Sex differences in the intensity of cross-sensitization between methylphenidate and amphetamine in adolescent rats

Effects of acute and chronic methylphenidate on prepulse inhibition: A sex difference study in Wistar rats

BACKGROUND

The utilization of methylphenidate (MPH) is experiencing a notable surge within the adult population. This growth can be attributed to two key factors: its recreational and cognitive enhancement purposes, as well as the rising prevalence of ADHD diagnoses within this population. This study examined acute and chronic oral MPH effects on attention in male and female Wistar rats. To this end, we used a prepulse inhibition (PPI) task, which is widely used to assess psychoactive drug effects in both humans and rodents. This task allowed us to evaluate changes in attention by analyzing sensorimotor gating associated with stimulus selection process.

METHODS

Animals were administered a clinically relevant dose of MPH (5 mg/kg) daily for seven days. The estrous cycle phases of the female rats were measured during behavioral sessions. The PPI task was conducted 20 min after drug administration on day 1 (acute), day 7 (chronic), and 48 h post-treatment.

RESULTS

Results indicated that both acute and chronic MPH treatment impaired PPI expression in male rats, but not in female rats, regardless of their estrous cycle phase. Furthermore, a differential effect of chronic MPH treatment on the PPI task was found in male rats. Specifically, on the seventh treatment day, the PPI effect was observed when animals undertook the PPI task for the first time but was impaired in those animals in which the initial PPI session occurred under the acute influence of the drug (day 1).

CONCLUSIONS

These findings suggest that the impact of MPH on sensorimotor gating responses may vary based on sex and task experience, possibly leading to state-dependent effects in healthy individuals.

The effect of environment on cross-sensitization between methylphenidate and amphetamine in female rats

Methylphenidate (MPD) and amphetamine (AMP) are both psychostimulants that are often used to treat behavioral disorders. More recently, it has also been increasingly used illicitly for recreation as well as to improve intellectual performance. Many factors such as age, gender, genetic background, and environment govern the development of behavioral sensitization to MPD and cross-sensitization with other drugs, which are experimental behavioral markers indicating potential of substance dependence and abuse. This study examines the effects of the environment and age when MPD was exposed in adulthood alone as well as in adolescence into adulthood on cross-sensitization with AMP in female SD rats by randomizing animals to either receive the drug in a home cage or a test cage during adolescence, adulthood, or both. In a 34 day experiment, 16 groups of animals starting in adolescence were treated with saline on experimental day one (ED1), followed by a 6 day (ED2-ED7) treatment with either saline, 0.6 mg/kg AMP, 0.6, 2.5, or 10.0 mg/kg MPD. Experimental groups were then subject to a 3-day washout period (ED8-ED10) and then a retreatment with the respective drug on ED11 in adolescence (P-38 to P-49). Experiments continued in the same animal groups now in adulthood (P-60) with a saline treatment (ED1), followed by the same sequence of treatments in adolescence (ED2-ED11;P-61 to P-69). A rechallenge with the same AMP or MPD dose was performed on ED11 (P-70) followed by a single exposure to 0.6 mg/kg AMP on ED12 (P-71) to assess for cross sensitization between MPD and AMP. Animals treated with MPD in both adolescence and adulthood and in the last experimental day of AMP (ED12) showed higher intensity of cross-sensitivity between MPD and AMP as compared to animals treated with MPD only in adulthood. AMP and MPD treatment in adolescence and into adulthood in the home or test cage resulted in significantly higher responses to the drug as compared to those treated only in adulthood. Overall, we conclude that environmental alteration and adolescent exposure to MPD appeared to increase the risk of cross-sensitization to AMP in female SD rats i.e, using MPD in adolescence may increase the probability of becoming dependent on drugs of abuse. This further indicates that age, sex, and environment all influence the response to MPD and AMP, and further work is needed to elucidate the risks associated with MPD and AMP use.

What are the consequences of Methylphenidate exposure for maternally separated rats?

Methylphenidate (MPH) abuse is prevalent among youth. Drug abuse results in pain perception. We sought to investigate whether Maternal separation (MS) prone to MPH addiction. The next question was whether MPH abusers with MS differ in pain perception. We investigated the impact of MS on addiction and drug reward as well as pain perception following 5 days of MPH injection in males and females rats. Initially, rats underwent MS protocol of 3 hr daily for 21 days. Conditioned place preference (CPP) test was an attempt to investigate whether MS rats experience more reward with MPH. The protocol consisted of 10 min habituation on Day 1, conditioning on Day 2-Day 6 (5 mg per kg MPH injection in drug compartment and saline in saline compartment with 4 hr gap between injections) and 10 min test on Day 7. Furthermore, using another group, differences in pain perception were investigated after 5 days of daily MPH injection with 5 mg per kg. Sensory pain sensitivity was tested on PND 39 using tail flick and hotplate in MS and control groups with and without MPH exposure. Results indicated that female rats are equally prone to addiction in CPP. On the other hand, MS males experience a higher reward in CPP. In tail flick test, female MS rats exposed to MPH show a lower sensory pain threshold with similar MPH exposure. Experiencing MPH similarly declined hotplate pain perception in MS and controls in the females. Males, on the other hand, did not show any difference in sensory pain tests. According to results one can argue MS is detrimental. MS males experience more reward with MPH, females are equally addiction prone and MS females experience more pain in tail flick. On the other hand pain threshold can decline in hotplate test for both control and MS females that received MPH.