Noxious stimulation accelerated the expression of c-fos protooncogene in cholecystokininergic and dopaminergic neurons in the ventral tegmental area

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

Stimulation of the ventral tegmental area increased nociceptive thresholds and decreased spinal dorsal horn neuronal activity in rat

Deep brain stimulation has been found to be effective in relieving intractable pain. The ventral tegmental area (VTA) plays a role not only in the reward process, but also in the modulation of nociception. Lesions of VTA result in increased pain thresholds and exacerbate pain in several pain models. It is hypothesized that direct activation of VTA will reduce pain experience. In this study, we investigated the effect of direct electrical stimulation of the VTA on mechanical, thermal and carrageenan-induced chemical nociceptive thresholds in Sprague-Dawley rats using our custom-designed wireless stimulator. We found that: (1) VTA stimulation itself did not show any change in mechanical or thermal threshold; and (2) the decreased mechanical and thermal thresholds induced by carrageenan injection in the hind paw contralateral to the stimulation site were significantly reversed by VTA stimulation. To further explore the underlying mechanism of VTA stimulation-induced analgesia, spinal cord dorsal horn neuronal responses to graded mechanical stimuli were recorded. VTA stimulation significantly inhibited dorsal horn neuronal activity in response to pressure and pinch from the paw, but not brush. This indicated that VTA stimulation may have exerted its analgesic effect via descending modulatory pain pathways, possibly through its connections with brain stem structures and cerebral cortex areas.

GABA site agonist gaboxadol induces addiction-predicting persistent changes in ventral tegmental area dopamine neurons but is not rewarding in mice or baboons

Dopamine neurons of the ventral tegmental area (VTA) are involved at early phases of drug addiction. Even the first in vivo dose of various abused drugs induces glutamate receptor plasticity at the excitatory synapses of these neurons. Benzodiazepines that suppress the inhibitory GABAergic interneurons in the VTA via facilitation of synaptic GABA(A) receptors have induced neuroplasticity in dopamine neurons due to this disinhibitory mechanism. Here, we have tested a non-benzodiazepine direct GABA site agonist 4,5,6,7-tetrahydroisoxazolol[4,5-c]pyridine-3-ol (THIP) (also known as gaboxadol) that acts preferentially via high-affinity extrasynaptic GABA(A) receptors. A single sedative dose of THIP (6 mg/kg) to mice induced glutamate receptor plasticity for at least 6 d after administration. Increased AMPA/NMDA receptor current ratio and increased frequency, amplitude, and rectification of AMPA receptor responses suggested persistent targeting of GluA2-lacking AMPA receptors in excitatory synapses of VTA dopamine neurons ex vivo after THIP administration. This effect was abolished in GABA(A) receptor δ(-/-) mice, which have a loss of extrasynaptic GABA(A) receptors. In behavioral experiments, we found neither acute reinforcement in intravenous self-administration sessions with THIP at relevant doses using a yoked control paradigm in mice nor in baboons using a standard paradigm for assessing drug abuse liability; nor was any place preference found after conditioning sessions with various doses of THIP but rather a persistent aversion in 6 mg/kg THIP-conditioned mice. In summary, we found that activation of extrasynaptic δ-subunit-containing GABA(A) receptors leads to glutamate receptor plasticity of VTA dopamine neurons, but is not rewarding, and, instead, induces aversion.

Projection-specific modulation of dopamine neuron synapses by aversive and rewarding stimuli

Midbrain dopamine (DA) neurons are not homogeneous but differ in their molecular properties and responses to external stimuli. We examined whether the modulation of excitatory synapses on DA neurons by rewarding or aversive stimuli depends on the brain area to which these DA neurons project. We identified DA neuron subpopulations in slices after injection of "Retrobeads" into single target areas of adult mice and found differences in basal synaptic properties. Administration of cocaine selectively modified excitatory synapses on DA cells projecting to nucleus accumbens (NAc) medial shell while an aversive stimulus selectively modified synapses on DA cells projecting to medial prefrontal cortex. In contrast, synapses on DA neurons projecting to NAc lateral shell were modified by both rewarding and aversive stimuli, which presumably reflects saliency. These results suggest that the mesocorticolimbic DA system may be comprised of three anatomically distinct circuits, each modified by distinct aspects of motivationally relevant stimuli.

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense-cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs.

Acute noxious stimulation modifies morphine effect in serotonergic but not dopaminergic midbrain areas

It is poorly understood if and how pain may modify the effect of opioids on neural systems that contribute to reward and addictive behavior. We hypothesized that the activation of ascending dopaminergic and serotonergic nuclei by morphine is modified by the presence of noxious stimulation. Immunohistochemical double-labeling technique with Fos was used to examine if an intraplantar formalin injection, an acute noxious input, changed the effect of morphine on dopaminergic neurons of the ventral tegmental area (VTA), and serotonergic neurons of the dorsal raphe nucleus (DR). Four groups of rats were analyzed: (1) control injected with normal saline s.c., (2) rats treated with formalin into the hind paw 30 min after normal saline injection, (3) rats injected with morphine sulfate s.c., and (4) rats treated with formalin into the hind paw 30 min after morphine injection (morphine/formalin). Following morphine injection, there was an increase in the number of dopaminergic neurons in the VTA with Fos immunolabeling. However, noxious stimulation did not detectably change morphine's effect on Fos expression in VTA dopamine neurons. In contrast, the number of serotonergic neurons containing Fos was increased in the morphine/formalin group compared to all other groups and this effect was topographically selective for the dorsal area of the DR at mid rostro-caudal levels. Therefore, morphine's activation of the VTA, which is associated with motivated behavior and reward seeking, appears similar in the context of pain. However, activation of the ascending serotonin system, which influences mood and has the capacity to modify reward pathways, appears different. In addition, these findings reveal interactions between nociceptive signaling and opioids that contrasts with the notion that opioids simply block access of nociceptive signaling to supraspinal structures.

Dopaminergic neurons expressing Fos during waking and paradoxical sleep in the rat

Formerly believed to contribute to behavioural waking (W) alone, dopaminergic (DA) neurons are now also known to participate in the regulation of paradoxical sleep (PS or REM) in mammals. Indeed, stimulation of postsynaptic DA1 receptors with agonists induces a reduction in the daily amount of PS. DA neurons in the ventral tegmental area were recently shown to fire in bursts during PS, but nothing is known about the activity of the other DA cell groups in relation to waking or PS. To fulfil this gap, we used a protocol in which rats were maintained in continuous W for 3h in a novel environment, or specifically deprived of PS for 3 days with some of them allowed to recover from this deprivation. A double immunohistochemical labeling with Fos and tyrosine hydroxylase was then performed. DA neurons in the substantia nigra (A9) and ventral tegmental area (A10), and its dorsocaudal extension in the periaqueductal gray (A10dc), almost never showed a Fos-immunoreactive nucleus, regardless of the experimental condition. The caudal hypothalamic (A11) group showed a moderate activation after PS deprivation and novel environment. During PS-recovery, the zona incerta (A13) group contained a significant number and percentage of double-labeled neurons. These results suggest that some DA neurons (A11) could participate in waking and/or the inhibition of PS during PS deprivation whereas others (A13) would be involved in the control of PS.

Dopamine transporters are involved in the onset of hypoxia-induced dopamine efflux in striatum as revealed by in vivo microdialysis

Although many studies have revealed alterations in neurotransmission during ischaemia, few works have been devoted to the neurochemical effects of mild hypoxia, a situation encountered during life in altitude or in several pathologies. In that context, the present work was undertaken to determine the in vivo mechanisms underlying the striatal dopamine efflux induced by mild hypoxaemic hypoxia. For that purpose, the extracellular concentrations of dopamine and its metabolite 3,4-dihydroxyphenyl acetic acid were simultaneously measured using brain microdialysis during acute hypoxic exposure (10% O(2), 1h) in awake rats. Hypoxia induced a +80% increase in dopamine. Application of the dopamine transporters inhibitor, nomifensine (10 microM), just before the hypoxia prevented the rise in dopamine during the early part of hypoxia; in contrast the application of nomifensine after the beginning of hypoxia, failed to alter the increase in dopamine. Application of the voltage-dependent Na(+) channel blocker tetrodotoxin abolished the increase in dopamine, whether administered just before or after the beginning of hypoxia. These data show that the neurochemical mechanisms of the dopamine efflux may change over the course of the hypoxic exposure, dopamine transporters being involved only at the beginning of hypoxia.

GABAergic projection from the ventral tegmental area and substantia nigra to the periaqueductal gray region and the dorsal raphe nucleus

Previous studies have shown that neurons in the ventral tegmental area (VTA) and substantia nigra (SN) project to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus (DR). Research has also shown that stimulation of neurons in the VTA/SN elicits cardiovascular depressor responses that are mediated by a projection to the PAGvl/DR. Anatomic and physiological experiments were done in the present study to determine the neurochemical identity of the VTA/SN projection to the PAGvl/DR. Experiments were done to characterize the origin and chemical nature of this projection by combining cholera toxin B tracing with immunofluorescence for the 67K isoform of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase. The PAGvl/DR region was found to receive a substantial input from neurons in the VTA, SN, and deep mesencephalic nucleus. The DR was preferentially innervated by neurons in the VTA, whereas the PAGvl was preferentially innervated by neurons in the SN. A proportion of neurons in the VTA and the reticular portion of the SN found to project to the PAGvl/DR were GAD positive. In addition, experiments were done in urethane-anesthetized rats to determine whether injections of a gamma-aminobutyric acid (GABA) antagonist in the region of the PAGvl/DR attenuated the cardiovascular depressor responses produced by glutamate stimulation of the VTA/SN. Injections of the GABA-blocking agent picrotoxin (2.5 nmol, 500 nl) into the PAGvl/DR eliminated the cardiovascular responses from stimulation of the VTA/SN (0.01 M, 50 nl). The results of the present investigation provide evidence for a GABAergic projection from the VTA/SN to the PAGvl/DR. This projection may be an important regulator of the PAGvl/DR, an area of the midbrain involved in the production of behavioral and physiological responses to pain and stress.

Different lipid profiles as constituencies of liquid formula diets do not influence pain perception and the efficacy of opioids in a human model of acute pain and hyperalgesia

Nutritional support and pain control by medication are often used concomitantly, but interactions are hardly investigated. A randomised, double-blind, cross-over study in ten right-handed volunteers was performed evaluating the influence of cholecystokinin (CCK)-excretion on the perception of pain in a standardised model. CCK-excretion was induced by a liquid formula diet with either long- or medium-chain triglycerides (LCT, MCT). Plasma samples were drawn over a 60 min period in 15-min intervals and CCK and somatostatin (SMS) were measured by radioimmunoassay (RIA). Gastric emptying was evaluated by C-13-breath testing. Transcutaneous electrical stimulation at a high current density (5 Hz, 70.1+/-5.8 mA) was used to provoke acute pain and stable areas of secondary mechanical hyperalgesia and pinprick allodynia for 2 h. Ongoing pain ratings as well as extension of pinprick-hyperalgesia and allodynia were compared between both liquid formula diets. In a second series of experiments, alfentanil (4.1+/-0.5 mg) was administered for 90 min using target-controlled infusions and measurements were performed as stated above. Oral administration of LCT as well as MCT may lead to different CCK blood levels, but we found no evidence for CCK-induced effects on pain sensation, touch-evoked allodynia, secondary hyperalgesia or morphine-induced anti-nociception in humans. In our studies, liquid formula diets did not influence acute pain perception or the efficacy of opioids in a human model of pain.

Oestrogen modulates cholecystokinin: opioid interactions in the nervous system

Responses of the nervous system to introceptive and extroceptive inputs depend upon the state of the brain. Oestrogen has the ability to modulate brain state and dramatically alter interactions among neural circuits to influence an organism's responses to given stimuli. Cholecystokinin (CCK) and endogenous opioid peptides (EOP) have a wide and parallel distribution in the nervous system. Their reciprocal interactions regulate a diverse physiology including reproduction, cortical function and nociception. The actions of CCK and EOP are diametrically opposed, in many regions. For example, when opioids inhibit reproductive behaviour or nociception, CCK facilitates. Because oestrogen is a powerful regulator of the expression of CCK and EOP, we examined whether oestrogen-state also modulated the interactions of these neuropeptides. In this paper we present new data and review previous work that demonstrates oestrogen modulation of functional CCK-opioid interactions that regulate reproductive behaviour, cortical function and nociception.

Immunohistochemical characterisation of Fos-positive cells in brainstem monoaminergic nuclei following intracranial self-stimulation of the medial forebrain bundle in the rat

Fos immunostaining was used as a marker of neuronal activity following intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB) in the rat, and was combined with immunostaining for tyrosine hydroxylase (TH), serotonin (5-HT), gamma-aminobutyric acid (GABA), or NR1 (one of the glutamate N-methyl- D-aspartate receptor subunits) for purposes of neurochemical identification. ICSS induced a significant but different degree of increase in the number of Fos-immunopositive (Fos+) cells in the six brainstem monoaminergic nuclei examined, which included the ventral tegmental area (VTA), substantia nigra pars compacta (SNc), dorsal raphe nucleus (DR), median raphe nucleus (MR), locus coeruleus (LC), and A7 noradrenaline cells. Densely labelled Fos+ cells were observed in the LC following ICSS, and many of these Fos+ cells were colocalized with TH. Similarly, many of Fos+ cells in the A7 and DR/MR were colocalized with TH and 5-HT, respectively. By contrast, a smaller number of Fos+ cells was detected in the VTA and SNc following the ICSS, and in these regions the majority of Fos+ cells were not colocalized with TH. Although results among regions quantitatively differed, the ICSS induced a significant increase in the number of double-labelled cells (GABA+/Fos+ or NR1+/Fos+) in all of the VTA, DR, and LC, in which the ICSS produced an ipsilaterally weighted increase in Fos-like immunoreactivity. These results suggest that ICSS of the MFB induces differential Fos expression within monoaminergic and GABAergic neurons in brainstem monoaminergic nuclei under modulation by glutamatergic afferents.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Expression of Fos immunoreactivity in the rat supraspinal regions following noxious visceral stimulation

We used immunohistochemical detection of the Fos protein to study the neuronal activation in the brain of methoxyfluorane-anesthetized rats after noxious deep somatic or visceral stimulation. The anesthesia was effective in triggering gene induction in many brain regions. Nevertheless, Fos appeared de novo in several brain nuclei following noxious stimulation in anesthetized animals. This could be of clinical relevance, as it suggests that the gas anesthetic does not suppress noxious stimulus-evoked reactivity in brain neurons. Two types of visceronociceptive stimuli were used to compare the effects of a diffuse visceral inflammation (peritoneal inflammation) with those of a more restricted inflammation (urinary bladder inflammation). In the same supraspinal areas, there were very few immunostained neurons in unstimulated controls, whereas Fos-positive cells were slightly more numerous in anesthetized controls and significantly more numerous after noxious stimulation. The peritoneal inflammation induced more Fos-labeled neurons than the restricted visceral stimulation. Labeled cells were found in these cases mainly in the ventrolateral medulla, parabrachial complex, dorsal raphe nucleus, periaqueductal gray, several hypothalamic and thalamic nuclei, amygdaloid complex, and cortex. Altogether these findings indicated that somatic and visceral inputs generally activate the same neuronal groups. However, a separation between the activation of somatic and visceral pathways was found in some brain nuclei, such as the parabrachial complex, hypothalamic, and thalamic nuclei.

Rewarding brain stimulation induces only sparse Fos-like immunoreactivity in dopaminergic neurons

In this study, c-fos immunohistochemistry was used to identify the brain regions activated by rewarding brain stimulation in rats. Rats had monopolar electrodes implanted in the medial forebrain bundle and were allocated to either a self-stimulation (n = 4), yoked stimulation (n = 4) or no stimulation (n = 6) group. In a single 1 h test session, each rat in the self-stimulation group made 1000 nose poke responses with each response followed by a 0.5 s train of brain stimulation. Rats in the yoked-stimulation group were paired with a partner in the self-stimulation group and received brain stimulation whenever their partner did. However, their nose poke responses did not trigger stimulation. This yoked procedure was thus used to identify any Fos-like immunoreactivity due to operant responding. Rats in the no stimulation group were placed in the same apparatus as the other rats but received no brain stimulation and were thus used to assess baseline Fos-like immunoreactivity. Results showed that stimulation increased Fos-like immunoreactivity in many areas of the brain in both the self-stimulation and yoked groups. The areas with the highest Fos-like immunoreactivity were ipsilateral to the electrode site and included the medial prefrontal cortex, lateral septum, nucleus accumbens (shell), the medial and lateral preoptic areas, bed nucleus of the stria terminalis, central amygdala, lateral habenula, dorsomedial hypothalamus, lateral hypothalamus and the anterior ventral tegmental area. Bilateral Fos-like immunoreactivity was evident in the nucleus accumbens core, paraventricular nucleus of the hypothalamus, the retrorubral fields and the locus coeruleus. A double-labelling procedure identifying both Fos and tyrosine hydroxylase was used to show that very few (< 5%) of the A10 dopamine cell bodies in the ventral tegmental area expressed Fos following brain stimulation. In contrast, most of the noradrenergic neurons of the locus coeruleus (A6), rubrospinal tract (A5) and pontine tegmental area (A7) were Fos positive. Overall, the results show that rewarding, brain stimulation induces Fos-like immunoreactivity in many forebrain regions but only sparsely in mesolimbic and mesocortical dopamine neurons. The similar patterns of Fos-like immunoreactivity seen in the self-stimulation and yoked-stimulation groups suggests that the operant responding for brain stimulation causes minimal Fos expression in itself.

Tachykinin NK-1 and NK-3 selective agonists induce analgesia in the formalin test for tonic pain following intra-VTA or intra-accumbens microinfusions

Experiments were designed to examine the analgesic effects induced by selective tachykinin receptor agonists microinfused into either the ventral tegmental area (VTA) or nucleus accumbens septi (NAS). Rats were tested in the formalin test for tonic pain following an injection of 0.05 ml of 2.5% formalin into one hind paw immediately after bilateral intra-VTA infusions of either the NK-1 agonist, GR-73632 (0.005, 0.05 or 0.5 nmol/side), the NK-3 agonist, senktide (0.005, 0.5 or 1.5 nmol/side), or saline. Two weeks later, the saline-treated rats were assessed in the tail-flick test for phasic pain after infusions of the tachykinin agonists. Tail-flick latencies were recorded following immersion of the tail in 55 degrees C hot water at 10 min intervals for 1 h immediately after intra-VTA infusions of either GR-73632 (0.5 nmol/side), senktide (1.5 nmol/side) or saline. In a second group of rats, the same effects were studied after infusions into the nucleus accumbens (NAS) of GR-73632 (0.005, 0.5 or 1.5 nmol/side), senktide (0.005, 0.5 or 1.5 nmol/side), or saline. In both the VTA and NAS, the NK-1 and the NK-3 agonists caused significant analgesia in the formalin test, although the NK-1 agonist appeared to be more effective. Naltrexone (2.0 mg/kg) pretreatment failed to reverse the analgesic effects in the formalin test induced by intra-VTA infusions of the substance P (SP) analog, DiMe-C7 (3.0 microg/side), GR-73632 (0.5 nmol/side), or senktide (1.5 nmol/side). Neither compound given at either site was effective in the tail-flick test. These findings suggest that SP-dopamine (DA) interactions within the mesolimbic DA system play an important role in the inhibition of tonic pain. Furthermore, they support our earlier ideas that activation of midbrain DA systems by SP might play a role in stress- and/or pain-induced analgesia.