Research report
An electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential pavlovian fear conditioning in the awake rabbit

https://doi.org/10.1016/S0166-4328(98)00102-8Get rights and content

Abstract

Recent research has suggested that the mesencephalic dopaminergic (DA) system is activated by stress. For example, alterations in DA metabolites have been found in the ventral tegmental area (VTA) following footshock and immobilization in the rat 15, 37. Furthermore, this activation appears selective to DA neurons within the VTA since no changes were observed within the substantia nigra 15, 16. While this research suggests that DA neurons in the VTA are activated by aversive events, there has been a paucity of electrophysiological research designed to examine the sensory response characteristics of these DA neurons, and in particular their response to stimuli which predict aversive events. The present study was conducted to investigate the response characteristics of DA neurons within the VTA of the awake rabbit to acoustic stimuli which, via Pavlovian aversive conditioning procedures, came to predict the occurrence of a mild shock to the pinna. 45% of the neurons meeting pre-established criteria for DA neurons demonstrated either significant excitation or inhibition to conditioned aversive stimuli. These neurons responded differentially to CS+ and CS− presentations. Some of these neurons (65%) demonstrated a greater increase in activity during the CS+ compared to the CS−, some (22%) demonstrated a greater decrease in activity during the CS+ compared to the CS− and some (13%) demonstrated a greater increase in activity during the CS− compared to the CS+. Further, conditioned heart rate responses in the rabbits occurred during the recording of a majority of these neurons. These overall results suggest that conditioned aversive stimuli can affect the firing of VTA DA neurons and that these neurons comprise a heterogenous population with respect to their response profiles.

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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