Research reportAn electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential pavlovian fear conditioning in the awake rabbit
Introduction
The mesencephalic dopaminergic (DA) system has been identified as a crucial neural system mediating the rewarding properties of various incentive stimuli, including food and water in deprived animals 2, 21, 49, sex 12, 34, electrical brain stimulation [36]and drugs of abuse (e.g. opiates and psychomotor stimulants 17, 45, 46). Additional research has suggested that this system is also activated by the stress of aversive events. This activation has been inferred from alterations in postmortem or in vivo levels of dopamine and its metabolites. For example, increased DA release and/or altered levels of DA metabolites have been reported in numerous brain areas following a variety of aversive events including footshock, immobilization, swim stress, and conditioned aversive stimuli. These areas include the ventral tegmental area (VTA) 15, 16, 25, the medial prefrontal cortex (mPFC) 1, 9, 18, 22, 27, the nucleus accumbens 1, 9, 18, 22and the amygdala 10, 22. Contradictory results, however, have also been reported that demonstrate no evidence of altered dopamine release or metabolism in each of these areas in response to some of these aversive events 1, 5, 6, 9, 22, 27, 33, 37.
These disparate results may be a function of the potential problems inherent in the techniques used in these studies to infer activation of DA neurons from metabolic by-products. For example, while some DA metabolites may be more accurate measures of dopamine release [47], others may be more representative of recently formed intracellular pools [41]. Further, the extent to which enhanced dopamine release or metabolism is observed in response to a particular aversive event will depend upon the exact time that neurochemical samples are collected. Indeed, Commissiong [11]has argued that changes in DA metabolites may have been falsely equated with changes in DA neuronal activity, and therefore electrophysiological monitoring of neuronal activity should be used to provide converging evidence to compliment research conducted using metabolic by-products.
While electrophysiological research has been directed at characterizing the response of DA neurons to rewarding stimuli 26, 28, 30, 38, 39, 40, little research has been directed towards characterizing their response to aversive events, and in particular conditioned stimuli predictive of aversive events. In one related study, Trulson and Preussler [44]characterized the response of VTA DA neurons in the cat to a tone which was paired with a corneal airpuff. They reported that a majority of these neurons demonstrated enhanced activity during tone presentations when compared to activity during a period of quiet waking. Although these data suggest that these neurons are activated by a conditioned aversive stimulus, an important question remains unanswered from this experiment. Since the activity of these neurons was not examined in response to presentations of a second tone of a different frequency that had not been paired with the airpuff, it is not possible to determine the extent to which this activity simply reflects the sensory responsiveness of these neurons rather than a specific response to the conditioned aversive characteristics of the stimulus. This is an important distinction in light of the data demonstrating that mesencephalic DA neurons are responsive to acoustic, visual and somatosensory stimuli 7, 19, 29, 42, 43.
Due to the lack of definitive results of previous experiments, the present study was conducted to characterize the response of individual DA neurons in the VTA of the rabbit to conditioned acoustic stimuli predictive of an aversive event. A Pavlovian differential fear conditioning paradigm was used to distinguish nonspecific neuronal responses to acoustic stimuli from specific responses to the aversive characteristics of conditioned acoustic stimuli.
Section snippets
Animals
Eight experimentally naive female New Zealand rabbits (Oryctolagus cuniculus) weighing from 2.3 to 2.6 kg at the start of the experiment were used. They were maintained on a 12:12 h light/dark cycle and given food and water ad libitum. Principles for the care and use of laboratory animals in research, as outlined in the Guide for the Care and Use of Laboratory Animals (National Institutes of Health, 1985), were strictly followed. All procedures were approved by the University of Vermont Animal
Heart rate conditioning
Single unit activity was recorded from histologically verified VTA neurons in six rabbits. The electrode placements in the remaining two rabbits were anterior to the VTA and these animals were excluded from the data analysis. Of the six rabbits included in the data analysis, all developed reliable differential heart rate responses to the CS+ and the CS− as shown in Fig. 1. A two-factor (CS-TYPE X DAYS) within-subjects analysis of variance (ANOVA) revealed a significant CS-TYPE effect (F
Discussion
The results of the present experiment demonstrated that VTA DA neurons exhibit a variety of response patterns to conditioned stimuli predictive of an aversive event during Pavlovian fear conditioning. That some of these neurons displayed discriminative responses to CS+ and CS− presentations in rabbits that also showed discriminative heart rate responses suggests that their activity is affected by conditioned aversive stimuli. It is important to note that non-discriminative responses to the two
Acknowledgements
This research was supported by National Science Foundation grant # IBN 9319699 awarded to Bruce S. Kapp.
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2019, Current Opinion in Behavioral SciencesCitation Excerpt :Perhaps the most important study implicating dopamine neuron firing in fear-related learning was performed by Guarraci and Kapp [25•]. In this study, they recorded from dopamine neurons of the VTA in an anesthetized rabbit and demonstrated that putative dopamine neurons respond selectively to an auditory CS+, but not to a CS− [25•]. Thus, dopamine neuron activity is associated with the degree of threat discrimination following fear conditioning.