Differential effects of noxious and non-noxious input on neurones according to location in ventral periaqueductal grey or dorsal raphe nucleus

Dysregulated neuromodulation in the anterior cingulate cortex in chronic pain

Chronic pain is a significant global socioeconomic burden with limited long-term treatment options. The intractable nature of chronic pain stems from two primary factors: the multifaceted nature of pain itself and an insufficient understanding of the diverse physiological mechanisms that underlie its initiation and maintenance, in both the peripheral and central nervous systems. The development of novel non-opioidergic analgesic approaches is contingent on our ability to normalize the dysregulated nociceptive pathways involved in pathological pain processing. The anterior cingulate cortex (ACC) stands out due to its involvement in top-down modulation of pain perception, its abnormal activity in chronic pain conditions, and its contribution to cognitive functions frequently impaired in chronic pain states. Here, we review the roles of the monoamines dopamine (DA), norepinephrine (NE), serotonin (5-HT), and other neuromodulators in controlling the activity of the ACC and how chronic pain alters their signaling in ACC circuits to promote pathological hyperexcitability. Additionally, we discuss the potential of targeting these monoaminergic pathways as a therapeutic strategy for treating the cognitive and affective symptoms associated with chronic pain.

Cerebellar modulation of memory encoding in the periaqueductal grey and fear behaviour

The pivotal role of the periaqueductal grey (PAG) in fear learning is reinforced by the identification of neurons in male rat ventrolateral PAG (vlPAG) that encode fear memory through signalling the onset and offset of an auditory-conditioned stimulus during presentation of the unreinforced conditioned tone (CS+) during retrieval. Some units only display CS+ onset or offset responses, and the two signals differ in extinction sensitivity, suggesting that they are independent of each other. In addition, understanding cerebellar contributions to survival circuits is advanced by the discovery that (i) reversible inactivation of the medial cerebellar nucleus (MCN) during fear consolidation leads in subsequent retrieval to (a) disruption of the temporal precision of vlPAG offset, but not onset responses to CS+, and (b) an increase in duration of freezing behaviour. And (ii) chemogenetic manipulation of the MCN-vlPAG projection during fear acquisition (a) reduces the occurrence of fear-related ultrasonic vocalisations, and (b) during subsequent retrieval, slows the extinction rate of fear-related freezing. These findings show that the cerebellum is part of the survival network that regulates fear memory processes at multiple timescales and in multiple ways, raising the possibility that dysfunctional interactions in the cerebellar-survival network may underlie fear-related disorders and comorbidities.

Anxiety disorders are a cluster of mental health conditions characterised by persistent and excessive amounts of fear and worry. They affect millions of people worldwide, but treatments can sometimes be ineffective and have unwanted side effects. Understanding which brain regions are involved in fear and anxiety-related behaviours, and how those areas are connected, is the first step towards designing more effective treatments.

A region known as the periaqueductal grey (or PAG) sits at the centre of the brain’s fear and anxiety network, regulating pain, encoding fear memories and responding to threats and stressors. It also controls survival behaviours such as the ‘freeze’ response, when an animal is frightened.

A more recent addition to the fear and anxiety network is the cerebellum, which sits at the base of the brain. Two-way connections between this region and the PAG have been well described, but how the cerebellum might influence fear and anxiety-related behaviours remains unclear.

To explore this role, Lawrenson, Paci et al. investigated whether the cerebellum modulates brain activity within the PAG and if so, how this relates to fear behaviours. Rats had electrodes implanted in their brains to record the activity of nerve cells within the PAG. A common fear-conditioning task was then used to elicit ‘freeze’ responses: a sound was paired with mild foot shocks until the animals learned to fear the auditory signal. In the rats, a subset of neurons within the PAG responded to the tone, consistent with those cells encoding a fear memory. But when a drug blocked the cerebellum’s output during fear conditioning, the timing of the PAG response was less precise and the rats’ freeze response lasted longer.

Lawrenson, Paci et al. concluded that the cerebellum, through its interactions with the brain’s fear and anxiety network, might be responsible for coordinating the most appropriate behavioural response to fear, and how long ‘freezing’ lasts.

In summary, these findings show that the cerebellum is a part of the brain’s survival network which regulates fear-memory processes. It raises the possibility that disruption of the cerebellum might underlie anxiety and other fear-related disorders, thereby providing a new target for future therapies.

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.

Nociception alterations precede motor symptoms in a progressive model of parkinsonism induced by reserpine in middle-aged rats

Nociception alterations are frequent non-motor symptoms of the prodromal phase of Parkinson's disease (PD). The period for the onset of symptoms and the pathophysiological mechanisms underlying these alterations remain unclear. We investigated the course of nociception alterations in a progressive model of parkinsonism induced by reserpine (RES) in rats. Male Wistar rats (6-7 months) received 5 or 10 subcutaneous injections of RES (0.1 mg/kg) or vehicle daily for 20 days. Motor evaluation and nociceptive assessment were performed throughout the treatment. At the end of the treatment rats were euthanized, the brains removed and processed for immunohistochemical analysis (TH and c-Fos). The RES-treated rats exhibited an increased nociceptive response to mechanical and chemical stimulation in the electronic von Frey and formalin tests, respectively. Moreover, these alterations preceded the motor impairment observed in the catalepsy test. In addition, the RES treatment reduced the TH-immunoreactivity in the ventral tegmental area (VTA) and increased the c-Fos expression in the ventral-lateral periaqueductal gray (vlPAG), rostral ventral medulla (RVM) and dorsal raphe nucleus (DRN) after noxious stimuli induced by formalin. Taken together, our results reinforce that nociceptive changes are one of the early signs of PD and monoamine depletion in basal ganglia can be involved in the abnormal processing of nociceptive information in PD.

Involvement of the Ventrolateral Periaqueductal Gray Matter-Central Medial Thalamic Nucleus-Basolateral Amygdala Pathway in Neuropathic Pain Regulation of Rats

The central medial nucleus (CM), a prominent cell group of the intralaminar nuclei (ILN) of the thalamus, and the ventrolateral periaqueductal gray matter (vlPAG) are two major components of the medial pain system. Whether vlPAG and CM are input sources of nociceptive information to the basolateral amygdala (BLA) and whether they are involved in neuropathic pain regulation remain unclear. Clarifying the hierarchical organization of these subcortical nuclei (vlPAG, CM, and BLA) can enhance our understanding on the neural circuits for pain regulation. Behavioral test results showed that a CM lesion made by kainic acid (KA) injection could effectively alleviate mechanical hyperalgesia 4, 6, and 8 days after spared nerve injury (SNI) surgery, with the symptoms returning after 10 days. Morphological studies revealed that: (1) the CM received afferents from vlPAG and sent efferents to BLA, indicating that an indirect vlPAG–CM–BLA pathway exists; (2) such CM–BLA projections were primarily excitatory glutamatergic neurons as revealed by fluorescence in situ hybridization; (3) the fibers originated from the CM-formed close contacts with both excitatory and inhibitory neurons in the BLA; and (4) BLA-projecting CM neurons expressed Fos induced by SNI and formed close contacts with fibers from vlPAG, suggesting that the vlPAG–CM–BLA indirect pathway was activated in neuropathic pain conditions. Finally, the vlPAG–CM–BLA indirect pathway was further confirmed using anterograde and monosynaptic virus tracing investigation. In summary, our present results provide behavioral and morphological evidence that the indirect vlPAG–CM–BLA pathway might be a novel pain pathway involved in neuropathic pain regulation.

Neuropathic pain-induced enhancement of spontaneous and pain-evoked neuronal activity in the periaqueductal gray that is attenuated by gabapentin

Neuropathic pain is a debilitating pathological condition that is poorly understood. Recent evidence suggests that abnormal central processing occurs during the development of neuropathic pain induced by the cancer chemotherapeutic agent, paclitaxel. Yet, it is unclear what role neurons in supraspinal pain network sites, such as the periaqueductal gray, play in altered behavioral sensitivity seen during chronic pain conditions. To elucidate these mechanisms, we studied the spontaneous and thermally evoked firing patterns of ventrolateral periaqueductal gray (vlPAG) neurons in awake-behaving rats treated with paclitaxel to induce neuropathic pain. In the present study, vlPAG neurons in naive rats exhibited either excitatory, inhibitory, or neutral responses to noxious thermal stimuli, as previously observed. However, after development of behavioral hypersensitivity induced by the chemotherapeutic agent, paclitaxel, vlPAG neurons displayed increased neuronal activity and changes in thermal pain-evoked neuronal activity. This involved elevated levels of spontaneous firing and heightened responsiveness to nonnoxious stimuli (allodynia) as well as noxious thermal stimuli (hyperalgesia) as compared with controls. Furthermore, after paclitaxel treatment, only excitatory neuronal responses were observed for both nonnoxious and noxious thermal stimuli. Systemic administration of gabapentin, a nonopioid analgesic, induced significant dose-dependent decreases in the elevated spontaneous and thermally evoked vlPAG neuronal firing to both nonnoxious and noxious thermal stimuli in rats exhibiting neuropathic pain, but not in naive rats. Thus, these results show a strong correlation between behavioral hypersensitivity to thermal stimuli and increased firing of vlPAG neurons in allodynia and hyperalgesia that occur in this neuropathic pain model.

Relative contribution of the dorsal raphe nucleus and ventrolateral periaqueductal gray to morphine antinociception and tolerance in the rat

The dorsal raphe nucleus (DRN) is embedded in the ventral part of the caudal periaqueductal gray (PAG). Electrical or chemical activation of neurons throughout this region produces antinociception. The objective of this manuscript is to determine whether the ventrolateral PAG and DRN are distinct antinociceptive systems. This hypothesis was tested by determining the antinociceptive potency of microinjecting morphine into each structure (Experiment 1), creating a map of effective microinjection sites that produce antinociception (Experiment 2) and comparing the development of antinociceptive tolerance to repeated microinjections of morphine into the ventrolateral PAG and DRN (Experiment 3). Morphine was more potent following cumulative injections (1.0, 2.2, 4.6 & 10 μg/0.2 μL) into the ventrolateral PAG (D₅₀  = 3.3 μg) compared to the lateral (4.3 μg) or medial DRN (5.8 μg). Antinociception occurred following 94% of the morphine injections into the ventrolateral PAG, whereas only 68.3% and 78.3% of the injections into the lateral and medial aspects of the DRN produced antinociception. Repeated microinjections of morphine into the ventrolateral PAG produced tolerance as indicated by a 528% difference in potency between morphine and saline pretreated rats. In contrast, relatively small changes in potency occurred following repeated microinjections of morphine into the lateral and medial aspects of the DRN (107% and 49%, respectively). These data indicate that the ventrolateral PAG and DRN are distinct antinociceptive structures. Antinociception is greater with injections into the ventrolateral PAG compared to the DRN, but this antinociception disappears rapidly because of the development of tolerance.

Analysis of morphine-induced changes in the activity of periaqueductal gray neurons in the intact rat

Microinjection of morphine into the periaqueductal gray (PAG) produces antinociception. In vitro slice recordings indicate that all PAG neurons are sensitive to morphine either by direct inhibition or indirect disinhibition. We tested the hypothesis that all PAG neurons respond to opioids in vivo by examining the extracellular activity of PAG neurons recorded in lightly anesthetized and awake rats. Spontaneous activity was less than 1Hz in most neurons. Noxious stimuli (heat, pinch) caused an increase in activity in 57% and 75% of the neurons recorded in anesthetized and awake rats, respectively. The same noxious stimuli caused a decrease in activity in only 17% and 6% of neurons recorded in anesthetized and awake rats. Systemic administration of morphine caused approximately equal numbers of neurons to increase, decrease, or show no change in activity in lightly anesthetized rats. In contrast, administration of morphine caused an increase in the activity of 22 of the 27 neurons recorded in awake rats. No change in activity was evident in the remaining five neurons. Changes in activity caused by morphine were surprisingly modest (a median increase from 0.7 to 1.3Hz). The small inconsistent effects of morphine are in stark contrast to the large changes produced by morphine on the activity of rostral ventromedial medulla (RVM) neurons or the widespread inhibition and excitation of PAG neurons treated with opioids in in vitro slice experiments. The relatively modest effects of morphine in the present study suggest that morphine produces antinociception by causing small changes in the activity of many PAG neurons.

Analysis of sleep disorders under pain using an optogenetic tool: possible involvement of the activation of dorsal raphe nucleus-serotonergic neurons

Background

Several etiological reports have shown that chronic pain significantly interferes with sleep. Inadequate sleep due to chronic pain may contribute to the stressful negative consequences of living with pain. However, the neurophysiological mechanism by which chronic pain affects sleep-arousal patterns is as yet unknown. Although serotonin (5-HT) was proposed to be responsible for sleep regulation, whether the activity of 5-HTergic neurons in the dorsal raphe nucleus (DRN) is affected by chronic pain has been studied only infrequently. On the other hand, the recent development of optogenetic tools has provided a valuable opportunity to regulate the activity in genetically targeted neural populations with high spatial and temporal precision. In the present study, we investigated whether chronic pain could induce sleep dysregulation while changing the activity of DRN-5-HTergic neurons. Furthermore, we sought to physiologically activate the DRN with channelrhodopsin-2 (ChR2) to identify a causal role for the DRN-5-HT system in promoting and maintaining wakefulness using optogenetics.

Results

We produced a sciatic nerve ligation model by tying a tight ligature around approximately one-third to one-half the diameter of the sciatic nerve. In mice with nerve ligation, we confirmed an increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep as monitored by electroencephalogram (EEG). Microinjection of the retrograde tracer fluoro-gold (FG) into the prefrontal cortex (PFC) revealed several retrogradely labeled-cells in the DRN. The key finding of the present study was that the levels of 5-HT released in the PFC by the electrical stimulation of DRN neurons were significantly increased in mice with sciatic nerve ligation. Using optogenetic tools in mice, we found a causal relationship among DRN neuron firing, cortical activity and sleep-to-wake transitions. In particular, the activation of DRN-5-HTergic neurons produced a significant increase in wakefulness and a significant decrease in NREM sleep. The duration of NREM sleep episodes was significantly decreased during photostimulation in these mice.

Conclusions

These results suggest that neuropathic pain accelerates the activity of DRN-5-HTergic neurons. Although further loss-of-function experiments are required, we hypothesize that this activation in DRN neurons may, at least in part, correlate with sleep dysregulation under a neuropathic pain-like state.

Common brain activations for painful and non-painful aversive stimuli

BACKGROUND

Identification of potentially harmful stimuli is necessary for the well-being and self-preservation of all organisms. However, the neural substrates involved in the processing of aversive stimuli are not well understood. For instance, painful and non-painful aversive stimuli are largely thought to activate different neural networks. However, it is presently unclear whether there is a common aversion-related network of brain regions responsible for the basic processing of aversive stimuli. To help clarify this issue, this report used a cross-species translational approach in humans (i.e. meta-analysis) and rodents (i.e. systematic review of functional neuroanatomy).

RESULTS

Animal and human data combined to show a core aversion-related network, consisting of similar cortical (i.e. MCC, PCC, AI, DMPFC, RTG, SMA, VLOFC; see results section or abbreviation section for full names) and subcortical (i.e. Amyg, BNST, DS, Hab, Hipp/Parahipp, Hyp, NAc, NTS, PAG, PBN, raphe, septal nuclei, Thal, LC, midbrain) regions. In addition, a number of regions appeared to be more involved in pain-related (e.g. sensory cortex) or non-pain-related (e.g. amygdala) aversive processing.

CONCLUSIONS

This investigation suggests that aversive processing, at the most basic level, relies on similar neural substrates, and that differential responses may be due, in part, to the recruitment of additional structures as well as the spatio-temporal dynamic activity of the network. This network perspective may provide a clearer understanding of why components of this circuit appear dysfunctional in some psychiatric and pain-related disorders.

Alterations in the central CRF system of two different rat models of comorbid depression and functional gastrointestinal disorders

Clinical evidence suggests comorbidity between depression and irritable bowel syndrome (IBS). Early-life stress and genetic predisposition are key factors in the pathophysiology of both IBS and depression. Thus, neonatal maternal separation (MS), and the Wistar-Kyoto (WKY) rat, a genetically stress-sensitive rat strain, are two animal models of depression that display increased visceral hypersensitivity and alterations in the hypothalamic-pituitary-adrenal axis. Corticotrophin-releasing factor (CRF) is the primary peptide regulating this axis, acting through two receptors: CRF1 and CRF2. The central CRF system is also a key regulator in the stress response. However, there is a paucity of studies investigating alterations in the central CRF system of adult MS or WKY animals. Using in-situ hybridization we demonstrate that CRF mRNA is increased in the paraventricular nucleus (PVN) of WKY rats and the dorsal raphé nucleus (DRN) of MS animals, compared to Sprague-Dawley and non-separated controls, respectively. Additionally, CRF1 mRNA was higher in the PVN, amygdala and DRN of both animal models, along with high levels of CRF1 mRNA in the hippocampus of WKY animals compared to control animals. Finally, CRF2 mRNA was lower in the DRN of MS and WKY rats compared to control animals, and in the hippocampus and amygdala of MS rats. These results show that the central CRF system is altered in both animal models. Such alterations may affect HPA axis regulation, contribute to behavioural changes associated with stress-related disorders, and alter the affective component of visceral pain modulation, which is enhanced in IBS patients.

Projections from the rat pedunculopontine and laterodorsal tegmental nuclei to the anterior thalamus and ventral tegmental area arise from largely separate populations of neurons

Cholinergic and non-cholinergic neurons in the brainstem pedunculopontine (PPT) and laterodorsal tegmental (LDT) nuclei innervate diverse forebrain structures. The cholinergic neurons within these regions send heavy projections to thalamic nuclei and provide modulatory input as well to midbrain dopamine cells in the ventral tegmental area (VTA). Cholinergic PPT/LDT neurons are known to send collateralized projections to thalamic and non-thalamic targets, and previous studies have shown that many of the afferents to the VTA arise from neurons that also project to midline and intralaminar thalamic nuclei. However, whether cholinergic projections to the VTA and anterior thalamus (AT) are similarly collateralized is unknown. Ultrastructural work from our laboratory has demonstrated that cholinergic axon varicosities in these regions differ both morphologically and with respect to the expression and localization of the high-affinity choline transporter. We therefore hypothesized that the cholinergic innervation to these regions is provided by separate sets of PPT/LDT neurons. Dual retrograde tract-tracing from the AT and VTA indicated that only a small percentage of the total afferent population to either region showed evidence of providing collateralized input to the other target. Cholinergic and non-cholinergic cells displayed a similarly low percentage of collateralization. These results are contrasted to a control case in which retrograde labeling from the midline paratenial thalamic nucleus and the VTA resulted in higher percentages of cholinergic and non-cholinergic dual-tracer labeled cells. Our results indicate that functionally distinct limbic target regions receive primarily segregated signaling from PPT/LDT neurons.

Neuropathic pain and the endocannabinoid system in the dorsal raphe: pharmacological treatment and interactions with the serotonergic system

We used a model of neuropathic pain consisting of rats with chronic constriction injury (CCI) of the sciatic nerve, in order to investigate whether endocannabinoid levels are altered in the dorsal raphe (DR) and to assess the effect of repeated treatment with (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate, a synthetic cannabinoid agonist, or N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM404), an inhibitor of endocannabinoid reuptake, on DR serotonergic neuronal activity and on behavioural hyperalgesia. CCI resulted in significantly elevated anandamide but not 2-arachidonoylglycerol levels in the DR. Furthermore, as well as thermal and mechanical hyperalgesia, CCI caused serotonergic hyperactivity (as shown by the increase of basal activity of serotonergic neurones, extracellular serotonin levels and expression of 5-HT1A receptor gene). Repeated treatment with either (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate or AM404 reverted the hyperalgesia and enhanced serotonergic activity induced by CCI in a way attenuated by N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide, a selective cannabinoid subtype 1 (CB1) receptor antagonist. Despite the elevated levels of anandamide following CCI, N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide did not produce hyperalgesia or any other effect on serotonergic neuronal activity when administered alone. Furthermore, the effects of AM404 were not accompanied by an increase in endocannabinoid levels in the DR. In conclusion, following CCI of the sciatic nerve, the endocannabinoid and serotonergic systems are activated in the DR, where repeated stimulation of CB1 receptors with exogenous compounds restores DR serotonergic activity, as well as thermal and mechanical nociceptive thresholds, to pre-surgery levels. However, an elevated level of endogenous anandamide in the DR does not necessarily contribute to the CB1-mediated tonic control of analgesia and serotonergic neuronal activity.

The medial pain system: neural representations of the motivational aspect of pain

In this article, we propose that the pathways mediating the motivational aspect of pain originate in laminae VII and VIII of the spinal cord, and in the deep layers of the spinal trigeminal complex, and projections from these areas reach three central structures where pain motivation is represented, the ventrolateral quadrant of the periaqueductal gray, posterior hypothalamic nucleus, and intralaminar thalamic nuclei. A final representation of the motivational aspect of pain is located within the anterior cingulate cortex, and this representation receives inputs from the intralaminar nuclei. Outputs from these representations reach premotor structures located in the medulla, striatum, and cingulate premotor cortex. We discuss pathways and structures that provide inputs to these representations, including those involved in producing involuntary (innate) and instrumental responses which occur in response to the recognition of stimuli associated with footshock and other nociceptive stimuli.

Modulation of noradrenergic and serotonergic transmission by noxious stimuli and intrathecal morphine differs in the dorsal raphe nucleus of anesthetized rat: in vivo voltammetric studies

We examined the effects of cutaneous noxious heat as well as the intrathecal administration of morphine on the oxidation current of peaks 1 and 2 in the dorsal raphe nucleus (DRN) of anesthetized rats. Differential normal pulse voltammetry with carbon fiber electrodes identified distinct oxidation currents at +120 mV (peak 1: catechol signals) and +280 mV (peak 2: 5-hydroxyindole signals). The catechol signal was significantly increased by 22.9 +/- 4.2% after applying cutaneous noxious heat at 52 degrees C. The 5-hydroxyindole signal was decreased by 39.8 +/- 4.3 and by 25.2 +/- 4.7% after stimulation with cutaneous noxious heat at 52 and 45 degrees C, respectively. A low dose of morphine (2.5 microg) potentiated the increase in the catechol signal and the decrease in the 5-hydroxyindole signal induced by noxious heat, and a high dose (10.0 microg) attenuated both. The effects of morphine at low (2.5 microg) and high doses (10.0 microg) were antagonized by naloxone (0.5 mg/kg, i.p.). These results indicate that noxious heat stimulation increased the catechol signal and decreased the 5-hydroxyindole signal in the DRN. The intrathecal administration of morphine affects the noxious stimulation-induced activity of noradrenergic and serotonergic neurotransmission in the DRN.

Behavioral sensitization to cocaine after a brief social stress is accompanied by changes in fos expression in the murine brainstem

The objective of the present study was to determine how c-fos gene expression in brainstem structures after a brief episode of social defeat stress is related to behavioral sensitization to cocaine challenge. Social stress was defined as defeat in a brief confrontation with an aggressive resident mouse and subsequent 20-min exposure to the resident's threats behind a protective screen. Mice were treated with cocaine (40 mg/kg, i.p.) immediately or 1 week after social defeat stress. Fos-like immunoreactive (Fos-LI) cell nuclei were analyzed in the ventral tegmental area (VTA), dorsal raphe nucleus (DR), periaqueductal grey area (PAG) and locus coeruleus (LC). One episode of social stress induced behavioral sensitization to cocaine as indicated by an augmented locomotor response to a challenge injection 7 days after a single defeat. In naive mice, social stress markedly increased the number of Fos-LI nuclei in the DR, PAG and LC, but not in the VTA. Similarly, cocaine administration resulted in a significantly increased number of Fos-LI nuclei in the same areas. Administration of cocaine immediately following social defeat significantly reduced the number of Fos-LI nuclei in the DR, PAG and LC. Cocaine-induced Fos expression returned in the PAG and DR, but not in the LC, 1 week after social stress. In conclusion, the present results suggest that the presence of brainstem Fos be related to the ability to express stress-induced behavioral sensitization to cocaine.

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.

Pharmacological and neuroanatomical evidence for the involvement of the anterior pretectal nucleus in the antinociception induced by stimulation of the dorsal raphe nucleus in rats

Several studies have shown that the anterior pretectal nucleus (APtN) is involved in descending inhibitory pathways that control noxious inputs to the spinal cord and that it may participate in the normal physiological response to noxious stimulation. Among other brain regions known to send inputs to the APtN, the dorsal column nuclei (DCN), pedunculopontine tegmental nucleus (PPTg), deep mesencephalon (DpMe), and dorsal raphe nucleus (DRN) are structures also known to be involved in antinociception. In the present study, the effects of stimulating these structures on the latency of the tail withdrawal reflex from noxious heating of the skin (tail flick test) were examined in rats in which saline or hyperbaric lidocaine (5%) was previously microinjected into the APtN. Brief stimulation of the PPTg, DpMe or DRN, but not the DCN, strongly depressed the tail flick reflex. The antinociceptive effect of stimulating the DRN, but not the PPTg or DpMe was significantly reduced, but not abolished, by the prior administration of the local anaesthetic into the APtN. The antinociception induced by stimulation of the PPTg or DpMe, therefore, is unlikely to depend on connections between these structures and the APtN. Similar inhibition of the effect of stimulating the DRN was obtained from rats previously microinjected with naloxone (2.7 nmol) or methysergide (2 nmol) into the APtN. Strongly labelled cells were identified in the DRN following microinjection of the fluorescent tracer Fast Blue into the APtN. These results indicate that the APtN may participate as a relay station through which the DRN partly modulates spinal nociceptive messages. In addition, they also indicate that endogenous opioid and serotonin can participate as neuromodulators of the DRN-APtN connection.

Neuropeptide FF receptors control morphine-induced analgesia in the parafascicular nucleus and the dorsal raphe nucleus

The ability of (1DMe)Y8Fa (D.Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), a selective neuropeptide FF analog resistant to enzymatic degradation, to control morphine-induced analgesia was investigated in rat after microinfusion into the dorsal raphe nucleus and the nucleus parafascicularis of the thalamus. Infusion of (1DMe)Y8Fa (2.5 nmol) in the nucleus raphe dorsalis did not modify the animal response in the tail-immersion test but significantly reversed analgesia induced by coinjected morphine (27 nmol). Similarly, (1DMe)Y8Fa (5 nmol) inhibited morphine effects in the hot-plate test after co-injection into the parafascicular nucleus. Furthermore, (1DMe)Y8Fa injected into the parafascicular nucleus attenuated analgesia induced by morphine injected into the nucleus raphe dorsalis and similarly, the neuropeptide FF analog in the nucleus raphe dorsalis decreased the effects of 27 nmol morphine injected in the parafascicular nucleus. The density of neuropeptide FF receptors did not decrease in the nucleus raphe dorsalis after lesion of serotonergic neurons by 5,7-dihydroxytryptamine. However, after this lesion, (1DMe)Y8Fa injected in the nucleus raphe dorsalis was no longer able to modify analgesic effects of morphine in hot-plate and tail-immersion tests. Similarly, the serotonin (5-HT) depletion induced by a systemic administration of para-chlorophenylalanine did not modify morphine analgesia microinjected into the nucleus raphe dorsalis and the parafascicular nucleus but blocked the ability of (1DMe)Y8Fa to reverse morphine effects in both nuclei. These data show that neuropeptide FF exerts anti-opioid effects directly into both the nucleus raphe dorsalis and the parafascicular nucleus and acts also at distance on opioid functions. Furthermore, anti-opioid effects of neuropeptide FF require functional serotonergic neurons although neuropeptide FF receptors are not carried on these neurons.

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.

Single-unit recordings at dorsal raphe nucleus in the awake-anesthetized rat: spontaneous activity and responses to cutaneous innocuous and noxious stimulations

In this study, we recorded the single-unit activity of the dorsal raphe nucleus (DRN) in rats tested first awake and, a few days later, anesthetized with sodium pentobarbital and recorded again. This was achieved by means of a small chronically implanted device supporting a 25 micron platinum-iridium wire as the recording electrode. In both the awake and anesthetized conditions, and in agreement with most of the studies performed at the DRN level, we found that a vast majority of the units, displaying small amplitude and long-duration action potentials, possessed a low level of spontaneous activity (0.2-4 Hz). Among these units, found in greater number under pentobarbital, it was possible to establish that this activity was regular or irregular, in accordance with the literature reports. However, as opposed to these studies, we determined that the 'regularity' was relative, only noticeable in more or less prolonged phases of activity. In particular, we never recorded the so-called 'clock-like' activity, largely reported as an unambiguous criterion for selecting the serotoninergic neurons. In both the awake and anesthetized conditions, the responses of the DRN neurons to peripheral mechanical innocuous and noxious stimulations were observed in only one-half of the units recorded and were weak in comparison to other results that we obtained at the nucleus raphe magnus level in previous studies. When present, these responses were excitation or inhibition, occurring during or after the stimulus application. These results question the direct involvement of the DRN in acute nociception.

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.

Dorsal raphe nucleus efferents: termination in peptidergic fields

This study was performed to identify the efferents of the dorsal raphe nucleus (DRN), particularly in regard to nociception. Phaseolus vulgaris leucoagglutinin (PHA-L) was microiontophoresed into the DRN and visualized immunocytochemically; PHA-L-immunoreactive (-IR) fibers and terminals were identified in the forebrain, hypothalamus, midline and intralaminar thalamus, habenula, periaqueductal gray, locus coeruleus, parabrachial nucleus, medullary raphe and reticular nuclei, and nucleus tractus solitarius. Dual immunocytochemistry was used to identify corticotropin-releasing factor, neurotensin, and enkephalin neurons in DRN terminal fields, in some cases in close proximity to PHA-L-IR terminals. Terminal fields were identified in regions that influence nociception, and the neuroactive substances identified in these terminal fields may play modulatory roles in nociception.

Extracellular characteristics of putative cholinergic neurons in the rat laterodorsal tegmental nucleus

The extracellular electrophysiological properties of neurons in the laterodorsal tegmental nucleus (LDT), a major source of cholinergic afferents to the thalamus, were studied in chloral hydrate-anesthetized rats. A combination of antidromic activation from the thalamus and histological verification of recording sites was used to correlate the identity of extracellular recordings in the rat LDT with cholinergic neurons in that region. All neurons antidromically activated by stimulation of the anteroventral thalamus were histologically verified to be within clusters of cholinergic (NADPH-d-positive) cells in the LDT or in the adjacent nucleus locus coeruleus (LC). The thalamically projecting LDT neurons had a homogeneous neurophysiological profile consisting of long duration action potentials (mean = 2.5 ms), slow conduction velocities (mean = 0.78 m/s), and lengthy chronaxie values (mean = 0.725 ms). The appearance and axonal characteristics of these neurons resembled those of noradrenergic LC neurons, but the two populations exhibited substantially different spontaneous activity patterns and sensory responsiveness. These characteristics may be useful in the preliminary identification of putative cholinergic neurons in vivo, and thereby provide a foundation for exploring the neuropharmacology, afferent modulation, sensory responsiveness and behavioral correlates of the brainstem cholinergic system.

Projections from the periaqueductal gray to the rostromedial pericoerulear region and nucleus locus coeruleus: anatomic and physiologic studies

Previous studies showed that the nucleus locus coeruleus (LC) receives two major afferent inputs from 1) nucleus paragigantocellularis and 2) nucleus prepositus hypoglossi, both in the rostral medulla. Recent reports suggested that the midbrain periaqueductal gray (PAG) projects to the rostromedial pericoerulear area and LC. Since the PAG is a major site for control of central antinociception, and since descending noradrenergic fibers have been implicated in pain modulation, we have investigated in detail the functional anatomy of projections from PAG to the dorsolateral pontine tegmentum. A combined anatomical and electrophysiological approach was used to assess the organization and synaptic influence of PAG on neurons in the rostromedial pericoerulear region and in LC proper. Injections of the tracer wheatgerm agglutinin conjugated to horseradish peroxidase encompassing LC proper and the rostromedial pericoerulear area retrogradely labeled neurons in PAG located lateral and ventrolateral to the cerebral aqueduct; injections restricted to LC proper did not consistently label PAG neurons. Deposits of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin into this same region of PAG labeled axons that robustly innervated the zone rostral and medial to LC. Only sparse fibers were observed in LC proper. Consistent with these results, focal electrical stimulation of LC antidromically activated only a few PAG neurons (6 of 100); all of these driven cells were located lateral and ventrolateral to the cerebral aqueduct. The majority of neurons in the rostromedial pericoerulear area were robustly activated by single pulse stimulation of PAG. In contrast, single pulse electrical stimulation of lateral PAG produced weak to moderate synaptic activation of some LC neurons; stimulation of ventrolateral PAG produced predominant inhibition of LC discharge, perhaps through recurrent collaterals subsequent to antidromic activation of neighboring LC cells. Taken together, these results indicate that PAG strongly innervates the region rostral and medial to LC, including Barrington's nucleus, but only weakly innervates LC proper. Although recent studies indicate that the dendrites of LC neurons ramify heavily and selectively in the rostromedial pericoerulear region, the results of the present physiological studies suggest that PAG preferentially targets rostromedial pericoerulear neurons rather than LC dendrites.

Periaqueductal gray spike trains recorded in frontal or horizontal mesencephalic brain slices from the rat

Spontaneous spike trains were recorded from the periaqueductal gray matter (PAG) in frontal and horizontal mesencephalic slices in order to compare spontaneous activity of the slice preparation to previous in vivo records. The firing rates resembled those recorded in vivo. They were low notwithstanding the fact that the slicing procedure removed tonic inhibitory input to the PAG. The firing rates increased caudo-rostrally, a fact that had not been reported in vivo, and were lower in frontal sections. Several different spike trains were found, described and classified according to their temporal firing patterns. The spike sequences were usually simple stochastic processes, distributed as a Gaussian function with or without stochastic deletion of spikes.

Functional activity mapping of the rat brainstem during formalin-induced noxious stimulation

Functional activity changes in 35 selected structures of the rat brainstem elicited by subcutaneous formalin injection in a forepaw were investigated by the [14C]2-deoxyglucose method in unanesthetized, freely moving animals. Experiments were initiated 2 min ("early" group) or 60 min ("late" group) after the injection. Treatment induced a significant increase of [14C]2-deoxyglucose uptake relative to controls in 17 structures of the "early" group, including portions of the bulbar, pontine and mesencephalic reticular formation, nucleus raphe magnus, median and dorsal raphe nuclei, the ventrolateral and dorsal subdivisions of the periaqueductal gray matter, deep layers of the superior colliculus and the anterior pretectal nucleus. Most changes were bilateral, with the exception of the increases observed in the nucleus reticularis paragigantocellularis and the lateral parabrachial area, which were contralateral, and the one in the mesencephalic reticular formation, which was ipsilateral to the injected paw. In pentobarbital-anesthetized rats a significant difference in metabolic activity values between formalin- and saline-injected animals was only detected at the medullary level. In the "late" unanesthetized formalin group functional activity levels were higher than controls in four structures, including the lateral reticular and paragigantocellular nuclei, contralaterally, and nucleus cuneiformis and ventrolateral periaqueductal gray matter, bilaterally. No between-groups difference was observed in visual or auditory structures. These results provide evidence for activation of several brainstem regions, which are conceivably involved in different sensory, motivational or motor circuits, during the initial phase of formalin-evoked noxious stimulation in unanesthetized animals. Functional changes blunted over time as did pain-related behavior integrated at the supraspinal level, but they persisted in some brainstem regions for which involvement in endogenous antinociceptive systems have been suggested. The mechanisms underlying these time-related changes need to be clarified.

Expression of c-fos-like protein as a marker for neuronal activity following noxious stimulation in the rat

C-fos is a proto-oncogene that is expressed within some neurons following depolarization. The protein product, c-fos protein, can be identified by immunohistochemical techniques. Therefore, c-fos expression might be used as a marker for neuronal activity throughout the neuraxis following peripheral stimulation. This study has analyzed patterns of c-fos expression in both control and anesthetized animals and in anesthetized rats subjected to various forms of peripheral stimulation. Labeled cells were counted in the spinal cord, brainstem, hypothalamus, and thalamus. Little c-fos immunoreactivity was found in control animals. Prolonged inhalational anesthesia increased the number of labeled cells at several brainstem sites. Noxious stimulation of anesthetized rats induced c-fos within the neuraxis in patterns consistent with data obtained from electrophysiological studies and in additional locations for which few direct electrophysiological data are available, such as the ventrolateral medulla, the posterior hypothalamic nucleus, and the reuniens and paraventricular thalamic nuclei. Gentle mechanical stimulation was ineffective in inducing c-fos-like protein. The data suggest that c-fos can be used as a transynaptic marker for neuronal activity following noxious stimulation. However, c-fos is expressed only in some kinds of neurons following peripheral stimulation, and it therefore may be an incomplete marker for nociresponsive activity. In addition, at least a few neurons express c-fos protein in the absence of noxious stimulation. Experiments analyzing c-fos expression must be designed with care, as both extraneous stimuli and anesthetic depth influence the results.

Dorsal raphe and nociceptive stimulations evoke convergent responses on the thalamic centralis lateralis and medial prefrontal cortex neurons

There is evidence for the existence of a descending pain suppression system, but also there are data supporting the hypothesis for the modulation of pain at higher central nervous system levels. In the present study we give evidence for a possible ascending pain modulation pathway which involves the dorsal raphe (DR), the centralis lateralis nucleus (CL) of the thalamus and the medial prefrontal cortex (PFCx). Urethane-anesthetized rats were used. Simultaneous single unit recordings were done in the CL and PFCx regions under noxious and DR stimulations. Cells responding to both types of stimuli exhibit duration responses directly related to the duration of the stimuli. Thus, from our results we conclude a DR influence upon CL and PFCx structures that are involved in the coding of nociceptive information. A possible route for an ascending pain modulation path is proposed.

Electrophysiological properties of ventromedial medulla neurons in response to noxious and non-noxious stimuli in the awake, freely moving rat: a single-unit study

The spontaneous and evoked activities of ventromedial medulla (VMM) neurons have been recorded in the chronic, awake, freely moving rat. The vast majority of neurons located at the level of the nucleus raphé magnus exhibited an irregular and variable (2-16 Hz) spontaneous activity and were activated by either cutaneous or auditory stimuli. Within this convergent neuronal class the neurons were activated by either cutaneous noxious and non-noxious inputs. The threshold for cutaneous activation was likely very low since a majority of units responded to air puffs, but the application of controlled brushing and pin-prick revealed that the VMM convergent neurons responded more for the noxious mechanical stimulation. Similar findings were found with pinch application. For both innocuous and noxious stimuli, the cutaneous receptive field was extremely extensive (almost all of the body); however, the application of the controlled brushing showed that for this innocuous stimulation, the most sensitive regions were the tail, back, snout and vibrissae and, to a lesser extent, the flank and paws. Preliminary experiments indicated that both the spontaneous and evoked activities of VMM convergent neurons were inhibited during stressful manipulations such as scruff lifting or defense reactions. These data contrast with other studies on VMM single unit recordings in anesthetized rats since the majority of these studies did not emphasize the VMM convergent group; in addition, with one exception, we did not find neurons exclusively driven by noxious inputs. Without excluding a role of the VMM convergent group in pain descending control systems, we proposed that this neuronal class is perhaps also involved in pain transmission or in general processess such as alertness and stress. Experiments are proposed in order to precisely determine the involvement of the VMM convergent neurons in alertness versus sensory discriminative aspects of nociception in the awake, freely moving rat.

Responses of rat lateral hypothalamic neurons to periaqueductal gray stimulation and nociceptive stimuli

The effects of dorsal periaqueductal gray (D-PAG) stimulation and noxious stimuli on neural activity in the lateral hypothalamic area (LHA) were investigated in 56 adult male anesthetized rats. Strong tail pinch was used as noxious stimulation. We examined 234 extracellular and 75 intracellular recordings of LHA responses to electrical stimulation of D-PAG. To determine neurotransmitter candidates, the effects of the opioid agonist, morphine, and its antagonist, naloxone were investigated by systemic administration and microelectrophoresis. Of 234 spontaneously firing LHA neurons, 70 (30%) were inhibited by D-PAG stimulation. Of these 70 neurons, 26/40 tested (65%) were glucose-sensitive, 16/19 (84%) were inhibited by morphine and 12/18 (67%) were inhibited by tail pinch. Glucose-sensitive neurons were selectively inhibited by morphine and tail pinch. Naloxone attenuated inhibitory responses to D-PAG stimulation, tail pinch and electrophoretic morphine. From intracellular recordings these polysynaptic inhibitory responses to D-PAG stimulation were considered to be inhibitory postsynaptic potentials (IPSPs) with 6.1 +/- 3.2 ms (mean +/- S.D.) latency and reversal membrane potential of about -78 mV. Since LHA glucose-sensitive neurons receive, selectively, both inhibitory opioid inputs from the D-PAG and inhibitory inputs through noxious stimulation, we suggest that D-PAG might be an intermediate site for transmission of noxious stimuli to the LHA.

Effects of thalamic parafascicular stimulation on the periaqueductal gray and adjacent reticular formation neurons. A possible contribution to pain control mechanisms

To investigate the mechanism of analgesic effect of electrical stimulation of the thalamic parafascicular nucleus (Pf), we studied modulations of neuronal activities in the periaqueductal gray (PAG) and the adjacent reticular formation (RF) in response to Pf electrical and peripheral noxious stimulations in the rat. Extracellular single unit activities were recorded from 129 neurons in the PAG and adjacent RF under light halothane anesthesia. Pf stimulation caused neuronal responses in approximately 80% of the PAG and adjacent RF neurons, and noxious stimulation in 75%, with predominant excitatory responses to either stimulation. When the responses to the two stimuli were tested in the same neurons (n = 69), 91% responding to noxious stimuli also responded to Pf stimuli, again with predominant excitatory responses to either stimulation. The PAG and adjacent RF neurons that were verified antidromically to project to the nucleus raphe magnus (NRM), showed a similar pattern of response (n = 20). These results suggest that a sizeable population of neurons in the PAG and adjacent RF receives excitatory effects from the Pf and noxious afferents, and that part of these neurons projects to the NRM, which inhibits the dorsal horn cells of the spinal cord (the descending pain suppression system). Thus, part of the mechanism of the analgesic effects of Pf stimulation is due to activation of the descending pain suppression system by exciting the PAG and adjacent RF neurons. A possible role of noxious afferents on the negative feedback to pain mediation through this descending system also has to be considered.

Analgesic effects of serotonin microinjection into nucleus raphe magnus and nucleus raphe dorsalis evaluated by the monosodium urate (MSU) tonic pain model in the rat

The effects of 5-hydroxytryptamine (5-HT) microinjection into the nucleus raphe magnus (NRM) and the nucleus raphe dorsalis (NRD) on tonic pain were studied using the monosodium urate (MSU) tonic pain model in the rat. For the NRM, 5-HT microinjection produced significant analgesic effects, which were antagonized by systemic naloxone administration and also by subsequent microinjection of naloxone into the NRM. For the NRD, systemic naloxone administration did not antagonize these analgesic effects, although 5-HT microinjection produced significant analgesic effects. Therefore, as far as tonic pain is concerned, it was suggested that neural transmission mediated by 5-HT in the NRM and NRD plays an antinociceptive action, but via different neural mechanisms.

Kainic acid microinjected into the cat raphe dorsal nucleus modulates the somatosensory evoked potentials and their cycles of excitability

Studies were made on the effect of the neuroexcitatory agent kainic acid, microinjected into raphe dorsal nucleus by glass micropipette and an air pressure system in doses ranging from 0.2 to 24.0 nmol (in volumes from 0.05 microliter to 0.47 microliter), on the somatosensory evoked potentials and their cycles of recovery (excitability) obtained from cortex (primary somatosensory and parietal associative), thalamus (ventral posterolateral nucleus and centre median nucleus), mesencephalic reticular formation and raphe dorsal nucleus. Kainic acid in doses higher than 3 nmol exerted an activating effect on the evoked potentials and their recovery cycles especially in thalamus and mesencephalic reticular formation. The analysis of these electrophysiological parameters revealed that the non-specific structures were involved to a larger extent in the activating effect of kainic acid than the specific ones. The morphological changes were not severe and were limited to a part of the raphe dorsal nucleus neurons. Our data indicate that kainic acid injected into raphe dorsal nucleus modulates (in direction of facilitation) the somatosensory evoked potentials and their cycles of excitability obtained in some brain structures. The results suggest that this nucleus is involved in the somatosensory information processing in a non-specific manner.

Firing properties of two types of nucleus raphe dorsalis neurons during the sleep-waking cycle and their responses to sensory stimuli

Spontaneous activity of the nucleus raphe dorsalis (NRD) neurons during the sleep-waking cycle and effects of sensory stimuli upon NRD neurons were studied in cats. Seventy-one neurons recorded within the NRD were classified into two groups with the use of the coefficient of variation of firing intervals during waking (W): 41 regularly firing (clock-like) and 30 irregularly firing (non-clock-like) neurons. The majority of clock-like and one-third of non-clock-like neurons showed a decrease in their firing rate during slow-wave sleep (SWS) compared with W. All neurons of both types displayed their lowest level of activity during paradoxical sleep. During the late phase of SWS, many clock-like neurons reduced their firing prior to the occurrence of pontogeniculo-occipital waves, whereas non-clock-like neurons did not show such a specific property. Clock-like neurons were totally unresponsive to nociceptive and non-nociceptive somesthetic stimuli, while about half of the non-clock-like neurons were driven by these stimuli. Half of the clock-like and one-third of the non-clock-like neurons were driven by click stimulation, and the majority of them showed an excitatory response. Some of the clock-like and non-clock-like neurons exhibited inhibitory and excitatory response to flash stimulation, respectively. The results of this experiment show that two types of neurons do exist in the NRD and suggest that they play a functionally different role in the brain.

Origins of afferents to visual suprageniculate nucleus of the cat

Small iontophoretic ejections of horseradish peroxidase (HRP) were made from recording-multibarrel micropipette assemblies in areas of the cat's suprageniculate nucleus (SGn) that contained visually responsive neurones. The sources of afferents of the SGn were determined by locating the labeled cell bodies of neurones that were presumed to send their axons to the area of the SGn containing the light-sensitive cells. The greatest concentration of labeled cell bodies was found in the granular insular cortex and the adjacent area of agranular insula. Most cells projecting to SGn from these areas were distributed in the middle and lower laminae. A second intensely labeled region was found in stratum opticum and stratum griseum intermediate of the superior colliculus. Other areas containing labeled cells that were distributed with intermediate density included the ventral thalamic nuclear complex (basal, medial, and lateral divisions), periaqueductal gray (PAG), zona incerta, and pretectal nuclei (posterior, medial, and anterior divisions). Sparsely labeled sites included the fields of Forel, substantia nigra (pars reticulata), peri-insular cortex, superior colliculus (profundum), lateral suprasylvian cortex (posterolateral lateral suprasylvian, PLLS and posteromedial lateral suprasylvian, PMLS), anterior ectosylvian cortex, thalamic reticular complex, nucleus of the optic tract, basal part of the ventromedial hypothalamic nucleus, and the pontine reticular nucleus (oralis) and adjacent reticular formation. Together with previous electrophysiological and neuroanatomical studies, the findings suggest that the SGn provides an integrating link between limbic structures and certain modalities of sensory information.

Prefrontal influences upon the midbrain: a possible route for pain modulation

After implanting stimulating electrodes in the prefrontal cortex (PFC) of adult male rats, the response to PFC stimulation was studied in widely scattered neurons of the midbrain. Subsequent testing was performed to determine if the firing rates of PFC-responsive neurons could also be altered by either a noxious stimulus (foot pinch) or the microiontophoretic administration of various neurotransmitter substances (methionine-enkephalin, ME; norepinephrine, NE; acetylcholine, ACh). Numerous mesencephalic neurons were identified which altered their spontaneous firing rates in response to PFC stimulation. Following PFC stimulation, most (71%) neurons decreased their firing rates. It was also noted that most (78%) PFC-responsive neurons were also responsive to noxious stimulation. Of these neurons, 65% altered their firing rates in a similar manner in response to both PFC and noxious stimuli. The remainder of the neurons which altered their firing rates in response to both PFC and noxious stimulation responded to the two types of stimuli in opposite manners. Of this latter type, it was found that when PFC and noxious stimuli were administered concurrently, PFC stimulation abolished the response to the noxious stimulus. It was also observed that the microiontophoretic administration of either ME or NE frequently (100% and 52% respectively) mimicked the response to PFC stimulation, thereby suggesting that these neurotransmitters may be involved in mediating the PFC influence upon neurons in the midbrain.

A reinvestigation of the analgesic effects induced by stimulation of the periaqueductal gray matter in the rat. II. Differential characteristics of the analgesia induced by ventral and dorsal PAG stimulation

This study consists of a detailed analysis of the analgesic effects induced by stimulation of the various parts of the periaqueductal gray matter (PAG) in the freely moving rat. In order to characterize the analgesia, two criteria are considered: (1) the evaluation of the degree of analgesia and behavioral side effects evoked during central stimulation; and (2) the presence of post-effects. Central stimulation (50 Hz sine waves) was delivered via bipolar concentric electrodes and analgesia was quantified by the change in the vocalization threshold induced by electrical stimulation of the tail. Within the ventral PAG, the vocalization threshold increased gradually with the intensity of the central stimulation, the degree of analgesia generally being powerful. There was no relationship between the strength of the analgesic effects and the motor disturbances also produced by stimulation of this region. Antinociceptive effects generally disappeared when the stimulation ceased. Only when the intensity of the stimulation was strong enough to induce very powerful analgesic effects were post-stimulation analgesic effects noticed. Within the dorsal and dorsolateral PAG as well as in the ventral region just surrounding the aqueduct, analgesia appeared suddenly, was generally less pronounced and was always concomitant with strong aversive reactions. In contrast with the analgesia from the ventral PAG, marked post-effects were observed. These latter characteristics were also obtained from stimulation of regions located outside the PAG (colliculi, intercollicular commissure and tectum adjacent to the dorsolateral PAG) although these zones were not extensively studied. By consideration of various data in the literature, it is concluded from this study, which clearly distinguishes stimulation-produced-analgesia (SPA) from ventral PAG versus dorsal PAG, that analgesia induced from this midbrain area involves at least two different neuronal substrates. Whilst the ventral PAG seems to be more preferentially involved in pain modulation, the authenticity of 'analgesia' triggered by stimulation of aversive regions (which are widely spread over the PAG) is questioned and proposals to explain the simultaneous appearance of analgesic effects and aversion are considered.

Limbic influence on the periaqueductal gray: a single unit study in the awake squirrel monkey

Single neurons of the periaqueductal gray (PAG) were studied during electrical stimulation of the amygdala and hippocampus. Fifty-one percent (34/67) of the units sampled throughout the rostrocaudal extent of the PAG were found to have a limbic influence. PAG neurons were characterized by low spontaneous firing rates (means = 4.94 spikes/sec). Units responded to basolateral amygdala stimulation primarily with short duration excitatory responses having a mean latency of 30 ms (range: 13.3-110 ms). Responses to corticomedial and lateral amygdala stimulation produced different patterns of activation including complex excitatory and inhibitory sequences. Only 10 units (15%) sampled in PAG responded to hippocampal stimulation with excitatory or tonic-inhibitory responses. The majority of responsive units (8) were to anterior hippocampal stimulation (latency range: = 20-75 ms). High frequency (9 Hz) basolateral amygdala stimulation recruited responses with increases in the probability of firing and a decrease in initial latency and latency variability.

Diencephalic connections of the raphé nuclei of the rat brainstem: an anatomical study with reference to the somatosensory system

The present study was undertaken to analyse in detail the connections of the various raphé nuclei with thalamic structures. Micropipettes filled with an aqueous solution of wheat-germ agglutinin conjugated to horseradish peroxidase were used to produce small iontophoretic deposits restricted to the various raphé nuclei in male Sprague Dawley albino rats. Tetramethyl benzidine was used as a chromogen to reveal both fiber terminals anterogradely labelled and retrogradely filled neurons. A detailed discussion of the possible cases of artefactual labelling using this technique is given. The present study confirms the results obtained previously in the cat that indicate that the various raphé nuclei project to different areas of the diencephalon. Related to the somatosensory system, the B3 area (nucleus raphé magnus) projects to the nucleus submedius and anterior intralaminar nuclei known to receive spinothalamic inputs, but not to the ventrobasal complex. The distribution of afferents from this nucleus suggests an innervation primarily of thalamic structures involved in the somatosensory system. The nucleus raphé medianus projects to the ventrobasal complex and the nucleus submedius , but the fact that its projections are widespread, including all thalamic sensory "relay" nuclei and the entire nucleus reticularis thalami, suggests that it could participate in a "nonspecific" system of control of different sensory modalities. The nucleus raphé dorsalis generally does not project to the thalamic nuclei believed to be involved in the somatosensory system.

Pontomedullary raphe neurons: intracellular responses to central and peripheral electrical stimulation

The responses of pontomedullary raphe neurons to electrical stimulation of the medullary reticular formation (MRF) and the mesencephalic ventral periaqueductal gray region (PAG) were studied using intracellular methods in chloralose-anesthetized cats. Single shock stimulation of PAG at the level of the trochelear nucleus evoked short latency, monosynaptic excitatory postsynaptic potentials (EPSPs) in antidromically identified raphe-spinal neurons. Similar large EPSPs were produced by medullary reticular stimulation of either side. The large majority of raphe-spinal neurons responded to sciatic nerve shock, and most responded to tooth pulp or forepaw shock as well; these responses were always bilateral. The responses of cells that could not be antidromically invaded from spinal cord were similar to those of raphe-spinal neurons, but tended to be more variable. Intracellular injection of horseradish peroxidase into electrophysiologically characterized cells revealed that most recordings were made from large and medium sized raphe neurons. These findings are discussed in the context of a potential role for pontomedullary raphe neurons in nociception.

Single unit studies of identified bulbospinal serotonergic units

Non-serotonergic bulbospinal neurons were identified by conduction velocities greater than 6 m/s. These units were found to fire at rates from 0 to 22 Hz, to respond to sensory stimuli with either excitation or inhibition, and to have unremarkable spike shapes. In iontophoretic experiments, both excitation and inhibition were observed in response to acetylcholine, norepinephrine and serotonin. Serotonergic bulbospinal neurons were identified by their conduction velocities below 6 m/s. These neurons, (which have been shown to be destroyed by 5,7-dihydroxytryptamine), exist as two groups: a a slower-conducting group with conduction velocities below 1.2 m/s, and a faster-conducting group with conduction velocities between 2 and 6 m/s. The neurons of the faster-conducting group were found to be similar to the non-serotonergic group in their firing, spike shapes and responses to sensory stimuli; while the units of the slower-conducting group were consistently found to fire between 0.03 and 6 Hz, to respond to sensory stimuli only with excitation, and to have distinctive spike shapes. Despite these differences, both groups of serotonergic units were found to be consistently inhibited by ACh, NE and 5-HT. In contrast to reports of serotonergic neurons in the midbrain, these units were not generally found to be inhibited by i.v. LSD. It is concluded that the serotonergic neurons of the medullary raphe are distinct both from the non-serotonergic neurons, and from serotonergic neurons in other parts of the brain.

Basic neurobiologic mechanisms of pain and analgesia

Information about tissue damaging, subjectively painful stimuli is transmitted to the central nervous system by specific receptors. Histologically, they are supposed to be free nerve endings connected to the spinal cord by thin myelinated (A delta or group III) and nonmyelinated (C or group IV) fibers. Nociceptive information is transferred to secondary cells mainly in the surface layers (lamina I and II) and the neck (lamina V) of the dorsal horn; it then ascends the anterolateral funiculus contralaterally in axons of the spinothalamic tract. One portion of this tract ends in the ventroposterior and posterior thalamus. It is assumed that it mediates the discriminative component of a pain sensation. The emotional-affective component of a pain sensation is supposed to be produced by that portion of the spinothalamic tract that terminates in the intralaminar nuclei and by the spinoreticular tract. The existence of a cortical pain center has not yet been proved, nor is it clear where in the cerebrum pain is consciously felt. The descending pain inhibiting systems, originating in the brain stem, may block the transfer of nociceptive information at a spinal level, probably using enkephalin as a transmitter. It is probable that they mediate morphine-induced analgesia. Lesions of tissue release endogenous substances, such as serotonin and certain prostaglandins, which sensitize receptors. The analgesic effect of nonsteroidal anti-inflammatory drugs, such as aspirin, can be explained by the inhibition of prostaglandin synthesis and the desensitization of nociceptors.

Unmyelinated nociceptive units in two skin areas of the rat

Responses of unmyelinated afferent fibres were investigated in two skin nerves of Sprague-Dawley rats. The units were tested as to whether they responded to mechanical probing of the skin, to controlled radiant heat stimulation, and/or to cooling of the skin (to less than or equal to 5 degrees C). Ninety-six units in a n. saphenaus and 129 units in a n. coccygealis were studied, which were identified as afferents by means of the above-mentioned stimuli. In both nerves mechano- and heat-sensitive units (CMHs) were most frequent (56% in n. saphenaus and 74% in n. coccygealis). There were, however, significantly more purely mechanoceptive units (CMs) in n. saphenaus than in n. coccygealis (30% vs 5%). In contrast cold-sensitive units (CCs) were more frequent in n. coccygealis. They constituted 25% of the afferent C-fibres in this nerve. When testing heat sensitivity of CMHs with ramp stimuli raising the temperature to 55 degrees C at a rate of 0.8 degree C/s, heat thresholds had a wide range of between 30 and 55 degrees C. Since CMHs with low heat thresholds had the highest discharge rates and the greatest dynamic sensitivity in the range of noxious temperatures, they most probably also had nociceptive functions. It was shown that the low heat thresholds of some CMHs were not due to sensitization by preceding heat stimuli. It is argued that low-frequency discharges (less than or equal to 2 Hz) observed in some nociceptive CMHs of the rat at non-noxious temperatures are insignificant for nociception. When comparing discharges during a first ramp heat stimulus to 50 degrees C (rise time 1 degree C/s) with those during a second stimulus of identical time course delivered 5-10 min later, 44% of the CMHs were sensitized, 24% were desensitized and the remainder were not clearly influenced. We did not find a significant correlation between initial heat thresholds and tendency to sensitization or desensitization.

Dorsal column input to inferior raphe centralis neurons

In anesthetized decerebellate cats with additional decerebration or decortication and with one of two types of cervical spinal lesions which either eliminated the dorsal half of the spinal cord or spared the dorsal funiculi, peripheral electric and localized natural stimuli activated neurons in the inferior raphe nuclear complex. Medial lemniscal as well as direct dorsal funicular stimulation was also effective. The majority of raphe neurons activated by stimulation of the dorsal funiculi were also discharged by ventrolateral funicular stimulation.

Enhancing GABAergic transmission reverses the aversive state in rats induced by electrical stimulation of the periaqueductal grey region

In a proposed rat model for anxiety (electrical stimulation of the periaqueductal grey region), progabide (a GABA agonist) and diazepam both increased the latency to escape to a safe compartment and also the current needed to induce the escape response (escape threshold). Furthermore, the effects of progabide and diazepam were greater than additive in their actions on the escape response as when given together in normally subliminal doses, the combination exerted a marked anti-aversive effect. These actions of the drugs alone or in combination could not be explained by non-specific motor effects. Blockade of GABA receptors by bicuculline greatly reduced or abolished the action of progabide and diazepam (single administration). Sodium valproate, which indirectly augments GABAergic transmission, also increased the escape latency and escape threshold whereas, in contrast, diphenylhydantoin accentuated the aversive effects of stimulation of the periaqueductal grey. Haloperidol increased the escape latency and threshold but not other signs of distress following central stimulation (vocalization, jumping) which were effectively blocked by progabide and diazepam. The action of haloperidol was completely explicable by an interference with motor mechanisms. These results are interpreted as an indication that GABA agonists have an anti-aversive action in this proposed rat model for anxiety and, furthermore, that GABA receptors at least partially mediate the actions of benzodiazepines in this model.