Somatosensory input to dopamine neurones of the monkey midbrain: responses to pain pinch under anaesthesia and to active touch in behavioural context

Revealing brain mechanisms of mTOR-mediated translational regulation: Implications for chronic pain

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mTOR is a major regulator of protein translation.

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mTOR serves an important role in neural plasticity.

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mTOR signalling in the brain as a pathology for neurological disorder is known.

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mTOR signalling in the brain as a chronic pain mechanism is understudied.

In the spinal cord, altered protein transcription and translation have received a lot of recent attention for their role in neural plasticity, a major mechanism leading to the development of chronic pain. However, changes in brain plasticity are also associated with the maintenance of pain symptoms, but these cellular mechanisms remain less clear. The mechanistic/mammalian target of rapamycin (mTOR) is a master regulator of protein synthesis, and controls several neuronal functions, including neural plasticity. While aberrant changes in mTOR signaling are associated with sensitization of the pain pathway (sensory neurons and spinal cord), there are various nervous system diseases that have pain as a comorbidity and altered mTOR activity in the brain. Here, we provide a brief review of mTOR changes in the brain that are associated with some neurological disorders and focus on how these changes may be relevant to the pain of the underlying condition and chronic pain itself.

Midbrain dopaminergic neuron activity across alternating brain states of urethane anaesthetized rat

Midbrain dopaminergic neurons are implicated in the control of motor functions and reward-driven behaviours. The function of this neuronal population is strongly connected with distinct patterns of firing - irregular or bursting, which either maintains basal levels of dopamine (DA) or leads to phasic release, respectively. Heterogeneity of dopaminergic neurons, observed on both structural and functional levels, is also reflected in different responses of DA neurons to changes in global brain states. Preparation of urethane anaesthetized animal is a broadly used model to study brain state dependent activity of neurons. Unfortunately activity of midbrain DA neurons across urethane induced cyclic, spontaneous brain state alternations is poorly described. To fulfil this gap in our knowledge we have performed simultaneous, extracellular recordings of the firing of single putative DA neurons combined with continuous brain state monitoring. We found that during slow wave activity, the firing rate of recorded putative DA neurons was significantly higher compared to firing rates during activated state, both in ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). In the presence of cortical slow waves, putative dopaminergic neurons also intensified bursting activity, but the magnitude of this phenomena differed in respect to the examined region (VTA or SNc). Our results show that activity of DA neurons under urethane anaesthesia is brain-state dependent and emphasize the importance of brain state monitoring during electrophysiological experiments.

A role for phasic dopamine release within the nucleus accumbens in encoding aversion: a review of the neurochemical literature

Survival is dictated by an organism's fitness in approaching positive stimuli and avoiding harm. While a rich literature outlines a role for mesolimbic dopamine in reward and appetitive behaviors, dopamine's involvement in aversion and avoidance behaviors remains controversial. Debate surrounding dopamine's function in the processing of negative stimuli likely stems from conflicting results reported by single-unit electrophysiological studies. Indeed, a number of studies suggest that midbrain dopaminergic cells are inhibited by the presentation of negative or fearful stimuli, while others report no change, or even an increase, in their activity. These disparate results may be due to population heterogeneity. Recent evidence demonstrates that midbrain dopamine neurons are heterogeneous in their projection targets, responses to environmental stimuli, pharmacology, and influences on motivated behavior. Thus, in order to assemble an accurate account of dopamine function during aversive stimulus experience and related behavior, it is necessary to examine the functional output of dopamine neural activity at mesolimbic terminal regions. This Review presents a growing body of evidence that dopamine release in the nucleus accumbens encodes not only reward, but also aversion. For example, our laboratory recently utilized fast-scan cyclic voltammetry to show that real-time changes in accumbal dopamine release are detected when animals are presented with predictors of aversion and its avoidance. These data, along with other reports, support a considerably more nuanced view of dopamine neuron function, wherein accumbal dopamine release is differentially modulated by positive and negative affective stimuli to promote adaptive behaviors.

Sensory regulation of dopaminergic cell activity: Phenomenology, circuitry and function

Dopaminergic neurons in a range of species are responsive to sensory stimuli. In the anesthetized preparation, responses to non-noxious and noxious sensory stimuli are usually tonic in nature, although long-duration changes in activity have been reported in the awake preparation as well. However, in the awake preparation, short-latency, phasic changes in activity are most common. These phasic responses can occur to unconditioned aversive and non-aversive stimuli, as well as to the stimuli which predict them. In both the anesthetized and awake preparations, not all dopaminergic neurons are responsive to sensory stimuli, however responsive neurons tend to respond to more than a single stimulus modality. Evidence suggests that short-latency sensory information is provided to dopaminergic neurons by relatively primitive subcortical structures - including the midbrain superior colliculus for vision and the mesopontine parabrachial nucleus for pain and possibly gustation. Although short-latency visual information is provided to dopaminergic neurons by the relatively primitive colliculus, dopaminergic neurons can discriminate between complex visual stimuli, an apparent paradox which can be resolved by the recently discovered route of information flow through to dopaminergic neurons from the cerebral cortex, via a relay in the colliculus. Given that projections from the cortex to the colliculus are extensive, such a relay potentially allows the activity of dopaminergic neurons to report the results of complex stimulus processing from widespread areas of the cortex. Furthermore, dopaminergic neurons could acquire their ability to reflect stimulus value by virtue of reward-related modification of sensory processing in the cortex. At the forebrain level, sensory-related changes in the tonic activity of dopaminergic neurons may regulate the impact of the cortex on forebrain structures such as the nucleus accumbens. In contrast, the short latency of the phasic responses to sensory stimuli in dopaminergic neurons, coupled with the activation of these neurons by non-rewarding stimuli, suggests that phasic responses of dopaminergic neurons may provide a signal to the forebrain which indicates that a salient event has occurred (and possibly an estimate of how salient that event is). A stimulus-related salience signal could be used by downstream systems to reinforce behavioral choices.

Phasic mesolimbic dopamine signaling precedes and predicts performance of a self-initiated action sequence task

BACKGROUND

Sequential reward-seeking actions are readily learned despite the temporal gap between the earliest (distal) action in the sequence and the reward delivery. Fast dopamine signaling is hypothesized to mediate this form of learning by reporting errors in reward prediction. However, such a role for dopamine release in voluntarily initiated action sequences remains to be demonstrated.

METHODS

Using fast-scan cyclic voltammetry, we monitored phasic mesolimbic dopamine release, in real time, as rats performed a self-initiated sequence of lever presses to earn sucrose rewards. Before testing, rats received either 0 (n = 11), 5 (n = 11), or 10 (n = 8) days of action sequence training.

RESULTS

For rats acquiring the action sequence task at test, dopamine release was strongly elicited by response-contingent (but unexpected) rewards. With learning, a significant elevation in dopamine release preceded performance of the proximal action and subsequently came to precede the distal action. This predistal dopamine release response was also observed in rats previously trained on the action sequence task, and the amplitude of this signal predicted the latency with which rats completed the action sequence. Importantly, the dopamine response to contingent reward delivery was not observed in rats given extensive pretraining. Pharmacological analysis confirmed that task performance was dopamine-dependent.

CONCLUSIONS

These data suggest that phasic mesolimbic dopamine release mediates the influence that rewards exert over the performance of self-paced, sequentially-organized behavior and sheds light on how dopamine signaling abnormalities may contribute to disorders of behavioral control.

Dopamine neurons code subjective sensory experience and uncertainty of perceptual decisions

Midbrain dopamine (DA) neurons respond to sensory stimuli associated with future rewards. When reward is delivered probabilistically, DA neurons reflect this uncertainty by increasing their firing rates in a period between the sensory cue and reward delivery time. Probability of reward, however, has been externally conveyed by visual cues, and it is not known whether DA neurons would signal uncertainty arising internally. Here we show that DA neurons code the uncertainty associated with a perceptual judgment about the presence or absence of a vibrotactile stimulus. We observed that uncertainty modulates the activity elicited by a go cue instructing monkey subjects to communicate their decisions. That is, the same go cue generates different DA responses depending on the uncertainty level of a judgment made a few seconds before the go instruction. Easily detected suprathreshold stimuli elicit small DA responses, indicating that future reward will not be a surprising event. In contrast, the absence of a sensory stimulus generates large DA responses associated with uncertainty: was the stimulus truly absent, or did a low-amplitude vibration go undetected? In addition, the responses of DA neurons to the stimulus itself increase with vibration amplitude, but only when monkeys correctly detect its presence. This finding suggests that DA activity is not related to actual intensity but rather to perceived intensity. Therefore, in addition to their well-known role in reward prediction, DA neurons code subjective sensory experience and uncertainty arising internally from perceptual decisions.

Putative γ-aminobutyric acid neurons in the ventral tegmental area have a similar pattern of plasticity as dopamine neurons during appetitive and aversive learning

Dopamine influences affective, motor and cognitive processing, and multiple forms of learning and memory. This multifaceted functionality, which operates across long temporal windows, is broader than the narrow and temporally constrained role often ascribed to dopamine neurons as reward prediction error detectors. Given the modulatory nature of dopamine neurotransmission, that dopamine release is activated by both aversive and appetitive stimuli, and that dopamine receptors are often localized extrasynaptically, a role for dopamine in transmitting precise error signals has been questioned. Here we recorded from ventral tegmental area (VTA) neurons, while exposing rats to novel stimuli that were predictive of an appetitive or aversive outcome in the same behavioral session. The VTA contains dopamine and -aminobutyric acid (GABA) neurons that project to striatal and cortical regions and are strongly implicated in learning and affective processing. The response of VTA neurons, regardless of whether they had putative dopamine or GABA waveforms, transformed flexibly as animals learned to associate novel stimuli from different sensory modalities to appetitive or aversive outcomes. Learning the appetitive association led to larger excitatory VTA responses, whereas acquiring the aversive association led to a biphasic response of brief excitation followed by sustained inhibition. These responses shifted rapidly as outcome contingencies changed. These data suggest that VTA neurons interface sensory information with representational memory of aversive and appetitive events. This pattern of plasticity was not selective for putative dopamine neurons and generalized to other cells, suggesting that the temporally precise information transfer from the VTA may be mediated by faster acting GABA neurons.

Sensory effects of intravenous cocaine on dopamine and non-dopamine ventral tegmental area neurons

Intravenous (iv) cocaine mimics salient somato-sensory stimuli in their ability to induce rapid physiological effects, which appear to involve its action on peripherally located neural elements and fast neural transmission via somato-sensory pathways. To further clarify this mechanism, single-unit recording with fine glass electrodes was used in awake rats to examine responses of ventral tegmental area (VTA) neurons, both presumed dopamine (DA) and non-DA, to iv cocaine and tail-press, a typical somato-sensory stimulus. To exclude the contribution of DA mechanisms to the observed neuronal responses to sensory stimuli and cocaine, recordings were conducted during full DA receptor blockade (SCH23390+eticloptide). Iv cocaine (0.25 mg/kg delivered over 10 s) induced significant excitations of approximately 63% of long-spike (presumed DA) and approximately 70% of short-spike (presumed non-DA) VTA neurons. In both subgroups, neuronal excitations occurred with short latencies (4-8 s), peaked at 10-20 s (30-40% increase over baseline) and disappeared at 30-40 s after the injection onset. Most long-(67%) and short-spike (89%) VTA neurons also showed phasic responses to tail-press (5-s). All responsive long-spike cells were excited by tail-press; excitations were very rapid (peak at 1 s) and strong (100% rate increase over baseline) but brief (2-3 s). In contrast, both excitations (60%) and inhibitions (29%) were seen in short-spike cells. These responses were also rapid and transient, but excitations of short-spike units were more prolonged and sustained (10-15 s) than in long-spike cells. These data suggest that in awake animals iv cocaine, like somato-sensory stimuli, rapidly and transiently excites VTA neurons of different subtypes. Therefore, along with direct action on specific brain substrates, central effects of cocaine may occur, via an indirect mechanism, involving peripheral neural elements, visceral sensory nerves and rapid neural transmission. Via this mechanism, cocaine, like somato-sensory stimuli, can rapidly activate DA neurons and induce phasic DA release, creating the conditions for DA accumulation by a later occurring and prolonged direct inhibiting action on DA uptake. By providing a rapid neural signal and triggering transient neural activation, such a peripherally driven action might play a crucial role in the sensory effects of cocaine, thus contributing to learning and development of drug-taking behavior.

Behavioral responses and Fos activation following painful stimuli in a rodent model of Parkinson's disease

In Parkinson's disease (PD), the motor dysfunction caused by degeneration of the nigrostriatal pathway is often associated with alterations of pain perception. This is likely related to the role that the nigrostriatal system may play in the processing of noxious, somatosensory stimuli. To further address this issue, we used a rodent model of PD, based on the unilateral, intrastriatal injection of neurotoxin 6-hydroxydopamine (6-OHDA). We investigated the effects of the nigrostriatal lesion on behavioral responses to pain tests designed to explore different aspects of nociception, such as the formalin test and the tail flick test; we also explored modifications in the expression of Fos protein, a marker of neuronal activation, in supraspinal nuclei involved in the integration of pain perception and stress-related behavior. Rats bearing the nigrostriatal lesion showed complex alterations in pain perception, including hyperalgesic responses to the tonic, inflammatory pain elicited by formalin injection, but only when the stimulus was delivered ipsilaterally to the lesion. This phenomenon was associated with delayed responses to the phasic, thermal stimulus induced by the tail flick test. The hyperalgesic response to the formalin test was accompanied by reduced Fos expression in the paraventricular nucleus of the hypothalamus, which is part of a network (the medial pain system) that mediates motivational-affective aspects of pain. Our results confirm that a unilateral alteration of central dopaminergic transmission disrupts the neural mechanisms underlying proper integration of painful stimuli, particularly in the hemibody ipsilateral to the dopaminergic denervation.

Regional cerebral perfusion differences between periventricular grey, thalamic and dual target deep brain stimulation for chronic neuropathic pain

Regional cerebral blood flow changes were evaluated in different subcortical brain targets following deep brain stimulation (DBS) for chronic pain. Three patients with intractable neuropathic pain were assessed; one had stimulating electrodes in the ventroposterolateral thalamic nucleus (VPL), one in the periventricular grey (PVG) area, and one had electrodes in both targets. Pain relief was achieved in all patients. Cerebral perfusion was measured by single-photon emission computed tomography to determine the effects of DBS. Comparison was made between individual scans using subtraction analysis. DBS consistently increased perfusion in the posterior subcortical region between VPL and PVG, regardless of the site of stimulation. Furthermore, thalamic and dual target DBS increased thalamic perfusion, yet PVG DBS decreased perfusion in the PVG-containing midbrain region and thalamus. Dual target stimulation decreased anterior cingulate and insular cortex perfusion. The study demonstrates regional differences in cerebral perfusion between three accepted and efficacious targets for analgesic DBS.

The ventral tegmental area revisited: is there an electrophysiological marker for dopaminergic neurons?

The ventral tegmental area (VTA) and in particular VTA dopamine (DA) neurons are postulated to play a central role in reward, motivation and drug addiction. However, most evidence implicating VTA DA neurons in these functions is based on indirect electrophysiological characterization, rather than cytochemical identification. These physiological criteria were first established in the substantia nigra pars compacta (SNc), but their validity in the VTA is uncertain. In the current study we found that while 88 +/- 2% of SNc neurons labelled by the neuronal marker NeuN were co-labelled for the catecholamine enzyme tyrosine hydroxylase (TH), a much smaller percentage (55 +/- 2%) of VTA neurons co-expressed TH. In addition, using in vitro whole-cell recordings we found that widely accepted physiological criteria for VTA DA neurons, including the hyperpolarization-activated inwardly rectifying non-specific cation current (I(h)), spike duration, and inhibition by DA D2 receptor agonists, do not reliably predict the DA content of VTA neurons. We could not distinguish DA neurons from other VTA neurons by size, shape, input resistance, I(h) size, or spontaneous firing rate. Although the absence of an I(h) reliably predicted that a VTA neuron was non-dopaminergic, and I(h)(-) neurons differ from I(h)(+) neurons in firing rate, interspike interval (ISI) standard deviation, and ISI skew, no physiological property examined here is both sensitive and selective for DA neurons in the VTA. We conclude that reliable physiological criteria for VTA DA neuron identification have yet to be determined, and that the criteria currently being used are unreliable.

Additional evidence for the involvement of the basal ganglia in formalin-induced nociception: the role of the nucleus accumbens

Identification of the brain areas that contribute to pain is an essential undertaking towards understanding persistent pain. Areas of the basal ganglia have been proposed to play important roles in nociception as previous studies have determined the involvement of the substantia nigra pars compacta and the dorsolateral striatum in pain. The purpose of the present study was therefore to expand upon these findings by determining the involvement of other areas of the basal ganglia such as the nucleus accumbens shell and core in formalin-induced nociception. It was found that injection of a local anaesthetic (bupivacaine) into the nucleus accumbens shell had no effect on formalin-induced nociception. However, injection into the nucleus accumbens core enhanced formalin-induced nociception. These results implicate the nucleus accumbens in the processing of pain and provide additional evidence for the involvement of the basal ganglia and possibly dopamine in pain.

Positron emission tomography study of a chronic pain patient successfully treated with somatosensory thalamic stimulation

Previous neuroimaging studies suggested that the neuronal network underlying the perception of chronic pain may differ from that underlying acute pain. To further map the neural network associated with chronic pain, we used positron emission tomography (PET) to determine significant regional cerebral blood flow (rCBF) changes in a patient with chronic facial pain. The patient is implanted with a chronic stimulation electrode in the left ventroposterior medial thalamic nucleus with which he can completely suppress his chronic pain. The patient was scanned in the following conditions: before thalamic stimulation (pain, no stimulation), during thalamic stimulation (no pain, stimulation) and after successful thalamic stimulation (no pain, no stimulation). Comparing baseline scans during pain with scans taken after stimulation, when the patient had become pain-free, revealed significant rCBF increases in the prefrontal (Brodmann areas (BA) 9, 10, 11 and 47) and anterior insular cortices, hypothalamus and periaqueductal gray associated with the presence of chronic pain. No significant rCBF changes occurred in thalamus, primary and secondary somatosensory cortex and anterior cingulate cortex, BA 24'. Significant rCBF decreases were observed in the substantia nigra/nucleus ruber and in the anterior pulvinar nucleus. During thalamic stimulation, blood flow significantly increased in the amygdala and anterior insular cortex. These data further support that there are important differences in the cerebral processing of acute and chronic pain.

Connections of the precommissural nucleus

The connections of the precomissural nucleus (PRC) have been examined with anterograde and retrograde axonal tracing methods in the rat. Experiments with cholera toxin B subunit (CTb) indicate that the PRC shares a number of common afferent sources with the dorsolateral periaqueductal gray (PAG). Thus, we have shown that the nucleus receives substantial inputs from the prefrontal cortex, specific domains of the rostral part of the lateral septal nucleus, rostral zona incerta, perifornical region, anterior hypothalamic nucleus, ventromedial hypothalamic nucleus, dorsal premammillary nucleus, medial regions of the intermediate and deep layers of the superior colliculus, and cuneiform nucleus. Moreover, the PRC also receives inputs from several PAG regions and from neural sites involved in the control of attentive or motivational state, including the laterodorsal tegemental nucleus and the ventral tegmental area. The efferent projections of the PRC were analyzed by using the Phaseolus vulgaris-leucoagglutinin (PHA-L) method. Notably, the PRC presents a projection pattern that resembles in many ways the pattern described previously for the rostral dorsolateral PAG in addition to projections to a number of targets that also are innervated by neighboring pretectal nuclei, including the rostrodorsomedial part of the lateral dorsal thalamic nucleus, the ventral part of the lateral geniculate complex, the medial pretectal nucleus, the nucleus of the posterior commissure, and the ventrolateral part of the subcuneiform reticular nucleus. Overall, the results suggest that the PRC might be viewed as a rostral component of the PAG, and the possible functional significance of the nucleus is discussed in terms of its connections.

Reward prediction in primate basal ganglia and frontal cortex

Reward information is processed in a limited number of brain structures, including fronto-basal ganglia systems. Dopamine neurons respond phasically to primary rewards and reward-predicting stimuli depending on reward unpredictability but without discriminating between rewards. These responses reflect 'errors' in the prediction of rewards in correspondence to learning theories and thus may constitute teaching signals for appetitive learning. Neurons in the striatum (caudate, putamen, ventral striatum) code reward predictions in a different manner. They are activated during several seconds when animals expect predicted rewards. During learning, these activations occur initially in rewarded and unrewarded trials and become subsequently restricted to rewarded trials. This occurs in parallel with the adaptation of reward expectations by the animals, as inferred from their behavioral reactions. Neurons in orbitofrontal cortex respond differentially to stimuli predicting different liquid rewards, without coding spatial or visual features. Thus, different structures process reward information processed in different ways. Whereas dopamine neurons emit a reward teaching signal without indicating the specific reward, striatal neurons adapt expectation activity to new reward situations, and orbitofrontal neurons process the specific nature of rewards. These reward signals need to cooperate in order for reward information to be used for learning and maintaining approach behavior.

The role of the basal ganglia in nociception and pain

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.