The monoamine-oxidase B inhibitor deprenyl increases selection of high-effort activity in rats tested on a progressive ratio/chow feeding choice procedure: Implications for treating motivational dysfunctions

Neuropsychological Effects of Antidepressants: Translational Studies

Pharmacological treatments that improve mood were first identified serendipitously, but more than half a century later, how these drugs induce their antidepressant effects remains largely unknown. With the help of animal models, a detailed understanding of their pharmacological targets and acute and chronic effects on brain chemistry and neuronal function has been achieved, but it remains to be elucidated how these effects translate to clinical efficacy. Whilst the field has been dominated by the monoamine and neurotrophic hypotheses, the idea that the maladaptive cognitive process plays a critical role in the development and perpetuation of mood disorders has been discussed since the 1950s. Recently, studies using objective methods to quantify changes in emotional processing found acute effects with conventional antidepressants in both healthy volunteers and patients. These positive effects on emotional processing and cognition occur without a change in the subjective ratings of mood. Building from these studies, behavioural methods for animals that quantify similar cognitive affective processes have been developed. Integrating these behavioural approaches with pharmacology and targeted brain manipulations, a picture is beginning to emerge of the underlying mechanisms that may link the pharmacology of antidepressants, these neuropsychological constructs and clinical efficacy. In this chapter, we discuss findings from animal studies, experimental medicine and patients investigating the neuropsychological effects of antidepressant drugs. We discuss the possible neural circuits that contribute to these effects and discuss whether a neuropsychological model of antidepressant effects could explain the temporal differences in clinical benefits observed with conventional delayed-onset antidepressants versus rapid-acting antidepressants.

The influence of reinforcement schedule on experience-dependent changes in motivation

The progressive ratio procedure is used across fields to assess motivation for different reinforcers, define the effects of experimental interventions on motivation, and determine experience-dependent changes in motivation. However, less is known about how operant training schedules affect performance on this widely utilized task. Here we designed an experiment to examine the effect of variable ratio versus fixed ratio training schedules of reinforcement on progressive ratio performance while holding other performance variables constant between groups. We found a robust increase in maximum ratio completed between the pretest and posttraining test highlighting a robust training effect on progressive ratio performance. However, it did not matter if the training was under a fixed or variable ratio schedule. Additionally, we show that neither individual rates during training nor extinction responding correlated with maximum ratio achieved during the sessions. Finally, we show that rates during the training sessions do correlate with extinction performance, suggesting that these variables measure a different aspect of performance that does not predict motivation.

Anhedonia as a central factor in depression: Neural mechanisms revealed from preclinical to clinical evidence

Anhedonia is one of the core symptoms of major depressive disorder (MDD), which is often inadequately treated by traditional antidepressants. The modern framework of anhedonia extends the definition from impaired consummatory pleasure or interest in rewards to a broad spectrum of deficits that impact functions such as reward anticipation, approach motivation, effort expenditure, reward valuation, expectation, and reward-cue association learning. Substantial preclinical and clinical research has explored the neural basis of reward deficits in the context of depression, and has implicated mesocorticolimbic reward circuitry comprising the nucleus accumbens, ventral pallidum, ventral tegmental area, amygdala, hippocampus, anterior cingulate, insula, orbitofrontal cortex, and other prefrontal cortex regions. Dopamine modulates several reward facets including anticipation, motivation, effort, and learning. As well, serotonin, norepinephrine, opioids, glutamate, Gamma aminobutyric acid (GABA), and acetylcholine are also involved in anhedonia, and medications targeting these systems may also potentially normalize reward processing in depression. Unfortunately, whereas reward anticipation and reward outcome are extensively explored by both preclinical and clinical studies, translational gaps remain in reward motivation, effort, valuation, and learning, where clinical neuroimaging studies are in the early stages. This review aims to synthesize the neurobiological mechanisms underlying anhedonia in MDD uncovered by preclinical and clinical research. The translational difficulties in studying the neural basis of reward are also discussed.

Sex differences in effort-related decision-making: role of dopamine D2 receptor antagonism

RATIONALE

Depressed individuals demonstrate debilitating symptoms, including depressed mood, anhedonia, and effort-related deficits. Effort-related decision-making can be measured through providing subjects with a choice between high effort/reward and low effort/reward options, which is a dopamine (DA)-dependent behavior. While previous research has shown sex differences in depression rates, this has not been examined within operant-based effort-related decision-making tasks nor has DA been shown to underlie this behavior in female rats.

OBJECTIVES

The current study investigated sex differences in an effort-related decision-making task prior to and following administration of the DA D2 receptor antagonist haloperidol (HAL).

METHODS

Adult rats were food restricted or fed freely and trained in an effort-related progressive ratio choice task. After stable responding, HAL was administered acutely (0.05-0.2 mg/kg) prior to testing.

RESULTS

Results indicate a significant effect of sex on training variables, with males having a greater number of lever presses, higher ratios, and longer active lever times. Pretreatment with HAL significantly reduced the same measures in both sexes for the high-valued reward, while increasing chow consumption in the food restricted males. Food restricted rats showed a greater number of total lever presses and achieved higher ratios; however, the effect in male food restricted rats was greatest.

CONCLUSIONS

These data suggest that, although there are sex differences in training, HAL decreases behavior across sexes, demonstrating that the D2 mechanism is similar in both sexes. These findings provide a better understanding of motivational dysfunction in both sexes and potential treatment targets for depression.

Protective effect of selegiline on cigarette smoke-induced oxidative stress and inflammation in rat lungs in vivo

Background

Cigarette smoke (CS)-induced build-up of oxidative stress is the leading cause of chronic obstructive pulmonary disease (COPD). Monoamine oxidases (MAOs) are novel sources of reactive oxygen species (ROS) due to the production of hydrogen peroxide (H₂O₂). However, it remains unclear whether MAO signaling is involved in CS-induced oxidative stress in vivo. This study aimed at investigating the impact of selegiline, a selective MAO-B inhibitor, on CS-induced lung oxidative stress and inflammation in vivo and its underlying mechanism.

Methods

Sprague Dawley rats were randomly divided into four groups: saline plus sham air (Saline/air), saline plus cigarette smoke (Saline/CS), selegiline plus sham air (Slg/air) and selegiline plus cigarette smoke (Slg/CS). Rats from Saline/air and Saline/CS groups were intraperitoneally injected with saline (2 mL/kg body weight) while rats from Slg/air and Slg/CS groups were injected with selegiline (2 mg/kg body weight) about 30 min prior to exposure daily. The Saline/air and Slg/air groups were exposed to atmospheric air while the Saline/CS and Slg/CS groups were exposed to mainstream CS generated from the whole body inExpose smoking system (SCIREQ, Canada) for twice daily (each for 1 hour with 20 cigarettes). After 7 days, rats were sacrificed to collect bronchoalveolar lavage (BAL) and lung tissues for the measurement of oxidative/anti-oxidative and inflammatory/anti-inflammatory makers respectively.

Results

CS caused significant elevation of MAO-B activity, reduction of total antioxidant capacity (T-AOC) and rGSH/GSSG ratio, and enhancement of superoxide dismutase (SOD) activity in rat lung. Selegiline significantly only reversed CS-induced elevation of MAO-B activity and reduction of rGSH/GSSG ratio. The CS-induced elevation of heme oxygenase-1 (HO-1) and NAD(P)H quinone dehydrogenase 1 (NQO1) expression via nuclear factor erythroid 2-related factor 2 (Nrf2) was also reversed by selegiline. Despite of CS-induced increase in total cell counts, especially the number of macrophages, selegiline had no effect. Selegiline attenuated CS-induced elevation of pro-inflammatory mediators (CINC-1, MCP-1 and IL-6) and restored CS-induced reduction of anti-inflammatory mediator IL-10 in BAL, which was driven through MAPK and NF-κB.

Conclusions

Inhibition of MAO-B may provide a promising therapeutic strategy for CS-mediated oxidative stress and inflammation in acute CS-exposed rat lungs.

Possible involvement of PI3K/AKT/mTOR signaling pathway in the protective effect of selegiline (deprenyl) against memory impairment following ischemia reperfusion in rat

Short-term cerebral ischemia led to memory dysfunction. There is a pressing need to introduce effective agents to reduce complications of the ischemia. Involvement of PI3K/AKT/mTOR signaling pathway has been determined in the neuroprotective effect of various agents. Selegiline (deprenyl) possessed neuroprotective properties. In this study global ischemia/reperfusion was established in rats. Selegiline (5 mg/kg for 7 consecutive days) administrated via intraperitoneal route. Possible involvement of PI3K/AKT/mTOR signaling pathway was evaluated using qRT-PCR, immunohistochemistry and histophatologic evaluations in the hippocampus. Spatial memory was evaluated by morris water maze (MWM). Results showed that ischemia impaired the memory and ischemic rats spent more time to find hidden platform in the MWM. Ischemia significantly decreased levels of PI3K, AKT and mTOR in the hippocampus. Histopathologic assessment revealed that the percent of dark neurons significantly increased in the CA1 area of the hippocampus of ischemic rats. Selegiline improved the memory as ischemic rats spent fewer time to find hidden platform in the MWM. Findings showed that selegiline increased the level and expression of PI3K, AKT and mTOR as well as decreased the proportion of dark neurons in the CA1 area of the pyramidal layer of the hippocampus. We concluded that selegiline, partially at least, through increases the expression of PI3K, AKT and mTOR as well as decreases the percent of dark neurons in the hippocampus could improve the memory impairment following the ischemia in rats.