Alpha2- and beta-adrenoceptors differentially modulate GABAA- and GABAB-mediated inhibition of red nucleus neuronal firing

Noradrenergic modulation of glutamate-induced excitatory responses in single neurons of the red nucleus: an electrophysiological study

The effect induced by noradrenaline (NA) on the spiking activity evoked by glutamate (Glu) on single neurons of the mesencephalic red nucleus (RN) of the rat was studied extracellularly. Long-lasting microiontophoretic applications of the amine induced a significant and reversible depression of the responsiveness of RN neurons to Glu. This effect was mediated by noradrenergic alpha2 receptors since it was mimicked by application of clonidine, an alpha2 adrenoceptor agonist, and blocked or at least reduced by application of yohimbine, an antagonist of NA for the same receptors. The effect appears homogeneously throughout the nucleus and is independent of the effect of NA on baseline firing rate. Application of isoproterenol, a beta adrenoceptor agonist, either enhanced or depressed neuronal responses to Glu in a high percentage (86%) of the tested neurons. Moreover, application of timolol, a beta adrenoceptor antagonist, was able to strengthen the depressive effects induced by NA application on neuronal responsiveness to Glu. Although these data suggest some involvement of beta adrenergic receptors in the modulation of neuronal responsiveness to Glu, the overall results indicate a short-term depressive action of NA, mediated by alpha2 receptors, on the responsiveness of RN neurons and suggest that stress initially leads to an attenuation of the relay function of the RN.

Modulation of muscimol state-dependent memory by α2-adrenoceptors of the dorsal hippocampal area

In the present study, the effects of bilateral intra-dorsal hippocampal (intra-CA1) injections of α2-adrenoceptor agonist and antagonist, on muscimol state-dependent memory were examined in mice. A single-trial step-down passive avoidance task was used for the assessment of memory retention in adult male NMRI mice. Administration of muscimol (0.1 μg/mouse, intra-CA1) 15 min before training or testing induced impairment of memory retention. Injection of the same dose of the drug 15 min before testing restored memory retention impaired under pre-training muscimol influence. Pre-test intra-CA1 administration of the α2-adrenoceptor agonist clonidine (0.5 and 1 μg/mouse) impaired memory retention, although the low dose of the drug (0.25 μg/mouse) did not affect memory retention. Pre-test intra-CA1 administration of the α2-adrenoceptor antagonist yohimbine (1 and 2 μg/mouse) improved memory retention, although the low dose of the drug (0.5 μg/mouse) did not affect memory retention. In other series of experiments, pre-test co-administration of certain doses of clonidine (0.125 and 0.25 μg/mouse, intra-CA1), doses which were ineffective when given alone, and muscimol (0.1 μg/mouse, intra-CA1) significantly inhibited muscimol state-dependent memory. Pre-test intra-CA1 administration of certain doses of yohimbine (0.25 and 0.5 μg/mouse), doses which were ineffective when given alone, improved pre-training muscimol (0.1 μg/mouse)-induced retrieval impairment. Moreover, pre-test co-administration of yohimbine (0.25 and 0.5 μg/mouse, intra-CA1) and muscimol (0.025 μg/mouse, intra-CA1), an ineffective dose, significantly restored the retrieval and induced muscimol state-dependent memory. It may be concluded that the α2-adrenoceptors of the dorsal hippocampal area play an important role in muscimol state-dependent memory.

Noradrenaline modulates neuronal responses to GABA in vestibular nuclei

The effects of noradrenaline (NA) on the inhibitory responses to GABA were studied in vivo in neurons of the vestibular nuclei of the rat using extracellular recordings of single unit electrical activity and a microiontophoretic technique of drug application in loco. NA application influenced GABA-evoked inhibitions in 82% of tested neurons, depressing them in 42% and enhancing them in 40% of cases. The more frequent action of NA on GABA responses was depressive in lateral and superior vestibular nuclei (50% of neurons) and enhancing in the remaining nuclei (56% of neurons). The most intense effect of NA application was the enhancement of GABA responses induced in a population of lateral vestibular nucleus neurons, characterized by a background firing rate significantly higher than that of other units. The alpha(2) noradrenergic receptor agonist clonidine mimicked the enhancing action of NA on GABA responses; this action was blocked by application of the specific alpha(2) antagonist yohimbine. The beta adrenergic agonist isoproterenol induced either depressive or enhancing effects on GABA responses; the former more than the latter were totally or partially blocked by application of the beta antagonist timolol. It is concluded that NA enhances GABA responses by acting on noradrenergic alpha(2) and to a lesser extent beta receptors, whereas depressive action involves beta receptors only. These results confirm the hypothesis that the noradrenergic system participates in the regulation of the vestibulospinal and the vestibulo-ocular reflexes and suggest that conspicuous changes of NA content in brain due to aging or stress could lead to a deterioration in the mechanisms of normal vestibular function.

Labetalol facilitates GABAergic transmission to rat periaqueductal gray neurons via antagonizing beta1-adrenergic receptors--a possible mechanism underlying labetalol-induced analgesia

Labetalol, a combined alpha1, beta1, and beta2 adrenoceptor-blocking drug, has been shown to have analgesic properties in vivo. To determine the underlying mechanisms, we examined its effects on GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) and spontaneous firings of rat ventrolateral periaqueductal gray (PAG) neurons, either mechanically dissociated, or in acute brain slices. These PAG neurons mediate opioid-mediated analgesia and pain transmission and are under tonic control of GABAergic interneurons. An increase in GABAergic transmission to these neurons yields an inhibitory hyperpolarized state and may interrupt pain signal transmission. Using patch clamp techniques, we found that labetalol reversibly increases the frequency of sIPSCs without changing their mean amplitude. This indicates that labetalol enhances GABAergic synaptic transmission by a presynaptic mechanism. Metoprolol, a specific beta1-adrenoceptor antagonist, also reversibly enhanced sIPSC frequency. In the presence of metoprolol, labetalol-induced increase in sIPSC frequency was significantly attenuated or even abolished. These results suggest that labetalol shares the same pathway as metoprolol in enhancing GABAergic transmission via an inhibition of presynaptic beta1-adrenoceptors. We further showed that labetalol reversibly reduced the firing rate of PAG neurons. This reduction was significantly attenuated in the presence of bicuculline, a selective antagonist of GABAA receptors. These data indicate that labetalol-induced inhibition of PAG cell firing is attributable to its potentiation of GABAergic transmission. Based on these data, we postulate that labetalol-induced analgesia is at least in part ascribed to its antagonistic effects on presynaptic beta1-adrenoceptors.

The short-lasting analgesia and long-term antihyperalgesic effect of intrathecal clonidine in patients undergoing colonic surgery

In this study, we investigated the antihyperalgesic effect of clonidine after surgery. Sixty patients undergoing right colic resection were studied. Patients were randomized to receive prior to general anesthesia a 2-mL intrathecal (IT) injection of 300 microg of clonidine or saline, or 10 mg of bupivacaine. General anesthesia was achieved using a target concentration propofol infusion and monitored using bispectral index. Postoperative analgesia was provided by morphine IV given through a patient-controlled analgesia device. Postoperative analgesia was assessed by morphine requirements and visual analog scale pain scores at rest, cough, and movement during the first 72 h. Mechanical hyperalgesia was measured by von Frey filaments. Patients were questioned regarding residual pain at 2 wk,1, 6, and 12 mo. The patient-controlled analgesia morphine requirements were significantly smaller in the IT clonidine group (31.5 +/- 12 versus 91 +/- 25.5 and 43 +/- 15 mg, respectively, in groups clonidine, saline, and bupivacaine: P < 0.05 at 72 postoperative hours). The area of hyperalgesia at 72 h was 3 +/- 5 cm(2) in the clonidine group versus 90 +/- 30 and 35 +/- 20 cm(2) in the saline and bupivacaine groups (P < 0.05). At 6 mo, fewer patients in the clonidine group experienced residual pain than in the saline group (0 of 20 versus 6 of 20, P < 0.05). We conclude that both intraoperative spinal clonidine and bupivacaine improve immediate postoperative analgesia. IT clonidine was, however, more potent than IT bupivacaine to reduce postoperative secondary hyperalgesia.

IMPLICATIONS

Spinal clonidine contributes to the reduction of secondary hyperalgesia in patients recovering from abdominal surgery.