Chronic peri-adolescent stress in humans increases risk to develop a substance use disorder during adulthood. Rats reared in social isolation during peri-adolescence (aSI; 1 rat/cage) period show greater ethanol and cocaine intake compared to group housed (aGH; 4 rats/cage) rats. In addition, aSI rats have a heightened dopamine response in the nucleus accumbens (NAc) to rewarding and aversive stimuli. Furthermore, single pulse electrical stimulation in slices containing NAc core elicits greater dopamine release in aSI rats. Here, we further investigated dopamine release kinetics and machinery following aSI. Dopamine release, across a wide range of stimulation intensities and frequencies, was significantly greater in aSI rats. Interestingly, subthreshold intensity stimulations also resulted in measurable dopamine release in accumbal slices from aSI but not aGH rats. Extracellular [Ca2+] manipulations revealed augmented calcium sensitivity of dopamine release in aSI rats. The readily releasable pools of dopamine, examined by bath application of Ro-04-1284/000, a vesicular monoamine transporter 2 (VMAT2) inhibitor, were depleted faster in aGH rats. Western blot analysis of release machinery proteins (VMAT2, Synaptogyrin-3, Syntaxin-1, and Munc13-3) showed no difference between the two groups. Tyrosine hydroxylase (TH) protein expression levels, however, were elevated in aSI rats. The greater dopamine release could potentially be explained by higher levels of TH, the rate-limiting step for dopamine synthesis. This augmented responsivity of the dopamine system and heightened dopamine availability post-aSI may lead to an increased risk of addiction vulnerability.

No abstract

Heightened novelty-seeking phenotypes are associated with a range of behavioural traits including susceptibility to drug use. These relationships are recapitulated in preclinical models, where rats that exhibit increased exploratory activity in novel environments (high responders-HR) acquire selfadministration of psychostimulants more rapidly compared to rats that display low novelty exploration (low responders-LR). Dopamine release dynamics in the nucleus accumbens (NAc) covaries with response to novelty, and differences in dopaminergic signalling are thought to be a major underlying driver of the link between novelty seeking and drug use vulnerability. Accumbal dopamine release is controlled by local microcircuits including modulation through glutamatergic and nicotinic acetylcholine receptor (nAChR) systems, but whether these mechanisms contribute to disparate dopamine signalling across novelty phenotypes is unclear. Here, we used ex vivo voltammetry in the NAc of rats to determine if α7 nAChRs contribute to differential dopamine dynamics associated with individual differences in novelty exploration. We found that blockade of α7 nAChRs attenuates tonic dopamine release evoked by low-frequency stimulations across phenotypes but that phasic release is decreased in LRs while HRs are unaffected. These stimulation frequency-and phenotype-dependent effects result in a decreased dynamic range of release exclusively in LRs. Furthermore, we found that differential α7 modulation of dopamine release in LRs is dependent on AMPA but not NMDA receptors. These results help to form an understanding of the local NAc microcircuitry and provide a potential mechanism for covariance of dopamine dynamics and sensitivity to the reinforcing effects of drugs of abuse.

Despite significant advances in understanding the neurobiology underlying substance use disorders (SUDs), about 20.3 million Americans still suffered from SUD in 2018. This is due, in part, to limited effectiveness of current treatment strategies. Development of more effective treatments may be advanced by the ability to identify markers of increased risk of developing SUD following initial drug use and to target treatments to this vulnerable subpopulation. This need to better understand individual differences in SUD vulnerability is demonstrated in that only 10–20% of individuals that experiment with drugs of abuse ultimately develop SUD. Some behavioral traits such as sensation seeking are predictive of increased vulnerability, but the neurobiology underlying these individual differences remains unclear. To investigate this neurobiology, sensation seeking can be modeled in rodents by examining locomotor response to a novel environment. Rodents that demonstrate higher locomotor response to an inescapable novel environment (high responders; HR) acquire self‐administration (SA) of drugs more rapidly and stably compared to low responders (LR). Phasic firing of mesolimbic dopamine (DA) neurons plays an integral role in encoding reward‐associations and occurs in response to unexpected rewards and reward‐predictive stimuli. HR rats show increased phasic DA signaling to reward‐predictive cues compared to LR rats. Nicotinic acetylcholine receptors (nAChRs) on DA terminals in the nucleus accumbens (NAc) modulate DA release in an action potential‐independent manner through acetylcholine signaling. Our lab has previously shown that locomotor response to a novel environment can predict nAChR modulation of phasic DA signals in the NAc. Specifically, desensitization or blockade of α6β2‐containing nAChRs within the NAc was found to augment phasic DA signal in brain slices of HR but not LR animals. In the present study, we used ex vivo fast‐scan cyclic voltammetry (FSCV) to further investigate the role of NAc nAChRs in modulating DA release in HRs versus LRs. We find that modulation of DA release by α7 nAChRs can be predicted by locomotor response to novelty at phasic‐like stimulations. Given the role that nAChRs play in Ca2+ entry into the cell, we additionally tested the possibility that differential Ca2+ utilization is one mechanism underlying differential DA modulation by nAChRs and utilized pharmacological manipulations to test possible voltage‐gated Ca2+ channel (VGCC) subtype‐specific effects. Here, we demonstrate that lowered Ca2+ concentration unmasks a diverging DA release profile between HRs and LRs at stimulation frequencies modeling phasic firing and find that P/Q‐type VGCCs appear to drive these individual differences in Ca2+ utilization. In sum, these data help form a more coherent understanding of the mechanisms underlying individual differences in vulnerability. Investigating these mechanisms allows us to better identify behavioral and neurochemical markers of substance abuse risk in humans and to ultimately st...

An estimated 70.5% of Americans aged 12 or older report having used an illicit substance at least once, while about 10‐20% of those individuals ultimately develop a substance use disorder (SUD). These data highlight substantial individual variability in the risk of developing SUD following initial drug use. Identifying markers of increased risk provides a significant opportunity to identify at‐risk populations and to aid in the prevention of the development of SUD. In preclinical rodent models, locomotor response to a novel environment predicts drug use vulnerability. Rodents that demonstrate higher locomotor response to an inescapable novel environment (high responders; HR) acquire self‐administration (SA) of drugs more rapidly and at lower doses compared to low responders (LR). Striatal dopamine (DA) signaling is critical for the acquisition of drug SA and tonic/phasic firing patterns of DA neurons encode information about salient environmental stimuli and rewards. HR rats show increased phasic DA signaling to reward‐predictive cues compared to LR rats. Dopamine release dynamics are tightly controlled by local modulation within the nucleus accumbens (NAc), including by nicotinic acetylcholine receptors (nAChRs), but the mechanisms by which this local modulation may contribute to differential DA release is not fully understood. Our lab has previously shown that locomotor response to a novel environment predicts nAChR modulation of phasic DA signals in the NAc. Specifically, desensitization or blockade of α6β2‐containing (α6β2*) nAChRs within the NAc was found to augment phasic DA signals in brain slices of HR rats, and reduce phasic DA signals in LR rats. However, these studies utilized non‐specific electrical stimulation to record DA release. In the present study, we used ex vivo fast‐scan cyclic voltammetry with an optogenetic approach to selectively stimulate DA neuron terminals or cholinergic interneurons (CINs) within the NAc to examine the specific contributions of acetylcholine (ACh) to the overall DA response in HR and LR rats. We find that, contrary to previous results with electrical stimulation, selective DA phasic stimulation results in lower DA release in HRs compared to LRs, while blockade of α6β2* nAChRs using light stimulation results in higher DA release in HRs. This suggests that additional circuitry is likely involved in differential DA signaling within the NAc. Given the tonic activity of CINs and presence of nAChRs on GABA interneurons, we next used pharmacological manipulations to examine the possibility of GABA signaling contributing to individual differences in DA release. Here, we find that antagonism of GABA receptors blocks the previously seen differential effects of α6β2* nAChR antagonism on DA release, suggesting that GABA interneurons do play a critical role in differential DA signaling in HRs versus LRs. In sum, these data help form a clearer understanding of the mechanisms underlying individual differences in DA signaling, which may be a primary driver of differences in sensitivit...