Abstract

We examined the effect of acute and chronic opioid treatment on synaptic transmission and μ-opioid receptor (MOR) endocytosis in cultures of naı̈ve rat hippocampal neurons. Opioid agonists that activate MOR inhibited synaptic transmission at inhibitory but not excitatory autapses. [d-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO), morphine, and methadone were all effective at blocking inhibitory transmission. These same drugs also reduced the amplitude of voltage-dependent Ca²⁺ currents in inhibitory but not excitatory neurons. Chronic treatment with all three opioids reduced the subsequent effects of a challenge with either the same drug or one of the others in individual autaptic neurons. Chronic treatment with DAMGO or methadone produced internalization of enhanced yellow fluorescent protein-tagged MOR expressed in hippocampal neurons within hours, whereas morphine produced internalization much more slowly, even when accompanied by overexpression of β-arrestin-2. We conclude that DAMGO, methadone, and morphine all produce tolerance in single hippocampal neurons. Morphine-induced tolerance does not necessarily seem to involve receptor endocytosis.