The STING-IRF3 pathway is involved in lipotoxic injury of pancreatic β cells in type 2 diabetes

Lipotoxic injury of pancreatic β cells is an important pathological feature in type 2 diabetes mellitus (T2DM). Stimulator of interferon genes (STING) can recognize its own DNA leaked into the cytoplasm from damaged mitochondria or nuclei of the host cell, thus activating its downstream factor interferon regulatory factor 3 (IRF3), causing inflammation and apoptosis. The STING-IRF3 signaling pathway is closely related to glycolipid metabolism, but its relationship with the lipotoxicity of pancreatic β cells has rarely been reported. Here, we investigated the role of the STING-IRF3 signaling pathway in lipotoxicity-induced inflammation, apoptosis, and dysfunction of pancreatic β cells. We examined the activation of STING and IRF3 in islets of db/db mice and identified the role of the STING-IRF3 signaling pathway in palmitic acid (PA)-induced lipotoxic injury of INS-1, a rat insulinoma cell line. STING and phosphorylated IRF3 including downstream interferon-β were upregulated in islets of db/db mice and PA-induced INS-1 cells. Gene silencing of STING or IRF3 ameliorated PA-induced INS-1 cell inflammation and apoptosis, and reversed impaired insulin synthesis. Additionally, PA induced downregulation of the phosphoinositide 3-kinase-AKT signaling pathway, and impaired high glucose-stimulated insulin secretion was reversed after knockdown of STING or IRF3. Our results suggest that activation of the STING-IRF3 pathway triggers inflammation and apoptosis of pancreatic β cells, leading to β-cell damage and dysfunction. Hence, inhibition of this signaling pathway may represent a novel approach for β-cell protection in T2DM.

Activation of the STING-IRF3 pathway promotes hepatocyte inflammation, apoptosis and induces metabolic disorders in nonalcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a common result of obesity and metabolic syndrome. Hepatocyte injury and metabolic disorders are hallmarks of NAFLD. Stimulator of interferon genes (STING) and its downstream factor interferon regulatory factor 3 (IRF3) trigger inflammatory reaction in response to the presence of cytosolic DNA. STING has recently been shown to play an important role in early alcoholic liver disease. However, little is known about the role of STING-IRF3 pathway in hepatocyte injury. Here, we aimed to examine the effect of STING-IRF3 pathway on hepatocyte metabolism, inflammation and apoptosis.

METHODS

We examined the activation of the STING-IRF3 pathway, a high-fat diet (HFD)-induced obese mouse model, and determined the role of this pathway in a free fatty acid (FFA)-induced hepatocyte inflammatory response, injury, and dysfunction in L-O2 human liver cells.

RESULTS

STING and IRF3 were upregulated in livers of HFD-fed mice and in FFA-induced L-O2 cells. Knocking down either STING or IRF3 led to a significant reduction in FFA-induced hepatic inflammation and apoptosis, as evidenced by modulation of the nuclear factor κB (NF-κB) signaling pathway, inflammatory cytokines, and apoptotic signaling. Additionally, STING/IRF3 knockdown enhanced glycogen storage and alleviated lipid accumulation, which were found to be associated with increased expression of hepatic enzymes in glycolysis and lipid catabolism, and attenuated expression of hepatic enzymes in gluconeogenesis and lipid synthesis.

CONCLUSIONS

Our results suggest that the STING-IRF3 pathway promotes hepatocyte injury and dysfunction by inducing inflammation and apoptosis and by disturbing glucose and lipid metabolism. This pathway may be a novel therapeutic target for preventing NAFLD development and progression.

Functional circuitry of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish

The valvula cerebelli of the mormyrid electric fish is a useful site for the study of cerebellar function. The valvula forms a part of the electrosensory-electromotor system of this fish, a system that offers many possibilities for the study of sensory-motor integration. The valvula also has a number of histological features not present in mammals which facilitate investigation of cerebellar circuitry and its plasticity. This initial study characterizes the basic physiology and pharmacology of cells in the valvula using an in vitro slice preparation. Intrinsic properties and synaptic responses of Purkinje cells and other cell types were examined. We found that Purkinje cells fire a small narrow Na(+) spike and a large broad Ca(2+) spike, generated in the axon initial segment and dendritic-soma region, respectively. Purkinje cells respond to parallel fiber inputs with graded excitatory postsynaptic potentials (EPSPs) and to climbing fiber inputs with all-or-none EPSPs. Efferent cells, Golgi cells, and deep stellate cells all fire a single type of large narrow spike and respond only to parallel fiber inputs. Both parallel fiber and climbing fiber responses in Purkinje cells appear to be entirely mediated by AMPA-type glutamate receptors, whereas parallel fiber responses in efferent cells and stellate cells include AMPA and NMDA components. In addition, a strong synaptic inhibition was uncovered in both Purkinje cells and efferent cells in response to the focal stimulation of parallel fibers. Dual cell recordings indicate that deep stellate cells contribute at least partially to this inhibition. We conclude that despite its unique histology, the local functional circuitry of the mormyrid valvula cerebelli is largely similar to that of the mammalian cerebellum. Thus, what is learned concerning the functioning of the mormyrid valvula cerebelli may be expected to be informative about cerebellar function in general.

Amyloid beta-protein fragment 31-35 forms ion channels in membrane patches excised from rat hippocampal neurons

Inside-out membrane patches excised from rat hippocampal neurons were used to test if ion channels could be formed by fragment 31-35 of amyloid beta-protein. The results showed: (1) after application of fragment 31-35 of amyloid beta-protein (5 microM) to either the inner or outer side of the patches, spontaneous currents could be recorded from those patches that had previously been 'silent'; (2) the fragment 31-35-induced conductance was cation-selective with a permeability ratio of P(Cs)/P(Cl)=23; (3) different levels of conductance, ranging from 25 to 500 pS, could be recorded in different patches, and in some cases, different conductances and spontaneous transitions among them could be recorded in a single patch; and (4) application of ZnCl(2) (1 mM) to the inner side of the patches reversibly blocked the newly formed channel activity; a similar effect was observed after application of CdCl(2) (1 mM). These results show that fragment 31-35 of amyloid beta-protein can insert into membrane patches from both sides and form cation-selective, Zn(2+)- and Cd(2+)-sensitive ion channels. It is proposed that fragment 31-35 in amyloid beta-protein might be the shortest active sequence known to date to form ion channels across neuronal membranes.

Opiate-like substances mediate norepinephrine-induced but not serotonin-induced antinociception at spinal level: reevaluation by an electrophysiological model of formalin test in rats

After subcutaneous injection of formalin (5%, 50 microl) into a hindpaw of rats, biphasic excitatory nociceptive discharges were recorded extracellularly in thalamic parafascicular neurons. Intrathecal (i.t.) administration of either norepinephrine (NE. 6 nmol, 10 microl) or serotonin (5-HT, 120 nmol, 10 microl) prior to the second phase significantly inhibited the second phase of the formalin-induced parafascicular nociceptive discharges. Intrathecal naloxone (Nal, 50 nmol, 10 microl) did not show any effect on the parafascicular nociceptive discharges. However, when i.t. Nal was given 5 min before NE, Nal prevented the NE antinociceptive effect. Pre-administration of Nal before 5-HT did not affect the antinociceptive effects of 5-HT on the second phase of nociceptive discharges. These results indicate that opiate-like substances are involved in the mediation of NE-induced antinociception. It is suggested that endogenous NE and 5-HT released from brainstem descending terminals at the spinal level carry out their antinociceptive actions differently.

Suppression of large conductance Ca2+-activated K+ channels by amyloid beta-protein fragment 31-35 in membrane patches excised from hippocampal neurons

To clarify the shortest essential active sequence in amyloid beta-protein (AbetaP) responsible for affecting neuronal electrophysiological properties, the effects of fragments 31-35 and 25-35 of AbetaP on the large conductance Ca(2+)-activated potassium (BK) channels were investigated in the "inside-out" membrane patches excised from hippocampal neurons of rats. After application of AbetaP 3l-35 (5 micromol/L, n=10), the mean P(o) and open frequency of BK channels decreased by 85.8+/-l3.5 percent;percent; (P<0.01) and 72.1+/-22.8 percent; (P<0.01), respectively, and the mean open time decreased by 41.l+/-l8.5 percent; (P<0.0l), while the mean current amplitude was not significantly affected (P>0.05). Application of AbetaP 25-35 (5 micromol/L) also induced a decrease of 85.5+/-22.l percent; (P<0.0l) in mean P(o) and of 5l.4+/-18.3 percent; (P<0.05) in mean open time within l~3 min after application. These results suggest that the functional alteration in BK channels elicited by AbetaP fragments may play an important role in the mechanisms underlying AbetaP neurotoxicity, and AbetaP 31-35 may be the shortest active sequence in AbetaP responsible for affecting the electrophysiological properties of neurons.

Long-term potentiation in hippocampus of rats is enhanced by endogenous acetylcholine in a way that is independent of N-methyl-D-aspartate receptors

By using extracellular recordings of field potential, the exact pathway by which the endogenous ACh influencing the induction of long-term potentiation (LTP) in CA1 area was analysed in slices of rat hippocampus. The results showed that: (1) the application of (-) huperzine A, an AChE inhibitor extracted from Chinese herb Qian Ceng Ta (Huperzia Serrata), could enhance the induction of LTP, while this drug showed little effect on the second components of multiple population spikes that were recorded in Mg(2+)-free medium and had proven to be N-methyl-D-aspartate (NMDA) receptor-mediated response; and (2) scopolamine, a muscarinic receptor antagonist, could significantly suppressed the induction of LTP, while most of the suppressive effect of scopolamine was blocked when slices were pretreated by bicuculline, a gamma-aminobutyric acid (GABA(A)) receptor antagonist. These results suggest that endogenous ACh potentiates the induction of LTP through the inhibition of GABAergic interneurons that modulate pyramidal neurons, but not through the activation of NMDA receptors located on pyramidal neurons.

Norepinephrine modulates single hypothalamic arcuate neurons via alpha(1)and beta adrenergic receptors

The effects of norepinephrine (NE) on the electrophysiological activities of single hypothalamic arcuate neurons were studied using extracellular recording of 385 neurons from 169 brain slices in rats. The results showed that: (1) of 236 neurons selected randomly and tested with NE application, 137 (58.0%) were excited, 67 (28.4%) were inhibited, and 32 (13.6%) failed to respond; (2) substitution of low Ca(2+)-high Mg(2+) artificial cerebrospinal fluid (ACSF) for normal ACSF abolished the NE-induced inhibitory effect but failed to abolish the excitatory effect; (3) both the NE-induced excitatory and inhibitory effects were antagonized partly by phentolamine, prazosin, and propranolol but not by yohimbine; (4) naloxone and glibenclamide, a blocker of adenosine triphosphate-sensitive (K(ATP)) channels, blocked the NE-induced inhibitory effect; and (5) neurons that were inhibited by NE were also inhibited by morphine and cromakalim, an agonist of K(ATP) channels, and moreover, the morphine-induced inhibitory effect could be blocked by glibenclamide, while the cromakalim-induced inhibitory effect was not blocked by naloxone. These results imply that: (a) NE excites arcuate neurons through a mechanism that is insensitive to lowering the extracellular Ca(2+) suggesting a direct postsynaptic response through alpha(1)- and beta-adrenergic receptors, while NE inhibits cells through at least an inhibitory interneuron in arcuate and so is dependent on a Ca(2+)-sensitive presynaptic release mechanism; and (b) the inhibitory interneuron may be opioidergic, being excited first through alpha(1)- and beta-adrenergic receptors, after which the released opioids inhibit the neurons being recorded with an involvement of activation of K(ATP) channels. This possibility needs to be substantiated in much more detail.

Neurokinin A, calcitonin gene-related peptide, and dynorphin A (1-8) in spinal dorsal horn contribute to descending inhibition evoked by nociceptive afferent pathways: an immunocytochemical study

Immunocytochemical technique was used to compare the contents of neurokinin A (NKA), calcitonin gene-related peptide (CGRP), and dynorphin A (1-8) (DynA) on two sides of the lumbar dorsal horn of rats in which the unilateral thoracic dorsalateral funiculus (DLF) was transected while formalin (0.2 ml, 0.5%) was injected equally into two hindpaws. The results showed that all the NKA-like, CGRP-like, and DynA (1-8)-like immunoreactivities were significantly lower in the superficial laminae of the dorsal horn on the side ipsilateral to the lesioned DLF than that on the side with intact DLF. This implies that peripheral noxious inputs activate the supraspinal descending inhibitory systems which in turn modulate the transmission of noxious message at the spinal level by changing the release of related neuropeptides.

Suppressive action produced by beta-amyloid peptide fragment 31-35 on long-term potentiation in rat hippocampus is N-methyl-D-aspartate receptor-independent: it's offset by (-)huperzine A

Extracellular recordings of field potential from CA1 region of rat hippocampal slices were used to observe the effects of a shorter synthetic fragment of beta-amyloid peptide (A beta31-35) on the induction of long-term potentiation (LTP) and the action of (-)huperzine A, a potent acetylcholinesterase (AChE) inhibitor on these processes was also observed. The results showed that: (1) 0.1 microM A beta31-35 suppressed the induction of LTP in a similar mode as the longer fragment A beta25-35, did, while they did not change the amplitude of the baseline population spike (PS); (2) when PSs were recorded separately in Mg2+-free medium, which unveils the N-methyl-D-aspartate (NMDA)-mediated responses, both A beta31-35 and A beta25-35 showed little effect on the components of multiple PSs; (3) two concentrations of 0.1 microM or 1.0 microM (-)huperzine A showed no effects on the PS amplitude while the latter could enhance the LTP and (4) co-administration of (-)huperzine A with 0.1 microM concentration could block most of the suppressive action induced by A beta31-35 or A beta25-35 upon the LTP. The results suggest that the shorter fragment A beta31-35, is long enough to suppress the induction of LTP and these two fragments might suppress the induction of LTP through a NMDA receptor-independent pathway that involves cholinergic terminals in hippocampus.

Peripheral nitric oxide contributes to both formalin- and NMDA-induced activation of nociceptors: An immunocytochemical study in rats

Nociceptive c-fos expressions in the dorsal horn following intraplantar injection of two kinds of algogenic agents combined with different doses of nitric oxide (NO) synthase inhibitor (N(omega)-nitro-L-arginine methyl ester (L-NAME)) or of NO donor (L-arginine) were used to explore if NO was involved in the activation of peripheral nociceptors. The results showed that: 1) combined injections of L-NAME with formalin into the plantar aspect of one hindpaw of normal rats elicited a dose-dependent suppression of c-fos expression as compared to that induced by formalin alone; 2) combined injections of L-arginine with formalin elicited considerable enhancement of c-fos expression when the dosages of L-arginine were less than 20 micromol, while it elicited marked suppression of c-fos expression when the dosages were in the range from 50 to 100 micromol; and 3) combined injection of L-NAME with NMDA, a selective agonist for NMDA receptors, into the hindpaw could also inhibit the NMDA-induced c-fos expression in the spinal dorsal horn. These results suggest that endogenously generated concentrations of nitric oxide may enhance the initiation of nociceptive inputs of peripheral nociceptors following local injection of formalin or NMDA.

Endogenous adenosine involved in the mediation of spinal antinociception produced by stimulating locus coeruleus

The focus of this study was to investigate whether spinal adenosine is involved in mediating descending nociceptive modulation by the locus coeruleus (LC). Nociceptive evoked responses in parafascicular (PF) neurons were studied before and after electrical stimulation of the LC as well as before and after intrathecal (i.t.) administration of phentolamine (Ph) or aminophylline (Aph), an adenosine receptor antagonist, and 5'ethylcarboxamidoadenosine (NECA), an adenosine agonist. The main results were as follows: (1) the nociceptive evoked responses recorded in PF neurons were suppressed by LC stimulation; (2) pretreatment with i.t., Ph (40 nmol) reversed the LC effects, i.e., the suppressive effect of LC stimulation on the PF nociceptive evoked responses was reversed in the presence of Ph; (3) smaller doses of i.t. Aph (120 nmol) blocked only the suppressive effect produced by LC stimulation, while larger doses (240 nmol) reversed the LC stimulation, i.e., the LC stimulation exerted a facilatatory effect; and (4) i.t. application of NECA, an adenosine agonist, suppressed the nociceptive discharges in PF neurons. The results suggest that spinal adenosine may be involved in the mediation of the spinal antinociceptive effect produced by LC stimulation.

Beta-amyloid peptide fragment 31-35 induces apoptosis in cultured cortical neurons

A synthetic fragment 31-35 of beta-amyloid peptide was used in cultured cortical neurons to examine whether this smaller sequence could trigger apoptotic degeneration in vitro by using morphological, biochemical and flow-cytometric examinations. The results showed that: (i) neurons treated with fragment 31-35 of beta-amyloid peptide exhibited membrane blebbing, compaction of nuclear chromatin, nuclear shrinkage and nuclear fragmentation; (ii) a typical DNA ladder was revealed by agarose gel electrophoresis following fragment 31-35 of beta-amyloid peptide exposure; (iii) the internucleosome DNA fragmentation was also detected by flow-cytometric examination following fragment 31-35 of beta-amyloid peptide exposure; and (iv) the DNA fragmentation induced by fragment 31-35 of beta-amyloid peptide in the above two examinations could be blocked by co-treatment with aurintricarboxylic acid or actinomycin D. It is suggested that fragment 31-35 of the beta-amyloid peptide may be a shorter sequence of beta-amyloid peptide responsible for triggering an apoptotic process in cultured neurons.

Interrelations of opioids with monoamines in descending inhibition of nociceptive transmission at the spinal level: an immunocytochemical study

This study was designed to reexamine a previous proposal of whether the opioid-like substances (OLS) being acting mainly as an intrinsic spinal mediator in the descending inhibition of nociception of the bulbospinally projecting NE-ergic, and/or 5-HT-ergic terminals in the dorsal horn by using an immunocytochemical method. The effects of intrathecal (i.t.) phentolamine (Ph), cyproheptadine (Cyp), and naloxone (Nal), administered separately or coadministered by two of them, on the expression of Fos-like-immunoreactive (FLI) neurons were observed on both sides of the lumbar dorsal horn of rats, in which equal volumes of formalin were injected into two hindpaws and the ipsilateral dorsolateral funiculus (DLF) was transected at the thoracic level antecedently. The results showed: (1) when rats were pretreated with i.t. saline, the number of nociceptive FLI neurons was significantly lowered 44% (p<0.01) on the side of the lumbar dorsal horn with intact DLF compared to the opposite side with sectioned DLF; (2) when rats were separately pretreated with i.t. Ph, Cyp and Nal, the reduction of FLI neurons on the DLF-intact side were decreased by 27% (p<0.01), 21% (p<0.01), and 25% (p<0.01), respectively; (3) when rats were pretreated with combined i.t. Ph+Cyp, the reduction on the intact side was eliminated almost completely (4%); (4) when rats were pretreated with combined i.t. Ph+Nal, the reduction on the intact side was 21% (p<0.01); and (5) when rats were pretreated with i.t. Cyp+Nal, the reduction on the intact side was 9.1%. These results suggest that: (1) nearly all the suppressive action exerted by the DLF-descending fibers are produced by the release of either NE or 5-HT as neurotransmitters at the spinal level; (2) most of the opioid-like substances act as an intrinsic spinal mediator mainly for the descending NE-ergic, but in a lesser extent for the 5-HT-ergic terminals in the dorsal horn circuitry; and (3) some OLS-ergic interneurons may only be activated by local nociceptive input.

Decreased expression of N-methyl-D-aspartate (NMDA) receptors in rat dorsal root ganglion following complete Freund's adjuvant-induced inflammation: an immunocytochemical study for NMDA NR1 subunit

The changes in expression of NMDA NR1, an essential functional subunit of N-methyl-D-aspartate (NMDA) receptors, were examined in the rat lumbar dorsal root ganglion (DRG) by immunocytochemical technique following injection of complete Freund's adjuvant (CFA) into the unilateral hindpaw. The results showed that there appeared a significant and several days-lasting down-regulation in expression of NMDA NR1 in the lumbar DRG following CFA injection, and especially, this change mainly appeared in the small and medium sized DRG neurons. It is suggested that a long-lasting flow of nociceptive inputs in nociceptors will elicit a decreased expression of NMDA receptors on primary afferent neurons and this plastic change would act as a factor leading towards the hyposensitization for nociception at the nociceptor level.

Involvement of endogenous opioids and ATP-sensitive potassium channels in the mediation of apomorphine-induced antinociception at the spinal level: a study using EMG planimetry of flexor reflex in rats

The effects of intrathecally (i.t.) administered naloxone or glibenclamide, a blocker of adenosine triphosphate-sensitive potassium (KATP) channels, on the antinociception produced by i.t. apomorphine were observed by an integrated electromyogram measurement of hindlimb flexor reflex in lightly pentobarbital-anesthetized rats. The results showed that i.t. apomorphine produced a significant and dose-dependent antinociception and that the antinociception produced by i.t. apomorphine could be blocked dose dependently by i.t. naloxone or glibenclamide. The results suggest that endogenous opioids and ATP-sensitive potassium channels might be sequentially involved in the mediation of apomorphine-induced antinociception at the spinal level.

[A quantitative study on the synaptic ultrastructural alterations in visual cortex in the maintenance of LTP]

The ultrastructure of synapses three hours after formation of long-term potentiation (LTP) was examined in the local microslices of visual cortical brain slices of 18-20 d rats. Slices without potentiating stimulation which were similarly incubated served as controls. The following structural changes were examined using a graph analyzer: (1) synaptic cleft width; (2) thickness of the postsynaptic densities(PSD); (3) length of the active zones; and (4) curvature of the synaptic interface. The number of synapses of different types in layer II/III of visual cortex was quantified by double-blind scoring procedures. The various counts were converted to the number of synapses per unit volume using stereological quantitation method. Analysis of variance was used for statistical evaluation. Our results suggest that field potentials reached their peak values at about one and a half hours after tetanus and could be maintained as long as three hours without decay. In comparison with the control groups, synaptic cleft width, thickness of PSD, surface density per unit volume(Sv) of the active zones, curvature of the synaptic interface, numeric density per unit volume (Nv) of all synapses, spine synapses and the Nv of perforated synapses were all increased significantly. These data suggest that the increase of Sv of active zones and the increase of the curvature of synaptic interface may be the morphological feature characterizing the maintenance of LTP, in addition to the formation of perforated synapses.

Nociceptive c-fos expression in supraspinal areas in avoidance of descending suppression at the spinal relay station

The number and distribution of Fos-like-immunoreactive neurons in different supraspinal brain areas induced by formalin injection into one hindpaw was estimated in rats with transected dorsal half of the spinal cord at the thoracic level in an attempt to avoid most of the descending modulatory actions. The results showed that: (i) after spinal lesion, the peripheral noxious inputs, going up mainly through the ventral spinal cord, elicited a more widespread and densely located Fos-like-immunoreactive neurons in subcortical areas, many of them showed no Fos expression when noxious stimulation was given in rats with intact spinal cord; (ii) at the same time, a small number of subcortical areas, such as the lateral ventroposterior thalamic nucleus and dorsal raphe nucleus, exhibited no significant increase of nociceptive Fos-like immunoreactive neurons after spinal lesion as compared to that with intact spinal cord; and (iii) there appeared a prominent expansion of cortical areas with densely located Fos-like-immunoreactive neurons in spinal-lesioned rats as compared with the limited labelled areas in the control group with intact spinal cord. These results indicate that: (i) in avoiding the spinally descending modulatory mechanisms, more widespread supraspinal and cortical neurons will be recruited and activated in response to the noxious stimulation; and (ii) the descending systems exert differential actions on the spinal targets which project nociceptive signals to different supraspinal regions. The implication of these facts is discussed.

c-Fos expression in NMDA receptor-contained neurons in spinal cord in a rat model of inflammation: a double immunocytochemical study

Double-labeling techniques were used to demonstrate the nociceptive activation of NMDA receptor-contained neurons in spinal dorsal horn by using c-Fos immunoreactivity as an indicator of this activation in a rat model of inflammation. About 25% and 55% of the c-Fos-immunoreactive neurons that were found in laminae I-II and lamina V showed NMDA receptor immunoreactivity, while about 4% and 11% of NMDA-receptor immunoreactive neurons in these two regions showed c-Fos immunoreactivity, respectively. The implication of the results was discussed.

Endogenous opioids and ATP-sensitive potassium channels are involved in the mediation of apomorphine-induced antinociception at the spinal level: a behavioral study in rats

The effects of intrathecally (i.t.) administered glibenclamide, a blocker of adenosine triphosphate-sensitive potassium (K(ATP)) channels, or naloxone on the antinociception produced by i.t. apomorphine or morphine were observed and analyzed in rats by tail-flick (TF) test. The results showed that: (1) i.t. apomorphine produced a significant and dose-dependent antinociception, (2) the antinociception produced by i.t. apomorphine could be blocked dose-dependently by i.t. glibenclamide or naloxone, (3) the antinociception produced by i.t. morphine could also be blocked dose-dependently by i.t. glibenclamide. The results suggest that endogenous opioids and ATP-sensitive potassium channels might be involved in the mediation of apomorphine-induced antinociception at the spinal level.

ATP-sensitive potassium channels mediate norepinephrine- and morphine-induced antinociception at the spinal cord level

The effects of intrathecally (i.t.) administered glibenclamide, a blocker of adenosine triphosphate-sensitive potassium ( KATP) channels, on antinociception produced by i.t. norepinephrine, morphine, or 5'-N-ethylcarboxamide adenosine, an adenosine agonist, were investigated using tail-flick assay. The results showed that: 1) i.t. norepinephrine (1 nmol), morphine (0.5 nmol) and 5'-N-ethylcarboxamide adenosine (0.5 nmol) elicited prolongation of tail-flick latency, 2) i.t. glibenclamide given in 2 different doses (5 and 10 nmol) exhibited no effects on tail-flick latency, 3) the antinociception produced by norepinephrine (1 nmol) and morphine (0.5 nmol) was blocked by glibenclamide in a dose-dependent manner, 4) glibenclamide failed to modulate the effects of 5'-N-ethylcarboxamide adenosine on tail-flick latency. These observations suggest that KATP channels may play an important role in norepinephrine- and/or morphine-induced antinociception at the spinal level.

Different GABA-receptor types are involved in the 5-HT-induced antinociception at the spinal level: a behavioral study

The effects of intrathecally (i.t.) administered GABA(A)-receptor antagonist picrotoxin or bicuculline on the antinociception produced by i.t. serotonin (5-HT), gamma-aminobutyric acid (GABA), muscimol--the GABA(A) agonist or baclofen--the GABA(B) agonist were investigated and compared using the tail-flick assay in rats. The results showed that 1) both i.t. picrotoxin (1.5 nmol) and i.t. bicuculline (0.5 nmol) exhibited a partial and later-emerged blockade on the antinociception produced by 5-HT (120 nmol) or GABA (1.5 nmol); 2) both i.t. picrotoxin and i.t. bicuculline, with the same dosages, completely blocked the antinociception produced by muscimol (1.0 nmol), but showed no effects on that produced by baclofen (0.3 nmol). The results suggest that GABA may mediate the 5-HT-induced antinociception at the spinal level, with the GABA(B)-receptors exhibiting the effect at the early-stage and the GABA(A)-receptors at the later stage of the 5-HT-induced antinociception.

ATP-sensitive K+ channels are involved in the mediation of intrathecal norepinephrine- or morphine-induced antinociception at the spinal level: a study using EMG planimetry of flexor reflex in rats

The effects of intrathecally (IT) administered glibenclamide (Gli), an ATP-sensitive K+ (KATP) channel blocker, on the antinociception produced by IT norepinephrine (NE), serotonin (5-HT), morphine (Mor), or adenosine agonist, 5'-N-ethylcarboxamide adenosine (NECA) were investigated using integrated EMG measurement of hindlimb flexor reflex (FR) in lightly pentobarbital-anesthetized rats. The results showed that: 1) NE (3, 6, or 12 nmol) or 5-HT (60, 120, or 240 nmol) each produced a dose-dependent suppression of FR EMG, respectively; 2) pretreatment with Gli (5, 10, or 20 nmol) antagonized the NE (6 nmol)-induced antinociception in a dose-dependent manner and failed to modulate the 5-HT (120 nmol)-induced suppression of FR EMG; 3) pretreatment with Gli (5, 10, or 20 nmol) also antagonize the Mor (2 nmol)-induced suppression of FR EMG in a dose-dependent manner; 4) pretreatment with naloxone (Nal, 60, 120, or 240 nmol) also antagonize the NE (6 nmol)-induced suppression of FR EMG in a dose-dependent manner; and 5) NECA (0.5, 1.0, or 2.0 nmol) produced a dose-dependent suppression of FR EMG, while pretreatment with Gli (5, 10, or 20 nmol) failed to modulate the NECA (1.0 nmol)-induced suppression of FR EMG. The results show that (a) ATP-sensitive K+ channels are involved in the NE- and Mor-induced antinociception but not 5-HT- or NECA-induced antinociception at the spinal level; (b) endogenous opioids might act as a successor of NE and then activate KATP channels to producing the antinociception.

Locus coeruleus modulates thalamic nociceptive responses via adrenoceptors

This study investigated the parafascicular (PF) neuronal nociceptive responses and their modulation following electrical stimulation of the locus coeruleus (LC) and intrathecal (i.t.) or intracerebroventricular (i.c.v.) administration of two alpha-adrenoceptor antagonists, the alpha2-antagonist, yohimbine, and the alpha1-antagonist, prazosin. The main results were as follows: (1) the nociceptive evoked discharges in PF neurons were suppressed by preceding stimulation of LC; (2) the suppressive effect of LC stimulation on PF neurons was replaced by a facilitatory effect following pretreatment of i.t. yohimbine in 14 units tested, while i.t. prazosin failed to alter the LC-induced suppression, even when the prazosin dose was doubled; (3) i.c.v. pretreatment with prazosin strengthened the suppressive effect of LC stimulation on PF neurons; (4) i.c.v. norepinephrine (NE) administration induced, in PF neurons, a biphasic response to noxious stimulation; an early, brief (about 10 min) inhibitory effect followed by a late, long-lasting facilitatory effect; and (5) i.c.v. pretreatment of yohimbine or prazosin prevented the inhibitory or facilitatory responses released by NE, respectively. These results provide evidence that: (1) the LC-descending projections exhibit a suppressive effect on nociceptive transmission at the spinal level through alpha2-receptors; and (2) the LC-ascending projections exhibit dual effects, facilitatory and inhibitory, at the medial thalamus (PF) level through alpha1- and alpha2-receptors, respectively.

Pre-emptive intrathecal Mk-801, a non-competitive N-methyl-D-aspartate receptor antagonist, inhibits the up-regulation of spinal dynorphin mRNA and hyperalgesia in a rat model of chronic inflammation

The effects of N-methyl-D-aspartate(NMDA) receptor antagonist, Mk-801, on the expression of spinal dynorphin (DYN) mRNA and the hyperalgesia induced by peripheral inflammation were studied by Northern analysis and behavioral test. Following an unilateral injection of complete Freund's adjuvant (CFA) into the rat hindpaw, there appeared a significant hyperalgesia of inflamed hindpaw and up-regulation of ipsilateral spinal DYN mRNA; while the pre-emptive and continuous intrathecal administration of Mk-801 (10 microg/microl per h) could significantly suppress both the hyperalgesia and the up-regulation of spinal DYN mRNA induced by peripheral inflammation. The results suggest that NMDA receptor activation may contribute to the development and maintenance of the thermal hyperalgesia that is associated with the up-regulation of DYN expression in spinal dorsal horn.

Nitric oxide contributes to both spinal nociceptive transmission and its descending inhibition in rats: an immunocytochemical study

The present study was designed to determine if nitric oxide (NO) was involved both in the dorsal horn responses to the primary nociceptive inputs and the descending inhibitory action on these responses. The first part of the experiments showed that when formalin was injected into one hindpaw, the nociceptive c-fos expression in the lumbar dorsal horn ipsilateral to the injection was suppressed dose-dependently by intrathecal (i.t.) administration of Nomega-nitro-L-arginine (L-NNA), a nitric oxide synthase inhibitor. In the second part of the study, the formalin injection was carried out into two hindpaws of the rats with a sectioned dorsal quadrant at the thoracic spinal level, in these rats, there was a significant suppression of c-fos expression in the dorsal horn on the side with intact dorsal quadrant, reasonably owing to the preservation of the spinally descending inhibitory fibers from the supraspinal level; furthermore, this suppression could be canceled following i.t. L-NNA administration. The results suggest that endogenous NO not only facilitates the perception of nociceptive inputs at the spinal level but also enhances the descending inhibition upon the spinal nociception.

Effects of intrathecal monoamine antagonists on the nociceptive c-Fos expression in a lesioned rat spinal cord

Effects of intrathecal (i.t.) administration of monoamine antagonists on formalin-induced neuronal c-Fos expression in two sides of the lumbar dorsal horn were observed in rats with unilateral transection of the dorsolateral funiculus at T11-12 level. The results showed that: 1) pretreated with i.t. normal saline (control) and then an equal volume of formalin was injected into the two hindpaws, the number of Fos-like immunoreactive neurons were 44% lower on the side of lumbar dorsal horn with intact dorsolateral funiculus (57 +/- 3.1 vs. 103 +/- 3.8). 2) Pretreatment with i.t. phentolamine (a non-selective alpha-adrenoceptor antagonist) caused an increase of Fos-like immunoreactive neurons on the intact side so showing only a reduction rate of 23% to the lesioned side (p < .01); 3) pretreatment with i.t. cyproheptadine (a 5-HT-receptor antagonist) caused a similar reduction rate of 21% (p < .01) of Fos-like immunoreactive neurons on the intact side; and 4) combined i.t. pretreatment with phentolamine and cyproheptadine caused a reduction of Fos-like immunoreactive neurons of only 4% on the intact side, namely, the differences in the number of Fos-like immunoreactive neurons on two sides of the lumbar spinal cord owing to the unilateral dorsolateral funiculus lesion were nearly abolished by i.t. coinjection of phentolamine and cyproheptadine. The results indicate that 1) peripheral noxious inputs can provoke a spinally-descending inhibitory effect on the spinal nociceptive transmission via the dorsolateral funiculus and 2) the descending fibers in dorsolateral funiculus exert their action mainly through the release of either norepinephrine or 5-HT at the spinal level.

Nitric oxide is involved in the formation of learning and memory in rats: studies using passive avoidance response and Morris water maze task

The study was designed to test whether blockade of endogenous NO production affects learning and memory formation in rats. The rats received an intracerebroventricular (i.c.v.) injection of the inhibitor of nitric oxide synthase, N omega-nitro-L-arginine (NAME), 30 min before each training, and were then tested in the one-trial passive avoidance response and Morris water maze task, respectively. The results showed that: (1) among four concentrations of drug used, only doses higher than 3 mumol of i.c.v. NAME impaired learning and memory formation significantly (p < 0.05) in both test models; (2) in Morris water maze task, the animals treated with highest dose of NAME (5 mumol) failed to learn while those treated with lower doses succeeded; (3) in Morris water maze task, the i.c.v. NAME, even in higher doses, did not affect the capacity of finding a visible platform. It is suggested that NO is involved in learning and memory formation by potentiating or facilitating mainly the acquisition process.

Peripheral NMDA receptors contribute to activation of nociceptors: a c-fos expression study in rats

By using immunocytochemical techniques, we demonstrated that: (1) unilateral injection of N-methyl-D-aspartate (NMDA; 10, 20, 50 micromol) into the plantar aspect of hindpaw of the normal waked rats elicited a dose-dependent increase of c-fos expression in the superficial laminae on the side of the spinal dorsal horn ipsilateral to the injection; (2) combined injection of MK-801, a non-competitive NMDA receptor antagonist, with formalin into the hindpaw could suppress formalin-induced c-fos expression in the dorsal horn, with a stronger suppression after higher doses of MK-801 being injected. These results strongly suggest that NMDA receptors might be located in the membrane of peripheral axonal terminals of the dorsal root ganglion neurons and might be involved in the activation of nociceptors.

Locus coeruleus stimulation modulates the nociceptive response in parafascicular neurons: an analysis of descending and ascending pathways

The nociceptive responses in parafascicular neurons (PF) were recorded and studied following electrical stimulation of locus coeruleus (LC) combined with intrathecal (IT) or intracerebroventricular (ICV) administration of phentolamine (Ph), an alpha-adrenoceptor antagonist. The results revealed the following. (1) Three different PF neuronal populations were observed according to their response pattern following noxious stimulation: nociceptive-on, nociceptive-off, and nonresponsive units. Only the nociceptive-on units were studied further. (2) The nociceptive discharges in majority of PF neurons (66/87) were inhibited by electrical stimulation of the LC. (3) The inhibitory effect of LC stimulation was prevented and even reversed by pretreatment of IT Ph (40 nmol) in 22 units, or by dorsolateral funiculi transection in 24 units tested. (4) The inhibitory effect of LC stimulation was strengthened by preadministration of ICV Ph (40 nmol) in 17 units tested. (5) ICV administration of norepinephrine (NE 30 nmol) resulted in PF neurons a biphasic response to nociceptive stimulation: an early brief inhibition and a late long-lasting facilitation. (6) Pretreatment of ICV Ph (40 nmol) prior to NE injection prevented the NE-induced biphasic response. The results suggest that stimulation of LC modulates the nociceptive response of PF neurons through both ascending and descending routes. These two diverse routes exert two different effects: a predominantly inhibitory role on the nociceptive transmission at the spinal cord level by descending NE-ergic fibers, and a facilitatory role on the responsiveness of PF to noxious inputs by ascending fibers.

Lateral hypothalamus: site involved in pain modulation

The present study is an attempt to examine the neuronal circuitry of a supraspinal site engaged in pain modulation. Five physiological measures were postulated as the criteria for defining a central nervous system site engaged in the circuitry of pain modulation. The lateral hypothalamus met these five measures: (i) 81% of the lateral hypothalamus neurons (247/304) responded to noxious stimuli using a single cell recording procedure; (ii) stimulation of the periaqueductal gray-dorsal raphe area or the habenula modulated 98% and 87% of the lateral hypothalamus noxious-evoked activity; (iii) microiontophoretically applied morphine modulated 77% of the lateral hypothalamus noxious evoked activity; (iv) electrical stimulation of the lateral hypothalamus produced behavioral analgesia proportional to the stimulus intensity as assessed by the tail flick assay; and (v) morphine application into the lateral hypothalamus produced behavioral analgesia in a dose-response manner using the tail flick assay. In conclusion, the lateral hypothalamus can be considered one of the pain modulation sites.

Sequential mediation of norepinephrine-and dopamine-induced antinociception at the spinal level: involvement of different local neuroactive substances

The effects of intrathecally (i.t.) administered opioid antagonist naloxone (Nal), adenosine antagonist aminophylline (Aph), and gamma-aminobutyric acid (GABAA)-receptor antagonist picrotoxin (PTX) or Bicuculline (BIC) on the antinociception produced by i.t. norepinephrine (NE), dopamine (DA), morphine (Mor), 5'-N-ethylcarboxamidoadenosine (NECA, an adenosine agonist) or muscimol (MUS, a selective GABAA-receptor agonist) were studied and compared using the tail-flick test in rats. The results showed that: (1) both i.t. NE (0.3, 0.5 and 1.0 nmol) and DA (5.5, 8.3 and 16.5 nmol) produced significant and dose-dependent increases in tail-flick latencies (antinociception); (2) both Nal (240 nmol) and Aph (120 nmol) blocked the antinociception produced by NE (1.0 nmol); (3) both Nal (240 nmol) and Aph (120 nmol) blocked the antinociception produced by Mor (0.5 nmol), but only Aph (120 nmol) blocked the antinociception produced by NECA (0.5 nmol), while Nal (240 nmol) did not; (4) neither Nal (240 nmol) nor Aph (120 nmol) altered the antinociception produced by DA (16.5 nmol); (5) both i.t. PTX (1.5 nmol) and BIC (0.5 nmol) completely blocked the antinociception produced by DA (16.5 nmol), but showed no effects on that produced by NE (1.0 nmol); and (6) both PTX and BIC blocked the antinociception produced by MUS (1.0 nmol). These results suggest that: (a) endogenous opiate and adenosine may be involved in the mediation of NE-induced, but not DA-induced, antinociception; (b) NE, opioid and adenosine may act in a sequential order in NE-induced antinociception at the spinal level; (c) endogenous GABA may be involved in the mediation of DA-induced antinociception through the GABAA-receptors, but is not involved in NE-induced antinociception at the spinal level.

Adenosine and opiate-like substances mediates antinociception at the spinal cord

The effects of intrathecally administered naloxone or aminophylline on the antinociception produced by intrathecal NE, 5-HT, morphine or adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) were observed in rats using the tail-flick test. The results show that: (1) the antinociception produced by NE with doses of 0.5 or 1.0 nmol could be completely blocked by both naloxone (240 nmol) and aminophylline (120 nmol); (2) neither naloxone (240 nmol) nor aminophylline (120 nmol) could alter the antinociception produced by 5-HT with doses of 60 or 120 nmol; and (3) the antinociception produced by morphine (0.5 nmol) could be blocked by both naloxone (240 nmol) and aminophylline (120 nmol), while the antinociception by NECA (0.5 nmol) could be blocked only by aminophylline (120 nmol), but not by naloxone (240 nmol). The results suggest that opiate-like substances (OLS) and adenosine are involved in the mediation of the NE-produced antinociception, but not in 5-HT-produced antinociception. Results also suggest that NE, OLS and adenosine may act in a sequential order in the performance of NE-induced antinociception at the spinal level.

Interaction of serotonin and norepinephrine in spinal antinociception

The interactions between different doses of serotonin (5-HT) and norepinephrine (NE) in in vivo experiments on rat spinal cord dorsal horn cells was investigated using the integrated electromyography (EMG) measurement of the nociceptive hindlimb flexor reflex (FR). The results indicate that (1) intrathecal (IT) administration of low doses of 5-HT (60 nmol) or NE (1.5 nmol) suppresses the nociceptive FR by 40% for 20 min, respectively; (2) administration of higher doses of 5-HT (240 nmol, IT) multiplies the suppression of the nociceptive FR by 80% for 40 min, and NE (15 nmol, IT) produces similar suppression of the nociceptive FR for 80 min; (3) concomitant administration of low doses of 5-HT (60 nmol, IT) and NE (1.5 nmol, IT) produces a summation of the nociceptive FR suppression both in amplitude and duration; (4) concomitant administration of the higher doses of 5-HT (240 nmol IT) with NE (15 nmol, IT) produces similar effect obtained as 5-HT given separately, and no summation was obtained as observed following the lower dosages; (5) serotonin (240 nmol, IT) given 40 min before NE (15 nmol, IT) attenuates the duration of the suppression induced by NE; (6) pretreatment with a selective 5-HT2 receptor antagonist ketanserin (60 nmol, IT) failed to abolish the 5-HT effects; (7) pretreatment with ketanserin prior to concomitant administration of the higher doses of 5-HT and NE prolongs the time duration of the nociceptive FR suppression.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats

The interrelations among norepinephrine (NE), serotonin (5-HT), opiate-like substances (OLS), and acetylcholine (ACh) were investigated by using electrophysiological method combining with intrathecal (i.t.) injection. The results show that: (1) pretreatment with i.t. naloxone (Nal) completely reversed the NE-induced suppression of nociceptive discharges in parafascicular (PF) neurons, but partially reversed that of induced by i.t. 5-HT; (2) pretreatment with i.t. atropine (Atr) completely reversed the suppression induced by either NE or 5-HT. The results suggest that OLS may act as a necessary mediator for NE-induced suppression on the spinal transmission of nociceptive inputs, while it is only partially involved in the 5-HT-induced suppression, and moreover, that endogenous ACh is necessary for the performance of nociceptive suppression induced by either spinal NE or 5-HT administration.

Morphine and norepinephrine-induced antinociception at the spinal level is mediated by adenosine

The purpose of this study was to examine whether adenosine or serotonin is involved in mediation of the antinociception produced by norepinephrine at the spinal cord level. Aminophylline, an adenosine receptor antagonist and naloxone given intrathecally (i.t.) were used to test the antinociception produced by i.t. norepinephrine, serotonin, morphine or the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) by using the tail-flick assay in rats. It was observed that (1) aminophylline blocked the antinociception produced by norepinephrine, but exhibited no effect on the antinociception produced by serotonin, (2) aminophylline blocked the antinociception produced by morphine similarly to naloxone, (3) aminophylline blocked the antinociception produced by NECA and (4) naloxone failed to block the antinociception produced by NECA and serotonin. The results suggest that adenosine is involved in mediation of the norepinephrine-produced antinociception at the spinal level and that norepinephrine and adenosine may act in a sequential manner in norepinephrine-induced antinociception.

Changes of spinal substance P, calcitonin gene-related peptide, somatostatin, Met-enkephalin and neurotensin in rats in response to formalin-induced pain

Changes of substance P (SP)-, calcitonin gene-related peptide (CGRP)-, somatostatin (SS)-, Met-enkephalin (Met-Enk)- and neurotensin (NT)- immunoreactive materials on two sides of the lumbar dorsal horn were inspected microscopically and quantified with a computer-assisted image processing system in rats with intact or totally transected spinal cord 2 h after injection of 0.2 ml of 0.5% formalin into the right hindpaw subcutaneously. The results showed that the SP-like immunoreactivity (SP-LI), CGRP-LI, SS-LI, Met-Enk-LI, and NT-LI were significantly higher in fibers and terminals in superficial laminae of the dorsal horn ipsilateral to the formalin injection in both of the experimental groups. It is supposed that the increased contents of these peptides reflect an increased biosynthesis, transport, and release of these peptides in primary afferents and spinal intrinsic neurons in response to the long-lasting inflow of noxious messages, and that these changes seem to be produced even in the condition when the supraspinal effects have been excluded.

Effects of descending inhibitory systems on the c-Fos expression in the rat spinal cord during formalin-induced noxious stimulation

This study provides morphological evidence for the activation of the descending modulatory control by nociceptive afferent pathways. Fos-like immunoreactivity in the spinal dorsal horn is used as an indicator of efficacy of transmission of noxious inputs at this level. Wistar rats were anesthetized with nembutal and the spinal dorsolateral funiculus was transected unilaterally at the level of T11,12. Two days later, an equal volume (0.2 ml) of formalin (5% in saline) was injected into the plantar aspect of two hindpaws. After 1 h of injection, rats were deeply anesthetized and killed for the immunocytochemical examination of Fos-like protein product by using an immunocytochemical technique. The results show that the mean number of Fos protein-like immunoreactive neurons is significantly lower in the superficial laminae and in the neck of the dorsal horn on the side ipsilateral to the intact dorsolateral funiculus than that on the opposite side (i.e. 18.4 +/- 1.0 vs 30.0 +/- 1.3 and 33.9 +/- 0.2 vs 56.8 +/- 1.7, respectively). We conclude that the peripheral noxious inputs (which ascend via the ventral half of the spinal cord in this study) can activate the supraspinal descending inhibitory systems, which in turn suppress the synthesis of Fos-like protein in the related dorsal horn neurons.

Norepinephrine and serotonin-induced antinociception are blocked by naloxone with different dosages

The effects of intrathecally (IT) administered naloxone (Nal) on the antinociception produced by IT norepinephrine (NE), serotonin (5-HT), or morphine (Mor) were observed and compared in rats using the tail-flick (TF) assay. The results show that: a) NE, 5-HT, and Mor in doses of 1 nmol, 240 nmol, and 0.5 nmol, respectively, produce similar increases in amplitude and time in TF latency (TFL); b) Nal treatment of 240 and 360 nmol has no effects on TFL; c) the antinociception produced by NE (1 nmol) can be blocked by Nal (240 nmol); d) antinociception produced by Mor (0.5 nmol) can also be blocked by Nal (240 nmol); e) 240 nmol of Nal does not affect the 5-HT (120 nmol)-produced antinociception, while 360 nmol of Nal show a delayed blockade to the 5-HT (120 nmol)-produced antinociception. The results suggest that endogenous opiate-like substances may be involved in both NE- or 5-HT-produced antinociception at the spinal level, and these effects may be mediated through different types of opiate receptors.

Morphological evidence for the activation of descending modulatory control by nociceptive afferent pathways: an immunocytochemical study

Immunocytochemical technique was used to compare the content of substance P (SP), Met-enkephalin (Met-Enk) and neurotensin (NT) on two sides of the lumbar dorsal horn of rats in which the unilateral dorsolateral funiculus was transected while formalin (0.2 ml, 5%) was injected equally into two hindpaws. The results showed that the SP-like immunoreactivity (SP-LI) and Met-Enk-LI were significantly higher and the NT-LI was significantly lower in the superficial laminae of dorsal horn on the side ipsilateral to the intact DLF than that on the opposite side, implying that peripheral noxious inputs can activate the supraspinal descending inhibitory systems which in turn modulate the transmission of noxious message at the spinal level by changing the activities of related peptidergic neurons.

Cerebellar stimulation modulates thalamic noxious-evoked responses

Parafascicular (PF) neurons responding to noxious stimuli and focal electrical stimulation of midbrain, diencephalon, and hypothalamic nuclei, which send projections to PF, modulates the PF spontaneous and noxious-evoked responses. Some cerebellar efferents ascend to PF. This investigation attempted to study the effect of cerebellar stimulation on spontaneous and noxious-evoked PF neuronal activity in rats. It was observed that 26% (73/280) of PF neurons responded to a noxious stimulus. The PF neuronal population exhibits two cell types according to their response pattern following the noxious stimulus. One type of PF neurons were excited (n = 53) and were classified as nociceptive-on cells. The second type of PF neurons responded to noxious stimulus by a decrease in the ongoing firing rate (n = 20) and were classified as nociceptive-off cells. The responses of these two types of nociceptively identified cells were tested following cerebellar lateral nucleus stimulation (Lat.N.S.) utilizing several current intensities. Lat.N.S. with lower intensities (0.1-0.2 mA) elicited suppression of both spontaneous and nociceptive-evoked discharges of the nociceptive-on neurons, although higher intensities (0.4-0.6 mA) elicited excitation on both discharges of this type of neuron. In contrast, Lat.N.S. induced a monophasic intensity-dependent suppression of both the spontaneous and the nociceptive-evoked discharges of the nociceptive-off neurons. The results indicate that Lat.N.S. modulates the nociceptive-evoked responses of PF neurons. The possible role and related pathways of cerebellum in modulating noxious input were discussed.

Suppression of nociceptive responses in parafascicular neurons by stimulation of substantia nigra: an analysis of related inhibitory pathways

A total of 166 neurons in parafascicular nucleus (PF) were studied, 85 from intact animals, 72 following dorsal spinal cord transection (D.Sp.C.X.), and 9 following complete transection of the spinal cord. Two patterns of nociceptive responses were identified following noxious stimulation and these responses were classified as 'nociceptive-on' and 'nociceptive-off' neurons, respectively. The effects of stimulating the substantia nigra (SNS) on the spontaneous and on the nociceptive evoked discharges were observed and compared in intact, D.Sp.C.X. and completely transected spinal cord rats. The results show that SNS significantly suppresses both the spontaneous and the nociceptive evoked discharges elicited by peroneal nerve stimulation. With an intact spinal cord, SNS suppressed both the spontaneous [-37 +/- 3.2% (P less than 0.05)] and the nociceptive evoked discharges [-52.8 +/- 2.8% (P less than 0.01)] of the 'nociceptive-on' cells respectively, while in the 'nociceptive-off' cells the same stimulation elicited an even more prominent suppression upon both discharges (-47.7 +/- 5.4%, P less than 0.01 and -64.9 +/- 5.0%, P less than 0.01), respectively. After D.Sp.C.X., the suppressive effects on the 'nociceptive-on' cells following SNS were diminished (-28.1 +/- 3.5% and -36.9 +/- 2.6%, respectively) but not abolished, while in the 'nociceptive-off' cells, the inhibitory effects on SNS were unchanged. In addition, the suppressive effects of SNS on the spontaneous activity of PF neurons in cases with completely cut spinal cords remains unchanged. These results suggest that SNS modulates the spontaneous and the noxious evoked responses of the PF neurons by way of supraspinal connections besides the previously described descending projecting pathways.

Serotonergic, noradrenergic and galaninergic projections to the nucleus parafascicularis

Immunocytochemical staining for serotonin (5-HT), tyrosine hydroxylase (TH) and galanin (GAL) was combined with horseradish peroxidase (HRP) retrograde tract-tracing technique to analyze the localizations of 5-HT-, catecholamine (CA)- and GAL-containing neurons in the brainstem which project to the nucleus parafascicularis (PF) in rats. It is demonstrated that most of the retrogradely HRP-labeled neurons (70%) in bilateral periaqueductal gray (PAG) and raphe nuclei are positively immunostained by antiserum to 5-HT, and that most of the retrogradely HRP-labeled neurons (over 80%) in bilateral locus coeruleus (LC) are positively immunostained by antisera to both TH and GAL. The possible functions of these PF-petal serotonergic, catecholaminergic (actually noradrenergic) and galaninergic projections are discussed.

D2 dopamine receptor involvement in spinal dopamine-produced antinociception

Experiments were performed on 79 lightly pentobarbital-anesthetized rats. Rats displayed a dose-dependent increase in tail-flick latencies following the injection of dopamine (DA) into the lumbar subarachnoid space through an intrathecal tube. Sulpiride, a D2-subtype receptor antagonist, antagonized the DA-induced analgesia (antinociceptive) effect; while SCH-23390, a D1-subtype receptor antagonist, had no effect even in a higher dose. To further investigate whether the well-known spinal serotonergic, noradrenergic and opioidergic receptor systems were involved in DA-induced antinociception, their antagonists, methysergide, phentolamine, and naloxone were tested respectively. The results showed that phentolamine, but not methysergide or naloxone, could block the DA-induced antinociception. The present data provide evidence that DA exerts antinociceptive effects through D2-subtype dopamine receptor(s) at the spinal level, and that spinal alpha-adrenergic receptors may mediate this effect.

Effects of intrathecal monoamine antagonists and naloxone on the descending inhibition of the spinal transmission of noxious input in rats: study with a new experimental model

An electrophysiological model has been developed to explore the transmitters and their relationships in the descending control of spinal transmission of noxious inputs. Nociceptive discharges were recorded extracellularly in parafascicular (Pf) neurons, and the caudal stump of longitudinally isolated dorsal half of the lower thoracic spinal cord was stimulated to simulate the descending volleys coming from the supraspinal structures. Nociceptive discharges in 34 Pf cells were markedly suppressed (83.2 +/- 13.9%) by the preceding spinal stimulation. Phentolamine, methysergide and naloxone were separately administered with random sequence in each of 25 cells by an intrathecal route to observe if the descending inhibition could be blocked by these drugs. The results suggested that in the dorsal spinal cord there exist at least two neurochemically different descending inhibitory fiber systems which comprise either the long descending adrenergic or serotonergic fibers and, most of the adrenergic fibers are succeeded by a propriospinal opioidergic neuron while a few of the serotonergic fibers have such a succession.

Focal dorsal raphe stimulation and pinnal electrical stimulation modulate spontaneous and noxious evoked responses in thalamic neurons

This study investigated the nocieceptive responses of single neurons within the nucleus parafascicularis (PF) thalami of the rat following two modes of electrical stimulation known to induce analgesia. It was found that both focal electrical dorsal raphe stimulation (DRS) and bilateral pinnal (ear) electrical stimulation (PES) converge on the same PF neurons, affecting both the spontaneous discharges and the noxious evoked responses toward these neurons. The effects of different stimulus current intensity, frequency and pulse duration were also examined. It was found that for both DRS and PES at pulse frequency of 10 Hz and current amplitude of 10 microA are the optimal parameters to modulate both the spontaneous and the noxious evoked responses. These stimuli produced prolonged effects related to the duration of stimulation. The external (PES) low current stimulation which was delivered below the sensory threshold was as effective in modulating noxious responses as the invasive DRS in intact animals and in animals with bilateral dorsolateral-funiculus ablation. It was observed that dorsal lateral funiculus ablation (DLFx) did not modify the DRS and the PES effects. These observations further support the existence of an ascending pain modulation pathway.

Effects of stimulation of superior colliculus on nociceptive unit discharges in parafascicular neurons and nocifensive reflex of hind limb in rat

Effects of stimulation of the superior colliculus (SC) on the spontaneous and nociceptive discharges of parafascicular (PF) neurons were investigated in 43 urethane-anesthetized rats. Two groups of PF cells were sampled according to their responses to noxious stimuli: 59 of them were 'nociceptive-on' and 12 'nociceptive-off'. Following stimulation of the intermediate and deep layers of SC, the firing rate of nociceptive discharge of 'nociceptive-on' cells was inhibited significantly (-76 +/- 5%, P less than 0.01) in 75% cases tested, while the nociceptive response of 'nociceptive-off' cells was disinhibited markedly (79 +/- 9%, P less than 0.01) in 67% cases by the same stimulation. In 30 animals of this series the latency of hind limb withdrawal reflex elicited by noxious skin heating was compared before and after SC stimulation. In 24 cases in which the stimulating electrodes were positioned exactly in intermediate-deep layers of SC, SC stimulation lengthened the latency by 62 +/- 8% (P less than 0.01), while in 6 cases in which the electrodes drifted from these areas, the latency was not changed following the same stimulation.

Habenulopetal catecholaminergic projections in the rat brain: a combined Fluoro-Gold/catecholamine fluorescence study

Habenulopetal catecholamine (CA)-containing neurons were determined using Fluoro-Gold retrograde tracing combined with a CA-fluorescence technique. A robust number of habenulopetal CA neurons were found mainly in the ipsilateral locus coeruleus (LC), while a considerable number of habenulopetal non-CA neurons were found in the LC, subcoeruleus, A5, A4 and A7 areas.

Habenular neuron responses to noxious input are modified by dorsal raphe stimulation

Single-cell experiments were undertaken to characterize the neurons of the rat habenular complex (Hab) which respond to noxious input and the extent to which the noxious responses can be modulated by dorsal raphe stimulation. The reported observations demonstrate that habenular neurons exhibit two response patterns to noxious stimulation termed 'nociceptive-on' and 'nociceptive-off'. In addition, it was demonstrated that dorsal raphe stimulation modulates the noxious input to the 'nociceptive-on' and to the 'nociceptive-off' cells. These findings demonstrated that the habenular complex responds to noxious input and, together with other findings reporting that both direct morphine application into the Hab and electrical stimulation of the Hab produced analgesia, suggest that the habenular complex is integrated into the pain regulatory circuitry.