Purpose: Lateral hypothalamic (LH) orexin neurons control diverse, survival-relevant homeostatic functions. We recently showed evidence in rats that cocaine exposure induces presynaptic plasticity in LH orexin (ORX) circuitry. Here we assessed the longevity of these changes beyond initial cocaine withdrawal periods. In addition, we have begun to assess potential sources of cocaine-induced presynaptic plasticity. Specifically, orexin neurons are hypothesized to make intra-LH synaptic contacts, including onto other orexin neurons, which may contribute to the plasticity events we described. Therefore, we undertook connectivity analyses of orexin neurons in drug-naïve animals.
Methods: In longevity experiments, orexin-GFP (n=20) mice were injected daily (7-days) with either cocaine (15mg/kg) or saline (control). Miniature excitatory postsynaptic currents (mEPSCs) recordings were made in LH neurons after 24 hrs, 7 days or 14 days of drug abstinence. In connectivity experiments, paired recordings were made from cocaine-naïve orexin-GFP neuron pairs or ChannelRhodopsin2/+ve orexin neurons during whole slice 488 nm photostimulation.
Results: Cocaine-induced increases in mEPSC frequency persisted for up to two weeks (26.89 ± 3.30 Hz – 24hrs, 25.60 ± 7.23 Hz – 7days, and 27.63 ± 2.94 Hz – 14 days, respectively) without affecting amplitude, rise or decay time. In paired recordings from orexin-GFP neurons we found no evidence of synaptic coupling (0/20 connections tested). Photostimulation reliably elicited action potential discharge in orexin-ChR2 neurons. However, we detected no change in spontaneous synaptic activity during light stimulation (0/8), implying a lack of connections between adjacent and recorded orexin-ChR2 neurons.
Conclusions: Cocaine-induced rewiring of LH-orexin circuits persists over a time frame highly relevant to addiction/relapse risk. Contrasting current models of orexin circuit function, we failed to detect evidence that orexin neurons provide a source of excitatory drive to LH neurons. Thus, if local orexin circuits do contribute to cocaine-induced synaptic plasticity, new denovo connections may have to form.