Main Research Findings

1) Treatment with Fencamfamine was found to block effects of latent inhibition induced by a pretreatment dose of the antipsychotic, risperidone.

2) Fencamfamine was shown to modulate sodium and potassium ATPase through activation of cyclic AMP and cyclic AMP-dependent proteins

Selected Data
1) The study performed by the research team of Alves et al investigated the effects of fencamfamine (FCF), an indirect dopaminergic psychostimulant, on latent inhibition (LI) in rats, as well as the ability of the atypical antipsychotic risperidone (RIS) to counteract any observed disruptions. Latent inhibition is a behavioral phenomenon in which prior exposure to a neutral stimulus reduces the ability to subsequently learn an association between that stimulus and an unconditioned stimulus. Because disruptions of LI are considered indicative of impaired attentional processing, this paradigm is widely used as an animal model of schizophrenia and psychosis [1].
The experiments employed naive male Wistar rats, each weighing approximately 300 grams at the start of the study. Animals were housed individually under controlled environmental conditions, including a 12-hour light/dark cycle and a stable temperature of 21 ± 1°C. To ensure motivation for the drinking-based behavioral task, rats were maintained on a water restriction schedule of approximately 23 hours, while food remained available ad libitum in the home cages. These conditions are standard for conditioning paradigms involving suppression of drinking behavior.
Two pharmacological agents were used including FCF which was dissolved in 0.9% saline. FCF was administered intraperitoneally at doses of 1.75, 3.5, and 7.0 mg/kg, allowing assessment of a dose–response relationship. RIS was also used and was initially dissolved in a small amount of acetic acid and then diluted in a 5.5% glucose solution. Two doses of 2.0 ,g/kg and 4.0 mg/kg were tested. All injections were delivered intraperitoneally in a volume of 1 ml/kg, and control animals received equivalent volumes of the respective vehicle solutions [1].
Behavioral testing was conducted in four operant conditioning chambers, each enclosed within sound-attenuating isolation boxes. A ventilation fan provided consistent background noise. Each chamber contained a removable drinking bottle, and licking behavior was recorded using a lickometer circuit, allowing precise measurement of drinking suppression. The conditioned stimulus (CS) consisted of a 5-second tone at 70 dB and 2.8 kHz generated by a Sonalert module. The unconditioned stimulus (US) was a scrambled foot shock of 1.0 mA intensity and 1-second duration. All stimulus presentations and data recordings were controlled by a computer-based system, ensuring experimental consistency and accuracy [1].
The LI paradigm was adapted from established protocols and consisted of four sequential phases, conducted at consistent times of day to minimize circadian influences. Days 1-5 were the baseline training phase. During this phase, each rat was placed in the operant chamber and allowed to drink from the bottle until 600 licks were completed. This training ensured stable baseline drinking behavior and habituation to the apparatus. After each session, animals were returned to their home cages and given 30 minutes of additional water access.
The preexposure phase occurred on day 6. To establish latent inhibition, rats were divided into preexposed (PE) and non-preexposed (NPE) groups. The water bottle was removed during this phase. PE animals received 30 presentations of the tone CS, each lasting 5 seconds, with an intertrial interval of 30 seconds. NPE animals were placed in the chamber for an equivalent duration but did not receive the tone, ensuring that any later differences in learning were attributable to prior stimulus exposure.
The conditioning phase took place on day 7 and all animals underwent classical conditioning. After a 5-minute acclimation period, each rat received two CS–US pairings, spaced 5 minutes apart. The tone CS was immediately followed by the foot shock US. Animals were removed from the chamber 5 minutes after the second pairing. This phase established the association between the previously neutral tone and the aversive stimulus [1].
During the testing phase on day 8, the water bottle was reintroduced, and rats were allowed to drink freely. When an animal completed 90 licks, the tone CS was presented and remained active until 10 additional licks were made. If the rat failed to complete these licks within 300 seconds, the session was terminated. A suppression ratio (SR) was calculated to quantify conditioned fear and latent inhibition. The SR was defined as the time taken to complete licks 80–90, referred to as the pre-CS period, divided by the time taken to complete licks 80–100, referred to as pre-CS plus CS period. Lower SR values indicate stronger suppression (greater conditioning), whereas higher SR values in PE animals relative to NPE animals indicate the presence of latent inhibition.
Three separate experiments examined the influence of FCF at 1.75, 3.5, and 7.0 mg/kg. Each dose group was subdivided into PE and NPE conditions, with corresponding saline control groups. FCF or saline was administered 15 minutes prior to both the preexposure and conditioning phases, targeting attentional and associative learning processes. To determine whether RIS could block FCF-induced disruptions of LI, two additional experiments were conducted using RIS doses of 2.0 and 4.0 mg/kg in combination with FCF at 3.5 mg/kg, the dose expected to produce maximal LI disruption. RIS was administered 45 minutes before FCF, which was then given 15 minutes prior to preexposure and conditioning. Control animals received saline at both time points (SAL+SAL). Each treatment condition again included PE and NPE subgroups [1].
Data were analyzed using two-way analysis of variance (2 × 2 ANOVA) with Preexposure (PE vs. NPE) and Drug Treatment as the main factors. Significant interactions between these variables were interpreted as evidence of drug-induced modulation of latent inhibition. This statistical approach allowed the researchers to determine both the presence of LI and the pharmacological effects of FCF and RIS [1].
2) This study completed by the research team of Ferreira et al investigated how FCF, a psychostimulant with dopaminergic activity, modulates Na⁺/K⁺-ATPase activity in the rat striatum, and whether this modulation occurs through cyclic AMP (cAMP) and protein kinase signaling pathways. To achieve this, the researchers designed a series of biochemical experiments using isolated rat brain tissue and pharmacological manipulations [2].
Male Wistar rats weighing between 350–400 grams were used as the experimental model. The animals were euthanized via decapitation under ether anesthesia to minimize distress and preserve brain tissue integrity. Immediately after sacrifice, the brains were removed, and the striatum, a brain region heavily involved in dopaminergic signaling, was carefully dissected. The striatal tissue was then sectioned into small slices measuring 0.3 × 0.3 × 1 mm, using a mechanical tissue chopper. These slices were washed thoroughly to remove debris and maintained at 4°C before being resuspended in a physiological buffer solution. This buffer contained key ions (NaCl, KCl, MgSO₄, CaCl₂) and HEPES to maintain pH at 7.4, approximating physiological conditions [2].
To assess enzyme activity, the researchers employed a permeabilized slice technique. Tissue slices were treated with freezing and thawing steps followed by incubation with alamethicin, a detergent that permeabilizes cell membranes without disrupting enzyme function. This ensured that ATP and other substrates could freely access intracellular enzymes. The ATPase activity assay relied on measuring the hydrolysis of radiolabeled ATP (γ-³²P-ATP). After incubation, the amount of inorganic phosphate (³²Pi) released from ATP hydrolysis was quantified using scintillation counting. This provided a direct measure of ATPase activity.
To distinguish Na⁺/K⁺-ATPase activity from other ATPases, such as Mg-ATPase, the researchers used ouabain, a specific inhibitor of Na⁺/K⁺-ATPase. By comparing total ATPase activity (without ouabain) to ouabain-insensitive activity (with ouabain), they calculated the specific Na⁺/K⁺-ATPase component. Protein content in each sample was measured using a colorimetric assay, allowing enzyme activity to be normalized per milligram of protein. Under the assay conditions, substrate concentrations were saturating, ensuring maximal enzyme activity and reliable comparisons.
A variety of pharmacological agents were used to probe the mechanism of FCF action. First, FCF was tested at concentrations ranging from 10 to 100 µM to establish dose-response effects. Dopamine (DA) was used at 10 µM as a comparison, since FCF is known to influence dopaminergic signaling. 8-बromo-cAMP, is a membrane-permeable analog of cAMP used to mimic intracellular cAMP signaling. Dopamine receptor antagonists including SCH 23390 and Sulpiride were used as D1 and D2 receptor antagonists, respectively. Finally, protein kinase inhibitor KT 5720 was used as a selective inhibitor of protein kinase A, PKA and protein kinase inhibitor KT 5823 was used as a selective inhibitor of protein kinase G, PKG [2].
Drugs were added to striatal slice suspensions and incubated under controlled conditions. In some experiments, tissues were pretreated with receptor antagonists or kinase inhibitors before exposure to FCF or other agents. Each experimental condition included multiple replicates of five samples per condition, and experiments were repeated independently to ensure reproducibility.
To evaluate whether FCF influences intracellular signaling pathways, levels of cyclic nucleotides, cAMP and cGMP, were measured using radioimmunoassay techniques. After drug treatment, supernatants from slice suspensions were collected and chemically processed to enhance assay sensitivity. Samples were acetylated and heated before being subjected to radioimmunoassay using commercially available kits. This allowed precise quantification of intracellular cAMP and cGMP concentrations [2].
The study included several key comparisons including dose-response effects of FCF on ATPase activity, the effects between FCF and dopamine, the combined treatment with FCF and dopamine to test for additive or shared mechanisms, the effects of receptor blockade D1 and D2 antagonists, the effects of kinase inhibition through PKA vs PKG, and the use of cAMPT analogs to mimic intracellular signaling. Control groups included untreated slices and slices treated with inhibitors alone to ensure that observed effects were specific to the experimental manipulations [2].

Discussion
1) The results of the study completed by Alves et al demonstrate that FCF exerts dose-dependent effects on LI in rats, and that these effects can be selectively reversed by RIS. The findings provide important insights into the dopaminergic mechanisms underlying attentional processes and support the utility of the LI paradigm as an animal model of psychosis.
Across all experiments, saline-treated control animals displayed a robust latent inhibition effect. Specifically, PE rats showed significantly higher suppression ratios (SRs) than NPE rats, indicating reduced conditioned fear to the tone due to prior exposure. This consistent difference confirmed the reliability of the experimental procedure and established a valid baseline against which drug effects could be assessed [1].

Figure 1: Mean suppression ratio in animals treated with saline vs animals treated with FCF
At the lowest dose of FCF, the LI effect remained intact. Statistical analysis revealed a significant main effect of preexposure, indicating that PE animals continued to show less conditioned suppression than NPE animals. However, there was no significant main effect of drug treatment and no interaction between preexposure and drug. These findings suggest that 1.75 mg/kg is below the threshold required to influence attentional or associative processes involved in LI.
The 3.5 mg/kg dose produced a markedly different outcome. At this dose, the difference between PE and NPE groups was abolished, indicating a complete disruption of latent inhibition. Statistical analyses revealed a significant main effect of preexposure, a significant main effect of drug treatment, and a significant Preexposure × Drug interaction. The absence of a difference between PE and NPE animals indicates that FCF interfered with the ability of rats to ignore previously irrelevant stimuli. This effect is characteristic of dopaminergic psychostimulants, such as amphetamine, and supports the hypothesis that FCF has psychotogenic-like properties [1].
Interestingly, the highest dose tested did not disrupt latent inhibition. Similar to the low-dose condition, statistical analyses showed a significant effect of preexposure but no significant drug effect or interaction. The persistence of LI at this dose suggests an inverse dose–response relationship, a phenomenon previously observed with other psychostimulants. One possible explanation is that high doses may induce stereotyped behaviors, which interfere with the conditioning process or alter neural activation patterns in dopaminergic pathways, thereby preserving LI.
To further explore the mechanisms underlying FCF-induced LI disruption, the study examined whether pretreatment with the atypical antipsychotic risperidone could reverse the effects of FCF at 3.5 mg/kg. Pretreatment with 2.0 mg/kg RIS failed to prevent the disruption of LI. Statistical analysis showed a significant Preexposure × Drug interaction, but neither the main effect of preexposure nor the main effect of drug reached significance. Behaviorally, PE and NPE animals did not differ, indicating that LI remained abolished. This suggests that the lower dose of RIS was insufficient to counteract the dopaminergic effects of FCF [1].
In contrast, 4.0 mg/kg RIS successfully blocked the disruptive effect of FCF, restoring latent inhibition. Statistical analyses revealed a significant main effect of preexposure, indicating that PE animals again exhibited reduced conditioned suppression compared to NPE animals. Additionally, there was no significant main effect of the drug, and no significant interaction between preexposure and the drug. These results demonstrate that the higher dose of RIS effectively antagonized the FCF-induced impairment of attentional processing. The restoration of LI is likely attributable to RIS’s dopamine D2 receptor antagonism, although its serotonin 5-HT2 receptor blockade may also contribute [1].
The study revealed an inverse dose–response curve for FCF’s effects on LI: only the intermediate dose disrupted LI, whereas both lower and higher doses left the phenomenon intact. This pattern parallels findings with amphetamine and other dopaminergic agents. The disruption at 3.5 mg/kg is consistent with enhanced mesolimbic dopamine transmission, particularly in the nucleus accumbens, which is known to modulate attentional gating. At higher doses, differential activation of striatal versus mesolimbic pathways or the emergence of stereotyped behaviors may attenuate this effect.
The ability of RIS to reverse FCF-induced LI disruption provides further validation of the LI paradigm as a predictive model for antipsychotic efficacy. Psychostimulant-induced impairments in LI are considered analogous to the positive symptoms of schizophrenia, such as impaired selective attention and aberrant salience attribution. The restoration of LI by an antipsychotic agent supports the relevance of this model for studying the neurobiological basis of psychosis and for screening potential therapeutic compounds [1].
In summary, the results demonstrate that saline-treated animals consistently exhibited robust latent inhibition. Fencamfamine at 3.5 mg/kg abolished LI, indicating disruption of attentional processing. Lower doses of 1.75 mg/kg and higher doses of 7.0 mg/kg did not affect LI, revealing an inverse dose–response relationship. Additionally, risperidone at 2.0 mg/kg failed to reverse the FCF effect, however risperidone at 4.0 mg/kg successfully restored LI, likely through dopaminergic and possibly serotonergic antagonism.
Collectively, these findings indicate that fencamfamine produces psychostimulant-like disruptions of latent inhibition mediated by dopaminergic mechanisms and that these effects can be counteracted by an atypical antipsychotic. The study thus strengthens the conceptual link between dopaminergic dysregulation and cognitive abnormalities associated with schizophrenia, while also reinforcing the value of the latent inhibition paradigm for psychopharmacological research [1].
2) The results of the study conducted by the research of Ferreira et al demonstrate that FCF significantly reduces Na⁺/K⁺-ATPase activity in the rat striatum through a mechanism involving dopamine signaling, cAMP, and PKA. Multiple lines of experimental evidence support this conclusion. FCF produced a clear dose-dependent inhibition of Na⁺/K⁺-ATPase activity across the tested concentration range of 10–100 µM) The inhibitory effect began at the lowest concentration of 10 µM and increased progressively, reaching a maximal reduction of approximately 51.5% at 100 µM. The calculated IC₅₀ value (the concentration required to inhibit enzyme activity by 50%) was approximately 4.7 × 10⁻⁵ M, indicating moderate potency. Importantly, FCF did not affect Mg-ATPase activity, demonstrating that its effect was selective for Na⁺/K⁺-ATPase rather than causing a general suppression of ATPase enzymes [2].
Time-course experiments revealed that the inhibitory effect of FCF persisted even after the drug was removed. Reduced Na⁺/K⁺-ATPase activity was observed consistently over a 5–60 minute period, indicating that FCF induces a lasting modification of enzyme function rather than a transient interaction. DA, tested at 10 µM, produced a similar degree of inhibition of Na⁺/K⁺-ATPase activity as high-dose FCF at a concentration of 100 µM. This suggests that FCF may act through dopaminergic mechanisms. When FCF and dopamine were applied together, their effects were not additive. The combined treatment did not produce greater inhibition than either drug alone. This nonadditive effect strongly suggests that both compounds act through a shared pathway or mechanism.
Both FCF and dopamine significantly increased intracellular levels of cAMP, while having no effect on cGMP levels. This indicates that the signaling pathway involved is specific to cAMP rather than cGMP. The increase in cAMP correlated with the observed decrease in Na⁺/K⁺-ATPase activity, supporting the idea that cAMP mediates the inhibitory effect. Further evidence came from experiments using a cAMP analog. This compound mimicked the effect of FCF, producing a similar reduction in Na⁺/K⁺-ATPase activity without affecting Mg-ATPase. This confirms that activation of cAMP signaling is sufficient to inhibit the enzyme [2].

Figure 2: Changes in NA+/K+-ATPase activity in response to DA and FCF in varying concentrations.
To determine whether dopamine receptors were involved, the researchers used selective D1 and D2 receptor antagonists including SCH 23390 and Sulpiride. Pretreatment with either antagonist completely blocked the inhibitory effect of FCF on Na⁺/K⁺-ATPase activity. This indicates that both D1 and D2 dopamine receptors are necessary for FCF’s action. These findings support the idea that FCF increases dopamine signaling, which then activates both receptor subtypes to produce downstream effects.
The involvement of protein kinases was tested using selective inhibitors for PKA and PKG including KT 5720 and KT 5823. Pretreatment with KT 5720 abolished the inhibitory effects of both FCF and 8-बromo-cAMP on Na⁺/K⁺-ATPase activity. In contrast, KT 5823 had no effect on these responses. Additionally, neither inhibitor alone altered enzyme activity, indicating that their effects were specific to blocking the signaling pathway rather than directly affecting the enzyme. These results demonstrate that PKA, but not PKG, is essential for mediating the inhibitory effect of FCF [2].
Taken together, the results support the following mechanism that FCF enhances dopaminergic signaling likely by increasing dopamine release or blocking reuptake. Additionally, dopamine activates D1 and D2 receptors in the striatum resulting in the increase in intracellular cAMP levels. These increased cAMP levels activate PKA which then phosphorylates components of the Na⁺/K⁺-ATPase system, and ultimately reduces enzyme activity. The lack of involvement of cGMP and PKG further confirms the specificity of the cAMP-PKA pathway.
The key conclusion of this study reports that FCF is a potent inhibitor of Na⁺/K⁺-ATPase in rat striatum, the effects were found to be is dose-dependent, selective, and sustained. Additionally FCF and dopamine were shown to share a common mechanism of action and all inhibitory effects were modulated by an increase in cAMP levels. Both D1 and D2 dopamine receptors were required for this mechanism and while PKA activation was essential, PKG was not involved. Overall, the findings provide strong biochemical evidence that FCF modulates neuronal ion transport through a dopamine-dependent cAMP-PKA signaling cascade, offering insight into its central nervous system effects [2].

Disclaimer

**LAB USE ONLY**
*This information is for educational purposes only and does not constitute medical advice. THE PRODUCTS DESCRIBED HEREIN ARE FOR RESEARCH USE ONLY. All clinical research must be conducted with oversight from the appropriate Institutional Review Board (IRB). All preclinical research must be conducted with oversight from the appropriate Institutional Animal Care and Use Committee (IACUC) following the guidelines of the Animal Welfare Act (AWA).

Citations

[1] Alves CR, Delucia R, Silva MT. Effects of fencamfamine on latent inhibition. Prog Neuropsychopharmacol Biol Psychiatry. 2002;26(6):1089-1093. doi:10.1016/s0278-5846(02)00241-5

[2] Pinto Ferreira M, DeLucia R, Luiz Aizenstein M, Glezer I, Scavone C. Fencamfamine modulates sodium, potassium-ATPase through cyclic AMP and cyclic AMP-dependent protein kinase in rat striatum. J Neural Transm (Vienna). 1998;105(6-7):549-560. doi:10.1007/s007020050078

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