Suppressive Effects of Transient Receptor Potential Melastatin 8 Agonist on Epileptiform Discharges and Epileptic Seizures

More than 30 percent of patients with epilepsy have inadequate control of seizures with drug therapy, but why this happens and whether it can be predicted are unknown. We studied the response to antiepileptic drugs in patients with newly diagnosed epilepsy to identify factors associated with subsequent poor control of seizures. We prospectively studied 525 patients (age, 9 to 93 years) who were given a diagnosis, treated, and followed up at a single center between 1984 and 1997. Epilepsy was classified as idiopathic (with a presumed genetic basis), symptomatic (resulting from a structural abnormality), or cryptogenic (resulting from an unknown underlying cause). Patients were considered to be seizure-free if they had not had any seizures for at least one year. Among the 525 patients, 333 (63 percent) remained seizure-free during antiepileptic-drug treatment or after treatment was stopped. The prevalence of persistent seizures was higher in patients with symptomatic or cryptogenic epilepsy than in those with idiopathic epilepsy (40 percent vs. 26 percent, P=0.004) and in patients who had had more than 20 seizures before starting treatment than in those who had had fewer (51 percent vs. 29 percent, P<0.001). The seizure-free rate was similar in patients who were treated with a single established drug (67 percent) and patients who were treated with a single new drug (69 percent). Among 470 previously untreated patients, 222 (47 percent) became seizure-free during treatment with their first antiepileptic drug and 67 (14 percent) became seizure-free during treatment with a second or third drug. In 12 patients (3 percent) epilepsy was controlled by treatment with two drugs. Among patients who had no response to the first drug, the percentage who subsequently became seizure-free was smaller (11 percent) when treatment failure was due to lack of efficacy than when it was due to intolerable side effects (41 percent) or an idiosyncratic reaction (55 percent). Patients who have many seizures before therapy or who have an inadequate response to initial treatment with antiepileptic drugs are likely to have refractory epilepsy.

Sensory nerve fibres can detect changes in temperature over a remarkably wide range, a process that has been proposed to involve direct activation of thermosensitive excitatory transient receptor potential (TRP) ion channels. One such channel--TRP melastatin 8 (TRPM8) or cold and menthol receptor 1 (CMR1)--is activated by chemical cooling agents (such as menthol) or when ambient temperatures drop below approximately 26 degrees C, suggesting that it mediates the detection of cold thermal stimuli by primary afferent sensory neurons. However, some studies have questioned the contribution of TRPM8 to cold detection or proposed that other excitatory or inhibitory channels are more critical to this sensory modality in vivo. Here we show that cultured sensory neurons and intact sensory nerve fibres from TRPM8-deficient mice exhibit profoundly diminished responses to cold. These animals also show clear behavioural deficits in their ability to discriminate between cold and warm surfaces, or to respond to evaporative cooling. At the same time, TRPM8 mutant mice are not completely insensitive to cold as they avoid contact with surfaces below 10 degrees C, albeit with reduced efficiency. Thus, our findings demonstrate an essential and predominant role for TRPM8 in thermosensation over a wide range of cold temperatures, validating the hypothesis that TRP channels are the principal sensors of thermal stimuli in the peripheral nervous system.

ThermoTRPs, a subset of the Transient Receptor Potential (TRP) family of cation channels, have been implicated in sensing temperature. TRPM8 and TRPA1 are both activated by cooling; however, it is unclear whether either ion channel is required for thermosensation in vivo. We show that mice lacking TRPM8 have severe behavioral deficits in response to cold stimuli. In thermotaxis assays of temperature gradient and two-temperature choice assays, TRPM8-deficient mice exhibit strikingly reduced avoidance of cold temperatures. TRPM8-deficient mice also lack behavioral response to cold-inducing icilin application and display an attenuated response to acetone, an unpleasant cold stimulus. However, TRPM8-deficient mice have normal nociceptive-like responses to subzero centigrade temperatures, suggesting the presence of at least one additional noxious cold receptor. Finally, we show that TRPM8 mediates the analgesic effect of moderate cooling after administration of formalin, a painful stimulus. Therefore, depending on context, TRPM8 contributes to sensing unpleasant cold stimuli or mediating the effects of cold analgesia.