Expression of the dopaminergic D1 and D2 receptors in the anterior cingulate cortex in a model of neuropathic pain.

BACKGROUND: The anterior cingulate cortex (ACC) has been related to the affective component of pain. Dopaminergic mesocortical circuits, including the ACC, are able to inhibit neuropathic nociception measured as autotomy behaviour. We determined the changes in dopamine D1 and D2 (D1R and D2R) receptor expression in the ACC (cg1 and cg2) in an animal model of neuropathic pain. The neuropathic group had noxious heat applied in the right hind paw followed 30 min. later by right sciatic denervation. Autotomy score (AS) was recorded for eight days and subsequently classified in low, medium and high AS groups. The control consisted of naïve animals.A semiquantitative RT-PCR procedure was done to determine mRNA levels for D1R and D2R in cg1 and cg2, and protein levels were measured by Western Blot. RESULTS: The results of D1R mRNA in cg1 showed a decrease in all groups. D2R mRNA levels in cg1 decreased in low AS and increased in medium and high AS. Regarding D1R in cg2, there was an increase in all groups. D2R expression levels in cg2 decreased in all groups. In cg1, the D2R mRNA correlated positively with autotomy behaviour. Protein levels of D2R in cg1 increased in all groups but to a higher degree in low AS. In cg2 D2R protein only decreased discretely. D1R protein was not found in either ACC region. CONCLUSIONS: This is the first evidence of an increase of inhibitory dopaminergic receptor (D2R) mRNA and protein in cg1 in correlation with nociceptive behaviour in a neuropathic model of pain in the rat.

Inflammatory nociception diminishes dopamine release and increases dopamine D2 receptor mRNA in the rat's insular cortex.

BACKGROUND: The insular cortex (IC) receives somatosensory afferent input and has been related to nociceptive input. It has dopaminergic terminals and D1 (D1R) -excitatory- and D2 (D2R) -inhibitory- receptors. D2R activation with a selective agonist, as well as D1R blockade with antagonists in the IC, diminish neuropathic nociception in a nerve transection model. An intraplantar injection of carrageenan and acute thermonociception (plantar test) were performed to measure the response to inflammation (paw withdrawal latency, PWL). Simultaneously, a freely moving microdyalisis technique and HPLC were used to measure the release of dopamine and its metabolites in the IC. Plantar test was applied prior, one and three hours after inflammation. Also, mRNA levels of D1 and D2R's were measured in the IC after three hours of inflammation. RESULTS: The results showed a gradual decrease in the release of dopamine, Dopac and HVA after inflammation. The decrease correlates with a decrease in PWL. D2R's increased their mRNA expression compared to the controls. In regard of D1R's, there was a decrease in their mRNA levels compared to the controls. CONCLUSIONS: Our results showed that the decreased extracellular levels of dopamine induced by inflammation correlated with the level of pain-related behaviour. These results also showed the increase in dopaminergic mediated inhibition by an increase in D2R's and a decrease in D1R's mRNA. There is a possible differential mechanism regarding the regulation of excitatory and inhibitory dopaminergic receptors triggered by inflammation.

Expression of muscarinic M1 and M2 receptors in the anterior cingulate cortex associated with neuropathic pain.

The anterior cingulate cortex (ACC) and muscarinic receptors modulate pain. This study investigates changes in the expression of muscarinic-1 and -2 receptors (M1R, M2R) in rats' ACC (cg1-rostral- and cg2-caudal) using a model of neuropathic pain by denervation, measured as autotomy score (AS) for 8 days. Changes were analysed with painful stimuli and with scopolamine into the ACC prior to this scheme. We used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence to determine M1R and M2R's mRNA and protein levels, respectively. Animals were divided in low, medium and high AS groups. Cg1 showed decreased mRNA levels for both M1R and M2R in the low AS group, as opposed to an increased expression in the medium and high AS groups. Both receptors correlated positively with AS in these groups. In the scopolamine-treated animals there was an increase in mRNA levels for both receptors in cg1, whereas in cg2, mRNA levels of M1R decreased in all the AS and scopolamine groups. The increased M2R mRNA in cg2 correlated with AS in the low, medium and high AS groups whereas all the scopolamine groups showed an increase. Immunoreactivity of the M2R in cg1 decreased in the medium AS group in comparison to controls but scopolamine treatment produced an increase in the medium scopolamine AS group compared to the medium AS group. The M1R in cg1 and both receptors in cg2 showed no immunoreactivity changes. These results highlight the role of the M2R in cg1 related to the degree of autotomy.

Dopamine receptors in the anterior insular cortex modulate long-term nociception in the rat.

The rostral agranular insular cortex (RAIC) receives dopaminergic projections from the mesolimbic system, which has been involved in the modulation of nociceptive processes. In this study we determined the contribution of dopamine D(1) and D(2) receptors in the RAIC regarding nociception processing in a neuropathic pain model, as well as inflammatory articular nociception measured as pain-induced functional impairment in the rat (PIFIR). Microinjection of vehicle or substances into the RAIC was performed after the induction of nociception. The groups were treated with: a dopamine D(1) receptor antagonist (SCH-23390), a dopamine D(1) receptor agonist (SKF-38393), a dopamine D(2) receptor agonist (TNPA) and a dopamine D(2) receptor antagonist (spiperone). Chronic nociception, induced by denervation, was measured by the autotomy score in which onset and incidence were also determined. The SCH-23390 and TNPA groups showed a decrease in the autotomy score and a delay on the onset as compared to control, whereas the PIFIR groups did not show statistical differences. This work shows the differential role of dopamine receptors within the RAIC in which the activation of D(2) or the blockade of D(1) receptors elicit antinociception.

The nightmare of a painful phantom / La pesadilla del fantasma doloroso

Summary: Pain has a representation in the central nervous system within some of the most ancient structures developed over the phylogenetical history. Also, it is the most important symptom for which patients seek medical attention. However, there are pathologies in which pain stops being the alarm and becomes a medical issue, perhaps the most dramatic example of this would be the presence of pain in a part of the body that is no longer present, this is known as 'phantom limb pain'. Over history, much attention has been given to pain understanding by well-known personalities and phantom limb itself was described first by Ambroise Paré in the 16th Century and later described by Silas Weir Mitchell in the 19th Century and thoroughly addressed by Livingston in the first half of the 20th Century. The non-painful phantom limb phenomena are reported by almost all amputees and there is pain in 50 to 80%, no matter the nature of the amputation. We face a puzzling public health problem. The major cause of limb amputations comes from vascular and neuropathic complications provoked by diabetes, followed in frequency by trauma, all astonishingly prevalent in the general working population in productive ages. A worrying cause of trauma and the phantom limb in some developing countries is caused by anti-personnel mines leading people, a majority of civilians with a considerable amount of children, to a living hell accompanied by a painful phantom. Phantom limb pain represents a challenge that involves a huge scope of study related to both public health and neuroscience. It is an entity that involves peripheral, central and psychological factors. Neuroscientific research has studied the system from the amputated peripheral nerve, where a neuroma develops, to the cerebral cortex in which there are changes in the somatosensory cortex after limb amputations, related to a decreased activation of the area that would represent the missing limb. Besides, this problem has caused the development of theories such as the 'neuromatrix' that is activated in the absence of peripheral sensory information. In this sense, the anterior cingulate cortex has been proposed as a key structure in the development of phantom nociception in animals. After a huge amount of research, only 30% of patients benefits for a good number of interventions. Phantom limb pain is a clear example that consciousness can indeed become ill and that the clinical frequency, importance and transcendence make research on the neuroscience of consciousness of vital importance. The painful phantom represents a tool and challenge in the neuroscientific field, opens the door for the study on consciousness, supports the need for improved healthcare and allows us to think about war and its consequences in the development of society.

Resumen: El dolor es representado en el Sistema Nervioso Central en algunas de las estructuras más antiguas desarrolladas a lo largo de la historia filogenética. Además, es el síntoma más importante por el que los pacientes buscan atención médica. Sin embargo, existen patologías en las que el dolor deja de ser la alarma y se convierte en un problema médico; probablemente el ejemplo más dramático sea la presencia de dolor en una parte del cuerpo que no está presente, esto se conoce como "miembro fantasma doloroso". A lo largo de la historia, se ha prestado mucha atención en el entendimiento del dolor por renombradas personalidades. El miembro fantasma fue descrito por primera vez por Ambroise Paré en el siglo XVI; en el siglo XIX hizo lo mismo Silas Weir Mitchell, y posteriormente fue revisado con mayor profundidad por Livingston a inicios del siglo XX. El fenómeno del miembro fantasma no doloroso es reportado prácticamente por todas las personas amputadas, y el doloroso en 50 a 80% de los casos, sin importar la naturaleza de la amputación. Nos encontramos frente un importante problema de salud pública. La principal causa de amputación de miembros viene de las complicaciones vasculares y neuropáticas de la diabetes, seguida en frecuencia por causas traumáticas, extremadamente frecuentes en la población económicamente activa. Una causa preocupante de trauma y del fantasma en algunos países en desarrollo son las minas anti-persona. Estas, llevan a los afectados, la mayoría civiles con un número considerable de niños, a un infierno viviente acompañado de un fantasma doloroso. El dolor por miembro fantasma representa un desafío que involucra un enorme campo de estudio relacionado con la salud pública y las neurociencias. Es una entidad que involucra factores periféricos, centrales y psicológicos. La investigación neurocientífica ha estudiado el Sistema desde el nervio periférico amputado donde se forma un neuroma, hasta la corteza cerebral en donde hay una activación disminuida del área que representa al miembro amputado en la corteza somatosensorial después de una amputación. Además, esto ha provocado el desarrollo de teorías como la "neuromatriz". En este sentido, la corteza anterior del cíngulo ha sido propuesta como una estructura clave en el desarrollo de la nocicepción fantasma en animales. Después de una gran cantidad de investigación, sólo 30% de los pacientes se ve beneficiado, gracias a un buen número de intervenciones. El dolor por miembro fantasma es un ejemplo claro de que la conciencia se puede enfermar y la frecuencia clínica, la importancia y la trascendencia de este problema hacen que la investigación en la neurofisiología de la conciencia sea de vital importancia. El fantasma doloroso representa una herramienta y un desafío en el campo neurocientífico; abre la puerta para el estudio de la conciencia, apoya la necesidad de mejorar los servicios de salud y nos permite meditar acerca de la guerra y las consecuencias que tiene para el desarrollo de las sociedades.

Dopamine and NMDA systems modulate long-term nociception in the rat anterior cingulate cortex.

The anterior cingulate cortex (ACC) plays a key role in pain processing. It has been reported that increased activity of glutamatergic projections into the ACC intensifies nociception; whereas dopaminergic projections inhibit it. The aim of this study was to evaluate the role of dopaminergic and NMDA systems of the ACC in the modulation of long-term nociception elicited by sciatic denervation in the rat. Score, onset and incidence of long-term nociception were measured by the autotomy behavior. The effects of a single microinjection into the ACC of different doses of dopamine (100 nM, 100 microM and 100 mM), a NMDA receptor antagonist (MK801 200 nM and 9.34 mM) and amantadine, a dopamine agonist and NMDA receptor antagonist (10, 100 and 1000 microM) were tested on long-term nociception. Dopamine diminished autotomy behavior in an inverse dose-dependent manner, with dopamine 100 nM as most effective concentration. MK801 and amantadine elicited a significant reduction on autotomy score. Prior injections of D1 and D2 receptor antagonists blocked the antinociceptive effects of amantadine on long-term nociceptive behavior. The present study suggests an interaction between dopaminergic and glutamatergic systems within the ACC in the genesis and maintenance of long-term nociception.

Scopolamine into the anterior cingulate cortex diminishes nociception in a neuropathic pain model in the rat: an interruption of 'nociception-related memory acquisition'?

The cingulate cortex plays a key role in the affective component related to pain perception. This structure receives cholinergic projections and also plays a role in memory processing. Therefore, we propose that the cholinergic system in the anterior cingulate cortex is involved in the nociceptive memory process. We used scopolamine (10 microg in 0.25 mircrol/saline) microinjected into the anterior cingulate cortex, either before thermonociception followed by a sciatic denervation, between thermonociception and denervation or after both procedures (n=10 each). The vehicle group (saline solution 0.9%, n=14) was microinjected before thermonociception. Chronic nociception was measured by the autotomy score, which onset and incidence were also determined. Group scopolamine-thermonociception-denervation (STD) presented the lowest autotomy score as compared to vehicle and group thermonociception-denervation-scopolamine (TDS) (vehicle vs. STD, p=0.002, STD vs. TDS, p=0.001). Group thermonociception-scopolamine-denervation (TSD) showed a diminished autotomy score when compared to TDS (p=0.053). STD group showed a delay in the onset of AB as compared to the rest of the groups. Group TSD presented a significative delay (p=0.048) in AB onset when compared to group TDS. There were no differences in the incidence between groups. The results show that nociception-related memory processed in the anterior cingulate cortex is susceptible of being modified by the cholinergic transmission blockade. When scopolamine is microinjected prior to the nociceptive stimuli, nociception-related memory acquisition is prevented. The evidence obtained in this study shows the role of the anterior cingulate cortex in the acquisition of nociception-related memory.