Inhibition of mammillary body neurons by direct activation of Group II metabotropic glutamate receptors.

The mammillary body is an important neural component of limbic circuitry implicated in learning and memory. Excitatory and inhibitory inputs, primarily mediated by glutamate and gamma-amino butyric acid (GABA), respectively, converge and integrate in this region, before sending information to the thalamus. One potentially overlooked mechanism for inhibition of mammillary body neurons is through direct activation of Group II metabotropic glutamate receptors (mGluRs). Here, whole-cell patch clamp recordings of in vitro slice preparations containing the mammillary body nuclei of the mouse were employed to record responses to bath application of pharmacological agents to isolate the direct effect of activating Group II mGluRs. Application of the Group II mGluR specific agonist, APDC, resulted in a hyperpolarization of the membrane potential in mammillary body neurons, likely resulting from the opening of a potassium conductance. These data suggest that glutamatergic inputs to the mammillary body may be attenuated via Group II mGluRs and implicates a functional role for these receptors in memory-related circuits and broadly throughout the central nervous system.