Biomedical subjects

T D White

Publications and source records attributed to T D White.

At least 55 records · Page 3Linked to original sources

PubMed1993

New discoveries of Australopithecus at Maka in Ethiopia.

The taxonomy of Australopithecus afarensis, the oldest known hominid species, has been a matter of debate since its description in 1978 (ref. 1). Some authorities regard all specimens assigned to A. afarensis as belonging to a single taxon whereas others regard the Tanzanian and Ethiopian specimens as each representing a different species. Further controversy surrounds the issues of sexual dimorphism and locomotion among these hominids. Resolution of these problems would shed light on hominid phylogeny in general and on the ancestry of later Australopithecus and Homo. Fossils discovered in the Afar of Ethiopia in 1990 constitute the first major addition to the 3-4 million year (Myr) hominid record since the 1970s. We report here the discovery of new fossils from Maka, dated to 3.4 Myr ago, which provide powerful support for the interpretation of A. afarensis as a single, ecologically diverse, sexually dimorphic, bipedal Pliocene primate species whose known range encompassed Ethiopia and Tanzania.

Animals↗

PubMed1993

NMDA-evoked adenosine release from rat cortex does not require the intermediate formation of nitric oxide.

Excitatory amino acids (EAAs) such as glutamate release the inhibitory neuromodulator adenosine from superfused rat cortical slices through the activation of both NMDA and non-NMDA EAA receptors. This study investigated the possibility that NMDA-evoked adenosine release may involve the intermediate formation of nitric oxide (NO). However, sodium nitroprusside did not evoke the release of adenosine, L-arginine did not augment and L-Nv-nitroarginine did not diminish NMDA-evoked adenosine release. It appears, therefore, that NMDA-evoked NO formation does not play a role in NMDA-evoked adenosine release in the cortex.

Adenosine↗

PubMed1993

Morphine-evoked release of adenosine from the spinal cord occurs via a nucleoside carrier with differential sensitivity to dipyridamole and nitrobenzylthioinosine.

We have investigated the potential role of a bi-directional nucleoside carrier in the release of endogenous adenosine from spinal cord synaptosomes by examining the effects of dipyridamole and nitrobenzylthioinosine (NBI) on evoked release of adenosine. When 40 pmol adenosine were added to synaptosomes, only 70 +/- 2% was recovered, suggesting 30% uptake of adenosine. Dipyridamole (0.1-10 microM) reduced this uptake and also increased basal adenosine release, probably due to inhibition of the re-uptake of adenosine derived from released nucleotide. In contrast, NBI (0.1-10 microM) had no effect on either uptake of added adenosine or on basal release of adenosine. Addition of K+ (24 mM) and morphine (10 microM) produced a 50-60% increase in the release of adenosine, and this was reduced 35-98% by both dipyridamole and NBI (0.01-10 microM). Dipyridamole (0.01-1 microM) had no effect on the release of nucleotides (detected as adenosine) induced by noradrenaline, 5-hydroxytryptamine (5-HT) and capsaicin (50 microM each), although 10 microM dipyridamole significantly reduced release evoked by noradrenaline and 5-HT. This latter effect of dipyridamole was determined not to be due to inhibition of ATP release when measured directly. Within the spinal cord, there is a removal system for adenosine which is dipyridamole-sensitive but NBI-insensitive. Release of adenosine, but not nucleotides, appears to occur via this carrier system. The inhibition of release by NBI, but its lack of effect on uptake, suggests the involvement of heterogeneous carrier molecules in adenosine uptake and release from the spinal cord.

Adenosine↗

PubMed1993

ATP release from dorsal spinal cord synaptosomes: characterization and neuronal origin.

The present study determined the Ca(2+)-dependence of the release of adenosine 5'-triphosphate (ATP) from dorsal spinal cord synaptosomes evoked by depolarization with K+ and capsaicin, and the effect of intrathecal capsaicin pretreatment, dorsal rhizotomy and intrathecal pretreatment with 6-hydroxydopamine on such release. Release of ATP evoked by K+ was Ca(2+)-dependent, while that evoked by capsaicin was Ca(2+)-independent. Capsaicin pretreatment (60 micrograms, 7 days), which lesions small diameter afferents, did not alter release of ATP evoked by either K+ or capsaicin. Dorsal rhizotomy, which lesions all afferents, produced a significant reduction in the amount of ATP released from the rhizotomized side compared to the intact side. Pretreatment with 6-hydroxydopamine (100 micrograms, 7 days) to destroy adrenergic nerve terminals, markedly reduced spinal cord noradrenaline levels, but did not alter the K(+)-evoked release of ATP. These results suggest that some K(+)-evoked release of ATP could originate from large but not small diameter afferent nerve terminals in the spinal cord. ATP does not appear to originate from small diameter afferents as, although ATP is released by in vitro exposure to capsaicin, such release occurs only at high concentrations, release is Ca(2+)-independent and it is unaltered by pretreatment with capsaicin. The bulk of the ATP released from the spinal cord does not originate from descending noradrenergic nerve terminals.

Adenosine Triphosphate↗

PubMed1993

Influence of calcium on the release of endogenous adenosine from spinal cord synaptosomes.

Intrathecal administration of Ca2+ has been shown to produce antinociception which is thought to be partially mediated by the release of adenosine. In the present study we have examined directly the effects of varying Ca2+ concentrations on the release of endogenous adenosine, measured by HPLC with fluorescence detection, from rat spinal cord synaptosomes. Although increasing the concentration of extracellular Ca2+ reduces the total amount of adenosine detected extrasynaptosomally, the component derived from the release of adenosine per se is actually augmented. This release of adenosine occurs from dorsal but not ventral spinal cord synaptosomes and appears to originate from capsaicin-sensitive nerve terminals. The Ca2+ ionophore A23187 also releases adenosine, but this release is due to the extrasynaptosomal conversion of released nucleotide(s) to adenosine, as it is markedly reduced by ecto-5'-nucleotidase inhibitors. Release of adenosine by A23187 occurs from both the dorsal and ventral spinal cord, and is not capsaicin-sensitive. Ethanol, used as a vehicle for the ionophore, releases adenosine which is a mixture of adenosine and nucleotide from both dorsal and ventral spinal cord synaptosomes. These observations provide direct support for behavioural studies which demonstrate that methylxanthines block antinociception produced by intrathecal administration of Ca2+.

Adenosine↗

PubMed1993

Involvement of calcium channels in depolarization-evoked release of adenosine from spinal cord synaptosomes.

The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 nM) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 mM) but not capsaicin (1 and 10 microM). This augmentation was dose dependent from 1 to 10 nM and was followed by inhibition of release from 30 to 100 nM. Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 mM) or capsaicin (1 and 10 microM). omega-Conotoxin inhibited K(+)-evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K(+)-evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+, N-type voltage-sensitive Ca2+ channels are primarily involved in K(+)-evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red.

Adenosine↗

PubMed1993

Morphine activates omega-conotoxin-sensitive Ca2+ channels to release adenosine from spinal cord synaptosomes.

Morphine-induced release of adenosine from the spinal cord is believed to contribute to spinal antinociception. Although this release is Ca2+ dependent, little is known of the nature of this dependence. In this study, the effects of the dihydropyridine L-type Ca2+ channel agonist Bay K 8644 and the antagonist nifedipine, the N-type Ca2+ channel antagonist omega-conotoxin, and ruthenium red, a blocker of Ca2+ influx induced by capsaicin, on release of adenosine evoked by morphine were determined. The effect of partial depolarization with a minimally effective concentration of K+ on morphine-evoked release of adenosine also was examined. Morphine 10(-5)-10(-4) M produced a dose-dependent enhancement of adenosine release from dorsal spinal cord synaptosomes. Following the addition of 6 mM K+ (total K+ concentration of 10.7 mM), 10(-6) M morphine also enhanced release, and an additional component of action at 10(-8) M was revealed. Release was Ca(2+)-dependent as it was not observed in the absence of Ca2+ and presence of EGTA. Bay K 8644 (10 nM) and nifedipine (100 nM) had no effect on the release of adenosine evoked by morphine, but omega-conotoxin (100 nM) markedly reduced such release in both the absence and the presence of the additional 6 mM K+. Morphine-evoked adenosine release was not altered in the presence of a partially effective dose of capsaicin, nor by ruthenium red.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗

PubMed1993

N-methyl-D-aspartate- and non-N-methyl-D-aspartate-evoked adenosine release from rat cortical slices: distinct purinergic sources and mechanisms of release.

Excitatory amino acids, acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors, release the inhibitory neuromodulator adenosine from superfused rat cortical slices. This study was initiated to investigate the possible purinergic sources and mechanisms of release for the adenosine release evoked by NMDA and non-NMDA receptor activation. Inhibition of the bidirectional nucleoside transporter with dipyridamole greatly enhanced adenosine release evoked by glutamate. NMDA, kainate, and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Inhibition of ecto-5'-nucleotidase with alpha, beta-methylene ADP and GMP had no effect on either kainate- or AMPA-evoked adenosine release, but it decreased glutamate- and NMDA-evoked adenosine release by 23 and 68%, respectively. A similar inhibition of NMDA-evoked adenosine release was observed with alpha, beta-methylene ADP alone, indicating that the inhibitory effect was not due to the reported competitive inhibition of NMDA receptors by GMP. Finally, NMDA-evoked adenosine release, but not kainate- or AMPA-evoked release, was Ca2+ dependent. These results indicate that activation of non-NMDA receptors releases adenosine per se in a Ca(2+)-independent manner. In contrast, NMDA receptor activation releases primarily a nucleotide that is subsequently converted extracellularly to adenosine; in this case, release is Ca2+ dependent. Although neither NMDA- nor non-NMDA-evoked adenosine release occurs via the nucleoside transporter, this transporter does appear to be a major route for removal of adenosine from the extracellular space.

5'-Nucleotidase↗

PubMed1993

Brain synaptosomal antibodies in systemic lupus erythematosus.

Nervous system involvement in systemic lupus erythematosus (SLE) has been linked to the production of autoantibodies that may bind to surface antigens on neuronal cells and cause cellular dysfunction. At present, little is known of the target antigens recognized by these antibodies. The aim of the present study was to examine reactivity to rat brain synaptosomes (RBS) in sera from patients with SLE. Sera from 73 unselected SLE patients and controls were studied. Crude RBS were prepared by differential centrifugation and enriched fractions of synaptosomes (SY-E), myelin (MY-E) and mitochondria (MI-E) were obtained by sucrose density centrifugation. Rat liver (RL) was used for control antigens. Wheat-germ lectin affinity chromatography yielded membrane-enriched fractions of RBS, RL and whole rat brain (WRB). Antibody binding was examined by Western blotting. IgM reactivity was detected in 12/73 sera (16%) and was directed to proteins of 62K, 48K 37K molecular weight. IgG reactivity was present in 5/73 sera (7%) to proteins of 52K, 48K, 37K and 29K molecular weight. Except for binding to the 52K and 37K proteins these autoantibodies were not detected in control sera. Reactivity was usually absent, or present in reduced amounts, in WRB and RL. Additional experiments revealed that binding of IgM to 62K was found predominantly in SY-E and MY-E fractions, 48K in SY-E and MI-E fractions and 37K in the SY-E fraction. Binding of IgG to 48K and 29K was detected in the SY-E and MI-E, but reactivity to 52K and 37K was restricted to the SY-E fraction. Thus, sera from SLE patients contain antibodies to synaptosomal antigens that may contribute to the neuropsychiatric manifestations of the disease.

Animals↗

PubMed1992

Directionality of phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens.

A dorsal approach to the eighth nerve and free-field stimulation were used to investigate the effect of sound direction and intensity on phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens. Tuning curves of 75 auditory neurons were analyzed (Fig. 2). Amphibian papillar neurons, but not basilar papillar neurons, exhibit significant phase locking to short tone bursts at the characteristic frequency (CF), the degree of phase locking (vector strength) decreasing with the neuron's CF (Figs. 3, 4 and 10E). Vector strength increases with sound pressure level to saturate about 20 dB above threshold, while the preferred firing phase is only slightly affected (Figs. 5 and 6). In contrast, sound direction hardly affects vector strength (Figs. 7, 8, 9A and 10A and C), but has a strong influence on the preferred firing phase (Figs. 7, 8, 9B and C, 10B and D): With respect to anterior tone presentation there are phase lags for ipsilateral and phase leads for posterior and contralateral presentation. Phase differences between both ears show a sinusoidal or cardioid/ovoidal directional characteristic; maximum differences are found with antero-lateral tone presentation (Fig. 11). The directionality of phase locking decreases with the neuron's CF (Fig. 10F) and only slightly changes with sound pressure level (Fig. 12). Thus, phase locking of amphibian papilla neurons can potentially provide intensity-independent information for sound localization.

Acoustic Stimulation↗

PubMed1992

Directional dependence of auditory sensitivity and frequency selectivity in the leopard frog.

Direction-dependent changes in frequency selectivity were documented in three populations of neurons in the auditory periphery of the leopard frog: low-, mid-, and high-frequency sensitive fibers. These changes were most pronounced in the mid-frequency (720-1199 Hz) sensitive fibers which exhibited positive shifts in characteristic frequencies (CFs) and concomitant narrowed bandwidths from posterior speaker presentations. Maximum sensitivities were observed for ipsilateral speaker presentations in these fibers. Low-frequency (63-500 Hz) sensitive fibers exhibited mean threshold shifts at their CFs of up to -7 dB with posterior speaker presentations. Low- and high-frequency sensitive fibers whose CFs closely correspond to the peaks of spectral energy of the species' mating call were the most sensitive (exhibited lowest thresholds) but showed little directional-dependent frequency selectivity. Directional-dependent variation in frequency selectivity and sensitivity in low- and mid-frequency sensitive fibers was attributed to pressure and phase differences impinging on the inner surface of the eardrum.

Animals↗

PubMed1992

Low-level N-methyl-D-aspartate receptor activation provides a purinergic inhibitory threshold against further N-methyl-D-aspartate-mediated neurotransmission in the cortex.

N-methyl-D-aspartate (NMDA) is 33 times more potent at releasing adenosine than it is at releasing [3H]norepinephrine from slices of rat parietal cortex. Consequently, maximal adenosine release occurs at levels of NMDA receptor activation which release little norepinephrine. The potential modulatory role of the adenosine released during NMDA receptor activation on NMDA-evoked [3H]norepinephrine release was investigated. The A1-selective agonist R-(-)N6-(2-phenylisopropyl)adenosine (10 microM) decreased 100 microM NMDA-evoked [3H]norepinephrine release by 27%; this was reversed by the P1 antagonist 8-phenyltheophylline (8-PT, 10 microM), indicating that NMDA-evoked norepinephrine release from cortical slices is susceptible to purinergic modulation. On the other hand, 8-PT had no effect on [3H]norepinephrine release evoked by 100 microM NMDA, suggesting that endogenous adenosine, released during NMDA receptor activation, does not modulate [3H]norepinephrine release. However, [3H]norepinephrine release precedes adenosine release, so that the released adenosine may not be temporally available to modulate [3H]norepinephrine release. Pretreatment with a concentration of NMDA (10 microM) which releases substantial endogenous adenosine, but very little [3H]norepinephrine decreased subsequent 100 microM NMDA-evoked [3H]norepinephrine release by 52%. 8-PT partially reversed this inhibition, indicating that prereleased adenosine, acting at P1 purinoceptors, modulated subsequent NMDA-evoked [3H]norepinephrine release. These results suggest that adenosine, released during submaximal NMDA receptor activation, may provide an inhibitory threshold which must be overcome in order for other NMDA-mediated processes to proceed maximally.

Adenosine↗

PubMed1991

Endogenous glycine modulates N-methyl-D-aspartate-evoked release of adenosine and [3H]noradrenaline from rat cortical slices.

Strychnine-insensitive, glycinergic modulation of N-methyl-D-aspartate (NMDA)-evoked adenosine and [3H]noradrenaline release was investigated in superfused rat cortical slices. 7-Chlorokynurenic acid (100 microM) significantly decreased 300 microM NMDA-evoked adenosine and [3H]noradrenaline release. The addition of exogenous glycine (100 microM) reversed 7-chlorokynurenic acid antagonism. Higher concentrations of NMDA (500 microM, 3 mM) overcame the 7-chlorokynurenic acid (100 microM) block of NMDA-evoked adenosine release but not the block of NMDA-evoked [3H]noradrenaline release. Addition of exogenous glycine (100 microM) alone did not augment either adenosine or [3H]noradrenaline release. Addition of exogenous glycine show that endogenous glycine, acting at a strychnine-insensitive glycine site on the NMDA receptor, is required for NMDA receptor-mediated release of adenosine and noradrenaline. The finding that non-competitive block of NMDA-evoked adenosine release by 7-chlorokynurenic acid could be overcome by high NMDA concentrations supports the suggestion that spare NMDA receptors exist for adenosine release. Furthermore, heterogeneous endogenous glycine concentrations within the cortical slices cannot account for the observation that NMDA is 33 times more potent at releasing adenosine than at releasing [3H]noradrenaline.

Adenosine↗

PubMed1991

Functional connections of the rat medial cortex and basal forebrain: an in vivo intracellular study.

Projections between the medial cortex and basal forebrain in the rat were demonstrated by intracellular recordings and the anterograde tracer Phaseolus vulgaris leucoagglutinin. Direct projections between these areas were indicated by antidromic action potentials, short latency (less than 5 ms) orthodromic potentials, and labeled axon terminals in the basal forebrain subsequent to iontophoresis of Phaseolus vulgaris leucoagglutinin into posterior cingulate cortex. High proportions of antidromic action potentials were encountered in responsive cortical neurons (66%) and basal forebrain neurons (97%). Antidromic latencies recorded in the basal forebrain (less than 1.0 ms) revealed fast ascending projections; cortical neurons showed both fast and slow descending projections (latencies of 0.3-3.7 ms). Relatively few synaptic potentials (none in the diagonal band of Broca) and sparse labeling of axon terminals observed in the basal forebrain indicated that the ascending projections may be the more physiologically important or, at least, densest pathway. Polysynaptic feedforward pathways were suggested through long latency (greater than 20 ms) inhibitory and excitatory postsynaptic potentials, the former being the more common response. Candidate inhibitory neurons were identified in both cortex and basal forebrain. Possible monosynaptic (less than 5 ms) inhibitory postsynaptic and antidromic responses in these cells provided evidence that candidate inhibitory neurons participate in the reciprocal pathways.

Action Potentials↗

PubMed1991

Forskolin and phosphodiesterase inhibitors release adenosine but inhibit morphine-evoked release of adenosine from spinal cord synaptosomes.

The effects of forskolin, Ro 20-1724, rolipram, and 3-isobutyl-1-methylxanthine (IBMX) on morphine-evoked release of adenosine from dorsal spinal cord synaptosomes were evaluated to examine the potential involvement of cyclic AMP in this action of morphine. Ro 20-1724 (1-100 microM), rolipram (1-100 microM), and forskolin (1-10 microM) increased basal release of adenosine, and at 1 microM inhibited morphine-evoked release of adenosine. Release of adenosine by Ro 20-1724, rolipram, and forskolin was reduced 42-77% in the presence of alpha,beta-methylene ADP and GMP, which inhibits ecto-5'-nucleotidase activity by 81%, indicating that this adenosine originated predominantly as nucleotide(s). Significant amounts of adenosine also were released from the ventral spinal cord by these agents. Ro 20-1724 and rolipram did not significantly alter the uptake of adenosine into synaptosomes. Although Ro 20-1724 and rolipram had only limited effects on the extrasynaptosomal conversion of added cyclic AMP to adenosine, IBMX, a phosphodiesterase inhibitor with a broader spectrum of inhibitory activity for phosphodiesterase isoenzymes, significantly inhibited the conversion of cyclic AMP to adenosine and resulted in recovery of a substantial amount of cyclic AMP. As with the non-xanthine phosphodiesterase inhibitors, IBMX increased basal release of adenosine and reduced morphine-evoked release of adenosine. Adenosine released by IBMX was reduced 70% in the presence of alpha,beta-methylene ADP and GMP, and release from the ventral spinal cord was 61% of that from the dorsal spinal cord. Collectively, these results indicate that forskolin and phosphodiesterase inhibitors release nucleotide(s) which is (are) converted extrasynaptosomally to adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

1-Methyl-3-isobutylxanthine↗

PubMed1991

Intracerebroventricular morphine releases adenosine and adenosine 3',5'-cyclic monophosphate from the spinal cord via a serotonergic mechanism.

Intracerebroventricular (i.c.v.) administration of morphine produces antinociception which is antagonized by intrathecal (i.t.) injection of adenosine receptor antagonists, suggesting that adenosine release from the spinal cord may partially mediate antinociception produced by supraspinal morphine. In the present study we have examined this hypothesis directly. Administration of 40 nmol of morphine i.c.v. acutely to rats increased tail-flick and hot-plate nociceptive latencies. This antinociception was antagonized by i.t. theophylline (12 nmol) and 8-phenyltheophylline (1.2 and 12 nmol), but was potentiated by a high dose of theophylline (120 nmol). The same dose of i.c.v. morphine increased the release of adenosine into i.t. perfusates by 40 to 60% above basal release values, and also released a nucleotide which was identified as cyclic AMP by using high performance liquid chromatography. This release of adenosine and cyclic AMP was reduced after i.t. pretreatment with 5,7-dihydroxytryptamine (100 micrograms) but not 6-hydroxydopamine (100 micrograms). Spinal release of purines induced by i.c.v. morphine also was reduced by i.t. perfusion with 50 microM methysergide whereas 50 microM phentolamine had no effect. These data suggest that i.c.v. morphine activates descending serotonergic pathways to release purines from the spinal cord, and that such release is secondary to release of 5-hydroxytryptamine. Extracellular adenosine may contribute significantly to antinociception produced by supraspinal morphine.

Adenosine↗

PubMed1990

Functional reciprocal connections of the rat entorhinal cortex and subicular complex with the medial frontal cortex: an in vivo intracellular study.

We used in vivo intracellular recording techniques in the rat in order to determine the properties of projections from the medial frontal cortex to the entorhinal cortex and subicular complex. Three main results were obtained. (1) A high proportion (65%) of neurons within the medial frontal cortex were antidromically activated at short latency (0.4-1.9 ms) by electrical stimulation of the entorhinal cortex or subicular complex. This provided physiological evidence for fast direct projections from the medial frontal cortex to the entorhinal cortex and subicular complex. (2) Clear excitatory postsynaptic potentials (EPSPs) were evoked in 8% of the cells within the entorhinal cortex, subicular complex, or adjacent cortices after electrical stimulation of the medial frontal cortex. (3) The most salient synaptic response was inhibition, as shown by the presence of inhibitory postsynaptic potentials (IPSPs) in 50% of the cells sampled. Similar results were obtained for the reciprocal pathway: 56% of the sampled cells in the entorhinal cortex or subicular complex responded with antidromic spikes to stimulation of the medial cortex; 4% of medial frontal neurons responded to stimulation of the entorhinal cortex or subicular complex with clear EPSPs, and 48% with IPSPs. The latencies of most synaptic responses, 15-25 ms, were inconsistent with monosynaptic activation. This suggests that oligosynaptic relays amplified the signal within or en route to their targets, and/or that cells with more slowly propagating axons were also present but not sampled by the intracellular electrodes. Finally, responsive fast-spiking cells (candidate inhibitory neurons) were encountered within target structures. The results provide evidence that these distant cortical regions are functionally connected in a reciprocal manner, and that both principal and inhibitory neurons are excited by this projection system.

Animals↗

PubMed1990

5-Hydroxytryptamine releases adenosine and cyclic AMP from primary afferent nerve terminals in the spinal cord in vivo.

5-Hydroxytryptamine (5-HT) releases a purine nucleotide, which is subsequently converted to adenosine, from primary afferent nerve terminals in vitro. This release may mediate spinal antinociception by 5-HT. In the present study, we have investigated whether release also occurs from the spinal cord in vivo using an intrathecal perfusion system in rats. Adenosine was quantitated using high performance liquid chromatography (HPLC) with fluorescence detection. Following perfusion of the spinal cord with 50 and 500 microM 5-HT, a 35-50% increase in the release of endogenous adenosine was observed. This release was completely blocked by 50 microM methysergide, and by intrathecal injection with 100 micrograms capsaicin 5-8 days prior to release experiments. Intrathecal perfusion with 50 and 500 microM 5-HT also released a nucleotide which eluted from the HPLC column at a retention time identical to that of cyclic AMP standards, and was reduced following incubation with cyclic AMP phosphodiesterase. This release of cyclic AMP also was eliminated following intrathecal pretreatment with capsaicin. In contrast to 5-HT, noradrenaline (NA, 500 microM and 5 mM) did not release adenosine or cyclic AMP from the intact spinal cord. These data demonstrate that release of nucleotide, probably cyclic AMP, and subsequent metabolism to adenosine, can be induced by 5-HT but not NA in vivo. This strengthens the hypothesis that release of adenosine from the spinal cord may mediate antinociception by intrathecal 5-HT but not NA.

Adenosine↗