Cocaine and regional brain monoamines in mice

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

Ontogeny of cocaine-induced behaviors and cocaine pharmacokinetics in male and female neonatal, preweanling, and adult rats

RATIONALE

Ontogenetic differences in the behavioral responsiveness to cocaine have often been attributed to the maturation of dopaminergic elements (e.g., dopamine transporters, D2High receptors, receptor coupling, etc.).

OBJECTIVE

The purpose of this study was to determine whether ontogenetic changes in cocaine pharmacokinetics might contribute to age-dependent differences in behavioral responsiveness.

METHODS

Male and female neonatal (PD 5), preweanling (PD 10 and PD 20), and adult (PD 70) rats were injected (IP) with cocaine or saline and various behaviors (e.g., locomotor activity, forelimb paddle, vertical activity, head-down sniffing, etc.) were measured for 90 min. In a separate experiment, the dorsal striata of young and adult rats were removed at 10 time points (0-210 min) after IP cocaine administration. Peak cocaine values, cocaine half-life, and dopamine levels were determined using HPLC.

RESULTS

When converted to percent of saline controls, PD 5 and PD 10 rats were generally more sensitive to cocaine than older rats, but this effect varied according to the behavior being assessed. Peak cocaine values did not differ according to age or sex, but cocaine half-life in brain was approximately 2 times longer in PD 5 and PD 10 rats than adults. Cocaine pharmacokinetics did not differ between PD 20 and PD 70 rats.

CONCLUSIONS

Differences in the cocaine-induced behavioral responsiveness of very young rats (PD 5 and PD 10) and adults may be attributable, at least in part, to pharmacokinetic factors; whereas, age-dependent behavioral differences between the late preweanling period and adulthood cannot readily be ascribed to cocaine pharmacokinetics.

Electrophysiological characteristics of paraventricular thalamic (PVT) neurons in response to cocaine and cocaine- and amphetamine-regulated transcript (CART)

Recent work has established that the paraventricular thalamus (PVT) is a central node in the brain reward-seeking pathway. This role is mediated in part through projections from hypothalamic peptide transmitter systems such as cocaine- and amphetamine-regulated transcript (CART). Consistent with this proposition, we previously found that inactivation of the PVT or infusions of CART into the PVT suppressed drug-seeking behavior in an animal model of contingent cocaine self-administration. Despite this work, few studies have assessed how the basic physiological properties of PVT neurons are influenced by exposure to drugs such as cocaine. Further, our previous work did not assess how infusions of CART, which we found to decrease cocaine-seeking, altered the activity of PVT neurons. In the current study we address these issues by recording from anterior PVT (aPVT) neurons in acutely prepared brain slices from cocaine-treated (15 mg/ml, n = 8) and saline-treated (control) animals (n = 8). The excitability of aPVT neurons was assessed by injecting a series of depolarizing and hyperpolarizing current steps and characterizing the resulting action potential (AP) discharge properties. This analysis indicated that the majority of aPVT neurons exhibit tonic firing (TF), and initial bursting (IB) consistent with previous studies. However, we also identified PVT neurons that exhibited delayed firing (DF), single spiking (SS) and reluctant firing (RF) patterns. Interestingly, cocaine exposure significantly increased the proportion of aPVT neurons that exhibited TF. We then investigated the effects of CART on excitatory synaptic inputs to aPVT neurons. Application of CART significantly suppressed excitatory synaptic drive to PVT neurons in both cocaine-treated and control recordings. This finding is consistent with our previous behavioral data, which showed that CART signaling in the PVT negatively regulates drug-seeking behavior. Together, these studies suggest that cocaine exposure shifts aPVT neurons to a more excitable state (TF). We propose that the capacity of CART to reduce excitatory drive to this population balances the enhanced aPVT excitability to restore the net output of this region in the reward-seeking pathway. This is in line with previous anatomical evidence that the PVT can integrate reward-relevant information and provides a putative mechanism through which drugs of abuse can dysregulate this system in addiction.

Cocaine tolerance in honey bees

Increasingly invertebrates are being used to investigate the molecular and cellular effects of drugs of abuse to explore basic mechanisms of addiction. However, in mammals the principle factors contributing to addiction are long-term adaptive responses to repeated drug use. Here we examined whether adaptive responses to cocaine are also seen in invertebrates using the honey bee model system. Repeated topical treatment with a low dose of cocaine rendered bees resistant to the deleterious motor effects of a higher cocaine dose, indicating the development of physiological tolerance to cocaine in bees. Cocaine inhibits biogenic amine reuptake transporters, but neither acute nor repeated cocaine treatments caused measurable changes in levels of biogenic amines measured in whole bee brains. Our data show clear short and long-term behavioural responses of bees to cocaine administration, but caution that, despite the small size of the bee brain, measures of biogenic amines conducted at the whole-brain level may not reveal neurochemical effects of the drug.

Dietary restriction mitigates cocaine-induced alterations of olfactory bulb cellular plasticity and gene expression, and behavior

Because the olfactory system plays a major role in food consumption, and because 'food addiction' and associated morbidities have reached epidemic proportions, we tested the hypothesis that dietary energy restriction can modify adverse effects of cocaine on behavior and olfactory cellular and molecular plasticity. Mice maintained on an alternate day fasting (ADF) diet exhibited increased baseline locomotion and increased cocaine-sensitized locomotion during cocaine conditioning, despite no change in cocaine conditioned place preference, compared with mice fed ad libitum. Levels of dopamine and its metabolites in the olfactory bulb (OB) were suppressed in mice on the ADF diet compared with mice on the control diet, independent of acute or chronic cocaine treatment. The expression of several enzymes involved in dopamine metabolism including tyrosine hydroxylase, monoamine oxidases A and B, and catechol-O-methyltransferase were significantly reduced in OBs of mice on the ADF diet. Both acute and chronic administration of cocaine suppressed the production of new OB cells, and this effect of cocaine was attenuated in mice on the ADF diet. Cocaine administration to mice on the control diet resulted in up-regulation of OB genes involved in mitochondrial energy metabolism, synaptic plasticity, cellular stress responses, and calcium- and cAMP-mediated signaling, whereas multiple olfactory receptor genes were down-regulated by cocaine treatment. ADF abolished many of the effects of cocaine on OB gene expression. Our findings reveal that dietary energy intake modifies the neural substrates underlying some of the behavioral and physiological responses to repeated cocaine treatment, and also suggest novel roles for the olfactory system in addiction. The data further suggest that modification of dietary energy intake could provide a novel potential approach to addiction treatments.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Chronic low-dose cocaine treatment during adolescence facilitates aggression in hamsters

Cocaine abuse during adolescence represents a significant health risk because of the potential for both acute and long-term negative physical and psychological sequelae, including increased aggressive behavior. This study examined the effects of chronic adolescent cocaine exposure on aggression in an animal model. It was hypothesized that chronic cocaine exposure during adolescence predisposes animals to heightened levels of aggressive behavior. To test this hypothesis, adolescent male golden hamsters (Mesocricetus auratus) were administered cocaine hydrochloride during their entire adolescent development (Postnatal Days 27-54) and then tested for offensive aggression using the resident-intruder model. Animals treated with low-dose cocaine during adolescence showed significantly elevated measures of offensive aggression (i.e., increased number of bites, attacks, and decreased latencies to bite), whereas measures of social communication, sexual motivation and motor activity remained constant. Cocaine-treated animals did not differ in body weight gain from controls, suggesting no dramatic physiological effects of adolescent cocaine exposure on body growth at the doses tested.

Caffeine and the olfactory bulb

Caffeine, a popular CNS stimulant, is the most widely used neuroactive drug. Present in coffee, tea, chocolate, and soft drinks as well as over-the-counter and prescription medications, it influences millions of users. This agent has achieved recent notoriety because its dependency consequences and addictive potential have been re-examined and emphasized. Caffeine's central actions are thought to be mediated through adenosine (A) receptors and monoamine neurotransmitters. The present article suggests that the olfactory bulb (OB) may be an important site in the brain that is responsible for caffeine's central actions in several species. This conclusion is based on the extraordinarily robust and selective effects of caffeine on norepinephrine (NE), dopamine (DA), and particularly serotonin (5HT) utilization in the OB of mice. We believe that these phenomena should be given appropriate consideration as a basis for caffeine's central actions, even in primates. Concurrently, we review a rich rodent literature concerned with A, 5HT, NE, and DA receptors in the OB and related structures along with other monoamine parameters. We also review a more limited literature concerned with the primate OB. Finally, we cite the literature that treats the dependency and addictive effects of caffeine in humans, and relate the findings to possible olfactory mechanisms.

A murine model of prenatal cocaine exposure: effects on the mother and the fetus

To develop and characterize a murine model for investigating the long-term effects of prenatal cocaine exposure, the present study established the route of drug administration and the doses to be used for pregnant C57BL/6 mice. Comparison of the effects of a high dose of cocaine (60 mg/kg) when gavaged or injected subcutaneously (SC) established patterns of pathology characteristic of administration route but no dominating logic for selecting one over the other route for prenatal studies; however, because of the fourfold greater brain levels, with no evidence of greater pathology, the SC route was selected. When injected daily during gestation days 12-18, the period of prenatal development of dopamine systems, cocaine at doses producing plasma concentrations consistent with its stimulatory effects reduced food ingestion and weight gains during pregnancy and fetal body and brain weights at term. The extent of these reductions was comparable to reports on babies exposed to cocaine prenatally. Furthermore, the present study suggests that maternal undernutrition is not a likely mediator of these perinatal effects and that differences in the amount of cocaine exposure may cause the contrasting effects of maternal cocaine noted in the human literature.

Striatal dopamine, dopamine transporter, and vesicular monoamine transporter in chronic cocaine users

Depletion of striatal dopamine (DA) has been hypothesized to explain some of the neurological and psychiatric complications of chronic use of cocaine, including increased risk for neuroleptic-precipitated movement disorders. We measured levels of DA, as well as two DA nerve terminal indices, namely, the DA transporter (DAT) and the vesicular monoamine transporter (VMAT2) in autopsied brain of 12 chronic cocaine users. Mean DA levels were normal in the putamen, the motor component of the striatum; however 4 of the 12 subjects had DA values below the lower limit of the control range. DA concentrations were significantly reduced in the caudate head (head, -33%; tail, -39%) with a trend for reduction in nucleus accumbens (-27%). Striatal DAT protein (-25 to -46%) and VMAT2 (-17 to -22%) were reduced, whereas DAT determined by [3H]WIN 35,428 binding was normal. In conclusion, our data suggest that chronic cocaine use is associated with modestly reduced levels of striatal DA and the DA transporter in some subjects and that these changes might contribute to the neurological and psychiatric effects of the drug.

Cocaine. Selective regional effects on central monoamines

Cocaine HCl (0, 10, or 50 mg/kg) was injected into adult male ICR mice ip. Thirty minutes later, the brains were removed, and nine regions were isolated: olfactory bulbs, olfactory tubercles, prefrontal cortex, septum, striatum, amygdala, hypothalamus, hippocampus, and thalamus. Using high-performance liquid chromatography, concentrations of norepinephrine, dopamine, serotonin, and their major metabolites and the metabolite/neurotransmitter ratios were determined as an indicator of utilization. Serotonergic systems responded most dramatically. 5HIAA/5-HT decreases were seen in all the brain regions, except the septum, hippocampus, and olfactory bulbs. In most instances, the alterations were dose-dependent. The most profound changes were seen in the amygdala, prefrontal cortex, hypothalamus, and thalamus. For noradrenergic systems, significant responses were seen only in the amygdala, prefrontal cortex, and hypothalamus, but then only at the lower dose. The dopaminergic responses were more complex and not always dose-dependent. The DOPAC/DA ratio was decreased only in the amygdala and striatum at the lower dose, and the olfactory tubercles at the higher dose. It was increased in the septum. The HVA/DA ratios were decreased in the amygdala, prefrontal cortex, and hypothalamus, but only at the lower dose (like MHPG/NE). The 3MT/DA ratio was decreased in the thalamus at the lower dose and in the olfactory tubercles at the higher dose, whereas it was increased in the prefrontal cortex at the lower dose. The HVA and DOPAC routes of degradation were both utilized only by the amygdala. Thus, cocaine produced its most comprehensive effects in this nucleus, as well as the greatest absolute percentage changes for all three of the monoamine systems studied.

Amygdala dopamine levels are markedly elevated after self- but not passive-administration of cocaine

The influence of cocaine on rat brain monoaminergic neurotransmitters (dopamine, serotonin, noradrenaline) and their metabolites, and on binding of [3H]WIN 35,428 and [3H]GBR 12,935 to the dopamine transporter was measured after 4 weeks of cocaine exposure. Cocaine (mean daily dose 9.25 +/- 0.48 mg/kg) was self-administered (responders) or passively received (yoked) during sessions which lasted for 1 h per day. As compared with the controls, mean dopamine and serotonin levels were significantly elevated (+ 107% and + 47%, respectively) in amygdala of responders, but not of yoked rats, sacrificed 1 h after the last cocaine session. Dopamine and metabolite levels were normal in all other brain areas examined, including striatum, nucleus accumbens and medial prefrontal cortex, at both 1 h and 4 weeks withdrawal from cocaine. [3H]WIN 35,428 and [3H]GBR 12,935 binding were unaltered after cocaine exposure. These data provide additional support for the involvement of the amygdala in the acquisition of drug seeking behavior associated with cocaine self-administration.