Several columns ago (“Schizophrenia, DISC1, and Animal Models,” Psychiatric Times, April 2008, page 22), I earnestly cautioned against the temptation to apply behavioral data from laboratory animals directly to the human experience.
I noted that the human cortex is the size of a baby blanket, whereas the mouse cortex is the size of a postage stamp. I explained that animal research historically works best when it acts as a guiding “flashlight” for human research, illuminating biological processes in which human-based investigations might reasonably succeed. I gave one example of such
research that explored the role of the “disrupted in schizophrenia 1” (DISC1) gene in schizophrenia and promised to give more. The subject of this column is a payment on that promise.
This month I will examine the relationship between alcohol use disorder, stress, and a neuropeptide called substance P (SP). The data that led directly to research with human subjects came from the mouse-based genetic manipulation of a gene called neurokinin-1 receptor (NK1R), the receptor for SP. To understand this research thread, I will need to review some basic biology behind a class of biochemicals called tachykinins, of which SP is its most famous member. I begin, however, with an attempt to understand the relationship between the experience of stress, relapse rates in alcohol-dependent populations, and how mouse research ended up helping a cohort of stressed-out patients.
Stress, relapse, and alcohol dependence
Of the many frustrating aspects of treating patients with alcohol dependence, the uncomfortably high relapse rate must rank highest on the list. There are many reasons for relapse, which can roughly be categorized into extrinsic and intrinsic trigger points. One of the best-characterized extrinsic triggers of relapse is environmental alcohol-associated cues. One of the best characterized intrinsic triggers is the patient’s experience of stress—especially if the patient is in a stress-susceptible population.
The relationship between stress and alcohol dependence has been studied extensively in animal models. Increases in alcohol consumption due to laboratory-induced alcohol dependence are always accompanied by an increased sensitivity to environmental stressors. The relationship has been characterized biochemically. Alcohol dependence can induce changes in the extrahypothalamic regions of the brain by a specific up-regulation of corticotropin-releasing hormone (CRH). The hypothalamus synthesizes many neuroactive molecules in response to aversive stimuli, including vasopressin and CRH. CRH is sent to the pituitary, activating downstream signals that eventually result in the arousal of the hypothalamic-pituitary-adrenal axis.
The relationship between stress and alcohol dependence is so strong that the neural systems mediating stress have been investigated as possible pharmacological targets. One promising lead involves the biochemistry behind SP, the neurotransmitter known to be involved in stress responses and drug reward. Another lead involves the investigation of the role the insular cortex (insula) plays in mediating these associating behaviors. The insula is a region that lies just beneath the central sulcus, under the operculi. This region has been shown to be involved in the subjective experience of cravings of many kinds and has been recently shown to be involved in the maintenance of addictive behaviors. Research in these systems, including ligand-receptor interactions and activation of the insula, form the bulk of this column.
SP is the most important member of a family of excitatory neuropeptides collectively called tachykinins (Figure). Synthesized by glial and neuronal cells in the central and peripheral nervous system, SP binds to the receptor NK1R. Together, both ligand and receptor are expressed in the hypothalamus, amygdala, and nucleus accumbens. That expression profile turns out to be important because these tissues are not only involved in mediating stress but they are also involved in mediating many rewarding responses related to addiction-forming behaviors.
Studies have shown just how powerful a role this system plays in stress responses—at least in mice. The researchers employed a technique known as genetic knockouts, a genetic engineering technology that deletes a specific gene early in development but does not otherwise impede normal growth. Mice can be created that are either heterozygous for the knocked-out gene (only one of the pair are affected) or homozygous. If the gene for the SP receptor is knocked out in mice, their responses to stress are greatly diminished. These data were confirmed with the development and administration of specific antagonists to the receptor (pharmacologically blocking the ability of SP to exert its effects on nerve cells). Using genetically unmodified animals, the same stress-dampening behaviors were observed when compared with controls.
Clearly, the responses to stress were being affected by interfering with SP-mediated biochemistries. What about rewarding behaviors in the genetically modified animals? Consistent with SP’s job description, these knockout mice demonstrated noticeably reduced rewarding behaviors in response to various drugs. The mice exhibited greatly inhibited response curves in opiate self-administration tests, for example. They also demonstrated a loss of conditioned place preference for opiates. Not only were these animals not responding to aversive stimuli, they were also not responding to normal rewarding cues.
Do these data indicate anything about alcohol-related behaviors? To conduct these experiments thoroughly, one would need to induce—or at least attempt to induce—alcohol-dependent behaviors in the genetically modified mice. There are established protocols for creating such behaviors in wild-type animals. What would happen if these wild types were compared with the genetically manipulated mice in standard alcohol-administration experiments?
The short answer is that the altered mice did not behave at all like the unmodified controls. Although the knockout mice had constant availability to alcohol, as did the controls, they displayed remarkably lower consumption levels. This diminution was shown even under conditions in which the concentrations of alcohol were high (3% to 15%). In these conditions, the pharmacological effects of ingesting alcohol typically trump other consumptive motivations, such as its taste. The animals were not buying it. They never established an alcohol dependency.
Interestingly, these reduced behaviors only showed up in the presence of the homozygous knockouts and not in the heterozygous group. If just one receptor gene was still active, the alcohol-associated behaviors typically observed in the wild-type animals returned with a vengeance in the heterozygous knockouts. Not only did these results serve as a convenient internal control, they also showed the great responsiveness of the system to alcohol.