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ORIGINAL ARTICLE
Year : 2013  |  Volume : 1  |  Issue : 1  |  Page : 28-32

Evaluation of neurobehavioral characteristics in actively lactating and nonlactating Wistar rats


1 Department of Physiology, Ladoke Akintola University of Technology, P.M.B 4000, Ogbomoso, Nigeria
2 College of Medicine, University of Ibadan, Ibadan, Oyo State, Nigeria

Date of Web Publication30-Dec-2013

Correspondence Address:
Akinloye Olanrewaju Oyekunle
Department of Physiology, Faculty of Basic Medical Sciences, Ladoke Akintola University of Technology, P. M. B 4000, Ogbomoso, Oyo State
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2348-0149.123960

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  Abstract 

Background : The processes of reproduction place a huge responsibility on the brain, hence the behavioral modulations observed in both the mother and the pups during this process owing to hormonal activities. Among such hormones are prolactin and oxytocin, which are majorly elaborated during lactation from the anterior pituitary and the hypothalamic nuclei. Recent in vivo data have implicated the receptors for these hormones in certain brain areas concerned with behavioral modulation. Aim : This study was therefore carried out to support existing documentation on the role of prolactin and oxytocin in neurobehavioral modulation. Materials and Methods : A total of 20 female Wistar rats were used for the study, 10 actively lactating and 10 nonpregnant, nonlactating. Neurobehavioral characteristics were evaluated in both groups using Open field and Hole-board tests. Results : In the Open field test, the result showed a significant (P < 0.05) increase in locomotor activity as well as rearing and grooming frequencies of lactating rats when compared with nonlactating rats. Likewise there was a significant increase (P < 0.05) in the mean number of head dips in lactating rats when compared with nonlactating rats. Discussion : Increased locomotor and exploratory behaviors exhibited by lactating animals is an indication of fearlessness and reduced anxiety while reduced locomotor and exploratory behaviors in nonlactating rats indicated fear and higher level of anxiety also lactating animals exhibited high level of head-dipping, which was indicative of low anxiety-like state while the nonlactating animals exhibited reduced head-dipping which was indicative of high anxiety-like state. Conclusion: The authors therefore suggest that, the behavioral deficit observed in nonlactating, nonpregnant group could be due to the reduced plasma concentration of hormones responsible for the process of lactation (prolactin and oxytocin).

Keywords: Hole-board, hypothalamus, lactation, open field, oxytocin, pituitary, prolactin


How to cite this article:
Oyekunle AO, Ibironke GF, Omotoso Y, Adu TS. Evaluation of neurobehavioral characteristics in actively lactating and nonlactating Wistar rats. Niger J Exp Clin Biosci 2013;1:28-32

How to cite this URL:
Oyekunle AO, Ibironke GF, Omotoso Y, Adu TS. Evaluation of neurobehavioral characteristics in actively lactating and nonlactating Wistar rats. Niger J Exp Clin Biosci [serial online] 2013 [cited 2019 Oct 17];1:28-32. Available from: http://www.njecbonline.org/text.asp?2013/1/1/28/123960


  Introduction Top


Lactation is the production of milk from the mammary gland. This physiological process provides significant nutritional, immunological, and developmental benefits to the young ones. This process is largely under the influence of two major hormones - Prolactin (PRL) and Oxytocin (OT). PRL discovered over eight decades ago [1],[2] is defined as a pituitary-secreted polypeptide hormone, which was named for its stimulatory action on lactation. Various animal models, including mice deficient for PRL [3] or for its receptor (PRL-R) [4] have unambiguously confirmed this. In addition to its critical actions in mammary gland function during pregnancy and lactation, the anterior pituitary hormone PRL exerts important actions within the brain. PRL is thought to gain access to the brain through a carrier mediated transport system [5] likely involving PRL-R in the choroid plexus. [6],[7] Likewise OT is synthesized in the neurons of the supraoptic and paraventricular nuclei (PVN) of the hypothalamus, which project to the posterior pituitary gland and release OT into the blood stream. [8] This peripherally acting OT has it receptor present in a number of tissues including uterus, [9] kidney, [10],[11] and brain. [12],[13] OT stimulates uterine contractions during parturition [9],[14] milk ejection during lactation [15],[16] and increases renal sodium excretion and glomerular filtration rates in rats. [17] This study is therefore aimed at investigating the neurobehavioral characteristics imposed by presence or absence of these hormones (PRL and OT).


  Materials and Methods Top


Animals

A total of 20 female Wistar rats were used for the study. The animals were grouped into two of 10 animals each. Group 1 consists of 10 healthy nonlactating, nonpregnant female animals (80-100 g) while group 2 consists of 10 actively lactating female animals (100-110 g). The animals were housed in preclinical animal house, College of Health Sciences, Ladoke Akintola University of Technology, Ogbomoso. They were maintained under standard laboratory conditions 12 h light/dark cycle at 23 ± 2°C and had free access to food (standard rat chow) and water throughout the experiment. All experimental protocol of the study was approved by the institutional animal ethical committee.

Neurobehavioral Studies

Open field test


The open field (OF) apparatus was constructed of a square area plywood (96 × 96 cm) with 60 cm high walls. One of the walls was made of Plexiglas to facilitate unobstructed view of the animal in the box. The floor was painted green and was divided into 16 squares by parallel and intersecting white lines. [18] Rats were placed singly in one corner of the OF and the following behaviors were scored visually during 30 minutes session at 10 minutes interval, locomotion, rearing and grooming frequencies. The maze was located in a 1.8 × 4.6 m test room and lit by a 60-watt red lamp for background lighting.

Procedure

Rats were carried to the test room in their home cages and were handled by the base of their tails at all times. Lactating rats were removed while feeding their pups and placed in the maze. Each animal was placed in the center of the maze and allowed to explore the apparatus for 30 minutes. After the 30 minutes test, rats were returned in their home cages and the OF was cleaned with 70% ethyl alcohol and permitted to dry between tests.

Behaviors Scored

Total locomotor activity


The frequency with which the animal crosses the grid lines with all the four paws and the frequency of rearing were taken as index of locomotor activity. [19]

Rearing

The frequency with which the animal stood on their hind legs in the maze. [20]

Grooming

The frequency of face washing and paw licking activities while stationary in the maze. [20]

Hole-Board Test

The rat hole-board was made of a wooden box with a floor 66 × 56 cm and 47 cm high. [21] In the floor there were 16 equally spaced holes 3.8 cm in diameter and 1 cm deep. The apparatus was mounted on wooden stand 6 cm above the floor of the test room.

Procedure

In the main experiment each animal (lactating or nonlactating) was placed singly in the center of the board, facing away from the observer and its behaviors recorded for 5 minutes. A head dip was scored if both eyes disappeared into the hole. [21] The holes in the board were individually numbered so that the number of different holes explored, as well as the total number of head-dips made, could be scored. Activity was measured by the amount of time each animal spent moving about on the hole-board (this measure excluded rearing and grooming). After each trial the floor of the apparatus was wiped and dried to remove traces of the previous path. Due to wide variation in motor activity and head-dipping throughout the day [22] the animals were only tested between 8.30 and 16.30 h. The results obtained were expressed as mean total number of head-dips. [23],[24]

Statistical Analysis

Results are expressed as mean ± SEM. The significant difference between experimental and control groups were estimated by Student's t-test.


  Results Top


[Table 1] showed the result obtained when the animals were allowed to explore the OF for 30 minutes at 10 minutes interval. Lactating rats exhibited significant (P < 0.05) locomotor activity going by the number of grid lines crossed when compared with nonlactating rats. Rearing and grooming frequencies were also significantly increased (P < 0.05) in the lactating rats when compared with nonlactating rats.
Table 1: Behavioral characteristics of nonlactating and lactating rats in open field test

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[Table 2] showed the result obtained from Hole-board test. There was significant increase (P < 0.05) in the number of head dips of lactating rats when compared with nonlactating rats.
Table 2: Behavioral characteristics of nonlactating and lactating rats in Hole-board test

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  Discussion Top


The OF tests [25],[26] have been used for measurement of emotionality and exploratory activities in rodents. The number of grid lines crossed and the frequency of rearing are usually used as measures of locomotor activity, but are also measures of exploration and anxiety. A high frequency of these behavios indicates increased locomotion and exploration and/or lower level of anxiety. [19],[27] Conversely reduced activity or exploration in a novel environment by a rodent indicates fear or higher level of anxiety. [25] From the result [Table 1], increased locomotor and exploratory behaviors exhibited by lactating animals is an indication of fearlessness and reduced anxiety while reduced locomotor and exploratory behaviors in nonlactating rats indicated fear and higher level of anxiety. The Hole-board apparatus offers a simple method of measuring the behavioral responses of a rodent to a novel environment and has been widely used for assessing drug effects. [21],[28] In general, high levels of head-dipping are interpreted as indicative of neophilia-low anxiety-like state, while low levels of head-dipping are assumed to result from a lack of neophilia or are assumed to reflect a high anxiety-like state in the animal. [29],[30] From the result [Table 2], lactating animals exhibited high level of head-dipping, which was indicative of low anxiety-like state, while the nonlactating animals exhibited reduced head-dipping, which was indicative of high anxiety-like state. The authors suggest that the differences in the behavioral characteristics exhibited by the animals in both procedures could be as a result of lactation in one group occasioned by elevated level of some hormones namely PRL and OT. Both hormones have been implicated in a wide range of physiological, behavioral, learning and memory, parturition and lactation, maternal, and sexual behavior (PRL [31],[32],[33] and OT [12],[13] ). Behavioral modulation by lactation is possible owing to the recent discovery of the receptors for these hormones in various parts of the brain. PRL is thought to gain access to the brain through a carrier-mediated transport system, [5] likely involving PRL receptors in the choroid plexus. [6],[7] PRL-R mRNA [34],[35],[36],[37] and protein [38],[39] have been identified in many hypothalamic nuclei. The nucleus involved in behavioral modulation is the ventromedial hypothalamus (VMH). [40],[41] Interestingly, PRL-R expression in the choroid plexus is markedly increased during pregnancy and lactation, [42] suggesting increased access of PRL to brain structures during these conditions. Similarly, levels of PRL-R protein in the hypothalamus appear to increase during lactation compared with nonpregnant rats. [43],[44],[45] These observations suggest that PRL may be a major regulator of hypothalamic function, particularly during pregnancy and lactation, when PRL levels are elevated. [46],[47] Likewise OT neurons of the PVN also send projections to many regions within the brain, including the hippocampus, amygdala, and hypothalamus, in which OT acts as a neurotransmitter involved in affiliative behaviors. [48] Oxytocin receptors (OTRs) are found in many brain regions, including the central nucleus of the amygdala (cAmyg) and the ventromedial nucleus of the hypothalamus (VMH) in which they are heterologously expressed. [49],[50] These two brain regions are critical components of neural circuits regulating distinct behavioral responses. The VMH is an important mediator of female sex behavior, [51] whereas the cAmyg is a portion of the limbic system that coordinates fear and anxiety responses. [52],[53],[54] The result of this study is consistent with existing documentation on behavioral modulations exerted by hormones promoting lactation majorly PRL and OT, however, the involvement of other hormones (pregnancy hormones) cannot be overruled completely.

In conclusion, behavioral modulation in female animal is a function of expression of reproductive hormones. The overall behaviors exhibited could be as a result of the hormones largely expressed. In this case the lactating animals tend to benefit emotionally from the physiological process of lactation by the increase in plasma concentration of PRL and OT.

 
  References Top

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    Tables

  [Table 1], [Table 2]


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