~ .... ~ ...... ' ..... ~ reviews Anatomyof CN$ opMdreceptors Alfred Mansour, Henry Khachaturian, Michael E. Lewis, Huda Akil and Stanley J. Watson AlfredMansour, There is a wide body of evidence to suggest the existence opioid peptides. Once we knew of only one opiate HudaAkil and Stanley of at least three distinct opioid receptor types in the receptor - now there have been suggestions of as J Watsonareatthe CNS, referred to as ~, 6, and K. This paper reviews many as nine 8-12. This review will focus on the three MentalHealth some of the findings that have led to this conclusion and widely accepted opioid receptor types which are ResearchInstitute, the anatomical distributions of these sites in the rat referred to as Iz, 6 and K, and will discuss their possible Universityof . Their relation to the opioid peptides and some of interactions with the pro-opiomelanocortin (POMC), Michigan,205 pro-enkephalin and pro-dynorphin pepfide systems. WashtenawPlace, the proposed functions mediated by these receptor AnnArbor, MI systems are also discussed. Several lines of evidence support the existence of 48109-0720, USA, multiple opioid receptors. One of the earliest findings HenryKhachatudan is Largely because of their clinical significance and to suggest this multiplicity was the demonstration by at the Departmentof recent technological advances, the opioid systems are Martin and his colleagues 13 that different classes of Anatomyand among the best understood peptide systems in the opiate drugs produced distinct behavioral syndromes Neurobiology, CNS. While it is difficult to pinpoint precisely what has and that tolerance to one group of opiates did not Un/versityof caused this interest, it is clear that the key elements result in cross-tolerance to another class of opiate Tennessee,College of included the discovery of opioid peptides in the CNS 1, drugs. On the basis of these findings, Martin and his Medicine,875 the description of their genes, mRNAs and colleagues proposed the existence of three types of MonroeAvenue, opioid receptors: ~ for morphine-like compounds, i< Memphis, TN38163, precursors z-4, the characterization of their USA,and Michael E. receptors 5, and the finding that these pepfides are for ketocyclazocine-like drugs and o for drugs Lewisis at Cephalon, released following certain forms of stimulation s . Since such as SKF 10,047. Further support for this position Inc., 145Brandywine it is well beyond the scope of this paper to review this came from studies using peripheral organ bioassays Parkway, West field comprehensively, we have chosen to concentrate where the relative potencies of several opioids and Chester,PA 19380, our efforts on the anatomical distribution of the opioid opiate antagonists varied with tissue system 14, again USA. receptors in the CNS and their relationships with the suggesting a heterogeneity of receptors. In addition, opioid pepfide systems. Having presented this investigators using these tissue systems suggested anatomical information, we will then discuss some of the existence of yet another receptor type, referred the functions thought to be mediated by these multiple to as 6, named for the mouse vas deferens bio- receptors. assay, where enkephalin peptides were found to be Historically, the discovery of opioid receptors 7 particularly potent b. The findings from these early preceded the isolation and characterization of the pharmacological experiments were further supported

Fig. 1. Darkfield autoradiograms of mu, delta and kappa receptors using horizontal rat brain sections. Note the three distinct receptor distributions, i~ sites were labelled with [ 3H]DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), 6 sites were labelled with [3H]DPDPE (n-Pen 2, D-Pen5- enkephalln), and x sites were labelled with [ZH]bremazocine in the presence of saturating concentrations of DAGO and DPDPE. The binding conditions were such that approximately 75%o of each receptor site was occupied 19.20' . These conditions were used to produce all the autoradiograms presented in this review. Abbreviations not listed in the centrefold include: CG, central gray; DR, dorsal raphe; f, fornix; Pir, piriform cortex; PRTX, parietal cortex; THL, ; 4V, fourth ventricle.

308 © 1988. Elsevier Publications, Cambridge 0378- 5912/88/$02.00 TINS, Vo~ 11, NO. 7, 1988 by homogenate binding and autoradiographic studies the neuraxis with particularly dense binding observed demonstrating I~, 6 and I( receptors as distinct opioid in limbic structures, thalamic nuclei and neural areas binding sites 1~-23, with o receptors being non- important for visceral functioning. To aid in the opioid in nature. In the following sections we present discussion of these sites and their comparative some of the anatomical evidence that has led to this distributions in other animals, we have provided conclusion and the functional implications of these parasagittal drawings (centerfold) and horizontal results. darkfield autoradiograms (Fig. 1) that summarize the distribution of the opioid receptors in the rat brain. Opioid 'receptor' distribution The parasagittal drawings are based on autoradio- Opioid receptors are widely distributed throughout graphic studies and represent qualitative differences

TABLE I. Distribution of opioid receptors and peptides in the rat brain

Receptors Peptides CNS Region I• 6 K POMC Pro-Enk Pro-Dyn I. Telencephalon Frontal cortex (laminar) + + + + + + 0 + + + Piriform cortex (laminar) + + + + + + 0 + + + Entorhinal cortex (laminar) + + + + + + 0 + + + + Amygdala Central nucleus 0 0 + + + + + + + + + + + + Medial nucleus +++ ++ ++ +++ +++ + Lateral nucleus ++++ +++ +++ ++ +++ + Hippocampal formation Hippocampus (laminar) + + + + + + 0 + + + + + Dentate gyrus (laminar) + + + + + 0 + + + + + Olfactory tubercle + + + + + + + 0 + + + + + + + + + + + + + + + + + + + + + (patchy) (ventral) Caudate-putamen + + + + + + + + + + + 0 + + + + + (patchy) (vent-lat) (vent-reed) (patchy) Globus pallidus + + + 0 ++++ +++ Medial septum + + + + + + + + + + + 0 Bed nucleus + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + II. 0 0 + + 0 + + + + Paraventricular nucleus 0 0 + + + + + + + + + + + + + + 0 0 + + + + + + + + + + + Ventromedial nucleus 0 + + + + + + + + + + Dorsomedial nucleus + 0 + + + + + + + + + + + Lateral hypothalamic area + 0 + + + + + + + + + + Thalamus Periventricular nucleus 0 0 + + + + + + + + + + + Central-medial nucleus + + + + + + + 0 + + + 0 Reuniens nucleus + + + + + + + 0 + + 0 Medial + + + + + + + 0 + + + 0 III. Mesencephalon Interpeduncular nucleus + + + + + + + + + + 0 + + + 0 (central) Substantia nigra Pars compacta + + + 0 0 + + + + Pars reticulata + + + + 0 + + + + + + + 0 + + + + + + Periaqueductal gray + 0 ++ ++++ +++ ++ (rostral-ventral) Sup./Inf. colliculi + + + + + + + + + + + + + Dorsal raphe nucleus + + 0 + + + + + + + + IV. Pons/medulla Parabrachial nucleus + + + 0 + + + + + + + + + + Nucleus raphe magnus + + 0 + + + + + + + Nucleus reticular + 0 + + + + + + + gigantocellularis Nucleus tractus solitarius + + + + + + + + + + + + + + + + + (caudal part) Lateral reticular nucleus + 0 + + + + + + + + Spinal trigeminal nucleus + + + 0 + + + + + + + + + + + V. Spinal cord Substantia gelatinosa + + + + + + + + + + + + + + + ++++ = very dense; +++ = dense; ++ = moderate; + = low; 0 = undetectable.

TINS, Vol. 11, No. 7, 1988 309 .... TCX/..... __ ~\STCX

DH -ce

VH

T~ PCX

STCX PCX FOX OB ENT

CRB

SG i'~DH

OTU VH AL ICa [] OPIOID mu opioid receptors are widely distributed throughout the , midbrain and hindbrain, mu opioid receptor sites are most RECEPTOR dense in the neocortex, caudate-putamen, nucleus accumbens, thalamus, hippo- campus, amygdala, inferior and superior DISTRIBUTION colliculi, nucleus tractus solitarius, spinal i trigeminal nucleus and dorsal horn. A moderate density of mu receptors is Schematic representation of mu, delta and kappa observed in the periaqueductal gray and opioid receptors in the rat brain as determined by , while relatively little binding is receptor autoradiography techniques. To facili- seen in the hypothalamus, preoptic area and tate the description of these distributions, the globus pallidus. The distribution of opioid receptor densities are colour- coded, with red= receptors corresponds well with their puta- very dense (++++), orange = dense (+++), green tive role in pain regulation and sensorimotor = moderate (++), and blue = light (+). These terms integration. are not meant to be quantitative and only provide a relative measure within a receptor distribution. In fact, since there are relatively few kappa sites in the rat, areas referred to as dense for kappa STCX binding may be equivalent in receptor number to areas of light or moderate mu or delta binding. Each map represents mutiple parasagittal levels through the rat brain and was reconstructed using the atlas of G. Paximos and C.Watson (The Rat Brain in Stereotaxic Coordinates, Academic Press, 1986).

Scientific Advisors: A. Mansour, Mental Health Research Institute, University of Michigan, Ann Arbor, MI 48109, USA and H. Khachaturian, Department of Anatomy and NeurobioloRy, University of Tennessee, Memphis, TN 88163, USA.

ABBREVIATIONS ABL, basolateral amygdaloid nucleus; AC, anterior commissure; ACB,| nucleus accumbens; ACE, central amygdaloid nucleus; AD, anterodorsal/ DH thalamus; AL, anterior lobe, pituitary; AME, medial amygdaloid nucleus;| AON anterior olfactory nucleus: ARC, arcuate nucleus, hypothalamus:/ "-'~ ce BST, bed nucleus stria terminalis; cc, corpus callosum: ce, central canalili CL, centrolateral thalamus; CM, centromedial thalamus; CPU, caudate- Iv. putamen; CRB, cerebellum: DG, dentate gyrus; DH, dorsal horn, spinal cord; DMH, dorsomedial hypothalamus; DPG. deep , superior colliculus; FCX, frontal cortex; ICa. Islands of Calleja; IGR intermediate granular layer, olfactory bulb; IL, intermediate lobe; IMD, intermediodorsal thalamus; ING, intermediate gray layer, superior colliculus; IP, interpedun- cular nucleus;IPL, intermediateplexiform layer, olfactory bulb; LC, locusco- eruleus; LD, laterodorsal thalamus; LHA, lateral hypothalamic area; LP, latero-posteriorthalamus; LRN, lateral reticularnucleus; LS, lateral septum; MD,dorsomedial thalamus; ME, median eminence; MG, medial geniculate; ML, ; MM, medial mammillary nucleus; MS, medial sep- turn; MV, medial vestibular nucleus; NDB, nucleus diagonal band; NL, neural lobe, pituitary; NRGC, nucleus reticularis gigantocellularis; NTS, nucleustractus solitarius;OB, olfactorybulb; OT, optic track; OTV, olfactory tubercle; PBN, parabrachial nucleus; PC, posterior commissure; PCX, parietal cortex; PN, pons; POA, preoptic area; PrS, presubiculum;PV, para- ventricular thalamus; PVN, para-ventricular hypothalamus; RD, dorsal raphe; RE, reuniens thalamus; RM, raphe magnus; RME, median raphe; SC, superior colliculus; scp, superior cerebellar peduncle; SG, substantia gelatinosa; SNC, substantia nigra, pars compacta; SNR, substantia nigra, pars reticulata;SNT, sensory nucleustrigeminal; SON, supraopticnucleus; STCX, striate cortex; STN, spinal trigeminal nucleus; SUG, superficialgray layer, superior colliculus; TCX, temporal cortex; VH, ventral horn, spinal cord; VL, ventrolateral thalamus; VM, ventromedial thalamus; VMH, ven- tromedial hypothalamus; VP, ventral pallidus; VPL, ventroposteriolateral thalamus; VPM, ventroposteriormedialthalamus; and Zl, . in relative densities within a receptor distribution and TABLE II. Relative proportions of the opioid receptor types are not meant to indicate absolute densities of a in several speciesa receptor type within a structure. Rather, they Species Iz 6 K indicate that within a receptor distribution, there is a relative abundance of this site in these structures. As Rat 52.5 64.1 12.6 can be seen from these figures, the distributions of (41%) (50%) (9%) Guinea-pig 17.1 17.7 34.4 the opioid receptors vary markedly in both their (25%) (25%) (50%) relative abundance across brain regions and their Mouse 32.5 81.0 17.6 specific localization. (25%) (62%) (13%) binding is observed in numerous nuclei and at Pigeon 17.7 12.4 100.2 several levels of the neuraxis, including the neocor- (14%) (10%) (76%) tex, caudate-putamen, septum, thalamus, hippocam- Human 22.7 27.0 29.2 pus, substantia nigra, inferior and superior colliculi, (29%) (34%) (37%) and nucleus tractus solitarius. The aOpioid receptor binding in several species as determined by distribution of 6 receptors, on the other hand, is more Scatchard analysis. The upper number of each ~air refers to fmol restricted and predominantly in forebrain structures, per mg tissue, p. receptors were labelled with [ H]DAGO, 6 sites such as neocortex, caudate-putamen, and amygdala. K with [3HIDPDPE and K receptors with [3H]bremazocine in the receptor autoradiography demonstrates yet a third presence of a 300-fold excess of unlabelled DAGO and DPDPE. Given the total amount of opioid binding, the numbers beneath receptor pattern, with sites localized in the preoptic in parentheses are the relative abundance of these sites within area, hypothalamus, median eminence, caudate- brain tissue. The values reported here reflect the binding of putamen, amygdala and nucleus tractus solitarius. human frontal cortex and forebrain tissue of the rat, guinea-pig, While there is some overlap in the localization of each mouse and pigeon. of these receptor types, their precise anatomical distributions vary markedly. Since a comprehensive By comparison with the telencephalon, diencephalic summary of each of the opioid receptor distributions structures show a predominance of ~ and K binding. In would be impractical in this format, only the most the thalamus, there is a predominance of ~t receptors salient comparisons will be discussed. More detailed in most of the nuclear divisions with the exception of information is provided in the figures and tables of this the zona incerta, periventricular, central lateral and review, as well as in several recent papers 19'z2'23. ventral posteromedial nuclei. Moderate to dense K The pattern of receptor binding in the neocortex labelling is seen in the medial regions of the thalamus demonstrates a number of features seen repeatedly in including the pefiventricular, mediodorsal, and reu- opioid receptor anatomy. For instance, while both niens nuclei. In the hypothalamus, the distribution and 6 sites are prominent in this tissue, their precise appears to be reversed with moderate to dense K distributions differ and generally appear complemen- labelling observed throughout most of the hypothala- tary. Although there are regional differences in mic nuclei and little or no Iz binding observed. A lamination, ~t sites are most prominent in layers I and notable exception is the mammillary nucleus which III/IV of frontal, parietal, and temporal cortex, contains dense numbers of ~ receptors and a whereas 8 receptors tend to be diffusely localized in moderate density of i< sites. layers II, III, V, and VI. A similar complementary More caudally in the mesencephalon and brainstem, pattern is observed in the olfactory bulb, where and K receptors show largely parallel distributions. binding is prominent in the glomerular layer and Both i~ and I< receptors are observed in the binding is densest in the external plexlform layer. K periaqueductal gray, superior and inferior colliculi, receptors are not as prominent in cortex and are light interpeduncular nucleus, raphe nuclei, locus coeru- to moderate in density in layers II, III, V, and VI of leus, parabrachial nucleus, nucleus tractus solitarius, frontal and parietal cortex. spinal trigeminal nucleus, and substantia gelatinosa of Limbic system structures, like neocortex, have a the spinal cord. The distributions vary markedly in the rich distribution of opioid receptors. For example, in substantia nigra where dense Ix binding is observed in the hippocampal formation there is a relative abund- the pars compacta, an area devoid of K receptors in the ance of ~ receptors in the pyramidal cell layer, stratum rat. Relatively low levels of ~ and i< binding are lacunosum-moleculare, and the molecular and granular observed in the pars reticulata. No opioid receptors are cell layers of the ventral dentate gyrus. Comparatively detected in the rat cerebellum. moderate to light 6 and K labelling is observed in these areas. The relatively low number of K sites observed in Opioid 'peptide-receptor' interactions the hippocampus is in marked contrast to the fairly rich Given the complexity of the distributions of both dynorphin innervafion observed in this region and is the opioid receptors and peptide systems in the consistent with other evidence that dynorphin pepfides CNS 25-27 , it may be premature to attempt to correlate may be active at ~ receptors in this region of rat a particular opioid peptidergic system with a receptor brain 24. type. Early investigators examining this issue made In contrast to the situation in the hippocampus, all the assumption that based on the relative selectivities three receptor types are densely distributed within of the opioid peptides, there should be a correspond- the caudate-putamen and nucleus accumbens, two ence between a particular opioid peptide and recep- major catecholaminergic projection regions. ~ recep- tor. For example, [~-endorphin is known to bind tors occur predominantly in the sub-callosal region of selectively to both ~t and 8 receptors, while dynorphin the caudate-putamen and in receptor 'patches' that Al-17 and [leu]enkephalin show affinity for i~ and 8 extend from the striatum to the nucleus accumbens. receptors, respectively. However, the problem with and K receptors, in contrast, are diffusely distributed this line of reasoning is that pro-enkephalin and pro- within these structures, being particularly dense dynorphin pepfides can bind to fz, 6 or K receptors ventrolaterally (6) or ventromedially (K). depending on the specific peptide product28'29 and

310 TINS, VoL 11, No. 7, 1988 species. Therefore, since there are many potential human, they represent at least a third of the total areas of peptide-receptor interaction, the more opioid receptor population. Forebrain tissue varies logical questions to be asked about any particular CNS across species, demonstrating predominantly K re- region are what peptide forms are present, and what ceptors in pigeon, predominantly ~5 receptors in are their receptor affinities. mouse, and a mixture of receptor sites in the guinea- The relative densities of ~t, 6 and K receptors are pig and human. compared in Table I with the relative densities of Anatomically, species differences can be observed POMC, pro-enkephalin and pro-dynorphin in each of within the distributions of each of the receptor types several CNS regions. This summary table provides a and at several levels of the neuraxis. In general, the qualitative indication of several good matches and distribution of opioid receptor types is well conserved many mismatches in the 'peptide-receptor' co- across species in brainstem and spinal cord areas and distribution. For example, in cortex, amygdala and varies most markedly in forebrain and midbrain caudate-putamen, there is a general match between structures. Examples of these differences are pro- the distribution of 6 and ~ receptors on the one hand, vided in Table III. When comparing rat and monkey at and pro-enkephalin and pro-dynorphin peptides on the the level of the hypothalamus (Fig. 2), for instance, other. Likewise, in the hypothalamus, there is a good the differences in the t~ and K distributions are quite correlation between pro-dynorphin and K sites in striking. In the rat hypothalamus and median emi- almost every nucleus. Furthermore, the ~ receptor nence there is a relative abundance of K sites and few, distribution in some amygdaloid nuclei, in the septum, if any, p. and 6 receptors, while in monkey, ~ sites are parabrachial nucleus, and nucleus tractus solitarius prominent in these areas. Similarly, in the rat there is corresponds well with the distribution of POMC in little r binding in the neocortex, whereas there is these same nuclei. Conversely, there is a striking lack dense K binding in monkey cortex. These differences of correlation between the distribution of POMC extend to other neural areas including the nigrostriatal peptides and ~ receptors in the cerebral cortex, system, hippocampus and cerebellum. While species hippocampus, caudate-putamen and thalamus, to differences are also observed in the distribution of 6 name a few. The same can be said about the receptors, their overall distribution appears to be far distribution of 8 receptors and pro-enkephalin in the more conserved across mammalian species, with globus pallidus, hypothalamus, substantia nigra, peri- dense binding found primarily in the cerebral cortex, aqueductal gray and parabrachial nucleus. In the globus caudate-putamen, and amygdala and low binding pallidus, thalamus and substantia nigra, there is a observed in most of the midbrain and brainstem. further lack of correlation between the distribution of As there appears to be a relatively low abundance pro-dynorphin and K receptors. of r receptors in the rat, numerous investigators have From the above, it appears that with the present used other animals such as the guinea-pig 3z'33 to study state of knowledge on opioid peptide-receptor these sites. Several studies have, in fact, shown that interactions, it is unwise to try to correlate a given the distribution of i~ sites in the guinea-pig more peptide precursor system with a particular receptor closely parallels that observed in the monkey, with type. Instead, the differential processing of each of dense labelling in deep layers of cortex, substantia the three precursors in each CNS nucleus may be a nigra, hippocampus and cerebellum, possibly making crucial regulatory mechanism whereby opioid pep- the guinea-pig a better model for studying K receptor- tides of varying lengths and receptor affinities are peptide interactions. released presynaptically and act as selective ligands at the ~t, 6 or K receptors, depending upon the relative Functional roles of the opioid receptor types abundance of these receptor types. While this view Despite a plethora of studies on the physiological may account for the complex anatomical interrelation- ships between the multiple opioid systems and TABLE III. Species differences in selected brain regions receptor types, there still remains the problem of the frequent inconsistencies in areas where there are Receptor Brain region Rat Monkey opioid receptors and no detectable peptides or the I~ Central nuc. amygdala 0 + + + reverse case of opioid peptides and no detectable Caudate + + + + + + receptors. Several possible reasons for peptide- (patchy) (diffuse) receptor mismatches have been extensively discus- Hypothalamus + + + + + sed by other investigators and might include limita- Median eminence 0 + + + tions in immunocytochemial detection of peptides in Periventricular thalamus 0 + + + + fine 'synaptic' terminals, non-functional or spare 6 Central nuc. amygdala 0 + + receptors, unrecognized low affinity or occupied Hippocampus + + + + + receptors or diffusion as part of the mechanism of (striatum moleculare) opioid peptide action27 ' 30--32. Median eminence 0 + + + K Frontal cortex + + + + Species differences in opioid receptor Hippocampus + + + + distributions (striatum lacu nosum-moleculare) The complexity of the opioid receptor and peptide Caudate + + + + + interactions is magnified when species differences are (diffuse) (patchy) considered. At the simplest level, species vary Globus pallidus + + + dramatically in the relative abundance of each of these Medial mammillary nucleus + + 0 sites. As can be seen from Table II, I< sites in the rat Cerebellum 0 + + + comprise approximately 10% of the total number Median eminence ++++ +++ of opioid receptor sites in the forebrain, whereas in Relative densities within a species: ++++ = very dense, +++ = dense, most other species, such as guinea-pig, monkey and ++= moderate, + = light, 0 = not detectable.

TINS, Vol. 11, No. 7, 1988 311 Fig. 2. Darkfieldautoradiograms of monkey and rat frontalsections taken at the level of the diencephalon. Note the markedspecies differences observed in p binding in the cortex and hypothalamus. In contrast, the distribution of 6 sites is well conserved in these divergent species. Abbreviations not listed in centrefold include: AMG, amygdala; CA1,2,3, fields I, 2, and 3 of Ammon’s horn; CAU, caudate; Cg, cingulate cortex; CL, claustrum; EN, endopiriform nucleus; 6 fornix; FPCX, frontal parietal cortex; HB, habenula; HYP, hypothalamus; LA, lateral amygdala; LH, ; mt, ; PCg, posterior cingulate cortex; PUT, putamen; VA, anterior ventral thalamus.

312 TINS, Vol. I I, No. 7, 1988 and behavioral effects of administer- TABLE IV. Proposed functions of the opioid receptor typesa ing diverse opieid pep tides, alka- Function Receptor types Anatomy loids and antagonists ~*, it is still difficult to draw firm conclusions Appetite modulation, eating regarding the functional roles of behavior ~,6andr Ventral tegmental area different opioid receptor types in Cardiovascular regulation ~, 6 and r Nucleus tractus solitarius Fluid balance r: diuresis Hypothalamus and/or pituitary; any neuronal system. For example, ~: antidiuresis also possibly kidney (r) the neural circuitry involved in the Endocrine responses Hypothalamus, possibly central control of cardiovascular Stimulatory effects pituitary function has been intensely studied, Growth 8 and the loci of opioid neurons and ACTH and r receptors within this circuitry is well Prolactin p~and r defined. However, it has proven Inhibitory effects very difficult to isolate the roles of Luteinizing hormone and 6 (?) the different opioid systems and/or K (also nucleus tractus solitarius) receptors within this circuitry be- p. and r Pain inhibition and 6 Supraspinal, spinal cause the physiological effects of 6 medullary reticular formation opioids are influenced by pharmaco- K spinal logical variables such as the dose, Respiration and 6: may mediate Brainstem stability, site of administration, and respiratory depression receptor specificity of the peptide Locomotor behavior ~: increased activity A9, A10 DA systems or alkaloid given, and by organismic K: sedation A10 DA systems (?) variables, such as whether the Thermoregulation ~: may mediate hypothermia Hypothalamus animal has been stressed, injured, 6: may mediate hyperthermia or anaesthetized 35. aThis table, which is not a complete summary, is modified from Holaday (1985) Endogenous While s'u-nilar problems have pla- Opioids and Their Receptors, Upjohn Publications. gued many functional studies of opioid receptor types, significant progress has been made in receptors 44 cannot be ruled out. Since the antidiuretic several systems (see Table IV). For example because of effects of I~ agonists do not appear to involve the immense background ofknowledge of the anatomy and vasopressin secretion4a, these opposing physiological functions of the mesencephalic dopaminergic neuronal actions are presumably mediated by I~ and r receptors systems, the study of their modulation by opioids coupled to different systems. began early36 and has been a particularly active area of The I~, 6 and K opioid receptor types have all been research. While there has been debate concerning implicated in the mediation of analgesia, but the details which (DA) neurons (A9 versus A10) are of their differential involvement are a source of lively involved in the motor stimulation effects of morphine in debate. Supraspinal mediation of opioid analgesia has rodents, it is still clear that both systems are power- frequently been attributed exclusively to I~ receptor fully influenced by opiate administration, and that the activation45 ' 46 , but recent evidence points to the opioid receptor types are differentially involved. For involvement of 6 receptors as well 47'48, particularly in example, direct administration of morphine into the the medullary reticular formation49. At the spinal A10 region results in a DA-dependent increase in level, selective t~, 6 and r agonists are all active in locomotor activity37, which is probably ~t-dependent visceral analgesia testing 48. However, r agonists since similar administration of DAGO (a i~-selective appear to be distinct in that they block mechanical compound) produces activation at doses 100-1000 nociceptive responses; this contrasts with the media- times less than those required for DPDPE (a 6- tion of thermal nociceptive responses by ~t and/or 6 selective drug) 38. These functional findings agree with receptors 5°. These differential actions may be related the observation of greater I~ than 6 radioligand binding to differences in the intraspinal distribution of the in the A10 region (ventral tegmental area; Table I). multiple opioid receptor types51. Based on studies of the interacting effects of differenti- Unfortunately, autoradiographic mapping studies ally selective opiates on DA metabolism, K receptors are not invariably consistent with functional studies, a have been suggested to modulate I~ receptor activation discrepancy that may be regarded as another form of of A10 neurons 39. 'mismatch'. An instructive example is given by the Electrophysiological studies of A9 neurons point to finding that ~t, 6 and r agonists stimulate feeding I~ and K opiates having opposite effects on cellular equipotently when microinjected into the ventral firing rates 4°, which parallel the opposite effects of tegmental area 52, a result which would not have been these opiates on motor behavior, i.e. activation and predicted from autoradiographic studies since the sedation, respectively. The inhibitory actions of K ventral tegmental area shows moderate I~ binding, but opiates on A9 neurons appear to be mediated at the little K and no detectable 6 binding (Table I). Such level of the striatum rather than the substantia functional mismatches may arise because autoradio- nigra 4°, which is consistent with the relative density of graphic studies cannot discriminate between uncoupled r radioligand binding in these structures (Table I). binding sites and fully functional receptors. Likewise, Opposing actions of r and I~ agonists have also been the relative 'efficiency' of functional coupling of the found on fluid regulation, these compounds having different receptor types cannot be determined from diuretic41 and antidiuretic effects42, respectively, r autoradiographic studies. diuresis appears to be due to inhibition of vasopressin Thus, while it is clear that autoradiographic maps of secretion43, probably via pituitary or hypothalamic K receptors may be useful in suggesting sites for receptors, although a diuretic effect via renal K functional studies, it is also apparent that mapping

TINS, Vol. 11, No. 7, 1988 313 Acknowledgements studies have so far been of limited value in predicting 11 Gouarderes, C., Attali, B., Audigier, Y. and Cros, J. (1983) Life Sci. 33 (Suppl. 1), 175-178 Thiswork was specific functional consequences of activating different 12 Grerel, J., Yu, V. and Sadee, W. (1985) J. Neurochem. 44, supportedby the NIH opioid receptor types. Nevertheless, since the 1647-1656 TrainingGrant anatomical distribution of the multiple receptor types 13 Martin, W. R., Eades, C. G., Thompson, J. A., Huppler, R. E. MH15794 (AM), is now clearly delineated, it may be worthwhile to use and Gilbert, P. E. (1976) J. Pharmacol. Exp. Ther. 197, 517- NIDA Grant this information to explicitly develop and test specific 532 14 Chang, K-J., Hazum, E. and Cuatrecasas, P. (1980) Trends DA02265 (HA), and hypotheses regarding the functional consequences of NIMH Grant Neurosci. 3, 160-162 MH422251 (SJWand activating these receptors via microinjections of 15 Robson, L. E. and Kosterlitz, H. W. (1979) Proc. R. Soc. 5er. B. 205,425-432 HA). We are also receptor-selective, stable agonists or antagonists into London 16 Smith, J. R. and Simon, E. J. (1980) Proc. NatlAcad. 5ci. US~ grateful to Adele areas with known peptide and receptor content. 77, 281-284 Henryand Came Future directions 17 James, I. F. and Goldstein, A. (1984) MoL Pharmacol. 25, 5erce/ for theirexpert 337-342 secretarialhelp. Given the ability to selectively label each of the 18 Goodman, R. R., Snyder, S. H., Kuhar, M.J. and Young, W. S. opioid receptor types, the fundamental cell biological (1980) Proc. Natl Acad. Sci. USA 77, 6239-6243 issue of differential receptor regulation, e.g. in 19 Mansour, A., Khachaturian, H., Lewis, M. E., Akil, H. and response to physiological and pharmacological Watson, S. J. (1987) J. Neurosci. 7, 2445-2464 manipulations s3, will be increasingly answered. These 20 Mansour, A., Lewis, M. E., Khachaturian, H., Akil, H. and Watson, S. J. (1986) Brain Res. 399, 69-79 studies will be aided greatly by the ability to study the 21 Morris, B. J. and Herz, A. (1986) Neuroscience 19, 839-846 genomic regulation of receptor expression, and the 22 McLean, S., Rothman, R. and Herkenharn, M. (1986) Brain recent cloning of the 5 receptor54 provides the first in Res. 378, 49-60 a new series of molecular tools to explore this level of 23 Tempel, A. and Zukin, R. S. (1987) Proc. NatlAcad. Sci. USA 84, 4308-4312 regulation. As molecular biological tools are further 24 Chavkin, C., Henriksen, S. J., Siggin, G. R. and Bloom, F. E. applied to the study of the opioid receptors, several (1985) Brain Res. 331,366-370 fundamental questions will be answered. For exam- 25 Khachaturian, H., Lewis, M. E., Schafer, M. and Watson, ple, as indicated earlier, binding studies 8-12 suggest S. J. (1985) Trends Neurosci. 8, 111-119 26 Petrusz, P., Merchenthaler, I. and Maderdrut, J. L. (1985) in that there may be more than the three opioid receptor Handbook of Chemical Neuroanatomy (Vol. 4) (Bj6rklund, A. types described here. Molecular biological techniques and H6kfelt, T., eds), pp. 273-334, Elsevier will aid in determining the number of receptor types 27 Lewis, M. E., Khachaturian, H. and Watson, S. J. (1985) present and whether they are derived from several Peptides 6 (Suppl. 1 ), 37-47 genes or are the product of differential post- 28 Quirion, R. and Weiss, A. S. (1983) Peptides4, 445-449 29 Corbett, A. D., Paterson, S. J., McKnight, A. T., Magnan, J. translational processing of a single gene product. The and Kosterlitz, H. (1982) Nature 299, 79-81 use of these tools at the anatomical level, via in-situ 30 Herkenham, M. and McLean, S. (1986) in Quantitative hybridization histochemistry, will permit the identifi- Receptor Autoradiography (Boast, C. A., Snowhill, E. W. and cation of populations of neurons that express the Altar, C. A., eds), pp. 137-171, Alan R. Liss different receptors, and by strategic combination of 31 Kuhar, M. J. (1985) Trends Neurosci, 8, 190-191 32 McLean, S., Rothman, R. R., Jacobson, A. E., Rice, K. C. and different methodologies it will be possible to identify Herkenharn, M. (1987) J. Comp. Neurol. 255,497-510 their possible synaptic inputs and their regulation at 33 Foote, R. W. and Maurer, R. (1986) Neuroscience 19, 847- the single cell level. Where will this approach lead us? 856 Perhaps by obtaining a new understanding of how 34 Olson, G. A., Olson, R. D. and Kastin, A. J. (1986) Peptides7, 907-933 opioid receptors can be synaptically regulated, 35 Fuerstein, G. (1985) Peptides 6, 51-56 together with deeper insight into the anatomical basis 36 Clouet, D. H. and Ratner, M. (1970) Science 168,854-856 of different opioid actions, we may ultimately be able 37 Joyce, E. M. and Iversen, S. D. (1979) Neurosci. Lett. 14, to define a new type of opioid pharmacology - the 207-212 selective expression of different opioid receptor types 38 Latimer, L. G., Duff, P., and Kalivas, P. W. (1987) J. Pharma- col. Exp. Ther. 241,328-337 in specific brain areas for the purpose of modifying 39 Kim, H. S., lyengar, S. and Wood, P. L. (1987) Life Sci. 41, the functions of the endogenous opioid peptide 1711-1715 systems, e.g. to alleviate pain without the danger of 40 Walker, J. M., Thompson, L. A., Frascella, J. and Friederich, addiction or respiratory depression. Such fanciful M. W. (1987) Eur. J. Pharmacol. 134, 53-59 goals may be less distant than they seem if future 41 Slizgi, G. R. and Ludens, J. H. (1982)J. PharmacoL Exp. Ther. 220, 585-591 molecular and functional studies of opioid receptors 42 Debodo, R. C. (1944) J. PharmacoL Exp. Ther. 82, 74-88 are thoughtfully integrated in the context of the 43 Leander, J. D., Zerbe, R. L. and Hart, J. C. (1985) J. chemical neuroanatomy of the CNS. 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