The question of feedback at the somadendritic region and antidepressant drug action

Blocking neurogenic inflammation for the treatment of acute disorders of the central nervous system

Classical inflammation is a well-characterized secondary response to many acute disorders of the central nervous system. However, in recent years, the role of neurogenic inflammation in the pathogenesis of neurological diseases has gained increasing attention, with a particular focus on its effects on modulation of the blood-brain barrier BBB. The neuropeptide substance P has been shown to increase blood-brain barrier permeability following acute injury to the brain and is associated with marked cerebral edema. Its release has also been shown to modulate classical inflammation. Accordingly, blocking substance P NK1 receptors may provide a novel alternative treatment to ameliorate the deleterious effects of neurogenic inflammation in the central nervous system. The purpose of this paper is to provide an overview of the role of substance P and neurogenic inflammation in acute injury to the central nervous system following traumatic brain injury, spinal cord injury, stroke, and meningitis.

The role of substance p in ischaemic brain injury

Stroke is a leading cause of death, disability and dementia worldwide. Despite extensive pre-clinical investigation, few therapeutic treatment options are available to patients, meaning that death, severe disability and the requirement for long-term rehabilitation are common outcomes. Cell loss and tissue injury following stroke occurs through a number of diverse secondary injury pathways, whose delayed nature provides an opportunity for pharmacological intervention. Amongst these secondary injury factors, increased blood-brain barrier permeability and cerebral oedema are well-documented complications of cerebral ischaemia, whose severity has been shown to be associated with final outcome. Whilst the mechanisms of increased blood-brain barrier permeability and cerebral oedema are largely unknown, recent evidence suggests that the neuropeptide substance P (SP) plays a central role. The aim of this review is to examine the role of SP in ischaemic stroke and report on the potential utility of NK1 tachykinin receptor antagonists as therapeutic agents.

Social separation and diazepam withdrawal increase anxiety in the elevated plus-maze and serotonin turnover in the median raphe and hippocampus

The present work aimed to evaluate the effects of social separation for 14 days (chronic stress) and of withdrawal from a 14-day treatment with diazepam (acute stress) on the exploratory behaviour of male rats in the elevated plus-maze and on serotonin (5-hydroxytryptamine) turnover in different brain structures. Social separation had an anxiogenic effect, evidenced by fewer entries into, and less time spent on the open arms of the elevated plus-maze. Separation also selectively increased 5-hydroxytryptamine turnover in the hippocampus and median raphe nucleus. Diazepam withdrawal had a similar anxiogenic effect in grouped animals and increased 5-hydroxytryptamine turnover in the same brain structures. Chronic treatment with imipramine during the 14 days of separation prevented the behavioural and neurochemical changes caused by social separation. It is suggested that the increase in anxiety determined by both acute and chronic stress is mediated by the activation of the median raphe nucleus-hippocampal 5-hydroxytryptamine pathway.

5-HT1A receptors in the dorsal hippocampus mediate the anxiogenic effect induced by the stimulation of 5-HT neurons in the median raphe nucleus

We evaluated the involvement of dorsal hippocampus (DH) 5-HT1A receptors in the mediation of the behavioral effects caused by the pharmacological manipulation of 5-HT neurons in the median raphe nucleus (MRN). To this end, we used the rat elevated T-maze test of anxiety. The results showed that intra-DH injection of the 5-HT1A/7 agonist 8-OH-DPAT facilitated inhibitory avoidance, an anxiogenic effect, without affecting escape. Microinjection of the 5-HT1A antagonist WAY-100635 was ineffective. In the elevated T-maze, inhibitory avoidance and escape have been related to generalized anxiety and panic disorders, respectively. Intra-MRN administration of the excitatory amino acid kainic acid, which non-selectively stimulates 5-HT neurons in this brain area facilitated inhibitory avoidance and impaired escape performance, but also affected locomotion. Intra-MRN injection of WAY-100635, which has a disinhibitory effect on the activity of 5-HT neurons in this midbrain area, only facilitated inhibitory avoidance. Pre-administration of WAY-100635 into the DH blocked the behavioral effect of intra-MRN injection of WAY-100635, but not of kainic acid. These results indicate that DH 5-HT1A receptors mediate the anxiogenic effect induced by the selective stimulation of 5-HT neurons in the MRN.

Long-term citalopram maintenance in mice: selective reduction of alcohol-heightened aggression

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) alleviate many affective disturbances in human clinical populations and are used in animal models to study the influence of serotonin (5-HT) on aggressive behavior and impulsivity.

OBJECTIVE

We hypothesized that long-term SSRI treatment may reduce aggressive behavior escalated by alcohol consumption in mice. Therefore, aggression was tested in male CFW mice to determine whether repeated citalopram (CIT) administration reduces alcohol-heightened aggression.

MATERIALS AND METHODS

Resident male mice self-administered alcohol by performing an operant response on a panel placed in their home cage that delivered a 6% alcohol solution. Mice repeatedly confronted an intruder 15 min after self-administration of either 1 g/kg alcohol (EtOH) or water (H(2)O). Aggressive behaviors were higher in most mice when tests occurred after EtOH intake relative to H(2)O. Once baseline aggression was established, animals were injected (i.p.) twice daily with 10 mg/kg CIT or saline (SAL) for 32 days. Every 4 days throughout the CIT treatment period, aggressive encounters occurred 6 h after CIT injections, with testing conditions alternating between EtOH and H(2)O intake.

RESULTS

Aggression was only modestly affected by CIT in the first 2 weeks of treatment. However, by day 17 of CIT treatment, alcohol-heightened aggressive behavior was abolished, while baseline aggression remained stable. These data lend support for the role of the 5-HT transporter in the control of alcohol-related aggressive behavior, and the time course of effects suggests that a change in density of 5HT(1A) autoreceptors is necessary before antidepressant drugs produce beneficial outcomes.

Pharmacological profile of the "triple" monoamine neurotransmitter uptake inhibitor, DOV 102,677

1. The molecular and behavioral pharmacology of DOV 102,677 is characterized. 2. This characterization was performed using radioligand binding and neurotransmitter uptake assays targeting the monoamine neurotransmitter receptors. In addition, the effects of DOV 102,677 on extracellular neurotransmitter levels were investigated using in vivo microdialysis. Finally, the effects of DOV 102,677 in the forced swim test, locomotor function, and response to prepulse inhibition was investigated.3. DOV 102,677 is a novel, "triple" uptake inhibitor that suppresses [(3)H]dopamine (DA), [(3)H]norepinephrine (NE) and [(3)H]serotonin (5-HT) uptake by recombinant human transporters with IC(50) values of 129, 103 and 133 nM, respectively. Radioligand binding to the dopamine (DAT), norepinephrine (NET), and serotonin (SERT) transporters is inhibited with k (i) values of 222, 1030, and 740 nM, respectively. DOV 102,677 (20 mg/kg IP) increased extracellular levels of DA and 5-HT in the prefrontal cortex to 320 and 280% above baseline 100 min after administration. DA levels were stably increased for the duration (240 min) of the study, but serotonin levels declined to baseline by 200 min after administration. NE levels increased linearly to a maximum of 348% at 240 min post-dosing. Consistent with these increases in NE levels, the density of beta-adrenoceptors was selectively decreased in the cortex of rats treated with DOV 102,677 (20 mg/kg per day, PO, 35 days). 4. DOV 102,677 dose-dependently reduced the amount of time spent immobile by rats in the forced swim test, a model predictive of antidepressant activity, with a minimum effective dose (MED) of 20 mg/kg and a maximal efficacy comparable to imipramine. This decrease in immobility time did not appear to result from increased motor activity. Further, DOV 102,677 was as effective as methylphenidate in reducing the amplitude of the startle response in juvenile mice, without notably altering motor activity. 5. In summary, DOV 102,677 is an orally active, "balanced" inhibitor of DAT, NET and SERT with therapeutic versatility in treating neuropsychiatric disorders beyond depression.

A Mathematical Model for Paroxetine Antidepressant Effect Time Course and Its Interaction with Pindolol

Although selective 5-HT reuptake inhibitors (SSRIs) block monoamine uptake within hours of administration to patients, their full clinical effect does not appear until 2-4 weeks after treatment onset. Pindolol, a betablocker with weak partial 5-HT1A receptor agonist activity has been shown to produce a more rapid onset of antidepressant action of SSRIs. However, the optimal dosing schedule of pindolol remains controversial. Building on a set-point model described previously for the hypothermic effect of 5-HT agonists, we have developed a model based on the concept of homeostatic control mechanisms, in which SSRIs exert their antidepressant effect by increasing the transduction set-point of the postsynaptic 5-HT1A receptor, and pindolol exerts its effect by increasing the rate of feedback mechanisms. The predictive distribution of the proportion of responders at each day of measurement (based on population simulation from the model) was not significantly different from the proportions observed in two published clinical trials, one with fluoxetine, the other with paroxetine alone or combined with pindolol. The model was applied to the simulation of paroxetine response (clinical score) time course with or without pindolol, after administration of different doses of each drug. The simulated total scores on the MADR scale obtained after treatment with paroxetine alone (20 mg/day) or paroxetine (20 mg/day) with different doses of pindolol (1.5, 7.5 and 37.5 mg/day) support that the reason for inconstant pindolol efficacy is that the 7.5 mg dose is too low. The model might be useful as a basis for clinical trial simulation.

Differential autoinhibition of 5-hydroxytryptamine neurons by 5-hydroxytryptamine in the dorsal raphe nucleus

5-Hydroxytryptamine neurons in the dorsal raphe nucleus are under autoinhibitory control by endogenous 5-hydroxytryptamine. Tonic activation of 5-hydroxytryptamine 1A autoreceptors was demonstrated in awake animals, but was inconsistently observed in anaesthetized animals and slice preparations, leading to questioning of its physiological significance. We re-evaluated autoinhibition in single-unit recordings from deeply seated 5-hydroxytryptamine neurons in slices in which endogenous 5-hydroxytryptamine bioavailability was restored by supplementing its precursor L-tryptophan. In these conditions, the application of the neutral 5-hydroxytryptamine 1A receptor antagonist WAY-100635 markedly increased 5-hydroxytryptamine neuron firing. Responses to WAY-100635 in single experiments ranged from a lack of effect to a several-fold increase in firing rate, suggesting that 5-hydroxytryptamine neurons in the dorsal raphe nucleus represent a heterogeneous population regarding their susceptibility to autoinhibition by endogenous 5-hydroxytryptamine.

Serotonergic neurons in the median raphe nucleus regulate inhibitory avoidance but not escape behavior in the rat elevated T-maze test of anxiety

RATIONALE

A wealth of evidence supports the involvement of the serotonergic neurons of the median raphe nucleus (MRN) in anxiety. However, it is presently unclear whether serotonergic pathways arising from this nucleus play distinguishing regulatory roles in defensive behaviors that have been associated with specific subtypes of anxiety disorders.

OBJECTIVES

To evaluate the role of the MRN serotonergic neurons in the regulation of two defensive behaviors, inhibitory avoidance and escape, which have been related, respectively, to generalized anxiety and panic disorders.

METHODS

Male Wistar rats were submitted to the elevated T-maze test of anxiety after intra-MRN administration of drugs that either non-selectively or selectively change the activity of the serotonergic neurons.

RESULTS

Intra-MRN injection of FG 7142 (0.04 and 0.08 nmol) and kainic acid (0.03 and 0.06 nmol), drugs that non-selectively stimulate the MRN serotonergic neurons, facilitated inhibitory avoidance acquisition, but impaired escape performance. Microinjection of muscimol (0.11 and 0.22 nmol), a compound that non-selectively inhibits the activity of the MRN serotonergic neurons, impaired inhibitory avoidance and facilitated escape performance. Both kainic acid and muscimol also changed rat locomotion in the open-field test. Intra-MRN injection of 8-OH-DPAT (0.6-15 nmol) and WAY-100635 (0.18-0.74 nmol), respectively an agonist and an antagonist of somatodendritic 5-HT(1A) receptors located on serotonergic neurons of the MRN, only affected inhibitory avoidance-while the former inhibited the acquisition of this behavior, the latter facilitated it.

CONCLUSION

MRN serotonergic neurons seem to be selectively involved in the regulation of inhibitory avoidance in the elevated T-maze. This result supports the proposal that 5-HT pathways departing from this nucleus play an important role in anxiety processing, with implications for pathologies such as generalized anxiety disorder.

Decrements in per pulse release of norepinephrine, antagonist potentiation of release and presynaptic receptor theory

In recent decades the theory that amine transmitter release at nerve terminals is routinely regulated through negative feedback systems sensing and responding to the instantaneous perineuronal concentration of previously liberated transmitter has assumed pre-eminence. However, observations indicate a major drop off in per pulse transmitter release when only two or four stimulation pulses are administered, reflecting the unexpectedly prompt operation of feedback inhibition. We explored this quandary in our understanding of control of transmitter release by axonal depolarization versus terminal feedback using isotopic norepinephrine and in vitro slices of rabbit hippocampus. A technique of continuous collection of superfusate over a 30min cycle of stimulation utilizing a wide range of intervals between individual stimulation pulses was used. Following simulation with single pulses even 60s apart or pseudo one pulses 150s apart a marked decline in per pulse release was noted. The deficits in per pulse release were not related to the number of pulse delivered at any time over the course of a 30min stimulation period. The pulse decrements were independent of the activity of neuronal uptake and of superfusion flow rate or even individual pulse duration and frequency. Presynaptic receptor antagonists, potentiated efflux near maximally with the second of only two pulses. Potentiations were independent of pulse number, pulse duration, or frequency. No linkage between perineuronal transmitter concentrations and the antagonist potentiation of release was found. However, the decrements in per pulse release with multiple pulses and the potentiation by alpha presynaptic antagonists occurred under the same test conditions. We conclude that the pulse deficit can be looked at as largely attributable to an enhanced efflux with delivery of the first pulse in a train of pulses, rather then to a pattern of progressively declining efflux linked to increasing extracellular transmitter levels as frequency and pulse number increase.

Amine neurochemistry and aggression in crayfish

A primary goal of our research is to explore proximate mechanisms important in recruiting adaptive social behaviors. For instance, if one of three different behaviors may be expressed in a particular set of circumstances, how do neurochemical mechanisms bias behavior towards the expression of one act in lieu of the other possibilities? In this article, we review recent results suggesting that serotonin may play such a role in the control of aggression in crayfish. First, we summarize techniques that have been optimized for sensitive characterization of neurochemical profiles in crayfish. Then, borrowing concepts from behavioral ecology, we review a framework for quantitative investigation, which regards behavior as a set of individual decisions, each with a particular probability for occurrence, a motivational context, and controlled by its own distinct neurochemical mechanisms.

Neurotransmitter release in an arterial preparation and the action of alpha-adrenoceptor antagonists

1 This study examined the potentiating effects of competitive antagonists of the adrenergic alpha2 receptors and of phenoxybenzamine (POB) an irreversible antagonist, on the stimulation-induced efflux of [3H]-noradrenaline in arterial tissue of rabbit. This was done to determine if the lack of concordance of efflux potentiation by antagonists with the expectations of presynaptic negative feedback theory can be attributed to increasingly successful competition from rising perineuronal transmitter concentrations, when stimulation parameters are increased, in the presence of a fixed concentration of competitive antagonist. 2 Tissues were stimulated with a fixed pulse number and frequency, to rule out confounding factors, and major alterations in the concentration of released transmitter were achieved through variations in the pulse duration. 3 Rauwolscine potentiated transmitter release less at the longest, rather than at the shortest pulse duration, and showed a potentiation of release that was indifferent to the quantities of released transmitter. This was also seen with POB although it binds covalently to the presynaptic receptor. 4 Noradrenaline inhibited stimulation-induced transmitter release confirming the presence of presynaptic alpha inhibitory sites. 5 Yohimbine potentiated transmitter release the same as did rauwolscine and POB, and protected the relevant sites against POB potentiation, confirming site identity. The combination of POB and rauwolscine had no greater effect than did either alone certifying that they acted similarly and that maximally effective concentrations of each were used. 6 Consequently, noradrenaline breakthrough of presynaptic receptor blockade does not explain the non-conforming observations made with competitive antagonists in tests of presynaptic theory.

The medical benefit of 5-HT research

5-HT research is now more than 50 years old, and it has generated a wealth of therapeutic agents, some of which have had a major impact on disease management. The 5-HT reuptake inhibitors (SSRIs) are among the most widely prescribed drugs for treating depression and a variety of other disorders including anxiety, social phobia and premenstrual dysphoria (PMD). The other major success stories of 5-HT research are the discovery of 5-HT1B/D receptor agonists for treating migraine and 5-HT3 receptor antagonists for chemotherapy and radiation-induced emesis. The role of 5-HT in the mechanism of action of antipsychotic agents remains a topic of intense research, which promises better treatments for schizophrenia in the future. Compounds interacting with 5-HT1F, 5-HT2C, 5-HT6 and 5-HT7 receptors are currently under investigation and may prove to have important therapeutic applications in the future.

Autoreceptors do not regulate routinely neurotransmitter release: focus on adrenergic systems

The theory that neurotransmitter release is regulated locally at the individual terminals of neurons has achieved a rapid and seemingly secure status in our understanding of neuronal function both in the periphery and in the central nervous system. This concept of negative feedback control through the monitoring of the perineuronal concentration of previously released transmitter has been extended to a multiplicity of transmitters and utilized to explain the mechanisms of action of diverse classes of drugs, ranging from antihypertensives to antidepressants. It is my view that negative feedback by terminal and by somadendritic receptors cannot account for the existing body of experimental work. Analyses of the profiles of action of agonists and antagonists, and of the per pulse release of transmitter in the absence of drugs in a variety if peripheral organ systems, as well as in superfused brain slices, demonstrates the need for alternate interpretations of the available data. Evidence is provided that the actions of agonists to inhibit transmitter release and that of antagonists to enhance release occur at different cellular loci and that the purported unitary action of these two classes that is so central to the validity of presynaptic theory is unsupportable.

Rate-independent inhibition by norepinephrine of 5-HT release from the somadendritic region of serotonergic neurons

Endogenous adrenergic drive regulates the firing rate of serotonergic neurons. However, advocates of feedback theory assert that 5-hydroxytryptamine (5-HT) released in the somatodendritic region of raphe neurons regulates both rate and release of 5-HT. Experiments were done to determine if the somatodendritic region might have receptors for norepinephrine that inhibit release of 5-HT independently of rate, as this would allow for discrete effects of norepinephrine on rate and release, even in the presence of functional feedback by 5-HT. The release of 5-HT at fixed frequencies of stimulation was substantially reduced when norepinephrine (1 and 3 x 10(-7) M) was present. Norepinephrine also inhibited the release of 3H-5-HT with delivery of a single stimulation pulse ruling out a remote action of the catecholamine. The alpha(1) antagonist prazosin did not modify the profile of norepinephrine inhibition. Further, the alpha(1) agonist phenylephrine had no effect on 3H-5-HT efflux. The alpha(2) antagonist yohimbine antagonized almost entirely the inhibition by norepinephrine at 1 Hz, and reduced it substantially at 3 Hz. Blockade of 5-HT(1) receptor sites with methiothepin did not reduce the inhibitory effect of norepinephrine on 3H-5-HT efflux. It is proposed that release of endogenous norepinephrine at synapses with 5-HT neurons could activate 5-HT neuron firing rate through alpha(1) receptors located at the soma and simultaneously short-circuit ongoing 5-HT feedback inhibition by inhibiting release through adrenergic alpha two receptors likely located at the dendrites.

Regarding the unitary theory of agonist and antagonist action at presynaptic adrenoceptors

1. The linkage between potentiation of field stimulation-induced noradrenaline release and blockade of the presynaptic inhibitory effect of exogenous noradrenaline by a presynaptic antagonist was examined in superfused rabbit aorta preparations. 2. Rauwolscine clearly potentiated the release of noradrenaline in response to 100 pulses at 2 Hz but reduced the capacity of noradrenaline to inhibit transmitter release to a questionable extent, and then only when comparisons were made with untreated, rather then to rauwolscine-treated, controls. 3. Aortic preparations exposed for 60 min to rauwolscine followed by superfusion with antagonist-free Krebs for 60 min retained the potentiation of stimulation-induced transmitter release but no antagonism of the noradrenaline-induced inhibition could be detected at either of two noradrenaline concentrations when comparisons were made with rauwolscine treated controls. 4. Comparisons of the inhibitory effect of exogenous noradrenaline (1.8 x 10-6 M) on transmitter efflux in the presence and absence of rauwolscine pretreatment revealed that the antagonist enhanced rather than antagonized the presynaptic inhibition by noradrenaline. 5 It is concluded that the unitary hypothesis that asserts that antagonist enhancement of transmitter release and its blockade of noradrenaline induced inhibition are manifestations of a unitary event are not supportable.

Pharmacology of antidepressants

Presently in the United States, 21 compounds have been approved by the Food and Drug Administration as antidepressants. Two additional drugs marketed outside the United States as antidepressants have been approved for obsessive-compulsive disorder. Nearly one half of all these compounds became available within the past 12 years, whereas the first antidepressant was available more than 40 years ago. After the clinical aspects of depression are introduced in this article, the pharmacology of the newer generation drugs is reviewed in relationship to the older compounds. The information in this review will help clinicians treat acute depression with pharmacological agents.

Current perspectives on the development of non-biogenic amine-based antidepressants

Compounds that inhibit the re-uptake and/or metabolism of biogenic amines (i.e. serotonin, norepinephrine, and dopamine) have been used to treat depression for more than 40 years. Selective re-uptake inhibitors, currently the most widely prescribed class of biogenic amine-based agents, are certainly safe and relatively easy to use, but do not exhibit either a faster onset of action or greater efficacy than their predecessors. An approach to overcome the limitations that may be inherent to these 'conventional' therapies is to circumvent the monoaminergic synapse. In this review, two potential antidepressant strategies are discussed that may converge with intracellular pathways impacted by chronic treatment with biogenic amine-based agents. Drugs emerging from these strategies may offer significant advantages over currently used antidepressants.

Autoregulation of neurotransmitter release at autonomic nerve terminals: a questionable theory

1. The evidence for feedback regulation of neurotransmitter release by means of autoreceptors is questioned. 2. Autoreceptor function must meet the expectations for feedback loops. However, this concordance has not been observed in most neuroeffector systems. 3. The characteristics of per pulse transmitter release with changes in the parameters of stimulation in several autonomic systems do not support the ongoing operation of negative feedback loop mediated by locally released transmitter. Also, the effects of antagonists and agonists often do not comply with feedback expectations. 4. Evidence is provided that agonists and antagonists act at different loci to achieve their inhibiting and potentiating effects on transmitter release. 5. Future efforts should be directed to exploring the mechanism(s) of antagonist action and to a system-by-system analysis of the evidence for and against autoreceptor operation.