Microprocesses in perception and personality

This paper utilizes an ontogenetically validated personality test, the Meta-Contrast Technique (MCT). The instrument focuses on the microdevelopment of perception (percept genesis) reflecting the process of individual adaptation to and construction of reality. Empirical evidence for the adequacy of the MCT paradigm in neuropsychological personality research is presented. In all, 45 patients were tested; thirty-three patients with various forms of supratentorial brain tumors were examined and compared with a group of 12 cerebrovascular disease (CVD) patients. Without the examiner's knowledge of degree of tumor malignancy, the MCT results allowed differentiation among tumors of various histological types. Uninformed of the final diagnosis, patients with highly malignant gliomas showed panic-related anxiety and schizoid-like regressions, in contrast both to patients with low and nonmalignant brain tumors and to the group of CVD patients.

A long-lived transient resulting from flash photolysis of hematoporphyrin in aqueous solution

A long-lived transient with a lifetime of several hundred microseconds was observed following the flash photolysis of aqueous solutions of hematoporphyrin buffered at pH 7.5. The transient-ground state difference absorption spectrum was determined 500 microseconds after flash photolysis. The yield of this species was found to increase with increasing hematoporphyrin concentration and it was also found to depend on the excitation wavelength. The lifetime of the species is not significantly affected by the presence of oxygen. Because the triplet state of hematoporphyrin is not the only long-lived species produced by flash photolysis of aqueous hematoporphyrin solutions, the observed triplet state extinction coefficients will be lower than the true value and hence the triplet state yields of hematoporphyrin determined by the flash photolysis, "complete conversion" technique, are only upper limits. The formation of the long-lived species is discussed in terms of electron transfer between the monomer partners in hematoporphyrin dimer and aggregates which are present in aqueous solutions of hematoporphyrin, particularly in concentrated solutions.

Space habitat contaminant growth models

This paper outlines the need for and role of Contaminant Growth Models (CGM) in designing space habitats and space mission operations. The tremendous complexity of the contamination problem and the current lack of data suggests that a "layered" approach to CGM development and utilization be made in conjunction with medical/toxicological research and risk assessments. Two space habitat contaminant examples: thermodegradation and hydrazine, are utilized to show how such a model might be useful in analyzing the environment of a space habitat. The examples also depict how NASA's interdisciplinary Center for Space Environmental Health (CSEH) will combine medical/toxicological research results with engineering environmental system design tools to set standards and help in the design, prediction, monitoring and safe control of space habitats. Since the CSEH project is just beginning, the contents of the paper are descriptive of the goal and the approach being taken, rather than conclusive in nature.

Hard metal interstitial pulmonary disease associated with a form of welding in a metal parts coating plant

We describe two cases of hard metal pulmonary disease (one fatal) in workers employed in the same area of a metal coating plant using the detonation gun process for applying a durable metal surface to metal parts. In this form of welding, a mixture of powdered metals, including tungsten carbide and cobalt, is heated by ignition of a flammable gas and propelled from the end of the "gun" at high temperature and velocity to form a welded metal coating. This process is done in an enclosed chamber and with each application, large volumes of fine aerosols are created. Inhalation exposure to hard metal may occur during the mounting and removal of the metal parts between applications, in spite of engineering controls and industrial hygiene surveillance. One of the cases presented with minimal chest x-ray abnormalities and an obstructive pattern on pulmonary function testing, although subsequent open lung biopsy showed diffuse interstitial pulmonary fibrosis. The fact that two cases of hard metal pulmonary interstitial disease occurred where thorough exposure control procedures and a surveillance program for cobalt were in place may indicate the need for revisions of the current technology used when hard metal is applied in the detonation gun process.

Proliferation of carcinogen-damaged hepatocytes during cell-cycle-dependent initiation of hepatocarcinogenesis in the rat

Hepatocyte proliferation and damage to DNA were characterized during the initiation phase of carcinogenesis in livers of rats that had received a single administration of the methylating agent methyl(acetoxymethyl)nitrosamine (DMN-OAc). Quiescent non-proliferating hepatocytes in intact livers did not appear to be susceptible to initiation by DMN-OAc, whereas proliferating hepatocytes in the S phase appeared to have greatest risk. To characterize the phenomenology of S-phase-dependent initiation further, the fractions of hepatocytes in the S and M phases of the cell cycle were enumerated at various times after treatment with DMN-OAc. Hepatocytes treated when in G1 experienced a delay of up to 20 h in the onset of S phase and a reduced rate of entry into the S and M cycle phases. Hepatocytes treated when in S phase experienced considerable delay in progression to mitosis due to part to inhibition of DNA replication. Hepatocytes treated when in late S/G2 also demonstrated a delay in progression into mitosis. The levels of 7-methylguanine and O6-methyldeoxyguanosine were quantified in the nuclear DNA of proliferating hepatocytes. The kinetics of removal of these lesions appeared to be first-order (half-life = 24 h). Hepatocyte risk of initiation was modeled by a function which summed over time the product of the fraction of hepatocytes in the S phase and the fraction of residual, unrepaired damage to DNA. For hepatocytes treated when in early G1, the time-weighted frequency of premutagenic DNA damage that was present during DNA replication was estimated to be less than half of that for hepatocytes treated when in early S. The results suggest that cell-cycle-dependent variation in sensitivity to initiation of hepatocarcinogenesis may be, in part, due to efficient removal of potentially carcinogenic lesions from DNA during an extended G1. The apparent high sensitivity of hepatocytes in late S/G2 suggests the contribution of additional factors.

Differences in adhesion to tissue culture plastic of clonally related transformed and control sublines from an epithelial cell strain

Clonally related sublines of the NAL1A lung epithelial cell strain were used in a comparison of the mechanism of attachment and the morphology of control and transformed epithelial cells. The initial attachment and spreading of the control cells on tissue culture plastic was shown to be dependent upon adsorption of serum vitronectin to the substratum. The alpha v subunit of the vitronectin receptor was detected in both the control and transformed cells by immunoprecipitation and immunoblot methods. The spontaneously transformed cells differed from the control cells in that, whereas attachment to tissue culture plastic could occur by binding to adsorbed vitronectin, the transformed cells could also become attached, with time, by a vitronectin-independent mechanism. Attachment by this vitronectin-independent reaction was inhibited by the protein synthesis inhibitor cycloheximide and also by the microtubule-disrupting drugs demicolcemid and nocodazole. The morphologies of attached control and transformed cells cultured on tissue culture plastic were disrupted by treatment with cytochalasin B, demicolcemid or nocodazole, indicating that the shape of these cultured epithelial cells is dependent upon the microtubule system as well as the actin filaments. These results show one important difference between the control and transformed cells, in that the transformed cells can attach to tissue culture plastic by a vitronectin-independent mechanism that involves new protein synthesis by the cell. Another interesting difference is that this vitronectin-independent attachment of the transformed cells was sensitive to inhibition by microtubule-disrupting agents. On the other hand, the attachment of either transformed or control cells to fibronectin- or vitronectin-coated surfaces was not affected by microtubule-disrupting agents.

Confocal laser scanning immunofluorescence microscopy of lamellar bodies and pulmonary surfactant protein A in isolated alveolar type II cells

We have determined the distribution of surfactant protein A (SP-A) in isolated type II cells from the lungs of rats by using immunofluorescence in conjunction with a laser scanning microscope fitted with a confocal aperture. Because of the very narrow depth of field of this microscope (less than 0.4 microns) in the confocal format, we were able to optically section type II cells and determine both the distribution of SP-A in the type II cell and the distribution of the lamellar bodies. The location of SP-A was determined by using fluorescein isothiocyanate-labeled secondary antibodies and the lamellar body distribution by using the lipid soluble fluorescent stain Phosphine 3R. SP-A was detected in the cytoplasm of type II cells as asymmetrically distributed punctate fluorescent bodies that resembled lamellar bodies in terms of size, number, and distribution within the cytoplasm of the cell. Most of the SP-A was located within bodies of the type II cell. Diffuse patches of fluorescence were seen in other cytoplasmic regions as well as the number of the cell. We conclude that SP-A is localized primarily, but not exclusively, in lamellar bodies of type II cells and that laser scanning microscopy is a much superior technique for the localization of SP-A than conventional microscopy in terms of both sensitivity and resolution.

Application of confocal scanning laser microscopy in experimental pathology

Confocal scanning laser microscopy (CSLM) represents an exciting new tool for scientific disciplines which focus on mechanistic studies such as experimental pathology. Enhanced resolution in the specimen plane and rejection of out-of-focus fluorescence flare allow analysis of specific nucleic acid sequences, enzymes, structural macromolecules, and cellular homeostasis utilizing fluorescent probes. Four different experimental applications are discussed which utilize CSLM to evaluate pathological processes at the subcellular, cellular, and tissue levels. Programmed cell death, or apoptosis, is a natural process of significance both during development and as a response to toxic stimuli. CSLM-imaging of nuclei of human B lymphoblastoid cells following exposure to a monofunctional alkylating agent suggests that the degradation of chromatin characteristic of apoptosis may occur in asymmetric patterns. Surfactant apoprotein-A is the major non-serum protein component of pulmonary surfactant and is essential for the extracellular function of surfactant. CSLM of alveolar type II cells suggests that apoprotein-A is present in both the cytoplasm, predominantly in lamellar bodies, and in the nucleus. The tumor promoter, phorbol myristate acetate, rapidly stimulated the formation of vacuoles in human neutrophils. CSLM using Lucifer Yellow as a probe suggests that cylindrical vacuoles are formed by fluid-phase pinocytosis. The blood-nerve barrier (BNB) in peripheral nerves may be an important target during toxin-induced neuropathies. Ricin-induced permeability of the BNB in the rat was rapidly visualized by CSLM as leakage of fluorescein isothiocynate (FITC)-dextran into the endoneurial compartment.

The role of inflammation in nonspecific abdominal aortic aneurysm disease

The predominant pathologic feature of abdominal aortic aneurysm is elastin destruction, and elastin destruction may be mediated by inflammation. In this investigation serial sections of abdominal aortic aneurysm specimens were selectively stained to study the relationship between inflammation and elastin degradation. In addition, soluble aortic extracts were examined for the presence of immunoglobulins. An inflammatory infiltrate was present in 8 of 10 of the abdominal aortic aneurysm specimens examined. The infiltrate was mononuclear, commonly located at the junction of the media and adventitia; it did not codistribute with loss of elastin. The presence of an inflammatory component in abdominal aortic aneurysm was associated with a large amount of immunoglobulin in soluble extracts from aneurysmal tissue compared to atherosclerotic and normal control extracts. This study further characterizes the microscopic pathology of abdominal aortic aneurysm and describes the presence of immunoglobulin in soluble tissue extracts. In addition, the possible role of inflammation in abdominal aortic aneurysm as it relates to protease expression is detailed.

EGF-receptor and extracellular matrix changes in mouse pulmonary carcinogenesis

Malignant Balb/c mouse lung cell clones related to alveologenic carcinoma exhibited low levels of epidermal growth factor (EGF) receptor activity compared to nonmalignant cell clones. Immunoprecipitation of cell homogenates and immunohistochemistry on urethane-induced lung tumors suggest that the absence of activity reflects decreased amounts of EGF receptor protein. Low levels of EGF receptor alone cannot cause neoplastic transformation, since a nonneoplastic cell cone, B5D3, exhibited low levels of EGF receptor despite its nontransformed phenotype. The reduced levels of EGF receptor in malignant clones have been mimicked by long-term (12 h) treatment of a nontransformed cell clone with 200 nM phorbol dibutyrate. The detection of mutated ras oncogene in the transformed cell lines, taken together with the EGF receptor findings, suggests that more than one alteration in the signal transduction pathway may be necessary for transformation in alveologenic adenoma and carcinoma cell systems. A further phenotypic feature of transformation, reduced expression of the extracellular matrix proteins fibronectin and laminin, may be mediated at the transcriptional level.

Clonal analysis of neoplastic transformation in cultured diploid rat liver epithelial cells

Expression of TGF-alpha mRNA, which correlates well with the ability of cells to condition medium with an EGF-like activity, clonally segregates best with tumorigenicity among the several single phenotypes considered in this study. The results of unreported studies in which we have analyzed the quantitative relationships between the expression of selected phenotypes and tumorigenicity, suggest that the elevated expression of myc and TGF-alpha mRNAs interact in their associations with tumor yield. These results suggest that elevated myc expression sensitizes hepatic epithelial cells to the possible tumorigenic action of TGF-alpha. This observation may explain why the correlation between the qualitative expression of TGF-alpha and tumorigenicity, described here, is not perfect. Conventionally applied markers of transformation in hepatocytes in vivo and in cultured liver epithelial cells in vitro that we studied -histochemical expression of GGT, ability to grow in medium containing low levels of calcium, and ability to grow in soft agar- clonally segregated with tumorigenicity poorly in liver epithelial cells transformed in vitro. We conclude that these phenotypes are not adequate markers for determining the lineage of hepatic epithelial neoplasms (including, probably hepatocellular cancers arising in vivo). This study appears to be the first to attempt to analyze clonally the association of these markers with tumorigenicity, and to quantify the sensitivity, specificity, and predictive value of the associations. Our study suggests that the relatively weak associations of these phenotypes with tumorigenicity may be related only to their stronger associations with expression of TGF-alpha, or to some other property that is strongly associated with tumorigenicity. Expression of TGF-alpha is more strongly associated with expression of GLC, for example, than is the GLC phenotype with tumorigenicity. At least for GLC, autocrine stimulation by TGF-alpha is likely, since EGF increases growth of WB cells in low calcium medium. This observation may explain the perfect correlation between expression of TGF-alpha and GLC. EGF also stimulates lactate dehydrogenase, pyruvate kinase, and glucose 6-phosphate dehydrogenase in WB cells. However, quantitative correlation between GGT activity and TGF-alpha is less strong. Thus, our data from these studies suggest that the tumorigenic phenotype of cultured hepatic epithelial cells is intimately dependent on the expression of the TGF-alpha gene, possibly producing autocrine stimulation of growth via the cells' EGF receptors. This is the most simple view of the potential relationship between TGF-alpha expression and tumorigenicity in liver epithelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and EBV-transformation of a minor population of normal human tonsillar B cells bearing a cold agglutinin-associated idiotope

Human autoantibodies which bind to red blood cells and cause agglutination in the cold, collectively comprise the cold agglutinins: a subset of these antibodies with specificity for the Ii carbohydrate antigen on the red cell surface bears cross-reacting idiotypic determinants. A monoclonal antibody has been raised which specifically inhibits the cold agglutination reaction, and has been used to probe normal lymphoid cell populations for the presence of B cells expressing the same idiotope. Although such cells comprise only 1-2% of the lymphoid cells in the tonsil, it has been possible to isolate them in good yield and purity using the antibody and magnetic beads. Released cells were infected with the Epstein-Barr (EB) virus and an idiotope-bearing line established. The IgM secreted by the line was found to agglutinate red cells in the cold indicating that cold agglutinin-producing cells were among this idiotope-positive population. The immunophenotype of the line, and the agglutinating power of the secreted IgM, have been compared with a similarly immortalized idiotope-bearing line established from neoplastic B cells of a patient with frank cold agglutination disease.

Outpatient percutaneous central venous access in cancer patients

A 1-year experience of percutaneous subclavian catheterization in outpatients with cancer was reviewed to document reliability, safety, and cost. There were 763 catheter insertions attempted with prospective documentation of complications in 664 consecutive patients. Catheter insertion was successful in 722 attempts (95%). There were only 13 pneumothoraces (2%). Thirty catheters required repositioning (4%). The average catheter duration was 191 days (range: 0 to 892 days). Fifty-six catheters (8%) were removed because of suspected infection. Documented catheter sepsis occurred in 21 patients (3%); catheter site infection occurred in 8 patients (1%). Thus, only 0.22 infections per catheter year occurred during this 382 catheter-year experience. The estimated cost of catheter insertion was $562, which is one-third the estimated cost for tunneled catheters ($1,403) and for reservoir devices ($1,738). In our experience, percutaneous subclavian catheterization is a reliable, cost-effective method compared with tunneled or reservoir devices, with an equivalent incidence of catheter-related infections. The cornerstone of our success with this program is a staff dedicated to catheter care and intensive patient education. In centers where a large number of patients require central venous access, percutaneous catheterization should be the technique of choice.

Enhanced invasiveness and metastatic potential of epithelial cell lines cultured in the presence of dimethyl sulphoxide

Several passage cycles of poorly metastatic malignant epithelial cells through immunosuppressed mice failed to induce enhanced metastasis-forming ability of cells derived from either the primary subcutaneous tumours or the resultant lung metastases. In vitro treatment of cultured malignant cells with dimethyl sulphoxide (DMSO) induced a reversible change in phenotype towards increased invasiveness but did not significantly increase metastasis formation. A cloned-cell line from a spontaneous in vitro transformant in the presence of DMSO was highly invasive and highly metastatic. In vitro treatment of cultured cells with 2% (v/v) DMSO produced alterations in morphology with decreased growth rate of all cell lines and decreased anchorage-independent colony formation in several malignant cell lines. All in vitro and in vivo effects were reversible following both short- and long-term (1 year) culture of cells in the presence of DMSO, suggesting epigenetic effects. These data support the concept of independent genetic controls for the invasiveness of tumours and the ability to form metastases.

Adhesion to laminin and expression of laminin in clonally related transformed and control sublines from an alveolar epithelial cell strain

Clonally related sublines of the NAL1A cell strain were used to examine the expression of laminin and the importance for the attachment and morphology of these control and spontaneously transformed alveolar epithelial cells. A laminin component was detected by immunoblot analysis in extracts of control cells cultured on plastic as a Mr 410,000 species consisting of disulfide-linked Mr 200,000 components. The laminin content of the malignant cells was reduced at least 40-fold as compared to that of the control cells. Cell attachment to laminin or to a laminin-like neurite promotion factor was compared with attachment to fibronectin and to extracellular matrix from bovine endothelial cells. Both control and transformed cells attached as well to laminin or neurite promotion factor as to fibronectin, in a serum-free adhesion assay. The control cells showed enhanced cell spreading on the surfaces coated with laminin, neurite promotion factor, or fibronectin. The transformed cells had very similar cell shape, as determined by phase contrast and scanning electron microscopy, when cultured on laminin or neurite promotion factor or fibronectin, as on tissue culture plastic.

Histopathology and cell culture characteristics of liver cells from grc- and grc+ rats given diethylnitrosamine

The histopathological response and cell culture characteristics of liver cells from the R16 (grc-) strain of rats, which carries an MHC-linked deletion, were examined one week after a single intraperitoneal injection of 200 mg/kg body weight diethylnitrosamine (DEN) and were compared with the response of liver cells from wild type (grc+) rats. The DEN exposure induced hydropic/vacuolar changes in the parenchymal cells and a limited proliferation of oval cells in the periportal areas of the livers of both grc+ and grc- rats. Primary culture of collagenase-digested livers consisted of parenchymal, bile ductular and oval-related cells as determined by cell-specific immunohistochemistry. Subpassaged cells from grc+ rats exhibited oval cell ultrastructural morphology, inducible histochemical staining for gamma-glutamyl transpeptidase (GGT), and DEN-associated onset of anchorage-independent growth. Primary cultures of liver cells from R16 rats consistently failed to form cell strains upon subpassage.

The histopathology of pulmonary reactions to drugs

Many pathologists and pulmonologists have trained and practiced in an era during which therapeutic options for patients have increased manyfold with regard to pharmacologic agents. Toxicity to the lung may be recognized as data accumulate in the form of case reports and clinical reviews to document the clinical and pathologic manifestations. For the histopathologist, paradoxically the more we learn the more difficult problems of interpretation become. Drug reactions previously described extensively at the autopsy now can be relatively easily interpreted in living patients with tissue obtained by open lung biopsy. This latter procedure is becoming less common with diffuse infiltrative lung disease in favor of the transbronchial biopsy in which sampling error abounds. There may be no disease or minimal change. The pathologic lesions may be duly noted and described, but then defy interpretation. An accurate differential diagnosis following biopsy may not solve the clinical problems for which a tissue diagnosis was sought. A specific diagnosis may be made that does not fit the clinical picture, or the real "answer" may be revealed distressingly later in the clinical-course or, more distressingly, at autopsy. The prudent histopathologist deals with such limitation by objectively stating the observable facets of tissue injury and synthesizing an interpretation. Notwithstanding the burgeoning literature on mechanisms of injury of toxic substances, the "state of the art" in 1990 for interpretation of pulmonary drug reactions still lies with the exacting judgment of experienced clinical physicians. Proper evaluation can result in exclusion of important mimickers of interstitial lung disease, usually infection and neoplasia; accurate categorization of tissue injury and comparison with known injurious responses to the drug in question may lead to a relatively specific diagnosis of drug-induced injury. Most drug reactions in the lung are classified as an interstitial pneumonia. This article illustrates and describes several interstitial reactions to injury that have few histologic features in common. However, because the lung can only react to injury in a limited number of ways, lesions that were thought at one time to be a specific clinicopathologic entity, for example, DIP, now evoke an ever-expanding differential diagnosis. In applying diagnostic criteria emphasis must be placed on the areas of overlap among tissue reactions as well. Specific agents may cause changes that are described as common for that agent, but a given patient may react with a variation or in a distinctly uncommon manner. There is enormous potential in this field for the laboratory investigation of drug injury.(ABSTRACT TRUNCATED AT 400 WORDS)

A solar erythemal radiation monitoring programme in New Zealand and Queensland, Australia

Solar erythemal radiation dose measurements carried out at five locations in Australasia from 43 degrees S to 19 degrees S during the southern summer of 1984-85 are reported. Monthly erythemal doses at the New Zealand sites reached a maximum of 8 x 10(4) joules/m2 for January and February 1985. During February 1985 this dose was the same as that measured at Townsville, Queensland, and slightly greater than the 7.3 x 10(4) joules/m2 at Rockhampton, Queensland. These results highlight the difficulty of calculating levels of solar erythemal radiation based on the latitude of the site because of local variations in cloud cover patterns.

Cytogenetic profile of in vitro spontaneously transformed and chemical tumor-derived cell lines related to the mouse lung alveologenic carcinoma

The karyotypes for a unique series of cloned untransformed and spontaneously and urethan-transformed mouse lung alveologenic carcinoma cell lines were compared by G-banding. All cell lines exhibited altered chromosome number accompanied by chromosomal aberrations. Both spontaneously and chemically transformed lines contained a higher proportion of cells carrying double minutes, short-arm chromosomes with no distinct banding pattern, and Robertsonian translocations between a variety of chromosomes. No common karyotypic abnormalities were observed among any of the transformed lines. These data suggest that enhanced genomic instability may lead to a critical subchromosomal molecular event(s) in generation of both spontaneously and chemically derived mouse lung alveologenic carcinoma.

Activated Ki-ras proto-oncogene in spontaneously transformed and chemical tumor-derived cell lines related to the mouse lung alveologenic carcinoma

An in vitro cell model of mouse lung alveologenic carcinoma consisting of preneoplastic nonmalignant cells, spontaneously transformed cells, and urethane-induced malignant cells was analyzed for phenotypic and genotypic changes associated with the transition to neoplasia. The polymerase chain reaction (PCR) was used to amplify the cDNA derived from the c-Ki-ras mRNA corresponding to exons 1 and 2 of the proto-oncogene. This approach allowed analysis of the gene transcription product rather than potentially unexpressed DNA. Direct sequencing of the PCR product identified a common single point mutation, alone or together with wild-type mRNA for c-Ki-ras, in all of the malignant clones. An A----G transition in the second base position of codon 61 was common to spontaneously malignant and chemical tumor-derived cell lines and to lines selected for lung metastatic behavior. The absence of the mutation in the nonmalignant cells suggests that the Ki-ras mutation may be a factor in onset or maintenance of the malignant phenotype and, at least in vitro, may be a late event in the transformation process.

Facilitation of acquisition and retention in preweanling but not postweanling rats by the presence of familiar home-nest material

A series of four experiments examined the effect of the presence of stimuli from the home nest on the acquisition and retention of aversively motivated behaviors in preweanling and adult rats. In Experiment 1, training in the presence of home-nest shavings facilitated acquisition of a T-maze discrimination to escape footshock for 16-day-old rats but not for adults. Experiment 2 demonstrated that the extent to which preweanlings were familiar with the home shavings determines the degree to which these stimuli facilitate spatial discrimination learning. When clean shavings were made more familiar than soiled home-nest stimuli (by changing the shavings every day) clean shavings enhanced discrimination performance, whereas no enhancement of learning by home shavings was observed. Experiment 3 extended the generality of the enhancement effect to a conditioned location aversion and examined the extent to which this facilitative effect was due to the tendency for home-nest shavings to elicit approach responses. Expression of the conditioned aversion was enhanced in subjects conditioned in the presence of home shavings, regardless of whether the home shavings were presented with the CS+, the CS-, or both. Experiment 4 determined that the enhanced expression of learning in the context of home-nest stimuli observed for preweanlings did not occur among subjects trained shortly after weaning. Collectively, these data suggest that whereas the enhancement of learning and retention by familiar home-nest stimuli enjoys generality across a number of conditioning situations, the effect may be limited to a relatively brief period during ontogeny.