Benzodiazepine inverse agonists augment long-term potentiation in CA1 and CA3 of guinea pig hippocampal slices

Effects of HMG-CoA reductase inhibitors on learning and memory in the guinea pig

Statins reduce the risk of death from cardiovascular disease in millions of people worldwide. Recent pharmacovigilance data has suggested that people taking statins have an increased risk of psychiatric adverse events such as amnesia and anxiety. This study aimed to investigate the possibility of statin-induced amnesia through animal models of memory and learning. We conducted extracellular field recordings of synaptic transmission in area CA1 of hippocampal slices to examine the effects of acute cholesterol lowering with lipid lowering drugs. We also assessed the effect of six weeks of simvastatin (2mg/kg/d) and atorvastatin (1mg/kg/d) treatment using the Morris water maze. Long Term Potentiation (LTP) was significantly diminished in the presence of 3µM atorvastatin or simvastatin and by the cholesterol sequestering agent methyl-β-cyclodextrin (MBCD). The effects were reversed in the MBCD but not the statin treated slices by the addition of cholesterol. In the water maze, statin treatment did not cause any deficits in the first five days of reference memory testing, but statin treated guinea pigs preformed significantly worse than control animals in a working memory test. The deficits observed in our experiments in water maze performance and hippocampal LTP are suggestive of statin induced changes in hippocampal plasticity. The effects on LTP are independent of cholesterol regulation, and occur at concentrations that may be relevant to clinical use. Our results may help to explain some of the behavioural changes reported in some people after beginning statin treatment.

GABAergic signaling by AgRP neurons prevents anorexia via a melanocortin-independent mechanism

The hypothalamic arcuate nucleus contains two anatomically and functionally distinct populations of neurons-the agouti-related peptide (AgRP)- and pro-opiomelanocortin (POMC)-expressing neurons that integrate various nutritional, hormonal, and neuronal signals to regulate food intake and energy expenditure, and thereby help achieve energy homeostasis. AgRP neurons, also co-release neuropeptide Y (NPY) and γ-aminobutyric acid (GABA) to promote feeding and inhibit metabolism through at least three possible mechanisms: (1) suppression of the melanocortin signaling system through competitive binding of AgRP with the melanocortin 4 receptors; (2) NPY-mediated inhibition of post-synaptic neurons that reside in hypothalamic nuclei; (3) GABAergic inhibition of POMC neurons in their post-synaptic targets including the parabrachial nucleus (PBN), a brainstem structure that relays gustatory and visceral sensory information. Acute ablation of AgRP neurons in adult mice by the action of diphtheria toxin (DT) results in precipitous reduction of food intake, and eventually leads to starvation within 6days of DT treatment. Chronic delivery of bretazenil, a GABA(A) receptor partial agonist, into the PBN is sufficient to restore feeding and body weight when AgRP neurons are ablated, whereas chronic blockade of melanocortin 4 receptor signaling is inadequate. This review summarizes the physiological roles of a neural circuitry regulated by AgRP neurons in control of feeding behavior with particular emphasis of the GABA output to the parabrachial nucleus. We also describe a compensatory mechanism that is gradually engaged after ablation of AgRP neurons that allows mice to continue eating without them.

L-655,708 enhances cognition in rats but is not proconvulsant at a dose selective for α5-containing GABAA receptors

The in vitro and in vivo properties of L-655,708, a compound with higher affinity for GABA(A) receptors containing an alpha5 compared to an alpha1, alpha2 or alpha3 subunit have been examined further. This compound has weak partial inverse agonist efficacy at each of the four subtypes but, and consistent with the binding data, has higher functional affinity for the alpha5 subtype. In a mouse hippocampal slice model, L-655,708 was able to enhance the long-term potentiation produced by a theta burst stimulation, consistent with a potential role for the alpha5 subtype in processes involving synaptic plasticity, such as learning and memory. When administered in a formulation specifically designed to achieve relatively constant plasma drug concentrations, and therefore maintain selective occupancy of alpha5- compared to alpha1-, alpha2- and alpha3-containing receptors (75+/-4% versus 22+/-10%, respectively), L-655,708 did not alter the dose of pentylenetetrazole required to induce seizures, indicating that the inverse agonist effects of L-655,708 at the alpha5 subtype are not associated with a proconvulsant liability. In the Morris water maze, L-655,708 enhanced performance not only during acquisition but also in a probe trial, demonstrating that this compound has cognition enhancing effects. These data further support the potential of alpha5-containing GABA(A) receptors as a target for novel cognition enhancing drugs.

Effects of GABA modulators on the repeated acquisition of response sequences in squirrel monkeys

The present study investigated the effects of positive and negative GABA(A) modulators under three different baselines of repeated acquisition in squirrel monkeys in which the monkeys acquired a three-response sequence on three keys under a second-order fixed-ratio (FR) schedule of food reinforcement. In two of these baselines, the second-order FR schedule and the discriminative stimuli for the response sequence were manipulated ("chain-strained" and "tandem-strained"). In the third baseline condition, response-independent tail shock was presented during acquisition of the response sequence. All of these baselines maintained high error levels and produced slow rates of acquisition. Under both the chain-strained and tandem-strained conditions, the positive GABA(A) modulator triazolam (0.0032-0.1 mg/kg) and the negative GABA(A) modulators beta-CCE (ethyl-beta-carboline-3-carboxylate; 0.01-1 mg/kg), beta-CCM (methyl-beta-carboline-3-carboxylate; 0.0032-0.1 mg/kg), and FG-7142 (methyl-beta-carboline-3-carboxamide; 0.18-10 mg/kg) dose-dependently decreased overall response rate compared to administration of saline (control). Under the same two conditions, triazolam and the negative GABA(A) modulators also increased the percentage of errors; however, the effects on accuracy frequently depended on the baseline condition and the particular modulator. In contrast, triazolam only decreased errors and enhanced acquisition in the presence of concurrent response-independent tail shock when compared to saline administration under this condition. The neutral GABA(A) modulator, flumazenil (1 mg/kg), had no effect on rate or accuracy of responding when administered alone, but antagonized the rate-decreasing and error-increasing effects produced by the negative GABA(A) modulators. Together, these data suggest that the effects of both the positive and negative GABAA modulators on acquisition can be similar in squirrel monkeys (i.e., both types of modulator may produce rate-decreasing and error-increasing effects) and that their effects on acquisition depend, in part, on the environmental conditions maintaining acquisition.

Effects of hippocampal injections of a novel ligand selective for the alpha 5 beta 2 gamma 2 subunits of the GABA/benzodiazepine receptor on Pavlovian conditioning

Benzodiazepine pharmacology has led to greater insight into the neural mechanisms underlying learning and anxiety. The synthesis of new compounds capable of modulating responses produced by these receptors has been made possible by the development of an isoform model of the GABA(A)/benzodiazepine receptor complex. In the current experiment, rats were pretreated with several concentrations of the novel ligand RY024 (an alpha 5 beta 2 gamma 2 -selective benzodiazepine receptor inverse agonist) in the hippocampus and were trained in a Pavlovian fear conditioning paradigm. RY024 independently produced fear-related behavior prior to training and, at the highest concentration, decreased the strength of conditioning observed 24 h after training. These data provide further evidence for the involvement of hippocampal GABA(A)/benzodiazepine receptors in learning and anxiety.

Density and pharmacology of alpha5 subunit-containing GABA(A) receptors are preserved in hippocampus of Alzheimer's disease patients

The anatomical localization and pharmacology of alpha5 subunit-containing GABA type-A receptors in the human hippocampal formation of Alzheimer's disease patients were studied with an alpha5 receptor selective ligand, [3H]L-655,708 and compared to age-matched human controls. Autoradiographic analyses revealed a heterogeneous distribution of [3H]L-655,708 binding sites in CA1-CA3 areas with high levels in stratum oriens, stratum pyramidale and stratum radiatum contrasting with low levels in stratum lacunosum. The highest quantity of alpha5 receptors was found in the molecular layer of the dentate gyrus. This pattern of expression was identical in both hippocampus from control and Alzheimer's disease subjects. Quantitative studies demonstrated that the number of [3H]L-655,708 binding sites is well preserved in Alzheimer's disease with only a moderate reduction (25-30%) in the CA1 subfield and entorhinal cortex. Furthermore, saturation and competition experiments with [3H]L-655,708 and representative benzodiazepine site ligands revealed that alpha5 receptors in Alzheimer's hippocampus have an alpha5beta2/3gamma2 pharmacology and structure as in control human brain.Overall, the data reported here provide evidence for a specific expression and relative sparing of alpha5 subunit-containing gamma-aminobutyric acid type-A receptors in the hippocampus of Alzheimer's patients.

Reduced long-term potentiation in hippocampal slices prepared using sucrose-based artificial cerebrospinal fluid

Sucrose-based artificial cerebrospinal fluid (aCSF) is sometimes used to prepare brain slices for in vitro electrophysiological experiments. This study compared the effect of preparing brain slices using chilled sucrose-based aCSF versus the conventional method using chilled aCSF on hippocampal synaptic plasticity. Brain slices from each treatment group were transferred to normal aCSF before electrophysiological recordings were made. The stimulus-response relationship of field excitatory postsynaptic potentials (fEPSPs) in the CA1 region was indistinguishable between the two treatment groups. However, the amount of LTP induced by either a &theta;-burst (four stimuli at 100 Hz repeated ten times at 200 ms intervals) or tetanic stimulation (100 Hz for 1 s) was significantly reduced in slices that had been prepared using sucrose-based aCSF. This was associated with reduced facilitation of the fEPSPs during the high frequency stimulus, reduced post-tetanic potentiation and short-term potentiation. In sucrose-cut slices the fEPSPs were slightly shorter in duration (29%, P<0.01), and during paired-pulse stimulation the broadening of the second fEPSP was enhanced. The LTP deficit in sucrose-cut slices was reversed by blocking GABA(A) receptor function with picrotoxin. These data suggest that the use of sucrose based aCSF better preserves GABA-mediated synaptic transmission, which limits the induction of LTP in hippocampal brain slices.

Benzodiazepines in perspective (II): The GABAA-Benzodiazepine Receptor Ligands

A huge number of natural and synthetic compounds modulate the function of the γ-aminobutyric acid type A receptor (GABAA-R) by interacting with several allosteric binding sites which may differ in the various GABAA-R subtypes. The benzodiazepine receptor (BDZ-R) is the most intensively studied allosteric site. It is the first allosteric modulatory site on a neurotransmitter receptor that has been found to mediate two opposite functions: facilitation and depression of GABAA-R function. The effects of BDZ-R ligands on behavior range from agonistic (anxiolytic, anticonvulsant, myore-laxant/ataxic and hypno-sedative effects) to inverse-agonistic (anxiety and panic, hypervigilance and convulsions). Of particular interest for the future are BDZ-R partial agonists, as they lack several of the undesired properties of classic full agonists. Furthermore the GABAA-R system shows a high plasticity. This polymorphism raises the possibility that ligands selective for distinct subtypes of BDZ-R may emerge as useful drugs. In both cases the possibility exists of achieving very subtle manipulations of GABAA-R function by using allosteric modulators.

Pharmacophore/receptor models for GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) via a comprehensive ligand-mapping approach

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

N-copine: a novel two C2-domain-containing protein with neuronal activity-regulated expression

Neuronal activity is often associated with changes in gene expression. By a two-dimensional cDNA-display system, restriction landmark cDNA scanning, we identified a novel gene whose expression in the hippocampus was up-regulated by kainate stimulation. The mRNA expression was detected only in brain and up-regulated by the stimulation evoking CA3-CA1 long-term potentiation. The encoded protein contains two copies of C2-domain, known as the Ca2+-binding domain of PKC-gamma, and shows 49% identity with human copine I. We designated this protein N-copine (neuronal-copine). N-copine may have a role in synaptic plasticity.

Modulation of long-term potentiation in CA1 region of mouse hippocampal brain slices by GABAA receptor benzodiazepine site ligands

Enhancement of GABAA receptor function with benzodiazepine (BZ) site agonists can disrupt memory formation and hippocampal synaptic plasticity. To investigate this further the effects of the agonist, flunitrazepam, were contrasted with that of the inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), on NMDA-dependent LTP induction in the CA1 region of mouse hippocampus. Under control conditions, a priming stimulus (10 stimuli at 100 Hz) potentiated e.p.s.p. slopes by 198%, and subsequent burst stimuli (4 x 10 events at 100 Hz every 20 sec) by 306%. This potentiation was blocked by the non-competitive NMDA receptor antagonist MK-801 and the glycine site antagonist L-701,324. Flunitrazepam (1 microM) alone caused a slight but significant reduction in e.p.s.p.s to 83% of control, suppressed LTP induced by priming stimuli (133%) and burst stimuli (188%), but not that induced by sustained high-frequency stimulation (2 x 100 events at 100 Hz, 20 sec apart). The suppression of LTP induction by flunitrazepam was blocked by the benzodiazepine site antagonist flumazenil. In contrast, the inverse agonist DMCM (100 nM) potentiated LTP formed by both priming (to 283%) and burst stimuli (to 477%). This was associated with an enhancement of paired pulse facilitation during the induction phase and the subsequent appearance of paroxysmal burst discharges. Therefore, in addition to improvements in learning and memory as a result of improved vigilance, benzodiazepine inverse agonists can have direct effects on synaptic processes thought to contribute to memory formation.

A novel benzodiazepine inverse agonist, S-8510, as a cognitive enhancer

1. Pharmacological actions of a novel benzodiazepine receptor ligand, S-8510 (2-(3-isoxazolyl)-3,6,7,9-tetrahydroimidazo[4,5-d]pyrano+ ++[4,3-b] pyridine monophosphate monohydrate), were examined in in vitro and in vivo studies. 2. S-8510 was characterized as a partial inverse agonist with a modest GABA ratio and low efficacy. 3. S-8510 ameliorated memory impairment induced by cholinergic deficit in the water maze paradigm of Wistar rats. 4. S-8510 augmented LTP of the Schaffer collateral/commissural fiber-CA1 synapses in the hippocampal slice preparations of SD rat. 5. S-8510 increased the extracellular levels of acetylcholine and noradrenaline in the hippocampus of Wistar rat. 6. S-8510 selectively potentiated pentylenetetrazol-induced convulsion without affecting minimal electroconvulsive shock- or strychnine-induced convulsion in ddY mice. 7. S-8510 failed to induce any sign of anxiety in the Wistar rat pro-conflict test. 8. S-8510 showed antidepressant-like pharmacological actions in ddY mice. 9. These results suggest that S-8510 can be used as a therapeutic drug for senile dementia, including Alzheimer's disease with little risk for inducing anxiety or convulsion.

Effects of beta-CCE on retention of aversively- and appetitively-motivated tasks in rats

Beta-carboline-3-carboxylic acid ethyl ester (beta-CCE; 1.0 or 5.0 mg/kg, i.p.) or vehicle control was administered to male Sprague-Dawley rats immediately after training in an aversively- or appetitively-motivated task. Aversively-motivated training consisted of a one-trial step-though inhibitory (passive) avoidance task with a 0.6 mA, 1.0 s foot shock. Retention was tested 21 days after training. The 5.0 mg/kg dose of beta-CCE significantly enhanced retention performance in the inhibitory avoidance task compared to the vehicle control. For appetitive training, the animals learned a T-maze for water reward. Retention was tested 48 h later, and neither dose of beta-CCE was found to affect retention performance. These results suggest that beta-CCE, when administered immediately after training, may alter retention performance of an aversively-motivated task but not an appetitive task.

Medial septal benzodiazepine receptors modulate hippocampal evoked responses and long-term potentiation

Infusion of benzodiazepine (BDZ) receptor ligands into the medial septum (MS) produces a bidirectional modulation of spatial memory retention. The present experiments sought to determine the effects of BDZ ligands upon synaptic responses and long-term potentiation (LTP) in the dentate gyrus following electrical stimulation of the angular bundle. Intraseptal infusion of the BDZ agonist, chlordiazepoxide, decreased the amplitude of the evoked population spike and increased paired-pulse facilitation at a 150-ms interstimulus interval (ISI) in a dose-dependent manner. Intraseptal infusion of the BDZ antagonist, flumazenil (10 nmol), enhanced the amplitude of the dentate population spike and also increased paired-pulse facilitation at the 150-ms ISI. There was no effect of either BDZ receptor ligand upon the slope of the rising phase of the evoked population excitatory postsynaptic potential (pEPSP). Intraseptal flumazenil also significantly enhanced the magnitude of dentate LTP induced by high-frequency stimulation of the angular bundle. Intraseptal chlordiazepoxide failed to alter LTP induction. These results indicate that intraseptal infusion of an amnestic dose of the BDZ ligand, chlordiazepoxide, decreases the excitatory responsiveness of the dentate gyrus to its synaptic input from entorhinal cortex. In contrast, the promnestic BDZ ligand, flumazenil, enhances dentate granule cell responsivity, and facilitates synaptic plasticity in the dentate gyrus network. Taken together these data suggest that the memory impairing and memory enhancing action of these compounds may be a function of their ability to alter hippocampal physiology during a critical phase of memory. The potential role of septodentate cholinergic and GABAergic projections in the present observation is discussed.

Midazolam inhibits long-term potentiation through modulation of GABAA receptors

Benzodiazepine drugs (BZ) are used for anxiety, insomnia, and seizures. They worsen memory, especially in large doses, but the mechanism of this action is uncertain. In micromolar concentrations, benzodiazepines have been shown to reduce long-term potentiation (LTP), which could be a cellular basis for their amnesic action. We have found that the LTP-inhibiting effects of BZ occur in the nanomolar concentrations attained in humans, and that this effect occurs through modulation of GABAA receptor function. We recorded extracellular synaptic input/output (I/O) curves for population spikes (PS) and EPSPs in rat hippocampal slices before and after induction of LTP. LTP increased maximal PS and EPSPs and shifted I/O curves for PS and EPSPs to the left, reflecting increased synaptic responsiveness after LTP. Curves relating EPSPs to PS were also shifted, so that after LTP larger PS were elicited for the same size EPSP (E-S potentiation). Midazolam (0.5 microM) markedly inhibited the left-shift in PS I/O curves due to E-S potentiation but did not significantly affect other parameters. 8-Phenyltheophylline (10 microM), an adenosine receptor antagonist, did not prevent midazolam inhibition of LTP. Bicuculline, a GABAA receptor antagonist, caused a dose-dependent antagonism of midazolam's LTP inhibition. Our results suggest that benzodiazepines reduce LTP primarily through reduction of E-S potentiation, and that this effect occurs through modulation of GABAA receptor function. This could in part account for the ability of benzodiazepines to disturb new memory formation.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

CCKB-receptor activation augments the long-term potentiation in guinea pig hippocampal slices

Effects of cholecystokinin octapeptide (CCK-8) on long-term potentiation (LTP) of CA1 synaptic transmission induced by tetanic stimulation of the input fibers were examined in guinea pig hippocampal slices. CCK-8 and a selective agonist for the CCKB receptor, non-sulfated CCK-8, dose-dependently augmented the magnitude of LTP. Concomitant application of a selective antagonist for the CCKB-receptor subtype, L-365,260 (3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4- benzodiazepine-3-yl)-N'-(3-methylphenyl)urea), completely blocked the augmentation of LTP induced by CCK-8, whereas a selective CCKA-receptor antagonist, L-364,718 (3S(-)-N-(2,3-dihydro-1-methyl-2- oxo-5-phenyl-1H-1,4-benzodiazepine)), had little effect. Thus, enhancement of LTP by CCK appears to be mediated by CCKB receptors. Furthermore, CCK-8 enhanced paired-pulse facilitation at a concentration of 10(-7) M without affecting the amplitude of the population spike induced by single stimulation. This effect was mimicked by a low dose of tetraethylammonium (TEA), a K+ channel blocker. Moreover, both CCK-8 and TEA reduced the late component of evoked field potentials. This late evoked potential was diminished by increasing the extracellular K+ concentration. It is suggested that CCK-8 reduces the K+ conductance in CA1 pyramidal neurons. This reduction in the K+ conductance might be related to enhancement of the LTP.

Role of somatostatin in the augmentation of hippocampal long-term potentiation by FR121196, a putative cognitive enhancer

N-(4-Acetyl-1-piperazinyl)-4-fluorobenzenesulfonamide (FR121196), a newly introduced putative cognitive enhancer of a derivative of piperazine, was investigated for its effects on long-term potentiation in guinea-pig hippocampal slices. The magnitude of long-term potentiation of population spikes recorded in CA3 pyramidal neurons was significantly augmented by perfusing FR121196 (10(-9)-10(-6) M) for 25 min before and during tetanic stimulation of the mossy fibers; the basal amplitude of population spikes before tetanus was hardly affected by the drug. The dose-response curve was bell-shaped with a maximal augmentation at 10(-7) M. Similar activity and bell-shaped dose-response curve were observed with methamphetamine (10(-8)-10(-6) M). Physostigmine (10(-8)-10(-6) M) also facilitated long-term potentiation of this pathway and the magnitude of augmentation was concentration-dependent. Scopolamine (10(-6) M) per se had little effect on the magnitude of long-term potentiation in the mossy fiber-CA3 pathway, but significantly attenuated its enhancement by FR121196 (10(-7) M) and physostigmine (10(-6) M), although it failed to influence that by methamphetamine (10(-7) M). In hippocampal slices from animals treated with cysteamine, which was shown to deplete hippocampal somatostatin, FR121196 (10(-7) M) hardly affected long-term potentiation generation, whereas physostigmine (10(-6) M) and methamphetamine (10(-7) M) augmented it significantly. These results suggest that FR121196 enhances the development of long-term potentiation in the mossy fiber-CA3 pathway through activation of somatostatinergic neurons in the hippocampal formation.