Activation requirements for metabotropic glutamate receptors.

It has been common experimentally to use high frequency, tetanic, stimulation to activate metabotropic glutamate receptors (mGluRs) in cortex and thalamus. To determine what type of stimulation is actually necessary to activate mGluRs we examined the effects of varying stimulation duration and intensity on activating mGluR responses. We used a thalamocortical and an intracortical slice preparation from mice and performed whole cell recordings from neurons in the ventral posterior medial nucleus or in layer 4 of primary somatosensory cortex (S1) while electrically stimulating in layer 6 of S1. Extracellular ionotropic glutamate receptor antagonists and GABAA receptor antagonists were used to isolate Group I or Group II mGluR responses. We observed that high frequency stimulation is not necessary for the activation of either Group I or Group II mGluRs. Either could be activated with as few as 2-3 pulses at stimulation frequencies around 15-20Hz. Additionally, increasing the number of pulses, intensity of stimulation, or stimulation frequency increased amplitude and duration of the mGluR response.