Opioid actions on rat anterior cingulate cortex neurons in vitro

Area of Science:

• Neuroscience
• Neurophysiology
• Pharmacology

Background:

• The anterior cingulate cortex (ACC) plays a crucial role in cognitive and emotional processing.
• Opioid receptors, particularly delta and mu subtypes, are known modulators of neuronal excitability.
• Understanding the specific actions of endogenous opioids like [Met5]enkephalin in the ACC is vital for deciphering its functional roles.

Purpose of the Study:

• To investigate the effects of [Met5]enkephalin on synaptic transmission and intrinsic membrane properties of rat anterior cingulate cortex layer V pyramidal neurons.
• To determine the specific opioid receptor subtypes involved in mediating these effects.
• To characterize the impact of [Met5]enkephalin on neuronal excitability.

Main Methods:

• Intracellular recordings from layer V pyramidal neurons in rat ACC slices.
• Application of [Met5]enkephalin and selective opioid receptor agonists/antagonents (DPDPE, DAMGO, naltrindole, CTOP).
• Evoked excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were measured, along with changes in membrane potential and conductance.

Main Results:

• [Met5]enkephalin reversibly reduced EPSP amplitude (EC50 ~800 nM) and IPSP amplitude (EC50 ~60 nM).
• These effects were mimicked by the delta-opioid agonist DPDPE and blocked by the delta-opioid antagonist naltrindole, indicating delta-receptor mediation.
• Fifty percent of pyramidal cells and five of seven nonpyramidal cells were hyperpolarized by [Met5]enkephalin, resulting from increased potassium conductance.
• Hyperpolarization of pyramidal cells was delta-receptor mediated, while nonpyramidal cell hyperpolarization was mu-receptor mediated.

Conclusions:

• [Met5]enkephalin exerts differential modulatory effects on synaptic transmission in the rat ACC, primarily through delta-opioid receptors.
• It reduces both excitatory and inhibitory inputs to layer V pyramidal neurons.
• [Met5]enkephalin induces hyperpolarization in a subset of pyramidal cells via delta-receptors and in nonpyramidal cells via mu-receptors, suggesting complex network modulation.