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Vol. 61, Issue 3, 578-585, March 2002
Department of Pharmacology, College of Medicine, National
Cheng-Kung University, Taiwan, Republic of China
In a previous study, we showed that type 1 cannabinoid
(CB1) receptor activation substantially depresses the
corticostriatal glutamatergic transmission onto striatal neurons in the
brain slice preparation. We now report that the adenylyl cyclase
activator forskolin and cAMP analog
(S)-p-8-(4-chlorophenythil)
adenosine-3',5'-monophosphorothioate (Sp-8-CPT-cAMPS)
strongly suppressed the synaptic depression induced by cannabimimetic
aminoalkylindole, WIN 55,212-2. Application of the cAMP-dependent
protein kinase (PKA) inhibitor KT5720 alone had no consistent effect on
basal synaptic transmission but the synaptic enhancement elicited by
forskolin was blocked. In addition, pretreatment of striatal slices
with either KT5720 or another PKA inhibitor, H89, completely abolished
the attenuation by forskolin on WIN 55,212-2-induced synaptic
depression. The effect of forskolin on CB1 receptor
function was still observed in a low Ca2+ bathing solution,
suggesting that the forskolin's action is not attributable to its
ability to saturate the presynaptic transmitter release processes. The
possibility that forskolin acted by increasing CB1 receptor
phosphorylation was confirmed by demonstrating that the
serine-phosphorylated component with CB1 receptors was
significantly increased after forskolin treatment. This forskolin
effect was markedly attenuated in the presence of KT5720. Moreover, the
activation of
-adrenergic receptors by isoproterenol mimics
forskolin to elicit a PKA-dependent inhibition of CB1
receptor function. Together, these observations indicate that the
presynaptic inhibitory action of CB1 receptors at
corticostriatal synapses could be negatively regulated by
cAMP/PKA-mediated receptor phosphorylation. This effect of PKA may play
a functional role in fine-tuning glutamatergic transmission at
corticostriatal synapses.
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