Selective Cholinergic Depletion in Medial Septum Leads to Impaired Long Term Potentiation and Glutamatergic Synaptic Currents in the Hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning

Impairment of LTP in 192-saporin infused rats.

<p>(A) Summary data for experiments in which LTP was induced by TBS and measured at 55–60 min after TBS. LTP in 192-saporin infuced rats (gray circles) was reduced compared to the saline infused controls (black circles). (B) Sample traces depicting LTP in saline infused control rats and lack of LTP in 192-saporin infused rats. Traces with gray lines represent those collected prior to TBS, during baseline recording, and the traces with black lines represent those taken 55–60 min after TBS. Calibration: 1 mV, 20 ms. (c) Bar chart showing drastic reduction of LTP in 192-saporin infused rats (gray) compared to the saline infused controls (black).</p

Cholinergic depletion in the MS leads to alterations in NMDA elicited synaptic single channel currents.

<p>Sample traces and respective current amplitude histograms of (A) non-infused control rats, (B) saline infused rats, and (C) 192-saporin infused rats show that channel open peak (right peaks in each histogram) in the 192-saporin data is reduced demonstrating a decrease in open probability (Calibration: each 4 pA, 200 ms). Dwell open time histograms of NMDA currents were best fitted with two terms for (D) non-infused, (E) saline infused, and (F) 192 saporin infused data. The NMDA elicited currents were confirmed by addition of APV to the extracellular solution to completely block these currents (Data not shown).</p

Inhibition of MS cholinergic pathway decreases the AMPA receptor activity in the CA1 pyramidal neurons of the hippocampus.

<p>(A) Representatives of AMPA receptor mediated sEPSC traces in controls and 192-saporin infused rats recorded at −65 mV membrane potential; Calibration: 40 pA, 1 s. The adjacent average traces depict the reduction in the amplitude in the MS cholinergic lesioned rats compared to control rats, which show no difference in amplitude; Calibration: 15 pA, 30 ms. (B) Cumulative fraction plot of sEPSC amplitude from the composite data shows the shift of 192-saporin curve to the left from the controls, indicating reductions in amplitude. (C) Cumulative fraction plots of sEPSC interevent intervals exhibiting increased values for 192-saporin infused rats suggesting decreased frequency. (D) Representative traces of AMPA receptor mediated mEPSCs for the three groups (Calibration: 10 pA, 1 s) and the mEPSCs (Calibration: 10 pA, 30 ms) to show reduced amplitude in the 192-saporin infused rats. (E) Cumulative fraction plots for amplitude and (F) interevent interval exhibiting reduced amplitude and frequency.</p

Burst analysis of AMPA and NMDA receptor single channel currents in synaptosomes.

<p>*<i>P</i><0.05,</p>#<p><i>P</i><0.01.</p

Infusion of 192-saporin results in lesioning of cholinergic neurons.

<p>Photomicrographs (50× magnification) of MS region immunostained with anti-ChAT antibody in (A) a saline infused rat and (B) in a rat infused with 192-saporin.</p