Enkephalin- and dynorphin-release produced by electrical stimulation of different frequencies in rat brain slices

Abstract

Endogenous opioid peptides contribute to the process of pain control and. Previous studies, in vivo, have shown that 2 and 100Hz peripheral electrical stimulation evokes the release of endogenous met-enkephalin (Mek) and dynorphin (Dyn) in cerebrospinal fluid, respectively, in a frequency-dependent manner. However, it is unknown whether opioidergic neurons in the brain areas related to analgesia could respond directly to electrical stimulation of differing frequencies. In the present study, the release of Mek and DynA from slices of rat spinal dorsal horn (DH) and arcuate nucleus (Arc), elicited by different frequencies of electrical stimulation, were investigated using radioimmunoassays and in vitro perfusion techniques. Slices were stimulated with electrical field stimulation (600 pulses, 0.2ms duration, and 10mA), in a frequency range of 0.5–200Hz. Frequencies at 2, 15, and 100Hz could increase the releases over basal level of Mek and DynA, but 2Hz was most potently to evoke Mek release in ARC slice, 100Hz was optimal to DynA release in DH slice. This frequency-dependent and region-specific release was unrelated to the ratio of the two peptides content. An intracellular calcium chelator inhibited Mek release induced by 2Hz, but did not affect the release of DynA at 100Hz from DH slices, implying that intracellular calcium ions in part mediate the release of Mek evoked by low frequency stimulation in DH slices. These data indicate that Mek and DynA peptidergic neurons can be stimulated by specific electrical frequencies in a frequency-dependent and region-specific manner, in which intracellular calcium ions may partially play some role. Thus, our findings may provide a new point of understanding about how opioidergic neurons code information.