Nitric oxide (NO) has been reported to be involved in the mechanisms of
pain generation throughout the nervous system. We examined the effects of intrathecally (i.t.) administered
nitric oxide synthase (NOS) inhibitors on the antinociceptive effects of
morphine and
endomorphin-1 during
acute pain and in chronic constriction injury (CCI)-exposed rats. We used
N(G)-nitro-l-arginine methyl ester (
l-NAME), a non-selective NOS inhibitor;
7-nitroindazole (7-NI) or 1-(2-trifluoromethyl-phenyl)-imidazole (TRIM), selective inhibitors of neuronal NOS (NOS1); and 1400W dihydrochloride, a selective inhibitor of inducible NOS (NOS2).
Morphine (0.5-2.5 μg) and
endomorphin-1 (2.5-20 μg) in
acute pain and
morphine (10-40 μg) and
endomorphin-1 (5-20 μg) after CCI-injury were combined with NOS inhibitors. For
acute pain, the ED50 for
endomorphin-1 (7.1 μg) was higher than that of
morphine (1.3 μg) in the tail-flick test. For
neuropathic pain, the ED50 value for
morphine was much higher (43.2 μg) than that of
endomorphin-1 (9.2 μg) in von Frey test. NOS inhibitors slightly influenced pain thresholds in both
pain models. Moreover, in
neuropathic pain, the effects of
morphine were more potentiated by
L-NAME, TRIM, 7-NI and 1400W (12×, 8.6×, 4.1× and 5.3×, respectively) than were the effects of
endomorphin-1 (2.7×, 4.3×, 3.4× and 2.1×, respectively) in the von Frey test.
Minocycline which is known to enhance the efficiency of
morphine in
neuropathic pain, decreased the
mRNA expression of NOS1 in the DRG and NOS2 and C1q in the spinal cord after CCI. Both NOS2 and IBA-1
protein levels in the spinal cord and NOS1, NOS2 and IBA1
protein levels in DRG decreased after
minocycline administration. In conclusion, our results provide evidence that both neuronal and non-neuronal NOS/NO pathways contribute to the behavioural
pain responses evoked by nerve injury. The NOS inhibitors regardless of the type of
pain enhanced
morphine antinociception and, to a lesser extent, altered the effects of
endomorphin-1, an
opioid ligand with a peptidergic structure.