Abstract:
مقدمه: نوجوانی یک دوره حساس تکاملی برای فرآیندهای رشد عصبی است. قرار گرفتن در معرض مواد افیونی در نوجوانی می‌تواند توانایی‌های شناختی را از طریق تغییرات پایدار در ساختار و عملکرد مغز تحت تاثیر قرار دهد. مطالعه حاضر به منظور تکمیل نتایج مطالعات قبلی (مواجهه مزمن و تحت مزمن با مرفین نوجوانی) با ارزیابی اثرات قرار گرفتن حاد در معرض مرفین در دوره نوجوانی بر فعالیت حرکتی و رفتار شبه اضطرابی در موش‌های صحرایی طراحی شد. روش کار: موش‌های صحرایی نر نوجوان نژاد ویستار به دو گروه دریافت‌کننده مرفین و سالین تقسیم شدند. حیوانات گروه مرفین در روز 31 پس از تولد، 2/5 میلی‌گرم بر کیلوگرم مرفین دو بار دریافت کردند. بعد از یک هفته از تزریق مرفین آزمون‌های رفتاری میدان باز (Open field test) و ماز بعلاوه مرتفع(Elevated plus maze) در طی چهار هفته در روزهای مشخصی تا دوره بزرگسالی بر روی حیوانات انجام شد. یافته‌ها: نتایج این مطالعه نشان داد تفاوت معنا‌داری در فعالیت حرکتی حیوانات بین گروه‌های سالین و مرفین وجود ندارد. همچنین مواجهه حاد با مرفین در دوره نوجوانی منجر شد تا پروفایل رفتاری ضد اضطرابی تا هفته اول بعد از تزریق مرفین مشاهده شود و این اثرات تا بزرگسالی ماندگار نبودند. نتیجه‌گیری: بلوغ سیستم عصبی می‌تواند تحت تأثیر سوء مصرف مواد در دوران رشد نوجوانی قرار گیرد و این اثرات ممکن است منجر به تغییرات رفتاری کوتاه مدت شود. مواجهه حاد با مرفین در دوره نوجوانی، اثرات ماندگاری بر تغییر مشخصات رفتاری شبه اضطرابی تا بزرگسالی نداشت.
Introduction
Adolescence is a critical period of development, transitioning from childhood to adulthood. During this period, extensive neural plasticity occurs in the central nervous system. These changes aim to establish individual independence and enhance cognitive capacity. Significant neurochemical changes also occur in the brain during adolescence. For example, the brain’s dopaminergic system is fundamentally reorganized, possibly associated with associative learning and motivational behaviors. In addition, dopamine receptors in both cortical and subcortical areas reach their highest levels during adolescence. Furthermore, higher cortical mechanisms do not fully control the heightened activity of the limbic system in adolescence. These distinct developmental pathways in different regions of the brain during adolescence may lead to increased risk-taking behaviors in adolescents and their engagement in addictive behaviors. Exposure to drugs at different stages of development can lead to lasting changes in the structure and function of the brain. However, it is important to note that mature brain systems can compensate for synaptic changes caused by drugs, whereas developing brain systems may incorporate these changes, leading to a stable event in cellular response. Research has explored morphine’s significant opioid impact on memory and learning in adults, yet few studies have examined the effects of acute morphine exposure on anxiety-like behaviors during adolescence. In our previous studies, we have examined the effects of short-term and long-term exposure to opioids during adolescence on learning and memory with behavioral and electrophysiological approaches up to adulthood. In addition, this study intend to investigate the effects of acute morphine exposure during adolescence on locomotor activity and anxiety-like behaviors from adolescence to adulthood.
Methods
In this study, adult male Wistar rats were used. The animals were housed under standard conditions with a 12-hour light/dark cycle and ad libitum access to food and water at 23±2°C. All experiments were conducted following ethical principles for working with laboratory animals and were approved by the Ethics Committee of the Research at Tarbiat Modares University with the ethical code (IR.Modares.REC.1398.052).
Morphine exposure during adolescence
Adolescent male Wistar rats were assigned to the saline and morphine groups to investigate the effects of morphine exposure. In the morphine group, rats were injected subcutaneously (s.c.) with 2.5 mg/kg of morphine sulphate (Temad, Iran) on postnatal day 31. Then, behavioral studies were conducted from adolescence to adulthood, measuring the animal’s locomotor activity and anxiety-like behaviors. The timeline related to this type of morphine exposure and the behavioral tests are shown in Figure 1.
Figure 1. Timeline of behavioral experiments in the group exposed to acute morphine during adolescence.
Behavioral tests
Open field test
The study focused on the locomotor activity of rodents using an Open Field Test (OFT). The apparatus consisted of a square enclosed area (50×50 cm) with a height of 30 cm and virtual lines marked on it. Locomotor activity was measured by evaluating the distance traveled during the 5 minutes. Additionally, the time of the animal’s presence in the center of the open field was measured to indicate anxiety-like behavior.
Elevated plus maze test
An elevated plus maze was used to evaluate the anxiety-like behavior of the animal. In this test, rats were allowed to freely explore the device, including two open arms (38×5 cm) and two closed arms (38×5×15 cm) with a central junction (5×5 cm) for 10 minutes. The time spent in open arms and the number of entries into open and closed arms were measured. The entrance was counted whenever the animal entered the arm with all four paws. An inverse relationship was observed between increased entry into open arms and time spent in open arms with anxiety-like behavior, and vice versa.
Investigating the effects of acute morphine exposure during adolescence on locomotor activity and anxiety-like behavior in the open field test
As shown in Figure 2a, no significant difference in the traveled distance was found between the group exposed to the acute morphine pattern and the saline group during four weeks (P>0.05). These data showed that acute morphine consumption in adolescence did not have a significant effect on the locomotor activity of animals from adolescence to adulthood. Another parameter measured in the open field test was the duration of being in the center of the open field (Figure 2b). In the first week after the morphine injection, the time of presence in the center of the open field in the morphine group showed a significant increase (P=0.0499) compared to the control group. However, during the following weeks, the group exposed to acute morphine compared to the control group did not have a significant difference in terms of the duration of their stay at the open field center (P>0.05).
Figure 2. The effects of acute morphine exposure during adolescence on locomotor activity and anxiety-like behavior in the open field test
Investigating the effects of morphine consumption with an acute exposure pattern during adolescence on anxiety-like behavior in the elevated plus maze test
This study analyzed three indicators of the time spent in the open arms and the number of entries into the open and closed arms as indicators of anxiety-like behavior to measure anxiety-like behaviors in the elevated plus maze. Two indicators of the time spent in the open arms (P=0.0368) and the number of entries to the open arms (P=0.0483) during the first week after morphine injection were significantly higher in the morphine group compared to the saline group. Nevertheless, the two groups had no significant difference in the following weeks (Figure 3a and b). These data showed that the group exposed to morphine with an acute pattern during adolescence had an anti-anxiety behavior pattern during the first week after morphine injection. Correspondingly, the data in Figure 3c showed no significant difference in the number of entries to the closed arms between the morphine and saline groups four weeks after receiving morphine (P>0.05).
Figure 3. The effects of morphine exposure during adolescence on anxiety-like behavior in the elevated plus maze test
Conclusion
In summary, the importance of these findings should be considered in the context of the importance of adolescence as a sensitive period of brain growth and development affected by environmental factors such as substance abuse. Finally, according to the results of the present study, future studies are needed to clarify the nature of the emotional experience of adolescents by examining receptors, signaling cascades, molecules, and brain structures involved in behavioral and cellular changes after exposure to morphine in adolescents. Ultimately, at the level of human societies, educational systems should pay special attention to teenagers and think of appropriate management programs to meet the psychological needs of this sensitive segment of society.
Ethical Considerations
Compliance with ethical guidelines
All experiments on animals complied with the ethical principles of working with laboratory animals approved by the Ethics Committee in Research of Tarbiat Modares University of Medical Sciences with a code of ethics (IR.Modares.REC.1398.052).
Authors’ contributions
Fatemeh Khani: Conducting experiments, analyzing data, and writing the manuscript. Ali Pourmotabbed: Edited the manuscript and consulted during the research. Hossein Azizi: Designed, supervised, and guided during the implementation of the research and final review of the manuscript.
Funding
This article is financially supported by the Faculty of Medical Sciences of Tarbiat Modares University.
Acknowledgments
The presenters are grateful to the Faculty of Medical Sciences of Tarbiat Modares University and the Faculty of Medicine of Kermanshah University of Medical Sciences.
Conflict of interest
The authors of this article have no conflict of interest.
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