Molecular biology and neurosurgery in the third millennium

The application of techniques in molecular biology to human neurosurgical conditions has led to an increased understanding of disease processes that affect the brain and to novel forms of therapy that favorably modify the natural history of many of these conditions. Molecular strategies are currently being either used or sought for brain tumors, stroke, neurodegenerative diseases, vascular malformations, spinal degenerative diseases, and congenital malformations of the central nervous system. Considering that the structure of deoxyribonucleic acid was ascertained by Watson and Crick as recently as 1953, the progress that has been made to implement molecular medicine in clinical practice has been meteoric. More than 2000 patients have been treated in approved gene therapy trials throughout the world. Many of these patients have been treated for neurological diseases for which conventional medical therapies have been of limited utility. As part of this continuing series on advances in neurosurgery in the third millennium, we first reflect on the history of the nascent field of molecular biology. We then describe the powerful techniques that have evolved from knowledge in this field and have been used in many publications in Neurosurgery, particularly within the past decade. These methods include commonly used techniques such as advanced cytogenetics, differential display, microarray technology, molecular cell imaging, yeast two-hybrid assays, gene therapy, and stem cell utilization. We conclude with a description of the rapidly growing field of bioinformatics. Because the Human Genome Project will be completed within 5 years, providing a virtual blueprint of the human race, the next frontier (and perhaps our greatest challenge) will involve the development of the field of "proteomics," in which protein structure and function are determined from the deoxyribonucleic acid blueprint. It is our conviction that neurosurgeons will continue to be at the forefront of the treatment of patients with neurological diseases using molecular strategies, by performing essential research leading to increased understanding of diseases, by conducting carefully controlled studies to test the effects of treatments on disease processes, and by directly administering (by neurosurgical, endovascular, endoscopic, or stereotactic means) the treatments to patients.

Behavioral evaluation of visceral pain in a rat model of colonic inflammation

A new rat model was established up to evaluate the antinociceptive effect of compounds in visceral pain. The test consisted in measuring the performance of rats in an aversive light stimulus avoidance experimental device. Rats with TNBS-induced colitis had a lower number of total active lever pressings and did not discriminate the active lever from the inactive one. Morphine (1 mg/kg, s.c.) and CI-977 (0.001 mg/kg, s.c.) treatment restored the level of pressing activity of animals and their ability to discriminate the active lever from the inactive one. Naloxone treatment antagonized the improvement of performance produced by morphine. The results obtained indicate that this behavioral paradigm may be used to evaluate the antinociceptive potential of compounds.

Fedotozine blocks hypersensitive visceral pain in conscious rats: action at peripheral kappa-opioid receptors

The effect of fedotozine on visceral hypersensitivity was evaluated in conscious rats. One hour after colonic irritation (0.6% acetic acid intracolonically), a 30 mmHg colonic distension was applied for 10 min. Irritation increased the number of abdominal contractions induced by colonic distension (23.4 +/- 4.1 versus 4.8 +/- 1.4 in saline-treated rats, P < 0.001). Facilitation of colonic pain was reversed in a dose-dependent manner by fedotozine ((+)-(-1R1)-1-phenyl-1-[(3,4,5-trimethoxy)benzyloxymethyl]-N ,N-dimethyl-n-propylamine), (+/-)-U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-1-pyrrolidinyl]cyclohexyl)benzen eacetamide) and morphine (respective ED50 values 0.67, 0.51 and 0.23 mg/kg s.c.). The kappa-opioid receptor antagonist, nor-binaltorphimine, abolished the effects of fedotozine and (+/-)-U-50,488H but not those of morphine. Low doses of naloxone (30 microg/kg s.c.) blocked the effect of morphine but not of fedotozine or (+/-)-U-50,488H. After intracerebroventricular administration, morphine was very potent (ED50 1.7 microg/rat), (+/-)-U-50,488H poorly active (58% of antinociception at 300 microg/rat) and fedotozine inactive up to 300 microg/rat. These results show that fedotozine blocks hypersensitive visceral pain by acting on peripheral kappa-opioid receptors in animals.

Response heterogeneity of 5-HT3 receptor antagonists in a rat visceral hypersensitivity model

Subcutaneous administration of granisetron (BRL 43694, endo-1-methyl-N-(9-methyl-9-azabicyclo[3.3.1.]non-3-yl-1 H-indazole-3-carboxamide) and zacopride (4-amino-N-(1-azabicyclo[2.2.2.]oct-3-yl)-5-chloro-2-methoxybenzamide), two 5-HT3 receptor antagonists, at doses ranging from 3 to 1000 micrograms/kg, inhibited abdominal contractions induced by distension (30 mmHg, 10 min) of irritated colon (0.6% acetic acid) in conscious rats with a bell-shaped dose-response curve. The ED50 of granisetron and zacopride were 17.6 and 8.2 micrograms/kg, respectively. In contrast, both tropisetron (ICS 205-930, (3-a-tropanyl)t-indole-3-carboxylic ester) and ondansetron (GR38032F, 1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1 H-imidazol-1-yl)methyl]-4 H-carbazol-4-one hydrocloride dihydrate) were inactive in this model. These data further support the concept of a heterogeneity in the potency of 5-HT3 receptor antagonists in modulating visceral hypersensitivity in conscious rats. This finding is in agreement with a reported efficacy of granisetron but not of ondansetron in patients with irritable bowel syndrome.

The V3 domain of SIVmac251 gp120 contains a linear neutralizing epitope

Antisera to 21 synthetic peptides containing hydrophilic sequences of simian immunodeficiency virus strain mac251 (SIVmac251) gp120 and gp32 were tested for the ability to neutralize SIVmac251. Goat antisera raised to peptides SP-1 and SP-1V containing the carboxy-terminal portion of the V3 domain of SIVmac251 gp120 between amino acids 327 and 339 inhibited syncytium formation (90% inhibition at a 1/1024 dilution) and cell killing of CEMx174 cells by SIVmac251 (50%) inhibition of cell killing at a dilution of 1/5832), SIVDeltaB670 (1/568), and SIVsmH4 (1/740). Neutralizing antibodies to SIVmac251, SIVDeltaB670, and SIVsmH4 could be adsorbed by peptides containing a neutralizing V3 sequence of SIVmac251 gp120 (GLVFHSQPIND, amino acids 329-339) but not by peptides lacking this sequence. This V3 neutralizing region corresponds to a homologous V3 neutralizing site within HIV-2 gp120 reported by Björling et al. 1991, Proc. Natl. Acad. Sci. USA 88, 6082-6086, 1994, J. Immunol. 152, 1952-1959). Antibodies in 20 of 31 sera obtained from rhesus macaques infected with SIVmac251 reacted with a peptide containing the entire V3 sequence of SIVmac251 gp120, whereas no sera contained antibodies reacting with the V3 neutralizing site between amino acids 329 and 339. Low levels of antibody-mediated recognition and subsequent lack of selective pressure against this linear V3 neutralizing site might in part explain why this region is not a dominant neutralizing site and also why sequences within V3 do not vary during the course of SIV infection.

Effect of fedotozine on the cardiovascular pain reflex induced by distension of the irritated colon in the anesthetized rat

The effect of fedotozine was evaluated in a model of colonic hypersensibility to balloon distension in anesthetized rats. Acetic acid (0.6%, intracolonically) significantly enhanced the hypotension reflex response to colonic distension (P < 0.05). At a noxious pain pressure (75 mm Hg), fedotozine ((+)-(-1R)-1-phenyl-1-[(3,4,5- trimethoxy)benzyloxymethyl]-N,N-dimethyl-n-propylamine) had no effect at 0.6 and 1 mg/kg i.v. in saline-treated rats and higher doses were required to produce antinociception (ED50 = 2.57 mg/kg i.v.). By contrast, fedotozine at 0.6 and 1 mg/kg i.v. displayed 38 and 54% antinociception (P < 0.05) respectively, in acetic acid-treated animals, leading to a decrease in its ED50 (1.15 mg/kg i.v.). Similar results were obtained with (+/-)-trans-N-methyl-N-[2-(pyrrolidinyl)-cyclohexyl]benzo[b]-thiophene- 4-acetamide (PD-117,302), a kappa-opioid receptor agonist, while the antinociceptive action of morphine and a kappa-opioid receptor agonist, trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1- pyrrolidinyl]cyclohexyl)benzenacetamide ((+/-)-U-50,488H), was identical in control and acetic acid-treated animals. Nor-binaltorphimine, a selective kappa-opioid receptor antagonist, reversed the enhanced antinociceptive activity of fedotozine and PD-117,302. It is concluded that acetic acid induces colonic hypersensibility to painful mechanical stimuli and that some but not all kappa-opioid receptor ligands can have enhanced efficacy in this pathological situation.

[Geo-linguistic mobility of the Francophone population of Quebec and Ontario]

"The relation between geographic and linguistic mobility among the population of French mother tongue in Ontario and Quebec [Canada] is explored. The results show that language behaviour, in particular the use of English as home language, is fairly regularly and coherently related to spatial behaviour. A distinct positive correlation between geographic and linguistic mobility is observed. It would seem, furthermore, that the ability of various linguistic environments to impose the dominant language is notably stronger when the dominant language is English." (SUMMARY IN ENG)

[Francophone migratory flows between Quebec and Ontario: toward spatio-linguistic polarization?]

"The link between linguistic mobility and spatial mobility is investigated through the analysis of francophone migratory flows between Quebec and Ontario during the 1981-1986 period. Using 1986 census data, the analysis shows that these flows are indeed part of a more polarized spatio-linguistic situation. However, the results also show that this phenomenon is closely related to another one, that is to the return migration flow toward Quebec. Due to this relation, the role of Quebec as a true migration pole for French population outside Quebec is more problematic." (SUMMARY IN ENG)

Pretransplant and posttransplant antibodies in human corneal transplantation

The purpose of this study was to measure the association between antibody formation and endothelial corneal allograft reactions in 533 consecutive corneal graft recipients. The median follow-up time of these recipients was 732 days. Pretransplant panel-reactive antibodies were not found to be associated with endothelial corneal allograft reactions. Out of 533 recipients, 239 developed posttransplant antibodies during the course of this study. The formation of posttransplant antibodies was frequent in recipients with pretransplant antibodies and in HLA-A,-B-incompatible recipients. Posttransplant antibodies most often appeared within the first six months after transplantation whereas endothelial allograft reactions most often occurred later. Out of 65 recipients who developed PPRA and underwent an allograft reaction, 53 had a PPRA peak prior to, or at about the time of, the allograft reaction. Corneal allograft reaction events diagnosed during the second and third year after surgery were correlated with PPRA formation during the first year after grafting. The 36-month reaction-free survival rate of transplants was estimated at 72% in recipients with PPRA compared with 86% in recipients without PPRA (log rank P value = 0.002). Furthermore, posttransplant antibody formation altered the outcome of corneal allografts in both HLA-A and -B-compatible and -incompatible recipients. These findings suggest that posttransplant antibody development represents a high risk of endothelial corneal allograft reactions.

Effects of feeding, fasting, and caerulein treatment on ornithine decarboxylase in rat pancreas

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in polyamine biosynthesis. We examined circadian variations in pancreatic ODC activity and time-course effects of caerulein in fed and fasted rats. Significant circadian variations in amount of ODC activity were observed. The highest values were obtained during the dark period (1855 +/- 406 pmoles CO2/h), and the lowest during the light period (359 +/- 84 pmoles CO2/h). Caerulein treatment induced hypertrophy and hyperplasia of the pancreas in fed rats; increases in pancreatic ODC activity preceded the rise in protein and DNA contents (447 +/- 44 pmoles CO2/h and 5573 +/- 893 pmoles CO2/h, 6 and 12 h after the first injection of caerulein, respectively). In fasted rats, pancreatic ODC activity was very low (149 +/- 37 pmoles CO2/h) and caerulein treatment induced a transient increase in this activity 12 h after the first injection; hypertrophy but not hyperplasia of the pancreas was observed. In caerulein-treated fasted rats, refeeding during the night following a 48 h fasting period was not enough to increase either ODC activity or DNA content. These findings demonstrate that nutritional status is an important factor in the regulation of ODC activity and, thereby, in caerulein-induced pancreatic growth.

[Altruism and the donor]

On December 20, 1988, the government of France passed a law to protect people who voluntarily participate in biomedical research. This article makes extensive reference to a major study, titled From Biology to Ethics, by Jean Bernard, a well-respected authority in the field of bioethics. The author looks at models proposed by Bernard, as examples for health volunteers, in particular, the blood donor and the self-experimenter. To set the tone of the article, she recalls the concept of altruism, as first proposed by Auguste Comte, then makes a linkage between his philosophy and Bernard's point of view. By trial and error, in their discussions, various ethics committees and the French State Council have agreed upon what constitutes fair compensation under the law. Unlike their Canadian counterparts, medical researchers in France have free access to volunteers who are not in perfect health--e.g., the elderly, people suffering from kidney deficiency, cirrhosis of the liver, etc.--but these "experimental subjects" receive no monetary compensation. Thus, healthy and less-than-healthy volunteers do not receive equal treatment under the law. This inequity, added to the fear of what amounts to a tax on the human body and the difficulty of ensuring just compensation, is giving rise to a great deal of uncertainty.

Human monoclonal antibody that recognizes the V3 region of human immunodeficiency virus gp120 and neutralizes the human T-lymphotropic virus type IIIMN strain

We describe a human IgG1 monoclonal antibody (N701.9b) derived by Epstein-Barr virus transformation of B cells from a human immunodeficiency virus-seropositive asymptomatic donor. This antibody was shown to recognize the principal neutralizing domain contained within the V3 region of gp120 of the MN strain of human immunodeficiency virus and MN-like strains, as determined by binding to the PB-1 fragment of MN gp120 and to synthetic peptides corresponding to the V3 region of MN and related virus strains. The epitope identified by monoclonal antibody N701.9b was mapped to a segment of V3 containing at least 7 amino acids (amino acids 316-322), which is located in the "tip" and "right" side of the V3 loop of the MN strain. Furthermore, this antibody manifested potent type-specific fusion-inhibitory activity against the MN strain but not against the IIIB or RF virus strains. This antibody also neutralized four virus isolates that had MN-like V3 region sequences and failed to neutralize three other strains containing unrelated V3 region sequences. Our findings confirm that the V3 region stimulates type-specific neutralizing antibody during natural human immunodeficiency virus infection in humans. The potential clinical use of this antibody is discussed.

Effects of pancreatic polypeptide on biliary flow and bile acid secretion stimulated by secretin and cholecystokinin in the conscious pig

Fourteen castrated male Large White pigs, weighing 42.5 +/- 1.0 kg, were fitted with biliary and duodenal fistulae for biliary secretion studies. Furthermore, catheters were placed in a carotid artery for blood sampling and in a jugular vein for peptide infusion. Bile was automatically restituted to the animals and continuously sampled for analysis on experimental days. Following an 8 day recovery period, infusion studies were performed after an overnight fast. After a 30 min basal period, sustained biliary flow and bile acid output were obtained and maintained throughout the assay with secretin (36 pmol/kg/h) and CCK-8 (600 pmol/kg/h) infusion. Then, 200, 400, 600, 800 or 1200 pmol/kg/h of porcine pancreatic polypeptide (PP) were infused for 60 min. Secretin plus CCK infusion was continued for 1 h after PP infusion was stopped. Each dose of PP was given on a separate day. Biliary flow was not affected by PP except for the dose of 400 pmol/kg/h. On the contrary, bile acid concentration and output decreased with the lowest dose of PP (200 pmol/kg/h). As soon as the first dose of PP was infused, bile acid concentration and output fell to about 60% of values obtained with secretin plus CCK. Plasma levels of PP were below or similar to postprandial values for 200, 400 and 600 pmol/kg/h and they were significantly larger with 800 and 1200 pmol/kg/h. Bile acid concentration and output did not return to values obtained with secretin plus CCK infusion after cessation of PP infusion. In conclusion, porcine PP given in physiological doses to the pig decreases bile acid output whereas biliary flow remains unaffected.

Polyvalent human immunodeficiency virus synthetic immunogen comprised of envelope gp120 T helper cell sites and B cell neutralization epitopes

In previous studies, we have used antisera raised to envelope (env)-gene-encoded synthetic peptides to identify a region of (HIV) glycoprotein (gp) 120 env protein designated SP10 that contains a type-specific neutralizing determinant. To develop a polyvalent, synthetic peptide inoculum that can evoke both neutralizing antibodies and T cell proliferative responses to more than one HIV isolate, synthetic peptides containing type-specific neutralizing determinants of gp120 from HIV isolates HTLV-IIIB (IIIB), HTLV-IIIMN (MN) and HTLV-IIIRF (RF) were coupled to a 16 amino acid T cell epitope (T1) of HIV-IIIB gp120 and used to immunize goats. Goat antisera to each T1-SP10 peptide derived from the SP10 region of gp120 of IIIB, MN, and RF neutralized HIV isolates IIIB, MN and RF in a type-specific manner. Moreover, peripheral blood T cells from immunized goats also proliferated in a type-specific manner to peptides derived from gp120 of IIIB, MN, and RF. When combined in a trivalent inoculum, T1-SP10 peptides from HIV-1 isolates IIIB, MN, and RF evoked a high titered neutralizing antibody response to isolates IIIB, MN, and RF in goats and as well induced immune T cells to undergo blast transformation in the presence of peptides derived from gp120 of all three HIV isolates. The T1 portion of the T1-SP10 construct was shown to induce a B cell antibody response against determinants within the T1 peptide in addition to inducing T cell proliferative responses in immune goat T cells. Moreover, the SP10 portion of the T1-SP10 constructs not only induced B cell antibody production but also induced type-specific T cell proliferative responses localized to the C-terminal variable sequences of the SP10 peptides. Finally, the T1-SP10 peptide construct induced memory T cell proliferative responses to native gp120 env protein. Thus, combinations of homologous SP10 region synthetic peptides containing type-specific neutralizing determinants and T cell epitopes of HIV gp120 may be useful in man to elicit high titered neutralizing B cell responses and, as well, T cell responses to more than one HIV isolate.

Polyvalent human immunodeficiency virus synthetic immunogen comprised of envelope gp120 T helper cell sites and B cell neutralization epitopes

In previous studies, we have used antisera raised to envelope (env)-gene-encoded synthetic peptides to identify a region of (HIV) glycoprotein (gp) 120 env protein designated SP10 that contains a type-specific neutralizing determinant. To develop a polyvalent, synthetic peptide inoculum that can evoke both neutralizing antibodies and T cell proliferative responses to more than one HIV isolate, synthetic peptides containing type-specific neutralizing determinants of gp120 from HIV isolates HTLV-IIIB (IIIB), HTLV-IIIMN (MN) and HTLV-IIIRF (RF) were coupled to a 16 amino acid T cell epitope (T1) of HIV-IIIB gp120 and used to immunize goats. Goat antisera to each T1-SP10 peptide derived from the SP10 region of gp120 of IIIB, MN, and RF neutralized HIV isolates IIIB, MN and RF in a type-specific manner. Moreover, peripheral blood T cells from immunized goats also proliferated in a type-specific manner to peptides derived from gp120 of IIIB, MN, and RF. When combined in a trivalent inoculum, T1-SP10 peptides from HIV-1 isolates IIIB, MN, and RF evoked a high titered neutralizing antibody response to isolates IIIB, MN, and RF in goats and as well induced immune T cells to undergo blast transformation in the presence of peptides derived from gp120 of all three HIV isolates. The T1 portion of the T1-SP10 construct was shown to induce a B cell antibody response against determinants within the T1 peptide in addition to inducing T cell proliferative responses in immune goat T cells. Moreover, the SP10 portion of the T1-SP10 constructs not only induced B cell antibody production but also induced type-specific T cell proliferative responses localized to the C-terminal variable sequences of the SP10 peptides. Finally, the T1-SP10 peptide construct induced memory T cell proliferative responses to native gp120 env protein. Thus, combinations of homologous SP10 region synthetic peptides containing type-specific neutralizing determinants and T cell epitopes of HIV gp120 may be useful in man to elicit high titered neutralizing B cell responses and, as well, T cell responses to more than one HIV isolate.

Effects of pancreatic polypeptide on the pancreatic exocrine secretion stimulated by secretin and cholecystokinin in the conscious pig

Fourteen castrated male Large White pigs, weighing 42.5 +/- 1.0 kg, were fitted with pancreatic and duodenal fistulae for pancreatic secretion studies. Moreover, catheters were placed in a carotid artery for blood sampling and in a jugular vein for peptide infusion. Pancreatic juice was automatically restituted to the animals and continuously sampled for analysis on experimental days. Following an 8-day recovery period, perfusion studies were performed after an overnight fast. After a 30-min basal period, sustained pancreatic flow and protein output were obtained and maintained throughout the assay with secretin (36 pmol/kg/h) and CCK-8 (600 pmol/kg/h) infusion. Then, 200, 400, 600, 800 or 1200 pmol/kg/h of porcine pancreatic polypeptide (PP) were infused for 60 min. Secretin + CCK infusion was continued for 1 h after PP infusion was stopped. Each dose of PP was given on a separate day. Neither pancreatic flow nor bicarbonate output were affected whatever the dose of infused PP. On the contrary, protein concentration and output decreased with the lowest dose of PP (200 pmol/kg/h) and the diminution was more pronounced with the other doses. With 600 pmol/kg/h as well as with 800 and 1200 pmol/kg/h of PP, pancreatic protein output fell to about 20% of values obtained with secretin + CCK. Plasma levels of PP were below or similar to postprandial values for 200, 400 and 600 pmol/kg/h and they were significantly larger with 800 and 1200 pmol/kg/h. Protein concentration and output returned to values obtained with secretin + CCK infusion after cessation of PP infusion. In conclusion, porcine PP given in physiological doses to the pig decreases pancreatic protein output whereas pancreatic flow remains unaffected.

Functional replacement of the HIV-1 rev protein by the HTLV-1 rex protein

Two evolutionarily distinct families of human retroviruses, the human immunodeficiency viruses (HIV) and the human T-cell leukaemia viruses (HTLV), have been defined (reviewed in ref. 1). Although these virus groups share tropism for human CD4+ T cells, they differ markedly in primary sequence, genetic organization and disease association (AIDS versus adult T-cell leukaemia), but show similar general strategies for the regulation of viral gene expression. Each encodes a protein able to trans-activate transcription from the homologous viral long terminal repeat (tat in HIV, tax in HTLV), although these proteins act by different mechanisms and do not appear to be interchangeable. Each virus also produces a second trans-acting protein that induces the expression of the unspliced messenger RNAs encoding the viral structural proteins (rev in HIV and rex in HTLV). Here we show that the rex protein of HTLV-I can functionally replace the rev protein of HIV-1 in transient expression assays. This genetic complementation by rex is adequate for the rescue of a replication-defective rev mutant of HIV-1. This unexpected shared function between the structurally distinct rex and rev proteins emphasizes the importance of this highly conserved pathway for the regulation of human retrovirus gene expression.

Patterns of language comprehension deficit in abused and neglected children

The similarity between factors associated with child abuse/neglect and those associated with language disability suggests that maltreated children are a population at risk for language problems. This study investigated the performance of three groups of abused/neglected children and a matched group of nonmaltreated subjects on three tests of language comprehension. Results revealed significant differences among groups for all measures. Severely neglected children obtained the lowest scores on all tests; the abused children consistently obtained lower scores than the controls; and generally neglected children showed the smallest difference in performance from the control group. These findings suggest a model for understanding relationships between abuse/neglect and language disability.

Antibodies that inhibit fusion of human immunodeficiency virus-infected cells bind a 24-amino acid sequence of the viral envelope, gp120

Antisera to recombinant human immunodeficiency virus (HIV) proteins containing the entire envelope, gp160, or the central portion of the envelope, PB1, can inhibit fusion of virally infected cells in culture. This fusion inhibition is HIV-variant specific--that is, anti-gp160-IIIB inhibits fusion of isolate HTLV-IIIB-infected cells but not of isolate HTLV-IIIRF-infected cells. Both anti-gp160 and anti-PB1 are completely blocked in fusion inhibition activity by the addition of PB1 protein. A 24-amino acid peptide (RP135, amino acids 307-330) completely blocks fusion inhibition activity of both antisera and also blocks the activity of serum from a chimpanzee infected with HTLV-IIIB. Thus, the principal epitope that elicits fusion-inhibiting antibodies is located in the central portion of gp120.

More on the geographical distribution of physicians

This paper presents more empirical evidence on the geographical distribution of physicians. Probit procedure and data for the Province of Quebec were used. The results are consistent with the standard location theory and therefore with those of Newhouse et al. (1982a). They also show that quality of leisure, distance to central city areas, average income and presence of a hospital are significant in explaining the probability that at least one physician (specialist or general practitioner) is present in a given town. Other variables were also considered.