Thomas K. Baumann

Human cutaneous C fibres activated by cooling, heating and menthol

Differential A‐fibre block of human peripheral nerves changes the sensation evoked by innocuous cooling (∼24°C) of the skin from ‘cold’ to ‘hot’ or ‘burning’, and this has been attributed to activity in unidentified unmyelinated fibres that is normally masked or inhibited by activity in Aδ cold fibres. Application of the TRPM8 agonist menthol to the skin evokes ‘burning/stinging’ as well as ‘cold’, and the unpleasant sensations are also enhanced by A‐fibre block. In this study we used microneurography to search for C fibres in human skin activated by cooling and menthol, which could be responsible for these phenomena. Afferent C fibres were classified by activity‐dependent slowing as Type 1A (polymodal nociceptor), Type 1B (mechanically insensitive nociceptor) or Type 2 (cold sensitive), and their responses to heating and cooling ramps were measured before and after topical application of menthol preparations (2–50%). The only C fibres activated by menthol were the Type 2 fibres, which discharged vigorously with innocuous cooling and were strongly activated and sensitized to cooling by menthol. Unlike an Aδ cold fibre, they continued to discharge at skin temperatures down to 0°C, and most (13/15) were also activated by heating. We propose that the Type 2 C fibres, although resembling Aδ cold fibres in their responses to innocuous cooling and menthol, have a more complex sensory function, colouring with a ‘hot‐burning’ quality the perceptions of low and high temperatures. Their bimodal thermoreceptive properties may help account for several puzzling psychophysical phenomena, such as ‘innocuous cold nociception’, ‘paradoxical heat’ and the thermal grill illusion, and also for some neuropathic pains.

Responses of adult human dorsal root ganglion neurons in culture to capsaicin and low pH

&NA; This study examined the responses of cultured adult human dorsal root ganglion (hDRG) neurons to protons and capsaicin, two substances known to produce pain and hyperalgesia in humans. Both substances were applied to each neuron and responses were examined under both voltage‐ and current‐clamp recording conditions. Sensitivity to protons was tested with rapid acidification of the extracellular fluid from pH 7.35 to 6.0. In neurons nominally clamped near −60 mV, low pH evoked a transient inward current which, in all 40 hDRG neurons tested, was followed by a more sustained inward current. The sustained current was associated with an increase in membrane conductance in 10 neurons, a decrease in 27 neurons, and no overt change in conductance (<10%) in 3 neurons. Current‐clamp recordings in the same neurons showed that the proton‐induced sustained net inward current caused a prolonged depolarization of the membrane potential in all 40 hDRG neurons. The prolonged depolarization was associated with action potential discharge in 5 neurons. Unlike low pH, capsaicin evoked a sustained net inward current in only a subset of neurons tested (10 nM: Symbol, 30 nM: Symbol, 100 nM: Symbol, and 10 &mgr;M: Symbol neurons tested). The capsaicin‐evoked currents were accompanied by an increase in membrane conductance in 15 neurons, a decrease in 2, and no overt change in conductance in 9 neurons. Capsaicin currents, like proton‐induced currents, resulted in prolonged depolarizations (10 nM: Symbol, 30 nM: Symbol, 100 nM: Symbol,d and 10 &mgr;M: Symbol neurons tested). The depolarization resulted in the discharge of action potentials in 14 neurons. It is concluded that, while both protons and capsaicin exert excitatory effects on human sensory neurons, multiple membrane mechanisms lead to the depolarization of cultured hDRG neurons by low pH. Inhibition of resting membrane conductances contributes to the responses to low pH in some hDRG neurons. Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available

Background potassium channel block and TRPV1 activation contribute to proton depolarization of sensory neurons from humans with neuropathic pain

Protons cause a sustained depolarization of human dorsal root ganglion (DRG) neurons [Baumann et al. (1996) Pain, 65, 31–38]. In the present study we sought to determine which ion channels are expressed in human DRG neurons that could mediate the sustained responses observed in the patch‐clamp recordings. RT‐PCR of material from the DRG tissue revealed the presence of mRNAs for a nonselective cation channel that is activated by protons (TRPV1) and background potassium channels that are blocked by protons (TASK‐1, TASK‐3 and Kir2.3). Highly acidic solution (pH 5.4) applied to cultured DRG neurons evoked prolonged currents that were associated with a net increase in membrane conductance. Consistent with the involvement of TRPV1, these proton‐evoked currents were blocked by capsazepine and were only found in neurons that responded to capsaicin with an increase in membrane conductance. Less acidic extracellular solution (pH 6.0) evoked such currents only rarely, but was able to strongly enhance the currents evoked by capsaicin. Capsazepine (1 µm) blocked the currents evoked by capsaicin at pH 7.35, as well as the potentiated responses to capsaicin at pH 6.0. In neurons that were not excited by capsaicin, moderate extracellular acidification (pH 6.0) caused a sustained decrease in resting membrane conductance. The decrease in membrane conductance by protons was associated with inhibition of background potassium channels. This excitatory effect of protons was not blocked by capsazepine. We conclude that in most neurons the sustained depolarization in response to moderately acidic solutions is the result of blocked background potassium channels. In a subset of neurons, TRPV1 also contributes.

Tremor control after pallidotomy in patients with Parkinson's disease: correlation with microrecording findings.

The goals of this study were to analyze the effect of pallidotomy on parkinsonian tremor and to ascertain whether an association exists between microrecording findings and tremor outcome. Forty-four patients with Parkinsons disease who had drug-induced dyskinesia, bradykinesia, rigidity, and tremor underwent posteroventral pallidotomy. Using a 1-mu-tip tungsten electrode, microrecordings were obtained through one to three tracts, starting 10 mm above the pallidal base. Tremor severity was measured on a patient-rated, 100-mm Visual Analog Scale (VAS), both preoperatively and 3 to 9 months (mean 6 months) postoperatively. Preoperatively, tremor was rated as 50 mm or greater in 24 patients (55%) and as less than 25 mm in 13 patients (30%). Postoperatively, tremor was rated as 50 mm or greater in five patients (11%) and less than 25 mm in 29 patients (66%). The difference was significant (p = 0.0001). Four patients (9%) had no postoperative tremor. Tremor improved by at least 50% in eight (80%) of 10 patients in whom tremor-synchronous cells were recorded (Group A) and in 12 (35%) of 34 patients in whom tremor-synchronous cells were not recorded (Group B). This difference was significant (p = 0.03). Tremor improved by at least 50 mm in all (100%) of the seven Group A patients with severe (> or = 50 mm) preoperative tremor and in nine (53%) of 17 Group B patients with severe preoperative tremor. This difference was also significant (p = 0.05). The authors prefer two conclusions: 1) after pallidotomy, tremor improves by at least 50% in two-thirds of patients with Parkinsons disease who have severe (> or = 50 mm on the VAS) preoperative tremor; and 2) better tremor control is obtained when tremor-synchronous cells are included in the lesion.

Vanilloid Receptor Expression and Capsaicin Excitation of Rat Dental Primary Afferent Neurons

Little is known about the molecular mechanisms that cause excitation of neurons which innervate the teeth. We investigated whether rat dental sensory neurons express the vanilloid (capsaicin) receptor (VR1). Dental sensory neurons were identified by retrograde transport of the fluorescent dye DiIC18 placed in maxillary molars. Patch-clamp recordings in culture showed that 65% of DiIC18-labeled rat trigeminal ganglion neurons are excited by capsaicin. Responders covered the entire range of cell sizes examined (soma diameter, 24 to 48 μm). All non-responders had a soma diameter > 33 μm. Capsazepine (1 μM) reduced the capsaicin-evoked membrane current (6/6) and depolarization (7/7 responders). RT-PCR amplified a 375-bp product from DiIC18-labeled neurons which was identical to that expected for VR1. Thus, many rat dental primary afferent neurons are excited by capsaicin, and the response appears to be mediated by VR1. These results suggest that pharmacological blockers of VR1 may provide significant relief of dental pain.

Characterization of Tusc5, an adipocyte gene co-expressed in peripheral neurons

Tumor suppressor candidate 5 (Tusc5, also termed brain endothelial cell derived gene-1 or BEC-1), a CD225 domain-containing, cold-repressed gene identified during brown adipose tissue (BAT) transcriptome analyses was found to be robustly-expressed in mouse white adipose tissue (WAT) and BAT, with similarly high expression in human adipocytes. Tusc5 mRNA was markedly increased from trace levels in pre-adipocytes to significant levels in developing 3T3-L1 adipocytes, coincident with several mature adipocyte markers (phosphoenolpyruvate carboxykinase 1, GLUT4, adipsin, leptin). The Tusc5 transcript levels were increased by the peroxisome proliferator activated receptor-gamma (PPARgamma) agonist GW1929 (1microg/mL, 18h) by >10-fold (pre-adipocytes) to approximately 1.5-fold (mature adipocytes) versus controls (p<0.0001). Taken together, these results suggest an important role for Tusc5 in maturing adipocytes. Intriguingly, we discovered robust co-expression of the gene in peripheral nerves (primary somatosensory neurons). In light of the marked repression of the gene observed after cold exposure, these findings may point to participation of Tusc5 in shared adipose-nervous system functions linking environmental cues, CNS signals, and WAT-BAT physiology. Characterization of such links is important for clarifying the molecular basis for adipocyte proliferation and could have implications for understanding the biology of metabolic disease-related neuropathies.

Potentiation of rabbit trigeminal responses to capsaicin in a low pH environment.

The sensitivity of 35 adult rabbit trigeminal ganglion neurons to low pH (pH 6.0), 10 microM capsaicin (CAP) and 10 microM capsaicin at low pH (CAP@pH6.0) was studied using voltage-clamp whole-cell recording techniques. Neurons responded to pH 6.0 with a transient inward current, followed by a more slowly activating (sustained) net inward current. Responses to capsaicin showed only a sustained current. Capsaicin caused an increase in membrane conductance, whereas responses to low pH were associated with either a net increase or decrease in conductance. A subset of neurons (n = 14) responded to CAP@pH6.0 with a sustained current which exceeded the sum of the peak sustained currents evoked by CAP and pH 6.0 applied singularly by approximately a factor of 4. The current was associated with a substantial increase in membrane conductance. The present results indicate that, in addition to a direct conductance activating effect, protons have the ability to enhance the current evoked by capsaicin.

Expression of VPAC2 receptor and PAC1 receptor splice variants in the trigeminal ganglion of the adult rat

PACAP and VIP are members of the VIP/secretin/glucagon family of peptides with neurotransmitter, neuroprotective, and neurotrophic functions. PACAP and VIP are known to be upregulated in primary sensory neurons following nerve injury, implying that these neuropeptides could be mediators of sensory transmission in neuropathic pain states. Nerve injury at the level of the trigeminal root is thought to be the prime cause of trigeminal neuralgia. Since cross-excitation (a chemically-mediated form of nonsynaptic transmission) within the TG is postulated to play a central role in trigeminal neuralgia, we studied the expression of PACAP and VIP receptors in the TG by RT PCR and immunocytochemistry. Of the three known receptors (PAC1, VPAC1 and VPAC2), RT PCR revealed the presence of mRNA for VPAC2 and several splice variants of the PAC1 receptor. Immunocytochemistry showed PAC1 and VPAC2 to be present in small-diameter TG neurons. Thus, PACAP and VIP are potential mediators of cross-excitation in the TG.

Functional Anatomy of the Pallidal Base in Parkinson's Disease

OBJECTIVE The purpose of this study is to define the morphology of the boundary between the globus pallidus and the ansa lenticularis (i.e., pallidal base) in humans. This information is important for surgeons who perform pallidotomy. METHODS Thirty-eight patients with Parkinsons disease underwent pallidotomy using microrecording techniques. The pallidal base was identified by the loss of neuronal single unit activity and by the change in background noise, as analyzed on the audio monitor and by fast Fourier transformation. RESULTS Three quarters of the patients had an abrupt transition of the background noise from neuronal to axonal activity. One quarter of the patients had multiple successive transitions of the background activity, over a distance of 0.4 to 2 mm (median, 1 mm). CONCLUSION We conclude that the pallidal base is not a smooth, sharp boundary between the globus pallidus and the ansa lenticularis. We propose two models that define the morphology of the pallidal base. One model depicts the pallidal base as a multifolded boundary that distinctly separates pallidal neurons from ansa lenticularis axons. Another model depicts the pallidal base as an indistinct transitional boundary between the globus pallidus and the ansa lenticularis, which contains axonal fibers intermixed with small clusters of pallidal neurons. We discuss the clinical relevance of these findings.

Enhancement of rat trigeminal ganglion neuron responses to piperine in a low-pH environment and block by capsazepine.

Both trigeminal and spinal ganglion neurons show a strong potentiation of responses to the irritant capsaicin in an acidic environment. The present study revealed that there is also a strong interaction between protons and piperine, another vanilloid irritant. We studied the mechanism of the interaction between protons and piperine. Whole-cell patch clamp recordings were performed on cultured adult rat trigeminal ganglion (TG) neurons voltage-clamped near their resting membrane potential (-60 mV). Piperine (10 microM) caused a sustained net inward current associated with either an increase or decrease in membrane conductance. When protons and piperine were co-applied, the membrane currents evoked in piperine-sensitive TG neurons far exceeded the algebraic sum of the responses to the two stimuli applied in isolation. Capsazepine blocked the response of TG neurons to piperine at both physiological and acidic pH. In the presence of capsazepine, responses to the mixture of piperine and protons resembled the response to the low pH stimulus applied alone. Capsazepine had no effect on the sustained proton-induced current. These findings suggest that protons enhance the piperine current by altering the vanilloid receptor/channel complex or increasing the length constant of the space clamp.