Elsevier

Behavioural Brain Research

Volume 281, 15 March 2015, Pages 24-31
Behavioural Brain Research

Research report
Tissue plasminogen activator modulates emotion in a social context

https://doi.org/10.1016/j.bbr.2014.11.022Get rights and content

Highlights

  • tPA KO mice showed delayed latency to the first contact with a stranger mouse.

  • tPA KO mice moved further in the space without a stranger than tPA Het mice.

  • tPA KO mice approached a cage without a stranger more frequently than Het mice.

  • tPA KO mice showed higher activity in the prefrontal cortex after social behavior.

  • Results suggest that tPA modulates social emotion through the prefrontal cortex.

Abstract

Tissue plasminogen activator (tPA) is known to play physiologically and pathologically crucial roles in the central nervous system. However, it is still obscure whether it affects social behavior. We investigated social behavior and neuronal activation after social stimulation in tPA knockout (KO) mice. In a resident-intruder test, the latency to the first contact was significantly delayed in tPA KO mice compared with that in tPA heterogenic (Het) mice. However, tPA KO mice spent significantly more time undertaking active behavior than tPA Het mice did. In a sociability test, tPA KO mice significantly spent more time and walked a greater distance in the chamber containing an empty cage than tPA Het mice. Furthermore, tPA KO mice approached an empty cage more frequently than tPA Het mice did. In a social novelty test, there was no difference in the duration spent sniffing a stimulator mouse between genotypes. However, tPA KO mice approached even a familiar mouse more frequently than tPA Het mice did. tPA KO mice spent similar durations in a chamber containing a familiar mouse and that containing an unfamiliar mouse, and tPA KO mice walked a significantly greater distance in the former chamber than tPA Het mice did. Furthermore, at the cingulate and prelimbic cortices, the number of cFos-positive cells was significantly increased in tPA KO mice compared with that in tPA Het mice after social stimulation. Our results suggest that tPA modulates emotion in a social context through the function of the prefrontal cortex.

Introduction

Tissue plasminogen activator (tPA) is a serine protease produced by endothelial cells and converts a zymogen, plasminogen, to active plasmin, which breaks down fibrin clots [1], [2]. This enzyme has been shown to be produced by neuronal cells in the central nervous system and to play physiologically and pathologically crucial roles [3], [4], [5], [6]. Pathologically, ischemic stroke causes the secretion of tPA and plasminogen at an injured region. The active plasmin, which is converted from inactive plasminogen by tPA, digests extracellular matrix proteins including laminin. The loss of extracellular matrix results in neuronal cell death [7], [8], [9]. Conversely, tPA deficiency attenuates neuronal degeneration and seizures induced by excitotoxin [10].

In the embryonic development of the brain, tPA contributes to neuronal migration and facilitates neurite outgrowth [11], [12], [13]. Extracellular proteolysis by tPA is crucial for neuronal plasticity, since tPA knockout (KO) mice show impaired plasticity and contrarily the overexpression of tPA enhances hippocampal long-term potentiation (LTP), which is a key event for memory formation [14], [15], [16]. Although the detailed mechanism by which tPA modulates neuronal plasticity has not been clarified, it is proposed that it affects the function of N-methyl-d-aspartate (NMDA) receptor via direct cleavage of the NR1 subunit and/or non-proteolytic interaction with the NR2 subunit of the NMDA receptor [17], [18], [19], [20], [21]. In addition, neuronal plasticity appears to be modulated by brain-derived neurotrophic factor maturated by the tPA/plasmin system [22], [23] and the activation of cAMP-dependent kinase via the interaction between tPA and its receptor protein, low-density-lipoprotein receptor-related protein [24]. Despite numerous reports suggesting that tPA modulates hippocampal plasticity as mentioned above, the abilities of tPA KO mice in terms of spatial learning and memory in the Morris water maze task are comparable to those of wild-type mice [14], whereas the overexpression of tPA enhances spatial learning in the same task [15]. Rather, tPA deficiency affects fear- and anxiety-related behaviors. Acute restraint stress induces tPA expression at the central and medial amygdala and increases anxiety behavior in wild-type mice but not in tPA KO mice [25]. Knockdown and overexpression of tPA at hippocampal neurons induce anxiogenic and anxiolytic behaviors, respectively [26]. The stress-induced tPA in the amygdala promotes the effect of corticotropin-releasing factor (CRF), a major stress mediator, on anxiety behavior [27]. The administration of CRF also enhances tPA activity in the bed nucleus of stria terminalis and potentiates acoustic startle response, whereas tPA deficiency attenuates the acoustic startle response [28]. Impaired performance in an active avoidance test and poor contextual fear conditioning in tPA KO mice are also reported [14], [29]. The reward system is also modulated by the tPA/plasmin system, since morphine-induced place preference and hyper-locomotion are significantly attenuated in tPA KO mice and plasminogen KO mice [30]. The tPA/plasmin system enhances the depolarization-evoked release of dopamine at the nucleus accumbens (NuAcc) [31]. Furthermore, tPA KO mice are reported to show highly exploratory behaviors and a decreased level of serotonin in some brain regions [32].

As described above, tPA plays roles in various brain regions such as hippocampus, amygdala, and bed nucleus of the stria terminalis, and this protease participates in a wide range of diverse behaviors including anxiety, fear conditioning, and rewarding behavior. These brain regions having tPA activity and the behavioral phenotype of tPA KO mice suggest that tPA may participate in social behaviors. Social behaviors are impaired in some developmental and psychiatric disorders, such as autism-spectrum disorders, schizophrenia, and post-traumatic stress disorder. However, molecular components that modulate social behaviors have remained obscure. In this report, we describe social behaviors and neuronal activation after the social interaction of tPA KO mice. To our knowledge, this is the first report indicating that tPA modulates emotion in a social context in rodents.

Section snippets

Animals

tPA-deficient mice and their wild-type counterparts, provided initially by Prof. D. Collen (University of Leuven, Leuven, Belgium), were bred in the Kinki University and Gunma University. Both heterozygous tPA+/− (Het) and homozygous tPA−/− (KO) mice, back-crossed to C57BL/6 [33], were kept in a standard cage under a 12 h light/dark cycle with access to food and water ad libitum. Behavioral tests were performed using 11–18-week-old male mice. The mice were housed individually one week before the

Resident-intruder test

Compared with tPA Het mice, tPA KO mice showed significantly delayed latency to the first contact (Fig. 1A; t(25) = −2.659, P < 0.05). Although statistical analyses showed no significant difference of overall behavioral pattern between genotypes, they detected significant interaction between genotype and behavioral pattern (Fig. 1B; ANOVA, main effect of genotype: F(1, 81) = 1.699, P = 0.196; interaction between genotype and behavioral pattern: F(2, 81) = 5.315, P < 0.01). The post hoc analysis revealed

Discussion

Here, we compared social behaviors and neuronal activation after social interaction between tPA KO and tPA Het mice. We used tPA Het mice as a control for the following reason. We attempted to understand the primary effect of tPA on social behavior. The neuronal circuits of tPA KO mice may be more similar to those of tPA Het mice than those of wild-type mice, since cell migration and neurite elongation are affected by tPA activity [11]. We could diminish the effect of developmental abnormality,

Acknowledgement

This research was supported in part by JSPS KAKENHI Grant Numbers 23591501 and 26461734.

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