Buspirone affects hippocampal rhythmical slow activity through serotonin1A rather than dopamine D2 receptors

Model of theta frequency perturbations and contextual fear memory

Theta oscillations in the hippocampal local field potential (LFP) appear during translational movement and arousal, modulate the activity of principal cells, and are associated with spatial cognition and episodic memory function. All known anxiolytics slightly but consistently reduce hippocampal theta frequency. However, whether this electrophysiological effect is mechanistically related to the decreased behavioral expression of anxiety is currently unclear. Here, we propose that a reduction in theta frequency affects synaptic plasticity and mnemonic function and that this can explain the reduction in anxiety behavior. We test this hypothesis in a biophysical model of contextual fear conditioning. First, we confirm that our model reproduces previous empirical results regarding the dependence of synaptic plasticity on presynaptic firing rate. Next, we investigate how theta frequency during contextual conditioning impacts learning. These simulations demonstrate that learned associations between threat and context are attenuated when learning takes place under reduced theta frequency. Additionally, our simulations demonstrate that learned associations result in increased theta activity in the amygdala, consistent with empirical data. In summary, we propose a mechanism that can account for the behavioral effect of anxiolytics by impairing the integration of threat attributes of an environment into the cognitive map due to reduced synaptic potentiation.

Is hippocampal theta frequency related to individual and sex differences in anxiety-like behaviour? An analysis in male and female Long-Evans rats

Hippocampal theta activity is a prominent slow (4-12 Hz) oscillatory activity pattern generated in the mammalian hippocampal formation. Based on evidence that anxiolytic drugs consistently decrease the frequency of hippocampal theta activity in rodents, hippocampal theta has been linked to anxiety states, leading to the influential theta suppression model of anxiolysis. Surprisingly, very few studies have examined whether hippocampal theta frequency relates to individual or sex differences in anxiety-like behaviour. Here, male and female rats were tested on the elevated plus maze (EPM) to quantify levels of defensive, anxiety-like behaviours. Females exhibited higher levels of open arm exploration (open arm entries and open arm time) compared to males, suggestive of reduced anxiety in females. Subsequently, reticular-elicited hippocampal theta activity was characterized in the same rats under deep urethane anesthesia. There was no sex difference in theta frequency over a range of stimulation intensities. Further, there were no significant correlations between behavioural measures of anxiety in the EPM and theta frequency among individual animals. Theta frequency did, however, decrease following systemic administration of the clinically-used anxiolytic agent buspirone (10 mg/kg). Together, these results suggest that theta frequency does not relate to levels of anxiety-like behaviours in the EPM in male and female rats, challenging the predictive validity of hippocampal theta activity as an index of anxiety in rodents.

Systemic administration of two different anxiolytic drugs decreases local field potential theta frequency in the medial entorhinal cortex without affecting grid cell firing fields

Neurons coding spatial location (grid cells) are found in medial entorhinal cortex (MEC) and demonstrate increasing size of firing fields and spacing between fields (grid scale) along the dorsoventral axis. This change in grid scale correlates with differences in theta frequency, a 6-10Hz rhythm in the local field potential (LFP) and rhythmic firing of cells. A relationship between theta frequency and grid scale can be found when examining grid cells recorded in different locations along the dorsoventral axis of MEC. When describing the relationship between theta frequency and grid scale, it is important to account for the strong positive correlation between theta frequency and running speed. Plotting LFP theta frequency across running speeds dissociates two components of this relationship: slope and intercept of the linear fit. Change in theta frequency through a change in the slope component has been modeled and shown experimentally to affect grid scale, but the prediction that change in the intercept component would not affect grid scale has not been tested experimentally. This prediction about the relationship of intercept to grid scale is the primary hypothesis tested in the experiments presented here. All known anxiolytic drugs decrease hippocampal theta frequency despite their differing mechanisms of action. Specifically, anxiolytics decrease the intercept of the theta frequency-running speed relationship in the hippocampus. Here we demonstrate that anxiolytics decrease the intercept of the theta frequency-running speed relationship in the MEC, similar to hippocampus, and the decrease in frequency through this change in intercept does not affect grid scale.

The lateral septum as a regulator of hippocampal theta oscillations and defensive behavior in rats

Hippocampal theta oscillations are linked to various processes, including locomotion, learning and memory, and defense and affect. The lateral septum (LS) has been implicated in the generation of the hippocampal theta rhythm, but its precise role in this process is not well understood. Here, we investigated the effects of direct pharmacological inhibition or disinhibition of the dorsal LS (dLS) on the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in urethane-anesthetized rats. We found that bilateral infusions of the GABAA receptor agonist muscimol into the dLS significantly increased theta frequency. Strikingly, intra-dLS infusions of the GABAA receptor antagonist GABAzine largely abolished reticularly elicited theta activity. We also locally injected these same compounds into the medial septum (MS) to test for neuroanatomical specificity. In contrast to the effects seen in the dLS, intra-MS infusions of muscimol had no effect on theta frequency, whereas intra-MS infusions of GABAzine increased theta frequency. Given the hypothesized role of hippocampal theta in behavioral defense, we also examined the effects of intra-dLS application of muscimol in two models of anxiety, the elevated plus maze and the novelty-induced suppression of feeding paradigm; both tests revealed clear, anxiolytic-like effects following muscimol infusions. The fact that dLS-muscimol increased theta frequency while also reducing anxiety-like behaviors challenges the influential theta suppression model of anxiolysis, which predicts a slowing of theta with anxiolytic compounds. More importantly, the experiments reveal a novel role of the LS, especially its dorsal aspects, as an important gating mechanism for the expression of theta oscillations in the rodent hippocampus.

Development of a theoretically-derived human anxiety syndrome biomarker

“Anxiety disorders” are extremely common; and are a major source of health costs and lost work days. Their diagnosis is currently based on clinical symptom check lists and there are no biological markers to diagnose specific syndromal causes. This paper describes: 1) a detailed theory of the brain systems controlling anxiolytic-insensitive threat-avoidance and anxiolytic-sensitive threat-approach — where, in specific brain structures, activity generates specific normal behaviours, hyperactivity generates abnormal behaviours, and hyper-reactivity (hypersensitivity to input) generates specific clinical syndromes; 2) a rodent model of systemic anxiolytic action (rhythmical slow activity), linked to the theory, that over a period of 40 years has shown predictive validity with no false positives or false negatives — and which is likely to assay the sensitivity of endogenous systems that control anxiety; and, 3) derivation from this rodent-based theory of a specific non-invasive biomarker (goal-conflict-specific rhythmicity) for the threat-approach system in humans. This new biomarker should allow division of untreated “anxiety” patients, with superficially similar clusters of symptoms, into distinct high scoring (syndromal) and low scoring groups with different treatment-responses. This would be the first theoretically-derived biomarker for any mental disorder and should: 1) predict treatment efficacy better than current symptom-based diagnoses; 2) provide a human single dose test of novel anxiolytics; 3) provide a starting point for developing biomarkers for other “anxiety” syndromes; and so, 4) greatly improve treatment outcomes and cost-effectiveness.

Novelty and anxiolytic drugs dissociate two components of hippocampal theta in behaving rats

Hippocampal processing is strongly implicated in both spatial cognition and anxiety and is temporally organized by the theta rhythm. However, there has been little attempt to understand how each type of processing relates to the other in behaving animals, despite their common substrate. In freely moving rats, there is a broadly linear relationship between hippocampal theta frequency and running speed over the normal range of speeds used during foraging. A recent model predicts that spatial-translation-related and arousal/anxiety-related mechanisms of hippocampal theta generation underlie dissociable aspects of the theta frequency-running speed relationship (the slope and intercept, respectively). Here we provide the first confirmatory evidence: environmental novelty decreases slope, whereas anxiolytic drugs reduce intercept. Variation in slope predicted changes in spatial representation by CA1 place cells and novelty-responsive behavior. Variation in intercept predicted anxiety-like behavior. Our findings isolate and doubly dissociate two components of theta generation that operate in parallel in behaving animals and link them to anxiolytic drug action, novelty, and the metric for self-motion.

Traumatic brain injury-induced cognitive and histological deficits are attenuated by delayed and chronic treatment with the 5-HT1A-receptor agonist buspirone

The aim of this study was to evaluate the potential efficacy of the serotonin(1A) (5-HT(1A)) receptor agonist buspirone (BUS) on behavioral and histological outcome after traumatic brain injury (TBI). Ninety-six isoflurane-anesthetized adult male rats were randomized to receive either a controlled cortical impact or sham injury, and then assigned to six TBI and six sham groups receiving one of five doses of BUS (0.01, 0.05, 0.1, 0.3, or 0.5 mg/kg) or saline vehicle (VEH, 1.0 mL/kg). Treatments began 24 h after surgery and were administered intraperitoneally once daily for 3 weeks. Motor function (beam-balance/beam-walk tests) and spatial learning/memory (Morris water maze) were assessed on post-operative days 1-5 and 14-19, respectively. Morphologically intact CA1/CA3 cells and cortical lesion volume were quantified at 3 weeks. No differences were observed among the BUS and VEH sham groups in any end-point measure and thus the data were pooled. Regarding the TBI groups, repeated-measures ANOVAs revealed that the 0.3 mg/kg dose of BUS enhanced cognitive performance relative to VEH and the other BUS doses (p<0.05), but did not significantly impact motor function. Moreover, the same dose conferred selective histological protection as evidenced by smaller cortical lesions, but not greater CA1/CA3 cell survival. No significant behavioral or histological differences were observed among the other BUS doses versus VEH. These data indicate that BUS has a narrow therapeutic dose response, and that 0.3 mg/kg is optimal for enhancing spatial learning and memory in this model of TBI. BUS may have potential as a novel pharmacotherapy for clinical TBI.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

A two-dimensional neuropsychology of defense: fear/anxiety and defensive distance

We present in this paper a picture of the neural systems controlling defense that updates and simplifies Gray's "Neuropsychology of Anxiety". It is based on two behavioural dimensions: 'defensive distance' as defined by the Blanchards and 'defensive direction'. Defensive direction is a categorical dimension with avoidance of threat corresponding to fear and approach to threat corresponding to anxiety. These two psychological dimensions are mapped to underlying neural dimensions. Defensive distance is mapped to neural level, with the shortest defensive distances involving the lowest neural level (periaqueductal grey) and the largest defensive distances the highest neural level (prefrontal cortex). Defensive direction is mapped to separate parallel streams that run across these levels. A significant departure from prior models is the proposal that both fear and anxiety are represented at all levels. The theory is presented in a simplified form that does not incorporate the interactions that must occur between non-adjacent levels of the system. It also requires expansion to include the dimension of escapability of threat. Our current development and these proposed future extensions do not change the core concepts originally proposed by Gray and, we argue, demonstrate their enduring value.

Effects of lidocaine reversible inactivation of the median raphe nucleus on long-term potentiation and recurrent inhibition in the dentate gyrus of rat hippocampus

Considering the fact that median raphe nucleus (MRN) constitutes one of the inputs of the hippocampus, the effects of reversible inactivation of MRN on long-term potentiation (LTP) and recurrent inhibition in the dentate gyrus (DG) of rat hippocampus, in vivo, were examined. Rats were anesthetized with urethane (1.5 g/kg, i.p.). MRN was temporarily suppressed by intra-MRN injection of lidocaine (0.5 microl, 2%). For LTP induction, eight episodes of high frequency stimuli (100 Hz) were delivered to the perforant path (PP), each consisting of 10 stimuli at 100 Hz. Population spikes (PS) and population excitatory post synaptic potentials ((P)EPSP) in DG were recorded 10 min before, and 5, 10, 20, 40, 60 and 120 min after tetanization. MRN inactivation itself had no effect on the amplitude of baseline responses. The PS amplitude and (P)EPSP slope in rats, injected with intra-MRN lidocaine, 5 min before tetanization, were not different from the control group. However, at 120 min PS amplitude was significantly higher than control. Lidocaine injection 5 min after tetanic stimuli caused a significant decrease in PS amplitude (10, 20 and 60 min) and (P)EPSP slope (20 and 40 min) after tetanization. The data showed that inactivation of MRN has no effect on LTP induction in the DG of hippocampus but it does affect its maintenance, and this effect depends on the pre- or post-tetanic inactivation. In the last part of this study, in order to investigate the effect of MRN on the efficacy of recurrent inhibition in the perforant-dentate synapses, paired pulse was applied to the PP at 10 and 20 ms interpulse intervals. Inactivation of MRN increased the amount of recurrent inhibition in the DG with 20 ms interpulse interval. This observation indicates that MRN inhibits the recurrent inhibition mechanism, which is in accordance with the suggested role of MRN neurons on inhibition of hippocampal GABAergic interneurons.

The effect of apomorphine and buspirone on regional cerebral blood flow during the performance of a cognitive task-measuring neuromodulatory effects of psychotropic drugs in man

Psychopharmacological activation, in conjunction with positron emission tomographic measurements of regional cerebral blood flow (rCBF), was used to investigate the neurotransmitter basis of a specific cognitive function in man. Monoaminergic neurotransmission was pharmacologically manipulated during performance of auditory - verbal memory tasks. Statistical parametric mapping was used to identify the brain sites of interaction between memory-induced increases in rCBF and active drugs. Memory task-induced increases in rCBF in the left prefrontal cortex were attenuated by apomorphine, a non-selective dopamine agonist, whilst buspirone, a serotonin1A partial agonist, augmented rCBF increases in this area. In addition, apomorphine and buspirone augmented memory-induced increases in rCBF centred in the posterior cingulate cortex, whilst buspirone alone attenuated rCBF increases in the retrosplenial cortex and posterior parahippocampal gyrus. These regionally selective interactions may represent neuromodulatory effects of monoaminergic neurotransmission on a specific cognitive function in man.

Separation anxiety syndrome in cats: 136 cases (1991-2000)

OBJECTIVE

To determine whether cats develop clinical signs typical of separation anxiety syndrome (SAS) and the type and frequency of applicable clinical signs in affected cats.

DESIGN

Retrospective study.

ANIMALS

136 cats with clinical signs typical of SAS in dogs. Subjects were evaluated during home visits.

PROCEDURE

Medical records of pet cats evaluated for behavior problems during a 10-year period were reviewed. Medical records of cats that displayed behaviors typical of dogs with SAS (eg, inappropriate elimination, excessive vocalization, destructiveness, or self-mutilation) were more extensively examined, and cats that displayed these behaviors only when separated from an apparent attachment figure were included in the study.

RESULTS

Behavior problems triggered by separation anxiety included inappropriate urination (96 cats), inappropriate defecation (48), excessive vocalization (16), destructiveness (12), and psychogenic grooming (8). Inappropriate defecation was identified in a significantly higher percentage of the neutered females in the study than in the neutered males. Seventy-five percent of the cats that urinated inappropriately urinated exclusively on the owner's bed. Psychogenic grooming was identified in 8 of the 40 neutered females but in none of the neutered males, whereas destructiveness was observed in 12 of the 92 neutered males but in none of the neutered females.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that cats can develop SAS. Sex and breed differences in the frequency of particular signs of SAS in cats may exist. Feline SAS should be considered in the differential diagnosis of anxiety-related misbehavior in cats.

Lidocaine reversible inactivation of the median raphe nucleus has no effect on reference memory but enhances working memory versions of the Morris water maze task

Numerous studies in the past have dealt with the role of serotonergic system lesions in tasks aimed at measurement of cognitive behavior, but the literature concerning the role of serotonin in cognition remains controversial. Rats with electrolytic lesions of the median raphe nucleus (MRN) were found to display a profound impairment in both the acquisition and retention of spatial memory task. In this study, the lidocaine inactivation was employed to evaluate the involvement of the rat's median raphe nucleus in reference and working memory versions of the Morris water maze (MWM) task. Lidocaine (0.5 microl, 2%) was injected through a single cannula aimed at the MRN; control groups were treated in the same way with a 0.5 microl injection of saline. In Experiment 1, rats were trained in a reference memory version of the MWM with two blocks of four trials per day for three consecutive days, with intra-cerebral injection made 5 min before training. No significant difference was found. In Experiment 2, intra-cerebral injection was applied immediately after two blocks of four trials, and in Experiment 3, the drug was injected 5 min before retention test in rats that had received eight trials per day on three consecutive days. Again, no significant difference between control and treatment groups was found. These results indicate that MRN has no role in acquisition, consolidation and retrieval of spatial reference memory. In subsequent experiments, rats were trained in a working memory version of the MWM task to find a new target position in trial 1, and retrieval was tested 75 min later. MRN inactivation 5 min before (Experiment 4) and immediately after the acquisition trial (Experiment 5) enhanced spatial working memory. It is concluded that normal activity of the MRN has no role in formation and retrieval of reference memory, but it has an inhibitory role in working memory. Our results are confirmed with other studies suggesting that the serotonergic system has a different role in long-term and short-term memory. Interaction with other neurotransmitter systems like acetylcholine may be involved in this case.

Effect of repeated ipsapirone treatment on hippocampal excitatory synaptic transmission in the freely behaving rat: role of 5-HT1A receptors and relationship to anxiolytic effect

The effects of acute and repeated treatment with the 5-HT1A receptor ligand ipsapirone on hippocampal excitatory synaptic transmission and in an ultrasonic vocalization anxiety test were investigated in the rat. Synaptic responses in the CA1 region of the dorsal hippocampus of alert, freely behaving male Wistar rats were reduced after acute injection of ipsapirone (1 or 2 mg/kg, i.p.). This effect was prevented by pretreatment with the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide trihydrochloride, 0.25 or 0.5 mg/kg, i.p.) but not by the 5-HT-depleting agent para-chlorophenylalanine (300 mg/kg per day for 3 days, i.p.). WAY-100635 (0.1-0.3 mg/kg, i.p.) also blocked the acute anti-aversive effects of ipsapirone (3 mg/kg, i.p.) in the anxiety test. Repeated administration of ipsapirone (1 or 2 mg/kg per day for 7-8 days, i.p.) produced a gradual reduction in baseline synaptic transmission which was transiently reversed by WAY-100635 (0.25 mg/kg, i.p.). Ipsapirone (1 mg/kg per day for 7 days) produced a gradual and sustained reduction in the duration of vocalizations in the anxiety test which paralleled the reduction in baseline synaptic responses in the same animals. The data indicate that with repeated administration of ipsapirone, a prolongation and enhancement of the 5-HT1A receptor-mediated reduction in hippocampal excitatory synaptic transmission occurs. This delayed effect may contribute to the sustained anxiolytic and/or antidepressant effect of ipsapirone.

Dextroamphetamine enhances "neural network-specific" physiological signals: a positron-emission tomography rCBF study

Previous studies in animals and humans suggest that monoamines enhance behavior-evoked neural activity relative to nonspecific background activity (i.e., increase signal-to-noise ratio). We studied the effects of dextroamphetamine, an indirect monoaminergic agonist, on cognitively evoked neural activity in eight healthy subjects using positron-emission tomography and the O15 water intravenous bolus method to measure regional cerebral blood flow (rCBF). Dextroamphetamine (0.25 mg/kg) or placebo was administered in a double-blind, counterbalanced design 2 hr before the rCBF study in sessions separated by 1-2 weeks. rCBF was measured while subjects performed four different tasks: two abstract reasoning tasks--the Wisconsin Card Sorting Task (WCST), a neuropsychological test linked to a cortical network involving dorsolateral prefrontal cortex and other association cortices, and Ravens Progressive Matrices (RPM), a nonverbal intelligence test linked to posterior cortical systems--and two corresponding sensorimotor control tasks. There were no significant drug or task effects on pCO2 or on global blood flow. However, the effect of dextroamphetamine (i.e., dextroamphetamine vs placebo) on task-dependent rCBF activation (i.e., task - control task) showed double dissociations with respect to task and region in the very brain areas that most distinctly differentiate the tasks. In the superior portion of the left inferior frontal gyrus, dextroamphetamine increased rCBF during WCST but decreased it during RPM (ANOVA F (1,7) = 16.72, p < 0.0046). In right hippocampus, blood flow decreased during WCST but increased during RPM (ANOVA F(1,7) = 18.7, p < 0.0035). These findings illustrate that dextroamphetamine tends to "focus" neural activity, to highlight the neural network that is specific for a particular cognitive task. This capacity of dextroamphetamine to induce cognitively specific signal augmentation may provide a neurobiological explanation for improved cognitive efficiency with dextroamphetamine.

Cerebral metabolic effects of serotonin drugs and neurotoxins

The functional effects of serotonin (5-HT) drugs and toxins on regional cerebral metabolic rates for glucose (rCMRglc) have been determined in rats with the in vivo, quantitative, autoradiographic [14C]2-deoxyglucose technique. Serotonin agents produced rCMRglc patterns different and more specific that one would predict from binding studies. At low doses 5-HT1 agonists reduced rCMRglc in limbic areas and at high doses increased rCMRglc in brain motor regions. The 5-HT2 agonists dose-dependently decreased rCMRglc in proencephalic areas and increased it in thalamic nuclei. 5-HT3 receptor antagonism resulted in rCMRglc decreases in limbic, auditory and visual areas and agents with 5-HT3 receptor activity increased rCMRglc in brain regions with high 5-HT3 receptor densities. Serotonin anxiolytics (e.g. azapirones) and antidepressants (e.g. tryciclic and non-tryciclic 5-HT reuptake inhibitors) reduced rCMRglc selectively in limbic areas and in brainstem monoaminergic nuclei. Dose, time from administration, receptor affinity, behavioral and neurochemical correlates, 5-HT system lesion and circulating glucocorticoid were all relevant factors in determining the rCMRglc effects of 5-HT drugs. Acutely neurotoxic amphetamines markedly increased rCMRglc in brain regions such as the nucleus accumbens that are thought to mediate amphetamine reinforcing properties; on the long term, toxic or electrolytic lesions or chronic treatment with 5-HT agonists produced minimal rCMRglc alterations in spite of marked and persistent changes in 5-HT function. In lesioned or chronically treated rats, acute challanges with 5-HT and non 5-HT agonists demonstrated specific deficits that were not detected in a resting state. Serotonin neuromodulation has been studied in humans by using positron emission tomography with 15O-water. Sequential measurements of regional cerebral blood flow (rCBF) were obtained during combined pharmacological challange with the 5-HT1A agonist buspirone and cognitive activation. Buspirone increased a memory related rCBF activation in task specific regions. This technique can provide a strong theoretical basis for the understanding of 5-HT drug mode of action in normal human brain and in neuropsychiatric diseases. Brain metabolism studies in animals will still be needed to elucidate the factors (e.g. pharmacokinetic and pharmacodynamic) relevant to the cerebral response to 5-HT drugs in humans.

Role of hippocampal 5-HT1A receptors on elevated plus maze exploration after a single restraint experience

Previous studies have shown that 2 h restraint stress induces deficits in open arm exploration of an elevated plus maze 24 h later. This effect was attenuated by a post-stress systemic injection of the 5-HT non-selective agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). To verify a possible involvement of hippocampal 5-HT1A receptors in this effect, rats were stereotaxically implanted with canulae in the dorsal hippocampus. Seven days later they received bilateral microinjections of 5-MeODMT (20 nmol/0.5 microliter) or saline. No difference was found on exploration of an elevated plus maze 24 h later. However, when treatments were performed immediately after 2 h of restraint stress, the drug was able to increase open arm exploration 24 h later. This effect was antagonized by a previous microinjection of (+)WAY-100135 (40 nmol/0.5 microliter), a selective 5-HT1A antagonist. The results suggest that hippocampal 5-HT1A receptors may attenuate stress behavioral consequences.

Role of 5-HT in stress, anxiety, and depression

There are conflicting results on the function of 5-HT in anxiety and depression. To reconcile this evidence, Deakin and Graeff have suggested that the ascending 5-HT pathway that originates in the dorsal raphe nucleus (DRN) and innervates the amygdala and frontal cortex facilitates conditioned fear, while the DRN-periventricular pathway innervating the periventricular and periaqueductal gray matter inhibits inborn fight/flight reactions to impending danger, pain, or asphyxia. To study the role of the DRN 5-HT system in anxiety, we microinjected 8-OH-DPAT into the DRN to inhibit 5-HT release. This treatment impaired inhibitory avoidance (conditioned fear) without affecting one-way escape (unconditioned fear) in the elevated T-maze, a new animal model of anxiety. We also applied three drug treatments that increase 5-HT release from DRN terminals: 1) intra-DRN microinjection of the benzodiazepine inverse agonist FG 4172, 2) intra-DRN microinjection of the excitatory amino acid kainic acid, and 3) intraperitoneal injection of the 5-HT releaser and uptake blocker D-fenfluramine. All treatments enhanced inhibitory avoidance in T-maze. D-Fenfluramine and intra-DRN kainate also decreased one-way escape. In healthy volunteers, D-fenfluramine and the 5-HT agonist mCPP (mainly 5-HT2C) increased, while the antagonists ritanserin (5-HT2A/2C) and SR 46349B (5-HT2A) decreased skin conductance responses to an aversively conditioned stimulus (tone). In addition, D-fenfluramine decreased, whereas ritanserin increased subjective anxiety induced by simulated public speaking, thought to represent unconditioned anxiety. Overall, these results are compatible with the above hypothesis. Deakin and Graeff have suggested that the pathway connecting the median raphe nucleus (MRN) to the dorsal hippocampus promotes resistance to chronic, unavoidable stress. In the present study, we found that 24 h after electrolytic lesion of the rat MRN glandular gastric ulcers occurred, and the immune response to the mitogen concanavalin A was depressed. Seven days after the same lesion, the ulcerogenic effect of restraint was enhanced. Microinjection of 8-OH-DPAT, the nonselective agonist 5-MeO-DMT, or the 5-HT uptake inhibitor zimelidine into the dorsal hippocampus immediately after 2 h of restraint reversed the deficits of open arm exploration in the elevated plus-maze, measured 24 h after restraint. The effect of the two last drugs was antagonized by WAY-100135, a selective 5-HT1A receptor antagonist. These results are compatible with the hypothesis that the MRN-dorsal hippocampus 5-HT system attenuates stress by facilitation of hippocampal 5-HT1A-mediated neurotransmission. Clinical implications of these results are discussed, especially with regard to panic disorder and depression.

Dose-dependent effects of buspirone on behavior and cerebral glucose metabolism in rats

In this study we compared the effects of the anxiolytic buspirone on behavior and regional cerebral metabolic rates for glucose (rCMRglc) with those of the reference serotonin (5-HT)1A agonist 8-hydroxy-2(di-N-propylamino)tetralin (DPAT). Behavioral effects were assessed by scoring the 5-HT syndrome. rCMRglc was measured in 56 brain regions by using the quantitative autoradiographic [14C]2-deoxyglucose technique, at 10 min after i.p. injection of DPAT (1 mg/kg) or buspirone (0.4, 4 and 40 mg/kg) in awake male Fischer-344 rats. Whereas DPAT produced an intense 5-HT syndrome, buspirone had no behavioral effect. A low dose (0.4 mg/kg) of buspirone reduced rCMRglc in 18 brain areas (32%), more markedly in limbic areas and raphe nuclei. These were the only rCMRglc effects buspirone had in common with the potent 5-HT1A agonist DPAT and suggest that low dose buspirone activates preferentially 5-HT1A receptors. Hence, this receptor subtype may mediate buspirone functional effects on the limbic system and, given the role of these brain areas in mood control, possibly buspirone therapeutic actions. High doses (4 and 40 mg/kg) of buspirone produced widespread rCMRglc decreases in 46 (82%) and 44 (79%) of the areas studied and increased rCMRglc in one brain area, the lateral habenula, that was not affected by DPAT or a low dose of buspirone. The topographic distribution and direction of rCMRglc changes by high doses of buspirone differ from those produced by the 5-HT1A agonist DPAT. Instead these changes resemble the rCMRglc effects of dopaminergic D2 antagonists like haloperidol and are consistent with some pharmacological and binding properties of buspirone.(ABSTRACT TRUNCATED AT 250 WORDS)

Food carrying in rats is blocked by the putative anxiolytic agent buspirone

The effects of the putative anxiolytic agent buspirone on food-handling behavior of laboratory rats were investigated. Rats trained to travel from a covered shelter to a food source were provided with food pellets of six sizes. Smaller pellets were eaten at the exposed food source, whereas larger pellets were carried back to the shelter for consumption. Subcutaneous administration of buspirone hydrochloride (0.2-2.0 mg/kg) reduced carrying of larger food pellets in a dose-dependent manner. Instead, these pellets were also eaten at the exposed food source. Carrying was maximally suppressed 1 h after drug administration. Handling of smaller pellets, travel times, and eating times were not affected by buspirone. Similar results have previously been obtained with diazepam. Buspirone appears to exert its effects through 5-HT1A and/or dopamine receptors, whereas diazepam interacts with benzodiazepine receptors. Thus, manipulations of distinct transmitter systems may have similar behavioral consequences on the food carrying responses of rats.

The role of 5-HT1A serotonin and D2 dopamine receptors in buspirone effects on cortical electrical activity in rats

The effects of buspirone (5 and 10 mg/kg i.p.), 8-OH-DPAT (0.25 mg/kg) and raclopride (2.5 mg/kg) on the EEG power spectra of the sensorimotor cortex were studied in freely moving rats. Buspirone (5 mg/kg) and 8-OH-DPAT produced selective slowing of the theta-activity. Buspirone (10 mg/kg) produced slowing of the theta-activity and increased the power of the alpha-band (9-11 Hz). Raclopride alone did not influence EEG power spectra. Simultaneous injection of 8-OH-DPAT and raclopride produced marked slowing of the theta-activity and increased the power of the alpha-band. The role of 5 HT1A and D2 dopamine receptors in buspirone effects on cortical electrical activity in rats was discussed.

5-HT1A receptor-mediated inhibition in the hippocampus of the alert rat--effects of repeated gepirone treatment

The effects of acute and repeated treatment with the 5-HT1A receptor ligand gepirone on hippocampal excitatory synaptic transmission were investigated. Recordings of the electrically evoked field population excitatory postsynaptic potentials (e.p.s.p.s) were made in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert male Wistar rats. Acute injection of gepirone reduced the e.p.s.p. amplitude in a transient dose-dependent (0.5-10 mg/kg, i.p.) manner. This effect was blocked by the 5-HT1A receptor antagonist MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl methylaminoethys]-8- azaspiro[4,5]decane-7,9-dione methyl sulphonate, 2 mg/kg, i.p.). Gepirone (1 mg/kg per day, i.p.) administered for 7 days produced a gradual reduction in the daily pre-injection baseline e.p.s.p. amplitude coupled with a concomitant reduction of the acute response to the drug. The chronic baseline reduction was transiently reversed by the 5-HT1A receptor antagonist spiroxatrine and complete recovery to pretreatment levels was observed 48 h after the last gepirone dose. The data indicate that with repeated administration, a prolongation and enhancement of the 5-HT1A receptor-mediated reduction in the e.p.s.p. by gepirone occurs. This delayed effect may contribute to the slow onset of therapeutic action of gepirone.

Effects of several 5-HT1A agonists on hippocampal rhythmical slow activity in unanesthetized rats

We examined the effect of 5-hydroxytryptamine (5-HT)1A agonists on walking related, atropine-resistant, rhythmical slow activity (wr-RSA) of the hippocampus in rats. Selective 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), flesinoxan, buspirone and ipsapirone significantly decreased the power value of 7-9 Hz band activity and the median frequency of wr-RSA. The order of potency was 8-OH-DPAT > flesinoxan = buspirone in power reduction. The 5-HT1A antagonists, (-)pindolol, (-)propranolol and spiperone, inhibited the effect of 8-OH-DPAT on wr-RSA. Pretreatment with parachlorophenylalanine did not abolish the effect of 8-OH-DPAT. These results indicate that 5-HT1A agonists reduce both power and median frequency values of wr-RSA through activation of post-synaptic 5-HT1A receptors in the forebrain in unanesthetized rats, in vivo.

Hippocampal 5-HT receptors and consolidation of stressful memories

It has been suggested that postsynaptic 5-HT1A receptors in the hippocampus, innervated by 5-HT neurons localized in the median raphe nucleus, mediate adaptive or coping responses to aversive events and that dysfunction of this system is related to symptoms of depression. To test this hypothesis we investigated the expression of c-fos mRNA in animals submitted to immobilization stress. The results showed that c-fos mRNA expression is significantly increased in the dentate gyrus and CA1-CA3 regions of the hippocampus after 30 min of forced restraint, suggesting that this structure is activated during stress. To investigate the role of 5-HT neurotransmission in the hippocampus on adaptation to aversive events we immobilized rats for 2 h and tested them 24 h later in an elevated plus-maze. Our results showed that the previous restraint period decreases exploration of open arms in the maze. This effect was reversed by bilateral microinjection of zimelidine (20 and 100 nmol), a 5-HT re-uptake blocker, or 8-OH-DPAT (3 nmol), a 5-HT1A agonist, into the dorsal hippocampus immediately after restraint. These results are compatible with the idea that postsynaptic 5-HT1A receptors located in the hippocampus participate in the development of tolerance to aversive events.

Measuring the neuromodulatory effects of drugs in man with positron emission tomography

Cognitive activation in conjunction with pharmacological challenge was used to demonstrate neuromodulation in man. Using positron emission tomography (PET), measurements of regional cerebral blood flow were made during the performance of memory tasks, before and after the administration of apomorphine (dopamine agonist), buspirone (5-HT1A partial agonist) or placebo. Drug effects on memory-induced increases in regional cerebral blood flow were assessed, on a voxel-by-voxel basis, using statistical parametric mapping. Increases of regional cerebral blood flow in response to the memory challenge were attenuated by apomorphine in the dorsolateral prefrontal cortex and augmented in the retrosplenial region of the posterior cingulate. Conversely, buspirone attenuated blood flow increases in the retrosplenial region. These interactions between drugs and a cognitive challenge can best be interpreted as neuromodulatory effects.

Pindolol antagonizes the effects on hippocampal rhythmical slow activity of clonidine, baclofen and 8-OH-DPAT, but not chlordiazepoxide and sodium amylobarbitone

Buspirone, benzodiazepines, barbiturates and ethanol all reliably reduce the frequency of reticular-elicited hippocampal rhythmical slow activity. In the present experiments we tested a number of drugs which are not usually used for treating generalized anxiety disorders but which have been reported to have some anxiolytic properties. Clonidine (0.3 mg/kg, i.p.), baclofen (6 mg/kg, i.p.) and 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) (2.5 mg/kg, i.p.) all reduced the frequency of rhythmical slow activity. The effect of all three drugs was reduced by the 5-hydroxytryptamine 1a antagonist pindolol (2 mg/kg, i.p.). Pindolol had no effect on the reduction in rhythmical slow activity produced by sodium amylobarbitone, as has been previously reported for the benzodiazepine chlordiazepoxide. Flumazenil (10 mg/kg, i.p.), a benzodiazepine receptor antagonist, reduced the effects of chlordiazepoxide (5 mg/kg, i.p.), but not buspirone (10 mg/kg, i.p.). A combination of the selective beta 1 adrenergic receptor antagonist metoprolol (20 mg/kg, i.p.) and the beta 2 adrenergic receptor antagonist ICI 118,551 (4 mg/kg, i.p.) did not reduce the effects of either buspirone (10 mg/kg, i.p.) or diazepam (1 mg/kg, i.p.). These data show that there are at least two separate routes through which anxiolytic agents reduce the frequency of hippocampal rhythmical slow activity. Buspirone, clonidine, baclofen and 8-OH-DPAT act via a system dependent on 5-hydroxytryptamine 1a receptor activation. Benzodiazepines act via activation of the benzodiazepine receptor and probably share with barbiturates action at the GABA-benzodiazepine-chloride ionophore complex but do not produce their effects, directly or indirectly, by 5-hydroxytryptamine 1a receptor activation.

Effects of long-term administration of anxiolytics on reticular-elicited hippocampal rhythmical slow activity

Buspirone is effective in treating clinical anxiety but, unlike classical anxiolytics, does not have anti-convulsant, sedative or muscle relaxant side-effects and does not interact with GABA. Buspirone may also differ from classical anxiolytics in requiring a period of 2 weeks or more to achieve its full therapeutic action. It has previously been shown that all anxiolytic drugs, including buspirone, reduce the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). The present experiments tested whether the time course of the effect of buspirone on rhythmical slow activity differed from that of the anxiolytic benzodiazepine chlordiazepoxide. Rats, implanted with reticular stimulation electrodes and subicular recording electrodes, received three intraperitoneal injections per day of buspirone (2.5 mg/kg), chlordizepoxide (5 mg/kg) or saline for 45 days. Both buspirone and chlordiazepoxide reduced the frequency of rhythmical slow activity on the first day of testing and Ro15-1788 (10 mg/kg) blocked the effects of chlordiazepoxide but not buspirone. There was no increase in the effect of buspirone with time. These results showed that, if the effect of anxiolytic drugs on rhythmical slow activity provides any basis for their clinical action, then some additional factors are required to explain both the delayed action of buspirone and the immediate action of classical anxiolytic drugs.

Neurochemically dissimilar anxiolytic drugs have common effects on hippocampal rhythmic slow activity

Previous experiments have shown that anxiolytic drugs reduce the frequency of hippocampal rhythmic slow activity, induced by high frequency stimulation of the reticular formation and flatten the function relating threshold septal stimulation to the frequency of driven rhythmic slow activity. All of the drugs involved are known to augment GABAergic transmission. The present experiments investigated the effects of the novel anxiolytic compound buspirone which, unlike conventional anxiolytics, does not interact with GABA, yet is a clinically effective anxiolytic. Buspirone (0.156-40 mg/kg, i.p.) was found to reduce the frequency of reticular-elicited rhythmic slow activity, in a similar manner to chlordiazepoxide (0.019-20 mg/kg, i.p.). Buspirone did not change the linearity of the voltage-frequency function. Buspirone (10 mg/kg, i.p.) also altered the threshold for septal driving of rhythmic slow activity, in a similar manner to classical anxiolytics. The combination of chlordiazepoxide (5 mg/kg, i.p.) with corticosterone (0.2 mg, s.c.) removed the minor differences between buspirone and chlordiazepoxide in both the septal and reticular tests. These results show that buspirone altered the control of rhythmic slow activity in the hippocampus, in a manner which appeared functionally equivalent to other anxiolytics but which depends on mechanisms which are likely to be neurally and pharmacologically distinct from those of other anxiolytic drugs.

Effects of long-term administration of imipramine on reticular-elicited hippocampal rhythmical slow activity

All anxiolytics so far tested show a common reduction in the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). The present experiments tested whether imipramine, an antidepressant drug which has also been used to treat generalized anxiety disorders, shares the common characteristics of anxiolytics on hippocampal RSA. Rats implanted with reticular stimulating electrodes and subicular recording electrodes received both acute and chronic injection of different doses of imipramine. Only relatively high doses (20 and 30 mg/kg, IP) of imipramine produced a reduction in RSA frequency after a single administration. Long-term administration of 20 mg/kg (but not 10 mg/kg, IP) imipramine induced an increase in baseline RSA frequency but there was no change in the acute frequency-reducing effect of the drug. These results suggest that changes in hippocampal RSA reflect different mechanisms of action for chronic versus acute treatment with antidepressant. It is possible that, at high doses, apparently anxiolytic effects of imipramine may be mediated by similar mechanisms to conventional anxiolytic drugs.