Lisa A. Teather

Posttraining estradiol injections enhance memory in ovariectomized rats : Cholinergic blockade and synergism

The present experiments examined acute posttraining estrogenic influences on memory in ovariectomized rats. In experiment 1 rats received a single 8-trial (30-s ITI) training session with a submerged escape platform located in the same quadrant of a circular water maze on all trials. Following trial 8, rats received a posttraining intraperitoneal injection of either an estradiol-cyclodextrin inclusion complex (0.1, 0.2, or 0.4 mg/kg) or saline. On a retention test session 24 h later, the escape latencies of rats given injections of estradiol (0.2 mg/kg) were significantly lower than those of saline-treated rats, indicating an enhancement of memory. Injections of estradiol delayed 2 h posttraining did not affect retention, demonstrating a time-dependent effect of estradiol on memory storage processes. In experiment 2a, posttraining injections of the cholinergic muscarinic receptor antagonist scopolamine (0.4 mg/kg) impaired memory in ovariectomized rats. In experiment 2b, the memory-enhancing effect of estradiol (0.2 mg/kg) was blocked by concurrent posttraining administration of a subeffective dose (0.1 mg/kg) of scopolamine, suggesting an interaction between estradiol and muscarinic cholinergic systems in memory modulation. In experiment 3a, posttraining injections of the cholinergic muscarinic receptor agonist oxotremorine (0.2 mg/kg) enhanced memory in ovariectomized rats. In experiment 3b, concurrent posttraining injection a subeffective dose of estradiol (0.1 mg/kg) and a subeffective dose of oxotremorine (0.1 mg/kg) enhanced memory, indicating a synergistic effect of estradiol and muscarinic receptor activation on memory.

Intra-hippocampal estradiol infusion enhances memory in ovariectomized rats.

OVARIECTOMIZED adult Long—Evans rats received an eight-trial training session in a hippocampal-dependent hidden platform water maze task. Following trial 8, rats received an intra-hippocampal injection of estradiol in a water soluble cyclodextrin inclusion complex (1.0, 2.0 or 5.0 μg/0.5 μl), or saline. Twenty-four hours later, the retention test escape latencies of rats administered posttraining intra-hippocampal injections of estradiol (5.0 μg) were significantly lower than those of saline treated rats, indicating a memory-enhancing effect of estradiol. Injections of estradiol (5.0 μg) given 2 h post-training had no effect on retention, indicating a time-dependent effect of estradiol on memory storage processes.

Double dissociation of hippocampal and dorsal-striatal memory systems by posttraining intracerebral injections of 2-amino-5-phosphonopentanoic acid.

Rats received an 8-trial training session on a spatial or cued task in a water maze, followed by a posttraining intracerebral injection of AP5 or saline. On a retention test 24 hr later, latency to mount the escape platform was used as a measure of memory. Intrahippocampal (10 micrograms), but not intra-dorsal striatal (2, 5, or 10 micrograms), injection of AP5 impaired memory in the spatial task. In contrast, intra-dorsal striatal (2 micrograms), but not intrahippocampal (2, 5, or 10 micrograms) injection of AP5 impaired memory in the cued task. Intracerebral injections of AP5 delayed 2 hr posttraining were ineffective. The findings indicate a double dissociation of the roles of the hippocampus and dorsal striatum in memory, a role for N-methyl-D-aspartate receptor function in posttraining memory processes, and a glutamatergic modulation of both hippocampal and dorsal striatal memory processes, suggesting that different forms of memory may share a similar neurochemical basis.

Bioactive lipids in excitatory neurotransmission and neuronal plasticity

Long-term potentiation (LTP), a model of activity-dependent synaptic plasticity and of certain forms of memory, comprises the persistent enhancement of excitatory neurotransmission that results from high-frequency activation. A presynaptic component of LTP is thought to be modulated by a retrograde messenger generated by the postsynaptic neuron. Arachidonic acid, nitric oxide, carbon monoxide and PAF have each been proposed as retrograde messengers in LTP, but arachidonic acid, unlike PAF, requires NMDA receptor activation. A PAF antagonist (BN 52021) that provides neuroprotection in ischemia-reperfusion displaces [3H] PAF bound to presynaptic membranes, blocks PAF-induced glutamate exocytosis and inhibits LTP. An antagonist selective for the intracellular PAF binding site (BN 50730) did not affect LTP, nor did BN 52021 modify NMDA currents. LTP was induced with weak synaptic stimulation coupled with postsynaptically administered enzyme resistant mcPAF. Theta-burst stimulation (10 min) after bath applications of mcPAF (1 microM) induced APV-independent LTP that was blocked by 5 microM BN 52021. When this antagonist was infused into the hippocampus before or immediately after training, it impaired memory of inhibitory avoidance training in the rat. Memory was not altered if the antagonist is infused 30 or 60 min after training. Moreover, mcPAF enhances memory on retention test performance of step-down inhibitory avoidance habituation and learning in rats. Also, memory was studied using a caudate nucleus-dependent cued water maze task. Rats received an 8 trial (30 s intertrial interval) training session in which a visible cued escape platform was located in a different quadrant of the maze of each trial. Following trial 8, the rats received a unilateral post-training intra-caudate injection of mcPAF (1 microgram/0.5 microliter), BN 52021 (0.5 microgram/0.5 microliter) or vehicle. On a retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. The retention test escape latencies of rats given mcPAF were significantly lower than those of the vehicle-injected controls, indicating a memory enhancing effect of mcPAF. Injection of mcPAF did not affect retention when administered 2 h post-training, indicating a time-dependent effect of mcPAF on memory. The latencies for animals injected with BN 52021 were significantly higher than those of the controls, indicating that antagonism of endogenous PAF impairs memory. The findings show that PAF plays a role in memory formation in a caudate-mediated cued discrimination task. Administration of BN 52021 2 h post-training had no affect on retention, indicating a time-dependent effect of endogenous PAF on memory formation. PAF, the most potent bioactive lipid known, modulates excitatory synaptic transmission, neuronal plasticity and memory. When PAF production is overstimulated as in seizures or ischemia, it becomes neurotoxic.

Posttraining Injections of MK-801 Produce a Time-Dependent Impairment of Memory in Two Water Maze Tasks☆

The role of glutamatergic N-methyl-D-aspartate (NMDA) receptors in memory storage processes was examined using systemic posttraining injections of MK-801. Male Long-Evans rats received an eight-trial (30-s ITI) training session on a spatial or cued water maze task. In the spatial task, a submerged escape platform was located in the same quadrant of the maze on all trials. In the cued task, a visible escape platform was located in a different quadrant of the maze on each trial. Following Trial 8 in both tasks, the rats received a posttraining intraperitoneal injection of the NMDA receptor antagonist MK-801 (0.025, 0.05, 0.1, or 0.2 mg/kg) or saline. On a retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. In both tasks, the retention test escape latencies of animals given MK-801 (0.05 and 0.1 mg/kg) were significantly higher than those of saline-injected controls, indicating a drug-induced impairment of memory. Injections of MK-801 (0.05 mg/kg) did not affect retention when administered 2 h posttraining in either task, indicating that the effects of MK-801 on retention are not due to an influence on non-mnemonic factors. Control experiments indicated that the memory impairing effects of MK-801 were due to an influence on memory for the type of discrimination training given (i.e., spatial or cued) and not due to an influence on a mnemonic strategy common to both tasks. The findings indicate a time-dependent role for NMDA receptor function in memory storage processes.

Environmental conditions influence hippocampus-dependent behaviours and brain levels of amyloid precursor protein in rats.

Sprague‐Dawley rats were reared in enriched (EC; group housing, exposure to stimulating objects, frequent handling) or restricted (RC; individual housing, no exposure to stimulating objects, minimal handling) environments starting on day 23 of life. At six months of age, they underwent behavioural tests to assess ‘cognitive’ and ‘stimulus‐response’ memory, selective attention, and inflammatory pain processing. Alterations in synapses and cell survival may occur as a result of environment differences; therefore we assessed the brain levels of several proteins implicated in neurite outgrowth, synaptogenesis, and cell survival. Brains were dissected and analysed for amyloid precursor protein (APP) and other synaptic and cytoskeletal proteins using Western blotting. The performance of EC animals in a hidden platform water maze task, and in a test of selective attention (both of which are thought to involve the hippocampus) was superior to that of RC animals. In contrast, performance of RC animals on two stimulus‐response tasks, the visible platform water maze test and simple visual discrimination (both of which are thought to be hippocampal independent) was indistinguishable from that of EC animals. Male EC rats displayed a different behavioural response to formalin during the inflammatory phase of nociception – the phase affected by hippocampal processing; a similar trend was observed in females. Female but not male RC rats exhibited elevated plasma corticosterone levels; adrenal weights were unaffected by environmental conditions. Region‐specific increases in brain levels of APP, neurofilament‐70 (NF‐70), and platelet‐activating factor receptor (PAF‐R) were found in EC rats. These data suggest that enriched animals manifest enhanced functioning of certain hippocampus‐mediated behaviours when compared with that of their restricted counterparts; and that brain levels of various synaptic and structural proteins involved in neurite outgrowth, cell survival, and synaptogenesis, are affected by environmental factors.

Dissociation of multiple memory systems by posttraining intracerebral injections of glutamate

The role of glutamatergic transmission in the memory functions of the hippocampus and the caudate-putamen was examined using posttraining intracerebral injections of glutamate. Rats received an eight-trial (30-sec intertriai interval) training session on a hidden or visible platform task in a water maze. In the hidden platform task, a submerged escape platform was located in the same quadrant of the maze on all trials. In the visible platform task, a visible escape platform was located in a different quadrant of the maze on each trial. Following Trial 8 in either task, rats received a posttraining intrahippocampal or intracaudate injection of glutamate or saline. On a two-trial retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. In the hidden platform task, the retention test escape latencies of rats receiving intrahippocampal injection of glutamate (2 µg) were significantly lower than those of vehicle-treated rats, indicating an enhancement of memory. Intracaudate injection (2, 5, or 10 µg) of glutamate had no effect on memory in the hidden platform task. In contrast, intracaudate injection of glutamate (5 µg) enhanced memory in the visible platform task, whereas intrahippocampal injection (1, 2, or 5 µg) had no effect. In both tasks and brain structures, intracerebral injections of glutamate had no effect on memory when delayed 2 h posttraining, indicating a time-dependent effect of glutamate on memory storage processes. The findings indicate a double dissociation of the roles of the hippocampus and the caudate-putamen in memory and suggest that glutamate modulates the distinct forms of memory mediated by these two brain structures.

Cyclooxygenase-2 mediates platelet-activating factor-induced prostaglandin E2 release from rat primary astrocytes

The phospholipid mediator platelet-activating factor (PAF), and its non-hydrolyzable analog methylcarbamyl-PAF (mc-PAF) increase prostaglandin E(2) (PGE(2)) release from astrocyte-enriched cortical cell cultures. Cyclooxygenase (COX) enzymes--of which there are two known isoforms--convert arachidonic acid to prostaglandin (PG) H(2) (PGH(2)), which is further metabolized to various PGs, including PGE(2). COX-1 is generally considered to contribute to cell homeostasis, whereas COX-2 is thought to mediate inflammatory/immune PG formation. In this study we examined the involvement of the COX isoforms in PAF-induced PGE(2) release. Treatment of cells with the non-specific COX inhibitor indomethacin, or the specific COX-2 inhibitor NS-398, prior to mc-PAF stimulation completely blocked the PAF-induced release of PGE(2); treatment with more selective COX-1 inhibitors (i.e. piroxicam and SC-560) failed to significantly do so. These data suggest that COX-2 is responsible for PAF-mediated PGE(2) release in primary astrocytes.

Platelet-activating factor increases prostaglandin E2 release from astrocyte-enriched cortical cell cultures

The phospholipid mediator platelet-activating factor (PAF) increased the release of prostaglandin E(2) (PGE(2)) from astrocyte-enriched cortical cell cultures in a concentration- and time-dependent manner. The nonhydrolyzable PAF analog methylcarbamyl-PAF (mc-PAF), the PAF intermediate lyso-PAF, and arachidonic acid (AA) also produced this effect. In contrast, phosphatidlycholine (PC) and lyso-PC, lipids that are structurally similar to PAF and lyso-PAF, had no effect on PGE(2) production, suggesting that PAF-induced PGE(2) release is not the consequence of nonspecific phospholipid-induced membrane perturbation. Antagonism of intracellular PAF binding sites completely abolished the ability of mc-PAF and lyso-PAF to mobilize PGE(2,) and attenuated the AA effect. Antagonism of the G-protein-coupled PAF receptor in plasma membranes had no significant effect on mc-PAF, lyso-PAF or AA-induced PGE(2) release. Based on the present findings, we hypothesize that intracellular PAF is a physiologic stimulus of PGE(2) production in astrocytes.

Inhibition of platelet-activating factor receptors in hippocampal plasma membranes attenuates the inflammatory nociceptive response in rats

Evidence suggests that platelet-activating factor (PAF) is a mediator in inflammatory-based pain. Using the biphasic formalin model in rats, we recently demonstrated that PAF antagonists which were selective for either intracellular or plasma membrane PAF receptors decreased the late-phase of the nociceptive response. Inasmuch as both of the PAF antagonists previously used were administered systemically, and reportedly are able to cross the blood-brain barrier, the anatomic locations at which PAF affects pain processing remained to be elucidated. Since PAF is required for hippocampal-dependent memory consolidation, and since the hippocampus has been shown to mediate the late-phase of formalin-induced nociception, the present study investigated the effects on nociception of administration of PAF antagonists within the hippocampus, and of using agents specific for either plasma membrane (BN 52021) or intracellular (BN 50730) PAF binding sites. Intrahippocampal injections of BN 52021 decreased the late-phase of the nociceptive response in a concentration-dependent manner. In contrast, intrahippocampal administration of BN 50730 had no effect on inflammatory nociception. These findings suggest that hippocampal plasma membrane PAF receptors, but not intracellular PAF binding sites, mediate tonic inflammatory pain processing in rats.