Glial cells modulate hippocampal synaptic plasticity in morphine dependent rats.

Drugs of abuse mediate adaptive mechanisms leading to alterations in the synaptic plasticity that has been shown to underlie addictive behaviors. Glial cells play a critical role in modulation of synaptic strength and are potentially involved in drug addiction. Chronic administration of morphine has been shown to increase glial cell activation; therefore, it is possible that altered neuroplasticity induced by drugs of abuse is in part mediated by the activity of glial cells. We investigated the effect of hippocampal glial inhibition on synaptic plasticity in morphine treated rats. The fluorocitrate (an inhibitor of glial cells) was microinjected into the CA1 area before morphine injection. The rats received subcutaneous (s.c.) injections of morphine sulfate (10 mg/kg) every 12 h for 9 days. Field excitatory postsynaptic potentials (fEPSP) were recorded from the stratum radiatum of the CA1 area following Schaffer collateral stimulation. Our results indicated that morphine treatment increases long-term potentiation (LTP) and inhibition of glial cells prevents morphine-induced LTP enhancement. Morphine exposed rats exhibited a resistance to LTD induction, whereas, pretreatment with fluorocitrate reduced this resistance. Glial inhibition did not affect LTP and LTD in the untreated animals. The paired pulse ratio (PPR) in inter stimulus intervals (ISI) of 80 ms in the morphine treated group was significantly higher than the control group, while glial inhibition significantly decreased the PPR in morphine treated rats. Our results suggest that morphine exposure modulates hippocampal short- and long-term synaptic plasticity and these alterations in neuronal activity are in part due to glial activity.

Hippocampal glial cells modulate morphine-induced behavioral responses.

Drugs of abuse cause persistent alterations in synaptic plasticity that is thought to underlie addictive-like behaviors. Although, the perisynaptic glial cells are implicated in metabolic maintenance and support of the nervous systems, accumulating evidence suggests that glial cells exert a modulatory action on synaptic functions and participate in synaptic plasticity. However, it is well-documented that glial cells are associated with the acquisition of rewarding effects of abused drugs. The role of hippocampal glial cells in addictive-like behaviors remains poorly understood. In this study, we investigated the role of hippocampal glial cells in morphine-induced behavioral responses including morphine dependence, tolerance to the antinociceptive properties of morphine, and conditioned place preference (CPP). Male rats received subcutaneous (s.c.) morphine sulfate (10 mg/kg) at an interval of 12 h for 9 days. To suppress glial cells activity, the animals received microinjection of fluorocitrate (FC, a metabolic inhibitor of glial cells) into the CA1 region before each morphine administration. The animals were assessed for morphine dependence by monitoring naloxone hydrochloride-induced precipitation of somatic signs of morphine withdrawal. The tolerance to the antinociceptive effects of morphine and morphine-induced CPP were measured in a separate set of experimental groups. We found animals receiving FC before morphine injection demonstrated a significant reduction in several signs of morphine withdrawal such as freezing, defecation, chewing, explosive running, ptosis, activity, scratching, wet dog shake, and writhing. Inhibition of glial cells caused a significant reduction of tolerance to the antinociceptive effect of morphine. Finally, intra-CA1 administration of FC decreased morphine-induced CPP. Our findings suggest that hippocampal glial cells may be involved in morphine-induced behavioral responses.