Evolutionary implications of pliocene hominid footprints.

Hominid footprints discovered at the Pliocene (3.6 to 3.8 million years ago) site of Laetoli in northern Tanzania represent the earliest evidence of bipedalism in human evolution. This new evidence emphasizes the mosaic pattern of human evolution.

Primitive hominid canine from Tanzania.

A primitive hominid canine recovered at Laetoli by Louis Leakey in 1935 and stored at the British Museum of Natural History adds to our knowledge of dental anatomy in Australopithecus. The specimen was the first adult Australopithecus recovered and the first ancient hominid discovered in eastern Africa.

A systematic assessment of early African hominids.

A large sample of Pliocene fossil hominid remains has been recovered from the African sites of Hadar in Ethiopia and Laetolil in Tanzania. These collections, dating approximately between 2.9 and 3.8 million years ago, constitute the earliest substantial record of the family Hominidae. This article assesses the phylogenetic relationships of the newly discovered fossil hominids and provides a taxonomy consistent with that assessment. A new taxon, Australopithecus afarensis, has been created to accommodate these Pliocene hominid fossils.

Suid evolution and correlation of African hominid localities.

Recently discovered Plio-Pleistocene vertebrate assemblages have allowed complete systematic revision of the sub-Saharan African Suidae. New phylogenies are proposed for the 7 genera and 16 species of fossil and extant representatives. Suids are common elements of African Plio-Pleistocene faunas, and their evolutionary trends, particularly in the species Mesochoerus limnetes and Metridiochoerus andrewsi, are of great correlative value. Suid data are employed in a refinement of stratigraphic correlations at Omo Shungura, Olduvai, and east of Lake Turkana (formerly East Rudolf) and in a correlation of East African and South African sites, with important implications for interpretation of hominid evolution. The suid record also bears significantly on questions of theoretical evolutionary biology.

African Homo erectus: old radiometric ages and young Oldowan assemblages in the Middle Awash Valley, Ethiopia.

Fossils and artifacts recovered from the middle Awash Valley of Ethiopia's Afar depression sample the Middle Pleistocene transition from Homo erectus to Homo sapiens. Ar/Ar ages, biostratigraphy, and tephrachronology from this area indicate that the Pleistocene Bodo hominid cranium and newer specimens are approximately 0.6 million years old. Only Oldowan chopper and flake assemblages are present in the lower stratigraphic units, but Acheulean bifacial artifacts are consistently prevalent and widespread in directly overlying deposits. This technological transition is related to a shift in sedimentary regime, supporting the hypothesis that Middle Pleistocene Oldowan assemblages represent a behavioral facies of the Acheulean industrial complex.

The extracellular fluid of solid carcinomas contains immunosuppressive concentrations of adenosine.

The purine nucleoside adenosine (9-beta-D-ribofuranosyladenine) inhibits a number of lymphocyte functions in vitro, including the ability of activated T lymphocytes and natural killer cells to adhere to and kill tumor targets. Solid tumors, such as adenocarcinomas of the lung and colon, are frequently hypoxic and are, therefore, likely to exhibit increased adenine nucleotide breakdown through the 5'-nucleotidase pathway, yielding adenosine. We examined whether the concentration of adenosine in the extracellular fluid of such tumors is adequate to cause immunosuppression. Murine tumors grown in syngeneic hosts or human tumors grown in immunodeficient nu/nu mice were subjected to microdialysis, and adenosine levels in the microdialysate were measured by high-performance liquid chromatography. Treatment of the tumor microdialysates with adenosine deaminase eliminated the adenosine peak. Recovery of adenosine ranged from 15 to 29%, depending on the microdialysis probe, and concentrations of adenosine in tumors ranged from 0.2 to 2.4 microM with a mean of 0.5 microM. In contrast, the adenosine concentration measured s.c. at the same location was 30 +/- 5 nM (mean +/- SE). Inclusion of the adenosine deaminase inhibitor coformycin (10 microM) and the adenosine kinase inhibitor 5'-iodotubercidin (0.1 microM) in the microdialysis perfusion buffer increased extracellular adenosine concentration in tumors to as high as 13 microM. These data show that extracellular adenosine levels in solid tumors are sufficient to suppress the local antitumor immune response and that interference with pathways of adenosine metabolism causes marked increases in tumor extracellular adenosine concentration.

Adenosine: a mediator of interleukin-1beta-induced hippocampal synaptic inhibition.

Interleukin-1 (IL-1) is a pleotrophic cytokine implicated in a variety of central activities, including fever, sleep, ischemic injury, and neuromodulatory responses, such as neuroimmune, and neuroendocrine interactions. Although accumulating evidence is available regarding the expression pattern of this cytokine, its receptors in the CNS, and its mechanistic profile under pathological levels, it is unclear whether this substance modulates central neurons under physiological concentrations. Further, in light of the functional and spatial overlap between the adenosine and IL-1 systems, it is not known whether these two systems are coupled. We report here that, in rat brain slices, brief application of sub-femtomolar IL-1beta causes a profound decrease of glutamate transmission, but not GABAergic inhibition, in hippocampal CA1 pyramidal neurons. This decrease by IL-1beta is prevented by pharmacological blockade of adenosine A1 receptors. In addition, we show that IL-1beta failed to suppress glutamate transmission at room temperature. Because the production and release of adenosine in the CNS is thought to be metabolically dependent, this observation suggests that one of the functions of IL-1beta is to increase the endogenous production of adenosine. Together, these data suggest for the first time that sub-femtomolar levels of IL-1 can effectively modulate glutamate excitation in hippocampal neurons via an adenosine-dependent mechanism.

Adenosine release may mediate spinal analgesia by morphine.

Spinal analgesia produced by morphine is blocked by methylxanthine adenosine receptor antagonists. In biochemical studies, morphine releases adenosine from spinal cord synaptosomes prepared from the dorsal spinal cord, as well as from the intact spinal cord in vivo. Adenosine release is reduced by intrathecal and neonatal pretreatment with capsaicin but not by intrathecal pretreatment with 6-hydroxydopamine or 5,7-dihydroxytryptamine, indicating that adenosine originates from small-diameter primary afferent neurons but not descending monoaminergic pathways. In this Viewpoint Jana Sawynok and colleagues review the evidence supporting the hypothesis that the spinal analgesic action of morphine is due to the release of adenosine from primary afferent nerve terminals and subsequent activation of A1 and A2 adenosine receptors.

Role of adenine compounds in autonomic neurotransmission.

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

N-methyl-D-aspartate, kainate and quisqualate release endogenous adenosine from rat cortical slices.

N-Methyl-D-aspartate, kainate, and quisqualate released endogenous adenosine from superfused slices of rat parietal cortex. N-Methyl-D-aspartate-evoked adenosine release was blocked by D,L-2-amino-5-phosphono-valeric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), indicating that it was receptor-mediated, although it did not show the expected potentiation in the absence of Mg2+. In contrast, N-methyl-D-aspartate-evoked release of [3H]noradrenaline from the same slices was markedly potentiated in Mg2(+)-free medium. Therefore, the lack of Mg2+ modulation of N-methyl-D-aspartate-evoked adenosine release was not due to depolarization-induced alleviation of the Mg2+ block in the slices. Kainate-evoked adenosine release was diminished by the non-specific excitatory amino acid antagonist, gamma-D-glutamyl-glycine, and kainate- and quisqualate-evoked adenosine release was diminished by 6,7-dinitroquinoxaline-2,3-dione, indicating that these agonists release adenosine by acting at non-N-methyl-D-aspartate receptors. Tetrodotoxin decreased N-methyl-D-aspartate- and kainate-evoked adenosine release by 40% and 19% respectively, indicating that release was mediated in part by propagated action potentials in the slices. Total release of adenosine by N-methyl-D-aspartate, kainate or quisqualate was not diminished in the absence of Ca2+. A second exposure to kainate following restoration of Ca2+ to slices previously depolarized in the absence of Ca2+ resulted in an amount of adenosine release equal to an initial release by slices in the presence of Ca2+, a result suggesting the presence of separate Ca2(+)-dependent and Ca2(+)-independent pools of adenosine. The present experiments demonstrate that activation of all three major subtypes of excitatory amino acid receptors in the cortex releases adenosine, possibly from separate Ca2(+)-dependent and -independent pools. Adenosine released from the cortex following excitatory amino acid stimulation may, by acting at inhibitory P1 purinoceptors, diminish excitatory neurotransmission and protect against excitotoxicity.

Activation of cholinergic and adrenergic receptors increases the concentration of extracellular adenosine in the cerebral cortex of unanesthetized rat.

Adenosine is an inhibitory neuromodulator in the CNS. For extracellular adenosine to play a physiological role in the brain, it must be present at effective concentrations. Acetylcholine and noradrenaline are known to play an important role in modulating the activity of cortical neurons, and they might have a role also in the release of adenosine in the cerebral cortex in vivo. We examined whether activation of cholinergic and adrenergic receptors affects extracellular adenosine levels in the cerebral cortex of unanesthetized rats using in vivo microdialysis. All drugs were administered locally within the cortex by reverse dialysis. Both acetylcholine and the acetylcholinesterase inhibitor neostigmine increased extracellular adenosine levels, and the effect of neostigmine was blocked by the nicotinic receptor antagonist mecamylamine. Both nicotine and the nicotinic receptor agonist epibatidine increased the concentration of extracellular adenosine. Activation of muscarinic receptors using the nonselective agonist oxotremorine and a selective M1 receptor agonist also increased extracellular adenosine levels. Noradrenaline and the noradrenergic reuptake inhibitor desipramine increased extracellular adenosine levels. The alpha(1)-adrenergic receptor agonist phenylephrine and the beta-adrenergic agonist isoproterenol increased extracellular adenosine levels, whereas the alpha(2)-adrenergic receptor agonist clonidine did not have an effect. These findings indicate that activation of specific cholinergic and adrenergic receptors can increase extracellular levels of adenosine in the cortex, and suggest that cholinergic and adrenergic receptor-mediated regulation of adenosine levels may represent a mechanism for controlling the excitability of cortical neurons.

Enhanced release of adenosine in rat hind paw following spinal nerve ligation: involvement of capsaicin-sensitive sensory afferents.

Modulation of endogenous adenosine levels by inhibition of adenosine metabolism produces a peripheral antinociceptive effect in a neuropathic pain model. The present study used microdialysis to investigate the neuronal mechanisms modulating extracellular adenosine levels in the rat hind paw following tight ligation of the L5 and L6 spinal nerves. Subcutaneous injection of 50 microl saline into the nerve-injured paw induced a rapid and short-lasting increase in extracellular adenosine levels in the subcutaneous tissues of the rat hind paw ipsilateral to the nerve injury. Saline injection did not increase adenosine levels in sham-operated rats or non-treated rats. The adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine and the adenosine deaminase inhibitor 2'-deoxycoformycin, at doses producing a peripheral antinociceptive effect, did not further enhance subcutaneous adenosine levels in the nerve-injured paw. Systemic pretreatment with capsaicin, a neurotoxin selective for small-diameter sensory afferents, markedly reduced the saline-evoked release of adenosine in rat hind paw following spinal nerve ligation. Systemic pretreatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic afferent nerves, did not affect release. These results suggest that following nerve injury, peripheral capsaicin-sensitive primary sensory afferent nerve terminals are hypersensitive, and are able to release adenosine following a stimulus that does not normally evoke release in sham-operated or intact rats. Sympathetic postganglionic afferents do not appear to be involved in such release. The lack of effect on such release by the inhibitors of adenosine metabolism suggests an altered peripheral adenosine system following spinal nerve ligation.

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.

Release of endogenous ATP during sympathetic nerve stimulation.

1 Vas deferens from guinea-pig was stimulated with a suction electrode and both contractions and release of endogenous ATP monitored 2 Release of ATP was tetrodotoxin-sensitive and increased when the number of stimuli was increased. 3 Release of ATP was not due to contraction of the muscle and persisted following block of contractions with prazosin and alpha, beta-methylene ATP. 4 These results indicate that stimulation of the sympathetic nerves in the vas deferens releases endogenous ATP presynaptically, supporting a cotransmitter function for ATP with noradrenaline.

5-hydroxytryptamine releases adenosine 5'-triphosphate from nerve varicosities isolated from the myenteric plexus of guinea-pig ileum.

5-Hydroxytryptamine (5-HT)-evoked release of ATP from nerve varicosities isolated from the myenteric plexus of guinea pig ileum was investigated. 5-HT released ATP from myenteric varicosities by a Ca2+-dependent mechanism. The EC50 for release of ATP was 7 X 10(-7) M 5-HT. 5-HT-evoked release of ATP was not blocked by tetrodotoxin (TTX), indicating that release was not initiated by the opening of Na+-channels in the isolated myenteric varicosities. Release of ATP by 5-HT was diminished to 56% of control values by in vivo pretreatment of the guinea-pig with 6-hydroxydopamine (6-OHDA, 250 mg kg-1, i.p.) for 24 h. 6-OHDA pretreatment caused extensive destruction of noradrenergic varicosities as indicated by an 87% loss of noradrenaline content. Quipazine (5 X 10(-6) M) and methysergide (10(-4) M) caused a small release of ATP and blocked subsequent 5-HT-induced release of ATP. Metergoline (2.5 X 10(-5) M), (+)-tubocurarine (7 X 10(-5) M) and cocaine (10(-4) M) decreased 5-HT-induced ATP release. 5-Methoxytryptamine (10(-4) M), picrotoxin (3.5 X 10(-6) M), spiroperidol (10(-6) M), morphine (1.3 X 10(-6) M) and phenoxybenzamine (3.7 X 10(-7) M) were ineffective. The results demonstrate a 5-HT-receptor-mediated release of ATP from noradrenergic and possibly non-adrenergic varicosities in the myenteric plexus of guinea-pig ileum. The 5-HT-induced release of ATP is consistent with a possible transmitter, cotransmitter or modulatory role for ATP in the myenteric plexus.

Classification of adenosine receptors mediating antinociception in the rat spinal cord.

Analogues of adenosine were injected intrathecally into rats implanted with chronic indwelling cannulae in order to determine a rank order of potency and hence characterize adenosine receptors involved in spinal antinociception. In the tail flick test L-N6-phenylisopropyl adenosine (L-PIA), cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamide adenosine (NECA) produced dose-related antinociception which attained a plateau level. NECA and CHA also produced an additional distinct second phase of antinociception. D-N6-Phenylisopropyl adenosine (D-PIA) and 2-chloroadenosine (CADO) had very little antinociceptive activity in this test. The rank order of potency in producing the plateau effect was L-PIA greater than CHA greater than NECA greater than D-PIA = CADO, while that for the second phase of antinociception was NECA greater than-CHA. Pretreatment with both theophylline and 8-phenyltheophylline (8-PT) antagonized antinociception produced by CHA, with 8-PT being at least an order of magnitude more potent than theophylline. Both antagonists produced a significant hyperalgesia in the tail flick test. L-PIA and CHA also produced methylxanthine-sensitive antinociception in the hot plate test. These results suggest that activation of A1-receptors in the spinal cord can produce antinociception. Activation of A2-receptors may produce an additional effect, but the relative activity of CHA in this component of activity is unusual.

Neural release of ATP and adenosine.

Release of ATP can be evoked from noradrenergic nerve varicosities isolated from guinea pig ileal myenteric plexus by depolarization with K+ and veratridine and during exposure to acetylcholine or 5-HT. Clonidine, however, modulates the release of [3H]noradrenaline without affecting the release of ATP. ATP is also released from noradrenergic sympathetic nerves in the vas deferens, where it mediates the initial depolarization and contraction in the smooth muscle. Factors that apparently modulate the release of noradrenaline do not produce corresponding effects on ATP release. The above results are best explained by the hypothesis that ATP and noradrenaline are stored in separate populations of vesicles within sympathetic nerves and that these pools are subject to differential presynaptic modulation. Depolarization of rat brain synaptosomes releases adenosine by a process that is mediated, at least in part, by efflux on the nucleoside transporter. Drugs that block the nucleoside transport (such as dipyridamole) reduce evoked adenosine release and may thereby diminish, rather than augment, the actions of adenosine at its receptors. Release of adenosine does not appear to be uniformly distributed throughout the brain insofar as release varies from synaptosomes prepared from different regions. Although the distribution of several markers for possible adenosine pathways in the brain, including adenosine release, do not show any consistent correlations, the non-uniform distribution for these markers suggests that adenosine may have differential functions in various brain regions.

Activation of metabotropic glutamate receptors increases extracellular adenosine in vivo.

In order to identify the mechanisms that would lead to increased levels of the inhibitory neuromodulator adenosine in the brain, we tested metabotropic glutamate receptor agonists for their ability to increase extracellular adenosine in the cortex of unanesthetized rat using in vivo microdialysis. The group I/II metabotropic glutamate receptor agonist trans-(+/-)- 1-amino-(1S,3R)-cyclopenyanedicarboxylic acid (I mM) increased extracellular adenosine as did the specific group I agonist (S)- 3,5-dihydroxyphenylglycine (DHPG; 1 mM). The evoked increase of adenosine by 1 mM DHPG was reduced by the group I antagonist (RS)- 1-aminoindan-1,5,-dicarboxylic acid. Activation of group II or III metabotropic receptors did not affect extracellular adenosine. These results suggest that activation of group I metabotropic receptors contributes to elevated extracellular adenosine levels in vivo.