Vav3 Is Involved in GABAergic Axon Guidance Events Important for the Proper Function of Brainstem Neurons Controlling Cardiovascular, Respiratory, and Renal Parameters

Vav3 is a phosphorylation-dependent activator of Rho/Rac GTPases that has been implicated in hematopoietic, bone, cerebellar, and cardiovascular roles. Consistent with the latter function, Vav3-deficient mice develop hypertension, tachycardia, and renocardiovascular dysfunctions. The cause of those defects remains unknown as yet. Here, we show that Vav3 is expressed in GABAegic neurons of the ventrolateral medulla (VLM), a brainstem area that modulates respiratory rates and, via sympathetic efferents, a large number of physiological circuits controlling blood pressure. On Vav3 loss, GABAergic cells of the caudal VLM cannot innervate properly their postsynaptic targets in the rostral VLM, leading to reduced GABAergic transmission between these two areas. This results in an abnormal regulation of catecholamine blood levels and in improper control of blood pressure and respiration rates to GABAergic signals. By contrast, the reaction of the rostral VLM to excitatory signals is not impaired. Consistent with those observations, we also demonstrate that Vav3 plays important roles in axon branching and growth cone morphology in primary GABAergic cells. Our study discloses an essential and nonredundant role for this Vav family member in axon guidance events in brainstem neurons that control blood pressure and respiratory rates.

Serotonin Type 1D Receptors (5HT1DR) are Differentially Distributed in Nerve Fibres Innervating Craniofacial Tissues

We tested the hypothesis that the 5HT1DR, the primary antinociceptive target of triptans, is differentially distributed in tissues responsible for migraine pain. The density of 5HT1DR was quantified in tissues obtained from adult female rats with Western blot analysis. Receptor location was assessed with immunohistochemistry. The density of 5HT1DR was significantly greater in tissues known to produce migraine-like pain (i.e. circle of Willis and dura) than in structures in which triptans have no antinociceptive efficacy (i.e. temporalis muscle). 5HT1DR-like immunoreactivity was restricted to neuronal fibres, where it colocalized with calcitonin gene-related peptide and tyrosine hydroxylase immunoreactive fibres. These results are consistent with our hypothesis that the limited therapeutic profile of triptans could reflect its differential peripheral distribution and that the antinociceptive efficacy reflects inhibition of neuropeptide release from sensory afferents. An additional site of action at sympathetic efferents is also suggested.

Sympathetic Modulation of Activity in Aδ- and C-Primary Nociceptive Afferents After Intradermal Injection of Capsaicin in Rats

Neuropathic and inflammatory pain can be modulated by the sympathetic nervous system. In some pain models, sympathetic postganglionic efferents are involved in the modulation of nociceptive transmission in the periphery. The purpose of this study is to examine the sensitization of Aδ- and C-primary afferent nociceptors induced by intradermal injection of capsaicin (CAP) to see whether the presence of sympathetic efferents is essential for the sensitization. Single primary afferent discharges were recorded from the tibial nerve after the fiber types were identified by conduction velocity in anesthetized rats. An enhanced response of some Aδ- and most C-primary afferent fibers to mechanical stimuli was seen in sham-sympathectomized rats after CAP (1%, 15 μl) injection, but the enhanced responses of both Aδ- and C-fibers were reduced after sympathetic postganglionic efferents were removed. Peripheral pretreatment with norepinephrine by intraarterial injection could restore and prolong the CAP-induced enhancement of responses under sympathectomized conditions. In sympathetically intact rats, pretreatment with an α1-adrenergic receptor antagonist (terazosin) blocked completely the enhanced responses of C-fibers after CAP injection in sympathetically intact rats without significantly affecting the enhanced responses of Aδ-fibers. In contrast, a blockade of α2-adrenergic receptors by yohimbine only slightly reduced the CAP-evoked enhancement of responses. We conclude that the presence of sympathetic efferents is essential for the CAP-induced sensitization of Aδ- and C-primary afferent fibers to mechanical stimuli and that α1-adrenergic receptors play a major role in the sympathetic modulation of C-nociceptor sensitivity in the periphery.

Involvement of sympathetic efferents but not capsaicin-sensitive afferents in nociceptin-mediated dual control of rat synovial blood flow

This study set out to examine the vasomotor effects of the opioid-like peptide nociceptin on knee joint capsular blood flow in urethane-anaesthetized rats. Topical application of nociceptin (10−15–10−8 mol) caused a progressive fall in joint perfusion that was significantly inhibited by the specific nociceptin receptor antagonist [Phe1-(CH2-NH)-Gly2] Nociceptin(1–13)-NH2 as well as the nonspecific opioid antagonist naloxone. To test whether this constrictor response was sympathetically mediated, we administered nociceptin in animals treated with guanethidine to produce sympathetic blockade or in the presence of the α-adrenoceptor antagonist phentolamine. Both guanethidine treatment and phentolamine coadministration attenuated the constrictor response to nociceptin. Inhibition of nociceptin-mediated vasoconstriction revealed a supplementary hyperemic response that persisted in animals whose knee joints were treated with 1% capsaicin to destroy the articular unmyelinated nerve supply. These results show that, in the rat knee, peripheral administration of nociceptin primarily causes a sympathetically mediated vasoconstriction. In addition, high-dose nociceptin produces a vasodilatatory response that is likely due to the direct action of nociceptin on vascular smooth muscle and not by a neurogenic mechanism.