Brian A. Baldo

Addiction to psychostimulants, including cocaine, causes widespread morbidity and mortality and is a major threat to global public health. Currently, no pharmacotherapies can successfully treat psychostimulant addiction. The neuroactive effects of cocaine and other psychostimulants have been studied extensively with respect to their modulation of monoamine systems (particularly dopamine); effects on neuropeptide systems have received less attention. Here, we employed mass spectrometry (MS) methods to characterize cocaine-induced peptidomic changes in the rat brain. Label-free peptidomic analysis using liquid chromatography coupled with tandem MS (LC-MS/MS) was used to describe the dynamic changes of endogenous peptides in five brain regions (nucleus accumbens, dorsal striatum, prefrontal cortex, amygdala, and hypothalamus) following an acute systemic cocaine challenge. The improved sensitivity and specificity of this method, coupled with quantitative assessment, enabled the identification of 1376 peptides derived from 89 protein precursors. Our data reveal marked, region-specific changes in peptide levels in the brain induced by acute cocaine exposure, with peptides in the cholecystokinin and melanin-concentrating hormone families being significantly affected. These findings offer new insights into the region-specific effects of cocaine and could pave the way for developing new therapies to treat substance use disorders and related psychiatric conditions.

Mu-opioid receptor (μ-OR) signaling in forebrain sites including nucleus accumbens (Acb) and ventromedial prefrontal cortex (vmPFC) modulates reward-driven feeding and may play a role in the pathophysiology of disordered eating. In preclinical models, intra-Acb or intra-vmPFC μ-OR stimulation causes overeating and vigorous responding for food rewards. These effects have been studied mainly in male animals, despite demonstrated sex differences and estrogen modulation of central reward systems. Hence, the present study investigated sex differences and estrogen modulation of intra-Acb and intra-vmPFC μ-OR-driven feeding behaviors. First, the dose-related effects of intra-Acb and intra-vmPFC infusions of the μ-OR-selective agonist, DAMGO, were compared among intact female, ovariectomized (OVX) female, and intact male rats. The DAMGO feeding dose-effect function was flattened in intact females relative to the robust, dose-dependent effects observed in OVX females and intact males. Thus, in intact females, intra-Acb DAMGO failed to elevate food intake relative to vehicle, while intra-vmPFC DAMGO elevated food intake, but to a smaller degree compared to males and OVX females. Next, to explore the possible role of estrogen in mediating the diminished DAMGO response observed in intact females, OVX rats were given intra-Acb or intra-vmPFC infusions of DAMGO either immediately after a subcutaneous injection of 17-beta-estradiol 3-benzoate (EB; 5 μg/0.1 mL) or 24 h after EB injection. Intra-Acb DAMGO effects were not changed at the immediate post-EB time point. At the delayed post-EB timepoint, significant lordosis was noted and the duration of intra-Acb DAMGO-driven feeding bouts was significantly reduced, with no change in the number of bouts initiated, locomotor hyperactivity, or Fos immunoreactivity in hypothalamic feeding and arousal systems. Similarly, EB failed to alter the motor-activational effects of intra-vmPFC DAMGO while reducing feeding. These findings indicate that delayed, presumably genomically mediated estrogen actions modulate the μ-OR-generated motivational state by reducing consummatory activity while sparing goal-approach and general arousal/activity. The results additionally suggest that EB regulation of consummatory activity occurs outside of forebrain-μ-OR control of hypothalamic systems.

The present study investigated immediate vs. delayed effects of estrogen replacement in ovariectomized (OVX) rats on hyperphagia elicited by GABA-A-agonist (muscimol) infusions into the nucleus accumbens shell (AcbSh). First, because intra-AcbSh muscimol-induced feeding has never been explored in OVX rats, a dose-effect curve was generated and compared to sham-operated males, the current point of reference in the literature. Muscimol (5, 10, 25, and 50 ng) increased food intake in both sexes, and both sexes reached the same asymptotic level of intake. Nevertheless, slopes of the linearized dose-effect functions for males and OVX females differed significantly, with females starting at a lower baseline and exhibiting a steeper slope. Next, the behavioral profiles of a behaviorally active, but non-maximal intra-AcbSh muscimol dose (25 ng) were examined in a separate group of OVX females at two time-points: immediately after injecting 17β-estradiol 3-benzoate (EB) subcutaneously (5 μg), and 24 h post-EB. Delayed, but not immediate, EB pre-treatment strongly suppressed, but did not eliminate, muscimol-driven food intake. However, EB did not change non-food-directed behaviors such as locomotion or rearing. These results demonstrate that feeding mediated by intra-AcbSh GABA-A receptors is delimited by delayed, but not rapid, effects of estradiol.

Subregions within insular cortex and medial prefrontal cortex (mPFC) have been implicated in eating disorders; however, the way these brain regions interact to produce dysfunctional eating is poorly understood. The present study explored how two mPFC subregions, the infralimbic (IL) and prelimbic (PRL) cortices, regulate sucrose hyperphagia elicited specifically by a neurochemical manipulation of the agranular/dysgranular region of gustatory insula (AI/DI). Using intra-AI/DI infusion of the mu-opioid receptor (µ-OR) agonist, DAMGO (1 µg), sucrose hyperphagia was generated in ad-libitum-maintained rats, while in the same rat, either the IL or prelimbic (PRL) subregion of mPFC was inactivated bilaterally with muscimol (30 ng). Intra-IL muscimol markedly potentiated AI/DI DAMGO-induced sucrose hyperphagia by increasing eating bout duration and food consumption per bout. In contrast, PRL attenuated intra-AI/DI DAMGO-driven sucrose intake and feeding duration and eliminated the small DAMGO-induced increase in feeding bout initiation. Intra-IL or -PRL muscimol alone (i.e., without intra-AI/DI DAMGO) did not alter feeding behavior, but slightly reduced exploratory-like rearing in both mPFC subregions. These results reveal anatomical heterogeneity in mPFC regulation of the intense feeding-motivational state engendered by µ-OR signaling in the gustatory insula: IL significantly curtails consummatory activity, while PRL modestly contributes to feeding initiation. Results are discussed with regard to potential circuit-based mechanisms that may underlie the observed results.

This study explored potentially dissociable functions of mu-opioid receptor (µ-OR) signaling across different cortical territories in the control of anticipatory activity directed toward palatable food, consumption, and impulsive food-seeking behavior in male rats. The µ-OR agonist, DAMGO ([D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin), was infused into infralimbic cortex (ILC), prelimbic cortex (PrL), medial and lateral ventral orbitofrontal cortices (VMO, VLO), and agranular/dysgranular insular (AI/DI) cortex of rats. Intra-ILC DAMGO markedly enhanced contact with a see-through screen behind which sucrose pellets were sequestered; in addition, rats having received intra-ILC and intra-VMO DAMGO exhibited locomotor hyperactivity while the screen was in place. Upon screen removal, intra-ILC and -VMO-treated rats emitted numerous, brief sucrose-intake bouts (yielding increased overall intake) interspersed with significant hyperactivity. In contrast, intra-AI/DI-treated rats consumed large amounts of sucrose in long, uninterrupted bouts with no anticipatory hyperactivity pre-screen removal. Intra-PrL and intra-VLO DAMGO altered neither pre-screen behavior nor sucrose intake. Finally, all rats were tested in a sucrose-reinforced differential reinforcement of low rates (DRL) task, which assesses the ability to advantageously withhold premature responses. DAMGO affected (impaired) DRL performance when infused into ILC only. These site-based dissociations reveal differential control of µ-OR-modulated appetitive/approach vs. consummatory behaviors by ventromedial/orbitofrontal and insular networks, respectively, and suggest a unique role of ILC µ-ORs in modulating inhibitory control.

Rationale Whereas reward-modulatory opioid actions have been intensively studied in subcortical sites such as the nucleus accumbens (Acb), the role of cortical opioid transmission has received comparatively little attention. Objectives The objective of this study is to describe recent findings on the motivational actions of opioids in the prefrontal cortex (PFC), emphasizing studies of food motivation and ingestion. PFC-based opioid effects will be compared/contrasted to those elicited from the Acb, to glean possible common functional principles. Finally, the motivational effects of opioids will be placed within a network context involving the PFC, Acb, and hypothalamus. Results Mu-opioid receptor (μ-OR) stimulation in both the Acb and PFC induces eating and enhances food-seeking instrumental behaviors; μ-OR signaling also enhances taste reactivity within a highly circumscribed zone of medial Acb shell. In both the Acb and PFC, opioid-sensitive zones are aligned topographically with the sectors that project to feeding-modulatory zones of the hypothalamus and intact glutamate transmission in the lateral/perifornical (LH-PeF) hypothalamic areas is required for both Acb- and PFC-driven feeding. Conversely, opioid-mediated feeding responses elicited from the PFC are negatively modulated by AMPA signaling in the Acb shell. Conclusions Opioid signaling in the PFC engages functionally opposed PFC➔hypothalamus and PFC➔Acb circuits, which, respectively, drive and limit non-homeostatic feeding, producing a disorganized and “fragmented” pattern of impulsive food-seeking behaviors and hyperactivity. In addition, opioids act directly in the Acb to facilitate food motivation and taste hedonics. Further study of this cortico-striato-hypothalamic circuit, and incorporation of additional opioid-responsive telencephalic structures, could yield insights with translational relevance for eating disorders and obesity.