THE SPONTANEOUS AND EVOKED RELEASE OF SPERMINE FROM RAT BRAIN in vitro

Abstract

The efflux of previously accumulated [3H]‐spermine from brain slices was measured using a continuous perfusion system. The spontaneous efflux was biphasic, consisting of an initial rapid efflux followed by a much slower release. The slices were depolarized by the addition to the medium of high potassium concentrations, ouabain or veratrine. At concentrations greater than 30 mM, potassium evoked a striking increase in the release of [3H]‐spermine. Following uptake in the presence of 5.7 × 10−9M[3H]‐spermine, K+‐evoked release was dependent on the presence of calcium ions. Release of spermine after uptake at 5.6 × 10−8m or 5.0 × 10−7m was not calcium‐dependent. The calcium‐dependent, K+‐stimulated release of spermine was inhibited in the presence of diphenylhydantoin (5 × 10−3m) or ruthenium red (10−3 m). Following uptake of 5.7 × 10−9m [3H]‐spermine in a sodium‐free medium, the calcium‐dependent, K+‐stimulated release was significantly inhibited. Ouabain (10−4m) caused a large but calcium‐independent increase in the efflux of [3H]‐spermine. Veratrine‐induced release was less substantial but was increased in a calcium‐free medium. Release evoked by veratrine was abolished in the absence of sodium. These results are discussed with respect to a possible ‘neurotransmitter’ or ‘neuromodulator’ role for spermine.

The NR2B-specific Interactions of Polyamines and Protons with theN-Methyl-d-aspartate Receptor

“…5, A and B). Previous measurements of the pH dependence of heterologous combinations of NR1A with either NR2A or NR2B show a trend toward subunit dependence for proton inhibition (39), whereas our experiments showed a slight difference in the IC 50 50 for E201D ϭ 50 Ϯ 6 nM, pH ϭ 7.3), whereas the mutation E201N (the residue found in ⑀ 1 ) demonstrated a pH dependence identical to that of 1A/⑀ 1 receptors. The pH dependence of NR1A/ E201A receptors was shifted to the left (IC 50 ϭ 160 Ϯ 20 nM, pH ϭ 6.8), whereas the greatest change was again seen with the E201R substitution (IC 50 ϭ 300 Ϯ 40 nM, pH ϭ 6.5).…”

“…Studies have shown that in normal brain, polyamines are not found in the synaptic cleft and are thus inaccessible to the extracellular face of NMDA receptors (48). Upon acidosis, which occurs during a hypoxic ischemic insult, polyamines synthesis is up-regulated (49), and there is a release of polyamines into the synaptic cleft where they can have direct interactions with the NMDA receptor (50). Polyamine stimulation of NMDA receptors is significantly enhanced at lower pH and at more depolarized membrane potentials (27,37), two of the characteristic conditions observed for the NMDA receptor during ischemia.…”

“…Like polyamines, proton sensitivity is altered by the presence or absence of the 5Ј-insert. NR1B (5Ј-insert present)-containing receptors show an EC 50 for proton inhibition of pH 6.3, whereas NR1A (lacking insert)-containing receptors have a greater proton sensitivity (EC 50 ϭ pH of 7.3) (39). Proton inhibition depends on NR2 subunit expression, although coexpression of either NR2A or NR2B subunits with NR1A yields receptors with a half-maximal pH inhibition of 7.3; NR2C-containing receptors are insensitive to protons (half-maximal pH ϭ 6.8) (39).…”

Spermine/spermidine is expressed by retinal glial (müller) cells and controls distinct K + channels of their membrane

“…This effect was probably caused by the known modulating activity of polyamines on glutamate receptors (Bowie and Mayer, 1995;Williams et al, 1991). In this context it should be noted that both spontaneous and evoked release of spm and spd from the rat brain has been observed in vitro (Harman and Shaw, 1981) and in vivo (Fage et al, 1992). As spm/spd stores of the rat brain have been mainly located to glial cells (Laube and Veh, 1997), the observed release was probably from glia.…”

Modulation of Neuronal Voltage-Activated Calcium and Sodium Channels by Polyamines and pH

“…6,7 It has been proposed that spermine and spermidine may also function as extracellular neuromodulators due to their interactions with a large number of ion channels in neurons. 8 In support of a neuromodulatory role, polyamines are present in neurons and manufactured from decarboxylation of ornithine, 9 are transported into synaptic vesicles, 10 are released from brain slices by depolarizing stimuli, 11 and may be taken up by glia from the extracellular space. 2,10 Polyamines have a wide range of actions on ion channels including both facilitation and block of NMDA-activated channels, 12,13 inhibition of two pore domain K + channels, 14 inhibition of voltage-activated Ca 2+ channels (VACC), 15 and inhibition of nonselective cation channels.…”