Effects of NMDA antagonists on the development and expression of tolerance to diazepam-induced motor impairment in mice

How chronic administration of benzodiazepines leads to unexplained chronic illnesses: A hypothesis

It is thought that an ill defined biochemical cascade may lead to protracted withdrawal symptoms subsequent to discontinuance of routine use of benzodiazepine class drugs and establish chronic illness in some patients. In this review, published findings are presented that support the novel concept that withdrawal from benzodiazepine class drugs can trigger elevated and sustained levels of a potent oxidant called peroxynitrite via potentiation of the L-type voltage-gated calcium channels, and in the later stages of withdrawal, via excessive N-methyl-D-aspartate receptor activity, as well. Potentiation of L-type voltage-gated calcium channels and excessive N-methyl-D-aspartate receptor activity both result in calcium influx into the cell that triggers nitric oxide synthesis. In pathophysiological conditions, such increased nitric oxide synthesis leads to peroxynitrite formation. The downstream effects of peroxynitrite formation that may occur during withdrawal ultimately lead to further peroxynitrite production in a system of overlapping vicious cycles collectively referred to as the NO/ONOO(-) cycle. Once triggered, the elements of the NO/ONOO(-) cycle perpetuate pathophysiology, perhaps including reduced GABA_A receptor functioning, that may explain protracted withdrawal associated symptoms while the vicious cycle nature of the NO/ONOO(-) cycle may explain how withdrawal becomes a chronic state. Suboptimal levels of tetrahydrobiopterin may be one risk factor for the development of the protracted withdrawal syndrome as this will lead to partial nitric oxide uncoupling and resultant peroxynitrite formation. Nitric oxide uncoupling results in superoxide production as calcium-dependent nitric oxide synthases attempt to produce nitric oxide in response to L-type voltage-gated calcium channel-mediated calcium influx that is known to occur during withdrawal. The combination of nitric oxide and superoxide produced, as when partial uncoupling occurs, react together in a very rapid, diffusion limited reaction to form peroxynitrite and thereby trigger the NO/ONOO(-) cycle. The NO/ONOO(-) cycle may explain the nature of the protracted withdrawal syndrome and the related constellation of symptoms that are also common in other illnesses characterized as NO/ONOO(-) disorders such as myalgic encephalomyelitis/chronic fatigue syndrome and fibromyalgia.

NMDA Receptors and NO:cGMP Signaling Pathway Mediate the Diazepam-Induced Sensitization to Withdrawal Signs in Mice

The goal of the present study was to examine the effects of N-methyl-aspartate (NMDA) receptor antagonists-memantine and ketamine and the drugs modifying the NO:cGMP pathway-NG-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), the endogenous precursor of NO-L-arginine, and the guanylyl cyclase inhibitor-methylene blue (MB) on the development of sensitization to withdrawal signs precipitated after chronic, interrupted treatment with diazepam, a benzodiazepine receptor agonist, in mice. To develop the sensitization, the mice were divided into groups: continuously and sporadically (with two diazepam-free periods) treated with diazepam (15 mg/kg, sc). To precipitate the withdrawal syndrome (clonic and tonic seizures, and death), pentylenetetrazole (55 mg/kg, sc) with the benzodiazepine receptor antagonist, flumazenil (5.0 mg/kg, ip), were administered after the last injection of diazepam or saline. Memantine (2.5, 5.0 mg/kg), and ketamine (2.5, 5.0 mg/kg), L-NAME (100, 200 mg/kg) and 7-NI (20 and 40 mg/kg), L-arginine (250, 500 mg/kg) and MB (5 and 10 mg/kg) were administered ip in sporadically diazepam-treated mice during the diazepam-free periods. Our results indicated that both NMDA receptor antagonists and drugs that inhibit the NO:cGMP pathway, except L-arginine (the endogenous donor of NO), attenuated the diazepam-induced sensitization to withdrawal signs in mice. Thus, NMDA receptors and the NO:cGMP pathway are involved in the mechanisms of sensitization to benzodiazepine withdrawal.

The Importance of L-Arginine:NO:cGMP Pathway in Tolerance to Flunitrazepam in Mice

The goal of the study was to investigate the effects of drugs modifying l-arginine:NO:cGMP pathway on the development of tolerance to flunitrazepam (FNZ)-induced motor impairment in mice. FNZ-induced motor incoordination was assessed on the 1st and 8th days of experiment, using the rotarod and chimney tests. It was found that (a) both a non-selective nitric oxide synthase (NOS) inhibitor: N^G-nitro-l-arginine methyl ester (l-NAME) and an unselective neuronal NOS inhibitor: 7-nitroindazole (7-NI) inhibited the development of tolerance to the motor-impairing effects of FNZ in the rotarod and the chimney tests and (b) both a NO precursor: l-arginine and a selective inhibitor of phosphodiesterase 5 (PDE5): sildenafil did not affect the development of tolerance to FNZ-induced motor impairment in mice. Those findings provided behavioural evidence that NO could contribute an important role in the development of tolerance to FNZ in mice.