J Physiol 588.10 Pain modulation by tethered toxins 1705 while we used a ramp from 25◦C to 0◦C). Nonetheless when the cooling gradient reached 10◦C we already observed reduced ﬁring discharges in Tg-t-MrVIa mice (Fig. 4E). Since studies on Nav1.9−/− mutants have not indicated any contribution of these channels to cold sensitivity (Amaya et al. 2006), the t-MrVIa-mediated block of TTX-R currents resulting in cold insensitivity is presumably accounted for by blockade of Nav1.8 exclusively. Studies on corneal C-ﬁbres have shown that many cold-sensitive afferents sustain discharge to thermal stimulation even when TTX is applied to the preparation (Brock et al. 1998). It is thus quite possible that the residual TTX-insensitive current generated in the cutaneous endings of cold-sensitive ﬁbres in the skin of trans- genic mice is sufﬁcient to preserve afferent responses to mild cooling. Environmental changes to innocuous cold temperatures are detected by the menthol TRPM8 receptor (Bautista et al. 2007). However, when temperatures become painful Nav1.8 seems to be not only essential to maintain the excitability of nociceptors (Zimmermann et al. 2007), but based on our results the activity of Nav1.8 is also required for nociceptors to detect noxious cold temperatures. Nociceptor-speciﬁc VGSCs are important therapeutic targets and major efforts have been made to screen for speciﬁc molecules to block their function. For instance, conotoxins are already used as analgesic drugs. The MrVIa-related conotoxin MVIIa (commercialised as Prialt or ziconotide) is a speciﬁc antagonist of N-type VGCCs, and is used to treat patients with extreme neuropathic pain that is insensitive to opioids (Thompson et al. 2006). In contrast to local application of such analgesic peptides, delivered intrathecally via infusion pumps to avoid severe side effects due to diffusion of the toxin into the CNS, genetic delivery of tethered toxins to deﬁned cell populations could become a viable alternate to prevent toxic effects and repetitive treatment. Future developments in the safety of gene transfer in humans offers considerable promise for the application of this novel tethered toxin technology to treat such extreme pain in humans. Extension of this approach to other peptide toxins together with the use of cell-speciﬁc genetic methods, may allow for speciﬁc intervention in deﬁned neural circuits in vivo and therefore potential treatment of diseases caused by channelopathies. Given the importance of VGSC function for AP propagation, the extensive use of TTX for functional inactivation of CNS neurones in vivo, and our demonstration that t-MrVIa can cell-autonomously inhibit VGSC function without resulting in misregulation of other ion channels or alterations in membrane potential, the extension of this methodology to other cell types and ion channels could provide a valuable new tool for the dissection of neural circuits. For instance, in a complementary study we have used other peptide C 2010 The Authors. Journal compilation C 2010 The Physiological Society toxins to block calcium inﬂux through the VGCCs Cav2.1 and Cav2.2 (Auer et al. 2010). Thus, depending on the speciﬁcity and mode of action of the natural toxin of choice, this approach can be used for neuronal silencing by either inhibiting VGCCs and blocking neurotransmitter release (Auer et al. 2010), or, as in this study, inhibiting VGSCs and reducing cell excitability. 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