Intrahippocampal Scopolamine Research Articles

Cholinergic Blockade Frees Fear Extinction from Its Contextual Dependency

Fears that are maladaptive or inappropriate can be reduced through extinction training. However, extinction is highly context-sensitive, resulting in the renewal of fear after shifts in context and limiting the clinical efficacy of extinction training. Lesion and inactivation studies have shown that the contextualization of extinction depends on the hippocampus. Parallel studies have found that intrahippocampal scopolamine (Scop) blocks contextual fear conditioning. Importantly, this effect was replicated with a noninvasive technique in which a low dose of Scop was administered systemically. We aimed to transfer the effects of this noninvasive approach to block the contextualization of fear extinction. Rats were tone fear conditioned and extinguished under various systemic doses of Scop or the saline vehicle. They were subsequently tested (off drug) for tone fear in a context that was the same (control subjects) or shifted (renewal group) with respect to the extinction context. The lowest dose of Scop produced a significant attenuation of fear renewal when renewal was tested either in the original training context or a novel context. The drug also slowed the rate of long-term extinction memory formation, which was readily overcome by extending extinction training. Scopolamine only gave this effect when it was administered during but not after extinction training. Higher doses of Scop severely disrupted extinction learning. We discovered that disrupting contextual processing during extinction with the cholinergic antagonist Scop blocked subsequent fear renewal. Low doses of Scop might be a clinically promising adjunct to exposure therapy by making extinction more relapse-resistant.

Intrahippocampal Norleucine 1-Angiotensin IV mitigates scopolamine-induced spatial working memory deficits

Depletion of cholinergic neurons in the hippocampus has been implicated in memory impairment and Alzheimer's Disease (AD). The brain angiotensin AT 4 receptor is co-localized with cholinergic neurons, and the AT 4 receptor has also been implicated in cognitive processing. The current investigation used the spatial win-shift version of the radial arm maze to determine the involvement of AT 4 receptors in spatial working memory formation. We initially established that intrahippocampal scopolamine significantly impaired the spatial working memory performance of Sprague-Dawley rats in the radial arm maze. We also demonstrated that subsequent intrahippocampal infusions of Norleucine 1-Angiotensin IV (Nle 1-AngIV) significantly prevented the scopolamine-induced deficit. Consistent with previously published data on long-term spatial memory, our findings suggest that activation of AT 4 receptors can compensate for impaired spatial working memory resulting from compromised muscarinic acetylcholine receptor function. We further demonstrate that the hippocampus is a site of action for Nle 1-AngIV-mediated cognitive improvement.

Excitotoxic median raphe lesions aggravate working memory storage performance deficits caused by scopolamine infusion into the dentate gyrus of the hippocampus in the inhibitory avoidance task in rats.

The interactions between the median raphe nucleus (MRN) serotonergic system and the septohippocampal muscarinic cholinergic system in the modulation of immediate working memory storage performance were investigated. Rats with sham or ibotenic acid lesions of the MRN were bilaterally implanted with cannulae in the dentate gyrus of the hippocampus and tested in a light/dark step-through inhibitory avoidance task in which response latency to enter the dark compartment immediately after the shock served as a measure of immediate working memory storage. MRN lesion per se did not alter response latency. Post-training intrahippocampal scopolamine infusion (2 and 4 microg/side) produced a more marked reduction in response latencies in the lesioned animals compared to the sham-lesioned rats. Results suggest that the immediate working memory storage performance is modulated by synergistic interactions between serotonergic projections of the MRN and the muscarinic cholinergic system of the hippocampus.

Low doses of 8-OH-DPAT prevent the impairment of spatial learning caused by intrahippocampal scopolamine through 5-HT(1A) receptors in the dorsal raphe.

1. We studied the effects of low doses of 8-OH-DPAT, a 5-HT(1A) receptor agonist, on the impairment of spatial learning caused by scopolamine injected into the CA1 region of the dorsal hippocampus of rats performing a two-platform spatial discrimination task. 2. Bilateral injections of 4 microg (in 1 microl) of scopolamine into the CA1 region of the dorsal hippocampus 10 min before each training session impaired choice accuracy with no effect on choice latency and errors of omission. 3. Administered subcutaneously 20 min before each training session, 8-OH-DPAT 10 and 30 (but not 3) microg kg(-1) did not modify choice accuracy but prevented the impairment by intrahippocampal scopolamine. 4. Injection of 1.0 microg (in 0.5 microl) of WAY 100635, a 5-HT(1A) receptor antagonist, into the dorsal raphe 5 min before scopolamine had no effect on choice accuracy and latency or errors of omission and did not modify the effect of scopolamine, but completely antagonized the effect of 10 and 30 microg kg(-1) 8-OH-DPAT on scopolamine-induced impairment of choice accuracy. 5. The results confirm previous findings that stimulation of presynaptic 5-HT(1A) receptors in the dorsal raphe attenuates the deficit of spatial learning caused by blockade of cholinergic excitatory input on hippocampal pyramidal cells. 6. Drugs that stimulate presynaptic 5-HT(1A) receptors such as 5-HT(1A) receptor partial agonists may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

Estrogen Replacement Enhances Acquisition of a Spatial Memory Task and Reduces Deficits Associated with Hippocampal Muscarinic Receptor Inhibition

A delayed matching-to-position (DMP) T-maze task was used to examine the effects of estrogen replacement on spatial learning and memory, as well as the ability of estrogen replacement to reduce performance deficits produced by acute systemic and intrahippocampal muscarinic cholinergic inhibition. Two experiments were performed. In Experiment 1, ovariectomized animals were trained to criterion on the DMP task and then tested with increased intertrial delays and following systemic scopolamine administration. The animals then received either continuous estrogen replacement or sham surgery and were retested beginning 10 days later. In Experiment 2, ovariectomized animals received guide cannulae implanted bilaterally into the hippocampus. Half of these animals also began receiving continuous estrogen replacement. Two months later, the animals were trained on the DMP task and then tested with increased intertrial delays and following systemic as well as intrahippocampal scopolamine administration. Animals received the same test battery 8 months later and were then immediately trained on a reversal task. The results indicate that estrogen-treated animals acquired the DMP task at a significantly faster rate than the ovariectomized, non-estrogen-treated controls. In addition, estrogen replacement significantly reduced deficits in DMP performance produced by intrahippocampal, but not systemic, scopolamine administration. This occurred when animals were tested after 3.5 months, as well as after 12 months, of continuous estrogen replacement. No evidence for an effect of estrogen replacement on spatial working memory or reversal learning was detected. These findings demonstrate that estrogen replacement can enhance acquisition of a spatial memory task and reduce performance deficits associated with hippocampal cholinergic impairment.

S 15535, a benzodioxopiperazine acting as presynaptic agonist and postsynaptic 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine.

1 The effect of S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine), an agonist at presynaptic and antagonist at postsynaptic 5-HT1A receptors, on the impairment of spatial learning caused by intrahippocampal scopolamine in a two-platform spatial discrimination task was studied. 2 Scopolamine (4.0 microg microl-1), injected bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. 3 Administered subcutaneously 30 min before each training session, S 15535 1.0 (but not 0.3) mg kg-1 did not modify choice accuracy but prevented its impairment by intrahippocampal scopolamine. 4 WAY 100635, a 5-HT1A receptor antagonist, injected into the dorsal raphe at 1.0 microg 0.5 microl-1 5 min before scopolamine, had no effect on choice accuracy and latency or errors of omission and did not modify the effect of scopolamine but completely antagonized the effect of S 15535 (1.0 mg kg-1) on scopolamine-induced impairment of choice accuracy. 5 The results confirm a previous report (Carli et al., 1998) that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe counteracts the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. 6 Drugs that stimulate action on presynaptic 5-HT1A receptors, such as S 15535 and other partial 5-HT1A receptors agonists, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

Stimulation of 5-HT1A receptors in the dorsal raphe reverses the impairment of spatial learning caused by intrahippocampal scopolamine in rats.

This study investigated the effect of stimulating 5-HT1A receptors in the dorsal raphe on the impairment of learning caused by 4 microg/microL scopolamine injected in the CA1 region of the dorsal hippocampus in rats performing a two-platform spatial discrimination task. At 1 (but not 0.2) microg/0.5 microL administered in the dorsal raphe on each acquisition training day 5 min before bilateral intrahippocampal injection of 4 microg/microL scopolamine, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, had no effect on choice accuracy and latency or errors of omission but completely antagonized the impairment of choice accuracy by intrahippocampal scopolamine. Administered into the dorsal raphe at 0.2 and 1 microg/0.5 microL, WAY 100635, a 5-HT1A receptor antagonist, had no effect on rats' performance or on the impairment caused by intrahippocampal scopolamine but dose-dependently antagonized the effect of 1 microg/0.5 microL 8-OH-DPAT on the scopolamine-induced deficit. The results show that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe reverses the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. Together with a previous study showing that blockade of postsynaptic hippocampal 5-HT1A receptors antagonized the effect of intrahippocampal scopolamine in the two-platform spatial discrimination task (Carli et al., 1995b), the results suggest that drugs with presynaptic stimulatory and postsynaptic blocking actions on 5-HT1A receptors, such as partial agonists at these receptors, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

The Effects of Intrahippocampal Scopolamine Infusions on Anxiety in Rats as Measured by the Black–White Box Test

Hippocampal cholinergic projections mediate attention to arousing stimuli as demonstrated by behavioral, electrophysiological, and endocrine studies. We recently reported that peripheral injections of the cholinergic antagonist scopolamine (SCOP) increased anxiety-like behaviour (ALB) in rats and we sought to investigate if this response might be hippocampally mediated. Adult male, Lister Hooded rats were implanted bilaterally with hippocampal cannulae 3 weeks prior to testing. On the test day, rats were injected with vehicle (VEH; artificial CSF at 3 μl), 15 or 30 μg SCOP, 20 min prior to being placed into the white chamber of the black–white box ( n = 10/group). Rats were scored for latencies to exit and reenter the white chamber, total time spent in the white chamber, intercompartmental crossings, and activity. SCOP at 30 μg significantly reduced time to exit the white arena, while both doses of SCOP elevated latencies to reenter the white chamber. There were no effects of SCOP on intercompartmental crossing, time spent in the white chamber, or on activity levels. Loss of hippocampal cholinergic function impairs processing of threatening stimuli that manifests itself as increased ALB.

WAY 100635, a 5-HT 1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal administration of scopolamine or 7-chloro-kynurenic acid

The effect of WAY 100635, a 5-HT 1A receptor antagonist, on the impairment of spatial learning caused by intrahippocampal administration of scopolamine, a cholinergic muscarinic receptor antagonist, or 7-chloro-kynurenic acid, an antagonist at the glycine site associated with the NMDA receptor complex, was studied in a two-platform spatial discrimination task. Scopolamine (4 μg/μl) or 7-chloro-kynurenic acid (3 μg/μl), administered bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. Administered subcutaneously at 1 (but not at 0.3) mg/kg 30 min before each training session, WAY 100635 did not modify the acquisition of spatial learning, but prevented the impairment of choice accuracy caused by intrahippocampal scopolamine or 7-chloro-kynurenic acid. These findings suggest that blockade of 5-HT 1A receptors can compensate the loss of cholinergic or NMDA-mediated excitatory input to pyramidal cells in the hippocampus. The mechanisms involved and the importance of these findings for the symptomatic treatment of memory disorders in man are discussed.

Working memory failure by combined blockade of muscarinic and beta-adrenergic transmission in the rat hippocampus.

INTRAHIPPOCAMPAL administration of the muscarinic acetylcholine receptor antagonist scopolamine at a dose of 3.2 micrograms/side significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in the working memory task with a three-panel runway setup, whereas 0.32 microgram/side scopolamine did not affect working memory errors. The beta-adrenoceptor antagonist propranolol (10 mg/kg, i.p.) had no effect on working memory error, but it produced a significant increase in working memory errors when administered in combination with intrahippocampal scopolamine at the behaviourally ineffective dose (0.32 microgram/side). The increase in working memory errors induced by intrahippocampal administration of 0.32 microgram/side scopolamine to rats treated with 10 mg/kg propranolol was decreased by concurrent injection of the cholinesterase inhibitor physostigmine (3.2 micrograms/side). D-Cycloserine (the partial agonist at the glycine bindings site on the NMDA receptor/channel complex) at a dose of 10 micrograms/side reduced the increase in working memory errors induced by intrahippocampal 0.32 microgram/side scopolamine combined with 10 mg/kg propranolol. These results suggest that neural mechanisms regulated cooperatively by hippocampal muscarinic and beta-adrenergic transmission underlie working memory performance, and that modification of NMDA function contributes to such interactive regulation of working memory processes in the hippocampus.

Dose-related impairment of spatial learning by intrahippocampal scopolamine: antagonism by ondansetron, a 5-HT 3 receptor antagonist

To study the role of hippocampal muscarinic receptors in spatial learning, various doses of scopolamine were injected bilaterally into the CA1 region of the dorsal hippocampus of rats trained in a two-platform spatial discrimination task. Scopolamine administered 10 min before each training session at doses ranging from 3.75 to 15 μg/μl impaired choice accuracy, had no effect on choice latency and increased the errors of omission only with 7.5 μg on day 1 and with 15 μg on days 1 and 2 of training. No dose affected choice accuracy or latency of a non-spatial visual discrimination task. A subcutaneous dose of 1 μg/kg ondansetron, a 5-HT 3 receptor antagonist, 30 min before each training session prevented the impairment of choice accuracy by intrahippocampal 3.75 gg scopolamine but 0.1 μg/kg ondansetron had no such effect. No dose of ondansetron by itself modified the acquisition of spatial learning. The results suggest that relatively low doses of scopolamine in the dorsal hippocampus selectively impair the acquisition of a spatial discrimination task, and that blockade of 5-HT 3 receptors prevents the deficit caused by the muscarinic antagonist. The utility of the deficit of spatial learning induced by intrahippocampal scopolamine for modelling some aspects of memory disturbances in Alzheimer's disease is discussed.

Blockade of 5-HT 1A receptors compensates loss of hippocampal cholinergic neurotransmission involved in working memory of rats

NAN-190, a selective 5-HT 1A receptor antagonist, had no effect on the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in the working memory task with a three-panel runway setup, when injected bilaterally at 0.32 or 1.0 μg/side into the dorsal hippocampas. Intrahippocampal administration of the muscarinic receptor antagonist scopolamine at 3.2 μg/side or the competitive NMDA receptor antagonist (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphoric acid (CPP) at 32 ng/side significantly increased the number of working memory errors. The increase in working memory errors induced by intrahippocampal scopolamine (3.2 μg/side) was reduced by concurrent infusion of 0.32 and 1.0 μg/side NAN-190, an effect that reached significance only for the 1.0 μg/side dose. In contrast, NAN-190 at 1.0 μg/side did not affect the increase in working memory errors when infused intrahippocampally together with 32 ng/side CPP. These results suggest that blockade of hippocampal 5-HT 1A receptors does not affect impairment of working memory resulting from block of NMDA receptor-mediated neurotransmission, but that it can compensate deficiency of septohippocampal cholinergic activity involved in working memory performance of rats.

( S)-WAY 100135, a 5-HT 1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

Scopolamine, 3.75 μg/μl infused bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy but had no effect on choice latency or errors of omission in rats trained in a two-platform spatial discrimination task. Administered subcutaneously at 3 and 10 mg/kg 30 min before each training session, N-tert-butyl-3-4-(2-methoxyphenyl)piperazin-1-yl-2-phenylpropanamide dihydrochloride (( S)-WAY 100135), a 5-HT 1A receptor antagonist, prevented the impairment of choice accuracy induced by intrahippocampal scopolamine. No subcutaneous dose of ( S)-WAY 100135 by itself modified the acquisition of spatial learning. Administered into the dorsal hippocampus 15 min before each training session, ( S)-WAY 100135 at doses of 0.2, 1 and 5 μg/μl did not affect the acquisition of spatial learning but dose dependently prevented the impairment of choice accuracy caused by scopolamine, 3.75 μg/μl infused into the same area. These findings suggest that blockade of 5-HT 1A receptors can compensate the loss of cholinergic excitatory input on pyramidal cells, probably by favouring the action of other excitatory transmitters.

Effects of intra-hippocampal scopolamine injections in a repeated spatial acquisition task in the rat.

The involvement of hippocampal cholinergic synapses in spatial discrimination learning was evaluated by locally administering scopolamine into the hippocampus. Sixteen 16-month-old male Lewis rats received bilaterally implanted cannulae aimed at the dorsal part of the hippocampus. The rats were trained on a repeated acquisition test in the Morris water-escape task. In this procedure the invisible platform is randomly moved from day to day to one of four possible locations. Thus, the rat has to learn to localize the platform from day to day. On each day the rats received four pairs of trials. Scopolamine injections (35 micrograms in 1 microliter per hippocampus) were given to one group (n = 8) on days 5 and 7. On days 6 and 8 all rats received saline injections. Place learning was retarded in the scopolamine-treated rats during the first swims of pairs of trials. During second swims the scopolamine-treated rats showed a general performance deficit, indicating that first and second swims were differentially affected. The data support the hypothesis that cholinergic neurotransmission in the dorsal hippocampus is involved in spatial learning processes.