-relative neurons were classified as SNNs (91.84%) having the evoked responses with long latency (≥50ms)and somatic non-nociceptive neurons (SNNNs, 8.16%) showing the evoked responses with short latency(<50ms). SNNs were further divided into specific SNNs (SSNNs, 92.22%) only showing the evoked responses with long latency and non-specific SNNs (NSSNNs, 7.78%) having the evoked responses with both long and short latencies. The modes of spontaneous biological electric activities of SNNs and SNNNs were found as follows: no-discharges in resting state, synaptic activities, occasional discharges, continuous tonic discharges and rhythmic discharges. Modes, frequencies, and amplitudes of spontaneous discharges of both SNNs and SNNNs were not obvious distinct. But 65.56% of SNNs had spontaneous firing which is significantly higher than that in SNNNs (12.50%,P<0.05). The result indicated that the excitability of the SNNs was higher than that of the SNNNs, and the SNNs were more susceptible to input of synaptic impulses than the SNNNs. The threshold intensities of SNNs for evoking the long latent noxious responses to stimulate SN were 0.4-0.5 mA. Those of SNNNs for evoking the short latent innocuous responses were 0.1-0.2 mA. According to evoked response latency, the responses of SNNs were classified into four groups: their mean latencies were 24.29±7.32ms ( n=7 ) ,66.61±13.44ms(n=31),145.77±36.73ms(n=13),293.53±48.63ms(n=17), respectively. The first one was responses with short latency. The others were responses of long latency. Different SNNs might have one kind or several kinds of different latency responses. But the latency of the same SNNs was relatively constant. Patterns of the responses of SNNs evoked by stimulating SN were divided into excitatory (61.11%), inhibitory (31.11%) and mixed (7.78%) groups. The evoked responses of SNNs with long latency were apparently inhibited by intravenous injection of morphine. However, SNNNs only had short latency responses (28.13±7.99ms, n=8), which were not inhibited by the action of morphine. Patterns of the evoked responses of SNNNs were excitatory (87.50%) and inhibitory (12.50%).SN is rich in Aδand C afferent fibers, which plays an important role in conducting impulses of pain. ACG belongs to the limbic system. However, no study has been done to examine whether ACG is one of the representative areas of the SN afferent pathway. In the present study, intracellular recording technique was used to record different latency evoked responses of neurons in contralateral ACG by electrically stimulating SN. The study provided the first data and suggested that the contralateral ACG is one of the representative areas of the SN afferent pathway. The different latency of the evoked responses of neurons in the ACG might be relative to both excitation of different fibers with different conducting velocity in SN and different synapses with different structure and function existed in the central areas of the SN afferent pathway. The results also indicated that the different fibers of SN were projected to different neurons in the ACG. The same neurons in the ACG produced several evoked responses with different latency induced by stimulating SN, which indicated that various fibers of SN converged at the same neurons of the ACG. According to literatures, it is assumed that evoked responses with long latencies might be driven by Aδand C fibers of SN, whereas evoked responses with short latencies by Aβfibers of SN. These results were similar to those in somatosensory cortex reported previously by other authors.Using extracellular recording techniques, Sikes and Vogt found SSNNs and NSSNNs in the ACG of rabbit. 27.68% of units tested in ACG responded to noxious transcutaneous electrical stimulation , 42.08 % of units tested responded to noxious mechanical and 19.15% of units tested responded to noxious heat stimuli. 78.57%of tested units responded to innocuous"tap"stimulus. Yamamura, et al reported about 40% neurons recorded with intracellular recording techniques from ACG in rat were classified as NSSNNs and remainders were SSNNs. In the present study, it was observed that the strength of the noxious stimulus of the SN used was strong enough to excite Aδand C fibers in SN and caused long-latency evoked responses in ACG and other responses, such as mydriasis. Evoked responses with long-latency in ACG were inhibited easily by injection of morphine. All these facts demonstrated long-latency evoked responses in ACG induced by stimulating SN are somatic nociceptive. The present studies further demonstrated the existence of SNNs in ACG. 16.16% of neurons tested were SNNs. Among 90 SNNs, 92.22% were SSNNs, and 7.78% were NSSNNs. These results provided valuable data for specific theory of pain sensation. The population differences of nociceptive neurons between above studies could be attributed to the differences in the species of experimental animals and the methods used.Yamamura has reported that neurons from the ACG had large receptive fields (RFs), usually bilateral, and some had RFs covering the whole body surface. These neurons showed excitatory or inhibitory responses following noxious stimulations of the receptive fields. However, features of the neuron responses in ACG evoked by stimulating SN have not been reported up to now. In this study, we observed the detailed differences of electrophysiological properties between SNNs and SNNNs in ACG. Compared with the evoked response of the SNNNs, the evoked responses of the SNNs had the specificities with higher threshold, longer latency, complex reactive patterns, and being inhibited by the action of morphine. Therefore, we thought SNNs and SNNNs had different electrophysiological properties,which provided new valuable data for specific theory of pain sensation.It is very excited to find the differences in electrophysiological properties between SNNs and VNNs in ACG. Compared with the evoked response of the SNNs, the evoked responses of the VNNs reported previously by our lab have the specificities with higher threshold, longer latency, and complex reactive patterns. Moreover, the percentage of VNNs is bigger than that of SNNs in ACG. These differences in electrophysiological properties between SNNs and VNNs in ACG might be electrophysiological basis and one of reasons for the differences in perception between visceral and somatic pain. The precise mechanisms of the differences in perception between visceral and somatic pain require further investigation on morphology of neuron, neurotransmitter, and other aspects.