Craniovascular application of capsaicin activates nociceptive thalamic neurones in the cat.

In cats anaesthetized with alpha-chloralose and urethane, extracellular recordings were made in ventrobasal thalamus from cells responding to electrical stimulation of the superior sagittal sinus and middle meningeal artery. Capsaicin, but not vehicle, evoked an increase in the firing rate of nociceptive cells (5 of 6 wide dynamic range and the only nociceptive specific cell). Non-nociceptive cells did not respond to either capsaicin or vehicle. Cells with long latencies to electrical stimulation were excited by capsaicin but cells with short latencies were not. Capsaicin-responsive cells were found in the ventroposteromedial nucleus and the medial nucleus of the posterior complex and mostly had receptive fields involving the first trigeminal division.     

Central sensitization in thalamic nociceptive neurons induced by mustard oil application to rat molar tooth pulp

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.     

Stimulation of cranial vessels excites nociceptive neurones in several thalamic nuclei of the cat

Summary Extracellular recordings were made in the thalamus of cats anaesthetized with chloralose and urethane following electrical, mechanical and chemical stimulation of the superior sagittal sinus or middle meningeal artery. Facial receptive fields were looked for using electrical and mechanical stimuli. The locations of fifty-six cells were verified histologically. Twenty six cells were located in the ventroposteromedial nucleus (VPM) and six in its ventral periphery (VPMvp). All units in VPM had facial receptive fields, usually involving the first trigeminal division. Cells with nociceptive receptive fields or responding to the craniovascular application of bradykinin were often found in the periphery or “shell” region of VPM. Other craniovascular nociceptive cells were found in VPMvp, in the posterior group and in the intralaminar complex. This study shows that craniovascular afferents in the cat project to several thalamic nuclei and implicate VPM especially in craniovascular nociception.     

The potentiation of analgesic activity of paracetamol plus morphine involves the serotonergic system in rat brain

OBJECTIVE AND DESIGN: We investigated the antinociceptive effect of subactive doses of paracetamol and morphine, given in combination. MATERIAL AND TREATMENT: Male Wistar rats were injected with paracetamol (50 or 100 mg/kg i.p.) and morphine (2, 3 or 5 mg/kg s.c.) 10 min later and subjected to algesimetric tests 20 min thereafter. METHODS: Pain threshold was evaluated in the hot-plate and formalin tests. 5-HT2 receptor binding capacity and 5-HT and 5-HIAA levels were measured in cortical and pontine areas of the brain by means of radioligand binding technique and by HPLC, respectively. Statistical analysis was done using Student-Neuman-Keul's test and 2 x 2 factorial analysis. RESULTS: Only when given in combination, paracetamol (100 mg/kg) and morphine (2 and 3 mg/kg) were able to evoke an antinociceptive effect in both tests associated with an increase in 5-HT levels and a decrease in 5-HT2 receptors in the cortex. These effects were prevented by i.p. pretreatment with naloxone (1 mg/kg i.p.). CONCLUSIONS: Subactive doses of paracetamol and morphine exert an analgesic effect when given in combination in the rat and indicate an involvement of both serotonergic and opiatergic systems.     

The activity of primate ventrolateral thalamic neurones during motor adaptation

Three monkeys were trained to perform stereotyped wrist movements to track a target (phase 1). Changing the gain between the wrist movement and visual display required the monkey to adapt its wrist movement. This adaptation consisted of progressive reduction of movement amplitude over a number of trials (phase 2) until a stereotyped movement accommodating the new gain was learned (phase 3). The experiment's aim was to investigate whether cerebellar thalamic neuronal discharge (ND) changed during motor adaptation and whether this change was related to scaling of kinematic parameters or movement error. Extracellular single-cell recordings were made from "wrist-related" neurones in the cerebellar thalamus (59) and the nucleus ventro-posterior lateralis caudalis (VPLc) (37) of each monkey while they performed the movement paradigm. Neurones were selected for further analysis (37/59 cerebellar thalamic and 23/37 VPLc) if phase-1 movements were stereotyped and motor adaptation occurred in phase 2 (according to statistical definitions). When the gain initially changed, there were positional errors in the form of overshoot. Adaptation to the new gain was achieved by a variety of strategies, including modification of the amplitude of kinematic parameters and positional error in addition to reduction of time to peak velocity and movement time. During stereotyped movements, most cerebellar thalamic neurones fired before movement onset and before VPLc neurones. During adaptation, this order of onset of firing was reversed, and cerebellar thalamic neurones discharged after VPLc neurones and close to the onset of movement. During motor adaptation, the mean rate of phasic ND rose in a large proportion of cerebellar thalamic and VPLc neurones, and the proportion of cerebellar thalamic neurones that encoded a signal about positional error and movement amplitude also increased. In addition, there is set-related activity in the discharge of a majority of cerebellar thalamic and VPLc neurones. This does not appear to be specifically related to motor adaptation, but is related to the movement amplitude. We have discussed the role of the cerebello-thalamo-cortical pathway in error detection in the light of the similarities between discharge patterns of cerebellar thalamic and VPLc neurones. We speculate that, when learned movements are performed, the discharge of cerebellar thalamic neurones occurs before movement, perhaps representing an efference copy of the intended movement. During adaptation, this signal is gated out, and later-arriving peripheral afferent input dominates cerebellar thalamic discharge.     

Intrathecally administered apomorphine or LY171555 reduces nociceptive responses recorded from ventrobasal thalamic neurones in urethane anaesthetised rats

The effects of intrathecal injections of the D2 agonists apomorphine and LY171555 were studied on responses of ventrobasal neurones responding to noxious stimulation in the urethane anaesthetised rat. At a dose of 75 micrograms/kg the D2 agonists had little effect on a total of 18 thalamic neurones. A higher dose (100 micrograms/kg) produced a reversible reduction of thalamic nociceptive responses on 15 occasions. A transient fall in blood pressure usually accompanied the intrathecal injection of the agonists. These results suggest that dopamine can modulate the transmission of nociceptive information to the thalamus and also suggests a role in the processing of autonomic function.     

Electrophysiological differences between nociceptive and non-nociceptive dorsal root ganglion neurones in the rat in vivo

Intracellular recordings were made from 1022 somatic lumbar dorsal root ganglion (DRG) neurones in anaesthetized adult rats, classified from dorsal root conduction velocities (CVs) as C, Aδ or Aα/β, and according to their responses to mechanical and thermal stimuli as nociceptive (including high-threshold mechanoreceptive (HTM) units), and non-nociceptive (including low-threshold mechanoreceptive (LTM) and cooling units). Of these, 463 met electrophysiological criteria for analysis of action potentials (APs) evoked by dorsal root stimulation. These included 47 C-, 71 Aδ- and 102 Aα/β-nociceptive, 10 C-, 8 Aδ− and 178 Aα/β-LTM, 18 C- and 19 Aδ- unresponsive, and 4 C-cooling units. Medians of AP and afterhyperpolarization (AHP) durations and AP overshoots were significantly greater for nociceptive than LTM units in all CV groups. AP overshoots and AHP durations were similar in nociceptors of all CV groups whereas AP durations were greater in slowly conducting, especially C-fibre, nociceptors. C-cooling units had faster CVs, smaller AP overshoots and shorter AP durations than C-HTM units. A subgroup of Aα/β-HTM, moderate pressure units, had faster CVs and AP kinetics than other Aα/β-HTM units. Of the Aα/β-LTM units, muscle spindle afferents had the fastest CV and AP kinetics, while rapidly adapting cutaneous units had the slowest AP kinetics. AP variables in unresponsive and nociceptive units were similar in both C- and Aδ-fibre CV groups. The ability of fibres to follow rapid stimulus trains (fibre maximum following frequency) was correlated with CV but not sensory modality. These findings indicate both the usefulness and limitations of using electrophysiological criteria for identifying neurones acutely in vitro as nociceptive.     

Depression by nicotine of pain-related nociceptive activity in the rat thalamus and spinal cord

Summary To assess the possible role of nicotinergic control in nociception and pain, experiments were carried out on rats under urethane anesthesia in which nociceptive activity was elicited by electrical stimulation of afferent C fibers in the sural nerve and recorded from single neurones in the thalamus and from ascending axons in the spinal cord. Intravenous administration of nicotine (0.01–0.5 mg/kg) depressed the nociceptive activity evoked in the thalamus and the spinal cord in a dose-dependent way. The maximum depression in thalamus and spinal cord was 40% of control activity and obtained at a dose of 0.025 mg/kg. Likewise, local administration of nicotine to the spinal cord by intrathecal injection (5, 10, and 30 g) reduced the nociceptive activity evoked in neurones of the thalamus and in ascending axons of the spinal cord, the maximum of the depression being 40% of control activity. The depressant effect of nicotine (0.05 mg/kg) was reduced by mecamylamine (1 mg/kg) but not by atropine (0.5 mg/kg). It is concluded that the antinociceptive effect of nicotine is due to a specific action of the alcaloid at the spinal level.     

Selective depression of nociceptive responses of dorsal horn neurones by SNC 80 in a perfused hindquarter preparation of adult mouse.

Detailed electrophysiological characterisation of spinal opioid receptors in the mouse has been limited due to various technical difficulties. In this study, extracellular single unit recordings were made from dorsal horn neurones in a perfused spinal cord with attached trunk-hindquarter to investigate the role of delta-opioid receptor in mediating nociceptive and non-nociceptive transmission in mouse. Noxious electrical shock, pinch and heat stimuli evoked a mean response of 20.8+/-2.5 (n=10, P<0.005), 30.1+/-5.4 (n=58, P<0.005) and 40.9+/-6.3 (n=29, P<0.005) spikes per stimulus respectively. In 5 of 22 cells, repetitive noxious electrical stimuli applied to the hindpaw for 20 s produced a progressive increase in spike number, the phenomenon known as 'wind-up' and/or hyperactivity. When the selective delta-opioid receptor agonist (+)-4-[(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80) was perfused for 8-10 min, these evoked nociceptive responses were reversibly depressed. SNC 80 (2 microM) depressed the nociceptive responses evoked by electrical shock, pinch and heat by 74.0+/-13.7% (n=8, P<0.01), 66.5+/-16.6% (n=10, P<0.01) and 74.1+/-17.0% (n=10, P<0.01) respectively. The maximum depression by 5 microM SNC 80 was 92.6+/-6.8% (n=3). SNC 80 at 5 microM also completely abolished the wind-up and/or hypersensitivity (n=5). The depressant effects of SNC 80 on the nociceptive responses were completely blocked by 10 microM naloxone (n=5) and 3 microM 17-(cyclopropylmethyl)-6,7-dehydro-4,5 alpha-epoxy-14 beta-ethoxy-5 beta-methylindolo [2',3':6',7'] morphinan-3-ol hydrochloride (HS 378, n=8), a novel highly selective delta-opioid receptor antagonist. Interestingly, HS 378 (3 microM) itself potentiated the background activity and evoked responses to pinch and heat by 151.8+/-38.4% (P<0.05, n=8), 34.2+/-6.1% (P<0.01, n=7) and 45.5+/-11.8% (P<0.05, n=5) respectively. In contrast, the responses of non-nociceptive dorsal horn neurones were not inhibited by SNC 80 at a dose of up to 10 microM (n=5).These data demonstrate that delta-opioid receptor modulate nociceptive, but not non-nociceptive, transmission in spinal dorsal horn neurones of the adult mouse. The potentiation of neuronal activity by HS 378 may reflect an autoregulatory role of the endogenous delta-opioid in nociceptive transmission in mouse.     

Effect of 5,7-dihydroxytryptamine on nociceptive sensitivity and on the analgesic effect of morphine

Department of Pharmacology, Academician I. P. Pavlov First Leningrad Medical Institute. Department of Morphology, Research Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Leningrad. (Presented by Academician of the Academy of Medical Sciences of the USSR A. V. Val'dman). Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 106, No. 9, pp. 311–314, September, 1988.     

Signalling pathways involved in the sensitisation of mouse nociceptive neurones by nerve growth factor

Nerve growth factor (NGF) causes a rapid sensitisation of nociceptive sensory neurones to painful thermal stimuli owing to an action on the heat and capsaicin receptor TRPV1 (formerly known as VR1). We have developed a new technique to study this rapid sensitisation of TRPV1 by monitoring the effects of NGF on the increase in intracellular calcium concentration ([Ca²⁺]_i) following exposure to capsaicin. Brief applications of capsaicin caused a rise in [Ca²⁺]_i, and NGF was found to enhance this rise in 37 % of capsaicin-responsive neurones within 2 min. Pathways responsible for transducing the sensitisation of TRPV1 by TrkA, the NGF receptor, were characterised by observing the effects of inhibitors of key members of NGF-activated second messenger signalling cascades. Specific inhibitors of the ras/MEK (mitogen-activated protein and extracellular signal-regulated kinases) pathway and of phospholipase C did not abolish the NGF-induced sensitisation, but wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase (PI3K), totally abolished the effect of NGF. Pharmacological blockade of protein kinase C (PKC) or calcium-calmodulin-dependent protein kinase II (CaMK II) activation also prevented NGF-induced sensitisation, while blockade of protein kinase A (PKA) was without effect. These data indicate that the crucial early pathway activated by NGF involves PI3K, while PKC and CaMK II are also involved, probably at subsequent stages of the NGF-activated signalling pathway.     

Central respiratory modulation of barosensitive neurones in rat caudal ventrolateral medulla

The sympathetic nerves that maintain blood pressure are modulated by the central respiratory generator. Neurones in the rostral ventrolateral medulla (RVLM) that drive this sympathetic nerve activity (SNA) also display central respiratory drive (CRD)-related activity, suggesting integration of respiratory and cardiovascular regulatory systems within the brainstem. Whether CRD-related activity in the RVLM is due to direct inputs from central respiratory neurones or modulation of cardiovascular-related neurones that influence the RVLM is not known. The caudal ventrolateral medulla (CVLM) contains GABAergic neurones that tonically inhibit presympathetic RVLM neurones and are essential for the production of numerous cardiovascular reflexes. The present study sought to determine whether cardiovascular-related GABAergic neurones in the CVLM display CRD-related activity. The firing patterns of individual barosensitive CVLM neurones were examined in relation to phrenic nerve activity in chloralose-anaesthetized, ventilated, neuromuscularly blocked, vagotomized rats. Histograms of phrenic-triggered CVLM neuronal activity showed that all baro-activated CVLM neurones displayed one of four patterns of CRD-related activity: (i) inspiratory peak ( n = 15), (ii) inspiratory depression ( n = 15), (iii) inspiratory peak with postinspiratory depression ( n = 10), and (iv) postinspiratory peak ( n = 9). A subset of each type of CVLM neurone was identified as GABAergic by individually filling the recorded neurone with biotinamide and observing expression of GAD67 mRNA by in situ hybridization ( n = 10). These data suggest that the activity of GABAergic neurones in the CVLM is regulated by cardiovascular and respiratory inputs, and baro-activated GABAergic CVLM neurones may contribute to CRD-related modulation of presympathetic RVLM neurones and SNA.     

Dorsal column inhibition of nociceptive thalamic cells mediated by gamma-aminobutyric acid mechanisms in the cat

Cells in posterior parts of the cat thalamus were investigated. Responses in single units excited by electrical stimulation in the lateral funiculus (LF), the dorsal column nucleus (DCN) or the canine tooth pulp (TP) were analysed. All cells had a spontaneous resting activity which could be increased by extracellular iontophoretic application of DL-homocysteic acid (DLH) and decreased by gamma-aminobutyric acid (GABA). No effect on the spontaneous firing rate was observed following iontophoresis of the selective GABA-antagonists, picrotoxin (GABA-A receptor antagonist) or saclofen (GABA-B receptor antagonist). However, the decreased firing following GABA application was partially blocked by picrotoxin but not by saclofen. A phasic inhibition induced by DCN stimulation in nociceptive thalamic cells is indicated since simultaneous administration of picrotoxin increased the evoked response. This type of inhibitory mechanism could not be detected following LF or TP stimulation. The extracellular activity evoked by electrical stimulation of LF or TP was significantly depressed by preceding electrical stimulation in the DCN. This inhibition was reversed by simultaneous administration of picrotoxin, indicating an involvement of GABA-A receptors. The reversal of the DCN-induced depression of the late responses following LF stimulation occurred after application of saclofen. It is suggested that this effect is partly mediated via GABA-B receptors. Results from the present study indicate an interaction in the thalamus between presumed low-threshold (DCN) and presumed nociceptive afferents (LF and TP) similar to that previously described in the spinal cord.     

The effect of medial thalamic lesions on emotionality,activity, and discrimination learning in the rat

Three experiments were conducted to examine the effects of medial thalamic lesions on emotionality, open-field activity, and discrimination learning. The first study revealed that rats with large medial thalamic lesions are significantly less responsive to capture and to handling than are sham-operated rats. The second study failed to produce any significant differences in open-field activity. A third study revealed that rats with large medial thalamic lesions, and with both small anterior and posterior medial thalamic lesions are impaired on the acquistion of an appetitive visual-tactile discrimination task, with the deficit being greatest in the large lesion group. It was concluded that the medial thalamus plays a role in the neural mediation of certain emotional responses and is critical in the acquisition of complex appetitive tasks.     

Enhancement of spontaneous synaptic activity in rat Purkinje neurones by ATP during development

The establishment of functional synaptic connections and activity is a pivotal process in the development of neuronal networks. We have studied the synaptic activity in the developing rat cerebellum, and the contribution mediated by purinergic receptors. The mean frequency of the spontaneous postsynaptic currents (sPSCs) recorded with the whole-cell patch-clamp technique from Purkinje neurones in acute brain slices at room temperature, increased fourfold from 4.4 ± 0.8 Hz at postnatal day 9/10 ( n = 23) to 17.8 ± 1.6 Hz at postnatal day 17–20 (p17–p20; n = 113; P < 0.01). ATP, which increased the frequency of sPSCs by up to 100%  (EC₅₀= 18 μ m )  in the third postnatal week, started to modulate the synaptic activity during the second postnatal week, which was determined by three processes: (1) the appearance of functional ATP receptors during p10–p12, (2) the enhancement of the sPSC frequency by endogenous ATP release becoming apparent after inhibition of ecto-ATPases by 6- N , N -diethyl-β,γ-dibromomethylene- d -adenosine-5-triphosphate (ARL67156; 50 μ m ) at p11–p12, and (3) with tonic stimulation of purinoceptors at p14, as revealed by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 μ m ). ATP had a similar effect at later stages (p24–p27) and at 35°C. Our results suggest that endogenous release of ATP starts to enhance the synaptic activity in Purkinje neurones by the end of the second postnatal week.     

Cytochrome oxidase activity in the rat retina following unilateral thalamic lesion

The present study has investigated the cytochrome oxidase (CO) activity in retinas of normal rats and rats which received central lesions at birth or young adult stage. The results show that a thalamic lesion which injured the retinal ganglion cell axons in young adult rats led to severe loss of CO activity in the ganglion, inner and outer plexiform layers in the retina contralateral to the lesion as compared to those of normal rats. In contrast, distinct CO-reactive bands and cells were clearly observed in corresponding laminae in retinas in which almost the entire population of retinal ganglion cells was eliminated by a neonatal thalamic lesion. These results indicate that retinal ganglion cells contribute significantly to the CO activity observed in the inner retinal laminae under normal but not under abnormal conditions, and suggest that considerable changes in the activity of the remaining neurons and possibly reorganization of neural circuitry within the retina in rats which received neonatal lesions has taken place, as revealed by CO histochemistry.