Spatial memory in nonhuman primates implanted with the subdural pharmacotherapy device.

This study investigated the possible influence of the Subdural Pharmacotherapy Device (SPD) on spatial memory in 3 adult, male bonnet macaques (Macaca radiata). The device was implanted in and above the subdural/subarachnoid space and cranium overlaying the right parietal/frontal cortex: a circuitry involved in spatial memory processing. A large test chamber, equipped with four baited and four non-baited food-ports at different locations, was used: reaches into empty food ports were counted as spatial memory errors. In this study of within-subject design, before SPD implantation (control) the animals made mean 373.3 ± 114.9 (mean ± SEM) errors in the first spatial memory test session. This value dropped to 47.7 ± 18.4 by the 8th session. After SPD implantation and alternating cycles of transmeningeal saline delivery and local cerebrospinal fluid (CSF) drainage in the implanted cortex the spatial memory error count, with the same port locations, was 33.0 ± 12.2 during the first spatial memory test session, further decreasing to 5.7 ± 3.5 by the 8th post-implantation session (P<0.001 for trend). Replacing transmeningeal saline delivery with similar delivery of the GABAA receptor agonist muscimol (1.0mM) by the SPD did not affect the animals' spatial memory performance, which in fact included at least one completely error-free session per animal over time. The study showed that complication-free implantation and use of the SPD over the parietal and frontal cortices for months leave spatial memory processes intact in nonhuman primates.

Subarachnoid pharmacodialysis for central nervous system disorders.

We postulate that alternating use of microcontroller-regulated delivery and drain pumps connected to one or more sealed subarachnoid fluid exchange ports can maintain localized bi-directional molecular fluxes across pia mater covering multiple diseased areas of the cerebral cortex or spinal cord. This system enables local irrigation with drugs and drainage of endogenous neurotoxic molecules normalizing regional neurochemistry and restoring physiological function. Viewing the pia mater as an endogenous dialysis membrane, testing this hypothesis requires demonstrating: (a) benefits of removing neurotoxic molecules from diseased cerebral cortical or spinal areas via subarachnoid dialysis, (b) neuropharmacological effects of subarachnoid drug delivery and (c) evaluating additive effects of combining the two, as a novel, "pharmacodialysis" procedure. Our supporting experimental data show that this procedure can drain proinflammatory cytokines from the neocortex to the subarachnoid cerebrospinal fluid in rats and can prevent focal seizures in monkeys through subarachnoid delivery of muscimol to their neocortex. Subarachnoid pharmacodialysis allows effective site-specific treatment and microcontroller-regulated timing responsive to the evolving course of a disease and can be performed with bedside systems or fully implanted devices. The procedure provides a two-pronged, combined therapy for traumatic brain and spinal cord injuries, cortically localized epilepsy, stroke and tumors as well as psychiatric disorders such as Alzheimer's disease with pathology of the association cortex. Therapeutic subarachnoid pharmacodialysis drainage of endogenous molecules from the neocortical interstitial space offers unprecedented opportunities to gain new insights into the neurochemistry of the human neocortex in real-life conditions over months or even years.

Long-term behavioral, electrophysiological, and neurochemical monitoring of the safety of an experimental antiepileptic implant, the muscimol-delivering Subdural Pharmacotherapy Device in monkeys.

OBJECT: The authors evaluated the extent to which the Subdural Pharmacotherapy Device (SPD), chronically implanted over the frontal cortex to perform periodic, localized muscimol-delivery/CSF removal cycles, affects overall behavior, motor performance, electroencephalography (EEG) activity, and blood and CSF neurochemistry in macaque monkeys. METHODS: Two monkeys were used to adjust methodology and 4 monkeys were subjected to comprehensive testing. Prior to surgery, the animals' behavior in a large test chamber was monitored, and the motor skills required to remove food pellets from food ports located on the walls of the chamber were determined. The monkeys underwent implantation of the subdural and extracranial SPD units. The subdural unit, a silicone strip integrating EEG electrodes and fluid-exchange ports, was positioned over the right frontal cortex. The control unit included a battery-powered, microprocessor-regulated dual minipump and radiofrequency module secured to the cranium. After implantation, the SPD automatically performed periodic saline or muscimol (1.0 mM) deliveries at 12-hour intervals, alternating with local CSF removals at 6-hour intervals. The antiepileptic efficacy of this muscimol concentration was verified by demonstrating its ability to prevent focal acetylcholine-induced seizures. During SPD treatment, the monkeys' behavior and motor performance were again monitored, and the power spectrum of their radiofrequency-transmitted EEG recordings was analyzed. Serum and CSF muscimol levels were measured with high-performance liquid chromatography electrochemical detection, and CSF protein levels were measured with turbidimetry. RESULTS: The SPD was well tolerated in all monkeys for up to 11 months. The behavioral study revealed that during both saline and muscimol SPD treatment, the monkeys could achieve the maximum motor performance of 40 food-pellet removals per session, as before surgery. The EEG study showed that local EEG power spectra were not affected by muscimol treatment with SPD. The neurochemical study demonstrated that the administration of 1.0 mM muscimol into the neocortical subarachnoid space led to no detectable levels of this compound in the blood and cisternal CSF, as measured 1-125 minutes after delivery. Total protein levels were within the normal range in the cisternal CSF, but protein levels in the cortical-site CSF were significantly higher than normal: 361 ± 81.6 mg/dl. Abrupt discontinuation of 3-month, periodic, subdural muscimol treatments induced withdrawal seizures, which could be completely prevented by gradually tapering off the subdural muscimol concentration from 1.0 mM to 0.12-0.03 mM over a period of 2 weeks. The monkeys' general health and weight were maintained. Infection occurred only in one monkey 9 months after surgery. CONCLUSIONS: Long-term, periodic, transmeningeal muscimol delivery with the SPD is essentially a safe procedure. If further improved and successfully adapted for use in humans, the SPD can be used for the treatment of intractable focal neocortical epilepsy affecting approximately 150,000 patients in the US.

Autoradiographic evidence for the transmeningeal diffusion of muscimol into the neocortex in rats.

Electrophysiological and behavioral studies have demonstrated that muscimol administered through the cranial meninges can prevent focal neocortical seizures. It was proposed that transmeningeal muscimol delivery can be used for the treatment of intractable focal neocortical epilepsy. However, it has not been proved that muscimol administered via the transmeningeal route can penetrate into the neocortex. The purpose of the present study was to solve this problem by using combined autoradiography-histology methods. Four rats were implanted with epidural cups over the parietal cortices. A 50 µL mixture of [³H] muscimol and unlabeled muscimol with a final concentration of 1.0mM was delivered through each cup on the dura mater. After a 1-hour exposure, the muscimol solution was removed and replaced with formalin to trap the transmeningeally diffused molecules. Then the whole brain was fixed transcardially, sectioned, with the sections subjected to autoradiography and thionine counterstaining. Results showed that (1) [³H] muscimol diffused through the meninges into the cortical tissue underlying the epidural cup in all rats. (2) [³H] muscimol-related autoradiography grains were distributed in all six neocortical layers. (3) [³H] muscimol-related autoradiography grains were localized to the cortical area underneath the epidural delivery site and were absent in the cerebral cortical white matter and other brain structures. This study provided evidence that muscimol can be delivered via the transmeningeal route into the neocortical tissue in a spatially controlled manner. The finding further supports the rationale of using transmeningeal muscimol for the treatment of intractable focal neocortical epilepsy.

An implantable triple-function device for local drug delivery, cerebrospinal fluid removal and EEG recording in the cranial subdural/subarachnoid space of primates.

Transmeningeal pharmacotherapy for cerebral cortical disorders requires drug delivery through the subdural/subarachnoid space, ideally with a feedback controlled mechanism. We have developed a device suitable for this function. The first novel component of the apparatus is a silicone rubber strip equipped with (a) fluid-exchange ports for both drug delivery and local cerebrospinal fluid (CSF) removal, and (b) EEG recording electrode contacts. This strip can be positioned between the dura and pia maters. The second novel component is an implantable dual minipump that directs fluid movement to and from the silicone strip and is accessible for refilling and emptying the drug and CSF reservoirs, respectively. This minipump is regulated by a battery-powered microcontroller integrating a bi-directional radiofrequency (RF) communication module. The entire apparatus was implanted in 5 macaque monkeys, with the subdural strip positioned over the frontal cortex and the minipump assembly secured to the cranium under a protective cap. The system was successfully tested for up to 8 months for (1) transmeningeal drug delivery using acetylcholine (ACh) and muscimol as test compounds, (2) RF-transmission of neocortical EEG data to assess the efficacy of drug delivery, and (3) local CSF removal for subsequent diagnostic analyses. The device can be used for (a) monitoring neocortical electrophysiology and neurochemistry in freely behaving nonhuman primates for more than 6 months, (b) determining the neurobiological impact of subdural/subarachnoid drug delivery interfaces, (c) obtaining novel neuropharmacological data on the effects of central nervous system (CNS) drugs, and (d) performing translational studies to develop subdural pharmacotherapy devices.

Periodic transmeningeal muscimol maintains its antiepileptic efficacy over three weeks without inducing tolerance, in rats.

Periodic transmeningeal administration of muscimol into the neocortical epileptogenic zone via a subdurally implanted device has been proposed for the treatment of intractable focal neocortical epilepsy. It is unknown whether such muscimol applications induce tolerance. The purpose of this study was to determine whether daily transmeningeal (epidural) muscimol applications into the rat parietal cortex induce tolerance to the antiepileptic effect of this drug. Rats were chronically implanted with an epidural cup and adjacent epidural EEG electrodes over the right parietal cortex. After recovery 1.0 mM muscimol was delivered into the implanted cortical area through the cup while the animal behaved freely, once per day for 4 consecutive days in each week, with each delivery followed within 3 min by the delivery of a seizure-inducing concentration of acetylcholine (Ach) into the same area. The study lasted for 3 weeks. In each week, one day was used to test the epileptogenicity of the examined cortical site by replacing muscimol with saline prior to Ach delivery. The duration of Ach-induced EEG seizures was measured in each experimental session to assess the antiepileptic efficacy of muscimol, while the rat's behavior was also monitored. The daily epidural muscimol pretreatments prevented Ach-induced EEG and behavioral seizures in all rats. This antiepileptic action did not diminish over time and was maintained throughout the 3-week test period. When muscimol was replaced with saline, the subsequent Ach administrations induced EEG and behavioral seizures. These results suggest that periodic transmeningeal administrations of a relatively low concentration of muscimol into the neocortex over three weeks do not induce tolerance to the localized antiepileptic effects of this drug.

Transmeningeal muscimol can prevent focal EEG seizures in the rat neocortex without stopping multineuronal activity in the treated area.

Muscimol has potent antiepileptic efficacy after transmeningeal administration in animals. However, it is unknown whether this compound stops local neuronal firing at concentrations that prevent seizures. The purpose of this study was to test the hypothesis that epidurally administered muscimol can prevent acetylcholine (Ach)-induced focal seizures in the rat neocortex without causing cessation of multineuronal activity. Rats were chronically implanted with a modified epidural cup over the right frontal cortex, with microelectrodes positioned underneath the cup. In each postsurgical experimental day, either saline or 0.005-, 0.05-, 0.5- or 5.0-mM muscimol was delivered through the cup, followed by a 20-min monitoring of the multineuronal activity and the subsequent delivery of Ach in the same way. Saline and muscimol pretreatment in the concentration range of 0.005-0.05 mM did not prevent EEG seizures. In contrast, 0.5-mM muscimol reduced the average EEG Seizure Duration Ratio value from 0.30±0.04 to 0. At this muscimol concentration, the average baseline multineuronal firing rate of 10.9±4.4 spikes/s did not change significantly throughout the 20-min pretreatment. Muscimol at 5.0mM also prevented seizures, but decreased significantly the baseline multineuronal firing rate of 7.0±1.8 to 3.7±0.9 spikes/s in the last 10 min of pretreatment. These data indicate that transmeningeal muscimol in a submillimolar concentration range can prevent focal neocortical seizures without stopping multineuronal activity in the treated area, and thus this treatment is unlikely to interrupt local physiological functions.

Multi-layer coils for efficient Transcutaneous Power Transfer.

TETS (Transcutaneous Energy Transfer System) has been successfully used for powering medical implants for different purposes such as for neural recordings and drug delivery. Yet, due to their low power transfer efficiency, these devices can cause unacceptable increase in skin temperature limiting their scalability to high power levels. Although, the efficiency of these systems can be improved by increasing coil diameter, in many cases this is not practical due to strict physical constraints on the coil diameter. In this paper, we investigate using multi-layer coils as secondary coils in the TETS to increase the power transfer efficiency, and thus allowing the delivery of the desired power safely for a longer period. Our experiments show a 5× increase in the duration of safe power delivery (not increasing the skin temperature more than 2 C) using multi-layer coils as the secondary coil compared to using single-layer coils even when there is a 50% misalignment in between primary and secondary coils. This increase in duration of safe power transfer is shown to be over 16× more when the coils are aligned. The improvement in the duration of safe power transfer is achieved without increasing the coil diameter and with a coil thickness of 2 mm.

A high-performance transcutaneous battery charger for medical implants.

As new functionality is added to the implantable devices, their power requirements also increase. Such power requirements make it hard for keeping such implants operational for long periods by non-rechargeable batteries. This result in a need for frequent surgeries to replace these batteries. Rechargeable batteries can satisfy the long-term power requirements of these new functions. To minimize the discomfort to the patients, the recharging of the batteries should be as infrequent as possible. Traditional battery charging methods have low battery charging efficiency. This means they may limit the amount of charge that can be delivered to the device, speeding up the depletion of the battery and forcing frequent recharging. In this paper, we evaluate the suitability of a state-of-the-art general purpose charging method called current-pumped battery charger (CPBC) for implant applications. Using off-the-shelf components and with minimum optimization, we prototyped a proof-of-concept transcutaenous battery charger based on CPBC and show that the CPBC can charge a 100 mAh battery transcutaneously within 137 minutes with at most 2.1°C increase in tissue temperature even with a misalignment of 1.3 cm in between the coils, while keeping the battery charging efficiency at 85%.

Evolution and prospects for intracranial pharmacotherapy for refractory epilepsies: the subdural hybrid neuroprosthesis.

Intracranial pharmacotherapy is a novel strategy to treat drug refractory, localization-related epilepsies not amenable to resective surgery. The common feature of the method is the use of some type of antiepileptic drug (AED) delivery device placed inside the cranium to prevent or stop focal seizures. This distinguishes it from other nonconventional methods, such as intrathecal pharmacotherapy, electrical neurostimulation, gene therapy, cell transplantation, and local cooling. AED-delivery systems comprise drug releasing polymers and neuroprosthetic devices that can deliver AEDs into the brain via intraparenchymal, ventricular, or transmeningeal routes. One such device is the subdural Hybrid Neuroprosthesis (HNP), designed to deliver AEDs, such as muscimol, into the subdural/subarachnoid space overlaying neocortical epileptogenic zones, with electrophysiological feedback from the treated tissue. The idea of intracranial pharmacotherapy and HNP treatment for epilepsy originated from multiple sources, including the advent of implanted medical devices, safety data for intracranial electrodes and catheters, evidence for the seizure-controlling efficacy of intracerebral AEDs, and further understanding of the pathophysiology of focal epilepsy. Successful introduction of intracranial pharmacotherapy into clinical practice depends on how the intertwined scientific, engineering, clinical, neurosurgical and regulatory challenges will be met to produce an effective and commercially viable device.

Comparison of the antiepileptic properties of transmeningeally delivered muscimol, lidocaine, midazolam, pentobarbital and GABA, in rats.

This study compared the potencies of epidurally delivered muscimol, lidocaine, midazolam, pentobarbital and gamma-aminobutyric acid (GABA) to prevent focal neocortical seizures induced by locally applied acetylcholine (Ach), in rats (n=5). An epidural cup was chronically implanted over the right somatosensory cortex in each animal, with epidural EEG electrodes placed posterior to the edge of the cup. After recovery, either artificial cerebrospinal fluid (ACSF; control solution) or one of the five drugs was delivered into epidural cup, followed by Ach administration into the cup to induce seizures. EEG seizure duration ratio was calculated for each drug delivery/seizure induction session to determine the potency of ACSF and the drugs to prevent the focal Ach-seizures. The concentration of all examined drug solutions was 1.0mM. ACSF, lidocaine, midazolam, pentobarbital and GABA all failed to prevent the Ach-induced neocortical EEG seizures, yielding EEG seizure duration ratios ranging from 0.41 to 0.80. In contrast, muscimol pretreatment fully prevented the development of ictal EEG in all animals. These results suggest that when used at low concentration muscimol was the best of the five drugs for transmeningeal pharmacotherapy trials for focal neocortical epilepsy.

Localized transmeningeal muscimol prevents neocortical seizures in rats and nonhuman primates: therapeutic implications.

PURPOSE: To determine whether muscimol delivered epidurally or into the subarachnoid space can prevent and/or terminate acetylcholine (Ach)-induced focal neocortical seizures at concentrations not affecting behavior and background electroencephalography (EEG) activity. METHODS: Rats (n = 12) and squirrel monkeys (n = 3) were chronically implanted with an epidural or subarachnoid drug delivery device, respectively, over the right frontal/parietal cortex, with adjacent EEG electrodes. Recordings were performed in behaving rats and chaired monkeys. Via the implants, either a control solution (artificial cerebrospinal fluid, ACSF) or muscimol (0.25-12.5 mm) was delivered locally as a "pretreatment," followed by the similar delivery of a seizure-inducing concentration of Ach. In five additional rats, the quantities of food-pellets consumed during epidural ACSF and muscimol (2.5 mm) exposures were measured. In a last group of four rats, muscimol (0.8-2.5 mm) was delivered epidurally during the ongoing, Ach-induced EEG seizure. RESULTS: In contrast to ACSF pretreatments, epidural muscimol pretreatment in rats completely prevented the seizures at and above 2.5 mm. In the monkeys, subarachnoid muscimol pretreatments at 2.5 mm completely prevented the focal-seizure-inducing effect of Ach, whereas similar deliveries of ACSF did not affect the seizures. Furthermore, 2.5 mm epidural muscimol left the eating behavior of rats intact and caused only slight changes in the EEG power spectra. Finally, muscimol delivery during Ach-induced EEG seizures terminated the seizure activity within 1-3 min. CONCLUSIONS: The results of this study suggest that muscimol is a viable candidate for the transmeningeal pharmacotherapy of intractable focal epilepsy.

Unmasking the CA1 ensemble place code by exposures to small and large environments: more place cells and multiple, irregularly arranged, and expanded place fields in the larger space.

In standard experimental environments, a constant proportion of CA1 principal cells are place cells, each with a spatial receptive field called a place field. Although the properties of place cells are a basis for understanding the mammalian representation of spatial knowledge, there is no consensus on which of the two fundamental neural-coding hypotheses correctly accounts for how place cells encode spatial information. Within the dedicated-coding hypothesis, the current activity of each cell is an independent estimate of the location with respect to its place field. The average of the location estimates from many cells represents current location, so a dedicated place code would degrade if single cells had multiple place fields. Within the alternative, ensemble-coding hypothesis, the concurrent discharge of many place cells is a vector that represents current location. An ensemble place code is not degraded if single cells have multiple place fields as long as the discharge vector at each location is unique. Place cells with multiple place fields might be required to represent the substantially larger space in more natural environments. To distinguish between the dedicated-coding and ensemble-coding hypotheses, we compared the characteristics of CA1 place fields in a standard cylinder and an approximately six times larger chamber. Compared with the cylinder, in the chamber, more CA1 neurons were place cells, each with multiple, irregularly arranged, and enlarged place fields. The results indicate that multiple place fields is a fundamental feature of CA1 place cell activity and that, consequently, an ensemble place code is required for CA1 discharge to accurately signal location.

Effects of subdural application of lidocaine in patients with focal epilepsy.

Antiepileptic drug (AED) delivery directly into the neocortex has recently been shown to be able to both prevent and terminate focal seizures in rats. The present clinical experiment aimed to test the local effects of lidocaine delivered onto the pia mater adjacent to epileptogenic zones in human patients. Administration of lidocaine resulted in a marked diminishment of spike counts on all patients, with a decremental effect of lidocaine on the faster frequency elements of individual spikes and overall testing epochs. The direct cortical application of lidocaine appears to affect local epileptogenic activity in human patients with intractable focal epilepsy.

Histological evidence for drug diffusion across the cerebral meninges into the underlying neocortex in rats.

Transmeningeal pharmacotherapy has been proposed to treat neurological disorders with localized pathology, such as intractable focal epilepsy. As a step toward understanding the diffusion and intracortical spread of transmeningeally delivered drugs, the present study used histological methods to determine the extent to which a marker compound, N-methyl-D-aspartate (NMDA), can diffuse into the neocortex through the meninges. Rats were implanted with bilateral parietal cortical epidural cups filled with 50 mM NMDA on the right side and artificial cerebrospinal fluid (ACSF) in the contralateral side. After 24 h, the histological effects of these treatments were evaluated using cresyl violet (Nissl) staining. The epidural NMDA exposure caused neuronal loss that in most animals extended from the pial surface through layer V. The area indicated by this neuronal loss was localized to the neocortical region underlying the epidural cup. These results suggest that NMDA-like, water soluble, small molecules can diffuse through the subdural/subarachnoid space into the underlying neocortex and spread in a limited fashion, close to the meningeal penetration site.

Transmeningeal delivery of GABA to control neocortical seizures in rats.

Transmeningeal drug delivery, using an implanted hybrid neuroprosthesis, has been proposed as a novel therapy for intractable focal epilepsy. As part of a systematic effort to identify the optimal compounds and protocols for such a therapy, this study aimed to determine whether transmeningeal gamma-aminobutyric acid (GABA) delivery can terminate and/or prevent neocortical seizures in rats. Rats were chronically implanted with an epidural cup and an adjacent EEG electrode in the right parietal cortex. While the rat was behaving freely, a seizure-inducing concentration of acetylcholine (Ach) was applied into the cup. In a seizure termination study, either artificial cerebrospinal fluid (ACSF) or GABA (0.25, 2.5, 25 or 50mM) was delivered into the exposed neocortical area during an ongoing seizure. In a seizure prevention study, either ACSF or 50mM GABA was delivered into the epidural cup before the application of Ach. Epidural delivery of 50mM GABA completely terminated ongoing Ach-induced EEG seizures and convulsions within 17-437s after its delivery. ACSF and lower concentrations of GABA did not produce this effect, but 25mM GABA reduced seizure severity. However, the used GABA concentration could not prevent the development, or affect the severity, of Ach-induced EEG seizures and convulsions. This study indicates that transmeningeal GABA delivery can be used for terminating neocortical seizures, but to achieve seizure prevention via this route either a more efficient GABA delivery method needs to be developed or other neurotransmitters/pharmaceuticals should be employed for this purpose.

Epidural pentobarbital delivery can prevent locally induced neocortical seizures in rats: the prospect of transmeningeal pharmacotherapy for intractable focal epilepsy.

PURPOSE: To determine whether epidural pentobarbital (PB) delivery can prevent and/or terminate neocortical seizures induced by locally administered acetylcholine (Ach) in freely moving rats. METHODS: Rats were implanted permanently with an epidural cup placed over the right parietal cortex with intact dura mater. Epidural screw-electrodes, secured to the cup, recorded local neocortical EEG activity. In the seizure-termination study, Ach was delivered into the epidural cup, and after the development of electrographic and behavioral seizures, the Ach solution was replaced with either PB or artificial cerebrospinal fluid (aCSF; control solution). In the seizure-prevention study, the epidural Ach delivery was preceded by a 10-min exposure of the delivery site to PB or aCSF. Raw EEG recordings, EEG power spectra, and behavioral events were analyzed. RESULTS: Ach-induced EEG seizures associated with convulsions, which were unaffected by epidural aCSF applications, were terminated by epidurally delivered PB within 2-2.5 min. Epidural deliveries of PB before Ach applications completely prevented the development of electrographic and behavioral seizures, whereas similar deliveries of aCSF exerted no influence on the seizure-generating potential of Ach. CONCLUSIONS: This study showed for the first time that epidural AED delivery can prevent, as well as terminate, locally induced neocortical seizures. The findings support the viability of transmeningeal pharmacotherapy for the treatment of intractable neocortical epilepsy.

Elevated lactate suppresses neuronal firing in vivo and inhibits glucose metabolism in hippocampal slice cultures.

Glucose is well accepted as the major fuel for neuronal activity, while it remains controversial whether lactate also supports neural activity. In hippocampal slice cultures, synaptic transmission supported by glucose was reversibly suppressed by lactate. To test whether lactate had a similar inhibitory effect in vivo, lactate was perfused into the hippocampi of unanesthetized rats while recording the firing of nearby pyramidal cells. Lactate perfusion suppressed pyramidal cell firing by 87.5+/-8.3% (n=6). Firing suppression was slow in onset and fully reversible and was associated with increased lactate concentration at the site of the recording electrode. In vivo suppression of neural activity by lactate occurred in the presence of glucose; therefore we tested whether suppression of neural firing was due to lactate interference with glucose metabolism. Competition between glucose and lactate was measured in hippocampal slice cultures. Lactate had no effect on glucose uptake. Lactate suppressed glucose oxidation when applied at an elevated, pathological concentration (10 mM), but not at its physiological concentration (1 mM). Pyruvate (10 mM) also inhibited glucose oxidation but was significantly less effective than lactate. The greater suppressive effect of lactate as compared to pyruvate suggests that alteration of the NAD(+)/NADH ratio underlies the suppression of glucose oxidation by lactate. ATP in slice culture was unchanged in glucose (1 mM), but significantly reduced in lactate (1 mM). ATP in slice culture was significantly increased by combination of glucose (1 mM) and lactate (1 mM). These data suggest that alteration of redox ratio underlies the suppression of neural discharge and glucose metabolism by lactate.

Detecting location-specific neuronal firing rate increases in the hippocampus of freely-moving monkeys.

The spatial properties of the firing of hippocampal neurons have mainly been studied in (a) freely moving rodents, (b) non-human primates seated in a moveable primate chair with head fixed, and (c) epileptic patients subjected to virtual navigation. Although these studies have all revealed the ability of hippocampal neurons to generate spatially selective discharges, the detected firing patterns have been found to be considerably different, even conflicting, in many respects. The present cellular electrophysiological study employed squirrel monkeys (Saimiri sciureus), which moved freely on the walls and floor of a large test chamber. This permitted the examination of the spatial firing of hippocampal neurons in nearly ideal conditions, similar to those used in rodents, yet in a species that belongs to the primate Suborder Anthropoidea. The major findings were that: (1) a group of slow-firing complex-spike cells increased their basal, awake firing rate more than 20-fold, often above 30 spikes/s, when the monkey was in a particular location in the chamber, (2) these location-specific discharges occurred consistently, forming 4-25 s action potential volleys, and (3) fast-firing cells displayed no such electrical activity. Thus, during free movement in three dimensions, primate hippocampal complex-spike cells do generate high-frequency, location-specific action potential volleys. Since these cells are components of the medial temporal lobe memory system, their uncovered firing pattern may well be involved in the formation of declarative memories on places.

Spatial memory performance of freely-moving squirrel monkeys.

Few experiments have addressed the problem of cognitive map formation in non-human primates. Therefore, a paradigm was developed to assess spatial memory formation in squirrel monkeys (Saimiri sciureus) moving freely in three dimensions. While moving on the walls and floor of a large test chamber, the animals learned to collect pieces of cereal from baited food-ports interspersed among non-baited ports. The cereal-pellets were not visible to the monkeys, so the animals needed to develop spatial memory to visit only the baited ports for food and avoid the non-baited ones. A session consisted of ten consecutive trials, and 3 successive sessions were conducted on each day for a 5-day period. For each trial, correct choices (CC; number of visited baited-ports) and incorrect choices (IC; number of visited non-baited ports) were registered, and spatial memory performance index (SMPI; ranging from 0.00 to 1.00) was calculated as follows: SMPI=(CC-IC)/CC. For each session, mean SMPI, session duration, total reaches into the non-baited ports, and total reaches into the baited ports were documented. In an 8-port task, where 4 food-ports were baited and 4 were non-baited, the mean SMPI was higher than 0 in the first session (day 1), indicating the development of short-term spatial memory. By the fifth session (day 2), this index was significantly higher than in the first session, indicating the build-up of long-term spatial memory. These changes were related to a significant decrease in the total reaches into the non-baited ports. At the same time, the duration of the sessions and the total reaches into the baited ports did not change significantly. This paradigm can be used for (1) studying cognitive map formation in primates, (2) examining the underlying cellular and molecular mechanisms in integrative neurobiological experiments, and (3) screening cognition-enhancer drugs in a monkey model.