Sex differences and ovarian hormones in animal models of drug dependence

Cocaine hydrolase encoded in viral vector blocks the reinstatement of cocaine seeking in rats for 6 months

BACKGROUND

Cocaine dependence is a pervasive disorder with high rates of relapse. In a previous study, direct administration of a quadruple mutant albumin-fused butyrylcholinesterase that efficiently catalyzes hydrolysis of cocaine to benzoic acid and ecgonine methyl ester acutely blocked cocaine seeking in an animal model of relapse. In the present experiments, these results were extended to achieve a long-duration blockade of cocaine seeking with a gene transfer paradigm using a related butyrylcholinesterase-based cocaine hydrolase (CocH).

METHODS

Male and female rats were allowed to self-administer cocaine under a fixed-ratio 1 schedule of reinforcement for approximately 14 days. Following the final self-administration session, rats were injected with CocH vector or a control injection (empty vector or saline), and their cocaine solutions were replaced with saline for 14 days to allow for extinction of lever pressing. Subsequently, they were tested for drug-primed reinstatement by administering intraperitoneal injections of saline (S), cocaine (C) (5, 10, and 15 mg/kg), and d-amphetamine according to the following sequence: S, C, S, C, S, C, S, d-amphetamine. Rats then received cocaine-priming injections once weekly for 4 weeks and, subsequently, once monthly for up to 6 months.

RESULTS

Administration of CocH vector produced substantial and sustained CocH activity in plasma that corresponded with diminished cocaine-induced (but not amphetamine-induced) reinstatement responding for up to 6 months following treatment (compared with high-responding control animals).

CONCLUSIONS

These results demonstrate that viral transfer of CocH may be useful in promoting long-term resistance to relapse to cocaine addiction.

Individual differences in psychostimulant responses of female rats are associated with ovarian hormones and dopamine neuroanatomy

Ovarian hormones modulate the pharmacological effects of psychostimulants and may enhance vulnerability to drug addiction. Female rats have more midbrain dopamine neurons than males and greater dopamine uptake and release rates. Cocaine stimulates motor behavior and dopamine efflux more in female than male rats, but the mediating mechanisms are unknown. This study investigated individual differences in anatomic, neurochemical, and behavioral measures in female rats to understand how ovarian hormones affect the relatedness of these endpoints. Ovarian hormone effects were assessed by comparing individual responses in ovariectomized (OVX) and sham adult female rats. Locomotion was determined before and following 10mg/kg cocaine. Electrically-stimulated dopamine efflux was assessed using fast cyclic voltammetry in vivo. Dopamine neuron number and density in substantia nigra (SN) and ventral tegmental area (VTA) were determined in the same animals using tyrosine-hydroxylase immunohistochemistry and unbiased stereology. Locomotor behavior and dopamine efflux did not differ at baseline but were greater in sham than OVX following cocaine. Cocaine increased dopamine release rates in both groups but uptake inhibition (K(m)) was greater in sham than OVX. Dopamine neuron number and density in SN and VTA were greater in shams. Sham females with the largest uterine weights exhibited the highest density of dopamine neurons in the SN, and the most cocaine-stimulated behavior and dopamine efflux. Ovariectomy eliminated these relationships. We postulate that SN density could link ovarian hormones and high-psychostimulant responses in females. Similar mechanisms may be involved in individual differences in the addiction vulnerability of women.

Reinstatement of methamphetamine seeking in male and female rats treated with modafinil and allopregnanolone

BACKGROUND

Sex differences in methamphetamine (METH) use (females>males) have been demonstrated in clinical and preclinical studies. This experiment investigated the effect of sex on the reinstatement of METH-seeking behavior in rats and determined whether pharmacological interventions for METH-seeking vary by sex. Treatment drugs were modafinil (MOD), an analeptic, and allopregnanolone (ALLO), a neuroactive steroid and progesterone metabolite.

METHOD

Male and female rats were trained to self-administer i.v. infusions of METH (0.05 mg/kg/infusion). Next, rats self-administered METH for a 10-day maintenance period. METH was then replaced with saline, and rats extinguished lever-pressing behavior over 18 days. A multi-component reinstatement procedure followed whereby priming injections of METH (1mg/kg) were administered at the start of each daily session, preceded 30 min by MOD (128 mg/kg, i.p.), ALLO (15 mg/kg, s.c.), or vehicle treatment. MOD was also administered at the onset of the session to determine if it would induce the reinstatement of METH-seeking behavior.

RESULTS

Female rats had greater METH-induced reinstatement responding compared to male rats following control treatment injections. MOD (compared to the DMSO control) attenuated METH-seeking behavior in male and female rats; however, ALLO only reduced METH-primed responding in females. MOD alone did not induce the reinstatement of METH-seeking behavior.

CONCLUSIONS

These results support previous findings that females are more susceptible to stimulant abuse compared to males, and ALLO effectively reduced METH-primed reinstatement in females. Further, results illustrate the utility of MOD as a potential agent for prevention of relapse to METH use in both males and females.

Transcriptional and epigenetic mechanisms of addiction

Investigations of long-term changes in brain structure and function that accompany chronic exposure to drugs of abuse suggest that alterations in gene regulation contribute substantially to the addictive phenotype. Here, we review multiple mechanisms by which drugs alter the transcriptional potential of genes. These mechanisms range from the mobilization or repression of the transcriptional machinery - including the transcription factors ΔFOSB, cyclic AMP-responsive element binding protein (CREB) and nuclear factor-κB (NF-κB) - to epigenetics - including alterations in the accessibility of genes within their native chromatin structure induced by histone tail modifications and DNA methylation, and the regulation of gene expression by non-coding RNAs. Increasing evidence implicates these various mechanisms of gene regulation in the lasting changes that drugs of abuse induce in the brain, and offers novel inroads for addiction therapy.

Ultrastructural analysis of sex differences in nucleus accumbens synaptic connectivity

Despite robust sex differences in behavioral responses to drugs of abuse, relatively little is known about structural sex differences in synaptic connectivity of reward circuits such as in the nucleus accumbens (NAc). Previously, we showed that distal dendritic spine density on medium spiny neurons in the NAc is higher in females than males, suggesting that sex differences in NAc excitatory synapses could play a role in differential behavioral responses to drugs. In the current study, we used electron microscopy and stereological counting methods to evaluate dendritic spine and shaft synapses, as well as tyrosine hydroxylase-immunoreactive (TH-IR) profiles, in the NAc core of male and female rats. We found an unanticipated rostro-caudal gradient in spine synapse density in females but not males, resulting in a sex difference favoring females in the caudal NAc core. The volume of the NAc was not different between males and females. We also found that the percentage of spines with large spine heads was greater in females in the rostral core. The density of shaft synapses was low compared to spine synapses, and sex differences were minor. The density of TH-IR profiles was not different between males and females, but females had a higher proportion of spines with large heads near TH suggesting a potential sex difference in dopaminergic modulation of large spine synapses. These findings underscore the importance of including both males and females in studies of reward circuitry, and of considering variation along the rostro-caudal axis of the NAc in future studies.

Baclofen has opposite effects on escalation of cocaine self-administration: increased intake in rats selectively bred for high (HiS) saccharin intake and decreased intake in those selected for low (LoS) saccharin intake

Rats selectively bred for high saccharin intake (HiS) self-administer more cocaine, escalate their cocaine intake during long access, and reinstate cocaine seeking at higher levels than those bred for low saccharin intake (LoS). The present study was conducted to determine if baclofen, an agonist at the GABA(b) receptor, has differential effects on the escalation of i.v. cocaine intake and reinstatement of cocaine-seeking in HiS and LoS rats. HiS and LoS rats self-administered cocaine during a 2-h daily short-access (ShA) phase for 3 days and then long-access (LgA) sessions for 21 days followed by a second ShA phase. One group of HiS and LoS rats received i.p. injections of 2.5 mg/kg baclofen (HiS+B and LoS+B, respectively), and other groups of HiS and LoS rats received saline (HiS+Sal and LoS+Sal) before each daily session. In a second experiment, HiS and LoS rats self-administered i.v. cocaine during 2-h sessions for 14 days followed by a 21-day extinction period. Baclofen (2.5 mg/kg, i.p.) or saline was administered before saline- or cocaine-primed reinstatement sessions. The HiS+B group escalated their cocaine self-administration and had increased cocaine infusions in the post-LgA ShA phase. The LoS+B group self-administered less cocaine throughout the entire LgA period compared to the LoS+Sal or HiS groups. Baclofen attenuated reinstatement of cocaine seeking in both the HiS and LoS rats with no phenotype differences. Thus, baclofen had opposite effects on cocaine intake in HiS and LoS rats during escalation; but similar effects during reinstatement. These results suggest that treatment effects might vary with individual differences (HiS vs. LoS) and the phase of drug-motivated behavior that is modeled.

Responding during signaled availability and nonavailability of iv cocaine and food in rats: age and sex differences

RATIONALE

Research suggests that age and sex are vulnerability factors for drug abuse. However, few studies have systematically examined their interaction.

OBJECTIVE

The purpose of the present study was to examine male and female, adult and adolescent rats under a procedure that measures responding during periods of signaled availability and nonavailability of iv cocaine and food reinforcers.

METHODS

Adolescent and adult rats lever pressed for iv infusions of cocaine or food pellets under a procedure with three components of signaled availability of the reinforcer alternating with two components of signaled nonavailability. Adolescent rats were removed and then later retested under the same conditions as adults, and a group of adult rats was also removed and retested after a similar number of days. A subset of rats earning cocaine infusions under the initial test was later retested with food pellets under the same behavioral task to assess the influence of prior cocaine exposure on subsequent responding for a nondrug reinforcer.

RESULTS

Adolescents (vs. adults) made more responses during periods of signaled iv cocaine availability and nonavailabiltiy, and adult females responded more than adult males during these periods. Responding during periods of signaled nonavailability of iv cocaine and food did not differ between the initial and subsequent retest conditions in adult rats. Further, adult males and females exposed to cocaine during adolescence responded more during periods of food availability compared to cocaine-naïve adults.

CONCLUSION

These results indicate that sex and age are vulnerability factors in cocaine abuse, and cocaine exposure during critical developmental stages can have long-lasting effects.

How generalized CNS arousal strengthens sexual arousal (and vice versa)

Heightened states of generalized CNS arousal are proposed here to facilitate sexual arousal in both males and females. Genetic, pharmacologic and biophysical mechanisms by which this happens are reviewed. Moreover, stimulation of the genital epithelia, as triggers of sex behavior, is hypothesized to lead to a greater generalized arousal in a manner that intensifies sexual motivation. Finally, launched from histochemical studies intended to characterize cells in the genital epithelium, a surprising idea is proposed that links density of innervation with the efficiency of wound healing and with the capacity of that epithelium to stimulate generalized CNS arousal. Thus, bidirectional arousal-related mechanisms that foster sexual behaviors are envisioned as follows: from specific to generalized (as with genital stimulation) and from generalized to specific.

The effects of oral micronized progesterone on smoked cocaine self-administration in women

There are currently no FDA-approved pharmacotherapies for cocaine abuse. Converging preclinical and clinical evidence indicates that progesterone may have potential as a treatment for cocaine-abusing women, who represent a growing portion of cocaine users. We have previously shown that oral progesterone reduced the positive subjective effects of cocaine in female cocaine users during the follicular phase of the menstrual cycle, when endogenous progesterone levels were low. To extend these findings, the present study assessed the effects of oral progesterone (150 mg BID) administered during the follicular phase on smoked cocaine self-administration in women relative to the normal follicular and luteal phases. Healthy, non-treatment seeking female cocaine smokers (N=10) underwent three 4-day inpatient stays, during: 1) a normal follicular phase; 2) a normal luteal phase; and 3) a follicular phase when oral progesterone was administered. During each stay, participants completed 4 self-administration sessions in which they first smoked a "sample" dose of cocaine (0, 12, 25 or 50 mg) and then had 5 opportunities at 14-minute intervals to self-administer that dose at a cost of $5 per dose. Expected cocaine dose effects on self-administration, subjective effects, and cardiovascular effects were observed. However, there was no effect of oral progesterone administration or menstrual cycle phase on cocaine self-administration. Thus, oral progesterone was not effective in reducing cocaine use in women under the current conditions. However, based on previous literature, further research assessing the role of oral progesterone for the treatment of cocaine dependence in women is warranted.

Females are more vulnerable to drug abuse than males: evidence from preclinical studies and the role of ovarian hormones

Human and animal research indicates the presence of sex differences in drug abuse. These data suggest that females, compared to males, are more vulnerable to key phases of the addiction process that mark transitions in drug use such as initiation, drug bingeing, and relapse. Recent data indicate that the female gonadal hormone estrogen may facilitate drug abuse in women. For example, phases of the menstrual cycle when estrogen levels are high are associated with enhanced positive subjective measures following cocaine and amphetamine administration in women. Furthermore, in animal research, the administration of estrogen increases drug taking and facilitates the acquisition, escalation, and reinstatement of cocaine-seeking behavior. Neurobiological data suggest that estrogen may facilitate drug taking by interacting with reward- and stress-related systems. This chapter discusses sex differences in and hormonal effects on drug-seeking behaviors in animal models of drug abuse. The neurobiological basis of these differences and effects are also discussed.

Role of progesterone in nicotine addiction: evidence from initiation to relapse

Nicotine addiction continues to be the main cause of preventable death in developed countries. Women and teen girls appear to be more vulnerable on certain aspects of nicotine addiction compared with men and boys. While the mechanism of gender differences in nicotine addiction is not yet clear, evidence suggests that while estrogen may underlie enhanced vulnerability in females, progesterone may protect females. Thus, progesterone may have therapeutic use for tobacco addiction, especially in female smokers. A greater understanding of the role of progesterone in nicotine addiction is important not only from a treatment standpoint, but also from a prevention standpoint: hormone transition phases, such as those that occur at adolescence, and during pregnancy and following birth, as well as following hormonal manipulation (e.g., using methods of hormonal birth control), may all contribute to changes in vulnerability to nicotine addiction. In this review, we summarize recent evidence from clinical and preclinical studies examining the role of progesterone in nicotine addiction focusing on its role during initiation of use and during later phases of the addiction process as a potential relapse prevention treatment. We conclude with future directions including further examination of progesterone as a potential intervention and treatment of nicotine addiction.

The effects of progesterone pretreatment on the response to oral d-amphetamine in Women

Stimulant abuse continues to be a problem, particularly for women. There is increasing preclinical and clinical evidence showing that the hormone progesterone attenuates the behavioral effects of cocaine, and this effect is primarily observed in females. The purpose of the present study was to determine if progesterone would also alter the behavioral effects of another stimulant, oral d-amphetamine (AMPH) in women. Eighteen normal non-drug abusing women completed eight outpatient sessions over two menstrual cycles. During the follicular phase of each cycle, women were administered AMPH (0, 10, 20 mg); in one cycle they were pretreated with oral micronized progesterone (200 mg) and in another cycle they were pretreated with placebo progesterone. Each session, participants completed a range of tasks including subjective measures of abuse liability, cognitive performance tasks, and behavioral measures of impulsivity and risk-taking. AMPH produced dose-related increases in positive subjective effects and these effects were enhanced by progesterone pretreatment. AMPH alone, or in combination with progesterone, had little effect on performance or behavioral measures of impulsivity. These results are in contrast with previous studies showing that progesterone attenuates the subjective response to cocaine and nicotine. Additional studies are needed to explore the modulatory role of progesterone on the effects of AMPH to determine whether progesterone has any clinical utility for AMPH abuse.

The role of progestins in the behavioral effects of cocaine and other drugs of abuse: human and animal research

This review summarizes findings from human and animal research investigating the influence of progesterone and its metabolites allopreganolone and pregnanolone (progestins) on the effects of cocaine and other drugs of abuse. Since a majority of these studies have used cocaine, this will be the primary focus; however, the influence of progestins on other drugs of abuse will also be discussed. Collectively, findings from these studies support a role for progestins in (1) attenuating the subjective and physiological effects of cocaine in humans, (2) blocking the reinforcing and other behavioral effects of cocaine in animal models of drug abuse, and (3) influencing behavioral responses to other drugs of abuse such as alcohol and nicotine in animals. Administration of several drugs of abuse in both human and nonhuman animals significantly increased progestin levels, and this is explained in terms of progestins acting as homeostatic regulators that decrease and normalize heightened stress and reward responses which lead to increased drug craving and relapse. The findings discussed here highlight the complexity of progestin-drug interactions, and they suggest a possible use for these agents in understanding the etiology of and developing treatments for drug abuse.