Long-term potentiation of glutamatergic pathways in the lamprey brainstem

Abstract

Plasticity of synaptic transmission has been investigated in the lamprey brainstem. In this preparation, neurons of the vestibular nuclei make monosynaptic connections to reticulospinal neurons of the posterior (PRRN) and middle (MRRN) rhombencephalic reticular nuclei. This glutamatergic projection shows a long-lasting enhancement in efficacy following a brief high frequency (50 Hz) train of stimuli (tetanus). Enhancement was found on the inputs from the octavomotorius intermediate (nOMI) and octavomotorius posterior (nOMP) nuclei to the reticulospinal neurons of the PRRN and MRRN, and persisted for the entire duration of the recordings. The enhancement was limited to the pathway that received tetanic stimulation for the inputs to any given reticulospinal neuron. It depended upon the activation of postsynaptic processes at least in part; dialysis of the reticulospinal neuron by recording with a whole-cell patch pipette prevented its induction. This dialysis-dependent abolition of enhancement was shown to be dependent on a change in Ca2+ concentration in the postsynaptic neuron. The enhancement was not affected by blockade of NMDA receptors with D,L-2-amino-5-phosphonopentanoate (AP5) but was prevented by the metabotropic glutamate receptor (mGluR) antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG). In conclusion, this study demonstrates that vestibular inputs to reticulospinal neurons are capable of undergoing long-term potentiation (LTP) and that this LTP shows synapse specificity. Furthermore, this LTP is activated by synaptic stimulation of a subtype of the m-GluR family and its induction is mediated by changing Ca2+ concentrations in the postsynaptic neuron.