Ventral Tegmental Area Dopamine Cell Activation during Male Rat Sexual Behavior Regulates Neuroplasticity and d-Amphetamine Cross-Sensitization following Sex Abstinence.

UNLABELLED: Experience with sexual behavior causes cross-sensitization of amphetamine reward, an effect dependent on a period of sexual reward abstinence. We previously showed that ΔFosB in the nucleus accumbens (NAc) is a key mediator of this cross-sensitization, potentially via dopamine receptor activation. However, the role of mesolimbic dopamine for sexual behavior or cross-sensitization between natural and drug reward is unknown. This was tested using inhibitory designer receptors exclusively activated by designer drugs in ventral tegmental area (VTA) dopamine cells. rAAV5/hSvn-DIO-hm4D-mCherry was injected into the VTA of TH::Cre adult male rats. Males received clozapine N-oxide (CNO) or vehicle injections before each of 5 consecutive days of mating or handling. Following an abstinence period of 7 d, males were tested for amphetamine conditioned place preference (CPP). Next, males were injected with CNO or vehicle before mating or handling for analysis of mating-induced cFos, sex experience-induced ΔFosB, and reduction of VTA dopamine soma size. Results showed that CNO did not affect mating behavior. Instead, CNO prevented sexual experience-induced cross-sensitization of amphetamine CPP, ΔFosB in the NAc and medial prefrontal cortex, and decreases in VTA dopamine soma size. Expression of hm4D-mCherry was specific to VTA dopamine cells and CNO blocked excitation and mating-induced cFos expression in VTA dopamine cells. These findings provide direct evidence that VTA dopamine activation is not required for initiation or performance of sexual behavior. Instead, VTA dopamine directly contributes to increased vulnerability for drug use following loss of natural reward by causing neuroplasticity in the mesolimbic pathway during the natural reward experience. SIGNIFICANCE STATEMENT: Drugs of abuse act on the neural pathways that mediate natural reward learning and memory. Exposure to natural reward behaviors can alter subsequent drug-related reward. Specifically, experience with sexual behavior, followed by a period of abstinence from sexual behavior, causes increased reward for amphetamine in male rats. This study demonstrates that activation of ventral tegmental area dopamine neurons during sexual experience regulates cross-sensitization of amphetamine reward. Finally, ventral tegmental area dopamine cell activation is essential for experience-induced neural adaptations in the nucleus accumbens, prefrontal cortex, and ventral tegmental area. These findings demonstrate a role of mesolimbic dopamine in the interaction between natural and drug rewards, and identify mesolimbic dopamine as a key mediator of changes in vulnerability for drug use after loss of natural reward.

Circadian Rhythms and Substance Abuse: Chronobiological Considerations for the Treatment of Addiction.

Reward-related learning, including that associated with drugs of abuse, is largely mediated by the dopaminergic mesolimbic pathway. Mesolimbic neurophysiology and motivated behavior, in turn, are modulated by the circadian timing system which generates ∼24-h rhythms in cellular activity. Both drug taking and seeking and mesolimbic dopaminergic neurotransmission can vary widely over the day. Moreover, circadian clock genes are expressed in ventral tegmental area dopaminergic cells and in mesolimbic target regions where they can directly modulate reward-related neurophysiology and behavior. There also exists a reciprocal influence between drug taking and circadian timing as the administration of drugs of abuse can alter behavioral rhythms and circadian clock gene expression in mesocorticolimbic structures. These interactions suggest that manipulations of the circadian timing system may have some utility in the treatment of substance abuse disorders. Here, the literature on bidirectional interactions between the circadian timing system and drug taking is briefly reviewed, and potential chronotherapeutic considerations for the treatment of addiction are discussed.

Diurnal rhythms in neural activation in the mesolimbic reward system: critical role of the medial prefrontal cortex.

Previous evidence suggests a circadian modulation of drug-seeking behavior and responsiveness to drugs of abuse. To identify potential mechanisms for rhythmicity in reward, a marker of neural activation (cFos) was examined across the day in the mesolimbic reward system. Rats were perfused at six times during the day [zeitgeber times (ZTs): 2, 6, 10, 14, 18, and 22], and brains were analysed for cFos and tyrosine hydroxylase (TH)-immunoreactive (IR) cells. Rhythmic expression of cFos was observed in the nucleus accumbens (NAc) core and shell, in the medial prefrontal cortex (mPFC), and in TH-IR and non-TH-IR cells in the ventral tegmental area (VTA), with peak expression during the late night and nadirs during the late day. No significant rhythmicity was observed in the basolateral amgydala or the dentate gyrus. As the mPFC provides excitatory input to both the NAc and VTA, this region was hypothesised to be a key mediator of rhythmic neural activation in the mesolimbic system. Hence, the effects of excitotoxic mPFC lesions on diurnal rhythms in cFos immunoreactivity at previously observed peak (ZT18) and nadir (ZT10) times were examined in the NAc and VTA. mPFC lesions encompassing the prelimbic and infralimbic subregions attenuated peak cFos immunoreactivity in the NAc, eliminating the diurnal rhythm, but had no effect on VTA rhythms. These results suggest that rhythmic neural activation in the mesolimbic system may contribute to diurnal rhythms in reward-related behaviors, and indicate that the mPFC plays a critical role in mediating rhythmic neural activation in the NAc.

Bidirectional interactions between the circadian and reward systems: is restricted food access a unique zeitgeber?

Reward is mediated by a distributed series of midbrain and basal forebrain structures collectively referred to as the brain reward system. Recent evidence indicates that an additional regulatory system, the circadian system, can modulate reward-related learning. Diurnal or circadian changes in drug self-administration, responsiveness to drugs of abuse and reward to natural stimuli have been reported. These variations are associated with daily rhythms in mesolimbic electrical activity, dopamine synthesis and metabolism, and local clock gene oscillations. Conversely, the presentation of rewards appears capable of influencing circadian timing. Rodents can anticipate a daily mealtime by the entrainment of a series of oscillators that are anatomically distinct from the suprachiasmatic nucleus. Other work has indicated that restricted access to non-nutritive reinforcers (e.g. drugs of abuse, sex) or to palatable food in the absence of an energy deficit is capable of inducing relatively weak anticipatory activity, suggesting that reward alone is sufficient to induce anticipation. Recent attempts to elucidate the neural correlates of anticipation have revealed that both restricted feeding and restricted palatable food access can entrain clock gene expression in many reward-related corticolimbic structures. By contrast, restricted feeding alone can induce or entrain clock gene expression in hypothalamic nuclei involved in energy homeostasis. Thus, under ad libitum feeding conditions, the weak anticipatory activity induced by restricted reward presentation may result from the entrainment of reward-associated corticolimbic structures. The additional induction or entrainment of oscillators in hypothalamic regulatory areas may contribute to the more robust anticipatory activity associated with restricted feeding schedules.

Neural correlates of arousal-induced circadian clock resetting: hypocretin/orexin and the intergeniculate leaflet.

In Syrian hamsters, some procedures for stimulating behavioural arousal (e.g. running in a novel wheel and sleep deprivation by gentle handling with minimal activity) markedly phase-advance circadian rhythms when applied during the middle of the daily rest period, while other arousal procedures do not (e.g. physical restraint, caffeine and modafinil). The neural basis for this differential effect of arousal procedures on clock resetting is unknown. We used c-fos expression as a marker for neuronal activation to determine whether these arousal procedures differentially activate two nonphotic inputs to the circadian system, the thalamic intergeniculate leaflet (IGL; a proposed nonphotic gateway to the circadian clock) and the hypothalamic hypocretin system (which depolarizes arousal-related cell groups throughout the brain and innervates both the IGL and the peri-suprachiasmatic nucleus region). c-FOS in hypocretin-1-immunoreactive neurons, in hypothalamic nonhypocretin neurons and in the IGL was significantly increased by novel wheel running, gentle handling and physical restraint, but only weakly by systemic injections of modafinil (300 mg/kg) or caffeine (75 mg/kg), at doses that are strongly alerting. Spatial analysis revealed few regional differences in the percentage of cells double-labelled for hypocretin-1 and c-FOS following each treatment. These results suggest that activation of hypocretin neurons (as in the restraint condition) is not sufficient to induce phase shifts, and that gating of arousal effects on circadian clock phase may be downstream from the hypocretin system and from IGL neurons activated by these procedures.

The dorsomedial hypothalamic nucleus is not necessary for the expression of circadian food-anticipatory activity in rats.

Restricted daytime feeding generates food-anticipatory activity (FAA) by entrainment of a circadian pacemaker separate from the light-entrainable pacemaker located in the SCN. The dorsomedial hypothalamic nucleus (DMH) has been proposed as the site of food-entrainable oscillators critical for the expression of FAA, but another study found no effects of complete DMH ablation on FAA. To account for these different results, the authors examined methodological factors, including (1) cage configuration and feeding method and (2) use of social cues. Intact and DMH-ablated rats were maintained on one 4-h daily meal in the middle of the light period, using caging and feeding methods matching those of Gooley et al. (2006). Rats with partial or complete DMH ablation were less nocturnal during ad lib food access but exhibited normal FAA during restricted feeding, as quantified by FAA magnitude, ratios, latency to appearance, duration, and precision. To evaluate the use of social cues, intact rats naive to restricted-feeding schedules were food deprived for 72 h on 4 tests. Daytime activity increased during food deprivation, but the magnitude and waveform of this activity was not influenced by the presence of food-entrained rats exhibiting robust FAA in adjacent cages. Thus, hungry intact rats do not use social cues to anticipate a daily mealtime, suggesting that DMH-ablated rats do not anticipate meals by reacting to sounds from food-entrained intact rats in adjacent cabinets. These results confirm our previous finding that the DMH is not critical for normal expression of FAA in rats, and this observation is extended to food restriction methodologies used by other labs. The methodological differences that do underlie discrepant results remain unresolved, as does the location of food-entrainable oscillators, input pathways, and output pathways critical for FAA.

Effects of food deprivation on locomotor activity, plasma glucose, and circadian clock resetting in Syrian hamsters.

Circadian rhythms in Syrian hamsters can be phase advanced by activity or arousal stimulated during the daily rest phase ("subjective day"). A widely used method for stimulating activity is confinement to a novel wheel. Some hamsters decline to run, and some procedures may reduce the probability of running. The authors evaluated food deprivation (FD) as a method to promote running. Given evidence that perturbations of cell metabolism or glucose availability may affect circadian clock function in some tissues or species, they also assessed the effects of FD on free-running circadian phase, resetting responses to photic and nonphotic stimuli and plasma glucose. In constant light, a 27-h fast significantly increased running in a novel wheel and marginally increased the average size of resulting phase shifts. FD, without novel wheel confinement, was associated with some very large phase shifts or disruption of rhythmicity in hamsters that spontaneously ran in their home wheels during the subjective day. Hamsters that ran only during the usual active phase (subjective night) or that were prevented from running did not exhibit phase shifts, despite refeeding in the mid-subjective day. Using an Aschoff Type II design for measuring shifts, a 27-h fast significantly increased the number of hamsters that ran continuously when confined to a novel wheel but did not affect the dose-response relation between the amount of running and the size of the resulting shift. A day of fasting also did not affect the size of phase delay or advance shifts to 30-min light pulses in the subjective night. Plasma glucose was markedly reduced by wheel running in combination with fasting but was increased by running in nonfasted hamsters. These results establish FD as a useful tool for stimulating activity in home cage or novel wheels and indicate that in Syrian hamsters, significant alterations in glucose availability, associated with running, fasting, and refeeding, have surprisingly little effect on circadian pacemaker function.

Nucleus accumbens NMDA receptor activation regulates amphetamine cross-sensitization and deltaFosB expression following sexual experience in male rats.

Sexual experience in male rats followed by a period of abstinence causes sensitization to d-Amphetamine (Amph) reward, evidenced by an increased conditioned place preference (CPP) for low doses of Amph. Moreover, sexual experience induces neural plasticity within the nucleus accumbens (NAc), including induction of deltaFosB, which plays a key role in Amph reward cross-sensitization. The NMDA receptor subunit NR1 is also upregulated by mating, but the functional relevance of NMDA receptors in sex experience-induced effects is unknown. Here, we examined the influence of intra-NAc MK 801 infusions on sex experience-induced NAc deltaFosB and cFos expression, as well as mating- and Amph-induced CPP in adult male rats. In experiment 1, males received MK 801 or saline into the NAc during each of 4 consecutive days of mating or handling and were tested for Amph CPP and experience-induced deltaFosB 10 days later. Intra-NAc MK 801 during sexual behavior prevented experience-induced increases in Amph CPP and NAc deltaFosB expression without affecting sexual behavior. In experiment 2, the effects of intra-NAc MK 801 on mating-induced CPP were examined by intra-NAc infusion of MK 801 or saline prior to mating on conditioning days. Intra-NAc MK 801 did not affect mating-induced CPP. Next, effects of intra-NAc MK 801 on mating-induced cFos immunoreactivity were examined. MK 801 prevented mating-induced cFos expression in NAc shell and core. Together, these results provide evidence that NAc NMDA receptor activation during sexual behavior plays a key role in mating-induced cFos and deltaFosB expression and subsequent experience-induced cross-sensitization to Amph reward.

Diurnal and circadian regulation of reward-related neurophysiology and behavior.

Here, we review work over the past two decades that has indicated drug reward is modulated by the circadian system that generates daily (i.e., 24h) rhythms in physiology and behavior. Specifically, drug-self administration, psychomotor stimulant-induced conditioned place preference, and locomotor sensitization vary widely across the day in various species. These drug-related behavioral rhythms are associated with rhythmic neural activity and dopaminergic signaling in the mesocorticolimbic pathways, with a tendency toward increased activity during the species typical wake period. While the mechanisms responsible for such cellular rhythmicity remain to be fully identified, circadian clock genes are expressed in these brain areas and can function locally to modulate both dopaminergic neurotransmission and drug-associated behavior. In addition, neural and endocrine inputs to these brain areas contribute to cellular and reward-related behavioral rhythms, with the medial prefrontal cortex playing a pivotal role. Acute or chronic administration of drugs of abuse can also alter clock gene expression in reward-related brain regions. Emerging evidence suggests that drug craving in humans is under a diurnal regulation and that drug reward may be influenced by clock gene polymorphisms. These latter findings, in particular, indicate that the development of therapeutic strategies to modulate the circadian influence on drug reward may prove beneficial in the treatment of substance abuse disorders.

Regulation of circadian rhythms in mammals by behavioral arousal.

Circadian rhythms in most mammals are synchronized to local time by phase and period resetting actions of daily light-dark cycles on a retino-recipient, light-entrainable circadian pacemaker, the suprachiasmatic nucleus (SCN). The SCN receives input from other brain regions, some of which mediate the phase and period resetting actions of behavioral arousal on circadian rhythms. We review historical milestones in the discovery of so-called "nonphotic" circadian clock resetting induced by environmentally stimulated arousal, or by feedback from clock-controlled rest-activity cycles. Topics include species generality, interactions between concurrent or successive photic and nonphotic inputs to the circadian clock, neural pathways, neurotransmitters, and clock cell responses that mediate resetting by behavioral arousal. The role of behavioral inputs to the circadian clock in determining the phase of entrainment to local time in natural environments is not well understood. Nonetheless, nonphotic effects are of sufficient magnitude to raise issues for the design of experiments in behavioral neuroscience (any procedure that is sufficiently arousing may alter the timing of circadian clocks that regulate dependent variables of primary interest). Nonphotic inputs to the clock may be exploited in strategies to reset or strengthen circadian rhythms in humans.

NMDA and PACAP receptor signaling interact to mediate retinal-induced scn cellular rhythmicity in the absence of light.

The "core" region of the suprachiasmatic nucleus (SCN), a central clock responsible for coordinating circadian rhythms, shows a daily rhythm in phosphorylation of extracellular regulated kinase (pERK). This cellular rhythm persists under constant darkness and, despite the absence of light, is dependent upon inputs from the eye. The neural signals driving this rhythmicity remain unknown and here the roles of glutamate and PACAP are examined. First, rhythmic phosphorylation of the NR1 NMDA receptor subunit (pNR1, a marker for receptor activation) was shown to coincide with SCN core pERK, with a peak at circadian time (CT) 16. Enucleation and intraocular TTX administration attenuated the peak in the pERK and pNR1 rhythms, demonstrating that activation of the NMDA receptor and ERK in the SCN core at CT16 are dependent on retinal inputs. In contrast, ERK and NR1 phosphorylation in the SCN shell region were unaffected by these treatments. Intraventricular administration of the NMDA receptor antagonist MK-801 also attenuated the peak in SCN core pERK, indicating that ERK phosphorylation in this region requires NMDA receptor activation. As PACAP is implicated in photic entrainment and is known to modulate glutamate signaling, the effects of a PAC1 receptor antagonist (PACAP 6-38) on SCN core pERK and pNR1 also were examined. PACAP 6-38 administration attenuated SCN core pERK and pNR1, suggesting that PACAP induces pERK directly, and indirectly via a modulation of NMDA receptor signaling. Together, these data indicate that, in the absence of light, retinal-mediated NMDA and PAC1 receptor activation interact to induce cellular rhythms in the SCN core. These results highlight a novel function for glutamate and PACAP release in the hamster SCN apart from their well-known roles in the induction of photic circadian clock resetting.

The transcription factor Runx2 is under circadian control in the suprachiasmatic nucleus and functions in the control of rhythmic behavior.

Runx2, a member of the family of runt-related transcription factors, is rhythmically expressed in bone and may be involved in circadian rhythms in bone homeostasis and osteogenesis. Runx2 is also expressed in the brain, but its function is unknown. We tested the hypothesis that in the brain, Runx2 may interact with clock-controlled genes to regulate circadian rhythms in behavior. First, we demonstrated diurnal and circadian rhythms in the expression of Runx2 in the mouse brain. Expression of Runx2 mRNA and protein mirrored that of the core clock genes, Period1 and Period2, in the suprachiasmatic nucleus (SCN), the paraventricular nucleus and the olfactory bulb. The rhythm of Runx2 expression was eliminated in the SCN of Bmal1(-/-) mice. Moreover, by crossbreeding mPer2(Luc) mice with Runx2(+/-) mice and recording bioluminescence rhythms, a significant lengthening of the period of rhythms was detected in cultured SCN of Runx2(-/-) animals compared to either Runx2(+/-) or Runx2(+/+) mice. Behavioral analyses of Runx2 mutant mice revealed that Runx2(+/-) animals displayed a significantly lengthened free-running period of running wheel activity compared to Runx2(+/+) littermates. Taken together, these findings provide evidence for clock gene-mediated rhythmic expression of Runx2, and its functional role in regulating circadian period at the level of the SCN and behavior.

Photic sensitivity for circadian response to light varies with photoperiod.

The response of the circadian system to light varies markedly depending on photic history. Under short day lengths, hamsters exhibit larger maximal light-induced phase shifts as compared with those under longer photoperiods. However, effects of photoperiod length on sensitivity to subsaturating light remain unknown. Here, Syrian hamsters were entrained to long or short photoperiods and subsequently exposed to a 15-min light pulse across a range of irradiances (0-68.03 µW/cm(2)) to phase shift activity rhythms. Phase advances exhibited a dose response, with increasing irradiances eliciting greater phase resetting in both conditions. Photic sensitivity, as measured by the half-saturation constant, was increased 40-fold in the short photoperiod condition. In addition, irradiances that generated similar phase advances under short and long days produced equivalent phase delays, and equal photon doses produced larger delays in the short photoperiod condition. Mechanistically, equivalent light exposure induced greater pERK, PER1, and cFOS immunoreactivity in the suprachiasmatic nuclei of animals under shorter days. Patterns of immunoreactivity in all 3 proteins were related to the size of the phase shift rather than the intensity of the photic stimulus, suggesting that photoperiod modulation of light sensitivity lies upstream of these events within the signal transduction cascade. This modulation of light sensitivity by photoperiod means that considerably less light is necessary to elicit a circadian response under the relatively shorter days of winter, extending upon the known seasonal changes in sensitivity of sensory systems. Further characterizing the mechanisms by which photoperiod alters photic response may provide a potent tool for optimizing light treatment for circadian and affective disorders in humans.

Diurnal variations in natural and drug reward, mesolimbic tyrosine hydroxylase, and clock gene expression in the male rat.

The impact of the circadian timing system upon behavior and physiology is pervasive, and previous evidence suggests a circadian modulation of drug-seeking behavior and responsiveness to drugs of abuse. To further characterize daily rhythms in reward and to extend these observations to natural reinforcers, diurnal variation in the rewarding value of sex and systemic amphetamine was assessed via the conditioned place preference paradigm. To identify potential mechanisms for rhythmicity in reward, levels of tyrosine hydroxylase (TH) and core clock proteins (Period1 and Bmal1) were examined across the day in the ventral tegmental area (VTA) and the nucleus accumbens (NAcc). During an initial training period, male rat sexual performance varied diurnally with a nadir near the light-to-dark transition. Diurnal rhythms also were evident for both mating and amphetamine-related reward. However, the rhythms for these particular stimuli exhibited differences in their pattern of timing, with sex reward showing a peak during the middark period and amphetamine reward exhibiting high points during the late night and midday with a nadir prior to the light-to-dark transition. A diurnal variation also was seen for the locomotor-activating effect of acute amphetamine administration with a peak during the late night. Western blot analyses revealed that Period1 and Bmal1 protein levels were rhythmic in the NAcc but not in the VTA. By contrast, TH protein levels were rhythmic in both the NAcc and VTA, but the peaks differed with that in the NAcc coinciding with the peak of sex reward and that in the VTA associated with the peak in amphetamine reward. Thus, it appears that both natural and drug-related reward vary in a diurnal fashion but differ in the timing of their peak and nadir levels. The phase relationships between reward rhythms and mesolimbic TH protein levels suggest that an increased capacity for the release of dopamine in the NAcc may underlie the rhythms in sex-related reward, while amphetamine-related reward occurs at a time when the likelihood of evoked NAcc DA release is relatively low.

Modafinil [2-[(diphenylmethyl)sulfinyl]acetamide] and circadian rhythms in syrian hamsters: assessment of the chronobiotic potential of a novel alerting compound.

In Syrian hamsters, behavioral procedures for inducing arousal (e.g., running in a novel wheel or gentle handling) can shift circadian rhythms when applied during the usual sleep period ("subjective day") and can attenuate phase shifts to light during the active period ("subjective night"). This raises the possibility that drugs that affect behavioral state may have "chronobiotic" potential. We characterized the effects of modafinil (2-[(diphenylmethyl)sulfinyl]acetamide), an atypical alerting compound, on circadian rhythms in male Syrian hamsters. Electroencephalogram recordings and video observations confirmed that modafinil dose dependently increases wakefulness at the expense of slow-wave and paradoxical sleep with no increase in locomotor activity per unit of time awake. Despite inducing arousal, modafinil at these doses (150 or 300 mg/kg), administered in the subjective day or early or late in the subjective night, did not perturb circadian phase. Modafinil (300 mg/kg) also had no effect on phase shifts to light exposure either early or late in the night and did not alter the size of phase shifts induced by running in a novel wheel for 3 h during the mid-day. Modafinil (300 mg/kg) did, however, decrease by approximately 50% the amount of novel wheel-stimulated running, moving leftward the dose-response relation between wheel revolutions and shift magnitude. These results indicate that, in Syrian hamsters, modafinil alone has no significant chronobiotic efficacy. Nevertheless, this agent may increase the sensitivity of the circadian pacemaker to nonphotic stimuli and may thus have some potential as a tool for promoting clock resetting in combination with behavioral strategies.