The role of dopamine in the nucleus accumbens in analgesia

Altered nucleus accumbens circuitry mediates pain-induced antinociception in morphine-tolerant rats

We investigated the effect of chronic administration of morphine on noxious stimulus-induced antinociception (NSIA) produced by intraplantar capsaicin injection. In the untreated (naive) rat, we previously found that NSIA depends on activation of dopamine, nicotinic acetylcholine, and mu- and delta-opioid receptors in nucleus accumbens. Rats chronically implanted with subcutaneous morphine pellets demonstrated tolerance to the antinociceptive effects of acute systemic morphine administration but did not show cross-tolerance to NSIA. Morphine pretreatment, however, significantly reduced NSIA dependence on intra-accumbens opioid receptors but not on dopamine or nicotinic acetylcholine receptors. As observed in naive rats, intra-accumbens microinjection of either the dopamine receptor antagonist flupentixol or the nicotinic receptor antagonist mecamylamine blocked NSIA in rats tolerant to the antinociceptive effects of morphine, but, in contrast to naive rats, intra-accumbens microinjection of either the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7 amide or the delta-receptor antagonist naltrindole failed to block NSIA. These findings suggest that although NSIA is dependent on nucleus accumbens opioid receptors in the naive state, this dependence disappears in rats tolerant to the antinociceptive effects of morphine, which may account for the lack of NSIA cross-tolerance. In separate experiments, intra-accumbens extracellular dopamine levels were measured using microdialysis. Dopamine levels increased after either capsaicin or systemic morphine administration in naive rats but only after capsaicin administration in morphine pretreated rats. Thus, intra-accumbens dopamine release paralleled antinociceptive responses in naive and morphine pretreated rats.

Routine sucrose analgesia during the first week of life in neonates younger than 31 weeks' postconceptional age

OBJECTIVE

To determine the efficacy of sucrose analgesia for procedural pain during the first week of life in preterm neonates in neonatal intensive care units on enhancing later clinical outcomes.

METHODS

A total of 107 preterm neonates who were born at <31 weeks' postconceptional age (PCA) entered this double-blind, randomized, controlled trial within 48 hours of birth at 3 level III university-affiliated neonatal intensive care units in Canada, and 103 completed the study. Sucrose (0.1 mL of 24%) or sterile water was administered orally up to 3 times, 2 minutes apart, for every invasive procedure during a 7-day period. Motor development and vigor, and alertness and orientation components of the Neurobehavioral Assessment of the Preterm Infant were measured at 32, 36, and 40 weeks' PCA; Score for Neonatal Acute Physiology was measured on the last day of intervention; and Neuro-Biological Risk Score (NBRS) was measured at 2 weeks of age and at discharge. Primary analyses of covariance were applied for each outcome to compare group differences followed by secondary analyses using standard linear regression within each group to determine predictors of outcomes.

RESULTS

Although there were no differences between the groups on any outcomes, there were significant dose-related effects within each group. In the sucrose group only, higher number of doses of sucrose predicted lower scores on motor development and vigor, and alertness and orientation at 36 weeks', lower motor development and vigor at 40 weeks', and higher NBRS at 2 weeks' postnatal age. Higher number of invasive procedures was predictive of higher NBRS both times in the water group.

CONCLUSIONS

Repeated use of sucrose analgesia in infants <31 weeks' PCA may put infants at risk for poorer neurobehavioral development and physiologic outcomes. Additional study is needed to determine the most appropriate age and duration of sucrose analgesia in preterm infants.

Dopamine D2 receptor binding in the human brain is associated with the response to painful stimulation and pain modulatory capacity

The pain modulatory role of dopamine D2 receptors of the human forebrain was studied by determining the association between dopamine D2 receptor binding potential and the response to experimental pain. Nineteen healthy male volunteers participated in a dopamine D2 receptor positron emission tomography study. The extrastriatal regions of interest studied with [11C]FLB 457 as radioligand (n = 11) were the anterior cingulum, the medial and lateral thalamus, the medial and lateral frontal cortex, and the medial and lateral temporal cortex. The striatal regions of interest studied with [11C]raclopride (n = 8) were the caudate nucleus and the putamen. The latency to the ice water-induced cold pain threshold and tolerance were determined in a separate psychophysical test session. Moreover, the cutaneous heat pain threshold and its elevation by concurrent cold pain in the contralateral hand were determined in each subject. Cold pain threshold was inversely correlated with D2 binding potential in the right putamen and the cold pain tolerance was inversely correlated with D2 binding potential in the right medial temporal cortex. The magnitude of heat pain threshold elevation induced by concurrent cold pain was directly correlated with D2 binding potential in the left putamen. Other correlations of D2 binding potentials in varying brain regions with sensory responses were not significant. A psychophysical control study (n = 10) showed that cold pain responses were identical in the right and left hand. The results indicate that dopamine D2 receptor binding potential in the human forebrain, particularly in the striatum, may be an important parameter in determining the individual cold pain response and the potential for central pain modulation. Accordingly, an individual with only few available D2 receptors in the forebrain is likely to have a high tonic level of pain suppression, combined with a low capacity to recruit more (dopaminergic) central pain inhibition by noxious conditioning stimulation.

CREB activity in the nucleus accumbens shell controls gating of behavioral responses to emotional stimuli

The transcription factor cAMP response element (CRE)-binding protein (CREB) has been shown to regulate neural plasticity. Drugs of abuse activate CREB in the nucleus accumbens, an important part of the brain's reward pathways, and local manipulations of CREB activity have been shown to affect cocaine reward, suggesting an active role of CREB in adaptive processes that follow exposure to drugs of abuse. Using CRE-LacZ reporter mice, we show that not only rewarding stimuli such as morphine, but also aversive stimuli such as stress, activate CRE-mediated transcription in the nucleus accumbens shell. Using viral-mediated gene transfer to locally alter the activity of CREB, we show that this manipulation affects morphine reward, as well as the preference for sucrose, a more natural reward. We then show that local changes in CREB activity induce a more general syndrome, by altering reactions to anxiogenic, aversive, and nociceptive stimuli as well. Increased CREB activity in the nucleus accumbens shell decreases an animal's responses to each of these stimuli, whereas decreased CREB activity induces an opposite phenotype. These results show that environmental stimuli regulate CRE-mediated transcription within the nucleus accumbens shell, and that changes in CREB activity within this brain area subsequently alter gating between emotional stimuli and their behavioral responses. This control appears to be independent of the intrinsic appetitive or aversive value of the stimulus. The potential relevance of these data to addiction and mood disorders is discussed.

Additional evidence for the involvement of the basal ganglia in formalin-induced nociception: the role of the nucleus accumbens

Identification of the brain areas that contribute to pain is an essential undertaking towards understanding persistent pain. Areas of the basal ganglia have been proposed to play important roles in nociception as previous studies have determined the involvement of the substantia nigra pars compacta and the dorsolateral striatum in pain. The purpose of the present study was therefore to expand upon these findings by determining the involvement of other areas of the basal ganglia such as the nucleus accumbens shell and core in formalin-induced nociception. It was found that injection of a local anaesthetic (bupivacaine) into the nucleus accumbens shell had no effect on formalin-induced nociception. However, injection into the nucleus accumbens core enhanced formalin-induced nociception. These results implicate the nucleus accumbens in the processing of pain and provide additional evidence for the involvement of the basal ganglia and possibly dopamine in pain.

Anti-nociceptive effect of neuropeptide Y in the nucleus accumbens of rats: an involvement of opioid receptors in the effect

The present study investigated the effect of neuropeptide Y on nociception in the nucleus accumbens of rats. Intra-nucleus accumbens administration of neuropeptide Y induced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. There were no significant changes in the HWL to both stimulation during 60 min after the administration of NPY to outside of the nucleus accumbens. The anti-nociceptive effect of NPY was blocked by subsequent intra-nucleus accumbens injection of the Y1 receptor antagonist neuropeptide Y 28-36, indicating that Y1 receptor is involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens. Furthermore, the anti-nociceptive effect of neuropeptide Y was attenuated by intra-nucleus accumbens administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats. Moreover, the neuropeptide Y-induced anti-nociception was attenuated by following intra-nucleus accumbens injection of the selective opioid antagonists nor-binaltorphimine and beta-funaltrexamine, but not by naltrindole, illustrating that mu- and kappa-opioid receptors, not the delta-opioid receptor, were involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats.

The role of substance P in depression: therapeutic implications

Substance P (for "powder"), identified as a gut tachykinin in 1931 and involved in the control of multiple other autonomic functions, notably pain transmission, is the focus of intense fundamental and clinical psychiatric research as a central neurotransmitter, neuromodulator, and immunomodulator, along with sister neurokinins A and B (NKA and NKB), discovered in 1984. Substance P is widely distributed throughout the central nervous system, where if is often colocalized with serotonin, norepinephrine, and dopamine. Many neurokinin (NK) receptor antagonists and agonists have been synthesized and some clinically tested. A double-blind study of MK869, a selective NK1 receptor antagonist that blocks the action of substance P, showed significant activity versus placebo and fewer sexual side effects than paroxetine in outpatients with major depression and moderate anxiety. Substance P, which is degraded by the angiotensin-converting enzyme (ACE), may mediate modulation of therapeutic outcome in affective disorders by functional polymorphism within the ACE gene: the D allele is associated with higher ACE levels and increased neuropeptide degradation, with the result that patients with major depression who carry the D allele have lower depression scores and shorter hospitalization. ACE polymorphism genotypinq might thus identify those patients with major depression likely to benefit from NK1 receptor antagonist therapy.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Olanzapine in the management of cancer pain

In cancer patients, cognitive impairment, psychological distress, and anxiety may accompany and aggravate pain. Neuroleptics are frequently used to control these symptoms and may be used to treat pain that has been unresponsive to more conventional approaches. Because of prominent side effects of traditional neuroleptics and conflicting data regarding their analgesic efficacy, their use in the treatment of pain remains controversial. Olanzapine, an atypical neuroleptic, might offer advantages because of its safer side effect profile. It has also been shown to have an independent antinociceptive activity in animals. The use of olanzapine in the management of cancer pain has not been previously described. We prospectively collected the data on 8 cancer patients with severe pain, uncontrolled in spite of aggressive opioid titration, who received olanzapine to treat severe anxiety and mild cognitive impairment. Patients did not meet criteria for delirium and their cognitive impairment was defined as cognitive disorder not otherwise specified (NOS) according to DSM-IV. Patients received 2.5 to 7.5 mg of olanzapine daily. In all patients, opioid requirements had escalated rapidly prior to starting olanzapine. Levels of pain, sedation, and opioid use were measured 2 days before and 2 days after olanzapine was started. Cognitive state was assessed daily. All 8 patients had marked reduction of the daily pain scores. The average daily opioid use decreased significantly in all patients. Cognitive impairment and anxiety resolved within 24 hours of initiating olanzapine. In these 8 patients, decreased pain scores and opioid requirements may have resulted from improvement in cognitive function and the known anxiolytic effect of olanzapine. Other mechanisms may include independent or adjuvant analgesic effects of olanzapine. We conclude that olanzapine may be useful in the treatment of patients with uncontrolled cancer pain associated with cognitive impairment or anxiety. Further studies to evaluate possible analgesic effect of olanzapine are needed.

Nicotine withdrawal hyperalgesia and opioid-mediated analgesia depend on nicotine receptors in nucleus accumbens

The nucleus accumbens, as part of the mesolimbic dopaminergic reward pathway, mediates both addiction to and withdrawal from substances of abuse. In addition, activity of substances of abuse such as opioids in the nucleus accumbens has been implicated in pain modulation. Because nucleus accumbens nicotinic receptors are important in nicotine addiction and because nicotinic activity can interact with opioid action, we investigated the contribution of nucleus accumbens nicotinic receptors to opioid-mediated analgesia/antinociception. The response of the nociceptive jaw-opening reflex to opioids was studied in the rat, both before and during chronic nicotine exposure. In nicotine-naive rats, intra-accumbens injection of the nicotinic receptor antagonist mecamylamine blocked antinociception produced by either systemic morphine, intra-accumbens co-administration of a mu- and a delta-opioid receptor agonist, or noxious stimulation (i.e., subdermal capsaicin in the hindpaw); intra-accumbens mecamylamine alone had no effect. The antinociceptive effect of either morphine or noxious stimulation was unchanged during nicotine tolerance; however, intra-accumbens mecamylamine lost its ability to block antinociception produced by either treatment. Intra-accumbens mecamylamine by itself precipitated significant hyperalgesia in nicotine-tolerant rats which could be suppressed by noxious stimulation as well as by morphine. These results indicate that nucleus accumbens nicotinic receptors play an important role in both opioid- and noxious stimulus-induced antinociception in nicotine-naive rats. This role was attenuated in the nicotine-dependent state. The suppression of withdrawal hyperalgesia by noxious stimulation suggests that pain can ameliorate the symptoms of withdrawal, thus suggesting a possible mechanism for pain-seeking behavior.

Antinociceptive interaction between spinal clonidine and lidocaine in the rat formalin test: an isobolographic analysis

Clinical and basic science studies suggest that spinal alpha-2-adrenergic receptor agonists and local anesthetics produce analgesia, but interaction between alpha-2-adrenergic receptor agonists and local anesthetics in the persistent pain model has not been examined. In the present study, using isobolographic analysis, we investigated the antinociceptive interaction of intrathecal clonidine and lidocaine in the rat formalin test. Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters, and were tested for paw flinch by formalin injection. Biphasic painful behavior was counted. Intrathecal clonidine (3-12 nmol) was administered 15 min before formalin, and intrathecal lidocaine (375-1850 nmol) was administered 5 min before formalin. To examine the interaction of intrathecal clonidine and lidocaine, an isobolographic design was used. Spinal administration of clonidine produced dose-dependent suppression of the biphasic responses in the formalin test. Spinal lidocaine resulted in dose-dependent transient motor dysfunction and the motor dysfunction recovered to normal at 10-15 min after administration. Spinal lidocaine produced dose-dependent suppression of phase-2 activity in the formalin test. Isobolographic analysis showed that the combination of intrathecal clonidine and lidocaine synergistically reduced Phase-2 activity. We conclude that intrathecal clonidine synergistically interacts with lidocaine in reducing the nociceptive response in the formalin test.

IMPLICATIONS

Preformalin administration of intrathecal clonidine and lidocaine dose-dependently produced antinociception in the formalin test. The combination of clonidine and lidocaine, synergistically produced suppression of nociceptive response in the persistent pain model.

Neuropeptide FF attenuates allodynia in models of chronic inflammation and neuropathy following intrathecal or intracerebroventricular administration

Experiments were conducted to explore the effects of Neuropeptide FF acting at spinal and supraspinal sites in models of chronic inflammatory or neuropathic pain and of acute pain. Neuropeptide FF was administered intrathecally (i.t.; 10.0, 25.0 and 50.0 nmol) or intracerebroventricularly (i.c.v.; 10.0, 12.5 and 15.0 nmol) either 24 h after inflammation-inducing injections of Freund's Complete Adjuvant in one hind paw or 7 days after unilateral sciatic nerve constriction. Evoked pain was assessed by measuring the withdrawal response threshold (in grams of pressure) to a mechanical stimulus applied to the plantar surface of the injured paw. Neuropeptide FF dose-dependently attenuated the allodynic response (i.e., withdrawal from a normally innocuous stimulus) to mechanical stimulation in the inflammatory and neuropathic model following i.t. (ED50=20.86 nmol and ED50=18.91 nmol, respectively) and i.c.v. (ED50=12.31 nmol and ED50=11.68 nmol, respectively) administration. Pretreatment with naloxone (2.0 mg/kg; s.c.) attenuated the anti-allodynic effect of i.t. or i.c.v. Neuropeptide FF in rats experiencing inflammatory, but not neuropathic pain. In contrast, Neuropeptide FF administered i.t. (10.0, 25.0 and 50.0 nmol) or i.c.v. (10.0, 12.5 and 15.0 nmol) had no effect on the response to acute thermal or mechanical stimulation. Neuropeptide FF injected i.t. or i.c.v. in inflamed or neuropathic rats did not produce any sign of motor dysfunction. These results suggest that Neuropeptide FF acting at spinal and supraspinal sites plays a role in modulating chronic, but not acute pain. Furthermore, the results suggest that the anti-allodynic effect of Neuropeptide FF is mediated indirectly by naloxone-sensitive opioid mechanisms in rats subjected to inflammatory, but not neuropathic pain.

Non-opioid antinociceptive effects of supraspinal histogranin and related peptides: possible involvement of central dopamine D(2) receptor

The antinociceptive effects of intracerebroventricular (ICV) administration of histogranin (HN) and related peptides were assessed in the mouse writhing and tail-flick assays. In the writhing test, the peptides displayed dose-dependent analgesic effects with an AD(50) of 23.9 nmol/mouse for HN and the following order for other peptides: HN-(7-15)<histone H4-(86-100) approximately HN approximately HN-(7-10)<[Ser(1)]HN<osteogenic growth peptide (OGP) approximately HN-(1-10). HN-(6-9) and HN-(8-10) did not show any significant analgesic activity at 50 nmol/mouse. The importance of the C- and N-terminal amino acids in the analgesic activity of the peptides was demonstrated by the prolonged effects of HN and [Ser(1)]HN ( approximately 30 min) compared with those of HN fragments (HN-(7-15), HN-(1-10) and HN-(7-10): 5-10 min). The analgesic activity of [Ser(1)]HN (50 nmol/mouse) was not affected by the coadministration of opioid (naloxone, 1 nmol/mouse), NMDA (CPP, 0.3 and MK-801, 0.3 nmol/mouse) and D(1) (SCH-23390, 0.5 nmol/mouse) receptor antagonists, but it was significantly antagonized by the coinjection of the D(2) receptor antagonist raclopride (0.5 nmol/mouse). In the mouse tail-flick assay, HN and related peptides (50 nmol/mouse) also showed significant analgesic activity (15-35% MPE). The analgesic effect of [Ser(1)]HN was dose-dependent and, at 75 nmol/mouse, lasted for up to 45 min, and was partially blocked by the coadministration of raclopride (1 nmol/mouse), but not naloxone (2 nmol/mouse). In the mouse rotarod assay, relative high doses (75-100 nmol/mouse) of HN and related peptides did not significantly affect motor coordination. These results indicate that supraspinal administration of HN and related peptides induce significant non-opioid analgesic effects devoid of motor activity by a mechanism that involves the participation of central dopamine D(2) receptors.

Electroacupuncture potentiates the antinociceptive effect of intrathecal endomorphin-1 in the rat formalin test

Endomorphin-1 is a novel endogenous mu-opioid peptide. In this study, we examined the effects of 2 Hz electroacupuncture in the rat tail flick test and the formalin test (a persistent noxious model). Moreover, we investigated if the electroacupuncture potentiated the effect of intrathecal endomorphin-1. The results demonstrated that electroacupuncture alone produced a significant antinociception in the tail flick test, but not in the formalin test, and that intrathecal endomorphin-1 dose-dependently suppressed the biphasic nociceptive behavior in the formalin test. Electroacupuncture enhanced the antinociceptive effect of intrathecal endomorphin-1 in the formalin test, resulting in a significant leftward shift in the dose-response curves for intrathecal endomorphin-1 antinociception. The enhanced effect was antagonized by intraperitoneal naltrexone. The study suggests that electroacupuncture may potentiate the intrathecal endomorphin-1 antinociception partially mediated by opioid receptors.