Elsevier

Brain Research

Volume 892, Issue 1, 16 February 2001, Pages 86-93
Brain Research

Research report
Prolonged fear responses in mice lacking dopamine D1 receptor

https://doi.org/10.1016/S0006-8993(00)03234-0Get rights and content

Abstract

Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D1-deficient mice (D1−/−) and their wild-type (D1+/+) and heterozygote (D1+/−) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45–90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D1−/− mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D1−/− mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D1 receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.

Introduction

Fear conditioning is an ideal model for studying emotional learning and memory in animals. It is a simple form of associative learning that occurs when a novel distinctive environment is paired with an aversive footshock stimulus. The animal uses some arbitrary cues from the environment to learn to associate it with the footshock. Through associative learning, these cues become conditioned stimuli capable of eliciting conditioned fear responses expressed as an aversion or freezing behavior to the environment even in the absence of footshock. Fear conditioning is a relevant animal model for phobias, post-traumatic stress disorder and anxiety in humans. It can be useful in understanding and identifying biochemical and molecular brain mechanisms that underlie the formation of aversive and unpleasant memories in animals. Like spatial learning and memory, contextual fear conditioning is generally believed to be a hippocampus-dependent form of learning and memory [1], [8], [17], [25], [30], [43], although there is evidence that the amygdala, entorhinal cortex, parietal cortex and nucleus accumbens are also involved in the acquisition and expression of conditioned fear [10], [23], [26], [29], [38], [39], [41].

The involvement of dopamine in fear conditioning has been widely documented [11], [16], [24], [44], [45]; however, little is known about the molecular mechanisms that underlie fear memory consolidation. Dopamine receptors are G-protein coupled receptors that exert their effects on second messengers and ion channels. To date five dopamine receptors have been cloned, termed D1-like (D1 and D5) and D2-like (D2, D3, and D4) [33], [37]. Dopamine D1-like receptors are coupled to G proteins (Gs) that cause stimulation of adenylyl cyclase activity and subsequently activate the cAMP and cAMP-dependent protein kinase A (PKA) signaling pathway that is important in the induction of long-term potentiation (LTP) necessary for memory consolidation of inhibitory avoidance learning [2], [3], [40]. Dopamine D1 receptor is expressed in the nucleus accumbens, the hippocampus and the amygdala brain regions that are involved in aversive learning and memory, therefore we hypothesize that it may play a role in fear learning and memory. The available evidence regarding the involvement of particular dopamine receptor subtypes in fear conditioning in animals is largely inconsistent. Both dopamine D1 and D2 receptors have been implicated in conditioned fear-motivated behaviors and aversive learning in rats and mice [2], [6], [7], [18], [19], [21], [22]. Dopamine D2 receptor agonists have been shown to impair passive avoidance learning, and both D1 and D2 receptors were shown to act synergistically in this impairment [19]. Intra-accumbens injection of the dopamine D2 receptor antagonist sulpiride has been shown to impair avoidance responding, whereas intra-hippocampal, intra-amygdaloid infusions and systemic injections of the dopamine D1 receptor antagonist SCH-23390 have been shown to differentially affect acquisition and expression of conditioned fear in rats [2], [3], [16], [18], [21]. Dopamine D1 receptor agonists were found to enhance passive avoidance and other manifestations of conditioned fear in rats [2], [3], [16]. Although the dopamine D1 receptor plays a role in spatial learning and memory [14], [42], the available evidence with regard to its role in fear conditioning is very limited and largely inconsistent due to the lack of highly selective ligands that can discriminate between individual receptor subtypes. The recent availability of the dopamine D1 receptor-deficient mice [12], [46] permits a more precise definition of the role of this receptor in aversive learning.

We used dopamine D1 receptor-deficient mice to investigate the potential involvement of the dopamine D1 receptor in acquisition and expression of fear-motivated behaviors using two fear-conditioning tasks. Our results reveal a significant role of the dopamine D1 receptor in the extinction of fear memory but no role in the acquisition and retention of contextual fear learning and memory.

Section snippets

Animals

Mice lacking the dopamine D1 receptor were generated by homologous recombination as described previously [12]. Wild-type (D1+/+), homozygote (D1−/−) and heterozygote (D1+/−) male offspring used in this study were derived from the mating of heterozygous mice. Genotype was determined by Southern blot analyses of genomic DNA [12]. All mice were 3–5 months of age, and were housed in groups of three per cage in a temperature-controlled room (22°C), maintained on a reversed 12:12-h dark–light cycle

Light–dark exploration

Mice from all genotypes consisting of D1−/−, D1+/+ and D1+/− littermates (n=8–10 per group) moved freely and spent approximately equal time in each of the compartments during the 2-min habituation trials given before the training session (data not shown). Similar results were also obtained from independent groups during 6- and 15-min habituation trials.

Passive avoidance conditioning

There was no impairment of acquisition or retention of passive avoidance response in dopamine D1−/− mice. During the training trials, all mice

Discussion

In the present study, we investigated the role of the dopamine D1 receptor in fear conditioning. Our data suggest that the dopamine D1 receptor is critical for fear extinction but is not involved in the acquisition or retention of conditioned fear responses. We have shown that D1−/− mice were normally capable of acquiring and expressing conditioned fear responses in two different paradigms. In the passive avoidance conditioning task, similar to D1+/+ and D1+/− mice, D1−/− mice quickly learned

Acknowledgements

The authors wish to thank Dr John Drago, Dr David Sibley and Dr Heiner Westphal for providing the dopamine D1 receptor knockout mice. This study was supported by the Medical Research Council of Canada Grant no. MT-12724 and the National Institute on Drug Abuse Grant no. DA-07223. Salary support for M.E. was from the University of Tripoli, Libya.

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