Natalia Malikowska-Racia

The serotonergic system remains a critical focus of neuropsychopharmacology due to its widespread influence on mood, cognition, and behavior. Despite the clinical success of selective serotonin reuptake inhibitors (SSRIs), their long-term efficacy is limited by receptor heterogeneity, desensitization, and compensatory adaptations. Recent advances suggest that ligands simultaneously modulating two serotonin (5-HT) receptor subtypes may offer superior therapeutic outcomes. This perspective summarizes progress in developing such dually acting compounds for CNS disorders, including Alzheimer’s and Parkinson’s disease, schizophrenia, and mood disorders. Clinically relevant examples include flibanserin (5-HT_1A receptor agonist/5-HT_2A receptor antagonist), pimavanserin (5-HT_2A/5-HT_2C receptors inverse agonist), and eltoprazine (5-HT_1A/5-HT_1B receptors partial agonist), alongside experimental 5-HT_2A/5-HT₆, 5-HT₃/5-HT₆ or TAAR1/5-HT_2C receptors ligands. Integrating structure–activity insights and clinical findings, we discuss challenges of rational dual modulation. Advances in biased signaling, targeting distinctive conformational states, and optopharmacology utilizing photochromic ligands may further enable the design of innovative dually acting agents with improved efficacy and safety profiles.

Rationale: Ketamine, known for its rapid-acting antidepressant effects, is often coadministered with other psychoactive drugs, including opioids and cannabinoids, in both clinical and recreational settings. We investigated whether ketamine (R-, S-, or racemic R, S-) isoforms alter the subjective effects of morphine or Δ ⁹ -tetrahydrocannabinol (THC) and whether the interoceptive effects of ketamine overlap with these drugs. Objectives: Rats were trained to discriminate morphine (6.4 mg/kg) or THC (3 mg/kg) from their respective vehicle controls. Subsequently, different ketamine isoforms were administered at doses of 5, 10, or 30 mg/kg either prior to or instead of the training drug. Results: In drug interaction tests, naltrexone (0.1 mg/kg) fully antagonized morphine’s effects, and rimonabant (0.32, 1, and 3.2 mg/kg) partially inhibited THC cue. However, ketamine isoforms did not fully block the discriminative stimulus effects of morphine or THC. In generalization tests, as expected, fentanyl (0.1 mg/kg) fully substituted for the morphine cue, and cannabidiol did not substitute (10 mg/kg) or partially substituted (3 and 50 mg/kg) for the THC cue. However, none of the ketamine isoforms fully substituted for the morphine or THC cue. Ketamine isoforms did not alter response rates in the morphine experiment, but suppressed responding in the THC experiment. Conclusions: These findings indicate that ketamine isoforms do not produce opioid- or cannabis-like subjective effects and are unlikely to fully mask the interoceptive effects of morphine or THC. However, their coadministration with cannabinoids may lead to pronounced sedative-like suppression. This warrants further investigation into the safety and interaction profiles of such drug combinations, particularly in therapeutic settings.

Background: Naltrexone is an opioid receptor antagonist that can modulate reward processing in opposite directions depending on the dose. Whether naltrexone similarly affects motivation remains unexplored. This study investigates the effects of naltrexone on behavioral measures of motivation and search for potential mechanisms, including the endogenous opioid pathway dependent on proopiomelanocortin (POMC). Methods: Male Sprague Dawley rats received naltrexone (0.01, 0.1, or 1 mg/kg, ip) for two weeks. During this period, rats were tested daily using a progressive ratio schedule of reinforcement (PR) test and effort-based choice (EBC) that address motivational vigor, directedness, and effort-based decision-making. After tests, the hypothalami were collected for proteomic analysis using data-independent acquisition (DIA). Results: Low-dose naltrexone (0.01 mg/kg; LDN) transiently increased PR response vigor without altering decision-making in EBC. At 0.1 mg/kg, but not at the high dose of 1 mg/kg, it impaired effort-based decision-making and goal-directedness. Proteomic analysis correlated LDN with the downregulation of a growth hormone (GH) pathway and altered G protein-coupled receptors (GPCR) signaling. Naltrexone’s intermediate dose predominantly impacted proteins involved in neural growth, while the 1 mg/kg dose affected proteins related to gene regulation. Conclusions: Different doses of naltrexone had varying effects on motivational measures and the rat’s hypothalamic proteome. Naltrexone 0.1 mg/kg impaired motivational directedness and effort-based decision-making that corresponds to reduced reward signaling due to opioid blockade. In contrast, LDN enhanced vigor, but only early in the treatment. Naltrexone had no effects on the POMC-dependent endogenous opioid pathway, suggesting that a different mechanism underlies its motivational effects.

Background: In clinical studies, psychedelics including psilocybin and D-lysergic acid diethylamide (LSD) demonstrate rapid and persistent antidepressant effects. Since the effective treatment with psychedelics is usually provided with psychotherapy, it is debatable whether their prolonged efficacy can be observed in infrahuman species. Preclinical reports on psychedelics' effects most often address their acute actions, and different tests and models provide inconsistent results. The goal of this study was to examine whether the treatment with psilocybin and/or LSD would demonstrate immediate and/or sustained antidepressant-like effects in the differential reinforcement of low-rate responding (DRL) schedule in rats. In contrast to the antidepressant screening tools, the DRL 72s test is known to detect antidepressants with high predictive validity as it differentiates clinically effective antidepressants from other psychoactive drugs in non-stressed animals. Methods: Adult male Sprague Dawley rats were injected over three consecutive days with psilocybin (1 mg/kg), LSD (0.08 mg/kg), or saline and then tested in DRL 72s for the following 4 weeks. Results: Treatment with psilocybin but not LSD demonstrated an immediate antidepressant-like effect, manifested as an increased number of reinforced presses and response efficiency. By contrast, neither of the drugs showed a long-term (up to 4 weeks following administration) antidepressant-like effect. Conclusions: Using DRL 72s schedule of reinforcement, we demonstrated the acute antidepressant-like effect of psilocybin but not of LSD, and failed to detect their persistent antidepressant-like efficacy. The present study suggests that the detection of long-lasting antidepressant-like activity in rats could be challenging and may require entirely novel behavioral methods.

Background: Although the terms “agonist” and “antagonist” have been used to classify sigma-1 receptor (σ₁R) ligands, an unambiguous definition of the functional activity is often hard. In order to determine the pharmacological profile of σ₁R ligands, the most common method is to assess their potency to alleviate opioid analgesia. It has been well established that σ₁R agonists reduce opioid analgesic activity, while σ₁R antagonists have been demonstrated to enhance opioid analgesia in different pain models. Methods: In the present study, we evaluated the pharmacological profile of selected σ₁R ligands using a novel object recognition (NOR) test, to see if any differences in cognitive functions between σ₁R agonists and antagonists could be observed. We used the highly selective PRE-084 and S1RA as reference σ₁R agonist and antagonist, respectively. Furthermore, compound KSK100 selected from our ligand library was also included in this study. KSK100 was previously characterized as a dual-targeting histamine H₃/σ₁R antagonist with antinociceptive and antiallodynic activity in vivo. Donepezil (acetylcholinesterase inhibitor and σ₁R agonist) was used as a positive control drug. Results: Both tested σ₁R agonists (donepezil and PRE-084) improved learning in the NOR test, which was not observed with the σ₁R antagonists S1RA and KSK100. Conclusions: The nonlinear dose–response effect of PRE-084 in this assay does not justify its use for routine assessment of the functional activity of σ₁R ligands. Graphical

Rationale: Ketamine and psilocybin belong to the rapid-acting antidepressants but they also produce psychotomimetic effects including timing distortion. It is currently debatable whether these are essential for their therapeutic actions. As depressed patients report that the “time is dragging,” we hypothesized that ketamine and psilocybin-like compounds may produce an opposite effect, i.e., time underestimation, purportedly contributing to their therapeutic properties. Objectives: Timing was tested following administration of (R)- and (S)-ketamine, and psilocybin, psilocin, and norpsilocin in the discrete-trial temporal discrimination task (TDT) in male rats. Timing related to premature responses, and cognitive and unspecific effects of compounds were tested in the 5-choice serial reaction time task (5-CSRTT) in the standard 1-s, and “easier” 2-s stimulus duration conditions, as well as in the vITI variant promoting impulsive responses. Results: (S)-ketamine (15 but not 3.75 or 7.5 mg/kg) shifted psychometric curve to the right in TDT and reduced premature responses in 5-CSRTT, suggesting expected time underestimation, but it also decreased the accuracy of temporal discrimination and increased response and reward latencies, decreased correct responses, and increased incorrect responses. While (R)-ketamine did not affect timing and produced no unspecific actions, it reduced incorrect responses in TDT and increased accuracy in 5-CSRTT, suggesting pro-cognitive effects. Psilocin and psilocybin produced mainly unspecific effects in both tasks, while norpsilocin showed no effects. Conclusions: Time underestimation produced by (S)-ketamine could be associated with its antidepressant effects; however, it was accompanied with severe behavioral disruption. We also hypothesize that behavioral disruption produced by psychedelics objectively reflects their psychotomimetic-like actions.

The increasing number of patients reporting depressive symptoms requires the design of new antidepressants with higher efficacy and limited side effects. As our previous research showed, 2-methoxyphenylpiperazine derivatives are promising candidates to fulfill these criteria. In this study, we aimed to synthesize a novel 2-methoxyphenylpiperazine derivative, HBK-10, and investigate its in vitro and in vivo pharmacological profile. After assessing the affinity for serotonergic and dopaminergic receptors, and serotonin transporter, we determined intrinsic activity of the compound at the 5-HT_1A and D₂ receptors. Next, we performed behavioral experiments (forced swim test, tail suspension test) to evaluate the antidepressant-like activity of HBK-10 in naïve and corticosterone-treated mice. We also assessed the safety profile of the compound. We showed that HBK-10 bound strongly to 5-HT_1A and D₂ receptors and presented antagonistic properties at these receptors in the functional assays. HBK-10 displayed the antidepressant-like effect not only in naïve animals, but also in the corticosterone-induced mouse depression model, i.e., chronic administration of HBK-10 reversed corticosterone-induced changes in behavior. Moreover, the compound’s sedative effect was observed at around 26-fold higher doses than the antidepressant-like ones. Our study showed that HBK-10 displayed a favorable pharmacological profile and may represent an attractive putative treatment candidate for depression.

Posttraumatic stress disorder (PTSD) has been associated with abnormal regulation of the hypothalamic-pituitary-adrenal gland axis (HPA). Women demonstrate a more robust HPA response and are twice as likely to develop PTSD than men. The role of sex hormones in PTSD remains unclear. We investigated whether post-trauma chronic treatment with the GABA-ergic agent tiagabine and dopamine-mimetic pramipexole affected the behavioral outcome and plasma levels of corticosterone, testosterone, or 17β-estradiol in female and male mice. These medications were investigated due to their potential capacity to restore GABA-ergic and dopaminergic deficits in PTSD. Animals were exposed to a single prolonged stress procedure (mSPS). Following 13 days treatment with tiagabine (10 mg/kg) or pramipexole (1 mg/kg) once daily, the PTSD-like phenotype was examined in the fear conditioning paradigm. Plasma hormones were measured almost immediately following the conditioned fear assessment. We report that the exposure to mSPS equally enhanced conditioned fear in both sexes. However, while males demonstrated decreased plasma corticosterone, its increase was observed in females. Trauma elevated plasma testosterone in both sexes, but it had no significant effects on 17β-estradiol. Behavioral manifestation of trauma was reduced by pramipexole in both sexes and by tiagabine in females only. While neither compound affected corticosterone in stressed animals, testosterone levels were further enhanced by tiagabine in females. This study shows sex-dependent efficacy of tiagabine but not pramipexole in a mouse model of PTSD-like symptoms and a failure of steroid hormones’ levels to predict PTSD treatment efficacy.