Synaptic plasticity dependent on N-methyl-D-aspartate (NMDA) receptors is thought to underlie certain types of learning and memory. In support of this, both hippocampal long-term potentiation and spatial learning in a watermaze are impaired by blocking NMDA receptors with a selective antagonist D(-)-2-amino-5-phosphonovaleric acid (AP5) or by a mutation in one of the receptor subunits. Here we report, however, that the AP5-induced learning deficit can be almost completely prevented if rats are pretrained in a different watermaze before administration of the drug. This is not because of stimulus generalization, and occurs despite learning of the second task remaining hippocampus dependent. An AP5-induced learning deficit is, however, still seen if the animals are pretrained using a non-spatial task. Thus, despite its procedural simplicity, the watermaze may involve multiple cognitive processes with distinct pharmacological properties; although required for some component of spatial learning, NMDA receptors may not be required for encoding the spatial representation of a specific environment.

This series of experiments investigated whether the NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5) could induce impairments of spatial learning across a dose range comparable to its impairment of hippocampal long-term potentiation (LTP) in vivo. Estimations of the extracellular concentration of D-AP5 in hippocampus using microdialysis were also made to compare whether these impairments occur at concentrations similar to those required to impair LTP in the in vitro hippocampal slice. Rats were chronically infused with D-AP5 into the lateral ventricle at a range of concentrations (0–50 mM) via osmotic minipumps. They were first trained to find and escape onto a hidden platform in an open-field water maze task. After the behavioral learning, they were anesthetized with urethane and an attempt was made to evoke and monitor hippocampal LTP. Extracellular samples of D-AP5 in hippocampus were then taken using microdialysis, and finally, the animals were killed and tissue samples dissected. The microdialysis and tissue samples were analyzed for D-AP5 content using HPLC with fluorescence detection. The results established, first, that D-AP5 impairs spatial learning in a linear dose-dependent manner, highly correlated with its corresponding impairment of hippocampal LTP in vivo. No concentration of D-AP5 was observed to block LTP without affecting learning. Second, the microdialysis estimates indicated that, subject to certain assumptions, D-AP5 causes these impairments at extracellular concentrations comparable to those that impair LTP in vitro. Third, comparison of the whole tissue and microdialysis samples revealed a concentration ratio of approximately 30:1, indicating that 97% of the intracerebral D-AP5 is inaccessible to the dialysis probes. Infusion of 20 mM EGTA was found to cause a sevenfold increase in D-AP5 in the dialysis perfusates, suggesting that at least part of the inaccessible D-AP5 is trapped by a calcium-dependent mechanism. Two further behavioral control studies indicated that the D-AP5-induced impairment of spatial learning is unlikely to be secondary to a drug- induced motor disturbance, and that the performance of the D-AP5 group whose concentration was just sufficient to block hippocampal LTP completely was statistically indistinguishable from that of a group of rats with bilateral hippocampal lesions induced by ibotenic acid. Taken together, these findings offer support for the hypothesis that activation of NMDA receptors is necessary for certain kinds of learning.

The immunosuppressive action of the drug FK506 involves inhibition of calcineurin in T-lymphocytes by a complex of FK506 and an FK506 binding protein, FKBP12, a member of the immunophilin protein family. The functional role of brain immunophilins is, however, unclear. We show here that FK506 is a powerful neuroprotective agent in an in vivo model of focal cerebral ischaemia when administered up to 60 min post-occlusion. The minimum effective neuroprotective dose is comparable with the immunosuppressant dose in humans, suggesting that FK506 may have clinical potential for the treatment of stroke. Although the related immunosuppressants rapamycin and cyclosporin failed to reduce brain damage, the finding that rapamycin pretreatment blocked the effect of FK506 confirms a role for immunophilins in the neuroprotective mechanism.

Extracellular (EC) adenosine, hypoxanthine, xanthine, and inosine concentrations were monitored in vivo in the striatum during steady state, 15 min of complete brain ischemia, and 4 h of reflow and compared with purine and nucleotide levels in the tissue. Ischemia was induced by three-vessel occlusion combined with hypotension (50 mm Hg) in male Sprague-Dawley rats. EC purines were sampled by microdialysis, and tissue adenine nucleotides and purine catabolites were extracted from the in situ frozen brain at the end of the experiment. ATP, ADP, and AMP were analyzed with enzymatic fluorometric techniques, and adenosine, hypoxanthine, xanthine, and inosine with a modified HPLC system. Ischemia depleted tissue ATP, whereas AMP, adenosine, hypoxanthine, and inosine accumulated. In parallel, adenosine, hypoxanthine, and inosine levels increased in the EC compartment. Adenosine reached an EC concentration of 40 microM after 15 min of ischemia. Levels of tissue nucleotides and purines normalized on reflow. However, xanthine levels increased transiently (sevenfold). In the EC compartment, adenosine, inosine, and hypoxanthine contents normalized slowly on reflow, whereas the xanthine content increased. The high EC levels of adenosine during ischemia may turn off spontaneous neuronal firing, counteract excitotoxicity, and inhibit ischemic calcium uptake, thereby exerting neuroprotective effects.

The effects of a number of biochemical and pharmacological manipulations on amphetamine (AMPH)-induced alterations in dopamine (DA) release and metabolism were examined in the rat striatum using the in vivo brain microdialysis method. Basal striatal dialysate concentrations were: DA, 7 nM; dihydroxyphenylacetic acid (DOPAC), 850 nM; homovanillic acid (HVA), 500 nM; 5-hydroxyindoleacetic acid (5-HIAA), 300 nM; and 3-methoxytyramine (3-MT), 3 nM. Intraperitoneal injection of AMPH (4 mg/kg) induced a substantial increase in DA efflux, which attained its maximum response 20-40 min after drug injection. On the other hand, DOPAC and HVA efflux declined following AMPH. The DA response, but not those of DOPAC and HVA, was dose dependent within the range of AMPH tested (2-16 mg/kg). High doses of AMPH (greater than 8 mg/kg) also decreased 5-HIAA and increased 3-MT efflux. Depletion of vesicular stores of DA using reserpine did not affect significantly AMPH-induced dopamine efflux. In contrast, prior inhibition of catecholamine synthesis, using alpha-methyl-p-tyrosine, proved to be an effective inhibitor of AMPH-evoked DA release (less than 35% of control). Moreover, the DA releasing action of AMPH was facilitated in pargyline-pretreated animals (220% of control). These data suggest that AMPH releases preferentially a newly synthesised pool of DA. Nomifensine, a DA uptake inhibitor, was an effective inhibitor of AMPH-induced DA efflux (18% of control). On the other hand, this action of AMPH was facilitated by veratrine and ouabain (200-210% of control). These results suggest that the membrane DA carrier may be involved in the actions of AMPH on DA efflux.