Gating of vagal inputs by sciatic afferents in nonspinally projecting neurons in the rat rostral ventrolateral medulla oblongata

Comparative effect of dense-and-disperse versus non-repetitive and non-sequential frequencies in electroacupuncture-induced analgesia in a rodent model of peripheral neuropathic pain

BACKGROUND

Neuropathic pain (NP) is a complex disease that remains challenging to treat. Low-frequency dense-and-disperse (DD) electroacupuncture (EA) has been used as adjuvant therapy for neuropathic pain; however, its analgesic effect decreases as stimulation time increases, or when it is repeatedly used. We hypothesized that a new frequency parameter could improve the effectiveness of EA, and aimed to compare the efficacy and duration of the analgesic effect between classic DD-EA and non-repetitive and non-sequential frequency (random frequency (RF)-EA) in neuropathic rats. Furthermore, the effect of RF-EA at local traditional acupuncture point locations versus auricular vagus nerve stimulation (aVNS) was evaluated.

METHODS

Male Wistar rats with peripheral neuropathy were subjected to a single session of DD-EA or RF-EA for 20 or 40 min at ST36 + GB34. An additional group of rats was treated with RF-EA for 20 min using aVNS at the appropriate ear point locations. Paw pressure test, von Frey filaments and spontaneous pain scores were evaluated. Sham-operated rats were used as controls.

RESULTS

In all, 20 min of RF-EA reversed hyperalgesia (for 24 h) and allodynia (for 8 h), showing a longer analgesic effect than DD-EA. Both RF-EA and DD-EA induced partial inhibition of spontaneous pain for 8 h. Forty minutes of DD-EA did not interfere with the NP phenomena; however, RF-EA induced significant long-term analgesia. aVNS induced an analgesic effect similar to local stimulation.

CONCLUSION

This pilot study shows that RF-EA at both local traditional acupuncture point and auriculotherapy point locations induces long-lasting analgesia in neuropathic rats, and more effectively so than classical DD-EA.

Anti-Inflammatory Effects of Acupuncture Stimulation via the Vagus Nerve

Although acupuncture therapy is widely used in traditional Asian medicine for the treatment of diverse internal organ disorders, its underlying biological mechanisms are largely unknown. Here, we investigated the functional involvement of acupuncture stimulation (AS) in the regulation of inflammatory responses. TNF-α production in mouse serum, which was induced by lipopolysaccharide (LPS) administration, was decreased by manual acupuncture (MAC) at the zusanli acupoint (stomach36, ST36). In the spleen, TNF-α mRNA and protein levels were also downregulated by MAC and were recovered by using a splenic neurectomy and a vagotomy. c-Fos, which was induced in the nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMV) by LPS and electroacupuncture (EAC), was further increased by focal administration of the AMPA receptor blocker CNQX and the purinergic receptor antagonist PPADS. TNF-α levels in the spleen were decreased by CNQX and PPADS treatments, implying the involvement of inhibitory neuronal activity in the DVC. In unanesthetized animals, both MAC and EAC generated c-Fos induction in the DVC neurons. However, MAC, but not EAC, was effective in decreasing splenic TNF-α production. These results suggest that the therapeutic effects of acupuncture may be mediated through vagal modulation of inflammatory responses in internal organs.

The rostral parvicellular reticular formation neurons mediate lingual nerve input to the rostral ventrolateral medulla

In rats that had been anesthetized by urethane-chloralose, we investigated whether neurons in the rostral part of the parvicellular reticular formation (rRFp) mediate lingual nerve input to the rostral ventrolateral medulla (RVLM), which is involved in somato-visceral sensory integration and in controlling the cardiovascular system. We determined the effect of the lingual nerve stimulation on activity of the rRFp neurons that were activated antidromically by stimulation of the RVLM. Stimulation of the lingual trigeminal afferent gave rise to excitatory effects (10/26, 39%), inhibitory effects (6/26, 22%) and no effect (10/26, 39%) on the RVLM-projecting rRFp neurons. About two-thirds of RVLM-projecting rRFp neurons exhibited spontaneous activity; the remaining one-third did not. A half (13/26) of RVLM-projecting rRFp neurons exhibited a pulse-related activity, suggesting that they receive a variety of peripheral and CNS inputs involved in cardiovascular function. We conclude that the lingual trigeminal input exerts excitatory and/or inhibitory effects on a majority (61%) of the RVLM-projecting rRFp neurons, and their neuronal activity may be involved in the cardiovascular responses accompanied by the defense reaction.

Patterning of somatosympathetic reflexes reveals nonuniform organization of presympathetic drive from C1 and non-C1 RVLM neurons

To determine the organization of presympathetic vasomotor drive by phenotypic populations of rostral ventrolateral medulla (RVLM) neurons, we examined the somatosympathetic reflex (SSR) evoked in four sympathetic nerves together with selective lesions of RVLM presympathetic neurons. Urethane-anesthetized (1.3 g/kg ip), paralyzed, vagotomized and artificially ventilated Sprague-Dawley rats (n = 41) were used. First, we determined the afferent inputs activated by sciatic nerve (SN) stimulation at graded stimulus intensities (50 sweeps at 0.5-1 Hz, 1-80 V). Second, we recorded sympathetic nerve responses (cervical, renal, splanchnic, and lumbar) to intensities of SN stimulation that activated A-fiber afferents (low) or both A- and C-fiber afferents (high). Third, with low-intensity SN stimulation, we examined the cervical SSR following RVLM microinjection of somatostatin, and we determined the splanchnic SSR in rats in which presympathetic C1 neurons were lesioned following intraspinal injections of anti-dopamine-β-hydroxylase-saporin (anti-DβH-SAP). Low-intensity SN stimulation activated A-fiber afferents and evoked biphasic responses in the renal, splanchnic, and lumbar nerves and a single peak in the cervical nerve. Depletion of presympathetic C1 neurons (59 ± 4% tyrosine hydroxylase immunoreactivity profiles lesioned) eliminated peak 2 of the splanchnic SSR and attenuated peak 1, suggesting that only RVLM neurons with fast axonal conduction were spared. RVLM injections of somatostatin abolished the single early peak of cervical SSR confirming that RVLM neurons with fast axonal conduction were inhibited by somatostatin. It is concluded that unmyelinated RVLM presympathetic neurons, presumed to be all C1, innervate splanchnic, renal, and lumbar but not cervical sympathetic outflows, whereas myelinated C1 and non-C1 RVLM neurons innervate all sympathetic outflows examined. These findings suggest that multiple levels of neural control of vasomotor tone exist; myelinated populations may set baseline tone, while unmyelinated neurons may be recruited to provide actions at specific vascular beds in response to distinct stressors.

Heterogeneous synaptic inputs from the ventrolateral periaqueductal gray matter to neurons responding to somatosensory stimuli in the rostral ventromedial medulla of rats

The ventrolateral cell column of the midbrain periaqueductal gray matter (vl-PAG) plays a major role in the attenuation of pain behaviour. It is established that this effect is exerted via modulation of neuronal activities in the rostral ventromedial medulla (RVM). Until recently it has been generally accepted that the vl-PAG exerts its modulatory effects upon RVM neurons through a direct monosynaptic pathway. However, recent data suggest that an additional indirect, di- or polysynaptic pathway may also exist. Using in vivo intracellular recordings we tested this hypothesis, by studying synaptic responses of somatosensory receptive RVM neurons evoked by electric stimulation of the vl-PAG in rats. RVM neurons were regarded as somatosensory receptive if they responded to electrical stimulation of the sciatic nerve. Most of the recorded RVM cells were excited by vl-PAG stimulation. Some of them responded with a short onset latency (3.6+/-0.9 ms) corresponding to monosynaptic excitation. All of these neurons were also excited by sciatic nerve stimulation at nociceptive intensities. In contrast to this, another proportion of the recorded RVM neurons responded with a four times longer (14.8+/-3 ms) onset latency to the vl-PAG stimulation, corresponding to polysynaptic modulation. All of these neurons were inhibited by sciatic nerve stimulation. The findings show that RVM neurons receive heterogeneous monosynaptic and polysynaptic inputs from the vl-PAG. The results also suggest that the monosynaptic and polysynaptic pathways modulate the activity of functionally distinct groups of RVM neurons.

Does the vagus nerve mediate the sixth sense?

Can sensations originating from the internal environment modulate attitude and behaviour? Can the feedback about the operation of the viscera provide a calming and relaxing influence? Information from the chest and abdomen is delivered continuously by the vagus nerve, the largest visceral sensory nerve in the body. Because various 'stress-related' diseases can be associated with impaired functions in sensory vagal fibres, a better understanding of how sensory vagal information is processed in the CNS might offer new strategies for the treatment and/or prevention of several disorders, including 'drug-resistant' forms of eating disorder, anxiety, chronic depression and epilepsy. A neuronal circuitry that has been suggested by experimental data to mediate sensory vagal inputs to those brain areas that are involved in the generation of 'stress-related' disorders is outlined.