Attenuation of morphine withdrawal symptoms by subtype‐selective metabotropic glutamate receptor antagonists

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Abstract
We have previously shown that chronic antagonism of group I metabotropic glutamate receptors (mGluRs), in the brain, attenuates the precipitated morphine withdrawal syndrome in rats. In the present investigation we assessed the effects of chronic antagonism of group II and III mGluRs on the severity of withdrawal symptoms in rats treated chronically with subcutaneous (s.c.) morphine. Concurrently with s.c. morphine we infused intracerebroventricularly (i.c.v.) one of a series of phenylglycine derivatives selective for specific mGluR subtypes. Group II mGluRs (mGluR2,3), which are negatively coupled to adenosine 3′: 5′‐cyclic monophosphate (cyclic AMP) production, were selectively antagonized with 2s, 1′s, 2′s‐2‐methyl‐2‐(2′‐carboxycyclopropyl) glycine (MCCG). Group III mGluRs (mGluR4,6,7 and 8), which are also negatively linked to cyclic AMP production, were selectively antagonized with α‐methyl‐l‐amino‐4‐phosphonobutanoate (MAP4). The effects of MCCG and MAP4 were compared with α‐methyl‐4‐carboxyphenylglycine (MCPG), which non‐selectively antagonizes group II mGluRs, as well as group I mGluRs (mGluR1,5) which are positively coupled to phosphatidylinositol (PI) hydrolysis. Chronic i.c.v. administration of both MCCG and MAP4 significantly decreased the time spent in withdrawal, MCPG and MCCG reduced the frequency of jumps and wet dog shakes and attenuated the severity of agitation. Acute i.c.v. injection of mGluR antagonists just before the precipitation of withdrawal failed to decrease the severity of abstinence symptoms. Rather, acute i.c.v. injection of MCCG significantly increased the time spent in withdrawal. Our results suggest that the development of opioid dependence is affected by mGluR‐mediated PI hydrolysis and mGluR‐regulated cyclic AMP production. British Journal of Pharmacology (1997) 120, 1015–1020; doi:10.1038/sj.bjp.0701000