Nitric oxide (NO) has been proposed as a new neurotransmitter/ neuromodulator in the central nervous system (CNS). In the present experiments we have examined the effects of inhibitors of NO synthase (NOS), the enzyme responsible for NO formation, on some mouse behaviour tests. In previous experiments, systemic injections of low doses of NOS inhibitors, induced anxiolytic effects in the elevated plus maze, whereas higher doses decrease maze exploration (Lino de Oliveira et al. 1997). This latter effect may involve motor effects of this compounds.
Male albino-Swiss mice (25-30 g) were housed in groups, in a temperature-controlled room with a 12 h light-dark cycle, with free access to food and water. The acute effects of the NOS inhibitors NG-nitro-L-arginine (L-NOARG, 10-80 mg kg-1 I.P.) and 7-nitroindazole (7-NIO, 3-30 mg kg-1 I.P.) on exploratory activity were analysed in an open field arena. Drug effects on catalepsy were examined in the hanging-bar and wire-ring test. Footprint pattern after treatment with the two NOS inhibitors was evaluated and the results compared with those obtained with the dopamine D2 receptor antagonist haloperidol (1-2 mg kg-1 I.P.). Sub-chronic (twice a day for 4 days) effects of L-NOARG (40 mg kg-1 I.P.) or 7-NIO (30 mg kg-1, I.P.) were also tested in the open field arena and catalepsy test. The experiments were carried out according to the Brazilian Society of Neuroscience and Behaviour guidelines for care and use of Laboratory animals. The animals were killed humanely.
L-NOARG and 7-NIO decreased locomotion and rearing in the open field arena. Both drugs induced catalepsy in the hanging-bar test but did not change footprint pattern. The cataleptic effect of L-NOARG in the hanging bar and wire-ring tests were highly correlated (r = 0.927). Acute administration of L-NOARG had an additive effect with haloperidol (dopamine D2-receptor inhibitor) and was potentiated by pre-treatment with WAY 100135 (a 5-HT1 receptor antagonist), ketanserin (5HT2a and α-adrenergic receptor antagonist) and amantadine (glutamatergic receptor antagonist). Pre-treatment with atropine sulphate or biperideno (muscarinic receptor antagonists) and apomorphine (D2 receptor blocker) inhibited L-NOARG-induced catalepsy. The exploratory and cataleptic effects of L-NOARG and 7-NIO provided evidence for tolerance after sub-chronic treatment. Neither drug changed footprint pattern. Sub-chronic L-NOARG treatment induced cross-tolerance to haloperidol catalepsy and an increase of NADPH-diaphorase neurons in the dorsal part of caudate, acumbens and segmental pedunculus pontinus nucleus (Del Bel & Guimaraes, 2000). There was a decrease in NADPH-diaphorase neurons in the substantia nigra compacta.
The results confirm that inhibition of neuronal NO formation induces an impairment of exploratory and motor behaviour and give support to the hypothesis that NO plays a role in motor behaviour control, probably modulating dopaminergic, serotonergic and cholinergic neurotransmission. This effect does not seem to involve aspects evaluated by footprint analysis, such as weight support, trunk stability and foot placement.
This work was supported by FAPESP and CNPQ.
All procedures accord with current local guidelines.