Taurine in the anterior cingulate cortex diminishes neuropathic nociception: A possible interaction with the glycineA receptor

Cortical Modulation of Nociception

Nociception is the neuronal process of encoding noxious stimuli and could be modulated at peripheral, spinal, brainstem, and cortical levels. At cortical levels, several areas including the anterior cingulate cortex (ACC), prefrontal cortex (PFC), ventrolateral orbital cortex (VLO), insular cortex (IC), motor cortex (MC), and somatosensory cortices are involved in nociception modulation through two main mechanisms: (i) a descending modulatory effect at spinal level by direct corticospinal projections or mostly by activation of brainstem structures (i.e. periaqueductal grey matter (PAG), locus coeruleus (LC), the nucleus of raphe (RM) and rostroventral medulla (RVM)); and by (ii) cortico-cortical or cortico-subcortical interactions. This review summarizes evidence related to the participation of the aforementioned cortical areas in nociception modulation and different neurotransmitters or neuromodulators that have been studied in each area. Besides, we point out the importance of considering intracortical neuronal populations and receptors expression, as well as, nociception-induced cortical changes, both functional and connectional, to better understand this modulatory effect. Finally, we discuss the possible mechanisms that could potentiate the use of cortical stimulation as a promising procedure in pain alleviation.

Putative Role of Taurine as Neurotransmitter During Perinatal Cortical Development

Neurotransmitters and neuronal activity affect neurodevelopmental events like neurogenesis, neuronal migration, apoptosis and differentiation. Beside glutamate and gamma-amino butyric acid, the aminosulfonic acid taurine has been considered as possible neurotransmitter that influences early neuronal development. In this article I review recent studies of our group which demonstrate that taurine can affect a variety of identified neuronal populations in the immature neocortex and directly modulates neuronal activity. These experiments revealed that taurine evoke dose-dependent membrane responses in a variety of neocortical neuron populations, including Cajal-Retzius cells, subplate neurons and GABAergic interneurons. Taurine responses persist in the presence of GABA(A) receptor antagonists and are reduced by the addition of strychnine, suggesting that glycine receptors are involved in taurine-mediated membrane responses. Gramicidin-perforated patch-clamp and cell-attached recordings demonstrated that taurine evokes depolarizing and mainly excitatory membrane responses, in accordance with the high intracellular Cl- concentration in immature neurons. In addition, taurine increases the frequency of postsynaptic GABAergic currents (PSCs) in a considerable fraction of immature pyramidal neurons, indicating a specific activation of presynaptic GABAergic networks projecting toward and exciting pyramidal neurons. In summary, these results suggest that taurine may be critically involved in the regulation of network excitability in the immature neocortex and hippocampus via interactions with glycine receptors.

Insular balance of glutamatergic and GABAergic signaling modulates pain processing

Neuroimaging studies of patients with chronic pain have shown that neurotransmitter abnormalities, including increases in glutamate and decreases in GABA, could be responsible for the cortical hyperactivity and hyperalgesia/allodynia observed in some pain conditions. These finding are particularly evident in the insula, a brain region known to play a role in both the sensory-discriminative and the affective-motivational aspects of pain processing. However, clinical studies are not entirely able to determine the directionality of these findings, nor whether they are causal or epiphenomenon. Thus, a set of animal studies was performed to determine whether alterations in glutamate and GABA are the result of injury, the cause of augmented pain processing, or both. Compared with controls, the excitatory neurotransmitters glutamate and aspartate are significantly higher in the rat insula after chronic constriction injury of the sciatic nerve (CCI). The CCI also produced significant increases in allodynia (mechanical and cold), thermal hyperalgesia, and nociceptive aversiveness. Unilateral microinjection of ionotropic glutamate receptor antagonists restored these nociceptive behaviors to preinjury values. Increasing endogenous levels of GABA or enhancing signaling at inhibitory glycinergic receptors had similar effects as the glutamate receptor antagonists. In naive rats, increasing endogenous levels of glutamate, decreasing endogenous levels of GABA, or blocking strychnine-sensitive glycine receptors in the insula significantly increased thermal hyperalgesia and mechanical allodynia. These data support the hypothesis that an altered balance of excitatory and inhibitory neurotransmitters in brain regions such as the insula occurs in chronic pain states and leads to augmented central pain processing and increased pain sensitivity.

Taurine as an Essential Neuromodulator during Perinatal Cortical Development

A variety of experimental studies demonstrated that neurotransmitters are an important factor for the development of the central nervous system, affecting neurodevelopmental events like neurogenesis, neuronal migration, programmed cell death, and differentiation. While the role of the classical neurotransmitters glutamate and gamma-aminobutyric acid (GABA) on neuronal development is well established, the aminosulfonic acid taurine has also been considered as possible neuromodulator during early neuronal development. The purpose of the present review article is to summarize the properties of taurine as neuromodulator in detail, focusing on the direct involvement of taurine on various neurodevelopmental events and the regulation of neuronal activity during early developmental epochs. The current knowledge is that taurine lacks a synaptic release mechanism but is released by volume-sensitive organic anion channels and/or a reversal of the taurine transporter. Extracellular taurine affects neurons and neuronal progenitor cells mainly via glycine, GABA(A), and GABA(B) receptors with considerable receptor and subtype-specific affinities. Taurine has been shown to directly influence neurogenesis in vitro as well as neuronal migration in vitro and in vivo. It provides a depolarizing signal for a variety of neuronal population in the immature central nervous system, thereby directly influencing neuronal activity. While in the neocortex, taurine probably enhance neuronal activity, in the immature hippocampus, a tonic taurinergic tone might be necessary to attenuate activity. In summary, taurine must be considered as an essential modulator of neurodevelopmental events, and possible adverse consequences on fetal and/or early postnatal development should be evaluated for pharmacological therapies affecting taurinergic functions.

Potential Benefits of Ameliorating Metabolic and Nutritional Abnormalities in People With Profound Developmental Disabilities

Background:

People with profound developmental disabilities have some of the most severe neurological impairments seen in society, have accelerated mortality due to huge medical challenges, and yet are often excluded from scientific studies. They actually have at least 2 layers of conditions: (1) the original disability and (2) multiple under-recognized and underexplored metabolic and nutritional imbalances involving minerals (calcium, zinc, and selenium), amino acids (taurine, tryptophan), fatty acids (linoleic acid, docosahexaenoic acid, arachidonic acid, adrenic acid, Mead acid, plasmalogens), carnitine, hormones (insulinlike growth factor 1), measures of oxidative stress, and likely other substances and systems.

Summary:

This review provides the first list of metabolic and nutritional abnormalities commonly found in people with profound developmental disabilities and, based on the quality of life effects of similar abnormalities in neurotypical people, indicates the potential effects of these abnormalities in this population which often cannot communicate symptoms.

Key messages:

We propose that improved understanding and management of these disturbed mechanisms would enhance the quality of life of people with profound developmental disabilities. Such insights may also apply to people with other conditions associated with disability, including some diseases requiring stem cell implantation and living in microgravity.

Congenital taurine deficiency in mice is associated with reduced sensitivity to nociceptive chemical stimulation

The amino acid taurine is required for development and functioning of the central and peripheral nervous system where it exerts osmoregulatory, neuromodulatory and anti-apoptotic actions. It is subject to cellular import by the taurine transporter slc6a6. Absence of the transporter and consequently, absence of taurine leads to several neurologic deficits and sensory losses. In a slc6a6 knock-out mouse model, consequences of congenital taurine deficiency were assessed in nociceptive sensory processes. The formalin assay, hot plate assay, and summated generator potentials in response to local nociceptive stimulation with gaseous CO2 were applied. Reduced responsiveness of slc6a6(-/-) mice to nociceptive stimulation was observed in particular to chemical nociceptive stimuli. Scl6a6 knock-out mice spent significantly less time licking the formalin injected paw and displayed smaller amplitudes of the nociceptive nasal mucosa potentials than wild-type mice (p=0.002 and 0.01 respectively). In contrast, withdrawal latencies on a hot plate did not significantly differ, suggesting that intracellular taurine deficits lead in particular to a hyposensitivity of nociceptive sensory neurons sensitive to noxious chemical stimulation. As hereditary absence of taurine affects biological processes of anatomical structure development, the altered nociceptive responses likely reflect consequences of compromised peripheral nervous system development.

Perinatal taurine exposure affects adult arterial pressure control

Taurine is an abundant, free amino acid found in mammalian cells that contributes to many physiologic functions from that of a simple cell osmolyte to a programmer of adult health and disease. Taurine's contribution extends from conception throughout life, but its most critical exposure period is during perinatal life. In adults, taurine supplementation prevents or alleviates cardiovascular disease and related complications. In contrast, low taurine consumption coincides with increased risk of cardiovascular disease, obesity and type II diabetes. This review focuses on the effects that altered perinatal taurine exposure has on long-term mechanisms that control adult arterial blood pressure and could thereby contribute to arterial hypertension through its ability to program these cardiovascular regulatory mechanisms very early in life. The modifications of these mechanisms can last a lifetime and transfer to the next generation, suggesting that epigenetic mechanisms underlie the changes. The ability of perinatal taurine exposure to influence arterial pressure control mechanisms and hypertension in adult life appears to involve the regulation of growth and development, the central and autonomic nervous system, the renin-angiotensin system, glucose-insulin interaction and changes to heart, blood vessels and kidney function.

Antinociceptive effect of intrathecal administration of hypotaurine in rat models of inflammatory and neuropathic pain

Hypotaurine is an intermediate in taurine biosynthesis from cysteine in astrocytes. Although hypotaurine functions as an antioxidant and organic osmolyte, its physiological role in the central nervous system remains unclear. This study used behavioral assessments to determine whether hypotaurine influenced nociceptive transmission in acute, inflammatory, and neuropathic pain. The tail flick, paw pressure, and formalin tests were performed in male Sprague-Dawley rats to examine the effects of the intrathecal administration of hypotaurine (100, 200, 400, 600 μg) on thermal, mechanical, and chemical nociception. Chronic constriction injury (CCI) to the sciatic nerve was induced in the rats, and the electronic von Frey test and plantar test were performed to assess the effects on neuropathic pain. To determine which neurotransmitter pathway(s) was involved in the action of hypotaurine, in this study, we examined how the antagonists of spinal pain processing receptors altered the effect of 600 μg hypotaurine. To explore whether hypotaurine affected motor performance, the Rotarod test was conducted. Hypotaurine had antinociceptive effects on thermal, mechanical, and chemical nociception in the spinal cord. In CCI rats, hypotaurine alleviated mechanical allodynia and thermal hyperalgesia. These effects were reversed completely by pretreatment with an intrathecal injection of strychnine, a glycine receptor antagonist. Conversely, hypotaurine did not affect motor performance. This study demonstrated that intrathecal hypotaurine suppressed acute, inflammatory, and neuropathic pain. Hypotaurine may regulate nociceptive transmission physiologically by activating glycinergic neurons in the spinal cord, and it is a promising candidate for treating various pain states.

Antinociceptive effect of intrathecal administration of taurine in rat models of neuropathic pain

PURPOSE

Taurine is the most abundant amino acid in many tissues. Although taurine has been shown to be antinociceptive, in this report, our focus is to elucidate the mechanism and action site on neuropathic pain. This study used behavioural assessments to determine whether taurine attenuates neuropathic pain in the spinal cord.

METHODS

Chronic constriction injury (CCI) to the sciatic nerve and streptozotocin-induced diabetic neuropathy were introduced to male Sprague-Dawley rats. We then assessed the antinociceptive effect of spinal injections of taurine (100, 200, 400, or 800 μg) using electronic von Frey, paw pressure, and plantar tests. To explore the effect of taurine on motor function, a rotarod test was performed, and in order to determine which neurotransmitter pathway is involved in taurine's action, we examined how several antagonists of spinal pain processing receptors altered the effect of taurine 400 μg in a paw pressure test.

RESULTS

Taurine alleviated mechanical allodynia, mechanical hyperalgesia, and thermal hyperalgesia in CCI rats and suppressed mechanical allodynia and hyperalgesia in diabetic rats. Significant effects were observed at 200 μg in both models. On the other hand, taurine dose-dependently affected motor performance, and a significant effect was seen at 400 μg. The antinociceptive effects were reversed completely by pretreatment with an intrathecal injection of strychnine, a glycine receptor antagonist.

CONCLUSION

The present study demonstrated that intrathecal administration of taurine attenuates different models of neuropathic pain, and these effects seem to be mediated by the activation of glycinergic neurotransmission. These findings suggest that taurine may be a candidate remedy for neuropathic pain.

Paracetamol fails to positively modulate and directly activate chloride currents in human α1-glycine receptors

Paracetamol (acetaminophen) is a widely used antipyretic and analgesic drug for mild or moderate pain states. As the primary site of action of paracetamol is still the subject of ongoing discussion, the focus of this study is the investigation of a potential mechanism which might contribute to its beneficial effects in the therapy of pain. Loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord plays a key role in the development of pain following inflammation or nerve injury. Inhibitory postsynaptic transmission in the adult spinal cord involves mainly glycine. In this study we investigated the interaction of paracetamol with strychnine-sensitive α(1)-glycine receptors (α(1)-GlyR). α(1)-GlyR subunits transiently expressed in HEK-293 cells were studied using the whole-cell patch-clamp technique and a piezo-controlled liquid filament fast application system. Paracetamol fails to show a positive allosteric modulatory effect in low nano- to micromolar concentrations and lacks direct activation in micromolar concentrations at the α(1)-GlyR. Consequently, the analgesic actions of paracetamol leading to pain relief appear to be mediated via other mechanisms, but not via activation of spinal glycinergic pathways.

Cerebral cortex modulation of pain

Pain is a complex experience encompassing sensory-discriminative, affective-motivational and cognitiv e-emotional components mediated by different mechanisms. Contrary to the traditional view that the cerebral cortex is not involved in pain perception, an extensive cortical network associated with pain processing has been revealed using multiple methods over the past decades. This network consistently includes, at least, the anterior cingulate cortex, the agranular insular cortex, the primary (SI) and secondary somatosensory (SII) cortices, the ventrolateral orbital cortex and the motor cortex. These cortical structures constitute the medial and lateral pain systems, the nucleus submedius-ventrolateral orbital cortex-periaqueductal gray system and motor cortex system, respectively. Multiple neurotransmitters, including opioid, glutamate, GABA and dopamine, are involved in the modulation of pain by these cortical structures. In addition, glial cells may also be involved in cortical modulation of pain and serve as one target for pain management research. This review discusses recent studies of pain modulation by these cerebral cortical structures in animals and human.