Autoradiographic distribution of [3H]acetylcholine binding sites in the cervical spinal cord of man and some other species

D3 Receptors and Restless Legs Syndrome

Restless Legs Syndrome (RLS) is a sensorimotor disorder that severely affects sleep. It is characterized by an urge to move the legs that is often accompanied by periodic limb movements during sleep (PLMS). RLS has a high prevalence in the population and is usually a life-long condition. While its origins remain unclear, RLS is initially highly responsive to treatment with dopaminergics that target the D3 receptor. However, over time patients often develop a gradual tolerance that can lead to the emergence of adverse effects and the augmentation of the symptoms. While the basal ganglia and the striatum control leg movements, the lumbar spinal cord is the gateway for the sensory processing of the symptoms and critical for the associated leg movements. D3 receptors are highly expressed in nucleus accumbens (NAc) of the striatum and the sensory-processing areas of the spinal dorsal horn. In contrast, D1 receptors are strongly expressed throughout the entire striatum and in the ventral horn of the spinal cord. Long-term treatment with D3 receptor full agonists is associated with an upregulation of the D1 receptor subtype, and D3 and D1 receptors can form functional heteromers, in which the D3R controls the D1R function. It is conceivable that the switch from beneficial treatment to augmentation observed in RLS patients after prolonged D3R agonist exposure may be the result of unmasked D1-like receptor actions.

Synaptic mechanisms underlying modulation of locomotor-related motoneuron output by premotor cholinergic interneurons

Spinal motor networks are formed by diverse populations of interneurons that set the strength and rhythmicity of behaviors such as locomotion. A small cluster of cholinergic interneurons, expressing the transcription factor Pitx2, modulates the intensity of muscle activation via 'C-bouton' inputs to motoneurons. However, the synaptic mechanisms underlying this neuromodulation remain unclear. Here, we confirm in mice that Pitx2+ interneurons are active during fictive locomotion and that their chemogenetic inhibition reduces the amplitude of motor output. Furthermore, after genetic ablation of cholinergic Pitx2+ interneurons, M2 receptor-dependent regulation of the intensity of locomotor output is lost. Conversely, chemogenetic stimulation of Pitx2+ interneurons leads to activation of M2 receptors on motoneurons, regulation of Kv2.1 channels and greater motoneuron output due to an increase in the inter-spike afterhyperpolarization and a reduction in spike half-width. Our findings elucidate synaptic mechanisms by which cholinergic spinal interneurons modulate the final common pathway for motor output.

Balanced cholinergic modulation of spinal locomotor circuits via M2 and M3 muscarinic receptors

Neuromodulation ensures that neural circuits produce output that is flexible whilst remaining within an optimal operational range. The neuromodulator acetylcholine is released during locomotion to regulate spinal motor circuits. However, the range of receptors and downstream mechanisms by which acetylcholine acts have yet to be fully elucidated. We therefore investigated metabotropic acetylcholine receptor-mediated modulation by using isolated spinal cord preparations from neonatal mice in which locomotor-related output can be induced pharmacologically. We report that M2 receptor blockade decreases the frequency and amplitude of locomotor-related activity, whilst reducing its variability. In contrast, M3 receptor blockade destabilizes locomotor-related bursting. Motoneuron recordings from spinal cord slices revealed that activation of M2 receptors induces an outward current, decreases rheobase, reduces the medium afterhyperpolarization, shortens spike duration and decreases synaptic inputs. In contrast, M3 receptor activation elicits an inward current, increases rheobase, extends action potential duration and increases synaptic inputs. Analysis of miniature postsynaptic currents support that M2 and M3 receptors modulate synaptic transmission via different mechanisms. In summary, we demonstrate that M2 and M3 receptors have opposing modulatory actions on locomotor circuit output, likely reflecting contrasting cellular mechanisms of action. Thus, intraspinal cholinergic systems mediate balanced, multimodal control of spinal motor output.

D3 and D1 receptors: The Yin and Yang in the treatment of restless legs syndrome with dopaminergics

Dopaminergic treatments targeting the D3 receptor subtype to reduce the symptoms of RLS show substantial initial clinical benefits but fail to maintain their efficacy over time. Sensorimotor circuits in the spinal cord are the gateway for the sensory processing of the symptoms and critical for the associated leg movements that relieve the symptoms and the periodic limb movements that often develop during sleep. There is a high preponderance of the inhibitory D3 receptor in the sensory-processing areas of the spinal cord (dorsal horn), whereas the motor areas in the ventral horn more strongly express the excitatory D1 receptor subtype. D3 and D1 receptors can form functional heteromeric ensembles that influence each other. In the spinal cord, long-term treatment with D3 receptor agonists is associated with the upregulation of the D1 receptor subtype and block of D1 receptor function at this stage can restore the D3 receptor effect. Alternate scenarios for a role of dopamine involve a role for the D5 receptor in regulating motor excitability and for the D4 receptor subtype in controlling D3-like effects. A model emerges that proposes that the behavioral changes in RLS, while responsive to D3 receptor agonists, may be ultimately be the result of unmasked increased D1-like receptor activities.

The antinociceptive effects of nicotinic partial agonists varenicline and sazetidine-A in murine acute and tonic pain models

Nicotinic agonists display a wide-range profile of antinociceptive activity in acute, tonic, and chronic pain models. However, their effectiveness is limited by their unacceptable side effects. We investigated the antinociceptive effects of two new α4β2* nicotinic partial agonists, varenicline and sazetidine-A, in acute thermal and tonic pain mouse models. Both drugs failed to induce significant effects in the tail-flick and hot-plate tests after subcutaneous administration. However, they blocked nicotine's effects in these tests at very low doses. In contrast to acute pain tests, varenicline and sazetidine-A dose-dependently induced an analgesic effect in the mouse formalin test after systemic administration. Their antinociceptive effects were mediated, however, by different nicotinic acetylcholine receptor (nAChR) subtypes. Sazetidine-A effects were mediated by β2* nAChR subtypes, whereas varenicline actions were attributed to α3β4 nAChRs. Moreover, low inactive doses of varenicline blocked nicotine's actions in phase II of the formalin test. Overall, our results suggest that the antagonistic actions of varenicline at low doses are mediated by β2*-nAChRs and at higher doses as an agonist by α3β4*-nAChRs. In contrast, both actions of sazetidine-A are mediated by β2*-nAChR subtypes. These results suggest that nicotinic partial agonists possess analgesic effects in a rodent tonic pain model and may provide a potential treatment for the treatment of chronic pain disorders.

Receptors involved in the antinociception of intrathecal melatonin in formalin test of rats

The authors examined the antinociceptive effect of melatonin in a nociceptive state and investigated a possible interaction with adrenergic or cholinergic receptors underlying this effect at the spinal level. Nociception was induced by a subcutaneous injection of 50 μl of a 5% formalin solution to the hindpaw of male Sprague-Dawley rats. The reversal effects of alpha-1 adrenoceptor antagonist (prazosin), alpha-2 adrenoceptor antagonist (yohimbine), muscarinic receptor antagonist (atropine) and nicotinic receptor antagonist (mecamylamine) on the activity of melatonin were assessed. Intrathecal melatonin reduced the flinching response during phase 1 and phase 2 in the formalin test. Intrathecal prazosin, yohimbine, atropine and mecamylamine increased the attenuating flinching response in both phases observed by intrathecal melatonin. Collectively, the present data suggest that intrathecal melatonin attenuates the facilitated state and acute pain evoked by formalin injection. Furthermore, the antinociception of melatonin is mediated through the alpha-1 adrenoceptor, alpha-2 adrenoceptor, muscarinic and nicotinic receptors in the spinal cord.

Interaction between midazolam and epibatidine in spinally mediated antinociception in rats

PURPOSE

Both gamma-aminobutyric acid (GABA)(A) receptors and nicotinic cholinergic receptors have important roles in antinociception in the spinal cord. The antinociceptive effects of midazolam (a GABA(A) agonist) and epibatidine (a nicotinic cholinergic agonist) in the spinal cord have been reported. The present study was performed to investigate the interaction between intrathecal midazolam and epibatidine.

METHODS

Sprague-Dawley rats with lumbar intrathecal catheters were tested for their tail withdrawal response to thermal stimulation (tail-flick test) or for their paw flinches in response to formalin injection (formalin test) after the intrathecal administration of epibatidine or a combination of midazolam and epibatidine. The combination doses were 1/16, 1/8, 1/4, 1/2, 1, 2, and 4 times the 50% effective dose (ED50) of each agent in each test. The interaction of midazolam and epibatidine was investigated by isobolographic analysis. Behavioral side effects were also investigated.

RESULTS

In the tail-flick test, the ED50 values of the combination were significantly higher than the theoretical additive values. In the formalin test, the ED50 values of the combination were significantly lower than the theoretical additive values in phase 1, but were not different from the theoretical additive values in phase 2.

CONCLUSION

The intrathecal combination of midazolam and epibatidine had antagonistic effects on thermal acute nociception, while the combination had synergistic effects on acute inflammatory nociception, with only additive effects on inflammatory-facilitated nociceptive responses.

Cholinergic mechanisms involved in the pain relieving effect of spinal cord stimulation in a model of neuropathy

The mechanisms underlying the pain relieving effect of spinal cord stimulation (SCS) on neuropathic pain remain unclear. We have previously demonstrated that suppression of tactile hypersensitivity produced by SCS may be potentiated by i.t. clonidine in a rat model of mononeuropathy. Since the analgesic effect of this drug is mediated mainly via cholinergic mechanisms, a study exploring the possible involvement of the spinal cholinergic system in SCS was undertaken. The effect of SCS was assessed with von Frey filaments in rats displaying tactile hypersensitivity after partial ligation of the sciatic nerve and both SCS-responding and non-responding as well as normal rats were subjected to microdialysis in the dorsal horn. Acetylcholine (ACh) was analyzed with HPLC before, during and after SCS. SCS produced significantly increased release of ACh in the dorsal horn in rats responding to SCS whereas the release was unaffected in the non-responding animals. Furthermore, the basal release of ACh was significantly lower in nerve lesioned than in normal rats. In another group of rats it was found that the response to SCS was completely eliminated by i.t. atropine and a muscarinic M(4) receptor antagonist while a partial attenuation was produced by M(1) and M(2) antagonists. Blocking of nicotinic receptors did not influence the SCS effect. In conclusion, the attenuating effect of SCS on pain related behavior is associated with the activation of the cholinergic system in the dorsal horn and mediated via muscarinic receptors, particularly M(4,) while nicotinic receptors appear not to be involved.

Peripheral bee venom's anti-inflammatory effect involves activation of the coeruleospinal pathway and sympathetic preganglionic neurons

There are several reports indicating that the locus coeruleus (LC) is capable of altering immune responses. Moreover, it is well established that the LC is the major source of descending noradrenergic system. Recently we have demonstrated that subcutaneous bee venom (BV) injection dramatically suppressed peripheral inflammation through activation of sympathetic preganglionic neurons (SPNs) leading to release of adreno-medullary catecholamines. Importantly, this 'BV-induced anti-inflammatory effect' (BVAI) is also associated with an increase of the activity of LC. Based on these data, present study examined whether BV-induced LC activation increased the activity of SPNs and this pathway played a role in BVAI using a zymosan-induced inflammatory air pouch model in mice. Unilateral BV injection into left hind limb produced anti-inflammation and specifically increased Fos expression in SPNs of the T7-T11 (which mainly project to adrenal medulla), but not those of the T1-T6 or T12-L2 spinal cord. 6-Hydroxydopamine-induced unilateral lesion of the contralateral, but not ipsilateral (to the BV injection site) LC significantly blocked BVAI and BV-induced Fos expression in SPNs. Additionally, intrathecal administration of idazoxan (alpha2-adrenoceptor antagonist), blocked BVAI. These results indicate that BV-induced activation of the contralateral LC-descending noradrenergic pathway increased the activity of SPNs that project to the adrenal medulla and this pathway is necessary for BVAI.

Reflex activity caused by laryngoscopy and intubation is obtunded differently by meptazinol, nalbuphine and fentanyl

BACKGROUND AND OBJECTIVE

To evaluate the different potencies of several opioids in obtunding reflex mechanisms of laryngoscopy and intubation.

METHODS

Three groups of patients (each n = 25, ASA 1-2) undergoing elective plastic surgery were randomly given meptazinol (2.5 mg kg-1), nalbuphine (0.3 mg kg-1) or fentanyl (5 microg kg-1) in a blinded fashion prior to laryngoscopy and intubation. This was followed by a standardized bolus induction of a barbiturate and a muscle relaxant. The response to laryngoscopy and intubation was studied, using blood pressure, heart rate and bispectral index.

RESULTS

With fentanyl, there was an increase of heart rate by 17%, and systolic blood pressure by 7% when compared to control. Bispectral index dropped an additional 8% when compared to 1 min after barbiturate induction. In the nalbuphine group there was a 16% increase in systolic blood pressure, and a 16% increase in heart rate when compared to control. Also, bispectral index increased by 18% when compared to 1 min after barbiturate injection. The group receiving meptazinol demonstrated no cardiovascular changes although bispectral index dropped by an additional 19% when compared to 1 min after barbiturate injection.

CONCLUSION

Meptazinol, appears to depress cardiovascular stimulatory effects and electroencephalogram arousal induced by laryngoscopy and intubation better than nalbuphine or fentanyl.

Substantial role of locus coeruleus-noradrenergic activation and capsaicin-insensitive primary afferent fibers in bee venom's anti-inflammatory effect

Several lines of evidence indicate significant interactions between the immune and nervous systems. Our recent study reveals that 'bee venom (BV) induced anti-inflammatory effect' (BVAI) was produced by sympathetic preganglionic neuronal activation and subsequent adrenomedullary catecholamine release in a zymosan-induced inflammation model. However, the specific peripheral input and the supraspinal neuronal systems that are involved in this BVAI remain to be defined. Here we show that subcutaneous BV injection into left hind limb significantly reduces zymosan-induced leukocyte migration and that this effect is completely inhibited by denervation of the left sciatic nerve. This BVAI was not affected by the destruction of capsaicin-sensitive primary afferent fibers using either neonatal capsaicin or resiniferatoxin (RTX) pretreatment. BV injection into the left hind limb significantly increased Fos expression in the contralateral locus coeruleus (LC) in non-inflamed mice. In zymosan-inflamed mice, BV injection produced a further increase in LC Fos expression as compared with non-inflamed mice. This BV-induced Fos increase in the LC was not affected by RTX pretreatment. Pharmacological blockage of central noradrenergic activity by either central chemical sympathectomy (i.c.v. 6-hydroxydopamine) or alpha2 adrenoceptor antagonism (i.c.v. idazoxan) completely blocked BVAI. Taken together, these results suggest that BVAI is mediated by peripheral activation of capsaicin-insensitive primary afferent fibers and subsequent central noradrenergic activation including the LC.

Engineering novel spinal circuits to promote recovery after spinal injury

We have developed an innovative way to establish a functional bridge around a spinal lesion. We disconnected the T13 nerve from its muscle targets, leaving the proximal end intact. The cut end was inserted either into an intact spinal cord, to assess regeneration of T13 axons into the cord and synapse formation with spinal neurons, or caudal to a hemisection at L2/3, to assess restoration of function below the injury. Four to 28 weeks later, anterograde tracers indicated that axons from the inserted T13 nerve regenerated into the ventral horn, the intermediate zone, and dorsal horn base, both in intact and hemisected animals. Antibodies to cholinergic markers showed that many regenerating axons were from T13 motoneurons. Electrical stimulation of the T13 nerve proximal to the insertion site 4 weeks or more after insertion into the intact cord evoked local field potentials in the intermediate zone and ventral horn, which is where T13 axons terminated. Stimulation of T13 in 71% of the animals (8 hemisected, 7 intact) evoked contraction of the back or leg muscles, depending on the level of insertion. Animals in which T13 was inserted caudal to hemisection had significantly less spasticity and muscle wasting and greater mobility at the hip, knee, ankle, and digits in the ipsilateral hindlimb than did animals with a hemisection only. Thus, T13 motor axons form novel synapses with lumbosacral motor circuits. Because the T13 motor neurons retain their connections to the brain, these novel circuits might restore voluntary control to muscles paralyzed below a spinal lesion.

Nicotinic Acetylcholinergic Receptors Regulate the Intraspinal Release of Acetylcholine in Male Rats

Activation of cholinergic receptors in the spinal cord increases the intraspinal release of acetylcholine (ACh) and produces potent analgesia. The mechanisms that regulate the release of spinal ACh are not fully known. In the present study, we investigated the role of nicotinic ACh receptors in the regulation of intraspinal ACh release. Using an in vivo intraspinal microdialysis technique, nicotine was administered alone and in combination with the nicotinic antagonists mecamylamine (50 microM), dihydro-beta-erythroidine (DbetaE) (500 microM) and methyllycaconitine (MLA) (40 nM). Administration of nicotine (1 microM-1 mM) produced a dose dependent increase of intraspinal ACh release, while 10 mM nicotine resulted in dramatic increase in ACh release followed by a decrease to baseline. Administration of mecamylamine or DbetaE also induced an increased ACh release while MLA caused a decreased release. Mecamylamine and DbetaE, but not MLA pretreatment attenuated the stimulatory effect of 100 microM nicotine on intraspinal ACh release. It is suggested that spinal ACh release is regulated by different nicotinic ACh receptors. These receptors may tonically regulate spinal ACh release either directly or indirectly via inhibitory interneurones. Some of these receptors may be desensitised by high nicotine concentrations leading to a reduction of ACh release.

Large cholinergic nerve terminals on subsets of motoneurons and their relation to muscarinic receptor type 2

The cholinergic C-bouton is a large nerve terminal found exclusively apposing motoneuron cell somata and proximal dendrites. The origin and function of the C-bouton is not known. An antiserum against the vesicular acetylcholine transporter was used to identify large cholinergic nerve terminals putatively of the C-type in close apposition to motoneuron cell somata. This type of nerve terminal was present in the rat spinal cord ventral horn, but only in some cranial motor nuclei. Fluoro-Gold tracing showed that subsets of spinal motoneuron cell somata were contacted by different numbers of putative C-boutons. Thus, motoneurons innervating an intrinsic foot muscle were contacted by about half the number of cholinergic terminals found on motoneurons of the predominantly fast-twitch gastrocnemius muscle. Slow-twitch soleus motoneurons showed an intermediate innervation. There was a strong correlation between the presence of putative C-boutons and muscarinic receptor 2 (m2)-like immunoreactivity (-LI) within a motor nucleus. By using confocal laser microscopy, the m2-LI appeared to be confined to the motoneuron cell membrane and strongly enriched beneath the C-type nerve terminal. Thus, our results suggested a differential distribution of large cholinergic C-boutons, depending on motoneuron type, and that the presence of this nerve terminal type is associated with m2-LI in the postsynaptic membrane.

Interactions of 5-HT2 receptor agonists with acetylcholine in spinal analgesic mechanisms in rats with neuropathic pain

Serotonin type 2 (5-HT(2)) receptors reportedly inhibit neuropathic pain in the spinal cord, but little is known about how spinal 5-HT(2) receptors might act against such abnormal sensitivity. We examined whether the cholinergic and tachykinin systems were involved in the antiallodynic effect of intrathecally administered 5-HT(2) receptor agonists in rats with nerve injury. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and determined by applying von Frey hairs to the left hindpaw. Effects of intrathecal pretreatment with 5-HT(2) receptor antagonists (ketanserin and RS-102221), muscarinic receptor antagonists (atropine and scopolamine), a choline uptake blocker (hemicholium-3), and an NK(1) receptor antagonist (L-706336) were assessed in rats subsequently given a 100- micro g intrathecal dose of a 5-HT(2) receptor agonist either alpha-methyl-5-HT or iododimethoxy aminopropane (DOI). Antiallodynic effects of 5-HT(2) receptor agonists were attenuated by the 5-HT(2A) receptor antagonist ketanserin (30 micro g), but not by the 5-HT(2C) receptor antagonist RS-102221 (40 micro g). Muscarinic receptor antagonists (30 micro g each), the choline uptake blocker (10 micro g), and the NK(1) receptor antagonist (30 micro g) also inhibited the antiallodynic effects of 5-HT(2) receptor agonists. Antiallodynic effects of intrathecally administered 5-HT(2) receptor agonists may be mediated by spinal release of acetylcholine induced via 5-HT(2A) and NK(1) receptors.

Intravenously administered oxotremorine and atropine, in doses known to affect pain threshold, affect the intraspinal release of acetylcholine in rats

Both systemically and intrathecally administered cholinergic agonists produce antinociception while cholinergic antagonists decrease pain threshold. The mechanism and the site of action of these substances are not known. In the present study it was hypothesized that systemically administered muscarinic agonists and antagonists modify nociceptive threshold by affecting intraspinal release of acetylcholine (ACh). Catheters were inserted into the femoral vein in rats maintained on isoflurane anaesthesia for administration of oxotremorine (10-300 microg/kg) and atropine (0.1, 10, 5000 microg/kg). Spinal microdialysis probes were placed intraspinally at approximately the C2-C5 spinal level for sampling of acetylcholine and dialysis delivery of atropine (0.1, 1, 10 nM). Additionally, the tail-flick behaviour was tested on conscious rats injected intraperitoneally with saline, atropine (10, 100 and 5000 microg/kg), or subcutaneously with oxotremorine (30, 100, 300 microg/kg). Subcutaneous administration of oxotremorine (30, 100, 300 microg/kg) significantly increased the tail-flick latency. These doses of oxotremorine dose-dependently increased the intraspinal release of acetylcholine. Intravenously administered atropine, in a dose that produced hyperalgesia (5000 microg/kg) in the tail-flick test, significantly decreased the intraspinal release of acetylcholine. Our results suggest an association between pain threshold and acetylcholine release in spinal cord. It is also suggested that an approximately 30% increase in basal ACh release produces antinociception and that a 30% decrease in basal release produces hyperalgesia.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

Role of spinal muscarinic and nicotinic receptors in clonidine-induced nitric oxide release in a rat model of neuropathic pain

Intrathecal administration of alpha(2) adrenergic agonists, such as clonidine, is capable of alleviating neuropathic pain. Recent studies suggest that spinal nitric oxide (NO) mediates the analgesic effect of intrathecal clonidine. Furthermore, compared to nicotinic receptors, spinal muscarinic receptors play a greater role in the analgesic effect of intrathecal clonidine. In the present study, we tested a hypothesis that clonidine-evoked NO release is dependent primarily on muscarinic receptors in the spinal cord after nerve injury. A rat model of neuropathic pain was induced by ligation of the left L(5)/L(6) spinal nerves. Using an in vitro spinal cord perfusion preparation, the effect of muscarinic and nicotinic receptor antagonists on clonidine-evoked nitrite (a stable product of NO) release was determined. Both muscarinic and nicotinic antagonists dose-dependently attenuated clonidine-elicited nitrite release. In spinal cords from the neuropathic rats, the inhibitory effect of muscarinic receptor antagonists (atropine and scopolamine) on clonidine-elicited nitrite release was more potent than that of nicotinic receptor antagonists (mecamylamine and hexamethonium). However, in spinal cords obtained from sham animals, the inhibitory effect of muscarinic and nicotinic antagonists did not differ significantly. These results indicate that muscarinic, as well as nicotinic, receptors mediate clonidine-induced NO release in the spinal cord. These data also suggest that after nerve injury, the cascade of activation of alpha(2) adrenergic receptors-muscarinic receptors-NO in the spinal cord likely plays a predominant role in the analgesic effect of intrathecal clonidine on neuropathic pain.

The effect of intrathecal magnesium sulphate on nociception in rat acute pain models

We examined the antinociceptive effect of intrathecally administered magnesium sulphate (MgSO4) in rats, using acute pain models including mechanical pressure, heat and subcutaneous formalin injection. According to the locomotion test 10 microliters of 6.2% MgSO4 did not produce motor paralysis. At the same dose, responses to pressure and heat were intact, compared with controls given saline. MgSO4 produced depression of pain responses only after the first 10 min in the formalin test. Our studies indicated that MgSO4 did not show remarkable antinociceptive effects in acute pain models.

Antinociceptive effect of intrathecal neostigmine evaluated in rats by two different pain models

The analgesic efficacy of cholinergic agonists and anticholinesterase agents has been widely recognized. The analgesic effect obtained by activating cholinergic mechanisms, however, seems to depend on the experimental pain model utilized for its evaluation. The antinociceptive effect of intraspinal neostigmine was examined in rats submitted concurrently to the tail flick and formalin tests. Neostigmine (8.25 and 16.5 nmol) produced a dose-dependent antinociceptive effect in the tail flick test (a model of phasic pain) and reduced the first phase (phasic pain) of the animal response to formalin also in a dose-dependent manner. The second phase (tonic pain) of the response to formalin, however, was slightly reduced after a longer period of time only by the higher dose of the anticholinesterase. The effect of neostigmine was not significantly different when the drug was injected into rats submitted exclusively to the tail flick test. The second phase of the animal response to formalin was slightly reduced by neostigmine (8.25 nmol) and strongly inhibited by the higher dose of the anticholinesterase when injection was made after the first phase. We conclude that phasic and tonic pain can both be controlled by high doses of neostigmine. In addition, we show that inhibition by a lower dose of neostigmine of the formalin-induced phasic pain did not prevent the subsequent occurrence of tonic pain produced by the irritant.

A comparison of multiple injections versus continuous infusion of nicotine for producing up-regulation of neuronal [3H]-epibatidine binding sites

Chronic nicotine exposure in the rat produces a characteristic increase in neuronal nicotinic binding sites in many brain regions. The conventional method for inducing such increases utilizes twice daily subcutaneous injections of a near maximal, sub-convulsive dose of nicotine. Alternatively, nicotine may be chronically infused via an osmotic mini-pump. However, little is known about how administration of nicotine by chronic infusion compares to multiple injections in producing nicotinic receptor upregulation. This study used [3H]-epibatidine, a high potency neuronal nicotinic agonist radioligand, to compare the increases in receptor levels in rat brain, spinal cord and trigeminal ganglion tissues following chronic nicotine administration via either twice daily injections (2 mg/kg s.c.) or an osmotic mini-pump (1 mg/kg/hr) for 10 days. All central and peripheral nervous system tissues examined demonstrated significant neuronal nicotinic receptor up-regulation following chronic infusion of nicotine. Only the cerebral cortex and hippocampus displayed significant up-regulation following nicotine administration by injections. Moreover, in all tissues studied, the receptor levels measured were significantly higher in the animals that received nicotine by chronic infusion compared with multiple injections. These data indicate that chronic infusion of nicotine is a convenient and efficacious alternative to multiple injections for producing neuronal nicotinic receptor up-regulation in both central and peripheral nervous tissues.

Spinal cholinergic and monoamine receptors mediate the antinociceptive effect of morphine microinjected in the periaqueductal gray on the rat tail, but not the feet

The antinociceptive effects of morphine (5 micrograms) microinjected into the ventrolateral periaqueductal gray were determined using both the tail flick and the foot withdrawal responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of appropriate antagonists was used to determine whether the antinociceptive effects of morphine were mediated by alpha 2-noradrenergic, serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray was reversed by intrathecal injection of the cholinergic muscarinic receptor antagonist atropine, but was not affected by the alpha 2-adrenoceptor antagonist yohimbine, the serotonergic receptor antagonist methysergide, or the opioid receptor antagonist naloxone. In contrast, the increase in the tail flick response latency produced by morphine was reduced by either yohimbine, methysergide or atropine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibits the nociceptive responses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic and alpha 2-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptors appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.

C-terminals on motoneurons: electron microscope localization of cholinergic markers in adult rats and antibody-induced depletion in neonates

C-terminals on motoneurons are defined as those accompanied by characteristic postsynaptic specializations termed subsurface cisterns. We have previously shown, by light microscope immunolabelling methods, that subsurface cisterns occur regularly beneath choline acetyltransferase- and acetylcholinesterase-containing boutons on motoneurons. In the present study, the cholinergic nature of C-terminals suggested by these results was further investigated by immunohistochemistry and electron microscopy in adult rats and in neonates treated with a murine monoclonal acetylcholinesterase antibody which was previously shown to cause immunological lesions of central cholinergic systems. In both the facial nucleus and lumbar segment of spinal cord of adult rats, C-terminals were seen intensely immunostained for the cholinergic markers choline acetyltransferase and acetylcholinesterase. Immunolabelled terminals made contact with either neuronal somata or large calibre dendrites, which were positive for the cholinergic markers, and exhibited club-shaped or thin elongated morphologies suggestive of terminal or en passant type synaptic interactions. The close relationship found between cholinergic markers and immunolabelled subsurface cisterns in adults was maintained on motoneurons of eight-day-old rats. While subcutaneous treatment of newborn rat with acetylcholinesterase antibody appeared to have no effect on the distribution of immunopositive subsurface cisterns in motoneurons when examined on postnatal day 8, the density of labelling for the two cholinergic markers around these neurons was reduced. Areas of neuropil immediately surrounding motoneurons in treated animals often showed signs of extensive swelling and deterioration indicative of a lesion event, and these motoneurons frequently displayed subsurface cisterns unapposed to C-terminals. These results support our earlier conclusion, based on light microscope investigation, that the majority if not all C-terminals are cholinergic in the areas investigated and demonstrate the potential utility of immunolesion methods in the study of C-terminal function.

Stenosis of central canal of spinal cord in man: incidence and pathological findings in 232 autopsy cases

The central canal of the spinal cord is generally regarded as a vestigial structure that is obliterated after birth in 70% to 80% of the general population. This report describes the first detailed histological study of the human central canal in 232 subjects ranging in age from 6 weeks' gestation to 92 years. Whole spinal cords were harvested at autopsy and sectioned serially from the conus medullaris to the upper medulla. Histological findings and morphometric analysis of the cross-sectional luminal area were used to grade stenosis at seven levels of the canal. Varying grades of stenosis were present at one or more levels in none (0%) of 60 fetuses, one (3%) of 34 infants, three (18%) of 17 children, 21 (88%) of 24 adolescents and young adults, 67 (96%) of 70 middle-aged adults, and all 27 adults aged 65 years or older (100%). The stenotic process was most pronounced in the thoracic segments of the canal and involved more levels with higher grades of stenosis in older individuals. Histological findings consisted of disorganization of the ependymal epithelium, formation of ependymal rosettes or microcanals, proliferation of subependymal gliovascular buds, and intracanalicular gliosis. These features are consistent with a pathological lesion involving ependymal injury and scarring and are less compatible with an involutional or degenerative process. Stenosis of the central canal probably influences the anatomical features of syringomyelia and may account for variations in cavity formation such as the prevalence of holocord syrinxes in children, the formation of focal and paracentral syrinxes in adults, and the rare incidence of syrinx formation in many older individuals with acquired lesions known to produce syringomyelia.

Distribution of cholinergic and dopaminergic receptors in rainbow trout pineal gland

The involvement of multiple receptors in modulating the function of the pineal gland was investigated by searching for dopaminergic and cholinergic receptors in trout pineal gland. Dopamine D1 and D2 receptors were measured using [3H]SCH23390 and [3H]spiperone, respectively. Muscarinic and nicotinic cholinergic receptors were measured using quinuclidinyl benzilate ([3H]QNB) and [3H]methylcarbamyl choline, respectively. High-affinity choline uptake sites were measured using [3H]hemicholinium-3. The distribution of dopaminergic receptors varied throughout the pineal gland in that the density of D2 receptors, which was higher than that of D1 receptors, was most abundant in the distal region, exhibiting a value of 112 +/- 17 fmol/mg tissue. The distribution of both muscarinic and nicotinic receptors was uniform throughout the pineal gland. However, the highest value for the high-affinity choline transporter (106 +/- 17 fmol/mg tissue) occurred in the proximal portion of the trout pineal gland. The results of these studies indicate that the pineal gland should not be viewed as a homogeneous tissue possessing identical density of various receptors. Furthermore, these results, along with previous data, are interpreted to suggest that different regions of pineal gland may indeed possess unique functions.

The effects of cholinoceptor agonists and antagonists on C-fibre evoked responses in the substantia gelatinosa of neonatal rat spinal cord slices

1. The effects of cholinoceptor agonists and antagonists were studied on neurones in the substantia gelatinosa (SG) of an in vitro spinal cord slice and nerve preparation from neonatal rats. 2. Bath application of carbachol (1-50 microM) reduced, in a dose-related manner, the amplitude and duration of the excitatory postsynaptic potentials (e.p.s.ps) evoked in response to nerve stimulation. 3. The latencies and stimulation thresholds required to evoke these e.p.s.ps suggested that the majority were due to C-fibre activation. 4. The reduction in e.p.s.p. amplitude and duration produced by carbachol was reversed by the muscarinic antagonists, atropine (in 8 out of 11 cells), pirenzepine (in 7 out of 9 cells) and methoctramine (in 8 out of 9 cells) and by the nicotinic antagonist mecamylamine (in 3 out of 7 cells). 5. Injection of small hyperpolarizing or depolarizing pulses was associated with no change in conductance in 19 out of 26 (73%) of cells tested, suggesting that an action at a site presynaptic to the neurone studied could account for part of the effect of carbachol. 6. It is proposed that some of the cholinoceptors associated with the e.p.s.p. depression are located on C-fibres.

Tonic cholinergic inhibition of spinal mechanical transmission

The present study examined the role of spinal cholinergic modulation of spinal mechanical and thermal transmission. Intrathecal administration of the cholinergic muscarinic receptor antagonists atropine or scopolamine in awake rats produced a dose-dependent decrease in the nociceptive mechanical withdrawal threshold of the rat tail. Pirenzepine, a selective muscarinic receptor type 1 antagonist, produced a similar effect at greater doses while mecamylamine, a nicotinic receptor antagonist, was without effect. The nociceptive tail flick (TF) reflex evoked by noxious heating was unaffected by the above drugs. Intrathecal administration of the cholinesterase inhibitor physostigmine produced a rapid, reversible and significant increase in the mechanical withdrawal threshold; TF latency was increased slightly but not significantly. Intrathecal administration of morphine, carbachol or clonidine all produced dose-dependent increases in TF latency; morphine and carbachol, but not clonidine, also increased the mechanical withdrawal threshold significantly. Intrathecal pretreatment with atropine reversed carbachol-produced increases in TF latency and the mechanical withdrawal threshold but did not affect increases in TF latency produced by intrathecal morphine or clonidine. The morphine-produced increase in the mechanical withdrawal threshold, however, was shifted rightward in a parallel fashion by intrathecal pretreatment with atropine. Intrathecal pretreatment with yohimbine did not affect the inhibitory effect of carbachol on either TF latency or the mechanical withdrawal threshold. These results suggest that a tonic, endogenous cholinergic muscarinic influence in the spinal cord, independent of spinal adrenergic mechanisms, modulates spinal mechanical transmission.

Spinal cholinergic and monoaminergic receptors mediate descending inhibition from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) both inhibit the nociceptive tail-flick (TF) reflex in rats. The present experiments were undertaken to determine the transmitter(s) at the level of the lumbar spinal cord mediating descending inhibition of the TF reflex produced by activation of the NGC/NGC alpha. Intrathecal administration of atropine (7.2-57.6 nmol) produced a dose-dependent increase in the electrical stimulation threshold required for inhibition of the TF reflex. Phentolamine (47.2 or 94.4 nmol) and methysergide (32 or 64 nmol) also increased the stimulation threshold significantly, but only at the greater doses. Neither naloxone (27.5 or 55 nmol) nor mecamylamine (49.1 or 98.2 nmol) affected stimulation thresholds for inhibition of the TF reflex. Stimulation at threshold intensities for inhibition did not change the blood pressure significantly at most sites of stimulation in the NGC/NGC alpha (25/39). Intrathecal administration of atropine, phentolamine or methysergide did not affect resting blood pressure or changes associated with stimulation in most cases, although inhibition of the TF reflex by stimulation in the NGC/NGC alpha was affected consistently by these pretreatments. Similarly, glutamate (100 nmol, 0.5 microliter/1.5 min) microinjection produced a short-lasting inhibition (4.63 +/- 0.70 min, n = 19) of the TF reflex. Glutamate microinjection produced both pressor and depressor effects which were not affected by intrathecal pretreatment. Inhibition of the TF reflex by glutamate was attenuated significantly by intrathecal pretreatment with atropine, scopolamine, phentolamine and methysergide, but not naloxone or mecamylamine. These findings suggest that either a descending or local spinal cholinergic system, together with descending serotonergic and noradrenergic systems, are involved in the centrifugal inhibition of spinal nociceptive transmission from the NGC/NGC alpha.

Choline acetyltransferase-immunoreactive profiles are presynaptic to primary sensory fibers in the rat superficial dorsal horn

The specific aim of this study was to search for morphological counterparts to the known antinociceptive effects of cholinomimetic drugs at the spinal cord level. For this, the light microscopic and ultrastructural distribution of choline acetyltransferase immunoreactivity was studied in laminae I-III of the rat cervical spinal cord. Immunoreactivity was present in cell bodies in lamina III, and in dendrites and axons of all three laminae. Immunoreactive axonal varicosities were often presynaptic to the central varicosities of type II synaptic glomeruli in lamina II and lamina III, less often presynaptic to the central elements of type I glomeruli in lamina II, and often presynaptic to dendrites in both type I and type II glomeruli. In addition, immunoreactive dendrites were often postsynaptic to the central varicosities of glomeruli of all morphological types. These results indicate that 1) primary sensory fibers excite cholinergic interneurons; 2) the acetylcholine released by the axon terminals of these interneurons modulates both nociceptive and non-nociceptive sensory information at the spinal cord level through both pre- and postsynaptic mechanisms. Furthermore, our results reinforce current ideas on reciprocal sensory interaction between thick and fine afferent fibers.

Behavioral effects after intrathecal administration of cholinergic receptor agonists in the rat

Behavioral effects of nicotine and cytisine, and the cholinesterase inhibitors physostigmine and 9-amino-1,2,3,4-tetrahydroacridine (THA), administered intrathecally (IT) at the lumbar level in the rat have been evaluated. Antinociceptive dose relationships were established using the tail immersion test. Total activity, locomotion and rearing were also measured in computerized test boxes. The nicotinic receptor antagonist, mecamylamine, and the muscarinic receptor antagonist, atropine, were used to study the selectivity of the effects. Physostigmine and THA significantly decreased total activity, locomotion and rearing as compared to control animals. The motor effects of physostigmine were completely antagonized only partly. Mecamylamine had no antagonistic effect. Nicotine did not affect any activity parameter. Cytisin reduced total activity and locomotion 1-6 min after dose. IT physostigmine, 15 micrograms, increased tail immersion latency for 30 min. No significant increase in response latency in this test was observed after the IT administration of nicotine or THA, whereas cytisine elicited a small increase. The IT administration of THA, nicotine and cytisine was also associated with gnawing, vocalization and hyperactivity and in the case of THA, diarrhoea. These effects were blocked by mecamylamine. Physostigmine antinociception as well as the behavioral effects including total activity, locomotion and rearing caused by physostigmine and by THA are most probably due to an action on spinal muscarinic receptors. Nicotinic receptors do not seem to be involved in spinal antinociception. Some aversive behavioral effects caused by the IT administration of nicotinic receptor agonists could, however, be attenuated by the spinal administration of the antagonist mecamylamine, which may indicate the involvement of nicotinic receptors in afferent sensory transmission.

Characterization of the antinociception induced by intrathecally administered carbachol

The antinociceptive effect of intrathecally administered carbachol at the L1/L2 level in the rat was evaluated using the tail immersion test. A dose dependent increase in the nociceptive reaction times was evident following intrathecal carbachol in the dose range of 2.5-15 micrograms. At doses of 20 micrograms and above, although still effective in the test, motor impairment was pronounced. The antinociception was antagonized with atropine, and with either pirenzepine (PZ) or AFDX 116, which are selective M1 and M2 muscarinic receptor blocking drugs, respectively. Spinal cholinergic pain modulation was also studied in rats pretreated with DSP4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine), which causes a selective depletion of the noradrenergic nerve fibres in the CNS. The increased latency times after spinal carbachol were attenuated in animals depleted of spinal noradrenaline by DSP4. In conclusion, spinal analgesia by carbachol in the rat may therefore be mediated through both M1 and M2 muscarinic receptor stimulation in the spinal cord. It is also concluded that this spinal cholinergic pain modulation is interacting with spinal noradrenergic nerve terminals, but that the mechanism of the interaction remains to be established.