The antidepressant agomelatine attenuates morphine-induced reinstatement but not self-administration or precipitated withdrawal

Abstract

Background:

Exogenous melatonin appears to have anti-addictive properties and was recently shown to improve mental health and metabolic measures in patients receiving chronic opioid maintenance therapy. Agomelatine is a marketed antidepressant which acts as a melatonin agonist. We evaluated its effects using a rat model of morphine-reinforced behavior.

Methods:

After pretreatment with noncontingent morphine, male Wistar rats were trained to self-administer intravenous morphine (1.0 mg/kg-injection) under a progressive-ratio schedule. Rats were pretreated with vehicle or agomelatine during extinction, reinstatement, and reacquisition of morphine-reinforced behavior.

Results:

Daily treatment with 10 mg/kg-day of agomelatine decreased the number of ratios completed and prolonged latency during morphine-induced reinstatement. There were no significant effects on cue-induced reinstatement, morphine self-administration, or naloxone-precipitated withdrawal. Treatment with 32 mg/kg-day of agomelatine caused postural changes. That dose prolonged withdrawal-induced loss of body weight and caused delayed reductions in food reinforcement.

Summary:

In addition to postural effects, high-dose agomelatine worsened the course of spontaneous withdrawal and produced nonspecific effects on food-reinforced behavior. When administered at a selective dose, agomelatine did not modify morphine self-administration or precipitated withdrawal, but decreased morphine-induced reinstatement. Our findings show potential detrimental effects of high-dose agomelatine, with reductions in opioid-seeking behavior after a lower, more selective dose.

1. Introduction

The number of drug overdoses has surged in the past decade and is currently the leading cause of accidental death in the United States (Mattson et al., 2021). More than 60 % of overdose deaths are attributed to opioids. With greater scrutiny on prescription opioids, many users have switched to cheaper and more readily available sources, including fentanyl and related compounds (Yeo et al., 2022). The rate of opioid overdose deaths in male military veterans doubled between 2010 and 2019 (Begley et al., 2022). Veterans with a prior nonfatal overdose have substantially higher mortality rates compared to other veterans or the general population (Warfield et al., 2021). Given the increasing prevalence of fatal overdose and other negative health outcomes associated with opioid abuse, new and innovative treatments are urgently needed.

Melatonin is an endogenous neurohormone and neurotransmitter that is primarily produced by the pineal gland and plays a role in establishing daily and seasonal rhythms, and possibly aging, mood, and substance use disorders (Das et al., 2022). Exogenous melatonin was recently shown to improve mental health and metabolic measures in patients receiving chronic opioid maintenance therapy (Ghaderi et al., 2019). In animals, supratherapeutic doses of exogenous melatonin can decrease signs of opioid withdrawal (Raghavendra and Kulkarni, 2000), tolerance (Lin et al., 2016), and drug preference (Han et al., 2008). Melatonin can attenuate cocaine self-administration and cue-induced reinstatement in rats (Takahashi et al., 2017). Interruption of the light-dark cycle in rats that increases oral morphine intake is associated with a decreased plasma concentration of melatonin (Garmabi et al., 2016). This finding, combined with observations of diminished morphine-induced conditioned-place preference after administration of exogenous melatonin, indicates that melatonin agonists may have a therapeutic role in opioid use disorders.

Agomelatine is a marketed antidepressant structurally related to melatonin (de et al., 2010). It acts as an agonist at melatonin type 1 and 2 receptors (Guardiola-Lemaitre et al., 2014) and an antagonist for serotonin type 2C receptors (5-HT_2CR). When administered at relatively low doses, such as those approved for use in patients, 5-HT_2CR agonists can increase stimulant (Johns et al., 2021) or opioid (Townsend et al., 2020) reinforced behaviors. In contrast, pretreatment with relatively high doses in animals can attenuate drug taking and also cause nonspecific effects (Collins et al., 2015; Kohut and Bergman, 2018; Panlilio et al., 2017).

Depression and substance use disorders often coexist and are typically associated with a worsened clinical course (Rappeneau and Bérod, 2017), supporting use of antidepressant compounds as a strategy to prevent relapse in substance use disorders. Overall, agomelatine has two potential mechanisms to prevent opioid-reinforced behavior, activating melatonin receptors and blocking the 5-HT_2CR. The current experiments were designed to make a preclinical assessment of whether this medication would be helpful as a treatment for individuals with opioid use disorders who are seeking to decrease drug use or achieve abstinence. We utilized well-characterized methods for intravenous morphine reinforcement, reinstatement, and naloxone-precipitated withdrawal to make this determination.

A significant number of patients become opioid-dependent after receiving medical prescriptions rather than through recreational use (Burke et al., 2020; Higgins et al., 2018). To model behavior in which dependence is established prior to voluntary use, animals were pretreated with noncontingent morphine prior to drug self-administration. Previous studies have shown that opioid pretreatment produces strong physical dependence and reliable opioid-reinforced behavior (Grasing et al., 2005; He and Grasing, 2006). Our hypothesis was that agomelatine would attenuate signs of withdrawal in dependent animals.

2. Materials and methods

2.1. Animals

Nine-week old, outbred Wistar rats (Crl:WI) were obtained from Charles River Laboratories (Raleigh, NC). To avoid menstrual influences on behavior (Mello et al., 2007; Vazquez et al., 2020), only males were included. Animals were individually housed under a reversed light-dark cycle (12 h of darkness beginning at 9:00 AM, followed by 12 lighted hours). To maintain consistent food reinforcement, daily intake was limited to 14 g of standard rat chow with ad libitum drinking water. Procedures were performed in accordance with the National Institutes of Health Guide for Care and Use of Laboratory Animals (2011) with protocols reviewed by the local animal care and use committee.

2.2. Operant conditioning

Daily sessions for drug- and food- reinforced behavior were conducted in sound-attenuated chambers during the dark phase of a reversed light-dark cycle as previously described (Grasing et al., 2019). A flashing stimulus light indicated the availability of reinforcement. Delivery of drug or food was signaled by an audible tone, followed by constant illumination of the stimulus light during a time-out period.

Prior to surgery for catheter implantation, rats were trained to respond for 45 mg food pellets under a progressive-ratio schedule (PR9–4). This ensured familiarity with operant responding by lever pressing before being exposed to drug treatments. Following at least seven days of recovery from surgery, rats were randomly assigned to respond for morphine (1.0 mg/kg-injection) or food (45 mg pellets). To obtain similar rates of reinforcement for either, schedule and time-out duration were adjusted as shown in Table 1 using Eq. (1):

$$\text{Response}\text{Requirement}=\text{Truncate}(\text{C}1*\text{Exp}[\text{C}2*\text{Step}]-\text{C}1+1)$$ (1)

where results are truncated to integer values and step is the number of ratios completed within that session, set to a value of one at the start of each daily session (Grasing et al., 2003). Values for constants 1 and 2 (C1 and C2) are shown in Table 1. For the schedules used, response requirement was incremented to values of 4 after step 9 (PR9–4) and 50 after step 15 (PR15–50). Sessions were terminated after 6 h, regardless of the elapsed time since the most recent ratio was completed.

Table 1.

Parameters for food and drug reinforced behavior. Reinforcement was provided according to the values below, where C1 and C2 are constants 1 and 2 used in Eq. (1).

Reinforcer	Schedule	C1	C2	Time out (s)
Morphine	PR9–4	10	0.035	1
Food	PR15–50	5	0.159	1000

Active and inactive lever responses were recorded over the different phases of study shown in Fig. 1. During Baseline, rats made response-contingent injections of morphine signaled by a tone after each self-administered injection. Contingent drug availability was indicated by a flashing cue light. Morphine self-administration was unavailable throughout five Extinction sessions, during which the tone and cue light were disabled. On the following day, both the tone and cue light were enabled with lever pressing recorded over 2 h of Cue-Induced Reinstatement. A single noncontingent intravenous injection of 7.5 mg/kg of morphine was then made to stimulate Drug-Induced Reinstatement, with lever pressing recorded over an additional 4 h in the presence of the tone and flashing cue light. Treatment with agomelatine or vehicle was continued during the next five sessions of morphine self-administration (Early Reacquisition), and discontinued during Late Reacquisition.

Fig. 1.

Study schema. Each animal was exposed to the sequence shown below. Rats received a daily intraperitoneal injection of agomelatine or vehicle during extinction, reinstatement, and early reacquisition; with the same dose administered over 11 consecutive days.

To accommodate animals to the treatment procedure, daily injections of vehicle were administered during baseline and late reacquisition. An entire cycle lasted at least 17 sessions, which could be extended in animals that took longer to establish stable morphine self-administration.

Latency was calculated as the mean of time intervals between reinforcers (or completed ratios) during a session. For example, an animal that earned 4 or 12 reinforcers over a six-hour session would have latency values of 1.5 and 0.5 h, respectively. Greater latency values correspond to diminished motivation to obtain reinforcement, a desirable feature for putative treatments of substance use disorders.

Responding during extinction or reinstatement is reported as the number of completed ratios. Ratios completed on active- and inactive-levers were independently calculated, excluding responses which would have occurred during administration of drug injections or time out periods.

2.3. Catheter placement

After initial training for food-reinforced responding was complete, rats were anesthetized with intraperitoneal ketamine and xylazine (50 and 4.0 mg/kg respectively). Micro-Renathane^® based catheters (Braintree Scientific, Braintree, Massachusetts) were implanted into the external jugular vein and exited to a back-mounted connector (C313G 3UP/SP, Plastic One, Roanoke, Virginia). Starting immediately after surgery, catheters were flushed each day with 0.1 ml of lock solution (15 % glycerol, 42 Units/ml of heparin, and 0.4 mg/ml of gentamicin).

2.4. Drug treatments

For morphine self-administration or naloxone-precipitated withdrawal, rats were pretreated with an intravenous infusion of morphine sulphate, donated by the National Institute on Drug Abuse. After being allowed to recover from surgery over one week, morphine was administered intravenously by infusion pump (Model A, Razel, Stamford, Connecticut), at a rate of 167 μL/h, 24 h per day, as rats were maintained in home cages with fluid swivels and tethers. Morphine dose was gradually increased over one week to a final value of 20 mg/kg-day (6.7, 8.0, 9.6, 11.6, 13.9, 16.7, 20.0 on days 1 to 7). Rats received 20 mg/kg-day of morphine for additional seven or more days.

For animals evaluated by drug self-administration, morphine pretreatment was discontinued immediately prior to the first self-administration session, with no further noncontingent morphine administered. Rats used to determine drug effects on naloxone-precipitated withdrawal continued to receive noncontingent morphine at 20 mg/kg over an additional four days. This infusion was paused over 3.5 h to allow for additional drug treatments and scoring (see below) and then restarted. Behavior was scored twice in each rat with 20 mg/kg of morphine delivered for at least 48 h prior to the second scoring procedure. Agomelatine or vehicle was administered prior to precipitated-withdrawal sessions in a random order as described below.

Agomelatine (TCI America, Portland, Oregon) has poor aqueous solubility. It was prepared as a suspension in 10 mg/ml hydroxyethyl cellulose and administered promptly by intraperitoneal injection. After at least three days of stable responding (a variance of 20 % or less in the number of ratios completed during each daily session), rats were randomly assigned to different treatments. Because antidepressant-like activity can be enhanced after repeated dosing with either melatonin (Raghavendra et al., 2000) or agomelatine (Bourin et al., 2004), animals received daily treatment with vehicle or agomelatine over 11 days administered 20 min prior to behavioral sessions.

A total of 13 rats received treatment with vehicle or agomelatine, with each animal receiving between one and four 11-day periods of agomelatine treatment (0, 10, or 32 mg/kg-day). Each period of agomelatine treatment was considered to be an independent observation. Animals that maintained patent catheters and stable self-administration received additional doses of vehicle or agomelatine in a random pattern, sometimes repeating one dose. Evaluating multiple treatments per subject minimized the number of animals that underwent surgery for catheter implantation and initial operant training.

2.5. Naloxone-precipitated withdrawal

An additional group of 19 rats received daily treatment with vehicle or agomelatine over 14 days to determine effects on precipitated withdrawal. Prior to behavioral scoring, morphine delivered by infusion was stopped (−2.5 h), rats injected with their daily dose of intraperitoneal vehicle or agomelatine (−0.5 h), followed by a second intraperitoneal injection of naloxone (0.3 mg/kg, 0 h). An observer blinded to drug treatment scored behavior for withdrawal signs under dim lighting while animals were food deprived.

Withdrawal was scored as a weighted sum of individual signs (Gellert and Holtzman, 1978), including loss of body weight expressed as a percentage exceeding 1.0 %; Escape attempts scored as 0.0 for one or fewer, 1.0 for 2 to 4, 2.0 for 5 to 9, and 3.0 for 10 or more events; and wet-dog shakes scored as 0.0 for none, 2.0 for 1 or 2, and 4.0 for 3 or more events. Additional signs were assessed as the number of events multiplied by a weighting factor of 2.0 for burrowing (burying or digging), diarrhea, flattened abdominal posture, fasciculation (teeth chatter), ptosis, stool boli, or swallowing movements; and 3.0 for abnormal posture, erection or ejaculation, or irritability (spontaneous vocalization). Weight loss was assessed over 1 h, with all other signs scored over a 30-minute period.

2.6. Statistical analysis

Statistical analyses were made using Systat software (version 13, Inc., San Jose, California). Normality was first evaluated using the Shapiro-Wilk test. Non-normally distributed measures were compared by Kruskal-Wallis ANOVA with post hoc comparisons made by Mann Whitney U tests. Normally-distributed measures were tested by analysis of variance (ANOVA), with post hoc comparisons made by paired t-tests. To evaluate the effects of agomelatine treatment, independent one-way analyses of variance were performed at each time point during extinction, reinstatement, and reacquisition phases; and during agomelatine treatment in food-reinforced animals. Because opioid-reinforced behavior has been well characterized in previous experiments, we did not test for statistical significance within or between phase (i.e., changes in number of ratios completed within extinction or between extinction and self-administration). F notation for ANOVA is only shown for instances in which statistically significant effects of agomelatine treatment were observed.

Based on a 40 % correlation between interrelated areas, an alpha (type I error) value of 0.026 was used for post hoc comparisons according to the correction by Sidak (Sankoh et al., 1997). This correction makes a partial adjustment for multiple comparisons.

3. Results

3.1. Body weight

Declines in body weight during different phases of self-administration are shown in Table 2. ANOVA showed significant effects of agomelatine dose [F_{(2, 135}) = 4.36, p < 0.02] and phase of treatment [F_{(4, 135}) = 2.76, p < 0.04]. Body weight declined significantly during extinction in animals treated with vehicle or either dose of agomelatine. Reductions in body weight persisted during reinstatement in only animals receiving high-dose agomelatine. In addition, some animals exhibited flattened posture in which the ventral surface of the body was in contact with the cage floor immediately after receiving 32 mg/kg of agomelatine.

Table 2.

Change in body weight. Body weight was recorded on a daily basis, immediately prior to injection of vehicle or agomelatine. Group means and standard error are shown for change from baseline after different treatments averaged across phase, expressed as a percentage.

Phase		Agomelatine dose (mg/kg)
		Vehicle	10	32
Subject number		11	10	9
Description	Extinction	−0.38 (0.13)*	−0.53 (0.17)**	−0.60 (0.16)**
	Reinstatement	−0.08 (0.14)	−0.18 (0.23)	−0.44 (0.18)*
	Reacquisition, active treatment	0.17 (0.14)	0.00 (0.18)	−0.40 (0.29)
	Reacquisition, post treatment	0.03 (0.14)	−0.25 (0.21)	−0.75 (0.52)

^*indicates a significant comparison with baseline values, with 1 to 2 symbols corresponding to p < 0.02 and 0.01, respectively.

3.2. Self-administration

Effects of agomelatine on morphine reinforcement are shown in Fig. 2. During baseline, rats maintained stable self-administration under a progressive-ratio schedule. Daily treatment with agomelatine did not modify the number of ratios completed during extinction or reacquisition. Afterwards, both vehicle- and agomelatine-treated animals reacquired self-administration of morphine at similar levels.

Fig. 2.

Morphine self-administration after chronic agomelatine treatment. Mean values and standard error are shown for 9 to 11 animals per condition. The number of ratios completed on active and inactive levers during different phases of study (baseline, extinction, and reacquisition of drug taking) is shown by the bottom and center panels. Upper panels show time to complete ratios (latency) for active levers during the same periods. Re-baseline corresponds to the late reacquisition period, during which agomelatine treatment was discontinued. * indicates p < 0.01 for comparison with vehicle-treated animals.

Inactive lever responding occurred at rates similar to those previously reported by our laboratory (Grasing et al., 2003; He and Grasing, 2006; Li et al., 2003). There was no significant effect of agomelatine on the number of ratios completed on inactive-levers at any time point. During the fourth extinction session, latency varied with agomelatine dose [F_{(2, 29)} = 3.83, p < 0.05]. Treatment with 10 but not 32 mg/kg-day of agomelatine caused a significant increase in latency [t(18) = 3.74, p < 0.01]. Otherwise, there were no significant effects of agomelatine treatment on latency values.

3.3. Reinstatement

Ratios completed varied significantly with agomelatine dose for morphine- but not cue- induced reinstatement [F_{(2, 27)} = 4.15 or 2.27, p < 0.03 and not significant, respectively], with no effect on inactive-lever values (Fig. 3). Post hoc comparisons indicated that 10 mg/kg of agomelatine decreased the number of ratios completed [t(19) = 2.90 p < 0.01] with 32 mg/kg ineffective [t(18) = 1.57, p not significant]. Based on comparisons with vehicle-treatment, reductions in morphine-induced ratios completed corresponded to 49.2 and 39.8 % for agomelatine doses of 10 and 32 mg/kg-day. Latency also varied significantly with agomelatine dose for morphine-induced reinstatement [F_{(2, 25}) = 5.18, p < 0.02], with no effects on cue-induced reinstatement. Compared to vehicle treatment, latency was prolonged during morphine-induced reinstatement after low- but not high- dose agomelatine [t(19) = 3.16 & 1.58, p < 0.01 and not significant, respectively].

Fig. 3.

Effects of agomelatine treatment on reinstatement. Non-reinforced responding (reinstatement) for subjects shown in the previous figure. Horizontal axis shows values following exposure to drug-associated light and tone (Cue) and 7.5 mg/kg of noncontingent morphine (M7.5). Upper and lower panels show results for the active and inactive levers. * indicates a significant comparison with vehicle-treated animals, p < 0.01.

3.4. Food-reinforced behavior

Changes in food-reinforced behavior after agomelatine treatment are shown in Fig. 4. Ratios completed varied significantly with agomelatine dose for only the second session following daily treatment [F_{(2, 27}) = 3.62, p < 0.05]. Relative to animals receiving vehicle, the 32 mg/kg dose of agomelatine caused small but significant decreases during that session [t(16) = 2.92, p < 0.01]. Otherwise, there were no significant effects on latency or inactive lever responding during food-reinforced behavior. Agomelatine treatment also did not modify body weight in food-reinforced rats.

Fig. 4.

Effect of agomelatine on food-reinforced responding. Mean values and standard error are shown for 6 to 12 animals per condition. * indicates p < 0.01 for comparison with vehicle-treated animals.

3.5. Naloxone-precipitated withdrawal

Given its melatonin agonist properties with potential to modify opioid withdrawal, we evaluated the effect of chronic agomelatine on precipitated withdrawal signs. Due to effects on body weight, posture, and food-reinforced behavior; the 32 mg/kg dose of agomelatine was not included. As shown in Table 3, opioid blockade produced robust signs in morphine-dependent rats. However, detailed measures of opioid withdrawal did not differ following agomelatine treatment.

Table 3.

Effect of agomelatine on precipitated withdrawal. Rats received daily treatment with either vehicle or 10 mg/kg of agomelatine with subject numbers indicated below. Behavior was scored by an individual blinded to drug treatment. Values in parentheses indicate standard error.

Measure	Agomelatine dose (mg/kg)
	Vehicle	10
Subject number	10	9
Number of signs	31.7 (3.4)	28.3 (7.3)
Total score	81.7 (7.9)	70.6 (17.3)
Loss of body weight	4.31 (0.50)	2.50 (0.72)
Flattened abdominal posture	11.7 (2.6)	13.2 (4.7)
Fasciculation or teeth chatter	0.100 (0.095)	0.222 (0.139)
Ptosis	3.80 (1.23)	2.11 (1.36)
Diarrhea	4.20 (1.30)	3.67 (1.35)
Stool boli	1.50 (0.57)	1.00 (0.63)
Abnormal posture	8.90 (1.60)	7.22 (2.03)
Ejaculation or genital attention	1.50 (0.47)	0.78 (0.31)

4. Discussion

Activation of melatonin receptors can decrease opioid-induced place-preference (Han et al., 2008) and stimulant-reinforced behavior (Takahashi et al., 2017). Therefore, melatonin agonists would be expected to have anti-addictive properties. The present study tested this hypothesis using a rat intravenous self-administration model. Unfortunately, daily treatment with agomelatine did not attenuate drug taking, even when administered at a relatively high dose with effects on body weight, posture, and food-reinforced behavior. Nonetheless, a lower dose of agomelatine that did not modify food-reinforced behavior attenuated morphine-induced reinstatement by approximately one-half while prolonging latency. There were no significant effects on cue-induced reinstatement.

Reinstatement has been put forth as an animal model of drug-seeking, in which non-reinforced behavior reflects an internal state that may predict a return to drug use (Epstein et al., 2006). Although reinstatement procedures are widely used, it is unclear whether medications that decrease reinstatement measures in rodents will translate into treatments that facilitate abstinence in humans (Strickland et al., 2022). Based on false-positive findings, the number of self-administered drug injections (drug taking) has been interpreted as the most clinically relevant measure (Comer et al., 2008; Haney and Spealman, 2008). Therefore, our results should be interpreted as less than favorable regarding agomelatine’s anti-addictive potential.

Conditioned-place preference relies on classical conditioning, measured by a subject’s preference for a location associated with a drug treatment (Tzschentke, 2007). In contrast, drug self-administration has been interpreted as operant (instrumental) learning (Jones and Comer, 2013). This difference may explain why melatonin attenuated morphine-induced conditioned-place preference (Han et al., 2008), but agomelatine which activates melatonin receptors failed to modify morphine-reinforced behavior in the present study. So far as we are aware, there has been no direct evaluation of melatonin on opioid-motivated behavior using an intravenous self-administration procedure.

Compared to fixed-ratio schedules for which response requirement remains constant, use of a progressive-ratio may be more likely to identify changes in drug-reinforced behavior (Lile et al., 2016; Stafford et al., 1998). Accordingly, the present study utilized progressive-ratio schedules to evaluate both food and morphine reinforcement. The PR9–4 schedule used was specifically designed for opioid self-administration (Grasing et al., 2003) and has been successful in previous work identifying decreases in drug taking after stimulant pretreatment (He and Grasing, 2006) or monoamine oxidase inhibition (Grasing and He, 2005). Our study also employed daily administration of agomelatine over an extended period rather than single-dose pretreatment, which is also recommended for evaluating potential treatments for substance use disorders (Banks et al., 2019). Potential therapeutic compounds with multiple mechanisms of action may be more likely to be effective (Townsend et al., 2021), such as noted above for agomelatine. Overall, the present study met several characteristics recommended for the evaluation of potential treatments for substance use disorders; including use of a progressive-ratio as opposed to a fixed-ratio schedule, chronic rather than acute administration of test compounds, and including an agent with multiple, complementary mechanisms of action.

Withdrawal states are key determinants of both opioid self-administration and the potential effects of therapeutic medications (Koob, 2009). As noted in the Introduction, activation of melatonin receptors can attenuate signs of opioid withdrawal in mice (Raghavendra and Kulkarni, 1999, 2000) and rats (Motaghinejad et al., 2015). In the present study, body weight declined during extinction sessions in which morphine self-administration was no longer available in both vehicle and agomelatine treated animals. This finding indicates that animals were opioid dependent during morphine self-administration, and underwent spontaneous opioid withdrawal during extinction. Rather than decreasing the severity of withdrawal, reductions in body weight were prolonged in rats receiving high-dose agomelatine (Table 2). Agomelatine was also ineffective in decreasing the severity of naloxone-precipitated withdrawal (Table 3). We conclude that agomelatine-induced activation of melatonin receptors was insufficient to attenuate withdrawal under the conditions studied. This may be due to differences in species or receptor subtype activation by melatonin and agomelatine.

Our interest in agomelatine arose after recent unsuccessful attempts to use the clinically-available 5-HT_2CR agonist lorcaserin as a treatment for opioid or stimulant use disorders (Negus and Banks, 2020). Several animal studies appearing to show beneficial actions of lorcaserin administered doses well above those approved for use in patients (Collins et al., 2018). When administered at lower, clinically-relevant doses; both preclinical and patient-based studies found predominantly negative effects which included increases in craving (Pirtle et al., 2019) and greater choices for drug by rhesus monkeys self-administering heroin under fixed-ratio-10 (Townsend et al., 2020) and humans self-administering cocaine under a progressive-ratio schedule (Johns et al., 2021). These findings highlight the importance of dose as a key factor for evaluating potential treatments in substance use disorders (Grasing et al., 2022). Despite the apparent utility of the 5-HT_2CR for stimulating drug use, we did not observe corresponding reductions after blockade with agomelatine in the present study, at least for opioid-reinforced behavior in rats.

Previous studies have shown beneficial effects of agomelatine in rodent models of mood (Tchekalarova et al., 2018b) and seizure (Tchekalarova et al., 2018a) disorders when delivered at a chronic dose of 40 mg/kg-day. Intriguingly, these findings were associated with a correction of melatonin rhythmicity and decreased inflammatory markers. Results of the current study show that agomelatine can have detrimental effects on body weight and food-reinforced behavior at a dose of 32 mg/kg-day and is behaviorally active at a significantly lower dose in a rat model of opioid reinstatement. Further study is needed to understand the full potential of agomelatine’s actions in substance use disorders.

Data availability

Data will be made available on request.