Opioid receptor modulation of mossy fiber synaptogenesis: independence from long-term potentiation

Long-term potentiation (LTP) at the mossy fiber-CA3 synapse of the rat hippocampus is an NMDA receptor-independent form of synaptic plasticity that is sensitive to opioid receptor antagonists [12]. In the present study, Timm's stain, a zinc detecting histological marker commonly used to infer synaptogenesis in the mossy fiber projection, was used to examine whether synaptogenesis occurs in response to mossy fiber LTP induction in the adult rat in vivo. Seven days following the induction of mossy fiber LTP by non-seizure-inducing high-frequency stimulation of the mossy fibers, a prominent band of Timm's staining appeared bilaterally in the infrapyramidal region of the stratum oriens in area CA3. Staining was more prominent on the side contralateral to the stimulation. Systemic administration of the opioid receptor antagonist naloxone, sufficient to block mossy fiber LTP induction, did not block the development of Timm's staining in the infrapyramidal region ipsilateral to stimulation, but it did block stimulation-induced increases in Timm's staining observed contralaterally. Systemic administration of (+/-) CPP, a competitive NMDA receptor-antagonist, by contrast, did not block the induction of LTP and did not alter the increase in Timm's staining observed either ipsilaterally or contralaterally. The increase in Timm's staining in the infrapyramidal region suggests that mossy fiber synaptogenesis occurs in response to non-seizure inducing stimulation. Synaptogenesis does not appear to be directly related to opioid receptor-dependent mossy fiber LTP induction, because it occurs in the presence of naloxone which blocks LTP. The mossy fiber synaptogenesis occurring contralaterally appears to be regulated by endogenous opioid peptides, because it is blocked by naloxone.