Nociceptive neurons of the raccoon lateral thalamus

Abstract

1. Responses to noxious mechanical and thermal stimuli were examined in 48 thalamic neurons in barbiturate or chloralose-anesthetized raccoons, with special attention to neurons whose peripheral receptive fields (RFs) included glabrous skin of the forepaw. Recording loci were in the core of the ventrobasal complex (VB; n = 32), its ventral or dorsal border (n = 5), or the medial division of the posterior nuclear group (POm; n = 11). 2. Twenty-one VB neurons and 7 POm neurons were classed as wide dynamic range (WDR), whereas 2 VB neurons and 4 POm neurons were classed as nociceptive specific (NS). Response properties of 14 light touch (LT) neurons located in VB were also examined. 3. WDR and NS neurons were not segregated, but rather were intermixed along the ventral and dorsal borders of VB, as well as in POm, and WDR and LT neurons were intermixed in the core of VB. Within the VB core, both LT and WDR neurons were somatotopically organized. 4. All WDR neurons had larger high-threshold than low-threshold RFs, and this difference was greater for POm neurons than for VB neurons. RF areas of LT neurons and low-threshold RF areas of WDR neurons were comparable to those previously reported for raccoon VB units. 5. Out of 25 WDR cells tested, 20 had heat thresholds > 53 degrees C; the range of thresholds in the remaining 5 was 49-53 degrees C. Four out of five NS neurons tested had heat thresholds > 53 degrees C; the threshold of the fifth was 51 degrees C. Of the six neurons with heat thresholds < or = 53 degrees C, two each were in the core of VB, along the border of VB, and in POm. 6. Sensitization to heat after a mild heat injury to the glabrous RF (53 degrees C for 90 s, or 55 degrees C for 30 s) occurred in 8 out of 16 neurons tested, and persisted for up to 2 h. Median thresholds decreased from > 53 degrees C before injury to 47 degrees C after injury, and responses to suprathreshold stimuli were enhanced. There was a significantly greater likelihood (P = 0.02) for sensitization to occur in POm neurons (6/7) than in VB neurons (2/9). 7. It is suggested that a small proportion of neurons located in VB and POm contribute to the sensation of heat pain. Furthermore, sensitization of these neurons may contribute to heat hyperalgesia after an injury to glabrous skin.

Squirrel monkey lateral thalamus. I. Somatic nociresponsive neurons and their relation to spinothalamic terminals

“…Nociresponsive and with surrounding innocuous cells are reminiscent of the VPL and Shi l Somatic Nociceptive Input to the Lateral Thalamus nociresponsive neurons. Both the raccoon (Simone et al, 1993) and the rat (Guilbaud et al, 1980) seem more similar to the primate in that nociresponsive cells are found within the core of VPL. However, the incidence of nociresponsive cells in areas equivalent to VP1 remain unknown in the macaque, rat, and raccoon.…”

“…Here, too, the majority of cells were described as WDR type although the relative locations of WDR type and nociceptivespecific (NS) type neurons were reported either as segregated (Yokota et al, 1988) or intermixed (ISnilIki and Mizumuna, 1983;Knitlki and Craig, 1985). Recently, Simone et al (1993) reported on the properties of nociresponsive cells in the raccoon lateral thalamus. They describe mostly WDR and some NS type cells within VP, dorsal and ventral borders of VP, and the medial division of the posterior nucleus (Porn).…”

Changes in the organization of the ventroposterior lateral thalamic nucleus after digit removal in adult raccoon

“…Neurons of the STT have not been studied physiologically in this animal, but, based on retrograde labeling studies, the STT is smaller than the SCT (Pubols and Haring, 1995). Neurons that are responsive to nociceptive stimulation are also spread widely throughout the VPL of the raccoon (Simone et al, 1993), suggesting that functional STT and SCT inputs are not restricted to regions where the anatomical terminations are dense (Craig and Burton, 1985). These studies also demonstrated that the RFs of neurons in the SCT pathway of the raccoon are larger than the RFs in its cuneate projection system.…”

Changes in the organization of the ventroposterior lateral thalamic nucleus after digit removal in adult raccoon

“…Neurons of the STT have not been studied physiologically in this animal, but, based on retrograde labeling studies, the STT is smaller than the SCT (Pubols and Haring, 1995). The possible importance of the SCT in the raccoon is also evidenced by its large input from the glabrous skin of the forepaw (Simone et al, 1993) and by a large proportion of neurons with high thresholds to mechanical stimulation (Hirata and Pubols, 1989;Simone and Pubols, 1991). These studies also demonstrated that the RFs of neurons in the SCT pathway of the raccoon are larger than the RFs in its cuneate projection system.…”