Neuropharmacology of the nucleus accumbens: systemic morphine effects on single-unit responses evoked by ventral pallidum stimulation

Circuit directionality for motivation: Lateral accumbens-pallidum, but not pallidum-accumbens, connections regulate motivational attraction to reward cues

Sign-tracking behavior, in which animals interact with a cue that predicts reward, provides an example of how incentive salience can be attributed to cues and elicit motivation. The nucleus accumbens (NAc) and ventral pallidum (VP) are two regions involved in cue-driven motivation. The VP, and NAc subregions including the medial shell and core, are critical for sign-tracking. Further, connections between the medial shell and VP are known to participate in sign-tracking and other motivated behaviors. The NAc lateral shell (NAcLSh) is a distinct and understudied subdivision of the NAc, and its contribution to the process by which reward cues acquire value remains unclear. The NAcLSh has been implicated in reward-directed behavior, and has reciprocal connections with the VP, suggesting that NAcLSh and VP interactions could be important mechanisms for incentive salience. Here, we use DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) and an intersectional viral delivery strategy to produce a biased inhibition of NAcLSh neurons projecting to the VP, and vice versa. We find that disruption of connections from NAcLSh to VP reduces sign-tracking behavior while not affecting consumption of food rewards. In contrast, VP to NAcLSh disruption affected neither sign-tracking nor reward consumption, but did produce a greater shift in animals' behavior more towards the reward source when it was available. These findings indicate that the NAcLSh → VP pathway plays an important role in guiding animals towards reward cues, while VP → NAcLSh back-projections may not and may instead bias motivated behavior towards rewards.

Ventral pallidum roles in reward and motivation

In recent years the ventral pallidum has become a focus of great research interest as a mechanism of reward and incentive motivation. As a major output for limbic signals, the ventral pallidum was once associated primarily with motor functions rather than regarded as a reward structure in its own right. However, ample evidence now suggests that ventral pallidum function is a major mechanism of reward in the brain. We review data indicating that (1) an intact ventral pallidum is necessary for normal reward and motivation, (2) stimulated activation of ventral pallidum is sufficient to cause reward and motivation enhancements, and (3) activation patterns in ventral pallidum neurons specifically encode reward and motivation signals via phasic bursts of excitation to incentive and hedonic stimuli. We conclude that the ventral pallidum may serve as an important 'limbic final common pathway' for mesocorticolimbic processing of many rewards.

Opioid limbic circuit for reward: interaction between hedonic hotspots of nucleus accumbens and ventral pallidum

Mu-opioid stimulation of cubic millimeter hedonic hotspots in either the nucleus accumbens shell (NAc) or the ventral pallidum (VP) amplifies hedonic "liking" reactions to sweetness and appetitive "wanting" for food reward. How do these two NAc-VP hotspots interact? To probe their interaction and limbic circuit properties, we assessed whether opioid activation of one hotspot recruited the other hotspot (neurobiologically) and whether opioid hedonic and incentive motivational amplification by either opioid hotspot required permissive opioid coactivation in the other (behaviorally). We found that NAc and VP hotspots reciprocally modulated Fos expression in each other and that the two hotspots were needed together to enhance sucrose "liking" reactions, essentially cooperating within a single hedonic NAc-VP circuit. In contrast, the NAc hotspot dominated for opioid stimulation of eating and food intake ("wanting"), independent of VP activation. This pattern reveals differences between limbic opioid circuits that control reward "liking" and "wanting" functions.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Effects of bilateral electrical stimulation of the ventral pallidum on acoustic startle

The ventral pallidum (VP) is believed to occupy a critical position between the limbic and the motor systems, for transferring motive information into motor commands. To estimate the time course of signaling from the VP to motor outputs, in the present study we examined the effects of bilateral electrical stimulation of the VP on the acoustic startle reflex in awake rats. When the interstimulus interval (ISI) between VP stimulation and acoustic stimulation was shorter than 5 ms, VP stimulation potentiated acoustic startle. When the ISI was longer than 5 ms, VP stimulation inhibited acoustic startle over a large range of ISIs with the maximum inhibition at ISIs between 15 and 25 ms. In contrast, bilateral electrical stimulation of the amygdala did not have a significant inhibitory effect on acoustic startle, but strongly augmented acoustic startle at shorter ISIs (0-10 ms). Compared to unilateral electrical stimulation of the inferior colliculus (IC), bilateral stimulation of the VP gave rise to a rightward shift of the ISI curve, indicating that the neural pathways conveying the inhibitory influence from the VP to the acoustic startle circuit are longer than those from the IC.

Opioid modulation of ventral pallidal inputs

While the ventral pallidum (VP) is known to be important in relaying information between the nucleus accumbens and target structures, it has become clear that substantial information processing occurs within the VP. We evaluated the possibility that opioid modulation of other transmitters contained in VP afferents is involved in this process. Initially, we demonstrated that opioids hyperpolarized VP neurons in vitro and suppressed spontaneous firing in vivo. The ability of opioids to modulate other transmitters was determined using microiontophoretically applied ligands and extracellular recordings of VP neurons from chloral hydrate-anesthetized rats. With neurons that responded to iontophoresed opioid agonists, the ejection current was reduced to a level that was below that necessary to alter spontaneous firing. This "subthreshold" current was used to determine the ability of mu opioid receptor (microR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. microR agonists decreased the variability and enhanced the acuity (e.g., "signal-to-noise" relationship) of VP responses to activation of glutamatergic inputs from the prefrontal cortex and amygdala. By contrast, microR agonists attenuated both the slow excitatory responses to substance P and GABA-induced inhibitions that resulted from activating the nucleus accumbens. Subthreshold opioids also attenuated inhibitory responses to stimulating midbrain dopaminergic cells. These results suggest that a consequence of opioid transmission in the VP is to negate the influence of some afferents (e.g., midbrain dopamine and accumbal GABA and substance P) while selectively potentiating the efficacy of others (e.g., cortical and amygdaloid glutamate). Interpreted in the context of opiate abuse, microR opioids in the VP may serve to diminish the influence of reinforcement (ventral tegmental area and nucleus accumbens) in the transduction of cognition (prefrontal cortex) and affect (amygdala) into behavior. This may contribute to drug craving that occurs even in the absence of reward.

Frontal lobe psychopathology: mania, depression, confabulation, catatonia, perseveration, obsessive compulsions, and schizophrenia

The frontal lobes can be subdivided into major functional neuroanatomical domains, which, when injured, surgically destroyed, or reduced in activity or volume, give rise to signature pathological and psychiatric symptomology. A review of case reports and over 50 years of research, including magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography scans, indicates that apathy, "blunted" schizophrenia, major depression, and aphasic-perseverative disturbance of speech and thought are associated with left lateral as well as bilateral frontal (and striatal) abnormalities. Impulsiveness, confabulatory verbosity, grandiosity, increased sexuality, and mania are associated with right frontal (as well as bilateral) disturbances. Gegenhalten, catatonia, and disturbances of "will" are indicative of medial frontal injuries. Disinhibitory states and obsessive-compulsive perseverative abnormalities are more frequently observed with orbital frontal lobe dysfunction, including frontal-striatal disturbances. These associations, however, are not always clear-cut as patients with the same diagnosis may demonstrate different symptoms that may be due to an additional abnormality in a different region of the brain. Moreover, as the frontal subdivisions are richly interconnected, and as frontal lobe abnormalities are not always discrete or well localized, a wide array of seemingly divergent waxing and waning symptoms may be manifest, sometimes simultaneously, including manic depression and what has been referred to as the "frontal lobe personality."

Neuronal responses in prefrontal cortex and nucleus accumbens during heroin self-administration in freely moving rats

Chronic multi-channel single unit recordings of neuronal responses in prefrontal cortex (PFC) and nucleus accumbens (NAc) were made in 9 male Sprague Dawley rats to determine patterns of neuronal activity during heroin self-administration. Up to 32 neurons were recorded simultaneously in these two brain regions while rats lever pressed on a continuous reinforcement schedule for intravenous infusion of heroin (30 microg/kg/infusion). The variety of neuronal responses observed before and after each self-administered heroin infusion can be classified according to the following categories: (1) neurons that increased or (2) decreased their activity immediately before the lever press; (3) neurons that increased or (4) decreased their activity after the heroin infusion; and, (5) neurons that did not alter their activity either before or after the lever press for heroin infusion. The majority (69% in the PFC and 65% in the NAc) of neurons sampled fell into this last category of no change, indicating that a selected fraction becomes active during this specific task. In general, NAc neurons displayed more post-heroin responses than PFC neurons while the proportion of neurons showing responses before the lever press was similar in the mPFC and the NAc. This initial description of the responses of PFC and NAc neurons during heroin self-administration suggests that the neuronal circuit of the mesocorticolimbic system is involved in heroin self-administration. This circuit appears to contribute both to the initiation of drug-seeking behavior (pre-lever press phasic neuronal responses), as well as the action of heroin infusion itself (post-infusion phasic neuronal responses) by activation of different subsets of neurons.

Ultrastructural immunocytochemical localization of mu-opioid receptors in rat nucleus accumbens: extrasynaptic plasmalemmal distribution and association with Leu5-enkephalin

mu-Opioid receptors and their endogenous ligands, including Leu5-enkephalin (LE), are distributed abundantly in the nucleus accumbens (NAC), a region implicated in mechanisms of opiate reinforcement. We used immunoperoxidase and/or immunogold-silver methods to define ultrastructural sites for functions ascribed to mu-opioid receptors and potential sites for activation by LE in the NAC. An antipeptide antibody raised against an 18 amino acid sequence of the cloned mu-opioid receptor (MOR) C terminus showed that MOR-like immunoreactivity (MOR-LI) was localized predominantly to extrasynaptic sites along neuronal plasma membranes. The majority of neuronal profiles containing MOR-LI were dendrites and dendritic spines. The dendritic plasma membranes immunolabeled for MOR were near sites of synaptic input from LE-labeled terminals and other unlabeled terminals forming either inhibitory or excitatory type synapses. Unmyelinated axons and axon terminals were also intensely but less frequently immunoreactive for MOR. Observed sites for potential axonal associations with LE included coexistence of MOR and LE within the same terminal, as well as close appositions between differentially labeled axons. Astrocytic processes rarely contained detectable MOR-LI, but also were sometimes observed in apposition to LE-labeled terminals. We conclude that in the rat NAC, MOR is localized prominently to extrasynaptic neuronal and more rarely to glial plasma membranes that are readily accessible to released LE and possibly other opioid peptides and opiate drugs. The close affiliation of MOR with spines receiving excitatory synapses and dendrites receiving inhibitory synapses provides the first direct morphological evidence that MOR selectively modulates postsynaptic responses to cortical and other afferents.

Neuropharmacology of the nucleus accumbens: iontophoretic applications of morphine and nicotine have contrasting effects on single-unit responses evoked by ventral pallidal and fimbria stimulation

Extracellular recordings within the nucleus accumbens (NAS) of halothane anesthetized rats have revealed that iontophoretically applied morphine and nicotine have contrasting effects on neuronal responses evoked by fimbria or VP stimulation. Iontophoretically applied morphine inhibited NAS single-unit responses evoked by VP stimulation but did not affect unit responses evoked by fimbria stimulation. In contrast, iontophoretically applied nicotine had no effect on NAS single-unit responses evoked by VP stimulation but inhibited single-unit responses evoked by fimbria stimulation. Spontaneously active NAS units were inhibited by iontophoretically applied morphine but were unaffected by nicotine. In addition, experiments were conducted to determine whether NAS unit responses to electrical stimulation of the VP were likely to involve cell body as opposed to axonal activations. Selective cell body stimulation by glutamate micro-infusions into the VP region excited spontaneously active VP single-units. Concurrently recorded NAS unit responses to electrical stimulation of the VP were also excited. These results are consistent with the idea that NAS evoked responses to VP electrical stimulation involve somal activation. Generally, these results suggest a specific neuropharmacological organization of the NAS. Analysis of the effects of morphine and nicotine on other NAS circuits will establish a systems level understanding of NAS responses to reinforcers.