Immunocytochemical localization of proopiomelanocortin WEBSTER H. PILCHER, M.D., PH.D., SHIRLEY A. JOSEPH, PH.D., AND JOSEPH V. MCDONALD, M.D.

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1 J Neurosurg 68: , 1988 Immunocytochemical localization of proopiomelanocortin neurons in human brain areas subserving stimulation analgesia WEBSTER H. PILCHER, M.D., PH.D., SHIRLEY A. JOSEPH, PH.D., AND JOSEPH V. MCDONALD, M.D. Division of Neurosurgery and The Neuroendocrine Unit, University of Rochester School of Medicine and Dentistry, Rochester, New York ~" The distribution of pro-opiomelanocortin (r adrenocorticotropic hormone, and 16-K) neurons and fiber projections was evaluated immunocytochemically in 50-~ thick cryostat sections of human diencephalon and midbrain. Specific attention was focused upon regions in which deep brain stimulation has been most effective in the relief of selected chronic pain syndromes. This study revealed a remarkable, nearly pointto-point correlation between clinically effective stimulation sites and the distribution of pro-opiomelanocortin fibers in the human brain. Of particular interest was the dense innervation of the periventricular stratum along the third ventricle, the parafascicular centromedian region of the thalamus, and the periaqueductal gray matter of the midbrain. This study provides anatomical support for the hypothesis that /3-endorphin-containing neuronal systems may contribute to stimulation analgesia in the human. KEY WOUDS 9 ~-endorphin 9 pro-opiomelanocortin 9 parafascicular nucleus 9 electrical stimulation 9 analgesia 9 pain O VER the past decade, the electrical stimulation of a variety of human diencephalic and mesencephalic regions via implantable electrodes has emerged as an effective clinical modality in the treatment of selected cases of chronic pain. 18,~9,36-39,52 In particular, the stimulation of restricted midline regions extending from the periventricular hypothalamus through the periaqueductal gray matter of the midbrain elicits in selected patients a profound "opioid" (that is, naloxone-antagonizable) analgesia. 2"9,'9 It is hypothesized that this occurs following the stimulationinduced release from nerve terminals of endogenous opioid peptides, including the most potent of the analgesic opioid peptides,/3-endorphin, a pro-opiomelanocortin (POMC) peptide. 3'4'12'2~ A wealth of data accumulated from investigations in the rat has identified an integrated neuronal system in the brain which is capable of powerfully reducing both the perception of pain and the attendant biobehavioral responses by actions at multiple levels of the nervous system. 7'8'14'26'29'35'47'48 This "descending pain control system" can be activated in vivo by impulses delivered through electrodes implanted in strategic locations in the brain. An important constituent of this system in the rat is thought to be a group of/3-endorphin-contain- ing POMC neurons with cell bodies located in the basal hypothalamus and projections which innervate many regions of the forebrain and brain stem. Studies in rats of the axonal projections of these neurons have revealed patterns of innervation of brain regions associated with descending pain control mechanisms, 15'22,4t'49 and have provided a theoretical framework for the initial human clinical trials of deep brain stimulation. 33'36 No information is available, however, with regard to the distribution of POMC fibers in the regions of the human brain which are purportedly involved in descending pain control following electrical stimulation. The objective of the present study, therefore, was to investigate for the first time the distribution of POMCimmunoreactive fibers in the human brain, with a specific focus upon the medial diencephalic-mesencephalic areas which have elicited analgesic effects in humans following electrical stimulation. Materials and Methods The diencephalic-mesencephalic cores of six human brains from subjects ranging in age from 6 to 72 years were obtained through the generous assistance of the Neuropathology Division at the University of Rochester Medical Center. The cause of death was "non- J. Neurosurg. / Volume 68/April,

2 W. H. Pilcher, S. A. Joseph, and J. V. McDonald neuropathological" in all cases; that is, the subjects died from trauma and/or cardiac arrest. These brains were fixed in toto in 10% formalin, Bouin's fixative, or "402" fixative (8% paraformaldehyde and 2% picric acid in 0.05 M phosphate-buffered saline (PBS), ph 8.0) for 4 to 6 weeks. The diencephalic-mesencephalic core was removed en bloc and postfixed in 402 fixative, with or without 4% sucrose, for an additional 2 weeks to assure proper penetration and fixation. The brain fragments were divided in frontal planes into two or three pieces with a knife, and then sectioned at 50 u on a freezingsliding microtome. Sections were collected in PBS and rinsed overnight with several changes of buffer to assure complete removal of fixative. Alternate sections were stained immunocytochemically with antisera generated in rabbits by us against synthetic bovine serum albumin-conjugated human adrenocorticotropic hormone (ACTH1.39) or against synthetic bovine serum albuminconjugated human/3-endorphin,.3,, as well as an antiserum against the 16-kD amino-terminal fragment of FIG. 1. Distribution of pro-opiomelanocortin perikarya (a, large dots) and fibers (a-f, small dots) as revealed in ICC-stained sections of human brain. These distribution patterns were plotted on selected frontal sections redrawn from Riley's Atlas. 4~ Sections range in a rostral-caudal direction from the level of the fornix columns (a) to the decussation of the trochlear nerve (0. Abbreviations: A = aqueduct; AD = anterodorsal nucleus thalamus; AT = anterior nucleus thalamus; BC = brachium conjunctivum; CM = centrum medianum; CN = caudate nucleus; CP = cerebral peduncle; CS = superior colliculus; DM = dorsomedial nucleus thalamus; F = fornix; FLM = medial longitudinal fasciculus; FR = fasciculus retroflexus; GP = globus pallidus; H = habenula; IC = internal capsule; LAM = medullary lamina thalamus; LF = lenticular fasciculus; LL = lateral lemniscus; LM = medial lemniscus; LV = lateral ventricle; M = mamillary body; MT = mamillothalamic tract; NMT = nucleus medialis thalami; OT = optic tract; PAG = periaqueductal gray matter; PC = posterior commissure; PF = parafascicular nucleus; SM = stria medullaris thalami; SN = substantia nigra; TC = tuber cinereum; TF = thalamic fasciculus; VLT = nucleus ventralis lateralis thalami; III= third ventricle; IIIN = oculomotor nerve; IVN = trochlear nerve. 622 J. Neurosurg. / Volume 68/April, 1988

3 Localization of pro-opiomelanocortin neurons in human brain the/3-endorphin precursor (16-K) which was generously donated by Dr. Richard Mains. 12'23"32 Absorption control studies for /3-endorphin and ACTH antisera revealed complete elimination of the immunocytochemical staining of respective antisera when antisera were preincubated with synthetic human /3-endorphin~_31 and human ACTHI_39. No absorption of staining was demonstrated following preincubation of ACTH antisera with human r of B- endorphin antisera with ACTHI_39, or when either of these antisera were preincubated with luteinizing hormone-releasing hormone, somatostatin, or a- and /%melanocyte-stimulating hormone. The absorption specificities of 16-K antisera have been described previously. 12 Several wells were stained with cresyl violet or Weil- Weigert stains to delineate nuclear boundaries and fiber tracts. Sections were incubated, with gentle agitation, at 4~ in appropriately diluted antisera (ACTH 1:2000, j3-endorphin 1:1000, and 16-K 1:2000) in PBS (1% bovine serum albumin and 1.2% Triton X) for 4 to 6 days, following which they were reacted using either the peroxidase-antiperoxidase or the avidin-biotin technique. 23'32 For the purposes of mapping, the distribution of perikarya and fibers was plotted on line diagrams adapted from Riley's Atlas. 4~ Results ~-Endorphin, ACTH, and 16-K Immunoreactivity It has been well established in the rat that/~-endorphin, ACTH, and 16-K are derived from a common precursor, pro-opiomelanocortin, ~2 and that immunocytochemical staining in rat hypothalamus using antisera directed against each of these peptides individually will reveal identical groups of neuronal perikarya and axonal projections. 22 Consistent with these studies, in the human brain antisera directed against r ACTH, and the 16-K precursor fragment stained a discrete population of neurons restricted to the infundibtdar nucleus (Figs. l c and 2), and a qualitatively identical fiber distribution throughout the diencephalon and mesencephalon. The staining of fibers was more intense when the a-acth antiserum was utilized; therefore, the a-acth antisera-stained sections were used for the purposes of definitive mapping and most photomicroscopy. Distribution of Neuronal Perikarya and Fiber Projections Immunoreactive neuronal perikarya were restricted to the infundibular nucleus of the human hypothalamus, at the inferior edge of the third ventricle, adjacent to the median eminence and pituitary stalk (Figs. la and 2). Both fusiform, bipolar-appearing neurons and multipolar neurons were identified within this nucleus (Fig. 2), were essentially confined to it, and gave rise to an extensive network of axonal projections which were found within hypothalamic, extrahypothalamic limbic, diencephalic, and mesencephalic sites. Within the hypothalamus a dense accumulation of immunoreactive fibers appeared to innervate the medial and lateral preoptic areas, the ventromedial and paraventricular nuclei, the lateral hypothalamic region, FIG. 2. Left: Photomicrograph of an immunocytochemically stained section of human hypothalamus showing pro-opiomelanocortin (POMC) perikarya in the infundibular nucleus. 35. Right: High-power magnification of neurons stained with antisera generated against human adrenocorticotropic hormone~.39 (upper) and the 16-K fragment of the 31-K precursor molecule (lower). Note the multipolar (upper) and bipolar (lower) neurons which synthesize the POMC neuropeptides J. Neurosurg. / Volume 68/April,

4 W. H. Pilcher, S. A. Joseph, and J. V. McDonald the mamillary nuclei, and the posterior hypothalamic area (Figs. la and b and 3). Extrahypothalamic projections were observed 1) passing rostrally into the prehypothalamic area including the region of the anterior commissure and the bed nucleus of the stria terminalis, 2) arching laterally over the optic tracts (Fig. la) toward the amygdala, and 3) projecting caudally and dorsally within the periventricular stratum of the third ventricle to innervate many important diencephalic and mesencephalic regions (Figs. lb and c, 4, and 5). Throughout its course, this caudally directed fiber aggregation is restricted, with few exceptions, to midline regions. At the level of the posterior hypothalamus, bundles of fibers pass dorsally and laterally to permeate the internal medullary lamina of the thalamus (Fig. 1 c). Most pertinent to the subject of this investigation is the accumulation of fibers visualized in the region of the diencephalic-mesencephalic junction. At this level numerous fibers are observed within the parafascicular nucleus, medial to the fasciculus retroflexus throughout much of its course (Figs. ld, 6, and 7). As the fasciculus approaches the habenula, fibers are also visualized lateral to it in the parafascicular-centromedian region, although the centromedian nucleus proper does not appear to be innervated by POMC fibers to a significant degree (Figs. ld and 7). This aggregation of fibers, medial and lateral to the fasciculus retroflexus, constitutes the most significant thalamic innervation by POMC neurons. Caudal to the parafascicular centromedial region at the level of the posterior commissure, POMC fiber projections appear to arch toward the dorsal periaqueductal gray matter which is abundantly supplied at the rostral mesencephalic levels. At more caudal levels the lateral and ventral periaqueductal gray matter, dorsal to the medial longitudinal fasciculus, is likewise permeated by POMC fibers (Figs. le and 8). These fibers pass laterally into the mesencephalic reticular formation, and at the pontomesencephalic junction embrace the brachium conjunctivum innervating the medial and lateral parabrachial nuclei (Fig. l f) as well as the more ventral Krlliker-Fuse nucleus and the medially located locus ceruleus. FIG. 3. Darkfield photomicrograph of the human brain sectioned in the frontal plane demonstrating the fiber distribution of pro-opiomelanocortin neuropeptides in the hypothalamus at the level of the fornix columns (F). A dense accumulation of fibers can be seen surrounding the lateral tuberal nucleus (TN) and within the midline periventricular stratum (arrows). IlI= third ventricle. 11. FIG. 4. Darkfield photomicrograph of the human brain at the mid-diencephalic level revealing the selective distribution of pro-opiomelanocortin fibers within midline regions along the ventricular system (arrows). III= third ventricle J. Neurosurg. / Volume 68/April, 1988

5 Localization of pro-opiomelanocortin neurons in human brain Discussion The concept that the mammalian brain contains intrinsic servomechanisms which down-regulate the perception of painful stimuli has received a great deal of attention in recent years. In 1968, Reynolds 35 demonstrated that electrical stimulation of midline diencephalic-mesencephalic sites in awake rats rendered them indifferent to a laparotomy incision. Following this seminal publication, a wealth of investigative data suggested the existence of a specific descending pain control system in the brain stem and spinal cord which is powerfully activated by the application of electrical stimuli 11'21'25'26'28'50 or the microinjection of opiates 16'25'5~ within midline diencephalic and mesencephalic sites. A major unanswered question involves the identity of the endogenous opioid peptide(s) which may be released from nerve endings following electrical stimulation in the medial diencephalon or the periaqueductal gray matter, and which may initiate the descending pain control cascade. 1'5'6'13'3~ The two principal candidates for this role are ~-endorphin, which is contained within nerve endings terminating in these regions, and met-enkephalin, a less potent analgesic opioid which is produced by neurons that have also been identified in this area in the rat. While endogenous analgesic opioid systems have been well characterized in the rat brain and implicated in the production of stimulation analgesia in this species, virtually nothing is known about the patterns of innervation in the human diencephalic and mesencephalic regions where electrical stimulation is effective. In fact, the contention in the clinical literature 38 that these electrode placements may stimulate the "human /3-endorphin system" is based solely upon extrapolation from anatomical work performed in the rat. This investigation was designed, therefore, to identify for the first time in the human brain the distribution of r phin-containing fibers in these areas and to address, using immunocytochemical methods, the hypothesis that the POMC system in the human may provide an anatomical substrate through which electrical stimulation may ameliorate chronic pain. FIG. 5. Darkfield photomontage of a more caudal level of the human diencephalon depicting the immunostained proopiomelanocortin fibers in the periventricular region (arrows9. SM = stria medullaris thalami; III= third ventricle, x 20, FIG. 6. Mid-sagittal section of the human brain demonstrating the pro-opiomelanocortin fiber system (arrows) and its trajectory from hypothalamic neurons to the parafascicular/centrum medianum region and to the mesencephalic periaqueductal gray matter. M = mamillary body; FR = fasciculus retroflexus; CM = centrum medianum nucleus; H = habenula; F = fornix. (Adapted from Haymaker W, Anderson E, Nauta WJH: The Hypothalamus. Springfield, II1: Charles C Thomas, 1969, p. 199, by courtesy of the publisher). J. Neurosurg. / Volume 68/April,

6 W. H. Pilcher, S. A. Joseph, and J. V. McDonald peptides in the cerebrospinal fluid (CSF), 3'4'20'27,45 and in several studies the pain relief was reversible with naloxone, 2'9'j9'39 providing support for the contribution of ~-endorphin systems to stimulation analgesia. While the rostral-caudal zone of effective stimulation appeared to traverse most of the diencephalon and mesencephalon, this region was quite restricted in a medial-lateral dimension. In fact, at the level of the periaqueductal gray matter or the parafascicular nucleus, an error in electrode placement of only several millimeters in the lateral direction was sufficient to render stimulation ineffective. 9'18'~9'38 FIG. 7. Darkfield photomontage of a frontal section of the human brain at the level of the caudal diencephalon demonstrating a dense innervation of the parafascicular nucleus (PF) by a discrete bundle of pro-opiomelanocortin fibers (open arrow). These fibers can also be visualized at this level within the internal medullary lamina and surrounding the medial nucleus of the thalamus (NMT) (white arrows). SM = stria medullaris thalami; CM = centrum medianum nucleus; III = third ventricle; FR = fasciculus retroflexus. 17. Human Clinical Investigations In early trials of deep brain stimulation 2'~9'38'39 for pain relief in humans, the mesencephalic periaqueductal gray matter (Figs. ld and f and 8), which had produced analgesia to acute pain in rats, was explored. Stimulation at this location provided minutes to hours of relief from chronic pain of peripheral origin; t9"38 however, the proximity of these electrode placements to the superior colliculi, the medial longitudinal fasciculus, and mesencephalic neuronal substrates mediating fear, startle, and aversive responses resulted in the production of undesirable side effects. 38's2 Thus, more rostral midline loci were utilized with increasing frequency, and the periventricular gray matter at the level of the posterior third ventricle (Figs. l d and 5) as well as the region of the thalamic parafascicular nucleus (Figs. ld and 7) were identified as optimally efficacious sites. 38'52 In these patients, pain relief was associated with a dramatic increase in the content of/3-endorphin-like Effective Stimulation Sites and POMC Fiber Distribution A careful review of effective stimulation sites reported in the clinical literature suggests a remarkable concordance with the distribution of POMC-immunoreactive fibers revealed by our investigation. For example, regions of the lateral, ventral, and dorsal periaqueductal gray matter between the colliculi, in which electrical stimulation produced elevations of CSF B- endorphin levels and pain threshold, 4't8'2~ were found to be discretely and densely populated by POMC fibers in this study (Figs. le and 8). The more rostral periventricular stratum adjacent to the posterior third ventricle, within and medial to the parafascicular nucleus, which has been identified as the most efficacious stimulation site, 2'19'38'52 was found to be most conspicuously and selectively populated by POMC fiber projections (Figs. l c and d and 7). Autopsy studies of treated patients support the concept that stimulation within the topographical domain of POMC fiber projections may be essential to the relief of certain forms of chronic pain. In Case 2 in the report by Richardson and Aki138 there was "very good" pain relief after placement of an electrode in the periventricular gray matter inferior to the habenula, a site corresponding to that in our Fig. ld. The patient in their Case 5 experienced "good" pain relief with electrode placement slightly lateral to the fasciculus retroflexus, within the parafascicular nucleus (see Fig. ld). In the study by Hosobuchi, et al., ~9 the patient in their Case 4, whose electrode was placed so that it traversed a region from a point immediately lateral to the habenula to the periventricular gray matter ventral to the habenula, experienced "complete relief' from leg and perineal pain, which was naloxone-reversible. Notably, these areas are densely innervated by POMC fibers (see Fig. ld). Their Case 2 had two electrodes implanted at the level of the posterior commissure. The medial electrode (corresponding to the location of POMC fibers depicted in Fig. l d, inferior to the posterior commissure) produced "complete analgesia" to chronic pain which was completely reversed by naloxone. The second electrode, located in a region devoid of POMC fibers (several millimeters lateral to the first), produced no analgesia whatsoever/8'~9 A similar phenomenon was observed in Case 5 of Hosobuchi, et al: an electrode located 626 J. Neurosurg. / Volume 68/April, 1988

7 Localization of pro-opiomelanocortin neurons in human brain FIG. 8. Darkfield photomontages of pro-opiomelanocortin (POMC) fiber distribution within the periaqueductal gray matter at rostral (left) and at more caudal (right) frontal planes. The POMC fibers are recognized as distinct bundles (arrows) surrounding the cerebral aqueduct (A). FLM = medial longitudinal fasciculus. 10. several millimeters lateral to an effective electrode elicited no analgesic effect. Boivie and Meyerson 9 reviewed five autopsy specimens and also supported the hypothesis that, in order to be effective, electrical stimulation must be restricted to a region of medial neuropil which corresponds to the projection field of POMC neurons as defined by this study. A noteworthy outcome of this investigation is identification of the selective distribution of POMC fibers within the thalamic parafascicular nucleus, a clinically optimal locus for the production of stimulation analgesia. Recent investigations have shown the parafascicular nucleus to be an important center for the integration of motor and behavioral responses to painful afferent inputs. 1~ Neurons in this region have diffuse receptive fields and respond to pain and temperature in an "on-off" rather than a graded fashion like ventrobasal thalamic neurons.'7 Parafascicular neurons are exquisitely sensitive to chronic pain 24 and receive substantial "paleospinothalamic" (spinoreticulothalamic) inputs. 1~ Of interest also is the high concentration within the parafascicular nucleus of opiate receptors and r POMC fibers, the exquisite sensitivity of these neurons to narcotics, 34 and the utility of the morphine-administration test in identifying candidates in whom parafascicular stimulation may be successful. ~8 Collectively, these studies suggest that the thalamic parafascicular nucleus is an opioid-innervated, opioidresponsive center for the integration of responses to paleospinothalamic ("protopathic") stimuli, such as oc- cur in chronic pain of peripheral origin. In contrast, the ventrobasal thalamic nuclei (ventral posterolateral, ventral posteromedial) which receive "epicritic" sensory inputs are nearly devoid of r fibers. Thus, our investigation demonstrates that diencephalic 13-endorphin systems in the human are distributed preferentially to areas of the medial thalamus which receive paleospinothalamic inputs. This preferential relationship of 13-endorphin fibers to paleospinothalamic rather than neospinothalamic systems is consistent with clinical reports that certain forms of "acute pain, "37 such as pinprick, are minimally affected by parafascicular nucleus stimulation, and that combined medial lemniscus/parafascicular stimulation is more effective in the treatment of pain syndromes like thalamic pain and deafferentation pain in which derangement of "epicritic" sensory modalities may occur. It is tempting, but obviously premature, to suggest on the basis of these data that electrical stimulation within the parafascicular nucleus may release /3-endorphin from POMC nerve terminals with the effect of selectively relieving chronic pain of peripheral origin. Conclusions Available evidence suggests that in the human diencephalon and mesencephalon the distribution of both POMC fibers and clinically effective electrode placements is remarkably restricted. Of even greater interest is the fact that there is a precise, nearly point-to-point, correspondence between the two. This precise correla- J. Neurosurg. / Volume 68/April,

8 W. H. Pilcher, S. A. Joseph, and J. V. McDonald tion supports the hypothesis suggested in the neurosurgical clinical literature 36 that relief of chronic pain by electrical stimulation in humans may result, in part, from activation of ~3-endorphin-containing neuronal systems. It is important to recognize, however, that other neurotransmitter and neuropeptide systems (for instance, serotonin, enkephalin, and substance P) may contribute significantly to the complex phenomenon of stimulation analgesia and that they merit careful consideration in future anatomical and physiological studies. 44 Acknowledgments The authors thank Carol Lewis and Mark Powell for their excellent technical assistance and Cynthia Nado for the illustrations. References 1. Abols IA, Basbaum AI" Afferent connections of the rostral medulla of the cat: a neural substrate for midbrain-medullary interactions in the modulation of pain. 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9 Localization of pro-opiomelanocortin neurons in human brain 33. Ray CD: Electrical and chemical stimulation of the CNS by direct means for pain control: present and future. Clin Neurosurg 28: , Reyes-Vazquez C, Dafny N: Microiontophoretically applied morphine and naloxone on single cell activity in the parafascicular nucleus of naive and morphine-dependent rats. J Pharmacoi Exp Ther 229: , Reynolds DV: Surgery in the rat during electrical analgesia induced by focal brain stimulation. Science 164: , Richardson DE: Analgesia produced by stimulation of various sites in the human beta-endorphin system. Appl Neurophysiol 45: , Richardson DE: Intracranial stimulation for the control of chronic pain. Clin Neurosurg 31: , Richardson DE, Akil H: Pain reduction by electrical brain stimulation in man. Part 1: Acute administration in periaqueductal and periventricular sites. J Neurosurg 47: , Richardson DE, Akil H: Pain reduction by electrical brain stimulation in man. Part 2: Chronic self-administration in the periventricular gray matter. J Neurosurg 47: , Riley HA: An Atlas of the Basal Ganglia, Brain Stem and Spinal Cord. Baltimore: Williams & Wilkins, Romagnano MA, Joseph SA: Immunocytochemical localization of ACTH~ 39 in the brainstem of the rat. Brain Res 276:1-16, Ruda MA, Coffield J, Dubner R: Demonstration of postsynaptic opioid modulation of thalamic projection neurons by the combined techniques of retrograde horseradish peroxidase and enkephalin immunocytochemistry. J Neurosci 4: , StrahlendorfJC, Strahlendorf HK, Barnes CD: Inhibition of periaqueductal gray neurons by the arcuate nucleus: partial mediation by an endorphin pathway. Exp Brain Res 46: , Sugimoto T, Takada M, Kaneko T, et al: Substance P- positive thalamocaudate neurons in the center medianparafascicular complex in the cat. Brain Res 323: , Tsubokawa T, Yamamoto T, Katayama Y, et al: Thalamic relay nucleus stimulation for relief of intractable pain. Clinical results and/3-endorphin immunoreactivity in the cerebrospinal fluid. Pain 18." , Ullan J: Cortical topography of thalamic intralaminar nuclei. Brain Res 328: , Watkins LR, Mayer DJ: Organization of endogenous opiate and nonopiate pain control systems. Science 216: , Watkins LR, Young EG, Kinscheck IB, et al: The neural basis of footshock analgesia: the role of specific ventral medullary nuclei. Brain Res 276: , Watson S J, Barchas JD, Li CH: B-lipotropin: localization of cells and axons in rat brain by immunocytochemistry. Proc Natl Acad Sei USA 74: , Yeung JC, Yaksh TL, Rudy TA: Concurrent mapping of brain sites for sensitivity to the direct application of morphine and focal electrical stimulation in the production of antinociception in the rat. Pain 4:23-40, Young EG, Watkins LR, Mayer DJ: Comparison of the effects of ventral medullary lesions on systemic and microinjection morphine analgesia. Brain Res 290: , Young RF, Kroening R, Fulton W, et al: Electrical stimulation of the brain in treatment of chronic pain. Experience over 5 years. J Neurosurg 62: , 1985 Manuscript received February 16, Accepted in final form September 18, This work was supported by the Louis Roncoli Foundation. Address reprint requests to: Webster H. Pilcher, M.D., Ph.D., Division of Neurosurgery, Box 661, University of Rochester School of Medicine and Dentistry, 601 Elrnwood Avenue, Rochester, New York J. Neurosurg. / Volume 68/April