The need for improved treatments for anxiety disorders has spawned numerous directions of research based on preclinical evidence and better understanding of the underlying causes of anxiety. Although there have been promising leads, too often those early promises do not come to fruition. Here, we set out three quite different approaches to the development of a safe and effective anxiolytic.

CRF receptor antagonists

Earlier, we described the important role of CRF in regulating the release of stress hormones by the HPA axis and its role as neurotransmitter in many brain regions of the emotion-processing circuits. Intracerebral injection of CRF causes signs of anxiety in a wide variety of animal tests. Exposure to stressful events increases CRF release in the amygdala along with increases in CRF mRNA. The anxiogenic (i.e., anxiety producing) effects accompanying alcohol withdrawal are reversed by CRF₁ antagonists in rodents. Further, altered levels of CRF in the CSF and dysfunction of CRF regulation of stress hormones are found in several anxiety disorders, including PTSD and panic disorder, and those abnormalities are reversed by effective treatment. Hence manipulating CRF function appears to provide a rational approach to treatment. Although there is strong preclinical evidence for the anxiolytic effects of CRF receptor antagonists in animal studies and many new CRF antagonists have been designed to improve their pharmacokinetic characteristics, solubility, and ability to penetrate the blood brain barrier, few have been clinically tested. Those trials that were completed were usually open-label (indicating that both researchers and participants know which treatment is being administered), and they used a limited number of subjects. Further, the dependent variables were focused primarily on symptoms of depression, with anxiety being of secondary consideration. In some cases, trials were terminated because of dangerous side effects. The most promising results showed that the CRF₁ receptor antagonist R121919 effectively reduced sleep disturbances and other anxiety related symptoms as well as decreasing symptoms of depression and suicidality. The effectiveness was considered to be equivalent to the antidepressant paroxetine. However because some individuals developed elevations in liver enzymes, further development was stopped (see Holsboer and Ising, 2008). At this point it is unclear whether CRF₁ receptor antagonists will be useful for treating individuals with particular anxiety disorders since there have been few trials looking at a single anxiety disorder. Unfortunately a multi-center, double-blind, placebo-controlled study using 260 patients with GAD found no significant difference in anxiety symptoms after 8 weeks with a CRF₁ antagonist (Pexacerfont) compared to placebo (Coric et al., 2010). Although these results are somewhat discouraging, further investment in developing new CRF antagonists is continuing.

Neuropeptide Y agonists

Neuropeptide Y is a 36-amino acid polypeptide that is found in many brain regions as well as being colocalized with NE in sympathetic neurons. It is the most abundant neuropeptide in the brain and its widespread distribution reflects its many functions including the modulation of emotional behavior. Significant concentrations of NPY are found in the frontal cortex as well as many limbic structures such as the amygdala, hypothalamus, hippocampus, and lateral septum. At present there are four known receptor subtypes (NPY₁, NPY₂, NPY₄, NPY₅) all of which are G protein–coupled.

Based on preclinical research, there is fairly consistent agreement that NPY has anxiolytic effects. Intracerebral administration of NPY into the central or basolateral nucleus of the amygdala, hippocampus, or lateral septum attenuates anxious behavior in animal testing. The anti-anxiety effects seem to rely on NPY₁ receptors since intra-amygdaloid injection of NPY or specific NPY₁ receptor agonists produces anxiolysis, but NPY₂ receptor agonists do not. Also, intra-amygdaloid blockade of the NPY1 receptor produces more anxious behavior in several rodent models. Further, knockout mice lacking the NPY₁ receptor show heightened anxiety. Of interest is the finding that NPY₂ agonists injected into the basolateral nucleus produce heightened anxiety while NPY₁ agonists have the predicted anti-anxiety effect. This result may reflect the fact that the NPY₂ receptor is an autoreceptor that when stimulated, causes a reduction in NPY release.

Consistent with the behavioral results, whole cell patch clamp electrophysiology has shown that pyramidal cells in the basolateral nucleus are hyperpolarized by NPY, an effect that is prevented by NPY₁ receptor antagonists. In the same tissue CRF, acting on CRF₁ receptors depolarizes the cells (Giesbrecht et al, 2010). It has been suggested that the two neuropeptides have opposing roles in regulating the excitatory output from the basolateral nucleus to brain regions that determine the nature of the emotional response. Pretreatment with NPY into the basolateral amygdala reduces the anxiogenic effect of activating CRF receptors in the same brain region, which shows a clear interaction between the two neuropeptides. An imbalance in the two neuropeptide systems could explain the development of mood disorders. (see Thorsell et al., 2010).

Evidence is more limited in human populations, although NPY levels are associated with greater psychological stress and are reduced in combat veterans with PTSD. The preclinical evidence suggests that clinical trials with mood disorder patients should proceed. However administration of neuropeptides is problematic because of unreliable penetration of the blood–brain barrier; innovative drug delivery strategies are needed to achieve significant bioavailability. In one ongoing study, intranasal application in healthy volunteers is being conducted to determine the extent of bioavailability using that route of administration. Subsequent research will examine the effect of intranasal NPY in patients with PTSD and panic disorder. A second possible approach is to develop nonpeptide ligands for the NPY₁ receptor that have better pharmacokinetic characteristics.

Glutamate antagonists

As you recall from Chapter 8, glutamate is the major excitatory transmitter and it is widely distributed in the nervous system. It may well be responsible for the excessive excitation of anxiety circuits that underlie anxiety disorders. Hence, it would seem reasonable to attempt to reduce glutamate activity to relieve anxiety symptoms. Unfortunately, although blocking glutamate ionotropic NMDA receptors produces anxiolytic effects in animals, use in humans is severely limited because NMDA antagonists produce significant adverse side effects including psychotic effects, memory dysfunction, ataxia, and neurodegeneration. Instead of targeting ionotropic receptors, the principal focus has been on selected metabotropic glutamate receptors (mGlu2 and mGlu3) because they are found in high concentration both pre- and postsynaptically in forebrain and limbic areas that modulate anxiety including the amygdala, hippocampus, and PFC. Those receptors on glutamate nerve terminals act as autoreceptors and reduce glutamate release, while others modify postsynaptic excitation by modulating ion channel function. mGlu heteroreceptors, which are located on the terminals of cells other than glutamatergic neurons, are responsible for reducing the release of several other neurotransmitters including GABA, DA, 5-HT, and others. Therefore, metabotropic glutamate receptors may reduce neuronal hyperexcitability by both direct and indirect means.

The prototypic mGlu2/3 receptor agonist LY354740 is selective for mGlu2 and mGlu3 receptors. It reduces fear-potentiated startle in both rodents and humans, increases exploration of the open arms of the elevated plus-maze, increases punished responding in conflict procedures like the water-lick suppression test, and reduces anxiety associated with drug withdrawal. Intra-amygdaloid injection reduced both acquisition and performance of fear-potentiated startle, an effect that could be blocked by system administration of a mGlu2/3 receptor antagonist. In addition, mGlu2/3 agonists also reduce concomitant excitation of the amygdala suggesting an important role for amygdaloid glutamate function in modulating anxiety. The finding that LY354740 failed to reduce anxiety in the elevated plus-maze when administered to both mGlu2 and mGlu3 knockout mice suggests that both receptors may be important for anxiolysis. Of clinical importance is the finding in animals that mGlu2/3 agonists do not produce ataxia, impair coordination, or potentiate the sedation produced by the barbiturate hexobarbital as do the BDZs. For more detail, the reader is directed to a review by Swanson and colleagues (2005).

As is often the case, results from human trials do not necessarily reflect what was found in preclinical studies. Initial clinical trials on patients with panic disorder did not show LY354740 to be effective in relieving symptoms. An important difference is that in rodent experiments administration is often intracerebral; when taken orally, as occurs with human patients, absorption from the gastrointestinal tract is poor, leading to low bioavailability. Modifying the molecule produced a derivative that showed a 10-fold increase in absorption. Disappointingly, the large clinical trial of the new agent was discontinued because rodent studies showed the development of seizures after chronic treatment with very high doses. Nevertheless, mGlu2/3 receptors are among the most promising new directions in drug development. Several modulators that potentiate the action of glutamate on the mGlu2/3 receptor by acting on a modulatory site on the receptor rather than directly at the glutamate site have been developed. In addition to Glu2 and Glu3, the other six glutamate metabotropic receptors have also been examined for anxiolytic effects and they are discussed elsewhere (Palucha and Pilc, 2007).

 

References

Coric, V., Feldman, H. H., Oren, D. A., Shekhar, A., Pultz, J., Dockens, R. C., Wu, X., Gentile, K. A., Huang, S. P., Emison, E., Delmonte, T., D’Souza, B. B., Zimbroff, D. L., Grebb, J. A., Goddard, A. W., and Stock, E. G. (2010). Multicenter, randomized, double-blind, active comparator and placebo-controlled trial of a corticotropin-releasing factor receptor-1 antagonist in generalized anxiety disorder. Depress. Anxiety, 27, 417–425.

Giesbrecht, C. J., Mackay, J. P., Silveira, H. B., Urban, J. H., and Colmers, W. F. (2010). Countervailing modulation of Ih by neuropeptide Y and corticotrophin-releasing factor in basolateral amygdala as a possible mechanism for their effects on stress-related behaviors. J. Neurosci., 30, 16970–16982.

Holsboer, F. and Ising, M. (2008). Central CRH system in depression and anxiety—evidence from clinical studies with CRH1 receptor antagonists. Eur. J. Pharmacol., 583, 350–357.

Palucha, A. and Pilc, A. (2007). Metabotropic glutamate receptor ligands as possible anxiolytic and antidepressant drugs. Pharmacol. Ther., 115, 116–147.

Swanson, C. J, Bures, M., Johnson, M. P., Linden, A. M., Monn, J. A., and Schoepp, D. D. (2005). Metabotropic glutamate receptors as novel targets for anxiety and stress disorders. Nat. Rev. Drug Discov., 4, 131–144.

Thorsell, A. (2010). Brain neuropeptide Y and corticotropin-releasing hormone in mediating stress and anxiety. Exp. Biol. Med., 235, 1163–1167.