Little and often? Maintaining continued performance in an automated T-maze for mice

Operant and maze tasks in mice are limited by the small number of trials possible in a session before mice lose motivation. We hypothesized that by manipulating reward size and session length, motivation, and hence performance, would be maintained in an automated T-maze. We predicted that larger rewards and shorter sessions would improve acquisition; and smaller rewards and shorter sessions would maintain higher and less variable performance. Eighteen C57BL/6J mice (9 per sex) acquired (criterion 8/10 correct) and performed a spatial discrimination, with one of 3 reward sizes (.02, .04, or .08 g) and one of 3 session schedules (15, 30, or 45 min sessions). Each mouse had a total of 360 min of access to the maze per night, for two nights, and averaged 190 trials. Analysis used split-plot GLM with contrasts testing for linear effects. Acquisition of the discrimination was unaffected by reward size or session length/interval. After-criterion average performance improved as reward size decreased. After-criterion variability in performance was also affected. Variability increased as reward size increased. Session length/interval did not affect any outcome. We conclude that an automated maze, with suitable reward sizes, can sustain performance with low variability, at 5-10 times faster than traditional methods.     

The Effects of Opioid Peptides on Dopamine Release in the Nucleus Accumbens: An In Vivo Microdialysis Study

An involvement of the mesolimbic dopamine (DA) system in mediating the motivational effects of opioids has been suggested. Accordingly, the present study employed the technique of in vivo microdialysis to examine the effects of selective mu-, delta-, and kappa- opioids on DA release in the nucleus accumbens (NAC) of anesthetized rats. Microdialysis probes were inserted into the NAC and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DO-PAC) and homovanillic acid (HVA), using a reverse-phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of selective mu-opioid [D-Ala2, N-methyl-Phe4, Gly5-ol]-enkephalin (DAMGO) or delta-opioid [D-Pen2, D-Pen5]-enkephalin (DPDPE) agonists, at doses that function as positive reinforcers in rats, resulted in an immediate and significant increase in extracellular DA. DOPAC and HVA levels were also significantly increased. The effects of DAMGO were blocked by the selective mu-antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) whereas those of DPDPE were blocked by the delta-antagonist allyl2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). In contrast to mu- and delta-agonists, the kappa-agonist N-CH3-Tyr-Gly-Gly-Phe-Leu-Arg-N-CH3-Arg-D-Leu-NHC2H5 (E-2078), a dynorphin analog that produces aversive states, decreased DA release in a biphasic manner. Norbinaltorphimine, a selective kappa-antagonist, could block this effect. These results demonstrate that mu-, delta-, and kappa-opioid agonists differentially affect DA release in the NAC and this action is centrally mediated.     

Replicator dynamics of reward & reputation in public goods games

Public goods games have become the mathematical metaphor for game theoretical investigations of cooperative behavior in groups of interacting individuals. Cooperation is a conundrum because cooperators make a sacrifice to benefit others at some cost to themselves. Exploiters or defectors reap the benefits and forgo costs. Despite the fact that groups of cooperators outperform groups of defectors, Darwinian selection or utilitarian principles based on rational choice should favor defectors. In order to overcome this social dilemma, much effort has been expended for investigations pertaining to punishment and sanctioning measures against defectors. Interestingly, the complementary approach to create positive incentives and to reward cooperation has received considerably less attention—despite being heavily advocated in education and social sciences for increasing productivity or preventing conflicts. Here we show that rewards can indeed stimulate cooperation in interaction groups of arbitrary size but, in contrast to punishment, fail to stabilize it. In both cases, however, reputation is essential. The combination of reward and reputation result in complex dynamics dominated by unpredictable oscillations.     

The stress-response-dampening effects of placebo

This experiment used both biological and self-report measures to examine how alcohol modifies stress responses, and to test whether the interaction between these two factors alters risk-taking in healthy young adults. Participants were divided into stress or no-stress conditions and then further divided into one of three beverage groups. The alcohol group consumed a binge-drinking level of alcohol; the placebo group consumed soda, but believed they were consuming alcohol; the sober group was aware that they were not consuming alcohol. Following beverage consumption, the stress group was subjected to the Trier Social Stress Test (TSST) while the no-stress group completed crossword puzzles; all participants subsequently completed a computerized risk-taking task. Exposure to the TSST significantly increased salivary levels of the hormone cortisol and the enzyme alpha-amylase, as well as subjective self-ratings of anxiety and tension. In the stress condition, both placebo and intoxicated groups reported less tension and anxiety, and exhibited a smaller increase in cortisol, following the TSST than did the sober group. Thus, the expectation of receiving alcohol altered subjective and physiological responses to the stressor. Neither alcohol nor stress increased risk taking, however the sober group demonstrated lower risk-taking on the computer task on the second session. These findings clearly demonstrate that the expectation of alcohol (placebo) alters subsequent physiological responses to stress.     

Lesions of orexin neurons block conditioned place preference for sexual behavior in male rats

The hypothalamic neuropeptide orexin (hypocretin) mediates reward related to drugs of abuse and food intake. However, a role for orexin in sexual reward has yet to be investigated. Orexin neurons are activated by sexual behavior, but endogenous orexin does not appear to be essential for sexual performance and motivation in male rats. Therefore, the goal of the current study was to test the hypothesis that orexin is critically involved in processing of sexual reward in male rats. First, it was demonstrated following exposure to conditioned contextual cues associated with sexual behavior in a conditioned place preference paradigm that cFos expression is induced in orexin neurons, indicating activation of orexin neurons by cues predicting sexual reward. Next, orexin-cell specific lesions were utilized to determine the functional role of orexin in sexual reward processing. Hypothalami of adult male rats were infused with orexin-B-conjugated saporin, resulting in greater than 80% loss of orexin neurons in the perifornical-dorsomedial and lateral hypothalamus. Orexin lesions did not affect expression of sexual behavior, but prevented formation of conditioned place preference for a sexual behavior paired chamber. In contrast, intact sham-treated males or males with partial lesions developed a conditioned place preference for mating. Orexin lesioned males maintained the ability to form a conditioned place aversion to lithium chloride-induced visceral illness, indicating that orexin lesions did not disrupt associative contextual memory. Overall, these findings suggest that orexin is not essential for sexual performance or motivation, but is critical for reward processing and conditioned cue-induced seeking of sexual behavior.     

Food hoarding,but not food intake,is attenuated by acute diazepam treatment in female Mongolian gerbils (Meriones unguiculatus)

This article is part of a Special Issue “Energy Balance”. Effects of γ-aminobutyric acid (GABA) on food hoarding are unknown in rodents, and the effects of energy balance and GABA have not been evaluated in females. To evaluate the role of food deprivation and GABA on food hoarding, female Mongolian gerbils were given i.p. injection of diazepam (1 mg/kg and 3 mg/kg, respectively), a GABA_A receptor agonist. Among food-deprived females, there was a bimodal pattern in the frequency of gerbils with different levels of food hoarding. High food hoarding (HFH) and low food hoarding (LFH) gerbils were analyzed. Diazepam blocked food deprivation-induced food hoarding in HFH gerbils, but not in LFH gerbils. This blockade was associated with increased cellular activation in selected brain areas, such as the nucleus accumbens (NAcc), caudate putamen (CP) and ventral tegmental area (VTA), which suggested that direct activation of GABA in the brain reward circuitry decreased food hoarding in HFH females. Moreover, diazepam increased Fos expression in field CA₂ and CA₃ of the hippocampus, but had no significant effect on Fos expression in field CA₁ and dentate gyrus (DG) of the hippocampus, indicating that the hippocampus has area-specific effects on food hoarding in HFH gerbils. Diazepam did not alter food intake in both HFH and LFH gerbils. In addition, serum corticosterone concentrations were higher in the HFH than in the LFH ones. Together, these data indicated that food deprivation increased food hoarding in female gerbils, diazepam reduced food deprivation-induced food hoarding in HFH gerbils, and that GABA might influence food hoarding via classical reward circuitry via the mesolimbic dopamine system and specific hippocampal areas.     

Neuropeptide Y-induced operant responding for sucrose is not mediated by dopamine

Neuropeptide Y (NPY) induces feeding in previously satiated animals after injection into the hypothalamus, especially the perifornical region (PFH). NPY also appears to have rewarding properties as evidenced by its ability to produce a conditioned place preference following injection into the nucleus accumbens (N.Acc), an effect ostensibly mediated by mesolimbic dopamine (DA). Since the progressive-ratio (PR) operant schedule has also been used to assess an organism’s motivation to respond for rewarding stimuli, we tested the possibility that NPY increases PR responding for sucrose pellets. Adult male rats were injected with NPY (0–235 pmol) bilaterally through cannulae aimed to terminate in the PFH. This produced a dose-dependent increase in the total number of responses made and the number of reinforcers earned. The DA receptor blocker, α-flupenthixol (FLU)(0–0.2 mg/kg intraperitoneally), attenuated both NPY(156 pmol)-induced and drug-free PR responding while having no effect on NPY(156 pmol)-induced free-feeding. FLU injected directly into the N.Acc (0–5 μg) also failed to reduce sucrose free-feeding. These results suggest that distinct reward mechanisms are activated during PFH NPY-induced feeding vs. PR responding, since FLU disrupted the latter but not the former.     

Male Syrian hamsters demonstrate a conditioned place preference for sexual behavior and female chemosensory stimuli

Sexual behavior is a natural reward for many rodent species, and it often includes chemosensory-directed components. Chemosensory stimuli themselves may also be rewarding. Conditioned place preference (CPP) is one paradigm frequently used to test the rewarding properties of a range of stimuli. Males and females of several rodent species show a CPP for sexual behavior; however, it is currently unknown whether sexual behavior can induce a CPP in male Syrian hamsters. As male Syrian hamsters are an animal model commonly used for investigation of the neurobiology of sexual behavior, understanding the rewarding components of sexual stimuli will better direct future research on brain regions and neurotransmitters involved in these behaviors. Experiment 1 tested the prediction that male hamsters show a CPP for sexual behavior. Female chemosensory stimuli are essential for the display of sexual behavior in male hamsters; however, the rewarding properties of female chemosensory stimuli contained in vaginal secretions (VS) are uncertain. Therefore, experiment 2 tested the prediction that male hamsters show a CPP for VS. This study is the first demonstration that both sexual behavior and VS induce a CPP in male hamsters. Thus, female chemosensory stimuli are a natural reward in a species that is dependent on these stimuli for reproductive fitness.     

Motivation and timing: clues for modeling the reward system

There is growing evidence that a change in reward magnitude or value alters interval timing, indicating that motivation and timing are not independent processes as was previously believed. The present paper reviews several recent studies, as well as presenting some new evidence with further manipulations of reward value during training vs. testing on a peak procedure. The combined results cannot be accounted for by any of the current psychological timing theories. However, in examining the neural circuitry of the reward system, it is not surprising that motivation has an impact on timing because the motivation/valuation system directly interfaces with the timing system. A new approach is proposed for the development of the next generation of timing models, which utilizes knowledge of the neuroanatomy and neurophysiology of the reward system to guide the development of a neurocomputational model of the reward system. The initial foundation along with heuristics for proceeding with developing such a model is unveiled in an attempt to stimulate new theoretical approaches in the field.     

The shared reward dilemma

One of the most direct human mechanisms of promoting cooperation is rewarding it. We study the effect of sharing a reward among cooperators in the most stringent form of social dilemma, namely the prisoner's dilemma (PD). Specifically, for a group of players that collect payoffs by playing a pairwise PD game with their partners, we consider an external entity that distributes a fixed reward equally among all cooperators. Thus, individuals confront a new dilemma: on the one hand, they may be inclined to choose the shared reward despite the possibility of being exploited by defectors; on the other hand, if too many players do that, cooperators will obtain a poor reward and defectors will outperform them. By appropriately tuning the amount to be shared a vast variety of scenarios arises, including the traditional ones in the study of cooperation as well as more complex situations where unexpected behavior can occur. We provide a complete classification of the equilibria of the n-player game as well as of its evolutionary dynamics.