Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

Neuropathic Pain Induced Alterations in the Opioidergic Modulation of a Descending Pain Facilitatory Area of the Brain

Opioids play a major role at descending pain modulation but the effects of neuropathic pain on the brain opioidergic system remain understudied. Since descending facilitation is enhanced during neuropathic pain, we studied the opioidergic modulation of the dorsal reticular nucleus (DRt), a medullary pain facilitatory area, in the spared nerve injury (SNI) model of neuropathic pain. We first performed a series of behavioral experiments in naïve-animals to establish the role of μ-opioid receptor (MOR) in the effects of endogenous and exogenous opioids at the DRt. Specifically, we showed that lentiviral-mediated MOR-knockdown at the DRt increased sensitivity to thermal and mechanical stimuli while the MOR agonist DAMGO induced the opposite effects. Additionally, we showed that MOR-knockdown and the pharmacological blockade of MOR by CTAP at the DRt decreased and inhibited, respectively, the analgesic effects of systemic morphine. Then, we performed in vivo microdialysis to measure enkephalin peptides in the DRt and evaluated MOR expression in the DRt at mRNA, protein and phosphorylated form levels by quantitative real-time PCR and immunohistochemistry, respectively. SNI-animals, compared to sham control, showed higher levels of enkephalin peptides, lower MOR-labeled cells without alterations in MOR mRNA levels, and higher phosphorylated MOR-labeled cells. Finally, we performed behavioral studies in SNI animals to determine the potency of systemic morphine and the effects of the pharmacologic and genetic manipulation of MOR at the DRt. We showed a reduced potency of the antiallodynic effects of systemic morphine in SNI-animals compared to the antinociceptive effects in sham animals. Increasing MOR-cells at the DRt of SNI-animals by lentiviral-mediated MOR-overexpression produced no effects on mechanical allodynia. DAMGO induced anti-allodynia only after MOR-overexpression. These results show that MOR inhibits DRt pain facilitatory actions and that this action contributes to the analgesic effects of systemic opioids. We further show that the inhibitory function of MOR is impaired during neuropathic pain. This is likely due to desensitization and degradation of MOR which are adaptations of the receptor that can be triggered by MOR phosphorylation. Skipping counter-regulatory pathways involved in MOR adaptations might restore the opioidergic inhibition at pain facilitatory areas.

GABA acting on GABAB receptors located in a medullary pain facilitatory area enhances nociceptive behaviors evoked by intraplantar formalin injection

The dorsal reticular nucleus (DRt) plays a key role in facilitation of nociceptive transmission at the spinal cord. In this study, we evaluated the mechanisms involved in GABA-mediated control of the DRt focusing on the role of local GABAB receptors. First, we used in vivo microdialysis to study the release of GABA in the DRt during the course of the formalin test. An increase of GABA levels in comparison with baseline values was detected in the second phase of the test. Because we previously showed that GABAB receptors are expressed by opioidergic DRt neurons, which respond to nociceptive stimuli and inhibit spinally projecting DRt neurons involved in descending pronociception, we then interfered with local GABAB receptors using gene transfer and pharmacological approaches. Lentiviral-mediated knockdown of GABAB1a expression decreased nociceptive responses during the second phase of the test. Local administration of the GABAB receptor antagonist CGP 35348 also decreased nociceptive responses in the second phase of the test, whereas the opposite was detected after injection of the GABAB agonist baclofen. Finally, we determined the GABAergic afferents of the DRt, namely those arising from its main brain afferents, which are located at the telencephalon and diencephalon. For that purpose, we combined retrograde tract-tracing from the DRt with immunodetection of glutamate decarboxylase, the GABA-synthesizing enzyme. The higher numbers of retrogradely labelled glutamate decarboxylase-immunoreactive neurons were located at insular, somatosensory, and motor cortices. Collectively, the results suggest that GABA acting on GABAB receptors may enhance pain facilitation from the DRt during inflammatory pain.

Norepinephrine and dopamine transmission in 2 limbic regions differentially respond to acute noxious stimulation

Central dopamine and norepinephrine regulate behavioral and physiological responses during rewarding and aversive stimuli. Here, we investigated and compared norepinephrine and dopamine transmission in 2 limbic structures, the ventral bed nucleus of the stria terminalis and the nucleus accumbens shell of anesthetized rats, respectively, in response to acute tail pinch, a noxious stimulus. Norepinephrine release in the ventral bed nucleus of the stria terminalis responded monophasically, increasing at the time of the tail pinch and remaining elevated for a period after its cessation. In contrast, dopamine transmission in the nucleus accumbens shell displayed a heterogeneous and time-locked response to tail pinch. For most trials, there was a suppression of extracellular dopamine concentration throughout the duration of the stimuli. At the termination of the stimuli, however, extracellular dopamine either recovered back to or spiked above the initial baseline concentration. These signaling patterns were more clearly observed after administration of selective catecholamine autoreceptor and transporter inhibitors. The results suggest that the opposing responses of these catecholamines can provide integration of noxious inputs to influence behavioral outputs appropriate for survival such as escape or fighting.

Frequency-dependent facilitation of synaptic throughput via postsynaptic NMDA receptors in the nucleus of the solitary tract

Hindbrain NMDA receptors play important roles in reflexive and behavioural responses to vagal activation. NMDA receptors have also been shown to contribute to the synaptic responses of neurons in the nucleus of the solitary tract (NTS), but their exact role remains unclear. In this study we used whole cell patch-clamping techniques in rat horizontal brain slice to investigate the role of NMDA receptors in the fidelity of transmission across solitary tract afferent-NTS neuron synapses. Results show that NMDA receptors contribute up to 70% of the charge transferred across the synapse at high (>5 Hz) firing rates, but have little contribution at lower firing frequencies. Results also show that NMDA receptors critically contribute to the fidelity of transmission across these synapses during high frequency (>5 Hz) afferent discharge rates. This novel role of NMDA receptors may explain in part how primary visceral afferents, including vagal afferents, can maintain fidelity of transmission across a broad range of firing frequencies. Neurons within the nucleus of the solitary tract (NTS) receive vagal afferent innervations that initiate gastrointestinal and cardiovascular reflexes. Glutamate is the fast excitatory neurotransmitter released in the NTS by vagal afferents, which arrive there via the solitary tract (ST). ST stimulation elicits excitatory postsynaptic currents (EPSCs) in NTS neurons mediated by both AMPA- and NMDA-type glutamate receptors (-Rs). Vagal afferents exhibit a high probability of vesicle release and exhibit robust frequency-dependent depression due to presynaptic vesicle depletion. Nonetheless, synaptic throughput is maintained even at high frequencies of afferent activation. Here we test the hypothesis that postsynaptic NMDA-Rs are essential in maintaining throughput across ST-NTS synapses. Using patch clamp electrophysiology in horizontal brainstem slices, we found that NMDA-Rs, including NR2B subtypes, carry up to 70% of the charge transferred across the synapse during high frequency stimulations (>5 Hz). In contrast, their relative contribution to the ST-EPSC is much less during low (<2 Hz) frequency stimulations. Afferent-driven activation of NMDA-Rs produces a sustained depolarization during high, but not low, frequencies of stimulation as a result of relatively slow decay kinetics. Hence, NMDA-Rs are critical for maintaining action potential generation at high firing rates. These results demonstrate a novel role for NMDA-Rs enabling a high probability of release synapse to maintain the fidelity of synaptic transmission during high frequency firing when glutamate release and AMPA-R responses are reduced. They also suggest why NMDA-Rs are critical for responses that may depend on high rates of afferent discharge.