Development and persistence of long-lasting behavioral sensitization to cocaine in female mice: role of the nNOS gene

Ghrelin activates the mesolimbic dopamine system via nitric oxide associated mechanisms in the ventral tegmental area

Besides enhanced feeding, the orexigenic peptide ghrelin activates the mesolimbic dopamine system to cause reward as measured by locomotor stimulation, dopamine release in nucleus accumbens shell (NAcS), and conditioned place preference. Although the ventral tegmental area (VTA) appears to be a central brain region for this ghrelin-reward, the underlying mechanisms within this area are unknown. The findings that the gaseous neurotransmitter nitric oxide (NO) modulate the ghrelin enhanced feeding, led us to hypothesize that ghrelin increases NO levels in the VTA, and thereby stimulates reward-related behaviors. We initially demonstrated that inhibition of NO synthesis blocked the ghrelin-induced activation of the mesolimbic dopamine system. We then established that antagonism of downstream signaling of NO in the VTA, namely sGC, prevents the ability of ghrelin to stimulate the mesolimbic dopamine system. The association of ghrelin to NO was further strengthened by in vivo electrochemical recordings showing that ghrelin enhances the NO release in the VTA. Besides a GABA_B -receptor agonist, known to reduce NO and cGMP, blocks the stimulatory properties of ghrelin. The present series of experiments reveal that ablated NO signaling, through pharmacologically inhibiting the production of NO and/or cGMP, prevents the ability of ghrelin to induced reward-related behaviors.

Sex chromosome complement influences vulnerability to cocaine in mice

Women acquire cocaine habits faster and are more motivated to obtain drug than men. In general, female rodents acquire intravenous cocaine self-administration (SA) faster and show greater locomotor responses to cocaine than males. Sex differences are attributed to differences in circulating estradiol. We used the four core genotype (FCG) mouse to ask whether sex chromosome complement influences vulnerability to cocaine's reinforcing and/or locomotor-activating effects. The FCG cross produces ovary-bearing mice with XX or XY genotypes (XXF, XYF) and testes-bearing mice with XX or XY genotypes (XXM, XYM). A greater percentage of gonadal females acquired cocaine SA via infusions into jugular catheters as compared with XYM mice, but XXM mice were not significantly different than any other group. Discrimination of the active versus inactive nose poke holes and cocaine intake were in general greater in gonadal females than in gonadal males. Progressive ratio tests for motivation revealed an interaction between sex chromosomes and gonads: XYM mice were more motivated to self-administer cocaine taking more infusions than mice in any other group. Locomotor responses to cocaine exposure revealed effects of sex chromosomes. After acute exposure, activity was greater in XX than in XY mice and the reverse was true for behavioral sensitization. Mice with XY genotypes displayed more activity than XX mice when given cocaine after a 10-day drug-free period. Our data demonstrate that sex chromosome complement alone and/or interacting with gonadal status can modify cocaine's reinforcing and locomotor-activating effects. These data should inform current studies of sex differences in drug use.

PDE4 Inhibition Restores the Balance Between Excitation and Inhibition in VTA Dopamine Neurons Disrupted by Repeated In Vivo Cocaine Exposure

Phosphodiesterase type 4 (PDE4) is a family of enzymes that selectively degrade intracellular cAMP. PDE4 inhibitors have been shown to regulate the rewarding and reinforcing effects of cocaine, but the underlying mechanisms remain poorly understood. Here we show that pretreatments with the PDE4 inhibitor rolipram attenuated cocaine-induced locomotor sensitization in mice. Repeated cocaine exposure in vivo caused a decrease in inhibitory postsynaptic currents (IPSCs) and an increase in the AMPAR/NMDAR ratio in ventral tegmental area (VTA) dopamine neurons in midbrain slices ex vivo. Cocaine exposure disrupted the balance between excitation and inhibition as shown by an increase in the excitation to inhibition (E/I) ratio. Rolipram pretreatments in vivo prevented cocaine-induced reductions in GABAergic inhibition but did not further increase cocaine-induced potentiation of excitation, leading to the restoration of a balance between excitation and inhibition and normalization of the E/I ratio. In support of this idea, we found that repeated cocaine exposure led to an increase in the single-unit action potential firing rate in vivo in VTA dopamine neurons, which was blocked by rolipram pretreatments. These results suggest that repeated cocaine exposure in vivo disrupts the balance between excitation and inhibition in VTA dopamine neurons, while PDE4 inhibition reestablishes the balance between excitation and inhibition through distinct mechanisms.

Adolescent exposure to cocaine, amphetamine, and methylphenidate cross-sensitizes adults to methamphetamine with drug- and sex-specific effects

The increasing availability, over-prescription, and misuse and abuse of ADHD psychostimulant medications in adolescent populations necessitates studies investigating the long-term effects of these drugs persisting into adulthood. Male and female C57Bl/6J mice were exposed to amphetamine (AMPH) (1.0 and 10 mg/kg), methylphenidate (MPD) (1.0 and 10 mg/kg), or cocaine (COC) (5.0 mg/kg) from postnatal day 22 to 31, which represents an early adolescent period. After an extended period of drug abstinence, adult mice were challenged with a subacute methamphetamine (METH) dose (0.5 mg/kg), to test the long-term effects of adolescent drug exposures on behavioral cross-sensitization using an open field chamber. There were no sex- or dose-specific effects on motor activity in adolescent, saline-treated controls. However, AMPH, MPD, and COC adolescent exposures induced cross-sensitization to a subacute METH dose in adulthood, which is a hallmark of addiction and a marker of long-lasting plastic changes in the brain. Of additional clinical importance, AMPH-exposed male mice demonstrated increased cross-sensitization to METH in contrast to the female-specific response observed in MPD-treated animals. There were no sex-specific effects after adolescent COC exposures. This study demonstrates differential drug, dose, and sex-specific alterations induced by early adolescent psychostimulant exposure, which leads to behavioral alterations that persist into adulthood.

cGMP Signaling, Phosphodiesterases and Major Depressive Disorder

Deficits in neuroplasticity are hypothesized to underlie the pathophysiology of major depressive disorder (MDD): the effectiveness of antidepressants is thought to be related to the normalization of disrupted synaptic transmission and neurogenesis. The cyclic adenosine monophosphate (cAMP) signaling cascade has received considerable attention for its role in neuroplasticity and MDD. However components of a closely related pathway, the cyclic guanosine monophosphate (cGMP) have been studied with much lower intensity, even though this signaling transduction cascade is also expressed in the brain and the activity of this pathway has been implicated in learning and memory processes. Cyclic GMP acts as a second messenger; it amplifies signals received at postsynaptic receptors and activates downstream effector molecules resulting in gene expression changes and neuronal responses. Phosphodiesterase (PDE) enzymes degrade cGMP into 5’GMP and therefore they are involved in the regulation of intracellular levels of cGMP. Here we review a growing body of evidence suggesting that the cGMP signaling cascade warrants further investigation for its involvement in MDD and antidepressant action.

The emergence of gonadal hormone influences on dopaminergic function during puberty

Adolescence is the developmental epoch during which children become adults-intellectually, physically, hormonally and socially. Brain development in critical areas is ongoing. Adolescents are risk-taking and novelty-seeking and they weigh positive experiences more heavily and negative experiences less than adults. This inherent behavioral bias can lead to risky behaviors like drug taking. Most drug addictions start during adolescence and early drug-taking is associated with an increased rate of drug abuse and dependence. The hormonal changes of puberty contribute to physical, emotional, intellectual and social changes during adolescence. These hormonal events do not just cause maturation of reproductive function and the emergence of secondary sex characteristics. They contribute to the appearance of sex differences in non-reproductive behaviors as well. Sex differences in drug use behaviors are among the latter. The male predominance in overall drug use appears by the end of adolescence, while girls develop the rapid progression from first use to dependence (telescoping) that represent a female-biased vulnerability. Sex differences in many behaviors including drug use have been attributed to social and cultural factors. A narrowing gap in drug use between adolescent boys and girls supports this thesis. However, some sex differences in addiction vulnerability reflect biologic differences in brain circuits involved in addiction. The purpose of this review is to summarize the contribution of sex differences in the function of ascending dopamine systems that are critical to reinforcement, to briefly summarize the behavioral, neurochemical and anatomical changes in brain dopaminergic functions related to addiction that occur during adolescence and to present new findings about the emergence of sex differences in dopaminergic function during adolescence.

Role of the nNOS gene in ethanol-induced conditioned place preference in mice

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity, and evidence suggests its role in a range of effects produced by alcohol in the central nervous system. The aim of the current study was to investigate the role of the nNOS gene in the development of ethanol-induced conditioned place preference (CPP) in mice. The CPP paradigm is designed to investigate the reinforcing properties of drugs of abuse and the development of maladaptive behaviors, such as conditioned response to drug-associated stimuli, after repeated drug exposure. Adult male and female wild type (WT) and nNOS knockout (KO) mice on a mixed B6;129S genetic background were trained by a morning saline session and afternoon ethanol (1, 2, and 3 g/kg; intraperitoneally) session for 4 days. Place preference in a drug-free state was recorded on the following day. Results show that WT males and females developed robust CPP, whereas nNOS KO mice did not (with the exception of female nNOS KO mice conditioned by 2 g/kg ethanol). The differential response of WT and nNOS KO mice was not due to genotypic differences in motor behavior. To investigate if the absence of the nNOS gene causes specific impairment in processing the motivational effect of ethanol or an overall impairment in associative learning, WT and nNOS KO mice were trained by LiCl (150 mg/kg) which causes conditioned place aversion (CPA). Results show that both WT and nNOS KO mice developed significant CPA. The findings that the absence of the nNOS gene impaired ethanol-induced CPP but not LiCl-induced CPA suggest that NO signaling has a specific role in processing the motivational effect of ethanol. Hence, inhibition of nNOS may attenuate the development of maladaptive behaviors associated with alcohol exposure.