The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

Abstract

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Figure 1

Two major pathways causing TRPV1 overactivity in sensory nerve endings: TRPV1 trafficking to the cell membrane and TRPV1 sensitization. A, ankyrin; C, C terminus; CCDKII, Ca²⁺-calmodulin-dependent kinase II; 5-HT, 5-hydroxytryptamine; N, N terminus; NGF, nerve growth factor; P, pore; PGs, prostaglandins; PI3K, phosphoinositide 3-kinase; PKA, protein kinase A; PKC, protein kinase C; PLC, phospholipase C; Src, Src kinase, TM, transmembrane domain.

Figure 2

Involvement of peripheral and central TRPV1 in thermoregulation.