Nobuyuki Fukushima

Low doses of naloxone produce analgesia in the mouse brain by blocking presynaptic autoinhibition of enkephalin release.

The involvement of presynaptic autoinhibition of Met-enkephalin release in naloxone-induced analgesia was studied. In both acetic acid writhing and tail-flick tests in mice, naloxone produced biphasic effects, analgesia at very low doses (1 microgram/kg s.c. or 1 ng intracisternal) and hyperalgesia at higher doses (100 micrograms/kg s.c. or 100 ng intracisternal). Morphine at 10(-6) to 10(-5)M depressed the high K+-evoked release of Met-enkephalin from slices of the rat brainstem by 12.5-55.9% of control, while naloxone at 10(-6)M significantly enhanced the release by 80.6%. These findings strongly suggest that in the mouse brain a very low dose of naloxone produces analgesia by blocking autoinhibition of enkephalin release.

Presynaptic opioid κ-receptor and regulation of the release of Met-enkephalin in the rat brainstem

Opioid kappa-agonists had much more potent inhibitory effects on the high K+-evoked Met-enkephalin release from rat brain slices than did the mu- or delta-agonists. The opioid kappa- antagonist, MR2266 enhanced the evoked release of Met-enkephalin to a greater extent than did mu- or delta-antagonists in vitro and had a potent analgesia in mice in vivo. These findings suggest that the release of Met-enkephalin may be regulated in vitro and in vivo, mainly by presynaptic kappa-receptor-mediated mechanisms.

The specific opioid κ-agonist U-50,488H inhibits low Km GTPase

Abstract DAGO, a specific opioid μ-agonists stimulated the low Km GTPase activity of membranes of the guinea-pig striatum at 1–1000 nM and this effect was antagonized by naloxone 10–100 nM. On the other hand, U-50,488H, a specific opioid κ-agonist inhibited the low Km GTPase activity of membranes of the guinea-pig cerebellum at 1–100 nM. As this effect was antagonized by MR2266 but not by naloxone, the findings suggest that opioid μ- and κ-receptor stimulation is probably linked to the stimulation and inhibition of low Km GTPase, respectively.

Uptake and release of kyotorphin in rat brain synaptosomes

Uptake and release of kyotorphin (Tyr-Arg) in rat brain synaptosomes were studied. Synthetic kyotorphin was taken up into crude synaptosomes (P2), in a temperature-dependent manner. The Km and Vmax of the uptake were 1.31 +/- 0.12 X 10(-4) M and 5.9 +/- 0.5 pmol/mg protein/min, respectively. Metabolic inhibitors such as dinitrophenol and iodoacetamide and ouabain which is known as an inhibitor of Na+ dependent uptake mechanism significantly inhibited the uptake. When the synaptosomes previously preloaded with synthetic kyotorphin at 10(-4)M were exposed to high K+ medium, kyotorphin was released in a Ca2+-dependent manner. These findings support the view that kyotorphin plays a role as neurotransmitter/neuroregulator.

A Met-enkephalin releaser (kyotorphin)-induced release of plasma membrane-bound Ca2+ from rat brain synaptosomes

The neuropeptide kyotorphin (Tyr-Arg) released 45Ca2+ from synaptosomal membrane preparations of the rat lower brainstem. This dipeptide also decreased plasma membrane-bound Ca2+ ([Ca2+]m), determined using chlorotetracycline, in the slice and synaptosome of the rat lower brainstem. Taken together with other data indicating that kyotorphin induces an increase in synaptosomal intracellular Ca2+ ([Ca2+]i) in a Ca2+ channel-independent manner, it is suggested that kyotorphin may mobilize the [Ca2+]m and thus increase the [Ca2+]i in the nerve terminals of the rat lower brainstem.