Physiological and behavioral evidence of a capsaicin sensitive TRPV-like channel in the medicinal leech

Abstract

Transient receptor potential vanilloid (TRPV) channels are found throughout the animal kingdom, where they play an important role in sensory transduction. In this study, we combined physiological studies with in vivo behavioral experiments to examine the presence of a putative TRPV-like receptor in the medicinal leech, building upon earlier studies in this lophotrochozoan invertebrate. The leech polymodal nociceptive neuron was activated by both peripheral and central application of the TRPV1-activator capsaicin in a concentration-dependent manner, with 100 μmol l −1 being the lowest effective concentration. Responses to capsaicin were inhibited by the selective TRPV1 antagonist SB366791. The polymodal nociceptive neuron also responded to noxious thermal stimuli (>40°C), and this response was also blocked by SB366791. Capsaicin sensitivity was selective to the polymodal nociceptor with no direct response being elicited in the mechanical nociceptive neuron or in the nonnociceptive touch-or pressure-sensitive neurons. Capsaicin also elicited nocifensive behavioral responses (withdrawals and locomotion) in a concentration-dependent manner, and these behavioral responses were significantly attenuated with SB366791. These results suggest the presence of a capsaicin-sensitive TRPVlike channel in the medicinal leech central nervous system and are relevant to the evolution of nociceptive signaling. KEY WORDS: Leech, TRPV, Invertebrate, Nociception INTRODUCTIONTransient receptor potential (TRP) channels are a diverse class of non-specific cation channels that are found throughout the animal kingdom (Damann et al., 2008). TRP channels are typically associated with sensory transduction and are often polymodal, such as the mammalian TRP-vanilloid 1 (TRPV1) channel, which responds to noxious thermal (>42°C), mechanical and chemical (low pH, capsaicin) stimuli (Szallasi and Blumberg, 1999). Capsaicin sensitivity varies greatly throughout metazoans, with mammals displaying the greatest response to capsaicin. Birds, once thought to be capsaicin insensitive, have been shown to respond to capsaicin at increased concentrations (30 μmol l −1 ) compared with mammals (Kirifides et al., 2004). Even among mammals, capsaicin sensitivity varies greatly between species, and this variance is further complicated by ontogentic shifts in capsaicin sensitivity within specific strains of mice (Holzer, 1991). Invertebrate sensitivity to capsaicin is no less complicated because ecdysozoans, such as Drosophila (Vriens et al., 2004) and Caenorhabditis elegans (Tobin RESEARCH ARTICLECenter for Brain and Behavior Research, Division of Basic Biomedical Sciences, Sanford, School of Medicine, University of South Dakota, Vermillion, SD 57069, USA. *Author for correspondence (bburrell@usd.edu)Received 23 June 2014; Accepted 7 October 2014 et al., 2002), are reported to be capsaicin insensitive, whereas lophotrochozoans, specifically the gastropod Megalobulimus (KalilGaspar et al., 2007) and the medicinal leech (Hirudo verbana, Carena 1820) (Pasto...

Physiological, pharmacological, and behavioral evidence for a TRPA1 channel that can elicit defensive responses in the medicinal leech

“…Since it has been shown that some invertebrates use their TRPA1 channel homologs for thermodetection of noxious heat (Neely et al, 2011), we repeated and expanded these experiments with the use of heat and HC030031. In experiments in which saline heated to 43°C was applied to the patch of skin in the body wall preparation, activity in the lN cell was observed, consistent with previous findings (Pastor et al, 1996;Summers et al, 2014). This activity was not attenuated by HC030031, but surprisingly, coapplication of this TRPA1 antagonist increased responsiveness to noxious heat ( Fig.…”

“…4A). In previous experiments studying the putative TRPV channel in the leech, peripheral application of capsaicin was also found to activate the P cells (Summers et al, 2014). However, this capsaicin-elicited activity could be blocked by the AMPA receptor antagonist CNQX, indicating that capsaicin was activating an unknown TRPV-containing afferent that was driving P-cell activity via glutamatergic synaptic transmission.…”

“…Previous studies have found that noxious heat (>43°C) activates lN cells and that this heat-induced activity can be attenuated with the TRPV antagonist SB366791 (Pastor et al, 1996;Summers et al, 2014). Since it has been shown that some invertebrates use their TRPA1 channel homologs for thermodetection of noxious heat (Neely et al, 2011), we repeated and expanded these experiments with the use of heat and HC030031.…”

“…capsaicin and H + ) (Damann et al, 2008). Recently, we published evidence of a capsaicin-sensitive TRPV-like channel in the medicinal leech (Summers et al, 2014). Another TRP channel involved in nociceptive signaling is the transient receptor potential ankyrin subtype 1 protein (TRPA1).…”

“…The leech possesses specific sensory neurons that detect light touch (T cells), sustained pressure (P cells) and both mechanical and polymodal nociceptive stimuli (N cells) (Nicholls and Baylor, 1968;Blackshaw et al, 1982;Pastor et al, 1996). Previous studies have found that the polymodal lateral N cells (lN cells) respond to capsaicin, H + and noxious heat and that this response can be blocked with the selective TRPV1 antagonist SB366791, suggesting the presence of a TRPV-like channel in the leech (Pastor et al, 1996;Summers et al, 2014).…”