Synaptic inhibition in the rat hippocampus in vivo following stimulation of the substantia nigra and ventral tegmentum.

Abstract

SUMMARY1. The effects of stimulating the substantia nigra (SN) or the ventral tegmental area (VTA) on the excitability of cells in the rat hippocampal formation have been investigated in vivo.2. A train of conditioning stimuli to either of the midbrain nuclei produced inhibition of evoked population spikes recorded in the CAI pyramidal cell layer of the hippocampus.3. These trains of pulses had no effect on the evoked synaptic field potential recorded in the stratum radiatum although they were effective in suppressing glutamate-induced firing of cells in the hippocampus. These observations suggest that the inhibition is mediated through a postsynaptic mechanism.4. The inhibition of the test population spike was observed at a latency of 50 ms after the conditioning train to either the SN or the VTA but did not reach a maximum until 300-500 ms and 500-750 ms post-conditioning, respectively. The total duration of the inhibition in each case was about 5 s.5. Following stimulation of the VTA, comparable levels of inhibition were recorded in the ventral and dorsal hippocampus. However, after stimulation of the SN, significantly less inhibition was observed in the ventral hippocampus.6. Unlike the effects on the commissural-evoked population spike in CAL, stimulation of SN or VTA had no effect on perforant path-induced granule cell excitability in the dentate gyrus.7. These results suggest that activity in the substantia nigra or ventral tegmental areas could have a powerful regulatory or feedback role in suppressing hippocampal excitability.

Ethanol acts via the ventral tegmental area to influence hippocampal physiology

“…The VTA-induced facilitation of dentate PSs reported here contrasts with a previous study demonstrating that Conditioning stimulation to the VTA had no effect on PPD responses (Spencer and Wheal, 1990). This discrepancy might be attributed to differences in the techniques used (400 Hz vs. 20 Hz) and differences in anesthetics used (halothane vs. urethane).…”

Ventral Tegmental Area Inactivation Suppresses the Expression of CA1 Long Term Potentiation in Anesthetized Rat

“…Consistent with our data, previous studies found no changes in the baseline synaptic response as well as PPF in the CA1 area of anesthetized rats by inactivation [21], electrical lesion or stimulation of the VTA [33] and in D1R depleted animals [6], [7]. A recent study clearly showed there are no differences in postsynaptic currents recorded by patch clamp, extracellular field EPSP s and paired-pulse ratios of field EPSP s in slices from sham-operated and VTA/SNc 6-OHDA-lesioned rats [22].…”

“…However, lidocaine also inactivates fibers of passage from raphe nucleus and locus coeruleus [33], [53] that might have produced the observed LTP impairment. Previous studies showed serotonin antagonists increase the magnitude and duration of CA1 LTP [54], [55], while its agonists block it [56], [57].…”