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

Linking Anxiolytic Action to Hippocampal "Theta"-A Personal History

This paper provides a personal history of work starting with the discovery that anxiolytic drugs reduce hippocampal theta frequency. It includes parallel work on septal elicitation of theta carried out in Jeffrey Gray's laboratory in Oxford; a statement of my original scientific perspective on the work; and a description of later work in my laboratory in New Zealand confirming the function of theta rhythmicity per se and its mediation of the effects of anxiolytic drugs on behavior. I finish with comments on risk management with such experiments and their use in larger scale theory development.

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.

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.

Anxiolytic- and antidepressant-like properties of ketamine in behavioral and neurophysiological animal models

Ketamine, a dissociative anesthetic agent, appears to have rapid antidepressant effects at sub-anesthetic doses in clinically depressed patients. Although promising, these results need to be replicated in double-blind placebo-controlled studies, a strategy thwarted by the psychoactive effects of ketamine, which are obvious to both patients and clinicians. Alternatively, demonstrations of the psychotherapeutic effects of ketamine in animal models are also complicated by ketamine's side-effects on general activity, which have not been routinely measured or taken into account in experimental studies. In this study we found that ketamine decreased "behavioral despair" in the forced swim test, a widely used rats model of antidepressant drug action. This effect was not confounded by side-effects on general activity, and was comparable to that of a standard antidepressant drug, fluoxetine. Interestingly, ketamine also produced anxiolytic-like effects in the elevated-plus-maze. Importantly, the effective dose of ketamine in the plus-maze did not affect general locomotion measures, in either the plus-maze or in the open field test. While the selective N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 also produced antidepressant-like and anxiolytic-like effects, these were mostly confounded by changes in general activity. Finally, in a neurophysiological model of anxiolytic drug action, ketamine reduced the frequency of reticularly-activated theta oscillations in the hippocampus, similar to the proven anxiolytic drug diazepam. This particular neurophysiological signature is common to all known classes of anxiolytic drugs (i.e. benzodiazepines, 5-HT1A agonists, antidepressants) and provides strong converging evidence for the anxiolytic-like effects of ketamine. Further studies are needed to understand the underlying pharmacological mechanisms of ketamine's effects in these experiments, since it is not clear they were mimicked by the selective NMDA antagonist MK-801.

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.

Different systems in the posterior hypothalamic nucleus of rats control theta frequency and trigger movement

Reduced frequency of theta activity is thought to compromise hippocampal function and so behavioural inhibition. The anxiolytic benzodiazepine chlordiazepoxide (CDP) reduces theta frequency when injected into the medial supramammillary nucleus (mSuM), posterior hypothalamic nucleus (PH) and dorsomedial hypothalamic nucleus (DMH). These hypothalamic effects on theta could underlie at least some behavioural effects of benzodiazepines. We have previously shown that in a fixed interval 60-s schedule (FI60), CDP injected into mSuM reduced both theta frequency and behavioural inhibition. The present experiments test the effect of injections into PH and DMH on theta and hippocampal-sensitive behaviour (FI60 and open field ambulation). Systemic CDP (5mg/kg i.p.) released, but PH/CDP (20microg in 0.5microl vehicle) suppressed FI responding, though they both reduced FI theta frequency. In the open field, both CDP i.p. and PH/CDP reduced ambulation, but only the systemic injection reduced ambulation theta frequency. Taken together with previous research, these results support a role for PH in the control of voluntary behaviour. They imply that this function may be suppressed, independently of theta, by benzodiazepines. An anxiolytic effect of PH/CDP in FI60 may, therefore, have been masked by a concurrent action of CDP on the PH motor system. DMH/CDP did not affect behaviour or theta in either experiment, despite the fact that this nucleus is involved in benzodiazepine mediation of risk assessment and the flight response. This suggests that, like the control of theta frequency by the hypothalamus, the neural mechanisms underlying anxiety are distributed in complex networks.

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.

Multiple hypothalamic sites control the frequency of hippocampal theta rhythm

Stimulation of a neural pathway originating in the brainstem reticular formation, with synapses in the medial hypothalamus, activates the hippocampal theta rhythm. The frequency of reticular-elicited theta is determined in the medial supramammillary nucleus (mSuM) completely in anaesthetised rats, but only partially when the animal is awake. We tested other medial hypothalamic sites for their capacity to control theta frequency in awake rats. Blockade of sodium channels (1 microl fast infusion of the local anaesthetic procaine, experiment 1) or increased inhibition by GABA (Chlordiazepoxide [CDP], experiment 2) was found to reduce or increase the frequency of reticular-elicited theta, depending on the precise site of injection, in the region of the dorsomedial hypothalamic nucleus (DMH) and the posterior hypothalamic nucleus (PH). A band of null sites for CDP was located in the region of the ventral border of PH and dorsal border of mSuM. Using 0.5 and 1 microl CDP, and slow infusions (experiment 3), it was found that effective PH sites were also separate from mSuM in the rostrocaudal direction. In experiment 4, the DMH/PH region was mapped with unilateral and bilateral slow infusions of 0.5 microl CDP. CDP significantly reduced frequency in medial (periventricular) and dorsal PH, but not DMH. Bilateral injections appeared to generally sum the usual effects of unilateral injection, producing effects of intermediate size. However, the absolute frequency change in any given site, or with any pair of sites, did not exceed 1 Hz, which is similar to what is seen with single injections in mSuM. Overall, it appears that at, any one time, theta frequency may be determined by a complex interplay between distinct but interacting modulatory regions in the medial hypothalamus.

Benzodiazepine receptors in the medial-posterior hypothalamus mediate the reduction of hippocampal theta frequency by chlordiazepoxide

Separate sub-populations of benzodiazepine receptors are involved in determining hippocampal theta frequency in freely moving rats. Systemic injection of the benzodiazepine chlordiazepoxide (CDP) reduced the frequency of theta elicited by stimulation of the brainstem reticular formation, and theta recorded during fixed interval (FI) bar pressing. Infusion of the benzodiazepine receptor antagonist flumazenil into the supramammillary nucleus (mSuM) or the posterior hypothalamic nucleus (PH) antagonised CDP in both cases. Flumazenil was similarly effective in mSuM and PH, suggesting that these nuclei are jointly involved in the coding of theta frequency during FI and during high levels of reticular activation. However, benzodiazepine receptors in other brain regions probably participate in determining the frequency of theta at other times.

Displacement of the binding of 5-HT(1A) receptor ligands to pre- and postsynaptic receptors by (-)pindolol. A comparative study in rodent, primate and human brain

Using receptor autoradiography we examined the displacement of the binding of [(3)H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and [(3)H][N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)cy clohexanecarboxamide. 3HCl] (WAY 100635) to 5-HT(1A) receptors by (-)pindolol in the brain of four different species, rat, guinea pig, monkey and human. (-)Pindolol completely displaced the binding of both tritiated ligands at 10(-6) M in all species and regions examined. The affinity of (-)pindolol for presynaptic 5-HT(1A) receptors in the dorsal raphe nucleus was similar to that observed in postsynaptic locations, such as hippocampus (areas CA1, CA3 and dentate gyrus) or entorhinal cortex. Affinity values (K(i)) were in the range 3.8 - 15.9 nM for [(3)H]8-OH-DPAT and 5.8 - 22.3 nM for [(3)H]WAY 100635. In human brain, the K(i) values using [(3)H]8-OH-DPAT as ligand were 10.8 nM in the dorsal raphe nucleus and 6.5 - 13.5 in postsynaptic sites. The present data do not support the hypothesis that (-)pindolol may displace 5-HT(1A) ligands preferentially from presynaptic 5-HT(1A) receptors in the dorsal raphe nucleus, as suggested by electrophysiological evidence. The affinity of (-)pindolol for human 5-HT(1A) receptors is below the mean plasma concentration attained in depressed patients treated with a combination of fluoxetine and pindolol, which indirectly supports an action of pindolol at 5-HT(1A) receptors in these patients.

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)

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.

Intra-hippocampal injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibits partial and generalized seizures induced by kindling stimulation in cats

This study assessed the effects of microinjections of a serotonin (5-HT) 1A agonist, 8-OH-DPAT, into the feline hippocampus (HIP) on seizure response induced by electrical stimulation of this region. Intra-HIP injection of 8-OH-DPAT (10 nmol) produced a significant elevation in the afterdischarge threshold of partial HIP seizures, with a significant reduction in the duration of focal afterdischarge. Similarly, microinjection of 8-OH-DPAT at the same dose significantly raised the seizure threshold for eliciting generalized seizures in HIP-kindled cats. The present results demonstrate that focally applied 8-OH-DPAT possesses a potent anticonvulsant action, and provide further evidence for the inhibitory role of 5-HT1A receptors in the generation of HIP seizures.