Real-Time, Simultaneous Detection of Dopamine and Met-Enkephalin Fluctuations in Rat Striatum Using Voltammetry

Many studies have investigated the role of striatal dopamine (DA) in reward-related learning and goal-directed behavior. However, met-enkephalin (M-ENK) and related endogenous opioid peptides remain far less studied, despite many lines of evidence linking these molecules to drug abuse and addiction. The opioid neuropeptides have proven difficult to directly detect in situ because they exist at low concentrations and are readily degraded by endogenous protease activity. As such, many fundamental questions regarding opioid peptide signaling remain unanswered, including, the timescale of release, the physiological concentration range, and the dynamics of the chemical release profile. In this work, fast-scan cyclic voltammetry (FSCV) is coupled with carbon-fiber microelectrodes to directly characterize DA and M-ENK signals evoked by local electrical stimulation in rat striatal tissue slices. Due to the overwhelming presence of catecholamine in striatum, an inhibitory Designer Receptor Exclusively Activated by Designer Drugs (DREADD) was used to chemogenetically minimize mesolimbic DA release. The data demonstrate a classic kinetic profile for DA governed by well-characterized release and reuptake kinetics. By contrast, the M-ENK signal demonstrates a biphasic release profile, with maximal evoked concentrations that rise in the extracellular space about 30 seconds after stimulation, significantly later than the DA signal simultaneously recorded at the same site. This is consistent with a 3-fold larger sphere of influence than that of DA and the ‘volume transmission’ concept of neuropeptide signaling. The data demonstrate how these neurotransmitters affect different cell populations in the striatum, to shape circuit output. Ultimately, the results provide direct data in support of widely accepted assumptions regarding neuropeptide signaling dynamics that will improve our understanding of longstanding medical problems that involve these systems, like pain and addiction.