Isomeric Carborane Neuromuscular Blocking Agents.

We synthesized a family of neuromuscular blocking agents (NMB) based on decamethonium, but containing a carborane cluster in the methylene chain between the two quaternary ammonium groups. The carborane cluster isomers o-NMB, m-NMB, and p-NMB were tested in animals for neuromuscular block and compared with agents used clinically: rocuronium and decamethonium. All three isomers caused reversible muscle weakness in mice as determined by grip strength and inverted screen tests, with a potency rank of p-NMB > rocuronium > decamethonium > m-NMB > o-NMB. The mechanism of action of the compounds was determined by using the in vitro rat phrenic nerve hemi-diaphragm preparation and electrophysiologic measurements in cells. Neostigmine reversed hemi-diaphragm weakness caused by the three isomers and rocuronium, but not succinylcholine. In electrophysiologic recordings of currents through acetylcholine receptor channels, the carborane compounds did not activate channel activity but did inhibit channel activation by acetylcholine. These results demonstrate that the carborane neuromuscular blocking agents are non-depolarizers in contrast to the depolarizing action of the parent compound.

Carborane-derived local anesthetics are isomer dependent.

Clinically there is a need for local anesthetics with a greater specificity of action on target cells and longer duration. We have synthesized a series of local anesthetic derivatives we call boronicaines in which the aromatic phenyl ring of lidocaine was replaced with ortho-, meta-, C,C'-dimethyl meta- and para-carborane clusters. The boronicaine derivatives were tested for their analgesic activity and compared with lidocaine using standard procedures in mice following a plantar injection. The compounds differed in their analgesic activity in the following order: ortho-carborane = C,C'-dimethyl meta-carborane > para-carborane > lidocaine > meta-carborane derivative. Both ortho-boronicaine and C,C'-dimethyl meta-boronicaine had longer durations of analgesia than lidocaine. Differences in analgesic efficacies are rationalized by variations in chemical structure and protein binding characteristics.

Ketamine induces hyperactivity in rats and hypersensitivity to nicotine in rat striatal slices.

The symptoms of schizophrenia can be modeled in rats through blockade of ionotropic glutamate receptors, which induces changes in central dopamine circuits. These circuits also contain nicotinic acetylcholine receptors that are activated by nicotine. A role for nicotine in the etiology of schizophrenia is supported by clinical observations of high tobacco use rates in individuals experiencing the psychopathology. The present study investigated the effect of the ionotropic glutamate receptor antagonist ketamine on the function of striatal nicotinic acetylcholine receptors to understand better the potential role of these receptors in schizophrenia. Ketamine (0.1-300 microM) was ineffective to evoke [3H] overflow from rat striatal slices preloaded with [3H]dopamine. Application of psychotomimetic ketamine concentrations (1-10 microM) to striatal slices augmented nicotine-evoked [3H] overflow. Finally, rats received ketamine (30-50 mg/kg) injections for 30 days, to model the development of the disorder, and hyperactivity was observed, although repeated ketamine treatment did not significantly alter nicotine-evoked [3H]dopamine overflow. These data indicate that the function of nicotinic acetylcholine receptors that mediate dopamine release are altered by ketamine, and support a role for nicotinic acetylcholine receptors in schizophrenia pathology.

NMDA receptor blockade augmented nicotine-evoked dopamine release from rat prefrontal cortex slices.

Nicotine evokes dopamine release through activation of nicotinic acetylcholine receptors, and tobacco cigarette smoking is more prevalent among individuals diagnosed with schizophrenia. Blockade of ionotropic glutamate (NMDA) receptors can induce changes in central dopamine and glutamate circuits, which models the symptoms of schizophrenia. The effect of the NMDA receptor antagonist, ketamine, on the effect of nicotine in rat prefrontal cortex was examined using a slice superfusion assay in which cortical slices were preloaded with [(3)H] dopamine. A wide range of ketamine concentrations (0.1-300 microM) did not evoke [(3)H] overflow from slices, indicating that NMDA receptor blockade did not induce dopamine release. Ketamine, at concentrations that model the symptoms of schizophrenia (1-10 microM), augmented the effect of nicotine (1-100 microM) to evoke [(3)H] overflow from slices and decreased the threshold nicotine concentration to evoke [(3)H] overflow. This indicates that NMDA receptor blockade increased the potency and efficacy of nicotine to evoke dopamine release from prefrontal cortex slices, suggesting that ketamine induced hypersensitivity to nicotine. The present results support a role for nicotinic acetylcholine receptors in the pathophysiology and treatment of schizophrenia.

The cannabinoid receptor agonists, anandamide and WIN 55,212-2, do not directly affect mu opioid receptors expressed in Xenopus oocytes.

A functional link between the cannabinoid and opioid receptor pathways has been proposed based on data showing that cannabinoid effects can be blocked by opioid receptor antagonists and that cannabinoids can bind to opioid receptors. To explore this link in more detail at the receptor level, we tested the hypothesis that cannabinoids directly activate or modulate mu opioid receptor function. The G-protein coupled mu opioid receptor, MOR-1, and its effector, the G-protein activated potassium channel, GIRK2 (Kir3.2), were expressed together in Xenopus oocytes and potassium currents measured using the two-electrode voltage clamp technique. The specific mu receptor agonist DAMGO activated potassium currents in oocytes expressing the mu receptor that were fully inhibited by the mu receptor antagonist, naloxone. The endogenous cannabinoid, anandamide, and the synthetic cannabinoid, WIN 55,212-2, had no direct effects on potassium currents in the oocytes expressing the mu receptor. The cannabinoids also had no effect on the magnitude of the potassium currents activated by DAMGO or on the desensitization kinetics of MOR-1 in the continued presence of DAMGO. Both WIN 55,212-2 and anandamide activated cannabinoid CB1 receptors when co-expressed with GIRK2 in the oocytes. We conclude that neither anandamide nor WIN 55,212-2 directly activate or modulate mu opioid receptor function in oocytes and that interactions of cannabinoids with mu opioid receptors are likely to be indirect.

Lobeline, a potential pharmacotherapy for drug addiction, binds to mu opioid receptors and diminishes the effects of opioid receptor agonists.

Lobeline diminishes the behavioral and neurochemical effects of nicotine and amphetamines, and is considered a potential pharmacotherapy for drug abuse and addiction. Lobeline has high affinity for nicotinic acetylcholine receptors and inhibits the function of vesicular monoamine and dopamine transporters. The present study investigated the less-explored interaction of lobeline and the endogenous opioid system. In guinea pig brain homogenates, lobeline displaced (K(i)=0.74 microM) the binding of [(3)H]DAMGO [(D-Ala(2), N-ME-Phe(4), Gly(5)-ol)-enkephalin]. In a functional assay system comprised of MOR-1 mu opioid receptors and GIRK2 potassium channels expressed in Xenopus oocytes, lobeline had no effect on the resting current, but maximally inhibited (IC(50)=1.1 microM) morphine- and DAMGO-activated potassium current in a concentration-dependent manner. In a second functional assay, lobeline-evoked [(3)H]overflow from rat striatal slices preloaded with [(3)H]dopamine was not blocked by naltrexone. Importantly, concentrations of lobeline (0.1-0.3 microM) that did not have intrinsic activity attenuated ( approximately 50%) morphine-evoked [(3)H]overflow. Overall, the results suggest that lobeline functions as a mu opioid receptor antagonist. The ability of lobeline to block psychostimulant effects may be mediated by opioid receptor antagonism, and lobeline could be investigated as a treatment for opiate addiction.

Sugar solution analgesia: the effects of glucose on expressed mu opioid receptors.

Glucose or sucrose solutions administered orally provide effective analgesia for procedural pain in neonates. Because analgesia with sugar solutions can be decreased by opioid receptor antagonists, we tested the hypothesis that glucose directly activates opioid receptors. Mu opioid receptors (MOR-1) were expressed in Xenopus oocytes, a well recognized expression system, and glucose was tested for possible agonist, antagonist, and modulatory effects on the receptor. In control experiments, 10 nM of Tyr-D-Ala-Gly-Me-Phe-Gly-ol (DAMGO), a synthetic enkephalin and specific mu agonist, activated the MOR-1, whereas 20 mM of glucose had no effect. In addition, glucose had no effect on the activation of the mu receptor by DAMGO. Finally, glucose did not modulate acute receptor desensitization induced by DAMGO. We conclude that glucose does not directly interact with MOR-1 in an in vitro expression system and that the purported interaction between glucose and the opioid system may be an indirect one, involving release of endogenous opioids.