Charles E. Schmidt College of Medicine
Florida Atlantic University
777 Glades Road
Boca Raton, FL 33431
For the past 30 years Dr. Lawrence Toll’s laboratory has studied neurotransmitter receptors and neuropeptides in the brain, focusing primarily on opioid and nicotinic systems. Dr. Toll’s research on opioid receptors has encompassed collaborations with medicinal and theoretic chemists to identify properties leading to abuse liability, as well as the synthesis and characterization of non-addicting analgesics. In 1995, Dr. Toll was involved in the discovery of “nociceptin”, the endogenous ligand for the NOP receptor, the fourth member of the opioid receptor family. This discovery has led to detailed studies of the NOP/nociceptin system and the investigation into the involvement of this system in both pain and reward. Dr. Toll pioneered the idea of a NOP/mu agonist as a potential analgesic with low abuse potential. More recent studies using a transgenic knock-in mouse with green fluorescent protein attached to the NOP receptor has permitted the investigation into changes in receptor level due chronic pain and the potential of NOP receptor agonists for treatment of chronic pain and as drug abuse medications. With respect to nicotinic acetylcholine receptors, Dr. Toll has developed selective ligands as potential smoking cessation medications. To accomplish these interdisciplinary goals, Dr. Toll’s laboratory has employed a variety of in vitro receptor binding and functional, behavioral, molecular biological, and imaging techniques. Dr. Toll’s research has been continually funded by the National Institute on Drug Abuse since 1988.
The NOP system in pain and drug abuse. The NOP receptor is the fourth member of the opioid receptor family. This receptor and its natural ligand nociceptin (also called N/OFQ) are found in many brain regions, the spinal cord, and many peripheral organs. In particular, this receptor system is highly expressed in pain pathways and brain regions involved in drug abuse. Over the past few years, we have worked with NOP-eGFP mice. These knock-in mice have a NOP receptor with an attached fluorescent protein (eGFP) attached to the C-terminal of the receptor. We have demonstrated that these receptors function like the wild type receptor but allow us to determine receptor location in the brain, spinal cord and peripheral tissues including dorsal root ganglia (DRG). We have determined that receptor levels decrease in certain brain regions, the spinal cord and DRG after chronic pain induced by spinal nerve ligation (SNL). We are now examining relationship between chronic pain and NOP receptor changes and determining how well NOP receptor agonists treat chronic pain symptoms, and on which symptoms (mechanical pain versus heat pain) agonists are most efficacious. These transgenic mice will also allow us to better understand the changes taking place in the body when exposed to chronic pain or to abused drugs to help in the development of novel medications for chronic pain and drug abuse. Experiments conducted in our lab, and others, have also demonstrated that nociceptin and small molecule NOP receptor agonists block the reward induced by morphine, cocaine, methamphetamine and alcohol, as determined by the conditioned place preference paradigm. However, we have now demonstrated that NOP receptor antagonists rather than agonists are effective for blocking drug seeking when using a different drug abuse paradigm, self-administration. We have proven this to be the case for self-administration of both alcohol and nicotine, but not cocaine. We are investigating this surprising dichotomy by examining the actions of NOP receptor antagonists on all aspects of self-administration, acquisition, maintenance, and relapse. The point of all of these studies is the development of medications for pain and drug abuse.
The acetylcholine receptor and drug abuse. Smoking is a huge problem in the US and worldwide, and is the primary cause of preventable death in the world. The ingredient in cigarettes that leads to addiction is nicotine. Nicotine binds to and activates a subtype of acetylcholine receptor, nicotinic acetylcholine receptors (nAChR), of which there are many subtypes. In collaboration with medicinal chemists from Torrey Pines and elsewhere, we have developed antagonists of two types of nAChR, called a4B2, and a3B4 nAChR. Although the a4B2 nAChR is by far the most prevalent in the brain, evidence by our group, and others, suggests that inhibitors of the a3B4 nAChR or the a4B2 nAChR may be sufficient to block drug seeking behavior not only of nicotine (in other words smoking), and potentially cocaine, morphine, alcohol, and other drugs of abuse as well. Using in vitro receptor binding and measuring functional activity in cells possessing nAChR we have identified high affinity and selective ligands for both of these nAChR subtypes. Furthermore, we have tested selective compounds in vivo using the self-administration paradigm in rats. Interestingly, the compounds selective for the a3B4 nAChR can block only nicotine self-administration, but the compound selective for the a4B2 nAChR can block self-administration of both nicotine and alcohol. We have also examined the comorbidity of smoking and chronic pain and we are asking the question why do people in pain smoke more? Preliminary data has demonstrated that nAChR subtypes levels change in DRG and brain regions after SNL-induced chronic pain, as does the sensitivity to nicotine in these animals. We are now pursuing our lead a4B2 nAChR antagonists to determine if they can be effective as smoking cessation medications, or medications for both smoking and alcohol abuse. This is particularly important because of the prevalence of this co-addiction. Ultimately, we hope to translate our basic research and our novel compounds into drug abuse medications.
NPQ-GALR3 system in pain: NPQ is a novel endogenous neuropeptide that we discovered based upon novel computational methodologies. We have determined that this peptide is involved in diverse biological actions including the regulation of micturition and antinociceptive activity. Recent literature has reported that NPQ peptide activates two G-protein coupled receptors, GALR2 and GALR3 formally known to be as the receptors for galanin. The identification of a receptor by which NPQ/spexin mediates its actions greatly facilitates the investigation of the properties of these peptides and allows the overall hypothesis that NPQ, rather than galanin is the major neuropeptide that mediates the actions of the GALR3, and that through the GALR3, NPQ has a role in acute, chronic, and inflammatory pain. Our goal is to understand whether NPQ peptides and their potential receptor, GALR3, are actually localized in the regions related to the pain management. So far, recent immunohistochemical experiments with antisera to NPQ and NPQ 53-70 have demonstrated that NPQ, and proNPQ/NPQ 53-70 are highly expressed in the PAG, LC and hypothalamus, which play roles in nociceptive responses. In order to explore the involvement of NPQ peptides in pain processing, as the next approach, we will also perform anatomical analyses to determine the localization of NPQ peptides and their putative receptor, GALR3 in the brain, spinal cord as well as the subpopulation of DRG neurons. Additionally, we will compare the distributions and levels of NPQ and GALR3 in the animal bearing persistent pain to confirm the context of NPQ and GALR3 to chronic pain. With these approaches, we try to understand the mechanism underlying the NPQ-GALR3 system in both acute and chronic pain.