Behavior and Fos activation reveal that male and female rats differentially assess affective valence during CTA learning and expression

Females are more affected by psychiatric illnesses including eating disorders, depression, and post-traumatic stress disorder than males. However, the neural mechanisms mediating these sex differences are poorly understood. Animal models can be useful in exploring such neural mechanisms. Conditioned taste aversion (CTA) is a behavioral task that assesses how animals process the competition between associated reinforcing and aversive stimuli in subsequent task performance, a process critical to healthy behavior in many domains. The purpose of the present study was to identify sex differences in this behavior and associated neural responses. We hypothesized that females would value the rewarding stimulus (Boost®) relative to the aversive stimulus (LiCl) more than males in performing CTA. We evaluated behavior (Boost® intake, LiCl-induced behaviors, ultrasonic vocalizations (USVs), CTA performance) and Fos activation in relevant brain regions after the acute stimuli [acute Boost® (AB), acute LiCl (AL)] and the context-only task control (COT), Boost® only task (BOT) and Boost®-LiCl task (BLT). Acutely, females drank more Boost® than males but showed similar aversive behaviors after LiCl. Females and males performed CTA similarly. Both sexes produced 55 kHz USVs anticipating BOT and inhibited these calls in the BLT. However, more females emitted both 22 kHz and 55 kHz USVs in the BLT than males: the latter correlated with less CTA. Estrous cycle stage also influenced 55 kHz USVs. Fos responses were similar in males and females after AB or AL. Females engaged the gustatory cortex and ventral tegmental area (VTA) more than males during the BOT and males engaged the amygdala more than females in both the BOT and BLT. Network analysis of correlated Fos responses across brain regions identified two unique networks characterizing the BOT and BLT, in both of which the VTA played a central role. In situ hybridization with RNAscope identified a population of D1-receptor expressing cells in the CeA that responded to Boost® and D2 receptor-expressing cells that responded to LiCl. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior.

Novel mucosal adjuvant, mastoparan-7, improves cocaine vaccine efficacy

Cocaine is one of the most potent and addictive psychostimulants known and there are no available pharmacotherapies to treat cocaine addiction. Here we describe a novel cocaine vaccine employing the mucosal adjuvant and mast cell-activating oligopeptide, mastoparan-7 (M7), to achieve optimal IgA antibody responses in mucosal secretions and effective induction of humoral immunity using a short immunization protocol. This formulation, using a hapten-carrier system to deliver cocaine as antigen, also reduced cocaine penetration of the blood brain barrier and protected mice from its psychoactive effects by reducing cocaine-induced locomotion. Surprisingly, the magnitude of cocaine-specific antibody titers induced by each adjuvant was not the major determinant of functional protection from cocaine challenge. A side-by-side comparison of the two haptens, cocaine and its analog GNC demonstrated that cocaine haptenation resulted in superior functional protection when used in combination with the novel mucosal adjuvant, M7. These results provide a new potential strategy for combatting cocaine addiction through mucosal vaccination.

Estradiol replacement enhances cocaine-stimulated locomotion in female C57BL/6 mice through estrogen receptor alpha

Psychostimulant effects are enhanced by ovarian hormones in women and female rodents. Estradiol increases behavioral responses to psychostimulants in women and female rats, although the underlying mechanism is unknown. This study utilized mice to investigate the time frame and receptor mediation of estradiol's enhancement of cocaine-induced behavior as mice enable parallel use of genetic, surgical and pharmacological methods. The spontaneous behavior of Sham and Ovariectomized (Ovx) female wildtype (WT) mice was determined during habituation to a novel environment and after cocaine administration. Ovx mice were replaced with vehicle (sesame oil) or 17β-estradiol (E2) for 2 days or 30 min prior to a cocaine challenge to investigate the time course of E2's effects. To examine receptor mediation of estradiol effects, Ovx mice replaced for 2 days with either the ERα-selective agonist PPT or the ERβ-selective agonist DPN were compared to Sham mice, and mice lacking either ERα (αERKO) or ERβ (βERKO) were compared to WT littermates. Ovx mice exhibited fewer ambulations during habituation than Sham females. Cocaine-induced increases in behavioral ratings were greater in Sham than in Ovx mice. Two days but not 30 min of E2 replacement in Ovx mice increased cocaine responses to Sham levels. PPT replacement also increased the cocaine response relative to vehicle- or DPN- treated Ovx mice. αERKO mice displayed modestly attenuated behavioral responses to novelty and cocaine compared to αWT littermates, but no behavioral differences were found between βERKO and βWT mice. These results suggest that E2 enhances cocaine-stimulated locomotion in mice predominantly through ERα.

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.

Intravascular food reward

Consumption of calorie-containing sugars elicits appetitive behavioral responses and dopamine release in the ventral striatum, even in the absence of sweet-taste transduction machinery. However, it is unclear if such reward-related postingestive effects reflect preabsorptive or postabsorptive events. In support of the importance of postabsorptive glucose detection, we found that, in rat behavioral tests, high concentration glucose solutions administered in the jugular vein were sufficient to condition a side-bias. Additionally, a lower concentration glucose solution conditioned robust behavioral responses when administered in the hepatic-portal, but not the jugular vein. Furthermore, enteric administration of glucose at a concentration that is sufficient to elicit behavioral conditioning resulted in a glycemic profile similar to that observed after administration of the low concentration glucose solution in the hepatic-portal, but not jugular vein. Finally using fast-scan cyclic voltammetry we found that, in accordance with behavioral findings, a low concentration glucose solution caused an increase in spontaneous dopamine release events in the nucleus accumbens shell when administered in the hepatic-portal, but not the jugular vein. These findings demonstrate that the postabsorptive effects of glucose are sufficient for the postingestive behavioral and dopaminergic reward-related responses that result from sugar consumption. Furthermore, glycemia levels in the hepatic-portal venous system contribute more significantly for this effect than systemic glycemia, arguing for the participation of an intra-abdominal visceral sensor for glucose.

Dopamine uptake inhibitors but not dopamine releasers induce greater increases in motor behavior and extracellular dopamine in adolescent rats than in adult male rats

Most life-long drug addiction begins during adolescence. Important structural and functional changes in brain occur during adolescence and developmental differences in forebrain dopamine systems could mediate a biologic vulnerability to drug addiction during adolescence. Studies investigating age differences in psychostimulant responses have yielded mixed results, possibly because of different mechanisms for increasing extracellular dopamine. Recent research from our laboratory suggests that adolescent dopamine systems may be most affected by selective dopamine uptake inhibitors. We investigated age-related behavioral responses to acute administration of several dopamine uptake inhibitors [methylphenidate, 1-{2-[bis-(4-fluorophenyl)methoxy]ethyl}-4-(3-phenylpropyl)piperazine (GBR12909), and nomifensine] and releasing agents [amphetamine and methylenedioxymethamphetamine (MDMA)] in adolescent and adult male rats. Methylphenidate and amphetamine effects on stimulated dopamine efflux were determined using fast-scan cyclic voltammetry in vivo. Dopamine uptake inhibitors but not dopamine releasing agents induced more locomotion and/or stereotypy in adolescent relative to adult rats. MDMA effects were greater in adults at early time points after dosing. Methylphenidate but not amphetamine induced much greater dopamine efflux in periadolescent relative to adult rats. Periadolescent male rats are particularly sensitive to psychostimulants that are DAT inhibitors but are not internalized and do not release dopamine. Immaturity of DAT and/or DAT associated signaling systems in adolescence specifically enhances behavioral and dopaminergic responses in adolescence.

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.

Androgen decreases dopamine neurone survival in rat midbrain

Clinical studies show that men are more likely to develop disorders affecting midbrain dopaminergic pathways, such as drug addiction and Parkinson's disease (PD). Although a great deal of focus has been given to the role of oestrogen in the maintenance of midbrain dopaminergic pathways, little is known about how testosterone influences these pathways. In the present study, we used stereological analysis of tyrosine hydroxylase-immunoreactive (TH-IR) cell bodies to determine how testosterone influences the dopaminergic cell bodies of the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA). Rats and mice were castrated at postnatal day (PN) 60, and these midbrain cell populations were counted on PN 90. One month after castration, TH-IR cell number had increased in the SNpc and VTA of rats and mice. Replacement with testosterone or the non-aromatisable analogue dihydrotestosterone (DHT) in castrated animals reduced TH-IR cell number in the SNpc and VTA in rats. In mice, the decrease of TH-IR cell number with testosterone or DHT replacement was observed only in the SNpc. The apparent increase in TH-IR neurone number after castration is not explained by an increase in TH expression because the number of nondopaminergic cells (TH-immunonegative, TH-IN) did not decrease proportionally after castration. TH-IN cell number did not change after castration or hormone replacement in rat or mouse SNpc or VTA. These findings suggest that testosterone may play a suppressive role in midbrain dopaminergic pathways.

Oestrogen receptors enhance dopamine neurone survival in rat midbrain

Previous findings in our laboratory and elsewhere have shown that ovariectomy of rats in adulthood attenuates cocaine-stimulated locomotor behaviour. Ovarian hormones enhance both cocaine-stimulated behaviour and increase dopamine overflow after psychomotor stimulants. The present study aimed to determine whether ovarian hormones have these effects in part by maintaining dopamine neurone number in the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) and to investigate the roles of specific oestrogen receptors (ERs) in the maintenance of mesencephalic dopamine neurones. To accomplish this goal, we used unbiased stereological techniques to estimate the number of tyrosine hydroxylase-immunoreactive (TH-IR) cell bodies in midbrain regions of intact, ovariectomised and hormone-replaced female rats and mice. Animals received active or sham gonadectomy on postnatal day 60 and received vehicle, 17beta-oestradiol (E(2)) or selective ER agonists propyl-pyrazole-triol (PPT, ERalpha) or diarylpropionitrile (DPN, ERbeta) for 1 month post-surgery. In both rats and mice, ovariectomy reduced the number of TH-IR cells in the SNpc and VTA. Replacement with E(2), PPT or DPN prevented or attenuated the loss observed with ovariectomy in both rats and mice. An additional study using ER knockout mice revealed that adult female mice lacking ERalpha had fewer TH-IR cells in midbrain regions than wild-type mice, whereas mice lacking ERbeta had TH-IR cell counts comparable to wild-type. These findings suggest that, although both ER subtypes play a role in the maintenance of TH-IR cell number in the SNpc and VTA, ERalpha may play a more significant role.

Are adolescents more vulnerable to drug addiction than adults? Evidence from animal models

BACKGROUND AND RATIONALE

Epidemiological evidence suggests that people who begin experimenting with drugs of abuse during early adolescence are more likely to develop substance use disorders (SUDs), but this correlation does not guarantee causation. Animal models, in which age of onset can be tightly controlled, offer a platform for testing causality. Many animal models address drug effects that might promote or discourage drug intake and drug-induced neuroplasticity.

METHODS

We have reviewed the preclinical literature to investigate whether adolescent rodents are differentially sensitive to rewarding, reinforcing, aversive, locomotor, and withdrawal-induced effects of drugs of abuse.

RESULTS AND CONCLUSIONS

The rodent model literature consistently suggests that the balance of rewarding and aversive effects of drugs of abuse is tipped toward reward in adolescence. However, increased reward does not consistently lead to increased voluntary intake: age effects on voluntary intake are drug and method specific. On the other hand, adolescents are consistently less sensitive to withdrawal effects, which could protect against compulsive drug seeking. Studies examining neuronal function have revealed several age-related effects but have yet to link these effects to vulnerability to SUDs. Taken together, the findings suggest factors which may promote recreational drug use in adolescents, but evidence relating to pathological drug-seeking behavior is lacking. A call is made for future studies to address this gap using behavioral models of pathological drug seeking and for neurobiologic studies to more directly link age effects to SUD vulnerability.

Novelty-induced locomotion is positively associated with cocaine ingestion in adolescent rats; anxiety is correlated in adults

The present studies assessed the roles of sex, age, novelty-seeking and plus-maze behavior on cocaine drinking in rats. Cocaine/saccharin solution was available in three daily, 5-hour sessions then a saccharin-only solution was also available in following sessions. In the one-bottle drinking phase, early and late adolescent males, post-natal day 28 (PN28) and PN42, consumed more cocaine/saccharin solution than young adults (PN65), but females did not exhibit significant age differences. Adolescents of both sexes consumed more cocaine/saccharin than adults during choice drinking. Saccharin availability in the two-bottle trials decreased cocaine/saccharin consumption in PN28 and PN65 rats. After a drug-free period, cocaine-stimulated locomotion was lower in cocaine/saccharin drinking than saccharin-only males, indicating tolerance. We tested the hypothesis that individual differences in pre-screened behavioral traits would correlate with cocaine/saccharin consumption in PN28 and PN65 male rats. High locomotor responses to novelty were associated with greater cocaine/saccharin drinking in adults in one-bottle sessions. In the subsequent choice drinking phase, correlations were age-specific. Adolescents with high novelty-induced locomotion and adults that spent less time on open arms of the elevated plus-maze drank more cocaine/saccharin. Thus, behavioral phenotypes correlated with individual differences in cocaine/saccharin consumption in an age-related manner.

Cocaine increases stimulated dopamine release more in periadolescent than adult rats

The neural mechanisms responsible for the enhanced adolescent vulnerability for initiating drug abuse are unclear. We investigated whether age differences in dopamine neurotransmission could explain cocaine's enhanced psychomotor effects in the periadolescent rat. Electrical stimulation of the medial forebrain bundle of anesthetized post-natal age 28 days (PN28) and PN65 rats elicited dopamine release in caudate nucleus and nucleus accumbens core before and after 15 mg/kg cocaine i.p. Extracellular dopamine concentrations were greater in PN65 than PN28 caudate following 20 and 60 Hz stimulations and in the PN65 nucleus accumbens following 60 Hz stimulations. Cocaine increased dopamine concentrations elicited by 20 Hz stimulations 3-fold in the adult, but almost 9-fold in periadolescent caudate. Dopamine release rate was lower in the periadolescent caudate although total dopamine clearance was similar to that of adults. The periadolescent caudate achieved adult levels of clearance by compensating for a lower V(max) with higher uptake affinity. Tighter regulation of extracellular dopamine by the higher uptake/release ratio in periadolescents led to greater increases after cocaine. In nucleus accumbens, dopamine release and V(max) were lower in periadolescents than adults, but uptake affinity and cocaine effects were similar. Immaturity of dopamine neurotransmission in dorsal striatum may underlie enhanced acute responses to psychostimulants in adolescent rats and suggests a mechanism for the greater vulnerability of adolescent humans to drug addiction.

A single high dose of cocaine induces differential sensitization to specific behaviors across adolescence

RATIONALE

Adolescence is a critical period for drug addiction. Acute stimulant exposure elicits different behavioral responses in adolescent and adult rodents. The same biological differences that mediate age-specific behavioral responsiveness to stimulants in rodents could contribute to increased addiction vulnerability in adolescent humans.

OBJECTIVES

This study compared the ability of a single high dose of cocaine (40 mg/kg) to induce behavioral sensitization to a challenge dose of cocaine (10 mg/kg) 24 h later in young adolescent postnatal day 28 (PN 28), mid-adolescent (PN 42), and young adult (PN 65) male rats. Horizontal activity was resolved into ambulatory and non-ambulatory movements. An observational behavioral rating was obtained by recording specific behaviors. We examined if individual behavioral responses to novelty and cocaine correlate with sensitization in each age group.

RESULTS

Single dose cocaine pretreatment induced behavioral sensitization to non-ambulatory horizontal activity, sniffing behaviors, and stereotypies in animals of all ages. Ambulatory sensitization was observed only in the youngest adolescents. Cocaine pretreatment caused greater increases in stereotypies in the young adolescents than in adults. The magnitude of the behavioral response to the initial cocaine treatment was positively correlated with the magnitude of sensitization in individual young adolescents. High levels of novelty-induced ambulatory activity only correlated with the magnitude of ambulatory sensitization in the youngest adolescents.

CONCLUSIONS

We conclude that a single high dose of cocaine produces age-specific patterns of behavioral sensitization. Young adolescent rats appear to be more sensitive than adults to some of the behavioral alterations induced by a single high dose of cocaine.

Sex differences in the neurochemical and functional effects of MDMA in Sprague-Dawley rats

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA; "Ecstasy") use has been associated with acute toxicities and persistent depletion of the neurotransmitter serotonin (5-HT).

OBJECTIVES

This study investigates whether sex differences in the acute and long-term effects of MDMA exist.

METHODS

Male and female rats received saline or 15 mg/kg MDMA, ip, bid for 4 days. Temperature was monitored on days 1 and 4. Locomotor activity was measured in a second cohort of animals on days 1 and 4 and after recovery on day 14. The effects of MDMA on performance in a plus maze task and brain levels of serotonin (5-HT) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in a third cohort of animals 2 weeks after the last MDMA treatment.

RESULTS

Locomotor activity and temperature increased after MDMA administration on day 1. The drug-induced increases in temperature but not locomotion attenuated with repeated MDMA administration. Male and female MDMA-treated rats spent less time in the open arms of the elevated plus maze and had less 5-HT and 5-HIAA in all brain regions 2 weeks after the end of treatment. Temperature effects of MDMA and persistent effects on plus maze and brain serotonin content were similar in males and females. In contrast, females exhibited markedly greater locomotor stimulation after acute MDMA and also showed sensitization to an acute challenge 2 weeks later.

CONCLUSIONS

MDMA elicits substantially greater locomotor activation in female rats than in males, but persistent effects on anxiety and serotonin content were similar in males and females.

Sex differences in neurochemical effects of dopaminergic drugs in rat striatum

Previous data indicate that dopamine neurotransmission is differently regulated in male and female rats. The purpose of the present study was to investigate the dopamine transporter and autoreceptor as potential loci responsible for this sex difference. Fast cyclic voltammetry at carbon-fiber microelectrodes was used to monitor changes in electrically evoked levels of extracellular dopamine in the striata of anesthetized male and female rats before and after administration of an uptake inhibitor, a dopamine D2 antagonist, or a D3/D2 agonist. Administration of 40 mg/kg cocaine ip increased electrically-evoked extracellular dopamine concentrations in both sexes, but to a significantly greater extent in female striatum at the higher stimulation frequencies. The typical antipsychotic, haloperidol, increased dopamine efflux in both sexes but the effect was twice as large in the female striatum. The D3/D2 agonist quinpirole induced an unexpected, transient increase in dopamine efflux following high-frequency stimulation only in females, and evoked dopamine was higher in females across this entire time course. More detailed analysis of cocaine effects revealed no fundamental sex differences in the interaction of cocaine with DAT in vivo or in synaptosomes. These results indicate that nigrostriatal dopamine neurotransmission in the female rat is more tightly regulated by autoreceptor and transporter mechanisms, perhaps related by greater autoreceptor control of DAT activity. Thus, baseline sex differences in striatal dopamine regulation induce different pharmacologic responses. These results contribute to understanding sex differences in stimulant-induced locomotor activity in rats and may have broader implications for neurologic disorders and their pharmacotherapies in humans.

Enhanced behavioral response to repeated-dose cocaine in adolescent rats

RATIONALE

Most lifelong drug addiction in humans originates during adolescence. Important structural and functional changes in the brain occur during adolescence, but there has been little direct study of how this impacts on drug abuse vulnerability. An emerging literature suggests that adolescents exhibit different behavioral responses to single doses of several addictive drugs, including ethanol, amphetamine, and cocaine. However, few studies have explored behavioral responses to the repeated dosing that is characteristic of human abuse of these substances.

OBJECTIVES

We have investigated age-related behavioral responses to acute "binge" cocaine treatment between adults and adolescents.

RESULTS

Adolescent rats displayed an exaggerated behavioral response to cocaine administered in two different binge patterns. Total locomotion after cocaine administration was the same in adolescents and adults. However, adolescent rats engaged in more intense stereotypic behaviors, including paw treading, head weaving, and focused sniffing than adult rats. These differences were observable following a modest dose of cocaine and became more robust following subsequent doses within a binge. Cocaine [corrected] brain levels were not significantly different between age groups during any of the exposure sessions.

CONCLUSIONS

These findings suggest that equivalent tissue concentrations of cocaine produce a greater behavioral response in young rats, and that adolescent animals display an apparent form of intrabinge sensitization.

Vaginal lavage attenuates cocaine-stimulated activity and establishes place preference in rats

Sex and estrous cycle stage affect psychostimulant responses in animals. Cycle stage is typically monitored by vaginal lavage. The present studies tested the hypothesis that vaginal lavage modifies behavioral responses to acute cocaine. Female rats were restrained by briefly holding the tail for either vaginal lavage or touching the thigh, or were undisturbed, for 7-10 days prior to testing. Although habituation to the open-field test chamber was equal in each group, repeated lavage decreased horizontal activity relative to naive rats following acute cocaine (10 mg/kg ip). Lavage and touch attenuated cocaine-stimulated vertical activity. A single lavage prior to testing did not affect cocaine-stimulated motor behavior. Estrous cycle influenced motor activity only in nonlavaged rats. The high cocaine-induced responding observed in proestrous and estrous nonlavaged rats was completely blocked by vaginal lavage. A separate experiment tested the ability of vaginal lavage to establish a conditioned place preference. Vaginal lavage immediately prior to the conditioning session, but neither lavage after conditioning nor touch before, induced a significant preference. These results suggest that vaginal lavage serves as a reinforcing stimulus and interacts with a neural substrate that mediates enhanced locomotor responses to cocaine during proestrus and estrus.

Functional selectivity of dopamine receptor agonists. I. Selective activation of postsynaptic dopamine D2 receptors linked to adenylate cyclase

Dihydrexidine (DHX), the first high-affinity D(1) dopamine receptor full agonist, is only 10-fold selective for D(1) versus D(2) receptors, having D(2) affinity similar to the prototypical agonist quinpirole. The D(2) functional properties of DHX and its more D(2) selective analog N-n-propyl-dihydrexidine (PrDHX) were explored in rat brain and pituitary. DHX and PrDHX had binding characteristics to D(2) receptors in rat striatum typical of D(2) agonists, binding to both high- and low-affinity sites and being sensitive to guanine-nucleotides. Consistent with these binding data, both DHX and PrDHX inhibited forskolin-stimulated cAMP synthesis in striatum with a potency and intrinsic activity equivalent to that of quinpirole. Unexpectedly, however, DHX and PrDHX had little functional effect at D(2) receptors expressed on dopaminergic neurons that mediate inhibition of cell firing, dopamine release, or dopamine synthesis. Quantitative receptor competition autoradiography demonstrated that DHX bound to D(2) receptors in striatum (predominantly postsynaptic receptor sites) with equal affinity as D(2) sites in the substantia nigra (autoreceptor sites). The data from these experiments, coupled with what is known about the location of specific dopamine receptor isoforms, lead to the hypothesis that DHX, after binding to D(2L) and D(2S) receptors, causes agonist-typical functional changes only at some of these receptors. This phenomenon (herein termed "functional selectivity") suggests that drugs may be targeted not only at specific receptor isoforms but also at separate functions mediated by a single isoform, yielding novel approaches to drug discovery.

Sex differences in cocaine-stimulated motor behavior: disparate effects of gonadectomy

Sex differences in biological substrates of drug use and addiction are poorly understood. The present study investigated sexual dimorphisms in motor behavior following acute cocaine administration (10, 20, or 40 mg/kg, i.p.). Cocaine increased stereotypy rating, horizontal and vertical activity in both sexes, and effects were always greater in females than males. A population analysis using data from multiple experiments indicated that horizontal activity scores were normally distributed in males but not in females. Gonadectomy induced disparate effects on cocaine-stimulated motor behavior. Population analysis indicated that castrated males exhibited more horizontal activity and stereotypy than shams. Ovariectomy did not affect cocaine-stimulated stereotypy but did attenuate horizontal activity in a subset of rats that had not been vaginally lavaged. In summary, gonadectomy effects were sex and behavioral topography specific and indicate that activational effects of gonadal hormones partially mediate the robust sex differences in cocaine-stimulated open-field behavior.

Sex, steroids, and stimulant sensitivity

The current study investigated ovarian modulation of the locomotor response to cocaine in rats. Ovariectomy in females lowered the response to cocaine (10 mg/kg i.p.), whereas castration did not change that of males. The locomotor responses of prepubertal males and females to cocaine were similar. However, the postpubertal sex difference resulted from a fall in cocaine-stimulated locomotion in males rather than a rise in females. Neonatal testosterone treatment of female rat pups decreased the response to cocaine in adulthood. These findings suggest that both the activational and organizational effects of gonadal steroids contribute to the greater response of females to cocaine.

Effect of ovarian hormones and estrous cycle on stimulation of the hypothalamo-pituitary-adrenal axis by cocaine

Cocaine is known to exert sexually dimorphic HPA axis effects in rats and to disrupt estrous cyclicity and/or fertility in rats, nonhuman primates, and humans. The present studies investigated the reciprocal interactions between ovarian hormones and HPA axis responses to cocaine. Thirty minutes after injection, cocaine (15 mg/kg i.p.) increased serum ACTH and corticosterone more in cycling than ovariectomized females or male rats. ACTH and corticosterone were highest in proestrus when estradiol was elevated. Cocaine did not alter serum estradiol in females or testosterone in males but did stimulate progesterone secretion in both sexes. Cocaine-stimulated progesterone secretion was significantly greater in females than in males or ovariectomized females, and greater in proestrous than diestrous 1 rats. Cocaine stimulated corticosterone and progesterone secretion in sham-adrenalectomized, but not adrenalectomized rats, indicating that the adrenal gland and not the ovary is the source of cocaine-stimulated progesterone. Estrogen influenced cocaine-stimulated progesterone secretion more than corticosterone, suggesting different release mechanisms for the two steroids in the adrenal. These results suggest that adrenally derived progesterone could contribute to cocaine-induced physiological changes, including inhibited gonadotropin release.

Partial, graded losses of dopamine terminals in the rat caudate-putamen: an animal model for the study of compensatory adaptation in preclinical parkinsonism

Procedures to lesion dopamine (DA) neurons innervating the rat caudate-putamen (CP) in a partial, graded fashion are described in this study. The goal is to provide a lesion model that supports intra-animal comparisons of voltammetric recordings used to investigate compensatory adaptation of DA neurotransmission. Lesions exploited the topography of mesostriatal DA neurons, microinjections of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial and lateral edges of the ventral mesencephalon containing DA cell bodies and microdissection of the CP into six regions. Analysis of tissue DA content in these regions by HPLC-EC demonstrated that 6-OHDA injected into the lateral substantia nigra results in a significantly greater loss of DA in lateral versus medial regions of the CP. The direction of the graded loss of DA was reversed (i.e. a medial to lateral lesion gradient) by the injection of 6-OHDA into the ventral tegmental area near the medial SN. Extracellular concentrations of electrically evoked DA could be measured across the mediolateral axis of the CP in a single animal using the technique of in vivo voltammetry. More importantly, graded decreases in the amplitude of evoked DA levels generally followed the direction of the tissue DA gradient in lesioned animals. These results suggest that the graded loss of DA terminals in the CP, coupled to a spatially and temporally resolved technique for monitoring extracellular DA, is a viable tool for investigating compensatory adaptation in the mesostriatal DA system.

Dopamine release and uptake are greater in female than male rat striatum as measured by fast cyclic voltammetry

The present studies investigated sexual dimorphisms in dopamine release and uptake using fast-scan cyclic voltammetry in anesthetized rats and in brain slices. Electrical stimulation of the medial forebrain bundle of anesthetized rats at high frequency (60 Hz) elicited significantly more extracellular dopamine in the caudate nucleus of females than males. This sex difference was apparent over a range of current intensities applied to the stimulating electrode. Local electrical stimulation of brain slices in vitro verified in vivo results as more extracellular dopamine was elicited by single and 10 pulse stimulations in the caudate nucleus of females. Kinetic analysis of in vivo and in vitro dopamine overflow data indicated that dopamine release (the concentration of dopamine released per stimulus pulse) and the maximal velocity of dopamine uptake are greater in female rats, but the affinity of the transporter for dopamine was the same in males and females. None of these three parameters varied across the female estrous cycle. Linear regression analysis of dopamine release versus maximal uptake velocity data indicated a significant association of release and uptake sites in each sex and regression lines for males and females virtually overlapped. One explanation for these results is greater dopamine neuron terminal density in female caudate nucleus. These sexual dimorphisms in dopaminergic neurotransmission provide a novel, plausible mechanism to explain robust sex differences in behavioral responses of rats to psychostimulant drugs and may have implications for human neurological disorders and drug abuse.

Effects of sex and gonadectomy on cocaine metabolism in the rat

The purpose of the current study is to determine whether sex differences in metabolism of cocaine (COC) exist that could contribute to the greater behavioral sensitivity of females to COC administration. To investigate this question, concentrations of COC and its two principle metabolites benzoylecgonine (BE) and ecgonine methyl ester (EME) were measured by gas chromatography/mass spectroscopy in brain and plasma collected from male and female rats that were sacrificed between 5 and 90 min after injection COC (15 mg/kg i.p.). COC concentrations did not differ in plasma or brain tissue of males and females, but sex-specific patterns of metabolite distribution were detected. BE was 2-fold higher in plasma and brain of males than females, whereas EME was much higher in brain and plasma of females. The influence of gonadal hormones on COC metabolite patterns were determined using gonadectomized and prepubertal rats. Castration of male or female rats did not alter brain or plasma COC, but did decrease BE concentrations. Seven-day-old pups injected with 15 mg/kg of COC had higher blood and brain COC than adults and relatively low levels of metabolites. No sex differences were found for COC, BE, or EME in brain or plasma of pups. These findings indicate that although gonadal steroids influence COC metabolism, these effects do not explain sex differences in COC-induced behaviors.

Dissociation of dopamine release in the nucleus accumbens from intracranial self-stimulation

Mesolimbic dopamine-releasing neurons appear to be important in the brain reward system. One behavioural paradigm that supports this hypothesis is intracranial self-stimulation (ICS), during which animals repeatedly press a lever to stimulate their own dopamine-releasing neurons electrically. Here we study dopamine release from dopamine terminals in the nucleus accumbens core and shell in the brain by using rapid-responding voltammetric microsensors during electrical stimulation of dopamine cell bodies in the ventral tegmental area/substantia nigra brain regions. In rats in which stimulating electrode placement failed to elicit dopamine release in the nucleus accumbens, ICS behaviour was not learned. In contrast, ICS was acquired when stimulus trains evoked extracellular dopamine in either the core or the shell of the nucleus accumbens. In animals that could learn ICS, experimenter-delivered stimulation always elicited dopamine release. In contrast, extracellular dopamine was rarely observed during ICS itself. Thus, although activation of mesolimbic dopamine-releasing neurons seems to be a necessary condition for ICS, evoked dopamine release is actually diminished during ICS. Dopamine may therefore be a neural substrate for novelty or reward expectation rather than reward itself.

Dopamine release and uptake rates both decrease in the partially denervated striatum in proportion to the loss of dopamine terminals

The present study tested the hypothesis that normal concentrations of extracellular dopamine are preserved in the partially denervated striatum without active compensatory changes in dopamine uptake or release. One to four weeks after adult rats were unilaterally lesioned with 6-hydroxydopamine, fast-scan cyclic voltammetry at Nafion-coated, carbon-fiber microelectrodes was used to monitor extracellular dopamine levels in vivo, under urethane anesthesia. Simultaneous voltammetric recordings were collected in the lesioned and contralateral control striata. Extracellular dopamine was elicited by bilateral electrical stimulation of the medial forebrain bundle. A 20 Hz stimulation evoked similar concentrations of extracellular dopamine in both lesioned and control striata, although tissue dopamine was decreased 30-70% in lesioned striata, as determined subsequently by HPLC-EC. However, kinetic analysis of the voltammetric recordings revealed that the concentration of dopamine released per stimulus pulse and Vmax for dopamine uptake decreased in proportion to the magnitude of the lesion. These data support the hypothesis that normal extracellular dopamine levels can be generated in the partially lesioned striatum in the absence of active neuronal compensation. These results also suggest that passive mechanisms involved in the regulation of extracellular dopamine play an important role in maintaining function during the preclinical or presymptomatic phase of Parkinson's disease.

Microelectrodes for the measurement of catecholamines in biological systems

Many of the molecules involved in biological signaling processes are easily oxidized and have been monitored by electrochemical methods. Temporal response, spatial considerations, and sensitivity of the electrodes must be optimized for the specific biological application. To monitor exocytosis from single cells in culture, constant potential amperometry offers the best temporal resolution, and a low-noise picoammeter improves the detection limits. Smaller electrodes, with 1-micron diameters, provided spatial resolution sufficient to identify the locations of release sites on the surface of single cells. For the study of neurotransmitter release in vivo, larger cylindrical microelectrodes are advantageous because the secreted molecules come from multiple terminals near the electrode, and the greater amounts lead to a larger signal that emerges from the Johnson noise of the current amplifier. With this approach, dopamine release elicited by two electrical stimulus pulses at 10 Hz was detected with fastscan cyclic voltammetry in vivo. Nafion-coated elliptical electrodes have previously been shown to be incapable of detecting such concentration changes without extensive signal averaging. In addition, we demonstrate that high-pass filtering (200 Hz) of cyclic voltammograms recorded at 300 V/s decreases the background current and digitization noise at these microelectrodes, leading to an improved signal. Also, high-pass filtering discriminated against ascorbic acid, DOPAC, and acidic pH changes, three common interferences in vivo.

Triadimefon and triadimenol: effects on monoamine uptake and release

Acute administration of the agricultural fungicide triadimefon produced a neurotoxic syndrome in rats characterized by increased motor activity, stereotyped behaviors, and altered monoamine metabolism. Triadimenol, a metabolite of triadimefon in mammals, plants, and soil, also increased motor activity in rodents. To test the hypothesis that triadimefon and triadimenol are indirect-acting dopamine agonists, the present studies examined their abilities to inhibit monoamine uptake, bind to the dopamine transporter, and stimulate dopamine efflux in rat brain tissue, in vitro. Both triazoles inhibited the uptake of dopamine in striatal synaptosomal preparations. Triadimefon was 100-fold less potent than GBR12909, a prototypical inhibitor of dopamine uptake (IC50 = 4.7 microM vs. 37.2 nM, respectively), and triadimenol was about three-fold less potent than triadimefon. Triadimefon also weakly inhibited the uptake of norepinephrine in cortical synaptosomes (IC50 = 22.4 microM), but neither compound blocked the uptake of serotonin in cortical synaptosomes (IC50s > 100 microM). Triadimefon and triadimenol had similar affinity for [3H]mazindol binding sites on the dopamine transporter (IC50s approximately 1-1.5 microM, only two- to threefold greater than GBR12909). Neither triadimefon nor triadimenol (0.01-100 microM) increased basal efflux of [3H]DA that had been preloaded into striatal minces in vitro. An unexpected result was that GBR12909 (10 microM) increased basal efflux of [3H]DA by 71%, suggesting that this compound has DA releasing properties. These data suggest that increased synaptic concentrations of dopamine due to inhibition of dopamine uptake may play an important role in the neurobehavioral effects of triadimefon and triadimenol.

Overoxidized polypyrrole-coated carbon fiber microelectrodes for dopamine measurements with fast-scan cyclic voltammetry

Thin films of overoxidized polypyrrole have been electro-chemically coated onto carbon fiber microelectrodes and used for dopamine measurements with background-substracted, fast-scan cyclic voltammetry at a scan rate of 300 V/s. The films were stable on the electrode surface only when the electrodes were scanned to high potentials (1400 mV vs SSCE) in pH 7.4 aqueous buffer. Dopamine sensitivity and ascorbate and dihydroxyphenylacetic acid (DOPAC) rejection at the overoxidized polypyrrole-coated electrode were compared to those at carbon fiber electrodes coated with Nafion, a perfluorinated ion-exchange material. At 300 V/s, the overoxidized polypyrrole-coated electrode was almost 3 times more sensitive to dopamine than an uncoated disk electrode. Furthermore, the films were as effective as Nafion in the attenuation of the response to ascorbate and DOPAC, common interferences of dopamine in vivo. Overoxidized polypyrrole-coated electrodes maintained a stable response to dopamine for several hours when implanted in the rat brain. The electrochemical deposition procedure was effective at both elliptical and cylindrical electrodes. This is in contrast to the dip-coating procedures employed with Nafion films that lead to nonuniform coatings at cylindrical electrodes.

Interhemispheric modulation of dopamine receptor interactions in unilateral 6-OHDA rodent model

A critical assumption in the unilateral 6-hydroxydopamine (6-OHDA) model is that interactions between the intact and denervated hemispheres do not influence the response to insult. The present study examined this issue by assessing the effects of unilateral substantia nigra 6-OHDA lesions in rats that previously had received corpus callosum transections, a treatment designed to minimize interhemispheric influences. Quantitative autoradiography in the caudate-putamen ipsilateral to the lesion revealed that corpus callosum transection did not alter the increase in D2-like receptors ([125I]-epidepride-labeled sites) that is induced by unilateral 6-OHDA lesion. There were no effects of either 6-OHDA lesion or transection on D1 receptor density ([125I]-SCH23982 autoradiography). As a functional endpoint, dopamine-stimulated cAMP efflux was measured in superfused striatal slices. In this paradigm, the net effect of dopamine (DA) represents a combination of D1 receptor-mediated stimulation and D2 receptor-mediated inhibition. 6-OHDA lesion increased cAMP efflux induced by exposure to 100 microM DA alone; corpus callosum transection did not alter this effect. An interaction between 6-OHDA lesion and transection status was revealed, however, by comparison of results obtained with DA alone vs. DA plus the D2 antagonist sulpiride (to block the D2 inhibitory effects of 100 microM DA). This comparison revealed two important effects of 6-OHDA lesion in rats with an intact corpus callosum: 1) a moderate decrease in dopamine D1 receptor-mediated stimulation; and 2) a dramatic decrease in the ability of D2 receptors to inhibit this stimulation. Corpus callosum transection prevented these effects of 6-OHDA. These results provide a biochemical demonstration of D1:D2 receptor uncoupling in unilateral 6-OHDA lesioned rats, and suggest that interhemispheric influences (e.g., contralateral cortico-striatal glutamatergic projections) may contribute to lesion-induced alterations in D1:D2 receptor interactions.

Triadimefon, a triazole fungicide, induces stereotyped behavior and alters monoamine metabolism in rats

Triadimefon, a triazole fungicide, has been observed to increase locomotion and induce stereotyped behavior in rodents. The present experiments designed to characterize the stereotyped behavior induced by triadimefon used a computer-supported observational method, and tested the hypothesis that these observed effects involved central dopaminergic systems. Adult male and female Sprague-Dawley rats were injected with triadimefon (0, 50, 100, and 200 mg/kg) in corn oil (2 ml/kg ip) 4 hr prior to behavioral assessment. The two lowest doses of triadimefon increased the frequency of locomotion and rearing, while the highest dose induced highly stereotyped behaviors, including backward locomotion, circling, and head weaving. Immediately after behavioral testing, the rats were sacrificed, and the striata and olfactory tubercles, terminal fields of the nigrostriatal and mesolimbic dopamine systems, respectively, were removed. Steady-state concentrations of the monoamines dopamine and serotonin and their metabolites were determined by HPLC-EC. In independent experiments, the direct effects of triadimefon on dopamine (D1 and D2) receptor binding and dopamine-sensitive adenylate cyclase activity were assessed in vitro using rat striata. Dopamine concentrations were increased in olfactory tubercles, but decreased in striatum. Concentrations of 5-hydroxyindoleacetic acid (the major metabolite of serotonin) were increased only in striatum, and only in animals treated with 200 mg/kg triadimefon. In vitro, triadimefon neither competed with D1 or D2 dopaminergic radioligands nor affected dopamine-stimulated adenylate cyclase activity. Together these behavioral and biochemical data lend support to the hypothesis that triadimefon may have actions similar to those produced by indirect-acting dopamine agonists.