T. Celeste Napier

Opioid Modulation of Ventral Pallidal Inputs

ABSTRACT: 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 (μR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. μR 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, μR 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, μR 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.

Methamphetamine-Induced Sensitization Differentially Alters pCREB and ΔFosB throughout the Limbic Circuit of the Mammalian Brain

Enhancements in behavior that accompany repeated, intermittent administration of abused drugs (sensitization) endure long after drug administration has ceased. Such persistence reflects changes in intracellular signaling cascades and associated gene transcription factors in brain regions that are engaged by abused drugs. This process is not characterized for the most potent psychomotor stimulant, methamphetamine. Using motor behavior as an index of brain state in rats, we verified that five once-daily injections of 2.5 mg/kg methamphetamine induced behavioral sensitization that was demonstrated (expressed) 3 and 14 days later. Using immunoblot procedures, limbic brain regions implicated in behavioral sensitization were assayed for extracellular signal-regulated kinase and its phosphorylated form (pERK/ERK, a signal transduction kinase), cAMP response element binding protein and its phosphorylated form (pCREB/CREB, a constitutively expressed transcriptional regulator), and ΔFosB (a long-lasting transcription factor). pERK, ERK, and CREB levels were not changed for any region assayed. In the ventral tegmental area, pCREB and ΔFosB also were not changed. pCREB (activated CREB) was elevated in the frontal cortex at 3 days withdrawal, but not at 14 days. pCREB levels were decreased at 14 days withdrawal in the nucleus accumbens and ventral pallidum. Accumbal and pallidal levels of ΔFosB were increased at 3 days withdrawal, and this increase persisted to 14 days in the pallidum. Thus, only the ventral pallidum showed changes in molecular processes that consistently correlated with motor sensitization, revealing that this region may be associated with this enduring behavioral phenotype initiated by methamphetamine. The present findings expand our understanding of the neuroanatomical and molecular substrates that may play a role in the persistence of druginduced sensitization.

The Effects of Psychostimulant Drugs on Blood Brain Barrier Function and Neuroinflammation

The blood brain barrier (BBB) is a highly dynamic interface between the central nervous system (CNS) and periphery. The BBB is comprised of a number of components and is part of the larger neuro(glio)vascular unit. Current literature suggests that psychostimulant drugs of abuse alter the function of the BBB which likely contributes to the neurotoxicities associated with these drugs. In both preclinical and clinical studies, psychostimulants including methamphetamine, MDMA, cocaine, and nicotine, produce BBB dysfunction through alterations in tight junction protein expression and conformation, increased glial activation, increased enzyme activation related to BBB cytoskeleton remodeling, and induction of neuroinflammatory pathways. These detrimental changes lead to increased permeability of the BBB and subsequent vulnerability of the brain to peripheral toxins. In fact, abuse of these psychostimulants, notably methamphetamine and cocaine, has been shown to increase the invasion of peripheral bacteria and viruses into the brain. Much work in this field has focused on the co-morbidity of psychostimulant abuse and human immunodeficiency virus (HIV) infection. As psychostimulants alter BBB permeability, it is likely that this BBB dysfunction results in increased penetration of the HIV virus into the brain thus increasing the risk of and severity of neuro AIDS. This review will provide an overview of the specific changes in components within the BBB associated with psychostimulant abuse as well as the implications of these changes in exacerbating the neuropathology associated with psychostimulant drugs and HIV co-morbidity.

Amphetamine-induced place preference and conditioned motor sensitization requires activation of tyrosine kinase receptors in the hippocampus.

The environmental context in which abused drugs are taken contribute to the drug experience and is a powerful and persistent stimulus to elicit memories of that experience even in the abstinent addict. Using amphetamine (AMPH) as the unconditioned stimulus, the present study compared two popular context-dependent paradigms in rats, conditioned motor sensitization (CMS) and conditioned place preference (CPP), to ascertain whether particular brain regions were differentially involved. The neuronal substrates underlying these context-dependent behaviors are poorly understood, but regulators of the neuronal plasticity that accompany learning, such as neurotrophic factors and their cognate tyrosine kinase receptors (e.g., TrkB), are credible candidates. We found a significant elevation of TrkB-like immunoreactivity specifically in CA3/dentate gyrus (DG) subregions of the hippocampus after AMPH (0.3 mg/kg)-induced CPP, but not in the delayed-paired (control) AMPH condition. A higher AMPH dose (1.0 mg/kg) induced both CPP and CMS and elevated TrkB in the CA3/DG as well as in the nucleus accumbens shell. The development of both conditioned behaviors was blocked by intra-CA3/DG infusion of the Trk inhibitor K-252a. These findings reveal that CPP and CMS are induced by different doses of AMPH and are associated with TrkB changes in particular brain regions. Moreover, Trk receptors in the hippocampus are critical mediators of the neuronal changes necessary for inducing both forms of conditioning. Thus, although these two conditioning models are distinct, because they are commonly regulated by the hippocampal Trk system, these receptors may be a therapeutic target for attenuating the significance of contextual cues that otherwise strengthen the addictive properties of abused drugs.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors.

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

ELECTROPHYSIOLOGICAL VERIFICATION OF THE PRESENCE OF D1 AND D2 DOPAMINE RECEPTORS WITHIN THE VENTRAL PALLIDUM

The ventral pallidum is a basal forebrain region recently shown to receive dopaminergic projections from the midbrain. Binding sites for the D1 and D2 dopamine receptor families have been identified within the ventral pallidum, yet the consequences of activating these receptors have not been studied. Thus, to characterize the physiological pharmacology of D1 and D2 receptor subtypes for the ventral pallidum, extracellular single‐neuron recording and microiontophoretic techniques were used in chloral hydrate‐anesthetized rats. Half of the 93 ventral pallidal neurons tested were sensitive to iontophoresis of dopamine (DA), and both rate increases and decreases were observed. Co‐iontophoresis of either the D1 antagonist SCH23390, or the D2 antagonist sulpiride, generally attenuated the DA‐induced rate changes. Like DA, about half of the ventral pallidal neurons tested were sensitive to the D1 agonist, SKF38393. Yet in contrast to DA, rate suppression was observed almost exclusively, and the magnitude of this decrease was greater than that produced by DA. SKF38393‐induced suppressions were antagonized by SCH23390, but not by sulpiride, demonstrating the specificity of the D1 agonist. Most of the neurons tested were not affected by quinpirole, but when responsive to the D2 agonist, rate increases were observed most often. The increases were antagonized by the D2 antagonist sulpiride, but not SCH23390, demonstrating that this response resulted from an activation of D2 receptors. These results support binding studies demonstrating that both D1 and D2 receptors are present in the ventral pallidum, and reveal that the independent activation of each of these is sufficient to alter neuronal activity.

Dopamine D1 and D2 receptor agonists induce opposite changes in the firing rate of ventral pallidal neurons

Selective dopamine D1 and D2 agonists were used to determine the contributions of each receptor subtype in the modulation of firing rate of ventral pallidum/substantia innominata (VP/SI) neurons. Administration of cumulative doses of the D2 agonist, quinpirole, decreased activity in 59% of the VP/SI cells tested. The decrease in firing rate was dose-dependent between 0.002-0.2 mg/kg i.v. and was blocked by the D2 antagonist, sulpiride (12.5 mg/kg i.v.). In addition, the magnitude and the distribution of responses of VP/SI neurons was not changed following administration of quinpirole as a single versus a divided cumulative dose of 0.1 mg/kg. In contrast, administration of the D1 agonist, SKF38393, excited 69% of the neurons sampled. Similar maximal responses were observed following administration of either a single or a divided cumulative dose of 3.2 mg/kg of SKF38393. The D1 receptor antagonist, SCH23390 (0.1-0.4 mg/kg i.v.) often attenuated the SKF38393-induced increases. The results illustrate that, (1) VP/SI neurons are sensitive to systemically administered dopamine agonists, (2) D1 or D2 receptor activation is sufficient to change the activity of these neurons and (3) these selective agonists mediate opposite effects on VP/SI neuronal activity. These differential responses contrast with effects observed for other dopaminoceptive brain regions, and distinguish VP/SI neurons from morphologically related neurons of the dorsal globus pallidus.

Dopamine in the rat ventral pallidum/substantia innominata: Biochemical and electrophysiological studies

Recent anatomical literature suggests that dopaminergic projections ascending from the midbrain terminate within the ventral pallidum/substantia innominata (VP/SI). The present investigation evaluated this possibility using standard biochemical and electrophysiologic approaches. Biochemical studies revealed that dopamine and its major metabolites are present within the rat VP/SI. Concentrations of these compounds were diminished greatly when dopaminergic neurons of the substantia nigra were destroyed. Electrophysiologic studies demonstrated that VP/SI neurons often respond to local applications of dopamine with a decrease in firing rate. These observations support the contention that dopamine regulates neuronal activity within the VP/SI and that cells of origin for at least a portion of this projection lie within the substantia nigra.

The neural substrates of amphetamine conditioned place preference: implications for the formation of conditioned stimulus–reward associations

Associations formed between conditioned stimuli and drug reward are major contributors in human drug addiction. To better understand the brain changes that accompany this process, we used immunohistochemistry for c‐Fos (a neuronal activity marker), synaptophysin (a marker for synaptogenesis) and tyrosine kinase B receptor (a neurotrophic factor receptor that mediates synaptic plasticity) to investigate the neural substrates of amphetamine‐induced conditioned place preference in rats. Conditioned place preference was induced by both 1.0 mg/kg and 0.3 mg/kg doses of amphetamine. Furthermore, amphetamine conditioning increased the density of c‐Fos‐immunoreactive cells and these cells were fully colocalized with the tyrosine kinase B receptor in the dentate gyrus, CA1 field and basolateral amygdala. Amphetamine conditioning increased the density of synaptophysin‐immunoreactive varicosities in all brain regions studied, except the nucleus accumbens shell and dorsolateral striatum. The degree of conditioned place preference was highly correlated with c‐Fos‐immunoreactive cell density in the basolateral amygdala and with the density of synaptophysin‐immunoreactive varicosities in all mesolimbic regions studied. The latter correlation was particularly impressive for the ventral pallidum and basolateral amygdala. The formation of conditioned stimulus–amphetamine reward associations is accompanied by tyrosine kinase B receptor expression in the basolateral amygdala and dentate gyrus, CA1 and CA3 fields of the hippocampus. These data therefore suggest that the formation of conditioned stimulus–reward associations requires, at least in part, activation of amygdalar–hippocampal circuits.

Linking Neuroscience With Modern Concepts of Impulse Control Disorders in Parkinson's Disease

Patients with Parkinsons disease (PD) may experience impulse control disorders (ICDs) when on dopamine agonist therapy for their motor symptoms. In the last few years, a rapid growth of interest for the recognition of these aberrant behaviors and their neurobiological correlates has occurred. Recent advances in neuroimaging are helping to identify the neuroanatomical networks responsible for these ICDs, and together with psychopharmacological assessments are providing new insights into the brain status of impulsive behavior. The genetic associations that may be unique to ICDs in PD are also being identified. Complementing human studies, electrophysiological and biochemical studies in animal models are providing insights into neuropathological mechanisms associated with these disorders. New animal models of ICDs in PD patients are being implemented that should provide critical means to identify efficacious therapies for PD‐related motor deficits while avoiding ICD side effects. Here, we provide an overview of these recent advances, with a particular emphasis on the neurobiological correlates reported in animal models and patients along with their genetic underpinnings.