Acetylcholine (ACh)-mediated synaptic transmission in the CNS is a key component of mnenomic function and substantial cognitive deficits can be induced with cholinergic antagonists that penetrate the CNS. Furthermore, the most widely prescribed therapies for disease-related cognitive decline, such as that present in Alzheimer’s disease, are agents designed to enhance cholinergic function. Despite this, relatively little work has been carried out on the neurophysiology of ACh-mediated synaptic responses in structures such as the hippocampus, which are central to normal mnemonic function. Using intracellular recording, a strongly desensitising muscarinic receptor-mediated EPSP has been recorded in hippocampal CA1 neurones in response to stimulation of stratum oriens (Morton & Davies, 1997). In this communication we describe extracellular recordings of this muscarinic EPSP and its modulation by pharmacological agents.
Male Lister-hooded rats were humanely killed by overdose of halothane, followed by decapitation, and hippocampal slices (400 µM thick) were prepared using standard methods. All recordings were made at approximately 32°C in a standard aCSF equilibrated with 95 % O2/5 % CO2. To inhibit glutamatergic and GABAergic synaptic transmission the aCSF was supplemented with 10 µM NBQX, 50 µM D-AP5 and 10 µM Gabazine. Recording electrodes, filled with aCSF, were placed in stratum pyramidale of area CA1. Trains of four stimuli (100 Hz, 30-40 V, 30-60 µs duration) were applied to stratum oriens every 5 min using a bipolar stimulating electrode. These stimuli consistently produced a negative-going potential that increased in amplitude with stimulus number and/or strength. This response was excitatory in nature as it often drove post-synaptic firing. Application of the muscarinic antagonist atropine (10 µM) eliminated a substantial component of the response, although an initial early component sometimes remained. The atropine-sensitive component determined by subtraction rose slowly to peak (time to peak = 1.3 ± 0.2 s) and had a duration of approximately 10.3 ± 1.6 s (n = 8); a time course in good agreement with the previously reported intracellular recordings of a muscarinic EPSP (Morton and Davies, 1997). Application of the cholinesterase inhibitor physostigmine (10 µM) greatly enhanced both the amplitude (630 ± 199 % increase) and duration (273 ± 142 % increase, n = 3) of the atropine-sensitive component.
This study outlines a facile method to record population hippocampal muscarinic EPSPs and may prove valuable in understanding the regulation and plasticity of this physiologically important conductance.