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

Behavioral, metabolic and functional brain changes in a rat model of chronic neuropathic pain: a longitudinal MRI study

Peripheral neuropathy often manifests clinically with symptoms of mechanical and cold allodynia. However, the neuroplastic changes associated with peripheral neuropathic pain and the onset and progression of allodynic symptoms remain unclear. Here, we used a chronic neuropathic pain model (spared nerve injury; SNI) to examine functional and metabolic brain changes associated with the development and maintenance of mechanical and cold hypersensitivity, the latter which we assessed both behaviorally and during a novel acetone application paradigm using functional MRI (fMRI). Female Sprague-Dawley rats underwent SNI (n=7) or sham (n=5) surgery to the left hindpaw. Rats were anesthetized and scanned using a 7 T MRI scanner 1 week prior to (pre-injury) and 4 (early/subchronic) and 20 weeks (late/chronic) post-injury. Functional scans were acquired during acetone application to the left hindpaw. (1)H magnetic resonance spectroscopy was also performed to assess SNI-induced metabolic changes in the anterior cingulate cortex (ACC) pre- and 4 weeks post-injury. Mechanical and cold sensitivity, as well as anxiety-like behaviors, were assessed 2 weeks pre-injury, and 2, 5, 9, 14, and 19 weeks post-injury. Stimulus-evoked brain responses (acetone application to the left hindpaw) were analyzed across the pre- and post-injury time points. In response to acetone application during fMRI, SNI rats showed widespread and functionally diverse changes within pain-related brain regions including somatosensory and cingulate cortices and subcortically within the thalamus and the periaqueductal gray. These functional brain changes temporally coincided with early and sustained increases in both mechanical and cold sensitivity. SNI rats also showed increased glutamate within the ACC that correlated with behavioral measures of cold hypersensitivity. Together, our findings suggest that extensive functional reorganization within pain-related brain regions may underlie the development and chronification of allodynic-like behaviors.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

Norepinephrinergic afferents and cytology of the macaque monkey midline, mediodorsal, and intralaminar thalamic nuclei

The midline and intralaminar thalamic nuclei (MITN), locus coeruleus (LC) and cingulate cortex contain nociceptive neurons. The MITN that project to cingulate cortex have a prominent innervation by norepinephrinergic axons primarily originating from the LC. The hypothesis explored in this study is that MITN neurons that project to cingulate cortex receive a disproportionately high LC input that may modulate nociceptive afferent flow into the forebrain. Ten cynomolgus monkeys were evaluated for dopamine-beta hydroxylase (DBH) immunohistochemistry, and nuclei with moderate or high DBH activity were analyzed for intermediate neurofilament proteins, calbindin (CB), and calretinin (CR). Sections of all but DBH were thionin counterstained to assure precise localization in the mediodorsal and MITN, and cytoarchitecture was analyzed with neuron-specific nuclear binding protein. Moderate-high levels of DBH-immunoreactive (ir) axons were generally associated with high densities of CB-ir and CR-ir neurons and low levels of neurofilament proteins. The paraventricular, superior centrolateral, limitans and central nuclei had relatively high and evenly distributed DBH, the magnocellular mediodorsal and paracentral nuclei had moderate DBH-ir, and other nuclei had an even and low level of activity. Some nuclei also have heterogeneities in DBH-ir that raised questions of functional segregation. The anterior multiformis part of the mediodorsal nucleus but not middle and caudal levels had high DBH activity. The posterior parafascicular nucleus (Pf) was heterogeneous with the lateral part having little DBH activity, while its medial division had most DBH-ir axons and its multiformis part had only a small number. These findings suggest that the LC may regulate nociceptive processing in the thalamus. The well established role of cingulate cortex in premotor functions and the projections of Pf and other MITN to the limbic striatum suggests a specific role in mediating motor outflow for the LC-innervated nuclei of the MITN.

Motor cortex stimulation in neuropathic pain. Correlations between analgesic effect and hemodynamic changes in the brain. A PET study

To investigate brain mechanisms whereby electrical stimulation of the motor cortex (MCS) may induce pain relief in patients with neuropathic pain, cerebral blood flow (CBF) changes were studied using H2O PET in 19 consecutive patients treated with MCS for refractory neuropathic pain. Patients were studied in three conditions, (a) before MCS (Baseline, stimulator stopped 4 weeks before), (b) during a 35-min period of MCS and (c) during a 75-min period after MCS had been discontinued (OFF). Compared to Baseline, turning on the stimulator was associated with CBF increase in the contralateral (anterior) midcingulate cortex (aMCC, BA24 and 32) and in the dorso-lateral prefrontal (BA10) cortices. The most important changes of CBF were observed in the 75 min after discontinuation of MCS (OFF). This post-stimulation period was associated with CBF increases in a large set of cortical and subcortical regions (from posterior MCC (pMCC) to pregenual (pg) ACC, orbitofrontal cortex, putamen, thalami, posterior cingulate and prefrontal areas) and in the brainstem (mesencephalon/periaqueductal grey (PAG) and pons). CBF changes in the post-stimulation period correlated with pain relief. Functional connectivity analysis showed significant correlation between pgACC and PAG, basal ganglia, and lower pons activities, supporting the activation of descending ACC-to-PAG connections. MCS may act in part through descending (top-down) inhibitory controls that involve prefrontal, orbitofrontal and ACC as well as basal ganglia, thalamus and brainstem. These hemodynamic changes are lengthened and might therefore underlie the long-lasting clinical effects that largely outlast the actual stimulation periods.

Differential modulation of nociceptive neural responses in medial and lateral pain pathways by peripheral electrical stimulation: a multichannel recording study

It is well accepted that peripheral electrical stimulation (PES) can produce an analgesic effect in patients with acute and chronic pain. However, the neural basis underlying stimulation-induced analgesia remains unclear. In the present study, we examined the pain-related neural activity modified by peripheral stimulation in rats. The stimulation frequency of pulses applied to needle electrodes in the hindlimb was 2 Hz alternating with 100 Hz, with 0.6 ms pulse width for 2 Hz and 0.2 ms for 100 Hz. The intensity of the stimulation was increased stepwise from 1 to 3 mA with each 1-mA step lasting for 10 min. The nociceptive neural and behavioral responses were examined immediately after the termination of stimulation. Using a multiple-channel recording technique, we simultaneously recorded the activity of many single neurons located in the primary somatosensory and anterior cingulate cortex (ACC), as well as the ventral posterior and medial dorsal thalamus in behaving rats. Our results showed that peripheral electrical stimulation significantly reduced the nociceptive responses in ventroposterior thalamus and somatosensory cortex, indicating an inhibition of nociceptive processing. In contrast, the analgesic stimulation produced a significant increase in mediodorsal thalamus while a less significant decrease in cingulate cortex, reflecting a complicated effect associated with combined antinociceptive activation and nociceptive suppression. These results support the idea that peripheral electrical stimulation can ultimately alter the pain perception by specifically inhibiting the nociceptive transmission in the sensory pathway while mobilizing the antinociceptive action in the affective pathway, thus to produce pain relief.

Efferent projections of ProTRH neurons in the ventrolateral periaqueductal gray

Our previous study has shown that prothyrotropin-releasing hormone (proTRH) gene expression is increased in the ventrolateral periaqueductal gray (PAG) neurons following precipitated morphine withdrawal and continues to be activated even 24 h after withdrawal. We have hypothesized that peptide products of proTRH may participate in the recovery from morphine withdrawal. To identify neuroanatomical substrates of the proposed action of proTRH-derived peptides originating from the ventrolateral PAG proTRH neurons, projections of these neurons were investigated by a series of anterograde and retrograde tract-tracing experiments. First, Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in the ventrolateral PAG in Sprague-Dawley rats. Following transport of the tracer, simultaneous immunolabeling for PHA-L and proTRH peptides was performed and mapped in discrete brain regions. PHA-L-immunoreactive (IR) fibers showing preterminal and terminal-like arborization that contained proTRH were identified in the dorsolateral and lateral PAG, deep layer of superior colliculus (CS), parafascicular nucleus (PF), ventromedial zona incerta (ZI) and at the border of the locus coeruleus (LC) and Barrington's nucleus. Scattered double-labeled fibers were present in the lateral septal nucleus, ventromedial preoptic nucleus, lateral hypothalamus, perifornical area and in the periventricular region at the diencephalon/midbrain junction. The retrogradely transported marker, cholera toxin beta-subunit (CTb) was then injected in the dorsolateral PAG, CS, PF, ZI and medial to the LC. Double-labeled perikarya for both CTb and proTRH in the ventrolateral PAG were found for each region injected with CTb, corroborating the findings by the anterograde tracing experiment. These studies demonstrate that proTRH neurons in the ventrolateral PAG project to several regions of the brain that are involved in autonomic and behavioral regulation and thereby, may function as an integrating center to coordinate responses to opiate withdrawal.

Extracellular signal-regulated kinases 1 and 2 phosphorylated neurons in the tele- and diencephalon of rat after visceral pain stimulation: an immunocytochemical study

We aimed at verifying whether extracellular signal-regulated kinases (erks) 1 and 2 are activated, i.e. phosphorylated, in forebrain neurons after visceral pain stimulation (VPS). Ether and urethane anaesthetized rats received an intraperitoneal injection of acetic acid or were left untreated (ECT, UCT). After 2 h the animals were perfused. Paraffin embedded brain sections immunoreacted with an antibody selective for the phosphorylated erks. The light microscope analysis revealed only a few labelled neurons in ECT, while in UCT, positive cells were detected. In VPS rats (VPSR) positive cells were mainly distributed in regions, such as the hypothalamic anterior and thalamic paraventricular midline nuclei, amygdala, hippocampal and parahippocampal, insular and perirhinal cortex, involved in nociception and/or visceral activities. Our data suggest an association of erks activation with the emotional component of nociception; moreover, they show that erks activation is not suppressed by anaesthesia.

Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and midline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in discrete regions of the PAG, and axonal labeling was examined in the thalamus. PHA-L was also placed into the dorsal raphe nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nuclei-lateral parafascicular, medial parafascicular, central lateral (CL), paracentral (PC), or central medial nucleus-or one of the midline thalamic nuclei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The distribution of CTb labeled neurons in the PAG was then mapped. All PAG regions (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to most of the other intralaminar and midline thalamic nuclei. PAG inputs to the VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventroposterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nociceptive-, and behavior-related forebrain circuits associated with defense and emotional responses.

Amygdaloid-thalamic interactions mediate the antinociceptive action of morphine microinjected into the periaqueductal gray

The bilateral administration of the serotonin receptor antagonist methysergide (2.5 microg, 5 microg, and 10 microg) into either the central nucleus of the amygdala (ACe) or nucleus parafascicularis thalami (nPf) produced dose-dependent inhibition of the antinociceptive action of ventrolateral periaqueductal gray (vPAG)-administered morphine. Unilateral administration of these doses of methysergide into either the ACe or nPf had no effect on morphine-induced antinociception. However, the combined unilateral administration of these doses of methysergide into the ACe and nPf produced dose-dependent inhibition of morphine antinociception that was identical to that observed after its bilateral administration into either site. This latter finding is interpreted as evidence that a functional interaction between the ACe and nPf supports the antinociceptive action of morphine administered into the vPAG.

Neuropeptide FF, pain and analgesia

Neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and the octadecapeptide neuropeptide AF (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe -NH2) were isolated from bovine brain, and were initially characterized as anti-opioid peptides. They can oppose the acute effects of opioids and an increase in their brain concentrations may be responsible for the development of tolerance and dependence to opioids. Numerous experiments suggest a possible neuromodulatory role for neuropeptide FF. A precursor protein has been identified, in particular in human brain. Neuropeptide FF immunoreactive neurons are present only in the medial hypothalamus, and the nucleus of the solitary tract, and in the spinal cord in the superficial layers of the dorsal horn and areas around the central canal. Depolarization induces a Ca2+-dependent release of neuropeptide FF immunoreactivity from the spinal cord. Neuropeptide FF acts through stimulation of its own receptors and high densities of specific binding sites are found in regions related either to sensory input and visceral functions or to the processing of nociceptive messages. In both isolated dorsal root ganglion neurons and CA1 pyramidal neurons of the hippocampus, neuropeptide FF has little effect of its own but reverses the effects of mu-opioid receptor agonists. In agreement with the hypothesized anti-opioid role of neuropeptide FF, supraspinal injection lowers the nociceptive threshold and reverses morphine-induced analgesia in rats. Furthermore, immunoneutralization of neuropeptide FF increases endogenous and exogenous opioid-induced analgesia. Similarly, microinfusion of neuropeptide FF or neuropeptide FF analogs into the nucleus raphe dorsalis, the parafascicular nucleus, or the ventral tegmental area has no effect on the nociceptive threshold but inhibits the analgesia induced by co-injected morphine. Furthermore, infusion of neuropeptide FF into the parafascicular nucleus or the nucleus raphe dorsalis reverses the analgesic effect of morphine infused into the nucleus raphe dorsalis or the parafascicular nucleus, respectively, demonstrating remote interactions between neuropeptide FF and opioid systems. By contrast, intrathecal administration of neuropeptide FF analogs induces a long lasting, opioid-dependent analgesia and potentiates the analgesic effect of morphine. Analgesic effects of neuropeptide FF after supraspinal injection could also be observed, for example during nighttime. In young mice, (1DMe)Y8Famide (D.Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), a neuropeptide FF analog, increases delta-opioid receptor-mediated analgesia. These findings indicate that neuropeptide FF constitutes a neuromodulatory neuronal system interacting with opioid systems, and should be taken into account as a participant of the homeostatic process controlling the transmission of nociceptive information.

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.

Antinociceptive activity of calcitonin and central cholinergic system: behavioural and neurochemical analyses

Behavioural and neurochemical analyses were carried out to investigate the relationship between the antinociceptive activity of porcine calcitonin (pCT) and central cholinergic system in mice and rats. Behavioural studies revealed that the antinociceptive activity of pCT encapsulated in sulphatide-containing liposomes injected intravenously into mice was significantly inhibited by atropine sulphate, but not by atropine methylnitrate, and potentiated by physostigmine, but not by neostigmine. Neurochemical studies using rat brain synaptosomes showed that pCT stimulated synaptosomal sodium-dependent high-affinity choline uptake, which was found to be closely associated with acetylcholine (ACh) synthesis (50-60%). This effect was concentration-dependent. In addition, pCT elicited a biphasic effect on ACh release from synaptosomes with an initial brief period of stimulation and subsequent prolonged inhibition. This stimulation was not affected by atropine sulphate, but markedly reduced by incubation in the presence of diltiazem or in a calcium-free medium, indicating that the modulation of ACh release by the peptide may be mediated by calcium fluxes across the synaptosomal membrane independent of cholinergic receptor activation. However, pCT does not affect the activity of synaptosomal acetylcholinesterase. Therefore, the behavioural study in vivo with the neurochemical analysis in vitro suggests that the central cholinergic system may be involved in the antinociceptive activity of calcitonin.

Mu-opiate receptor binding is up-regulated in mice selectively bred for high stress-induced analgesia

Pain perception and sensitivity to opiate analgesics strongly depend on genotype. Mice selectively bred for high (HA) and low (LA) swim stress-induced analgesia display markedly divergent morphine analgesia, a difference that appears to be determined by one or at the most two major genes. In an attempt to provide candidate genes mediating the supranormal analgesia displayed by HA mice, we performed mu-opiate receptor binding on 27th generation HA, LA, and control (C) mice using [3H]naloxone. HA mice were found to have significantly higher whole-brain receptor density (Bmax) than LA mice in whole brain homogenates; no significant difference in affinity (Kd) was observed. Quantitative autoradiography confirmed the line difference in whole-brain receptor binding. In the medial thalamus, a brain area implicated in ascending pathways of pain inhibition, HA mice were found to display significantly higher [3H]naloxone binding than C mice (a 64% increase) and LA mice (a 128% increase). No significant line differences were observed in any other brain locus. Thalamic mu receptors may therefore play an important role in a central 'volume control' mechanism of pain inhibition, and underlie individual differences in the responses of mice to opiate analgesic drugs.

Serotonergic projections from the midbrain periaqueductal gray and nucleus raphe dorsalis to the nucleus parafascicularis of the thalamus

By a double-labeling method combining the retrograde tracing of horseradish peroxidase and the immunocytochemical technique, serotonin-like immunoreactive neurons in the midbrain periaqueductal gray (PAG) and nucleus raphe dorsalis (DR) of the rat were observed to send projection fibers to the nucleus parafascicularis of the thalamus bilaterally with an ipsilateral dominance. These serotonin-containing projecting neurons were observed mainly at the middle-caudal levels of the ventrolateral subdivision of the PAG and less at the middle-rostral levels of the DR.

Neurophysiological, pharmacological and behavioral evidence for medial thalamic mediation of cocaine-induced dopaminergic analgesia

These studies examined the effects of cocaine on thalamic neurons that respond maximally either to noxious or to innocuous somatic stimulation. Cocaine attenuated high intensity electrically-evoked nociceptive responses of all 25 units studied in the parafascicular and central lateral nuclei of the medial thalamus. A dose of 1 mg/kg intravenously (i.v.) suppressed medial thalamic unit discharge evoked by both noxious somatic stimulation (49.4 +/- 8.7% of control response) and spinal cord stimulation (76.2 +/- 6.6% of control response). The effect of cocaine on unit responses to noxious somatic stimulation was dose-related in the range of 0.3-3.5 mg/kg i.v. and was attenuated by eticlopride, a D-2 selective dopamine receptor antagonist. Morphine also suppressed noxious somatic evoked responses of medial thalamic units in a dose-dependent manner. Units in the lateral (ventrobasal) thalamus (n = 4) that responded only to innocuous stimuli were not affected by cocaine at doses up to 3.5 mg/kg i.v. Ibotenic acid lesions in the parafascicular nucleus of the medial thalamus attenuated the analgesic effect of cocaine in the formalin test. These results suggest that both cocaine and the parafascicular nucleus interact with dopaminergic mechanisms that attenuate nociceptive spinal projections to the medial thalamus.

Serotonin and substance P afferents to parafascicular and central medial nuclei

Potent analgesia is elicited by electrical stimulation of the periaqueductal gray (PAG), dorsal raphe nucleus (DRN) and intralaminar thalamus. Horseradish peroxidase conjugated wheat germ agglutinin (HRP-WGA) was stereotaxically pressure injected into the parafascicular (PF) or central medial (CM) nucleus to identify brainstem afferents to the intralaminar thalamus. WGA-immunoreactive (-ir) neurons were identified in the DRN, PAG and lateral dorsal tegmentum (LDTg) after PF and CM injections. Many retrogradely labeled cells in the DRN and ventral PAG were also serotonin-ir, and a portion of WGA-ir cells in the LDTg were substance P-ir. These results substantiate previous studies implicating the intralaminar thalamus and periaqueductal region, as well as serotonin and substance P, in antinociception.

Deep brain stimulation: a review of basic research and clinical studies

Deep brain stimulation for pain control in humans was first used almost 30 years ago and has continued to receive considerable attention. Despite the large number of clinical reports describing pain relief, numerous studies have indicated that the results of these procedures vary considerably. In addition, many neurosurgeons find the procedures unpredictable, and considerable disagreement still exists regarding important issues related to the technique itself. This review gives an historical overview of the relevant basic and clinical literature and provides a critical examination of the clinical efficacy, choice of stimulation sites, parameters of stimulation, and effects on experimental pain. Finally, we give suggestions for future research that could more definitively determine the usefulness of deep brain stimulation for pain control.

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.

Dorsal raphe stimulation, 5-HT and morphine microiontophoresis effects on noxious and nonnoxious identified neurons in the medial thalamus of the rat

In single cell experiments, the characterization of the responses of medial thalamic neurons to noxious and nonnoxious stimulation was made to examine the effects of two substances involved in pain, morphine and 5-HT, and the action of one pain suppressor mechanism, dorsal raphe stimulation. Single cell activity was recorded in urethane anesthetized rats. Tail pinch and tail immersion in hot water were used as nociceptive stimuli. Skin strokes, air puffs and hair brushing were used as nonnociceptive stimuli. Morphine, 5-HT microiontophoresis and dorsal raphe stimulation were performed in all the recorded units. Fifty-eight percent from 61 medial thalamic recorded units responded both to noxious and nonnoxious stimulation; whereas only 18% and 24.6% of the units responded exclusively to noxious and nonnoxious stimulation, respectively. The noxious responding units were located in the most posterior portions of the medial thalamus. Dorsal raphe stimulation and 5-HT ejection prevented the excitation elicited by noxious input. Morphine ejection prevented both the noxious and nonnoxious input in medial thalamus, in a different population as compared to dorsal raphe stimulation or 5-HT ejection. These findings support the existence of a pain ascending mechanism mediated by an opioid-serotonergic interaction in the medial thalamus of the rat.

Light and electron microscopic study of galanin-immunoreactive nerve fibers in the rat posterior thalamus

Light and electron microscopic immunocytochemistry was used to study certain cell groups in the posteromedial thalamus which contain galanin-immunoreactive (GAL-IR) fibers. The nuclei subparafascicularis pars parvicellularis (SPFpc) and parafascicularis (PF) contain a dense network of GAL-IR fibers which form basketlike structures around unstained cells. The periventricular area also contains numerous GAL-IR fibers and these also occasionally form basketlike structures. The GAL-IR terminal fields continue caudally in the mesodiencephalic junction and merge with other GAL-IR fibers in the dorsal aspects of the substantia nigra and around the dorsolateral tip of the medial lemniscus. Ultrastructural analysis of the GAL-IR basketlike structures revealed that GAL-IR terminals make numerous synapses with the cell bodies and proximal dendrites of SPFpc neurones. These results suggest that the activity of cells in the SPFpc and PF nuclei may be strongly influenced by galanin-containing nerve fibers probably originating in the spinal cord.

Dissociated mesencephalic responses to medial and ventral thalamic nuclei stimulation in rats. Relationship to analgesic mechanisms

To investigate the mechanism of analgesia noted with electrical stimulation of the thalamic sensory relay nucleus and medial thalamus, modulations of neuronal activities in the periaqueductal gray matter (PAG) were studied in response to electrical stimulations of the ventroposterolateral nucleus (VPL) and parafascicular nucleus (Pf) and to peripheral noxious stimulations in rats. Extracellular single-unit activities were recorded from 102 neurons in the PAG and the adjacent area in animals under halothane anesthesia. A large population (83%) of the PAG neurons reacted to Pf stimulations with a predominantly excitatory response, whereas smaller numbers (43%) responded to VPL stimulations. There was a significant correlation between the response characteristics of Pf and noxious stimulations, whereas no correlation was found between VPL and noxious stimulations. The PAG neurons that were verified antidromically to project to the nucleus raphe magnus showed a similar pattern of response. The excitatory response to the Pf stimulation was partially attenuated by systemic administration of naloxone, whereas that to the VPL stimulation was not affected. These results suggest that part of the analgesic mechanism of medial thalamus stimulation involves activation of the descending pain suppression system by exciting the PAG neurons through the opioid system, while the analgesia produced by sensory relay nucleus stimulation does not involve the PAG neurons or the opioid system.