Divergent effects of L-arginine-NO pathway modulators on diazepam and flunitrazepam responses in NOR task performance

Neuromodulatory role of L-arginine: nitric oxide precursor against thioacetamide-induced-hepatic encephalopathy in rats via downregulation of NF-κB-mediated apoptosis

The aim of the present study was to investigate the impact of arginine (ARG), a nitric oxide (NO) precursor, on thioacetamide (TAA)-induced hepatic encephalopathy (HE) in rats by injection of TAA (100 mg/kg, i.p) three times per week for six consecutive weeks. TAA-injected rats were administered ARG (100 mg/kg; p.o.) concurrently with TAA for the six consecutive weeks. Blood samples were withdrawn, and rats were sacrificed; liver and brain tissues were isolated. Results of the present study demonstrated that ARG administration to TAA-injected rats revealed a restoration in the serum and brain ammonia levels as well as serum aspartate transaminase, alanine transaminase, and alkaline phosphatase and total bilirubin levels as well as behavioral alterations evidenced by restoration in locomotor activity, motor skill performance, and memory impairment. ARG showed also improvement in the hepatic and neuro-biochemical values, pro-inflammatory cytokines, and oxidative stress biomarkers. All these results were confirmed by histopathological evaluation as well as ultrastructural imaging of the cerebellum using a transmission electron microscope. Furthermore, treatment with ARG could ameliorate the immunological reactivity of nuclear factor erythroid-2-related factor 2 (Nrf2) and cleaved caspase-3 proteins in the cerebellum and hepatic tissues. From all the previous results, it can be fulfilled that ARG showed a beneficial role in modulating the adverse complications associated with TAA-induced HE in rats via reducing hyperammonemia and downregulating nuclear factor kappa B (NF-κB)-mediated apoptosis.

Diazepam induces retrograde facilitation of object recognition and object location memory in male mice

Benzodiazepines are widely prescribed for patients suffering from anxiety and insomnia. Although amnesic effects of benzodiazepines are commonly known as side effects, it has also been reported that these drugs improve memory for information learned before drug intake, a phenomenon called retrograde facilitation. However, the retrograde effects of benzodiazepines on cognitive performances in rodents remain controversial. It should be considered that studies on diazepam-induced retrograde facilitation in humans have been conducted using a recall paradigm focused on short-term memory, whereas these studies in rodents have been conducted using memory tasks that mainly target long-term memory and/or require negative or positive reinforcers. In the current study, we investigated whether diazepam, a benzodiazepine, induces retrograde facilitation for object recognition memory and spatial memory in mice, using a novel object recognition test and an object location test, respectively. These tests are available for short-term memory and do not require any reinforcer. The mice treated with diazepam retained object recognition memory for at least 180 min and spatial memory for at least 150 min. In contrast, vehicle-treated control mice retained object recognition memory for 120 min but not 150 min or longer, and spatial memory for 90 min but not 120 min or longer. These data clearly demonstrated diazepam-induced retrograde facilitation for both object recognition and spatial memories in mice. The present study is expected to contribute to the elucidation of the neural basis of retrograde facilitation.

Modification of NO-cGMP Pathway Differentially Affects Diazepam- and Flunitrazepam-Induced Spatial and Recognition Memory Impairments in Rodents

This study investigated the influence of sildenafil and methylene blue (MB), two modulators of the nitric oxide (NO)-cyclic guanosine-3',5'-monophosphate (cGMP) pathway on amnesic effects of two benzodiazepines (BZs) (diazepam (DZ) and flunitrazepam (FNZ)), in rodents-mice and rats. In the modified elevated plus maze (mEPM) and novel object recognition (NOR) tests, MB given ip at a dose of 5 mg/kg 5 min prior to DZ administration (0.25 or 1 mg/kg, sc) enhanced/induced memory impairment caused by DZ. When MB (2.5, 5, and 10 mg/kg) was applied 5 min prior to FNZ administration (0.05 and 0.1 mg/kg), an effect was opposite and memory impairment induced by FNZ was reduced. When sildenafil (2.5 and 5 mg/kg, ip) was applied 5 min prior to DZ, we observed a reduction of DZ-induced memory deficiency in the mEPM test. A similar effect of sildenafil was shown in the NOR test when the drug was applied at doses of 1.25, 2.5, and 5 mg/kg prior to DZ. In the mEPM test, sildenafil at abovementioned doses had no effects on FNZ-induced memory impairment. In turns, sildenafil administered at doses of 2.5 and 5 mg/kg increased the effect of FNZ on memory impairment in the NOR test. In conclusion, the NO-cGMP pathway is involved differentially into BZs-induced spatial and recognition memory impairments assessed using the NOR and mEPM tests. Modulators of the NO-cGMP pathway affect animal behavior in these tests in a different way depending on what benzodiazepine is applied.

Role of nitric oxide synthase on brain GABA transaminase activity and GABA levels

In an attempt to clarify the controversial role of nitric oxide (NO) in seizures, the effects of NO on brain GABA transaminase (GABA-T) activity and GABA levels were investigated. To this aim, the effects of the substrate (l-arginine) and inhibitors (Nω-nitro-l-arginine methyl ester, 7-nitroindazole) of NO synthase (NOS) on GABA-T activity and GABA levels in vitro and ex vivo were analyzed. In vitro NO diminished GABA-T activity and increased GABA. Ex vivo NO modified GABA-T activity and GABA levels biphasically. Inhibition of endothelial and neuronal NOS (eNOS and nNOS) had opposite effects on GABA-T activity and GABA levels, even during seizures induced by pentylenetetrazole. Different effects of NO on GABA-T activity and on GABA levels, depending on the NOS isoform involved, may explain its contradictory role in seizures, the endothelial NOS acting as an anticonvulsant and the neuronal NOS as a proconvulsant. nNOS inhibitors may represent a new generation of antiepileptics.

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.

Decreased hippocampal homoarginine and increased nitric oxide and nitric oxide synthase levels in rats parallel training in a radial arm maze

L-homoarginine (hArg) is derived from enzymatic guanidination of lysine. It was demonstrated that hArg is a substrate for nitric oxide (NO) synthesis, blocks lysine transport and inhibits the uptake of arginine into synaptosomes and modulates GABA responses ex vivo. As there is limited information on its physiological roles in the brain, the aim of the study was to show whether hippocampal or frontal lobe (FL) hArg is paralleling training in the radial arm maze (RAM) or NO formation. Hippocampi and FL of male Sprague-Dawley rats were taken from trained or yoked in a RAM. Then hArg and metabolites, NO and NO synthase (NOS) were determined by standard methods. The animals learned the task in the RAM showing significant reduction of working memory errors. hArg showed decreased levels in both brain regions of trained animals as compared to yoked animals. Nitrate plus nitrite (NOx) concentrations and NOS activity were significantly increased in hippocampi, F(1,36) = 170.5; P ≤ 0.0001 and FL, F(1,36) = 74.67; P ≤ 0.0001 of trained animals as compared to yoked animals. Levels of hArg were negatively correlated with NOx in hippocampus (r = -0.6355; P = 0.0483) but not in FL and with lysine in the FL (r = -0.6650; P = 0.0358). NOx levels were positively correlated with NOS in both the hippocampus (r = 0.7474; P = 0.0129) and FL (r = 0.9563; P ≤  0.0001). These novel findings indicate that hArg is linked to NO formation in hippocampus but not in FL and is paralleling spatial memory in the RAM.

l-NAME differential effects on diazepam and flunitrazepam responses of rats in the object recognition test

BACKGROUND

The present study was undertaken to better understand possible interaction(s) between a non-selective nitric oxide inhibitor: N(G)-nitro-l-arginine methyl ester (l-NAME) and benzodiazepines (BZs) in recognition memory.

METHODS

The study was carried out on adult male albino Wistar rats. A novel object recognition (NOR) task was used to evaluate memory process.

RESULTS

Combined administration of l-NAME (50mg/kg, ip) with a threshold dose of DZ (0.25mg/kg) induced amnesic effects in rats, participating in the NOR test. On the other hand, following a combined administration of l-NAME (100mg/kg, ip) with flunitrazepam (FNZ; 0.1mg/kg), it was found out that l-NAME inhibited the amnesic effects of FNZ on rats in the NOR test.

CONCLUSIONS

The obtained results suggest that suppressed NO synthesis may lead to a facilitation of DZ-induced memory impairment but surprisingly may prevent amnesic effect after FNZ in rats, submitted to NOR task.

The effects of L-arginine on spatial memory and synaptic plasticity impairments induced by lipopolysaccharide

Background:

An important role of nitric oxide (NO) in neuroinflammation has been suggested. It is also suggested that NO has a critical role in learning and memory. Neuro-inflammation induced by lipopolysaccharide (LPS) has been reported that deteriorates learning and memory. The effect of L-arginine (LA) as a precursor of NO on LPS-induced spatial learning and memory and neuronal plasticity impairment was evaluated.

Materials and Methods:

The animals were grouped into: (1) Control, (2) LPS, (3) LA-LPS, and (4) LA. The rats received intraperitoneally LPS (1 mg/kg) 2 h before experiments and LA (200 mg/kg) 30 min before LPS. The animals were examined in Morris water maze (MWM). Long-term potentiation (LTP) from CA₁ area of the hippocampus was also assessed by 100 Hz stimulation in the ipsilateral Schaffer collateral pathway.

Results:

In MWM, time latency and traveled path were higher in LPS group than the control group (P < 0.001) whereas in LA-LPS group they were shorter than LPS group (P < 0.001). The amplitude and slope of field excitatory postsynaptic potential (fEPSP) decreased in LPS group compared to control group (P < 0.05 and P < 0.01) whereas, there was not any significant difference in these parameters between LPS and LA-LPS groups.

Conclusion:

Administration of LPS impaired spatial memory and synaptic plasticity. Although LA ameliorated deleterious effects of LPS on learning of spatial tasks, it could not restore LPS-induced LTP impairment.