Inhibition of cyclic AMP production by alpha 2-adrenoceptor stimulation in the guinea-pig spinal cord slices

Norepinephrine reduces heat responses of cutaneous C-fiber nociceptors in Sprague–Dawley rats in vitro

Nociceptors are excited or sensitized by many inflammatory mediators as well as by elevation of tissue temperature. We have shown that there is a facilitatory synergistic interaction between norepinephrine (NE) and bradykinin (BK) on cutaneous C-fiber nociceptors in normal Lewis rats. These interactions may play an important role in the mechanism of sympathetically maintained pain. In the present experiment, using skin-saphenous nerve in vitro preparations, we tested the effect of NE on the activity of nociceptive fibers and their response to heat in normal Sprague-Dawley rats. For comparison with the previous data on Lewis rats, we also examined the effect of NE on BK response. NE (10(-5) or 10(-6) M) did not excite nociceptive fibers before repeated heat stimuli or BK superfusion (10(-5) or 10(-6) M) to the receptive field. In contrast, after a few applications of heat or BK, NE excited 20-43% of nociceptive fibers to similar magnitudes. We also found that NE sensitized subsequent BK responses, but somewhat unexpectedly that it suppressed subsequent heat responses. This occurred regardless of the presence or absence of NE-induced excitation. These results suggest different mechanisms of NE modification to the BK and heat responses of cutaneous C-fiber nociceptors.

Intrathecal ketorolac enhances antinociception from clonidine

Although both alpha2-adrenergic agonists and cyclooxygenase inhibitors produce analgesia, their exact sites of action and interaction remain unclear. A previous report demonstrated a surprising inhibition of antinociception in rats from intrathecal clonidine by co-administered ketorolac. There are no other reports of interaction between these two classes of analgesics. We therefore reexamined this interaction, determining the effect of intrathecal clonidine and ketorolac alone and in combination in normal rats. Clonidine, but not ketorolac, produced antinociception to noxious hind paw thermal stimulation. The addition of ketorolac significantly enhanced the effect of clonidine, indicating a synergistic interaction for analgesia. Although the reasons for the discrepancy between this and the previous report are unclear, these results are consistent with previous studies that indicate an antinociceptive action of intrathecal alpha2-adrenergic agonists in the normal condition, a lack of such effect for cyclooxygenase inhibitors, and positive reinforcing effects of these two systems when co-stimulated.

IMPLICATIONS

Spinal injection of the alpha2-adrenergic agonist clonidine and the cyclooxygenase inhibitor ketorolac results in a synergistic interaction for antinociception in normal animals, suggesting that the combination of these drugs will enhance rather than detract from the analgesia of either alone.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Alpha2-adrenoceptor-mediated presynaptic inhibition in bulbospinal neurons of rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) controls sympathetic tone via excitatory bulbospinal neurons. It is also the main target of alpha2-adrenoceptor (alpha2-AR) agonists used for treatment of hypertension. In this study, we examined the synaptic mechanisms by which alpha2-AR agonists may inhibit the activity of RVLM bulbospinal neurons. We recorded selectively from RVLM bulbospinal neurons in brain stem slices of neonate rats (P5-P21) using the patch-clamp technique (holding potential -70 mV). alpha2-ARs were activated by norepinephrine (NE, 30 microM) in the presence of the alpha1-adrenoceptor blocker prazosin. NE induced modest outward currents (5-28 pA) in 70% of the cells that were blocked by barium and by the alpha2-AR antagonist 2-methoxyidazoxan. The magnitude of this current was not correlated with the tyrosine hydroxylase immunoreactivity of the neurons. Mono- and oligosynaptic excitatory postsynaptic currents (EPSCs) or monosynaptic inhibitory postsynaptic currents (IPSCs) were evoked by focal electrical stimulation. In all cells, NE decreased the amplitude of the evoked EPSCs in the absence or presence of barium (49 and 70%) and decreased the amplitude of the evoked IPSCs (64 and 59%). The effect of NE on EPSC amplitude was blocked by 2-methoxyidazoxan. Focal stimulation produced a 1- to 2-s EPSC afterdischarge (probably due to activation of interneurons) that was 53% inhibited by NE. In the presence of tetrodotoxin, NE decreased the frequency of miniature EPSCs by 74%. In short, alpha2-AR stimulation produces weak postsynaptic responses in RVLM bulbospinal neurons and powerful presynaptic inhibition of both glutamatergic and GABAergic inputs. Thus the inhibition of RVL bulbospinal neurons by alpha2-AR agonists in vivo results from a combination of postsynaptic inhibition, disfacilitation, and disinhibition.

Differential distribution of alpha2A and alpha2C adrenergic receptor immunoreactivity in the rat spinal cord

alpha2-Adrenergic receptors (alpha2-ARs) mediate a number of physiological phenomena, including spinal analgesia. We have developed subtype-selective antisera against the C termini of the alpha2A-AR and alpha2C-AR to investigate the relative distribution and cellular source or sources of these receptor subtypes in the rat spinal cord. Immunoreactivity (IR) for both receptor subtypes was observed in the superficial layers of the dorsal horn of the spinal cord. Our results suggest that the primary localization of the alpha2A-AR in the rat spinal cord is on the terminals of capsaicin-sensitive, substance P (SP)-containing primary afferent fibers. In contrast, the majority of alpha2C-AR-IR was not of primary afferent origin, not strongly colocalized with SP-IR, and not sensitive to neonatal capsaicin treatment. Spinal alpha2C-AR-IR does not appear to colocalize with the neurokinin-1 receptor, nor is it localized on astrocytes, as evidenced by a lack of costaining with the glial marker GFAP. However, some colocalization was observed between alpha2C-AR-IR and enkephalin-IR, suggesting that the alpha2C-AR may be expressed by a subset of spinal interneurons. Interestingly, neither subtype was detected on descending noradrenergic terminals. These results indicate that the alpha2-AR subtypes investigated are likely expressed by different subpopulations of neurons and may therefore subserve different physiological functions in the spinal cord, with the alpha2A-AR being more likely to play a role in the modulation of nociceptive information.

The alpha2a adrenergic receptor subtype mediates spinal analgesia evoked by alpha2 agonists and is necessary for spinal adrenergic-opioid synergy

Agonists acting at alpha2 adrenergic and opioid receptors have analgesic properties and act synergistically when co-administered in the spinal cord; this synergy may also contribute to the potency and efficacy of spinally administered morphine. The lack of subtype-selective pharmacological agents has previously impeded the definition of the adrenergic receptor subtype(s) mediating these effects. We therefore exploited a genetically modified mouse line expressing a point mutation (D79N) in the alpha2a adrenergic receptor (alpha2aAR) to investigate the role of the alpha2aAR in alpha2 agonist-evoked analgesia and adrenergic-opioid synergy. In the tail-flick test, intrathecal administration of UK 14,304, a nonsubtype-selective alpha2AR agonist, had no analgesic effect in D79N mice, whereas the analgesic potency of morphine (intrathecal) in this assay was not affected by the mutation. The mutation also decreased alpha2-agonist-mediated spinal analgesia and blocked the synergy seen in wild-type mice with both the delta-opioid agonist deltorphin II and the micro-opioid agonist [D-ALA2,N-Me-Phe4, Gly-ol5]-Enkephalin (DAMGO) in the substance P behavioral test. In addition, the potency of spinally administered morphine was decreased in this test, suggesting that activation of descending noradrenergic systems impinging on the alpha2aAR contributes to morphine-induced spinal inhibition in this model. These results demonstrate that the alpha2aAR subtype is the primary mediator of alpha2 adrenergic spinal analgesia and is necessary for analgesic synergy with opioids. Thus, combination therapies targeting the alpha2aAR and opioid receptors may prove useful in maximizing the analgesic efficacy of opioids while decreasing total dose requirements.

Dissociation of hypnotic-anesthetic actions of alpha 2 agonists from cyclic AMP in the rat

alpha 2 adrenergic agonists are used clinically for their anesthetic, analgesic, and sympatholytic actions in surgical patients. All alpha 2 adrenergic receptors, when activated by alpha 2-adrenergic agonists, are able to inhibit adenylate cyclase. We have examined the alpha 2-adrenoceptor-mediated anesthetic actions of dexmedetomidine, a highly selective alpha 2-adrenergic agonist, after pretreatment of the animals with rolipram, a cyclic AMP (cAMP)-specific phosphodiesterase inhibitor, cAMP accumulation and monoamine turnover were measured in the locus coeruleus (LC) and hippocampus (HC) following administration of rolipram [275 mg/kg, intraperitoneally (IP)] and dexmedetomidine (100-500 mg/kg, IP). The hypnotic response to dexmedetomidine was also measured in these animals. In other experiments, rats were stereotactically cannulated in the LC with an indwelling catheter, and after the second day, the tail-flick analgesic response to dexmedetomidine (3.5 mg/0.2 ml LC), following rolipram (275 mg/kg, IP) pretreatment, was assessed. In the presence of elevated cAMP levels, the hypnotic, analgesic, and sympatholytic effects of dexmedetomidine persisted. These data suggest that adenylate cyclase activity does not mediate the cellular responses to alpha 2-adrenergic agonists but instead may act in concert with other alpha 2-adrenoceptor-coupled effector mechanisms to transduce the anesthetic actions of these agents.

Effects of cAMP on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rat

Involvement of cAMP in the generation of respiratory rhythm was studied in newborn rat brainstem-spinal cord preparations. The respiratory rhythm was monitored by C4 inspiratory activity and/or pre-inspiratory (Pre-I) activity of neurons in the rostral ventrolateral medulla; previously suggested to be primary rhythm generating neurons which have pacemaker properties. The effects of four cAMP-increasing agents (forskolin, IBMX, Db-cAMP, and 8-Br-cAMP) on this neuronal activity were examined. Perfusion with forskolin (3-10 microM) increased the burst rate of C4 inspiratory activity in 20 of 32 preparations, but in 8 of those the increase was preceded by transient depression. The facilitation of the respiratory rhythm was greater whenever the burst rate before forskolin treatment was lower. The Pre-I neuron burst rate, which was recorded together with C4 activity, predominantly increased with forskolin treatment. The effects of IBMX, Db-cAMP and 8-Br-cAMP were similar to those of forskolin, but they were slightly less potent. Long-lasting depression of the respiratory rhythm (C4 and Pre-I activity) by clonidine, which might decrease intracellular cAMP level via alpha 2-receptors, was reversed by forskolin. To investigate the direct effects of the cAMP-increasing agents on Pre-I neurons, Pre-I activity was isolated by blocking the chemical synaptic transmission by incubation in a low Ca solution (0.2 mM Ca2+, 5 mM Mg2+). Forskolin (5-10 microM), IBMX (5-10 microM), Db-cAMP (0.2-0.4 mM), and 8-Br-cAMP (0.4-0.75 mM) all enhanced the burst rate of isolated Pre-I neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of substance P on respiratory rhythm and pre-inspiratory neurons in the ventrolateral structure of rostral medulla oblongata: an in vitro study

The pre-inspiratory (Pre-I) neurons which fire in the pre- and usually also during the post-inspiratory phase are located in the ventrolateral structures of the rostral medulla. They are suggested as primary rhythm generating neurons for respiration. These have been studied in isolated brainstem-spinal cord preparations from newborn 0-5-day-old rats. We have found that application of substance P (SP) enhanced the respiratory rhythm as measured by C4 ventral root and pre-I neuronal activities. Furthermore, the effect of SP was dependent on basal respiratory rate. An increase of the Pre-I and C4 burst rate by SP was clearer when the basal respiratory rhythm was somewhat lower. Moreover, long lasting depression of respiratory rate after the application of the alpha 2-agonist clonidine was reversed by SP. On the other hand, an inhibitory effect appeared in preparations with a higher basal respiratory rate, while the Pre-I burst rate tended to increase during SP perfusion. During chemical synaptic transmission blockade by perfusion with low Ca2+, high Mg2+ solution, a pre-I burst retained or completely blocked was found to be enhanced or reactivated by SP perfusion. The results suggest a direct postsynaptic action of SP, which could strongly stimulate burst generating properties of Pre-I neurons.

Inhibition of cAMP production by alpha 2-adrenoceptor stimulation in rabbit retina

Adenylate cyclase activity in rabbit retinal homogenates can be stimulated directly by forskolin or through a receptor-mediated mechanism by vasoactive intestinal peptide (VIP). In contrast the alpha 2-adrenoceptor agonists clonidine and UK-14,304 reduce the basal cAMP level slightly. This was more evident following application of forskolin and VIP where the decrease of cAMP caused by clonidine and UK-14,304 is dose-dependent. The alpha 2-adrenoceptor agonist response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor, isobutylmethylxanthine, suggesting the involvement of a Gi-protein. Clonidine and UK-14,304 attenuation of elevated cAMP levels can be inhibited by the alpha 2-receptor antagonist yohimbine and phentolamine but not by the specific alpha 1-receptor antagonist, prazosin. Serotonergic, cholinergic and beta-adrenergic receptor antagonists were without effect. The results demonstrate that alpha 2-adrenergic receptors in the retina exert inhibitory effects on adenylate cyclase activity mediated by an inhibitory guanine nucleotide regulating protein.

Antinociceptive actions of alpha 2-adrenoceptor agonists in the rat spinal cord: evidence for antinociceptive alpha 2-adrenoceptor subtypes and dissociation of antinociceptive alpha 2-adrenoceptors from cyclic AMP

The antinociceptive actions of intrathecal injections of two alpha 2-adrenergic agonists, UK-14,304 and guanfacine, were investigated in rats after pretreatment of the animals with the noradrenaline neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) 14 days in advance. The chronic noradrenaline depletion induced by DSP4 caused a marked increase in sensitivity of the antinociceptive action of UK-14,304 in the tail-flick test. By contrast, the antinociceptive effect of guanfacine was not appreciably affected by the DSP4 treatment. The antinociceptive effects of both UK-14,304 and guanfacine were blocked by intraperitoneal injections of yohimbine, a result indicating that both drugs induced their actions by activating alpha 2-adrenoceptors. Both UK-14,304 and guanfacine were found to reduce the production of cyclic AMP (cAMP) in the spinal cord, as determined using an in vitro radioisotopic method. The cAMP inhibitory effects of both agonists were effectively blocked by yohimbine, but not by prazosin, a finding indicating the alpha 2-adrenergic nature of the response. However, the cAMP inhibitory effect of UK-14,304 was not potentiated by pretreatment with DSP4, a finding in marked contrast with the strong potentiation of the antinociceptive action of UK-14,304 induced by the chronic depletion of endogenous noradrenaline. Moreover, intrathecal injections of forskolin, which increased the endogenous levels of spinal cord cAMP fivefold, did not modify the antinociceptive effects of UK-14,304 or guanfacine in neither normal nor DSP4-treated animals. It is suggested that there exist pharmacologically differing alpha 2-adrenergic receptor pathways capable of mediating antinociceptive effects at the level of the spinal cord. The cAMP inhibitory actions of spinal cord alpha 2-adrenoceptors appear not to be directly linked with the antinociceptive actions of these receptors.

Functional supersensitivity of antinociceptive spinal cord alpha 2-adrenoceptors, induced by depletion of endogenous noradrenaline, is associated with an enhanced sensitivity for guanine nucleotide regulation of 3H-clonidine binding

In spinal cords from normal mice, 3H-clonidine bound to alpha 2-adrenoceptors with an apparant Kd of 4.6 nM and capacity (Bmax) of 430 fmol/mg protein. The non-hydrolyzable GTP analogue Gpp(NH)p was found to dose-dependently reduce the binding of 3H-clonidine the effect of Gpp(NH)p being essentially maximal at 10(-4) M. Gpp(NH)p was found to reduce the apparent affinity as well as the apparent number of binding sites for 3H-clonidine; the Kd of 3H-clonidine being 16 nM and the Bmax 300 fmol/mg protein when 10(-4) M of the nucleotide was present. In mice pretreated with the noradrenaline neurotoxin DSP4 for 14 days prior to assay, the specific binding of nearly saturating concentrations (10-30 nM) of 3H-clonidine was virtually the same as for control mice. However, for the DSP4 treated animals the down-regulation of 3H-clonidine binding induced by 10(-4) M Gpp(NH)p was significantly higher than for control mice. Thus, at a 3H-clonidine concentration of 10 nM the down-regulation induced by Gpp(NH)p was 64.1 +/- 2.6% for the DSP4 versus 58.9 +/- 2.3% for the control mice (P less than 0.05). At a 3H-clonidine concentration of 30 nM the down-regulation was 52.4 +/- 1.4% for DSP4 versus 41.8 +/- 4.1% for control mice (P less than 0.05). DSP4 pretreatment also potentiated the antinociceptive effect of intrathecal clonidine, as assessed by the tail-flick and hot plate tests. It is suggested that the functional supersensitivity induced by DPS4 for clonidine is related to the increased sensitivity for guanine nucleotide regulation of the spinal cord alpha 2-adrenoceptors.

Alpha 2-adrenoceptors mediate inhibition of cyclic AMP production in the spinal cord after stimulation of cyclic AMP with forskolin but not after stimulation with capsaicin or vasoactive intestinal peptide

In slices obtained from the ventral and the dorsal guinea pig spinal cord both forskolin and vasoactive intestinal peptide (VIP) caused a dose-dependent stimulation of the production of cyclic AMP. By contrast capsaicin stimulated cyclic AMP formation only in the dorsal cord; no effect was observed in the ventral cord. The alpha 2-adrenergic agonist UK-14,304 dose-dependently inhibited the production of cyclic AMP in both the dorsal and ventral aspects of the cord when the formation of cyclic AMP had been stimulated with 3 microM forskolin, the maximal inhibition amounting to 25-32%. Also the basal (i.e., unstimulated) production of cyclic AMP was inhibited, the inhibition amounting to about 16-18%. However, after stimulation of cyclic AMP formation in the dorsal cord with capsaicin, UK-14,304 was virtually ineffective in inhibiting the accumulation of cyclic AMP. Also, when the formation of cyclic AMP was stimulated with VIP, UK-14,304 was virtually ineffective in inhibiting the formation of cyclic AMP both in the ventral and the dorsal parts of the cord. When cyclic AMP production had been stimulated with forskolin the ability of UK-14,304 to inhibit the formation of cyclic AMP was not attenuated by capsaicin, either in the ventral or in the dorsal cord. The results are discussed with the notion that cyclic AMP inhibitory spinal cord alpha 2-adrenoceptors are located on cells accessible to stimulation of cyclic AMP with forskolin but not with capsaicin or VIP.