Calcium-sensitive accumulation of norepinephrine in rat cerebral cortex

Area of Science:

• Neuroscience
• Neurochemistry
• Cell Biology

Background:

• Norepinephrine is a key neurotransmitter in the central nervous system.
• Its accumulation in nerve terminals is primarily mediated by the high-affinity, sodium-dependent transporter (uptake1).
• The role of other accumulation mechanisms at high extracellular concentrations remains less understood.

Purpose of the Study:

• To investigate norepinephrine accumulation mechanisms in rat cerebral cortex synaptosomes.
• To characterize a non-uptake1 dependent accumulation process.
• To determine the potential physiological significance of this novel accumulation pathway.

Main Methods:

• Incubation of rat cerebral cortex crude synaptosomal preparations with [3H]-norepinephrine.
• Assessment of accumulation in the presence and absence of uptake1 inhibitors.
• Evaluation of the effect of calcium ions and specific inhibitors (normetanephrine, metaraminol).
• Fractionation of synaptosomes using density gradient centrifugation.

Main Results:

• A rapid, calcium-dependent norepinephrine accumulation (CSA) was observed when uptake1 was inhibited.
• CSA was not affected by normetanephrine but was inhibited by metaraminol.
• CSA was localized predominantly in the synaptosomal fraction.
• At high norepinephrine concentrations (1.0 microM), CSA constituted the majority of total accumulation.

Conclusions:

• A distinct calcium-sensitive accumulation (CSA) of norepinephrine exists in rat brain synaptosomes.
• CSA is functionally different from the canonical high-affinity uptake1 transporter.
• Given its prevalence at high concentrations, CSA may be a significant contributor to noradrenergic neurotransmission in vivo.