[3H]dexmedetomidine, an alpha 2-adrenoceptor agonist, detects a novel imidazole binding site in adult rat spinal cord.

Binding properties of [3H]dexmedetomidine [(+)-(S)-4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole] as an agonist-type radioligand for alpha 2-adrenoceptors were characterised for the first time in tissues relevant to its analgesic (spinal cord from neonatal or adult rats) and behavioural (rat cerebral cortex) actions. In membranes of rat cerebral cortex (KdHigh 0.2 +/- 0.03 nM, KdLow 8.8 +/- 1.4 nM with Bmax High 130 +/- 11 fmol/mg protein, RHigh 16%) and neonatal spinal cord (KdHigh 0.3 +/- 0.04 nM, KdLow 14 +/- 3.7 nM with Bmax High 290 +/- 40 fmol/mg protein, RHigh 25%) Gpp(NH)p modifies the biphasic binding to monophasic and binding is competed with specifically by alpha 2-adrenoceptor compounds. Binding to rat cerebral cortex is not modified by pretreatment with the noradrenergic neurotoxin, DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). In contrast, [3H]dexmedetomidine binding to adult rat spinal cord membranes is more complex and both saturation analysis and competition experiments indicate the presence of a non-adrenergic component of binding (about 40% of total binding) which is sensitive to imidazole-type compounds. This non-adrenergic component of [3H]dexmedetomidine binding can be defined as a novel type of imidazole binding site such that, of the imidazoline I1 or I2 receptor ligands, only cimetidine has relatively high affinity. In conclusion, [3H]dexmedetomidine shows very complex binding characteristics that limit its use as an agonist-type radioligand for alpha 2-adrenoceptors but it may be a useful tool for imidazoline receptor characterisation.

alpha 2A/D-Adrenoceptor subtype predominates also in the neonatal rat spinal cord.

alpha 2-Adrenoceptors are remarkably regulated by developmental factors. In this study alpha 2-adrenoceptor subtypes have been characterised in neonatal and adult rat spinal cords. In saturation experiments, a 5% proportion of [3H]rauwolscine binding has a high affinity component, representing the alpha 2C-subtype in both tissues. Competition studies with [3H]RX821002 indicate that in both tissues the alpha 2A/D subtype is expressed similarly.

Isoflurane and an alpha 2-adrenoceptor agonist suppress nociceptive neurotransmission in neonatal rat spinal cord.

Analgesia is an important component of general anesthesia. alpha 2-adrenoceptor agonists such as clonidine and dexmedetomidine are effective analgesics at the spinal level, and furthermore, they reduce the volatile anesthetic requirement. In order to probe a possible spinal-level contribution to general anesthetic-induced analgesia, the effects of dexmedetomidine were tested in an isolated spinal cord preparation. The effects of dexmedetomidine were compared with those of isoflurane, and dexmedetomidine-isoflurane interactions were explored. The test response was a nociceptive-related slow ventral root potential (slow VRP) recorded from the isolated neonatal rat spinal cord in response to electrical stimulation of a dorsal root. At 0.2-1.28 vol%, isoflurane reversibly depressed the slow VRP. At a lower concentration (0.14 vol%), isoflurane increased the slow VRP in three of five preparations. At 1.0-1.28 vol%, isoflurane also depressed the monosynaptic reflex. Recovery on washout usually was to a level greater than control. The N-methyl-D-aspartate (NMDA) receptor antagonist (DL)-2-amino 5-phosphonovalerate (10 microM) prevented the rebound to levels above control on isoflurane washout. The earlier components of the slow VRP were more sensitive to isoflurane than were the later. Dexmedetomidine (0.5-10 nM) depressed the slow VRP and had no effect on the monosynaptic reflex. The slow VRP depends on both substance P and glutamate NMDA-receptor-mediated neurotransmission; isoflurance and dexmedetomidine depressed responses to both substance P and NMDA. Although the two agents depress responses to the same neurotransmitters, there is no evidence that they act at the same cellular site(s). There was no significant interaction between dexmedetomidine and isoflurane. The results suggest that isoflurane exerts marked inhibitory effects on spinal neurotransmission, depressing both substance P and glutamate-mediated pathways. There is a possible biphasic effect on the NMDA receptor. To the extent that nociception depends on these neurotransmitters, isoflurane may be expected to exert profound analgesic effects at the spinal level. By blocking responses to strongly arousing stimuli, these effects may contribute to general anesthesia. Suppression of nociceptive neurotransmission at the spinal level may contribute to dexmedetomidine's anesthetic-sparing properties as well as to analgesia by this agent.

The alpha 2-adrenoceptor agonist dexmedetomidine increases the apparent potency of the volatile anesthetic isoflurane in rats in vivo and in hippocampal slice in vitro.

Alpha 2-adrenoceptor agonists such as clonidine are sedatives and enhance the effectiveness of several different kinds of anesthetics. This study was performed to quantitate the effect of dexmedetomidine, a novel alpha 2-adrenoceptor agonist, on the action of the volatile anesthetic agent isoflurane in rats in vivo. A separate set of experiments in rat hippocampal slices was designed to determine whether isoflurane and dexmedetomidine exerted similar effects on synaptic transmission in vitro and to examine the interaction between the two agents. In vivo, dexmedetomidine (100 micrograms/kg i.p.) reduced isoflurane minimum alveolar anesthetic requirement (MAC), determined by loss of response to tail pinch, by approximately 90%. In hippocampal CA1 neurons, on the other hand, there was a relatively small potentiation of the effects of isoflurane at the maximally effective dexmedetomidine concentration (1 nM). The hippocampal CA1 area, at least in the slice preparation, may thus not be representative of the CNS site(s) at which alpha 2 adrenoceptor agonists lessen anesthetic requirement in vivo.

Alpha 2-adrenoceptors inhibit a nociceptive response in neonatal rat spinal cord.

Alpha 2-Adrenoceptors mediate analgesia in vivo. The present study explored the actions of the alpha 2-adrenoceptor agonists dexmedetomidine and clonidine on a nociceptive response in isolated neonatal rat spinal cord. Stimulation of a dorsal root generates a slow ventral root potential (slow VRP) at the corresponding ipsilateral ventral root. The slow VRP meets several criteria for a nociceptive response. Dexmedetomidine (10 nM) and clonidine (200 nM) depressed the slow VRP by approximately 80%. Dexmedetomidine's action was approximately linear over the concentration range 0.5-500 nM, whereas clonidine (20 nM-5 microM) exerted biphasic effects. The profile of agonist and antagonist effectiveness characterized the receptor(s) as alpha 2-adrenoceptors; the subtype could not be identified as either alpha 2A or alpha 2B. Naloxone pretreatment partially blocked dexmedetomidine's effect, suggesting a possible endogenous opiate involvement. Dexmedetomidine (0.5-2.0 nM) also depressed the VRP evoked by application of substance P to the cord, implicating postsynaptic as well as possible presynaptic actions. At high concentrations, dexmedetomidine (50-500 nM) depressed the monosynaptic reflex, probably through non-alpha 2-receptor(s). Results from the neonatal spinal cord correlate well with those from in vivo analgesia studies. They suggest an important direct spinal contribution to alpha 2-adrenoceptor-mediated analgesia.