Ana Rita Costenla

Decrease of adenosine A1 receptor density and of adenosine neuromodulation in the hippocampus of kindled rats.

Adenosine is a neuromodulator that has been proposed to be a major endogenous anticonvulsant acting via A1 receptors. We tested if implementation of kindling through stimulation of the amygdala affected A1 receptor‐mediated neuromodulation in hippocampal slices taken from rats 4 weeks after the last stage 5 seizure. The A1 receptor agonist, N6‐cyclopentyladenosine (CPA) (6–100 nm), inhibited field excitatory postsynaptic potential (fEPSP) slope with an EC50 of 19.1–19.5 nm in control and sham‐operated rats, but was less potent in kindled rats (EC50 = 42.7 nm). This might result from a decreased number of A1 receptors in hippocampal nerve terminal membranes, because A1 receptor immunoreactivity decreased by 28 ± 3% and the binding density of the A1 receptor agonist [3H]R‐PIA decreased from 1702 ± 64 to 962 ± 78 fmol/mg protein in kindled compared with control rats. The tonic inhibition of hippocampal synaptic transmission by endogenous adenosine was also lower in kindled rats, because A1 receptor blockade with 50 nm 1,3‐dipropyl‐8‐cyclopentyladenosine (DPCPX) enhanced fEPSP slope by 23 ± 3% and θ‐burst‐induced long‐term potentiation by 94 ± 4% in control rats but was virtually devoid of effects in kindled rats. The evoked release of adenosine from hippocampal slices or nerve terminals was 56–71% lower in kindled rats probably due to the combined decrease in the capacity of adenosine transporters and decreased release of adenosine 5′‐triphosphate (ATP), which was partially compensated by a higher extracellular catabolism of ATP into adenosine in kindled rats. These results indicate that, although adenosine might inhibit the onset of epileptogenesis, once kindling is installed, the efficiency of the adenosine inhibitory system is impaired.

Enhancement of LTP in Aged Rats is Dependent on Endogenous BDNF

Long-term potentiation (LTP), considered the neurophysiological basis for learning and memory, is facilitated by brain-derived neurotrophic factor (BDNF), an action more evident when LTP is evoked by weak θ-burst stimuli and dependent on co-activation of adenosine A2A receptors (A2AR), which are more expressed in aged rats. As θ-burst stimuli also favor LTP in aged animals, we hypothesized that enhanced LTP in aging could be related to changes in neuromodulation by BDNF. The magnitude of CA1 LTP induced by a weak θ-burst stimuli delivered to the Schaffer collaterals was significantly higher in hippocampal slices taken from 36 to 38 and from 70 to 80-week-old rats, when compared with LTP magnitude in slices from 4 or 10 to 15-week-old rats; this enhancement does not impact in cognitive improvement as aged rats revealed an impairment on hippocampal-dependent learning and memory performance, as assessed by the Morris water maze tests. The scavenger for BDNF, TrkB-Fc, and the inhibitor of Trk phosphorylation, K252a, attenuated LTP in slices from 70 to 80-week-old rats, but not from 10 to 15-week-old rats. When exogenously added, BDNF significantly increased LTP in slices from 4 and 10 to 15-week-old rats, but did not further increased LTP in 36 to 38 or 70 to 80-week-old rats. The effects of exogenous BDNF on LTP were prevented by the A2AR antagonist, SCH58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine). These results indicate that the higher LTP magnitude observed upon aging, which does not translate into improved spatial memory performance, is a consequence of an increase in the tonic action of endogenous BDNF.

Enhanced role of adenosine A2A receptors in the modulation of LTP in the rat hippocampus upon ageing

Adenosine neuromodulation depends on a balanced activation of inhibitory A1 (A1R) and facilitatory A2A receptors (A2AR). Both A1R and A2AR modulate hippocampal glutamate release and NMDA‐dependent long‐term potentiation (LTP) but ageing affects the density of both A1R and A2AR. We tested the effects of selective A1R and A2AR antagonists in the modulation of synaptic transmission and plasticity in rat hippocampal slices from three age groups (young adults, 2–3 month; middle‐aged adults, 6–8 months; aged, 18–20 months). The selective A2AR antagonist SCH58261 (50 nm) attenuated LTP in all age groups, with a larger effect in aged (−63 ± 7%) than in middle‐aged adults (−36 ± 9%) or young adult rats (−36 ± 9%). In contrast, the selective A1R antagonist DPCPX (50 nm) increased LTP magnitude in young adult rats (+42 ± 6%), but failed to affect LTP magnitude in the other age groups. Finally, in the continuous presence of DPCPX, SCH58261 caused a significantly larger inhibition of LTP amplitude in aged (−71 ± 45%) than middle‐aged (−28 ± 9%) or young rats (−11 ± 2%). Accordingly, aged rats displayed an increased expression of A2AR mRNA in the hippocampus and a higher number of glutamatergic nerve terminals equipped with A2AR in aged (67 ± 6%) compared with middle‐aged (34 ± 7%) and young rats (25 ± 5%). The results show an enhanced A2AR‐mediated modulation of LTP in aged rats, in accordance with the age‐associated increased expression and density of A2AR in glutamatergic terminals. This age‐associated gain of function of A2AR modulating synaptic plasticity may underlie the ability of A2AR antagonists to prevent memory dysfunction in aged animals.

Caffeine, adenosine receptors, and synaptic plasticity.

Few studies to date have looked at the effects of caffeine on synaptic plasticity, and those that did used very high concentrations of caffeine, whereas the brain concentrations attained by regular coffee consumption in humans should be in the low micromolar range, where caffeine exerts pharmacological actions mainly by antagonizing adenosine receptors. Accordingly, rats drinking caffeine (1 g/L) for 3 weeks, displayed a concentration of caffeine of circa 22 microM in the hippocampus. It is known that selective adenosine A1 receptor antagonists facilitate, whereas selective adenosine A2A receptor antagonists attenuate, long term potentiation (LTP) in the hippocampus. Although caffeine is a non-selective antagonist of adenosine receptors, it attenuates frequency-induced LTP in hippocampal slices in a manner similar to selective adenosine A2A receptor antagonists. These effects of low micromolar concentration of caffeine (30 microM) are maintained in aged animals, which is important when a possible beneficial effect for caffeine in age-related cognitive decline is proposed. Future studies will still be required to confirm and detail the involvement of A1 and A2A receptors in the effects of caffeine on hippocampal synaptic plasticity, using both pharmacological and genetic approaches.

Adenosine modulates synaptic plasticity in hippocampal slices from aged rats

Adenosine is known to modulate synaptic plasticity in the hippocampus of young animals through activation of adenosine A1 receptors. The objective of the present study is to investigate whether the modulatory role of adenosine on phenomena of synaptic plasticity is maintained or modified in the hippocampus of aged animals. We compared the effects of the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 50 nM), on paired-pulse facilitation (PPF), long-term depression (LTD), long-term potentiation (LTP) and depotentiation elicited in hippocampal slices taken from young adult (5-6 weeks) and old (2 years old) male Wistar rats. DPCPX attenuated PPF both in young (1.64 +/- 0.05 vs. 1.76 +/- 0.05%, n = 6) and in old rats (1.33 +/- 0.05 vs. 1.55 +/- 0.1%, n = 6). LTD was only observed in the presence of DPCPX in both young (21.3 +/- 0.6%, n = 4) and old rats (14.4 +/- 0.9%, n = 6). LTP induced by high-frequency stimulation (HFS) was not significantly different in young and old animals, in the presence or in the absence of DPCPX. A larger depotentiation was observed in the presence of DPCPX in young rats (27.6 +/- 4.4% vs. 16.8 +/- 4.7%, n = 7) as well as in old rats (41.3 +/- 5.1% vs. 16.1 +/- 2.7%, n = 6). LTP induced by theta-burst stimulation was observed only in the presence of DPCPX (53.9 +/- 4.9%, n = 5) in young rats, but could be obtained either in the control solution (81.8 +/- 17.9%, n = 7) or in the presence of DPCPX (98.5 +/- 24.2%, n = 7) in old rats. The modulatory role of endogenous adenosine on synaptic plasticity is generally maintained in aged animals. Drugs interfering with adenosine A1 receptor effects could then be used in old animals to modify synaptic plasticity with relevant behavioural consequences.

Adenosine A 2A receptor blockade reverts hippocampal stress-induced deficits and restores corticosterone circadian oscillation

Maternal separation (MS) is an early life stress model that induces permanent changes in the central nervous system, impairing hippocampal long-term potentiation (LTP) and spatial working memory. There are compelling evidences for a role of hippocampal adenosine A2A receptors in stress-induced modifications related to cognition, thus opening a potential window for therapeutic intervention. Here, we submitted rats to MS and evaluated the long-lasting molecular, electrophysiological and behavioral impairments in adulthood. We then assessed the therapeutic potential of KW6002, a blocker of A2A receptors, in stress-impaired animals. We report that the blockade of A2A receptors was efficient in reverting the behavior, electrophysiological and morphological impairments induced by MS. In addition, this effect is associated with restoration of the hypothalamic-pituitary-adrenal axis (HPA-axis) activity, as both the plasma corticosterone levels and hippocampal glucocorticoid receptor expression pattern returned to physiological-like status after the treatment. These results reveal the involvement of A2A receptors in the stress-associated impairments and directly in the stress response system by showing that the dysfunction of the HPA-axis as well as the long-lasting synaptic and behavioral effects of MS can be reverted by targeting adenosine A2A receptors. These findings provide a novel evidence for the use of adenosine A2A receptor antagonists as potential therapy against psychopathologies.

A functional role for adenosine A3 receptors: modulation of synaptic plasticity in the rat hippocampus

Adenosine modulates hippocampal synaptic plasticity, namely long-term potentiation (LTP) and long-term depression (LTD), through activation of A1 and A2A receptors. We now report a novel role for the recently described adenosine A3 receptor in the regulation of synaptic plasticity in the CA1 area of hippocampal slices. Activation of adenosine A3 receptors by (1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-p-yl]-1-deoxy-N-methyl-beta-D-ribofuranuronamide (Cl-IBMECA) (100 nM) increased the magnitude of theta-burst induced LTP (from 1.2+/-0.6% in the control solution to 25.5+/-0.8% in the presence of Cl-IBMECA) and attenuated LTD (from 30.0+/-5.5% decrease in the control solution to 13.6+/-6.6% decrease in the presence of Cl-IBMECA). The selective adenosine A3 receptor antagonist, MRS 1191 (5-10 microM), prevented the effects of Cl-IBMECA. These findings indicate a functional role for adenosine A3 receptors in the modulation of synaptic plasticity.

Maternal separation impairs long term-potentiation in CA1-CA3 synapses and hippocampal-dependent memory in old rats

Exposure to chronic stress during the neonatal period is known to induce permanent long-term changes in the central nervous system and hipothalamic-pituitary-adrenal axis reactivity that are associated with increased levels of depression, anxiety, and cognitive impairments. In rodents, a validated model of early life stress is the maternal separation (MS) paradigm, which has been shown to have long-term consequences for the pups that span to adulthood. We hypothesized that the early life stress-associated effects could be exacerbated with aging, because it is often accompanied by cognitive decline. Using a MS model in which rat pups were separated from their mothers for 3 hours daily, during postnatal days 2-14, we evaluated the long-term functional consequences to aged animals (70-week-old), by measuring synaptic plasticity and cognitive performance. The baseline behavioral deficits of aged control rats were further exacerbated in MS animals, indicating that early-life stress induces sustained changes in anxiety-like behavior and hippocampal-dependent memory that are maintained much later in life. We then investigated whether these differences are linked to impaired function of hippocampal neurons by recording hippocampal long-term potentiation from Schaffer collaterals/CA1 synapses. The magnitude of the hippocampal long-term potentiation induced by high-frequency stimulation was significantly lower in aged MS animals than in age-matched controls. These results substantiate the hypothesis that the neuronal and endocrine alterations induced by early-life stress are long lasting, and are able to exacerbate the mild age-associated deficits.

Persistence of the neuromodulatory effects of adenosine on synaptic transmission after long-term potentiation and long-term depression

Adenosine modulates long-term potentiation (LTP) and long-term depression (LTD) in the hippocampus. We tested whether induction of LTP or LTD might reciprocally modify the role of adenosine as an inhibitory modulator of synaptic transmission in the hippocampus. The experiments were performed on hippocampal slices of the rat. Two separate sets of the Schaffer pathway were alternately stimulated. Evoked field excitatory postsynaptic potentials (fEPSPs) were recorded extracellularly from CA1 stratum radiatum. Long-term potentiation (LTP) was induced by high-frequency stimulation and long-term depression (LTD) by low-frequency stimulation. The inhibitory effects of the adenosine analogue, 2-chloroadenosine (CADO, 0.1-5 microM), on the fEPSP slope were similar in both pathways (EC(50)=0.72 (95% confidence intervals: 0.50-1.1) microM and EC(50)=0.84 (0.55-1.3) microM, n=6). After induction of LTP in the test pathway, a second concentration-response curve was obtained. CADO was significantly less potent as compared to the first concentration-response curve, however the inhibitory effects of CADO were still similar in the potentiated pathway (EC(50)=2.2 (1.6-3.1) microM) and in the control pathway (EC(50)=2.1 (1.5-3.0) microM, n=6). The inhibitory effects of CADO (0.1-5 microM) were also not significantly different in the pathway where LTD was previously induced (EC(50)=1.7 (1.5-2.0) microM), compared to the control non-depressed pathway (EC(50)=1.7 (1.4-2.0) microM, n=6). In conclusion, the neuromodulatory action of adenosine seems to be maintained in the presence of substantial variations in long-term synaptic efficiency during LTP or LTD.

Enhanced LTP in aged rats: Detrimental or compensatory?

ABSTRACT Age‐dependent memory deterioration has been well documented and yet an increase in rat hippocampal LTP upon aging has been reported. This poses the question of whether the enhanced LTP is a cause or an attempt to compensate the memory deficits described in aged rats. Hippocampal slices from young, adult and aged Wistar rats were pre‐incubated, with an NMDA receptor (NMDAR) antagonist, memantine (1 &mgr;M, 4 h), and hippocampal LTP was evaluated. The results show that memantine significantly decreases the larger LTP magnitude recorded in hippocampal slices from aged rats without compromising LTP recorded in slices from young and adult animals. To unveil the impact of in vivo administration of memantine, different doses (1, 5 and 10 mg/kg/day) or saline vehicle solution were intraperitoneally administered, for 15–20 days, to both young and aged animals. Memantine did not significantly affect neither the place learning of young animals, evaluated by Morris Water Maze, nor LTP recorded from hippocampal slices from the same group of animals. However, memantine (5 and 10 mg/kg/day) significantly decreased the large LTP recorded in hippocampal slices from aged animals. Moreover, aged animals treated with memantine (10 mg/kg/day) showed a significantly compromised place learning when compared to aged control animals. Overall, these results suggest that the larger LTP observed in aged animals is a compensatory phenomenon, rather than pathological. The finding that age‐dependent blockade of LTP by a NMDAR antagonist leads to learning deficits, implies that the increased LTP observed upon aging may be playing an important role in the learning process. HighlightsLTP recorded in hippocampal slices from aged rats is larger than in young rats.Memantine decreases hippocampal LTP in aged animals and leads to learning deficits.Larger LTP observed in aged animals is a compensatory phenomenon.