The Relation between Increasing Anxiety and Prolactin-Releasing Peptide in Rats

1. Introduction

PrRP, also known as prolactoliberin, is a hypothalamic neurohormone obtained from a bovine hypothalamic extract and enhances prolactin production in a rat pituitary adenoma cell line and lactating rat pituitary cells. The Prl-RG injection also boosted plasma prolactin levels in male rats and activated concentrations in female rats throughout all stages of the menstrual cycle, including proestrus, estrus, and metestrus ( 1 ). The fact that the structural formation of the PrRP is highly consistent across a wide range of animal species suggests that this peptide plays a vital role in body organisms. PrRP has been shown to control the intake of food and energy expenditure, but it may also have a role in stress sensitivity, reproduction, cardia productivity, secretion of endocrine components, and lately, neuroprotective characteristics, among others. Arrange of inputs, particularly those involving the periphery, control the creation of PrRP in the brain and affect its activity ( 2 - 4 ).

In addition to binding to the G-protein coupled receptor (GPR10) ( 5 ), PrRP also shows a lower affinity for the neuropeptide FF (NPFF) receptor type 2 ( 6 ). The role of other neuropeptides, including leptin ( 4 ), cholecystokinin ( 7 ), or neuropeptide Y ( 8 , 9 ), in the actions of PrRP, is also critical to its success. Novel PrRP analogs with connected fatty acids and modifications in the chain of amino acids were synthesized in structure-activity relationship research to defeat the blood-brain barriers and enhance consistency and bioavailability from the peripheral circulation, thereby displaying attractive therapeutic candidates ( 10 ).

Anxiety disorders are the most frequent kind of mental illness and usually begin in a person's adolescence or early adulthood. Anxiety, worry, and perceived risk avoidance are the most common symptoms. Those who suffer from anxiety disorders have problems with brain systems that react to threats. Anxiety disorders are linked to genetic, environmental, and epigenetic variables ( 11 ). Anxiety disorders often coexist with other mental and physical problems, including depression and other psychiatric conditions. Generally, comorbidity is associated with more intense symptoms, a higher level of clinical load, and more difficulties in medication. Individuals may reduce the enormous burden of sickness from anxiety disorders by detecting the condition early, accurately administering appropriate therapy, and scaling up treatment as necessary. When treating depression and other mood disorders, patients may benefit from evidence-based psychotherapies like cognitive behavioral therapy and psychoactive drugs like serotonergic substances ( 12 ).

The current study was performed to identify if prolactin-releasing peptide (PrRP) increased anxiety clinical features in rats.

2. Materials and Methods

2.1. Animals

The study included 114 Wistar handling-acclimated male rats (160gm, 2 months old). The rats were housed under standard conditions with five rats per cage. Before the initiation of the experiment, the rats were exposed to a handling method for 2 weeks every day for 5 mins to avoid responding to the stress of being handled during the experiment. The rats were divided randomly into three major groups (38-control animals (38C), and 38-PrRP animals (38P), both were examined using the EPM test to test for stress-related signs, such as fear of height (5 mins duration for each rat). After the experiment ended for each rat, the maze was wiped with H₂O to get rid of the previous rat smell. The tests were performed between 13:00 to 17:00 of the day. Then, a week later, 38(19-PrRP animals (19P) and 19-control animals (19C)) were examined using the SP test conducted between 13:00 to 16:00 of the day.

2.2. Experiment

Fifteen minutes before EPM, the 38C received intranasal 0.9%-10µl NaCl (per nostril), and 38P received intranasal 10^-10mol/l-10µl PrRP (per nostril), and the anxiety-related signs, such as time spent in open arms (less time means more anxious), during the EPM test were recorded. The 19P and 19 Creceived 10^-10mol/l-10µlPrRPand0.9%-10µlNaCl, respectively, (intranasal, per nostril, and 15 minutes before the SP test, where a stranger rat was placed in a specific cage in front of each of the 19P and 19C animals in a separate cage, in which both cages provided visual and olfactory but no confrontational contact).

2.3. Statistical test

The data were processed-analyzed using a P<0.05-level-of-significance-based t-test. Mean±standard deviation (M±SD) was used to represent analyzed data.

3. Results and Discussion

The results showed that PrRP significantly (P<0.05) decreased the time spent by the treated rats on the open arms with little access to these open arms and fewer hanging times (Table 1). In addition, PrRP revealed significant (P<0.05) decreases in the time spent close to the stranger rat, which means increased anxiety levels (Table 2).

PrRP at low concentrations causes an elevation in anxiety in rats but does not influence their locomotor activity, according to this research. This suggests that PrRP has anxiety-inducing instead of a sedative action. The impact of PrRP on social preference is believed to be directly linked to a shift in anxiety levels. If an animal has a high level of anxiety because of their genetic disposition, they prefer to be with a stranger rather than a known animal ( 13 , 14 ). Surviving and paying attention to friends and human workers are the criteria for the new environment of the experimental rats that they focus on ( 15 , 16 ). As a general rule, rats with high baseline anxiety prefer to stay close to their cagemates rather than go out into the surroundings. Because neophobia is less prominent in rats with minimal initial anxiety, they may engage in exploratory behavior toward an unknown animal. In such context, Ben-Ami Bartal, Rodgers ( 17 ) detected that rats released stranger rats from a trap similarly to their friendrats.

Many questions remain unsolved about PrRP-psychiatric effects, although it has been demonstrated that it may interact with central nervous system domains that govern emotions and stress response.GPR10isthereceptorthathasastrongaffinityforPrRP.GPR10mRNAisfound in the thalamus, hypothalamus, and postrema region of rat brain. It has been detected in the parabrachial nucleus, the nucleus accumbens, and the hippocampus, which is all engaged in different processes, such as the body's reaction to stress and the nociceptive system ( 18 ).

Electric pain stimulation activates the neurons in the median eminence that produce PrRP ( 19 ). Knockout mice of the PrRP gene exhibit higher blood glucose and corticosterone levels in response to immobility stress than control animals ( 20 ). Stress activates PrRP-nergic neural pathways in the nucleus of the solitary tract ( 21 ). McConn, Tachibana ( 22 ) reported that PrRP changed the level of food intake in their tested quails, which is linked to alterations in the hypothalamic corticotropin-releasing factor and the gene that encodes the neuropeptide Y receptor gene.

The current findings revealed that prolactin-releasing peptide increases anxiety and decreases sociality in the studied male rats.

Authors' Contribution

Study concept and design: K. G. G.

Acquisition of data: K. G. G.

Analysis and interpretation of data: M. S. K.

Drafting of the manuscript: B. H. A. A.

Critical revision of the manuscript for important intellectual content: K. G. G., M. S. K. and R. A. J.

Statistical analysis: M. S. K.

Administrative, technical, and material support: M. S. K.

Ethics

The study design was approved by the ethics committee of the University of AL-Qadisiyah, Al-Qadisiyah, Iraq.

Conflict of Interest

The authors declare that they have no conflict of interest.

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