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

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

Distribution of and organisation of dorsal horn neuronal cell bodies that possess the muscarinic m2 acetylcholine receptor

Cholinergic systems in the dorsal horn are involved in antinociception but little is known about the organisation of receptors that mediate this process. In this study we examined immunocytochemical properties of dorsal horn neuronal cell bodies that express the m2 muscarinic acetylcholine receptor. Tissue was examined with confocal laser scanning microscopy and quantitative analysis performed. Immunoreactive cells were found throughout the dorsal horn and in lamina X. Quantitative analysis revealed that 22% of neuronal somata in the dorsal horn possess the receptor. The greatest concentration of cells was found in deeper laminae (IV-VI) and around lamina X. A proportion of cholinergic cells (labelled with an antibody against choline acetyltransferase) were immunoreactive for the receptor (approximately, 40% of dorsal horn cells and 44% of lamina X cells). Populations of presumed inhibitory interneurons also displayed immunoreactivity for the receptor. Between 27-34% of cells immunoreactive for GABA, nitric oxide synthase and the somatostatin receptor(2A) expressed the receptor but only 8% of parvalbumin-immunoreactive cells displayed receptor immunoreactivity. Cells labelled with neurotensin, which belong to a subgroup of excitatory neurons, displayed no receptor immunoreactivity. A small number neurokinin-1 receptor-immunoreactive cells in lamina I possessed m2 immunoreactivity but 42% of laminae III/IV neurokinin-1 cells possessed it. This study shows that a significant proportion of cell bodies in the dorsal horn express the muscarinic m2 acetylcholine receptor. The receptor is present on some cholinergic neurons and therefore may function as an autoreceptor. It is associated with inhibitory local circuit neurons and may have a role in the modulation of specific inhibitory systems. It is also found on a proportion of projection cells that possess the neurokinin-1 receptor. This could be the basis of some of the antinociceptive actions of acetylcholine.

Evaluation of muscarinic agonist-induced analgesia in muscarinic acetylcholine receptor knockout mice

Centrally active muscarinic agonists display pronounced analgesic effects. Identification of the specific muscarinic acetylcholine receptor (mAChR) subtype(s) mediating this activity is of considerable therapeutic interest. To examine the roles of the M(2) and M(4) receptor subtypes, the two G(i)/G(o)-coupled mAChRs, in mediating agonist-dependent antinociception, we generated a mutant mouse line deficient in both M(2) and M(4) mAChRs [M(2)/M(4) double-knockout (KO) mice]. In wild-type mice, systemic, intrathecal, or intracerebroventricular administration of centrally active muscarinic agonists resulted in robust analgesic effects, indicating that muscarinic analgesia can be mediated by both spinal and supraspinal mechanisms. Strikingly, muscarinic agonist-induced antinociception was totally abolished in M(2)/M(4) double-KO mice, independent of the route of application. The nonselective muscarinic agonist oxotremorine showed reduced analgesic potency in M(2) receptor single-KO mice, but retained full analgesic activity in M(4) receptor single-KO mice. In contrast, two novel muscarinic agonists chemically derived from epibatidine, CMI-936 and CMI-1145, displayed reduced analgesic activity in both M(2) and M(4) receptor single-KO mice, independent of the route of application. Radioligand binding studies indicated that the two CMI compounds, in contrast to oxotremorine, showed >6-fold higher affinity for M(4) than for M(2) receptors, providing a molecular basis for the observed differences in agonist activity profiles. These data provide unambiguous evidence that muscarinic analgesia is exclusively mediated by a combination of M(2) and M(4) mAChRs at both spinal and supraspinal sites. These findings should be of considerable relevance for the development of receptor subtype-selective muscarinic agonists as novel analgesic drugs.

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.

Spinal interneuronal systems: identification, multifunctional character and reconfigurations in mammals

This review focuses on the flexibility of operation of spinal interneuronal networks and their multifunctional character in mammals. It concerns, in particular, two ways in which spinal interneuronal networks may be functionally reorganised, namely by modulating the synaptic actions of primary afferents by monoamines and by GABAergic presynaptic inhibition. The evidence will be reviewed for topographical and target-related differences in modulatory effects in various interneuronal networks and these will be related to differences in the intrinsic properties of different functional types of interneurones in these networks and to the role played by them.

M2-receptor subtype does not mediate muscarine-induced increases in [Ca(2+)](i) in nociceptive neurons of rat dorsal root ganglia

Multiple muscarinic receptor subtypes are present on sensory neurons that may be involved in the modulation of nociception. In this study we focused on the presence of the muscarinic receptor subtypes, M2 and M3 (M2R, M3R), in adult rat lumbar dorsal root ganglia (DRG) at the functional ([Ca(2+)](i) measurement), transcriptional (RT-PCR), and translational level (immunohistochemistry). After 1 day in culture exposure of dissociated medium-sized neurons (20-35 micrometer diam) to muscarine was followed by rises in [Ca(2+)](i) in 76% of the neurons. The [Ca(2+)](i) increase was absent after removal of extracellular calcium and did not desensitize after repetitive application of the agonist. This rise in [Ca(2+)](i) may be explained by the expression of M3R, which can induce release of calcium from internal stores via inositoltrisphospate. Indeed the effect was antagonized by the muscarinic receptor antagonist atropine as well as by the M3R antagonist, 4-diphenylacetoxy-N-(2 chloroethyl)-piperidine hydrochloride (4-DAMP). The pharmacological identification of M3R was corroborated by RT-PCR of total RNA and single-cell RT-PCR, which revealed the presence of mRNA for M3R in lumbar DRG and in single sensory neurons. In addition, RT-PCR also revealed the expression of M2R, which did not seem to contribute to the calcium changes since it was not prevented by the M2 receptor antagonist, gallamine. Immunohistochemistry demonstrated the presence of M2R and M3R in medium-sized lumbar DRG neurons that also coexpressed binding sites for the lectin I-B4, a marker for mainly cutaneous nociceptors. The occurrence of muscarinic receptors in putative nociceptive I-B4-positive neurons suggests the involvement of these acetylcholine receptors in the modulation of processing of nociceptive stimuli.

Spinal modulation of the muscle pressor reflex by nitric oxide and acetylcholine

Static contraction of skeletal muscle activates the sympathetic nervous system, which in turn increases cardiovascular function. These changes are mediated, in part, by a reflex arising from the contracting muscle. This reflex is termed the exercise pressor reflex or, more simply, the muscle pressor reflex (MPR). Over the past few years, studies have been performed investigating the sensory processing that occurs in the dorsal horn of the spinal cord as it pertains to the MPR. Several putative neurotransmitters and receptors have been implicated in mediating the MPR at the level of the dorsal horn. In addition, several receptor systems have been shown to modulate the MPR at the dorsal horn. We have recently performed studies investigating the potential modulatory role of dorsal horn nitric oxide (NO) and acetylcholine (ACH) on the MPR. Along these lines, our experiments suggest that NO enhances the excitability of dorsal horn cells receiving input from muscle afferent neurons, while ACH decreases the MPR when its concentration in the dorsal horn is elevated. The purpose of this manuscript is to review recently published findings from our laboratory and apply this information in an effort to better understand the integration of sensory input that occurs in the dorsal horn as it pertains to cardiovascular regulation. This review is also designed to stimulate questions as to how these two neurochemicals exert their actions and whether or not they represent or can represent important physiological mechanisms involved in regulating the dorsal horn integration of the MPR.

Spinal cholinergic inhibition of the pressor response to skeletal muscle activation

The purpose of this study was to delineate the role of the cholinergic pathway within the spinal cord in the reflex cardiovascular responses to muscle activity. Based on dose-response experiments, we microdialyzed a 0.1 mM solution of the acetylcholinesterase inhibitor neostigmine into the L7 level of the dorsal horn of anesthetized cats to determine its effects on the mean arterial blood pressure (MAP) and heart rate (HR) responses to static muscle contraction or passive stretch. The peak responses to 1-min contractions and stretches were reduced from control levels after 1 h of drug administration. In four experiments, the cardiovascular responses returned to control levels after a 2-h recovery period. Perfusion of the cholinergic receptor antagonist atropine accentuated the peak MAP response to muscle contraction. By contrast, atropine administration had no effect on the peak MAP adjustment to passive muscle stretch. These data support the hypothesis that increased acetylcholine (ACh) concentrations in the spinal cord inhibit the reflex cardiovascular responses to static muscle contraction. Further, the results suggest that the spinal cholinergic system is activated by metabolic changes in skeletal muscle, but likely unaffected by mechanical muscle changes.

Muscarinic M2-receptors in rat thoracic dorsal root ganglia

The occurrence and distribution of the muscarinic M2-receptor subtype (M2R) was investigated in rat thoracic dorsal root ganglia (DRG). Messenger RNA for M2R was demonstrated by RT-PCR in total RNA from DRG. Immunoreactivity to M2R-protein was localized to 26% of sensory neurons, the majority of them (85%) belonging to the size class of 25-40 microm in diameter. Double-labeling (immuno)histochemistry revealed that all M2R-immunoreactive neurons bind the lectin, I-B4, whereas they are generally devoid of substance P-immunoreactivity. These data show the presence of M2R on a subpopulation of presumably nociceptive primary afferent neurons, thereby extending previous pharmacological and electrophysiological studies that indicated a role of M2R and/or M4R in inhibition of calcium channel currents in rat sensory neurons (Wanke, E., Bianchi, L., Mantegazza, M., Guatteo, E., Macinelli, E. and Ferroni, A., Muscarinic regulation of Ca2+ currents in rat sensory neurons: channel and receptor types, dose-response relationships and cross-talk pathways. Eur. J. Neurosci., 6 (1994) 381-391).

Sensitization of pain pathways in the spinal cord: cellular mechanisms

Sensitization is manifested as an increased response of neurones to a variety of inputs following intense or noxious stimuli. It is one of the simplest forms of learning and synaptic plasticity and it represents an important feature of nociception. In the spinal cord, repeated stimulation (at constant strength) of dorsal root afferents including nociceptive C fibres can elicit a progressive increase in the number of action potentials generated by motoneurones and interneurones. This phenomenon is termed "action potential windup" and is used as a cellular model of pain sensitization developing at the level of the central nervous system. Understanding the mechanisms responsible for windup generation might allow clarification of the cellular mechanisms of pain signalling and development of new strategies for pain treatment. Action potential windup is observed in a minority of cells only, indicating that certain cell-specific mechanisms are responsible for its generation. The most reliable index to predict windup generation is the rate at which the membrane potential is depolarized during repetitive stimulation. This phenomenon has been proposed to be due to gradual recruitment of NMDA receptor activity, to summation of slow excitatory potentials mediated by substance P (and related peptides) or to facilitation of slow calcium channels by metabotropic glutamate receptors. Little is known about the role of synaptic inhibition in windup, although it should not be underestimated. Each theory per se is unable to account for all the experimental observations. Since NMDA receptors are involved in many forms of synaptic plasticity, additional mechanisms such as summation of slow peptidergic potentials, facilitation of slow Ca2+ currents and disinhibition are proposed as necessary to impart specificity to pain-induced sensitization. These additional mechanisms might be species specific and change during development or chronic pain states.