Muscarinic presynaptic modulation in GABAergic pallidal synapses of the rat

Abstract

The external globus pallidus (GPe) is central for basal ganglia processing. It expresses muscarinic cholinergic receptors and receives cholinergic afferents from the pedunculopontine nuclei (PPN) and other regions. The role of these receptors and afferents is unknown. Muscarinic M 1 -type receptors are expressed by synapses from striatal projection neurons (SPNs). Because axons from SPNs project to the GPe, one hypothesis is that striatopallidal GABAergic terminals may be modulated by M 1 receptors. Alternatively, some M 1 receptors may be postsynaptic in some pallidal neurons. Evidence of muscarinic modulation in any of these elements would suggest that cholinergic afferents from the PPN, or other sources, could modulate the function of the GPe. In this study, we show this evidence using striatopallidal slice preparations: after field stimulation in the striatum, the cholinergic muscarinic receptor agonist muscarine significantly reduced the amplitude of inhibitory postsynaptic currents (IPSCs) from synapses that exhibited short-term synaptic facilitation. This inhibition was associated with significant increases in paired-pulse facilitation, and quantal content was proportional to IPSC amplitude. These actions were blocked by atropine, pirenzepine, and mamba toxin-7, suggesting that receptors involved were M 1 . In addition, we found that some pallidal neurons have functional postsynaptic M 1 receptors. Moreover, some evoked IPSCs exhibited short-term depression and a different kind of modulation: they were indirectly modulated by muscarine via the activation of presynaptic cannabinoid CB 1 receptors. Thus pallidal synapses presenting distinct forms of short-term plasticity were modulated differently.

Diverse Short-Term Dynamics of Inhibitory Synapses Converging on Striatal Projection Neurons: Differential Changes in a Rodent Model of Parkinson’s Disease

“…IPSCs amplitudes were measured from basal line to peak from the first response (IPSC 1 ). The STSP curves were graphed as the amplitude of the n th IPSC normalized by the amplitude of the first IPSC (IPSC 1 ) and fitted to a generic multiplication of exponential functions after normalization: Exponentials functions correspond to facilitating and depressing plasticity, respectively [ 33 ], because both processes appear to occur at the same time in any synapse [ 3 ]. A represents the initial steady state value that the recovery process tends to approach.…”