Convergent, not serial, striatal and pallidal circuits regulate opioid-induced food intake.

Mu opioid receptor (MOR) signaling in the nucleus accumbens (NAcc) elicits marked increases in the consumption of palatable tastants. However, the mechanism and circuitry underlying this effect are not fully understood. Multiple downstream target regions have been implicated in mediating this effect but the role of the ventral pallidum (VP), a primary target of NAcc efferents, has not been well defined. To probe the mechanisms underlying increased consumption, we identified behavioral changes in rats' licking patterns following NAcc MOR stimulation. Because the temporal structure of licking reflects the physiological substrates modulating consumption, these measures provide a useful tool in dissecting the cause of increased consumption following NAcc MOR stimulation. Next, we used a combination of pharmacological inactivation and lesions to define the role of the VP in hyperphagia following infusion of the MOR-specific agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) into the NAcc. In agreement with previous studies, results from lick microstructure analysis suggest that NAcc MOR stimulation augments intake through a palatability-driven mechanism. Our results also demonstrate an important role for the VP in normal feeding behavior: pharmacological inactivation of the VP suppresses baseline and NAcc DAMGO-induced consumption. However, this interaction does not occur through a serial circuit requiring direct projections from the NAcc to the VP. Rather, our results indicate that NAcc and VP circuits converge on a common downstream target that regulates food intake.

Contributions of the amygdala and medial prefrontal cortex to incentive cue responding.

Reward-seeking behavior is controlled by neuronal circuits that include the basolateral nucleus of amygdala (BLA), medial prefrontal cortex (mPFC), nucleus accumbens (NAc) and ventral tegmental area. Using a discriminative stimulus (DS) task in which an intermittently presented cue (DS) directs rats to make an operant response for sucrose, we previously demonstrated that dopamine receptor antagonism in the NAc reduced reinforced cue responding, whereas general inactivation of the NAc increased behavioral responding in the absence of the cue. Because they send major glutamatergic projections to the NAc, the BLA and mPFC may also contribute to reward-seeking behaviors modulated by the NAc. In this study we compare the effects of BLA and mPFC inactivation on rats' performance of a DS task. BLA inactivation by combined GABA(A) and GABA(B) agonists impaired cue responding with minimal effects on operant behavior in the absence of cues. Dorsal medial prefrontal cortex (dmPFC) inactivation also inhibited cue-evoked reward-seeking. In contrast, ventral medial prefrontal cortex (vmPFC) inactivation disinhibited responding to unrewarded cues with less influence on reinforced cue responding. These findings demonstrate that the BLA and dmPFC facilitate cue-evoked reward-seeking, whereas, in the same task the vmPFC exerts inhibitory control over unrewarded behaviors.

Opposing effects of intra-nucleus accumbens mu and kappa opioid agonists on sensory specific satiety.

Mu opioid (MOP) agonists acting in the nucleus accumbens (NAcc) robustly enhance consumption of palatable foods. In addition, the effect on consumption of palatable foods produced by MOP agonists acting in the NAcc depends on both recent flavor exposure and the availability of a choice between different-flavored foods. In contrast, kappa opioid (KOP) agonists have variable effects on feeding and KOP agonists have MOP opposing behavioral actions when microinjected at several brain sites. We previously demonstrated that NAcc MOP agonists reverse the devaluation (satiety) effect of pre-feeding for a given flavor; in fact, NAcc MOP agonists selectively increase consumption of a recently sampled food. In contrast, in the present study, we found that the selective KOP agonist U50488 injected into the NAcc of rats reduced consumption of a recently sampled flavor while increasing consumption of the flavor that was not pre-fed. Intra-NAcc U50488 did not affect overall consumption or flavor preference in the absence of pre-feeding. The present data, in conjunction with our previous findings, highlight the robust and opposing role of NAcc MOP and KOP opioid receptors in palatability-based food choice and consumption and raise the possibility that an endogenous KOP agonist acting in the NAcc contributes to the phenomenon of sensory specific satiety.

Nucleus accumbens opioid signaling conditions short-term flavor preferences.

Opioid signaling in the nucleus accumbens (NAcc) strongly modulates flavor-based food choice. To further investigate the role of opioid signaling in taste reward, we used a sensory specific satiety (SSS) paradigm to devalue specific flavors of nutritionally identical food pellets in rats. In the NAcc, infusion of a mu opioid (MOP) receptor selective agonist selectively increased consumption of a pre-fed flavor, thus reversing the SSS effect. Conversely, blockade of endogenous opioid signaling with the opioid antagonist naltrexone selectively decreased consumption of a recently consumed flavor, potentiating the SSS effect. No enhancement of consumption was observed if a delay of 3 h was imposed following the intra-NAcc MOP agonist indicating that there were no long-term changes in flavor preference. If a delay was introduced between the initial flavor exposure and the intra-NAcc MOP agonist infusion, pellet consumption was increased non-selectively (irrespective of flavor) suggesting that close temporal contiguity between flavor experience and NAcc opioid action is critical for the opioid effect on flavor preference. In contrast to opioid effects, inactivating NAcc neurons by local microinjection of muscimol (a GABAA agonist) increased consumption of both the pre-fed and non-pre-fed flavors equally. These results demonstrate that opioids released in the NAcc during consumption of palatable foods produce a selective and transient increase in preference for a recently sampled flavor.

Nucleus accumbens opioids regulate flavor-based preferences in food consumption.

Opioid signaling in the nucleus accumbens (NAcc) regulates feeding behavior, having particularly strong effects on consumption of highly palatable foods. Since macronutrient content may contribute to palatability, it is uncertain whether opioid regulation of food consumption is based primarily on its macronutrient content or its flavor per se. In order to isolate the effect of flavor, we manipulated opioid signaling in the NAcc in rats and quantified consumption of differently flavored but nutritionally identical pellets. When pellets of either flavor were presented alone, microinjection of d-Ala(2),N,Me-Phe(4),Gly-ol(5)-enkephalin (DAMGO (a mu opioid receptor (MOP) agonist)) into the NAcc increased consumption of pellets of both flavors equally. When both flavors of pellets were presented simultaneously, however, DAMGO in the NAcc selectively increased, while naltrexone (a non-selective opioid antagonist) in the NAcc selectively decreased, consumption of the more preferred flavor. Systemic naltrexone injection had no flavor specific effects, decreasing consumption of both flavors equally. Non-selective inactivation of NAcc neurons by local microinjection of muscimol (a GABA(A) agonist) increased consumption of both the more- and less-preferred flavors equally. These results indicate that opioid signaling directly regulates a subset of NAcc neurons that can selectively enhance consumption of preferred palatable foods based exclusively on flavor cues.

Nucleus accumbens dopamine release is necessary and sufficient to promote the behavioral response to reward-predictive cues.

The nucleus accumbens is part of the neural circuit that controls reward-seeking in response to reward-predictive cues. Dopamine release in the accumbens is essential for the normal functioning of this circuit. Previous studies have shown that injection of dopamine receptor antagonists into the accumbens severely impairs an animal's ability to perform operant behaviors specified by predictive cues. Furthermore, excitations and inhibitions of accumbens neurons evoked by such cues are abolished by inactivation of the ventral tegmental area, the major dopaminergic input to the accumbens. These results indicate that dopamine is necessary to elicit neural activity in the accumbens that drives the behavioral response to cues. Here we show that accumbens dopamine release is causal to the rats' reward-seeking behavioral response by demonstrating that dopamine in this structure is both necessary and sufficient to promote the appropriate behavioral response to reward-predictive cues.

Kappa opioids inhibit physiologically identified medullary pain modulating neurons and reduce morphine antinociception.

Microinjection of kappa opioid receptor (KOR) agonists into the rostral ventromedial medulla (RVM) attenuates mu-opioid receptor mediated antinociception and stress-induced analgesia, yet is also reported to have an analgesic effect. To determine how KOR agonists produce both antinociceptive and antianalgesic actions within the RVM, the KOR agonist U69593 was microinjected directly into the RVM while concurrently monitoring tail flick latencies and RVM neuronal activity. Among RVM neurons recorded in vivo, two types show robust changes in activity just prior to the nocifensive tail flick reflex: ON cells burst just prior to a tail flick and their activity is pronociceptive, whereas OFF cells pause just prior to the tail flick and their activity is antinociceptive. Although RVM microinjection of U69593 did not affect tail flick latencies on its own, it did attenuate the on cell burst, an effect blocked by co-injection of the KOR antagonist, nor-binaltorphimine (nor-BNI). Furthermore, U69593 inhibited ongoing activity in subsets of OFF cells (4/11) and NEUTRAL cells (3/9). Microinjection of U69593 into the RVM also attenuated morphine antinociception and suppressed the excitation of off cells. Together with previous in vivo and in vitro studies, these results are consistent with the idea that KOR agonists can be either pronociceptive through direct inhibition of OFF cells, or antianalgesic through both postsynaptic inhibition and presynaptic inhibition of glutamate inputs to RVM OFF cells.

Basolateral amygdala lesions impair both cue- and cocaine-induced reinstatement in animals trained on a discriminative stimulus task.

Drug-associated environmental cues can maintain drug use and contribute to relapse even after long periods of abstinence. We investigated the ability of sensory stimuli that signaled periods of reward availability to sustain cocaine self-administration and trigger the reinstatement of reward-seeking behavior. We demonstrate that lesions of the basolateral amygdala (BLA), a structure strongly implicated in attributing salience to environmental stimuli, significantly reduced the power of predictive cues to elicit reward-seeking behavior. In daily training sessions, a 20 s discriminative stimulus (DS) was presented to rats on a variable interval schedule. If five lever presses were recorded during the DS-on period, then cocaine (0.5 mg/kg) and a conditioned stimulus (CS) were simultaneously delivered. After training, half the animals received excitotoxic lesions of the BLA with quinolinic acid; the other half received saline. Compared with sham-lesioned animals, rats with BLA lesions earned fewer cocaine injections and were less accurate in responding to the DS in the first few days following the lesion. However, they maintained the same cocaine intake as sham-lesioned animals when the DS requirement was lifted. Finally, after seven extinction sessions, reinstatement was measured in response to: 1) i.v. cocaine infusion, 2) DS, 3) CS, 4) a familiar, but non-rewarded cue (S-) or 5) no stimulus. In sham-lesioned animals, cocaine and the DS, but not the CS or the S-, triggered reinstatement. BLA lesions abolished DS-induced reinstatement and significantly attenuated cocaine-induced reinstatement. These results demonstrate 1) that when tested under the same conditions, a discriminative cue which signals reward availability is a more robust trigger of reward-seeking than a Pavlovian CS which signals reward delivery and 2) that the BLA contributes to reinstatement in response to these discriminative cues.

Roles of alpha1- and alpha2-adrenoceptors in the nucleus raphe magnus in opioid analgesia and opioid abstinence-induced hyperalgesia.

Noradrenaline and alpha-adrenoceptors have been implicated in the modulation of pain in various behavioral conditions. Noradrenergic neurons and synaptic inputs are present in neuronal circuits critical for pain modulation, but their actions on neurons in those circuits and consequently the mechanisms underlying noradrenergic modulation of pain remain unclear. In this study, both recordings in vitro and behavioral analyses in vivo were used to examine cellular and behavioral actions mediated by alpha1- and alpha2-adrenoceptors on neurons in the nucleus raphe magnus. We found that alpha1- and alpha2-receptors were colocalized in the majority of a class of neurons (primary cells) that inhibit spinal pain transmission and are excited during opioid analgesia. Activation of the alpha1-receptor depolarized whereas alpha2-receptor activation hyperpolarized these neurons through a decrease and an increase, respectively, in potassium conductance. Blockade of the excitatory alpha1-receptor or activation of the inhibitory alpha2-receptor significantly attenuated the analgesia induced by local opioid application, suggesting that alpha1-receptor-mediated synaptic inputs in these primary cells contribute to their excitation during opioid analgesia. In the other cell class (secondary cells) that is thought to facilitate spinal nociception and is inhibited by analgesic opioids, only alpha1-receptors were present. Blocking the alpha1-receptor in these cells significantly reduced the hyperalgesia (increased pain) induced by opioid abstinence. Thus, state-dependent activation of alpha1-mediated synaptic inputs onto functionally distinct populations of medullary pain-modulating neurons contributes to opioid-induced analgesia and opioid withdrawal-induced hyperalgesia.

The affective component of pain in rodents: direct evidence for a contribution of the anterior cingulate cortex.

Numerous human and animal studies indirectly implicate neurons in the anterior cingulate cortex (ACC) in the encoding of the affective consequences of nociceptor stimulation. No causal evidence, however, has been put forth linking the ACC specifically to this function. Using a rodent pain assay that combines the hind-paw formalin model with the place-conditioning paradigm, we measured a learned behavior that directly reflects the affective component of pain in the rat (formalin-induced conditioned place avoidance) concomitantly with "acute" formalin-induced nociceptive behaviors (paw lifting, licking, and flinching) that reflect the intensity and localization of the nociceptive stimulus. Destruction of neurons originating from the rostral, but not caudal, ACC reduced formalin-induced conditioned place avoidance without reducing acute pain-related behaviors. These results provide evidence indicating that neurons in the ACC are necessary for the "aversiveness" of nociceptor stimulation.

Kappa opioid receptor inhibition of glutamatergic transmission in the nucleus accumbens shell.

Microinjection of kappa-opioid receptor agonists into the nucleus accumbens produces conditioned place aversion. While attention has focused primarily on the inhibition of dopamine release by kappa-receptor agonists as the synaptic mechanism underlying this effect, recent anatomical studies have raised the possibility that regulation of noncatecholaminergic transmission also contribute. We have investigated the effects of kappa-receptor activation on fast excitatory synaptic transmission in an in vitro slice preparation using whole cell voltage-clamp or extracellular field recordings in the shell region of the nucleus accumbens. The kappa-receptor agonist U69593 produces a pronounced, dose-dependent inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) that can be reversed by 100 nM nor-BNI. Furthermore, U69593 causes an increase in the paired-pulse ratio as well as a decrease in the frequency of spontaneous miniature events, suggesting a presynaptic site of action. Despite anatomical evidence for kappa-receptor localization on dendritic spines of nucleus accumbens neurons, no electrophysiological evidence of a postsynaptic effect was found. This presynaptic inhibition of excitatory synaptic transmission in the nucleus accumbens shell provides a novel mechanism that may contribute to the kappa-receptor-mediated aversion observed in intact animals.

Bi-directional changes in affective state elicited by manipulation of medullary pain-modulatory circuitry.

The rostral ventromedial medulla contains three physiologically defined classes of pain-modulating neuron that project to the spinal and trigeminal dorsal horns. OFF cells contribute to anti-nociceptive processes, ON cells contribute to pro-nociceptive processes (i.e. hyperalgesia) and neutral cells tonically modulate spinal nociceptive responsiveness. In the setting of noxious peripheral input, the different cell classes in this region permit bi-directional modulation of pain perception (analgesia vs hyperalgesia). It is unclear, however, whether changes in the activity of these neurons are relevant to the behaving animal in the absence of a painful stimulus. Here, we pharmacologically manipulated neurons in the rostral ventromedial medulla and used the place-conditioning paradigm to assess changes in the affective state of the animal. Local microinjection of the alpha(1)-adrenoceptor agonist methoxamine (50.0 microg in 0.5 microl; to activate ON cells, primarily), combined with local microinjection of the kappa-opioid receptor agonist U69,593 (0.178 microg in 0.5 microl; to inhibit OFF cells), produced an increase in spinal nociceptive reactivity (i.e. hyperalgesia on the tail flick assay) and a negative affective state (as inferred from the production of conditioned place avoidance) in the conscious, freely moving rat. Additional microinjection experiments using various concentrations of methoxamine alone or U69, 593 alone revealed that the rostral ventromedial medulla is capable of eliciting a range of affective changes resulting in conditioned place avoidance, no place-conditioning effect or conditioned place preference (reflecting production of a positive affective state). Overall, however, there was no consistent relationship between place-conditioning effects and changes in spinal nociceptive reactivity. This is the first report of bi-directional changes in affective state (i.e. reward or aversion production) associated with pharmacological manipulation of a brain region traditionally associated with bi-directional pain modulation. We conclude that, in addition to its well-described pain-modulating effects, the rostral ventromedial medulla is capable of modifying animal behavior in the absence of a painful stimulus by bi-directionally influencing the animal's affective state.

A cellular mechanism for the bidirectional pain-modulating actions of orphanin FQ/nociceptin.

Orphanin FQ/nociceptin (OFQ/N) and its receptor share substantial structural features and cellular actions with classic opioid peptides and receptors, but have distinct pharmacological profiles and behavioral effects. Currently there is an active debate about whether OFQ/N produces hyperalgesia or analgesia. Using a well-defined brainstem pain-modulating circuit, we show that OFQ/N can cause either an apparent hyperalgesia by antagonizing mu opioid-induced analgesia or a net analgesic effect by reducing the hyperalgesia during opioid abstinence. It presumably produces these two opposite actions by inhibiting two distinct groups of neurons whose activation mediates the two effects of opioid administration. OFQ/N antagonism of the hyperalgesia may have significance for the treatment of opioid withdrawal and sensitized pain.

Pain modulation: expectation, opioid analgesia and virtual pain.

To summarize, although there are multiple potential target nuclei for modulating pain transmission and several candidate efferent pathways that exert modulatory control, the most completely described pain modulating circuit includes the amygdala, PAG, DLPT and RVM in the brainstem. Through descending projections, this circuit controls both spinal and trigeminal dorsal horn pain transmission neurons and mediates both opioid and stimulation produced analgesia. Several different neurotransmitters are involved in the modulatory actions of this circuit, which exerts bi-directional control of pain through On cells that facilitate and Off cells that inhibit dorsal horn nociceptive neurons. There is evidence that this circuit contributes to analgesia in humans and may be activated by acute stress or the expectation of relief. Conversely, through the facilitating effect of On cells, this circuit is theoretically capable of generating or enhancing perceived pain intensity. Such an effect could provide a physiological mechanism for the pain enhancing actions of mood, attention and expectation.

Brainstem pain modulating circuitry is sexually dimorphic with respect to mu and kappa opioid receptor function.

We have previously shown that activation of kappa opioid receptors within the rostral ventral medulla in lightly anesthetized rats has an anti-mu opioid analgesic action in male rats. Microinjections of the kappa opioid receptor agonist, U69593, attenuated the increase in tail-flick latency produced by activation of mu opioid receptors located within the ventrolateral periaqueductal gray. There are sex differences in the pain modulating potency of opioid analgesics, including kappa opioid agonists. In the present study, we examined whether activation of kappa opioid receptors within the rostral ventral medulla in lightly anesthetized female rats produces an anti-mu opioid analgesic effect similar to that found in males. We found that in the RVM the same dose of kappa opioid receptor agonist that reduces mu receptor-mediated increase in tail-flick latency in male rats produces a moderate increase in tail-flick latency in female rats. Additionally, we discovered that female rats are significantly more sensitive to the mu opioid agonist, DAMGO, injected into the ventrolateral periaqueductal gray. The results indicate that these two brain structures, which mediate the analgesic effects of opioids, are sexually dimorphic with regard to opioid receptor function.

Delta opioid receptor mediated actions in the rostral ventromedial medulla on tail flick latency and nociceptive modulatory neurons.

The rostral ventromedial medulla (RVM) is critical for the modulation of dorsal horn nociceptive transmission. Three classes of RVM neurons (ON, OFF, and NEUTRAL) have been described that have distinct responses to noxious stimuli and mu opioid receptor (MOR) agonists. The present study in barbiturate anesthetized rats investigated the effects of the delta 2 opioid receptor (DOR2) agonist, [D-Ala2]deltorphin II (DELT), microinfused into the RVM on the tail flick reflex and activity of RVM neurons. Tail flick latencies increased dose-dependently after administration of DELT (0.6 nmol and 1.2 nmol). Furthermore, DELT inhibited the tail flick related increase in ON cell activity and shortened the tail flick related pause in OFF cell activity. The activity of NEUTRAL cells was not affected. The antinociceptive effects and corresponding changes in ON and OFF cell activity produced by DELT were antagonized by the DOR2 antagonist, naltriben methanesulfonate, administered at the same site. These DOR2 mediated effects on noxious stimulation-evoked changes in RVM neuronal activity are similar to those reported for MOR agonists and suggest that both DOR2 and MOR produce analgesia through activation of OFF cells.

A locus and mechanism of action for associative morphine tolerance.

Repeated administration of an opioid in the presence of specific environmental cues can induce tolerance specific to that setting (associative tolerance). Prolonged or repeated administration of an opioid without consistent contextual pairing yields non-associative tolerance. Here we demonstrate that cholecystokinin acting at the cholecystokinin-B receptor is required for associative but not non-associative morphine tolerance. Morphine given in the morphine-associated context increased Fos-like immunoreactivity in the lateral amygdala and hippocampal area CA1. Microinjection of the cholecystokinin B antagonist L-365,260 into the amygdala blocked associative tolerance. These results indicate that cholecystokinin acting in the amygdala is necessary for associative tolerance to morphine's analgesic effect.

Highly delta selective antagonists in the RVM attenuate the antinociceptive effect of PAG DAMGO.

The present study tested the hypothesis that endogenous opioid peptides acting at the delta-opioid receptor (DOR) in the rostral ventromedial medulla (RVM) contribute to the antinociception elicited by the mu-opioid receptor (MOR) agonist DAMGO in the midbrain periaqueductal gray (PAG). Following microinjection of DAMGO into the PAG, either the highly selective DOR antagonist TIPP[psi] or the DOR2 antagonist naltriben (NTB) was microinjected into the RVM. Both TIPP[psi] (1.0 microg) and NTB (5.0 ng) significantly attenuated the analgesic effect of PAG DAMGO but had no effect when given before PAG saline. These results confirm and extend previous studies suggesting that PAG mu-opioids activate a descending system with a DOR mediated endogenous opioid link in the RVM.

Causalgia and reflex sympathetic dystrophy: does the sympathetic nervous system contribute to the generation of pain?

The striking response of causalgia and reflex sympathetic dystrophy (RSD) to sympatholytic procedures together with signs of autonomic nervous system abnormalities suggest that the sympathetic efferent system can generate or enhance pain (sympathetically maintained pain, SMP). This concept is supported by human and animal experiments indicating that sympathetic activity and catecholamines can activate primary afferent nociceptors. Some clinical evidence, however, calls the SMP concept into question and alternative explanations have been advanced. In this review, we describe the clinical features of causalgia and RSD and the evidence for sympatholytic efficacy. The major barrier to proving the SMP concept is that all available sympatholytic procedures are problematic. We conclude that, although the weight of current evidence supports the SMP concept and its relevance to causalgia and RSD, it remains unproven by scientific criteria. More careful adherence to diagnostic criteria and well-controlled trials of sympatholysis are needed to finally settle the issue.