Understanding Substance P

The substance P receptor is a G protein-coupled receptor, in many respects similar to other well-studied receptors in psychiatry, particularly monoamine receptors. As we learn more about neurotransmitters and their receptors in the brain, one of the basic assumptions we have made has been challenged -- that the receptors for neurotransmitters exist primarily, if not exclusively, within the synapse. Recently, this has been discovered to not be true, and neurons can have receptors on membranes throughout their cell bodies and dendrites. This is particularly true for the substance P receptor, which exists all over the cell body of its neurons.

Substance P is 1 of a group of neuropeptides called tachykinins. There are 3 known members of this group: substance P, neurokinin (NK) A, and NK B. The receptors for these 3 neuropeptides are called NK1, NK2, and NK3, respectively. Thus, when the substance P receptor is discussed, it refers to the NK1 receptor. The NK2 receptor has yet to be found in brain tissue, and the NK3 receptor has some affinity for substance P, but approximately 30 orders of magnitude less.

Because the substance P receptor is a prototypical G protein-coupled receptor, it has many similarities to other receptors in terms of its protein sequence. It was only when antibodies were developed to a unique area in its structure that we were able to learn a great deal about its location (eg, finding it all over neuron bodies). It has also been discovered through the use of labeled antibodies that the substance P receptor (and other similar kinds of receptors, such as those from monoamines) is often internalized into the cell via a budding process in which a lysosome is formed, the neurotransmitter is degraded, and the membrane fragment is recycled back into the cell membrane.

Substance P is not only present in brain, it is also present in spinal cord tissue. It was in this area that the issue of how far substance P can diffuse to different layers of neurons was examined. It has been recently discovered that neurotransmitters not only affect those cells across the synapse into which they are released, but can diffuse to surrounding neurons and bind to receptors on those cells. Brown and colleagues^[1] looked at how far substance P diffuses through rat spinal tissue, and found that in a normal resting state the neurotransmitter only diffuses through 2 or 3 levels of cells. They then created an inflamed condition upstream from the area of the spine that they examined and discovered that an inflammatory state increases both the amount of substance P and the number of substance P receptors. They also found that in this condition, substance P is able diffuse a full 5 layers.

These authors were also interested in the effects of substance P in the amygdala. Approximately 10% of all the neurons in the amygdala are substance P neurons. The authors created rats with no functional substance P receptors in their amygdala by binding substance P with a compound known to be toxic to these neurons and injecting it directly into the rats' amygdala. This technique allows researchers to dissect out specific neurons in animal models and literally change the animals' phenotype. When the rats had their substance P neurons chemically removed from their amygdala, it had very little effect on their locomotion, but they behaved significantly less anxiously in controlled laboratory tests.

NK1 Receptors and PET Imaging

Visualizing and quantifying the NK1 receptors in the human brain through PET imaging has allowed scientists to understand the impact of these receptors in the brain. There are recent developments^[2] in the understanding of a high-affinity radiolabeled ligand that binds to substance P receptors and the validation of this ligand through various studies. Data from postmortem studies and animal models -- particularly the guinea pig, which shows significant similarity to humans in this regard -- show substance P receptors primarily in the limbic system, with particularly high concentrations in the striatum, caudate, putamen, amygdala, brain stem, and in the solitary nucleus. Positron emission tomography (PET) scan studies reinforce this. The ligand was also shown to be a valid indicator of substance P receptors when PET scans were performed both before and after the administration of a substance P antagonist. The ligand showed very little binding after the administration of the antagonist, showing that the receptors to which it would normally bind were blocked. Although humans do show some binding in visual cortex, there is essentially no substance P binding in the cerebellum. Thus, a 3-compartment model of ligand can be developed, with the tracer either in the blood, nonspecifically present but not bound in the brain (eg, in the cerebellum) and bound to substance P receptors. This radioligand tracer shows great potential in the further study of the role and activity of substance P in the brain.

Evidence for Antidepressant and Anxiolytic Activity of Substance P Antagonists

Although substance P was first discovered in 1931, it is only in the last 10 years that we have had nonpeptide antagonists of its receptor to study.^[3] The original studies with substance P in pharmacology were mostly focused on pain and analgesia. Later studies looked at emesis, and finally at depression and anxiety. Preclinical substance P animal studies are somewhat complicated by the fact that most of the substance P antagonist compounds under investigation had very different affinities for receptors in the brains of rats (the animal most commonly used for these kinds of studies) than those in humans. As a result, these compounds require different animal pharmacologic models. The development of a strain of "knockout" mice without NK1 receptors has been helpful to these studies. Still, each new compound requires a species with receptor affinities similar to those in to humans in order to do preclinical studies.

The original application of nonpeptide antagonist substance P compounds was pain and analgesia. It was felt that substance P antagonists held great promise as pain medications because it was well documented that substance P was released in pain states and inflammation, and its neurons seem to follow pain tracts in afferent fibers. Although many preclinical studies were positive and encouraging, clinical trials of substance P antagonists to treat pain have been very disappointing.

The best-studied substance P antagonist is called MK 869. This medication has proven to be very effective in the treatment of emesis, particularly when it results from cancer treatment. This was studied because substance P has been found in emetic centers in the brain stem, and intravenous infusions of substance P often cause vomiting. Studies have shown that substance P antagonists are effective in the treatment of emesis from acute and delayed side effects of cancer treatments.

Substance P causes a "fight or flight" response, and there is evidence of substance P antagonists blocking this stress response via blockade of substance P receptors in the amygdala. There are multiple animal models providing evidence for this. Guinea pig pups that are separated from their mothers make vocalizations that seem to result from increased substance P released in their internal amygdala. Substance P antagonists inhibit these vocalizations. More direct evidence has come from cats who manifest rage behavior when their medial hypothalamus is stimulated. The medial hypothalamus has direct projections to the medial amygdala. Substance P antagonists as well as antidepressants block this behavior. Similar effects have been noted in hamsters with forced intruders in their cages and in mice forced to swim. There appears to be no direct interaction between substance P antagonists and antidepressants; substance P antagonists seem to work at sites unrelated to monoamines.

Other areas of the brain that have been implicated in substance P activity are the dorsal raphe nucleus and an area of the thalamus called the habenula, which has the highest density of substance P receptors. The habenula inhibits firing of the dorsal raphe nucleus. The dorsal raphe consists of approximately 50% serotonin neurons and 50% substance P neurons. Rats dosed with fluoxetine increase firing of both kinds of neurons, but substance P antagonists only increase substance P activity. When both drugs are given together, the substance P antagonist modulates the effects of the fluoxetine, increasing the response in both kinds of neurons.

Tyrosine hydroxylase, the rate-limiting step in the synthesis of norepinephrine, has been looked at in postmortem brains and was found to be increased in chronic stressful situations and decreased with the chronic administration of antidepressant medication. Substance P antagonists are known to reduce the stress response, and in those knockout mice bred without a NK1 receptor, there is considerably less tyrosine hydroxylase found by immunoreactivity.

The Efficacy and Safety of MK-869, an NK-1 Antagonist, in Patients With Major Depressive Disorder

Substance P is found mostly in peripheral afferent nerve fibers and in the gastrointestinal system. It is thought to be the primary neurotransmitter for nociceptive information. Because of this, substance P antagonists were first thought to be useful in the treatment of pain conditions. MK 869 was discovered by Merck Research Laboratories as the first nonpeptide substance P antagonist with possible human utility. MK 869 has a half-life of approximately 15 hours.^[4] It was investigated in the treatment of dental pain, osteoarthritis, neuropathic pain, and migraine, all with negative results. MK 869 was then studied as a treatment for emesis, and was shown to be effective in combination with the usual double treatment of dexamethasone and ondansetron.

Cutler and colleagues^[5] studied MK 869 for depression and anxiety in a 6-week, randomized, double-blind trial with 3 arms, each with 70 patients. Patients in the first arm were given placebo, the second MK 869 300 mg/day , and the third used paroxetine 20 mg. Patients needed to have a Hamilton Depression Scale rating above 22 and Hamilton Anxiety Scale rating of above 15. The study showed that MK 869 was equal in efficacy to paroxetine and superior to placebo on both scales. The time course for recovery was similar with both active drugs. There was only a 9% rate of adverse events with MK 869, compared with a 19% rate with paroxetine. Of particular note was the rate of sexual dysfunction: only 3% of patients reported sexual dysfunction with MK 869, compared with 26% with paroxetine. Since MK 869 seemed to be effective and had a better side-effect profile than established antidepressants, there were possibilities not only for monotherapy but for combination therapy with other antidepressants. He also commented that the drug may possibly be effective in the treatment of other psychiatric illnesses, such as panic disorder, obsessive-compulsive disorder, generalized anxiety disorder, and posttraumatic stress disorder. He noted that the phase 2 dosing study that was done subsequent to this study was a failure due to a very high placebo effect, but his group remains optimistic about the future of this and similar compounds.

References

1. Mantyh P. Understanding substance P and the substance P receptor. Presented at the XXIInd Congress of the Collegium Internationale Neuro-Psychopharmacologicum (CINP); July 10, 2000; Brussels, Belgium. Abstract S.51.
2. Brown JL, Liu H, Maggio JE, Vigna SR, Mantyh PW, Basbaum AI. Morphological characterization of substance P receptor-immunoreactive neurons in the rat spinal cord and trigeminal nucleus caudalis. J Comp Neurol. 1995;356:327-344.
3. Hietala J, Sciberras D, Goldberg M, et al. PET imaging of NK1 receptors in living human brain. Int J Neuropsychopharmacol. 2000;3(suppl 1):S6. Abstract S.05.2.
4. Rupniak NMJ, Carlson E, Smith D, et al. Evidence for antidepressant and anxiolytic activity of substance P antagonists in preclinical assays. Int J Neuropsychopharmacol. 2000;3(suppl 1):S6. Abstract S.05.3.
5. Cutler NR, Kramer MS, Reines SA, Sramek JJ. Single site results from a multicenter study of efficacy and safety of MK-869, an NK-1 antagonist, in patients with major depressive disorder. Int J Neuropsychopharmacol. 2000;3(suppl 1):S7. Abstract S.05.4.