Intravenous administration of progesterone and the onset of receptivity in female hamsters

Progesterone-facilitated receptivity is believed to be a genomically mediated event. However, in contrast to estrogen, progesterone's effects occur rapidly. This experiment was designed to examine the temporal onset of receptivity in hamsters following intravenous (IV) administration of progesterone. Ovariectomized female hamsters were tested for their responsiveness to 10 micrograms estradiol benzoate (EB) plus 0, 50, or 200 micrograms progesterone, administered subcutaneously (SC). The hamsters were tested for sexual receptivity at 0.5, 1, 1.5, 2, 3 and 4 h. Three days later they were fitted with jugular catheters. Two days after catheter implantation they received 10 micrograms EB SC; 48 h later they were tested for receptivity and then received an IV injection of 0, 50, or 200 micrograms progesterone in 0.2 ml propylene glycol. Following the IV P injection the animals were again tested at 0.5, 1, 1.5, 2, 3 and 4 h. Progesterone administration (either SC or IV) resulted in an increase in total lordosis duration (TLD) over time. TLD had significantly increased from the pre-progesterone levels by 2 h after 50 micrograms progesterone IV. In contrast, 50 micrograms progesterone SC did not cause an increase in TLD until 3 h postinjection. Moreover, in comparison to SC administration, the effects of IV progesterone were short-lived; TLD was significantly decreased by 4 h after injection. Following 200 micrograms progesterone, the latency to respond was shorter than with 50 micrograms progesterone (1.5 vs. 2 h). There was no difference in the onset of receptivity as a function of route of administration at the 200 micrograms dose.(ABSTRACT TRUNCATED AT 250 WORDS)     

Capsaicin suppresses substance P-induced joint inflammation in the rat

Intra-articular injection of 20 micrograms substance P in rat knee joints results in a pronounced inflammatory response. However, prior intra-articular injection of 1% capsaicin solution (1-5 weeks previously) virtually abolishes this response. This is not a neurotoxic effect of capsaicin on nerve fibres as denervation of the knee produces no alteration of the response to injected substance P. The potent effect of capsaicin on substance P-mediated inflammation cannot be attributed to depletion of mast cells by this treatment as the mast cell degranulator compound 48/80 injected into capsaicin pre-treated knees still gives rise to a marked inflammatory response. Compound 48/80 does not activate nerve fibres to cause release of substance P as it is equally effective in eliciting an inflammatory response in the presence of 100 micrograms of the substance P antagonist D-Pro4, D-Trp7,9,10 SP(4-11) in the synovial cavity. The results suggest that capsaicin may act by depleting substance P receptors in joint tissue.     

Modulation of immune response in vitro and in vivo by human granulocyte factors

The adherence of granulocytes induces secretion of specific granule contents. The secreted proteins were termed granulocyte factors (GF). The experiments in vivo provide evidence that GF play an essential role in the stimulation of PFC in BALB/c mice immunized with SRBC when applied before challenge three times (5 micrograms per mouse), but 50 micrograms per mouse given in the same way diminishes the response. To elucidate this discrepancy, the effect of GF on the generation of suppressor cells (SC) and helper cells (HC) in vitro has been investigated. Antigen specific nonadherent SC or HC were induced in vitro using CBA mice spleen cells incubated with 100 micrograms/ml or 0.1 mg/ml of TNP-KLH, respectively, for 4 days. GF in concentrations of 0.1 to 1 microgram/ml abolish antigen specific SC generation. SC and HC activity was tested in cooperative cultures. Antigen specific SC in delayed hypersensitivity (DTH) to BCG were induced in an in vitro system as above using normal BALB/c spleen cells and 100 micrograms/ml PPD. Nonadherent suppressor cells were transferred intravenously into cyclophosphamide (CY)-treated syngeneic recipients. The recipients were immunized to BCG immediately after the cell transfer. DTH was measured by foot-pad reaction. This reaction was positive to PPD in CY treated mice immunized to BCG, while it was suppressed by the transfer of in vitro induced SC. When the SC were induced in the presence of 1 microgram/ml GF, the suppression was abrogated. The higher GF concentrations stimulated SC activities when they were measured in response to a nonrelated antigen and in specific anti-PPD response, but the HC inhibition could not be excluded.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dogs hypersensitive to house dust mite

Active intradermal skin reaction, respiratory resistance and respiratory patterns were examined in five dogs after intradermal injection or inhalation of house dust mite (Dermatophagoides farinae) extract and the results obtained were as follows: 1. Three out of the five dogs exhibited positive skin reactions against house dust mite extract solution at concentrations of 0.1 micrograms/ml to 10 micrograms/ml. 2. Inhalation of mite extract at a concentration of 1 mg/ml by the dogs that had positive skin reactions produced a marked increase in respiratory resistance and changes in spirogram. These results suggest that the dogs that exhibited positive skin reactions were spontaneously sensitized to D. farinae.     

The effect of intra-articular capsaicin on nerve fibres within the synovium of the rat knee joint

The aim of this study was to establish the effects of intra-articular capsaicin (pelargonic acid vallinylamide) on synovial innervation of the rat knee. Rats were sacrificed 1, 2, 4 and 7 days after intra-articular injection of capsaicin and joint tissues stained with either conventional haematoxylin and eosin (H and E) or with specific antibodies to the calcitonin gene-related peptide (CGRP), substance P (both of which are markers for primary afferent fibres), the C-flanking peptide of neuropeptide Y (CPON) (localised in postganglionic sympathetic fibres), or protein gene product 9.5 (a pan-neuronal marker). At lower concentrations (0.1% and 0.25%), capsaicin produced no change in peptide staining pattern or histological appearance. At 0.5% capsaicin, there was complete loss of nerve fibres showing positive staining for CGRP and substance P at all time points. Staining for CPON and protein gene product 9.5 was still present, but decreased, 1 and 2 days after treatment and virtually absent at 4 and 7 days. These findings provide evidence for partially selective denervation induced by 0.5% capsaicin, in contrast to 1% capsaicin which abolished staining for all peptide markers, indicating a total ablation of nerve fibres. A consistent but unexpected finding was the presence of a severe inflammatory response in joints treated with 0.5% and 1% capsaicin. An influx of polymorphonuclear leucocytes was found to occur within 4 h of injection, with progressive appearance of mononuclear cells after this time. We conclude that it is difficult to specifically deplete sensory nerve fibres from the synovium by means of local capsaicin injection. Although selective loss of staining for sensory nerve fibres could be achieved by injection of 0.5% capsaicin, there was progressive non-specific loss of post-ganglionic autonomic fibres which may be related to the severe inflammatory response provoked by the higher doses of capsaicin.     

The Calcification of Fibrous Pleural Lesions Produced in Guinea Pigs by the Injection of Chrysotile Asbestos Dust

In guinea-pig pleural granulomas produced by the injection of chrysotile asbestos dust, the initial cellular lesions were replaced by old acellular fibrous tissue within 15-18 months. After this period calcification of the fibrous tissue commonly occurred. The first sign of calcification was always the formation among the collagen fibres of large numbers of spherical lamellated bodies. These structures stained positive for acid mucopolysaccharides, and electron-microscope observations showed that they were formed around a core of asbestos dust. Calcium in the form of apatite crystals was also often present in these structures. Shortly after the formation of lamellar bodies the mucopolysaccharide normally present on the collagen fibres, was removed and instead it formed a thick coating around any free asbestos dust in the fibrous area. The first signs of general calcium deposition were seen both in contact with collagen fibres and on the surface of the coated asbestos dust. Eventually calcium deposits filled all the tissue spaces and enclosed collagen, coated asbestos dust and lamellar bodies in a solid mass of calcification.     

Sensitisation of articular afferents in normal and inflamed knee joints by substance P in the rat

To examine whether substance P (SP) influences the response properties of fine articular afferents in normal and acutely inflamed joints, single units were recorded from the rat knee during normal and noxious joint rotations. Only three of 39 units were activated by a single bolus injection of 0.1 mM SP. However, 35% (7/20) of the nerve fibres from the normal joint and 21% (4/19) of the units from the inflamed joint significantly increased their responses to movements after the SP injection. This was most prominent during noxious movements in normal joints, whereas in inflamed joints increase of responses occurred mainly during normal movements. These data indicate that SP may also be involved in the process of sensitisation of primary afferents during an inflammation.     

Time course of effect of capsaicin on ultrastructure and histochemistry of substance P-immunoreactive nerves associated with the cardiovascular system of the guinea-pig

Capsaicin, a neurotoxin which depletes substance P from primary afferent nerve fibres, was injected systematically into adult guinea pigs. The effects of capsaicin were studied by immunohistochemistry, electron microscopy and radioimmunoassay at times from 5 min to 1 year. Within 5 min after a single injection of capsaicin (50 mg/kg) substance P immunofluorescence appeared less intense and less homogeneous than normal (i.e. it appeared granular). Large nerve trunks remained evident, but there were fewer fine single nerve fibres. With increasing time there was a progressive decrease in the number of immunoreactive fibres; by 4 h there was a marked reduction in the number of fibres and by 24 h only an occasional fibre was evident. In animals sacrificed 2 or more hours after treatment large brightly fluorescent swellings were seen in many nerves. Depletion of substance P-immunoreactivity persisted for as long as 365 days after treatment. Electron microscopy revealed alterations in capsaicin-sensitive nerve fibres within 5 min after treatment. Many fibres appeared swollen and there was disruption of their internal morphology, e.g. loss of microtubules and filaments and presence of an amorphous flocculent material in the axons. With increasing time after treatment, electron-dense profiles, indicative of degenerating nerve fibres, were commonly seen associated with Schwann cells. These findings demonstrate that the effects of systemic administration of capsaicin to adult guinea pigs occur rapidly in capsaicin-sensitive nerve fibres. The long lasting depletion of substance P-containing fibres is due to their degeneration.     

Calcification of subcutaneously implanted type I collagen sponges. Effects of formaldehyde and glutaraldehyde pretreatments.

Although collagen-containing implants are widely used in various surgical applications, there has been relatively little attention paid to the possibility that this type of biomaterial may undergo pathologic calcification which could compromise its function. The present study reports for the first time the calcification of a series of implants of purified collagen sponges prepared with graded degrees of aldehyde-induced cross-linkages (assessed by shrinkage-temperature, wetting time, and collagenase digestibility). Type I collagen sponges were pretreated with either glutaraldehyde (0.1% to 2.0% aqueous solution, for 5-180 minutes) or formaldehyde (as vapors for 15 minutes to 15 hours), and implanted subcutaneously for 21 days in weanling rats. Although specimens not pretreated with either aldehyde reagent and the formaldehyde sponges pretreated for 15 minutes were resorbed without evidence of calcification, all other aldehyde-pretreated implants mineralized. The degree of calcification did not correlate with extent of cross-linking. Formaldehyde-pretreated implants calcified more extensively (Ca2+ = 87.8 +/- 2.8 micrograms/mg, mean +/- standard error of the mean; n = 58) than did glutaraldehyde-pretreated implants (Ca2+ = 40.9 +/- 1.4 micrograms/mg; n = 52). It is concluded that both glutaraldehyde- and formaldehyde-pretreated Type I collagen sponges calcify after subdermal implantation in young rats. Although aldehyde pretreatment of Type I collagen sponge implants is a prerequisite for their eventual mineralization, the threshold level of aldehyde-induced cross-linking required to potentiate their maximal pathologic calcification is low.     

Effect of alpha lipoic acid co‐administration on structural and immunohistochemical changes in subcutaneous tissue of anterior abdominal wall of adult male albino rat in response to polypropylene mesh implantation

Polypropylene mesh is commonly used in the treatment of abdominal hernia. Different approaches were addressed to improve their tissue integration and consequently reduce long‐term complications. This study aimed to investigate the effect of alpha‐lipoic acid (ALA) co‐administration on structural and immunohistochemical (IHC) changes in the subcutaneous tissues of the anterior abdominal wall of the adult rat in response to polypropylene mesh implantation. Forty adult male albino rats were divided into: group I (control), group II (receiving ALA), group III (polypropylene mesh implantation) and group IV (mesh implantation + ALA co‐administration). After 4 weeks, subcutaneous tissue samples were prepared for light microscopy and IHC study of CD34 as a marker for angiogenesis. In groups I and II rats, positive CD34 expression was demonstrated by IHC reaction, localized to endothelial cells lining small blood vessels. Group III showed an excess inflammatory reaction, deposition of both regular and irregularly arranged collagen fibres around mesh pores and few elastic fibres. CD34‐positive was detected not only in cells lining small blood vessels but also in other cells scattered in the connective tissue indicating angiogenesis. In group IV, ALA co‐administration resulted in less inflammatory reaction, regular collagen deposition, enhanced elastic fibres synthesis and a significant increase in CD34‐positive cells and small blood vessels reflecting improved angiogenesis. ALA co‐administration with polypropylene mesh implantation controlled the inflammatory reaction, helped regular collagen deposition, enhanced elastic fibres synthesis and improved angiogenesis in the subcutaneous tissue of anterior abdominal wall of adult albino rats, suggesting a possible role of ALA in optimizing mesh integration in subcutaneous tissue.     

An ultrastructural study of the macrophages of the carrageenan-induced granuloma in the rat lung

The intralobular injection of 0.17 ml of 2 per cent. carrageenan into the rat lung induced an inflammatory granulomatous response. This inflammation was characterized in the early stages by immediate polymorphonuclear (PMN) leucocyte infiltration into the alveoli. Within 2 days the PMN's began to disappear and were replaced by carrageenan-containing macrophages. Alveolar macrophages actively phagocytised the injected carrageenan and were the dominant cell type in the alveoli for the length of this study (365 days). These low-turnover alveolar macrophages, which with light microscopy stained pinkish-red metachromatically with toludine blue due to their carrageenan content, underwent changes in vacuole morphology as well as changes in size and shape. Throughout the course of this inflammation, these macrophages had carrageenan-containing vacuoles which could be seen undergoing fusion to form larger vacuoles which at times constituted half the size of the entire macrophage cytoplasm. The carrageenan in the vacuoles was an amorphous flocculent appearing material for the initial 14 days but changed to a more fine filamentous form for the remainder of the study. This was probably due to its partial digestion by lysosomal enzymes. The general composition of the cytoplasm remained fairly constant during the 365 days. Pinocytotic vesicles and free ribosomes were abundant and the many mitochondria were small and rounded. Smooth endoplasmic reticulum lysosomes, and Golgi apparati, although not prominent, were present but the most striking feature of the cytoplasm was the presence of numerous lamellar bodies (phagocytosised surfactant). Some of the macrophages increased greatly in overall size (five to seven times) compared to their initial size. A few isolated macrophages could be seen degenerating but no general necrosis was seen. Except for one isolated case, no epithelioid cells were observed in this carrageenan inflammation. Fibrosis, if present at all, was very localised and was only evident at day 340 post-injection. This fibrosis generally involved one or two carrageenan-containing macrophages encapsulated by a few collagen fibres. No widespread fibrosis was ever observed in this study which confirmed earlier histological and biochemical investigations.     

SO2-induced bronchopathy decreases airway sensitization with intratracheal ovalbumin in the rat

We investigated in this study the effect of SO2-induced bronchopathy on airway sensitization to ovalbumin in the rat. Sprague-Dawley rats were immunized with a single intratracheal injection of ovalbumin (OA) 100 micrograms in 0.1 ml PBS or 0.1 ml Bordetella pertussis (BP) heat-killed vaccine (6.5 X 10(9) cells X ml-1). The rats were primed immediately after SO2 exposure (60 h, 200 ppm; group I, n = 16) and three months after exposure was achieved (group II, n = 24), then compared to a control group exposed to air (group III, n = 30). Airway sensitization was evaluated by the in vitro contractile response to antigen challenge using paired tracheal rings. Specific IgE level was determined with PCA reactions. No significant difference was found in the maximal contractile responses to carbamyl choline within and between each group. Excepted in animals of group III, OA alone was not found able to sensitize the airways. When OA was used in association with BP, sensitization of the airways occurred, but this occurrence was found to depend upon a previous SO2 exposure: 73.3% in group III, 41.7% in group II and 25% in group I were sensitized. In addition, only five animals (BP + OA injected rats of group III) displayed a PCA positive reaction. It is concluded that: 1) the concomitant intratracheal injection of BP with OA increased the occurrence of specific airway sensitization, 2) a previous chronic exposure to SO2 decreased the specific tracheal smooth muscle sensitization to intratracheal ovalbumin. This decrease persisted, although slighter, when immunization was done three months after the exposure to SO2 was stopped.     

On the interactions between macrophages and developmental stages of Schistosoma mansoni: the cytotoxic mechanisms involved in macrophage-mediated killing of schistosomula in vitro

In an in vitro cytotoxicity assay, mouse adherent peritoneal exudate macrophages (APEM), harvested 8-10 weeks post Schistosoma mansoni infection caused sizable (greater than 90%) specific killing of schistosomula. This cidal effect was not diminished by the addition of scavengers of oxidative burst products to the cytotoxicity assay, albeit macrophages from schistosome-infected mice produced more H2O2 than did macrophages from non-infected mice. Of inhibitors of lysosomal enzyme function and release added to the cytotoxicity assay, trypan blue (1 mg/ml) fully abolished the schistosomulicidal effect; hydrocortisone (100 micrograms/ml) was partly effective, and gold salts (1 mg/ml) were ineffective. A cidal effect was not apparent in the absence of L-arginine nor in the presence of excess (greater than 400 micrograms/ml) L-arginine, L-lysine or L-ornithine. Arginase (5 U/ml) totally abrogated the schistosomulicidal effect. The findings suggest that a macrophage protein of a lysosomal origin, dependent on arginine for its reaction and/or production, may be involved in the in vitro killing of schistosomula by macrophages from S. mansoni-infected mice.     

Experimental Calcification of the Myocardium: Ultrastructural and Histochemical Investigations

Focal areas of calcification are frequent in rat myocardium 30 and 60 days after administration of dihydrotachysterol. These areas are PAS-positive, stain deeply with alcian blue and show high affinity for colloidal iron. Calcification is almost completely confined to intracellular structures. Small clusters of needle-shaped crystals are first found in apparently undamaged mitochondria in undamaged myocardial cells. When all the mitochondria are calcified, the cell degenerates, and inorganic crystals are laid down in relationship with its myofilaments. In other myocardial cells, clusters of amorphous or finely granular inorganic substance are found in both mitochondria and myofibrils. Both structures show signs of advanced degeneration. Inorganic substance has only occasionally been found within the structures of the sarcoplasmic reticulum. These structures do not seem to be involved in myocardial calcification under the present experimental conditions. Calcification of myocardial cells gives rise to a cellular reaction. Many macrophagic cells surround the calcified areas, which are rapidly reabsorbed. The present results show that myocardial mitochondria are actively engaged in controlling the intracellular concentration and movement of calcium ions. Their role in the myocardial contraction-relaxation cycle and the possible mechanism of myocardial calcification are discussed.     

Differential immunohistochemical localization of cytokeratins and collagen types I and III in experimentally-induced cirrhosis

Liver cirrhosis was induced in male Wistar rats by subcutaneous injection (1 ml of 30 g/l) of an aqueous solution of thioacetamide. Using the indirect immunoperoxidase technique, high molecular weight keratins were localized in bile ducts and ductules. Low molecular weight cytokeratins were present in regenerating hepatocytes in active cirrhosis; bile ducts were unstained. These results suggest that cytokeratin staining may be useful in distinguishing bile duct epithelium and hepatocytes in hepatobiliary diseases. Anticollagen type III antibody stained hepatocytes and thin connective tissue fibres, while anticollagen type I antibody stained thicker fibres and some sinusoidal cells but not hepatocytes. Collagens were usually undetectable in normal liver cells. It is suggested, therefore, that hepatocytes may play a major role in collagen type III production which precedes the deposition of collagen type I. By contrast, collagen type I may be produced by fibroblasts and some cells along sinusoids (e.g. perisinusoidal fat-storing cells) after liver injury.     

Immunoreactive calcitonin gene-related peptide and substance P coexist in sensory neurons to the spinal cord and interact in spinal behavioral responses of the rat

Using immunohistochemistry evidence was obtained for the coexistence of calcitonin gene-related peptide (CGRP)- and substance P (SP)-like immunoreactivity in spinal sensory neurons. Analysis of caudally directed biting and scratching (CBS) behavior was carried out after intrathecal administration of CGRP and SP alone or in combination. Thus, SP (up to 20 micrograms) alone caused CBS only for a few minutes after injection, whereas SP (10 micrograms) plus CGRP (20 micrograms) caused a response with a duration up to 40 min. CGRP (20 micrograms) alone had no effects in this model. These findings provide support for a possible interaction of the two peptides at synapses in the dorsal horn of the spinal cord.     

Neurogenic inflammation caused by wool fabric in the rat; possible mediation by substance P

We have measured plasma extravasation in response to stimulation of the rat hind footpad with wool fabric. The stimulus was delivered using a constantly weighted disc of wool rotating at 4 revolutions per minute for 30 min on the base of blisters induced by vacuum. The protein content of prestimulated blister base perfusate was 38 +/- 2 micrograms/ml in a 30-min collection. With stimulation this increased to 80 +/- 10 micrograms/ml (P less than 0.001; Student's paired t-test). No increase in plasma extravasation was observed in adult animals pretreated as neonates with capsaicin indicating that the plasma extravasation observed was neurogenically mediated. Plasma extravasation was also observed with substance P added to the perfusate. The threshold for substance P-induced plasma extravasation was 2 X 10(-11) mol/30 min. These results suggest that neurogenic inflammation induced by the mechanical stimulus of wool fabric may be mediated by substance P. This reaction may be involved in wool-induced prickle and itch noted in some individuals.     

2-Deoxy-D-glucose inhibits intracellular multiplication and promotes intracellular killing of Legionella pneumophila in A/J mouse macrophages.

Legionella pneumophila can grow intracellularly in A/J mouse macrophages. 2-Deoxy-D-glucose (2dG) (0.1, 1, and 10 mM) inhibited intracellular multiplication and promoted intracellular killing of L. pneumophila dose dependently when it was added to the culture medium of macrophage monolayers, whereas it did not inhibit the bacterial growth in buffered yeast extract broth, which was used for an L. pneumophila culture. The effect of 2dG was reversible because the surviving bacteria resumed intracellular multiplication after the washing away of 2dG from the culture. The effect of 2dG was also competitively inhibited by high concentrations of glucose. The inhibitory effect of 2dG was not attributed to the inhibition of bacterial phagocytosis by macrophages. Furthermore, sodium fluoride (0.1 and 1 mM), cycloheximide (0.1 and 1 microgram/ml), and tunicamycin (1, 2, and 5 micrograms/ml) did not promote the killing of L. pneumophila in macrophages, implying that the inhibitory effect of 2dG cannot be attributed to the inhibition of glycolysis, protein synthesis, and protein glycosylation in macrophages. We suggest that 2dG promotes intracellular killing of L. pneumophila by activating some novel killing mechanism of macrophages.     

The skin inflammatory response of the badger (Meles meles).

Twenty-five badgers, captured in an area where they had been implicated in outbreaks of bovine tuberculosis, received intradermal inoculations of control medium, 150 micrograms phytohaemagglutinin (PHA), 40 units streptokinase/10 units streptodornase (SK/SD), 200 micrograms purified protein derivative of Mycobacterium bovis (PPD), Freund''s incomplete adjuvant (IFA), and Freund''s complete adjuvant (CFA), each in 0.1 ml of inoculum. The reactions were assessed by skinfold thickness and skin histology 30 min--7 days after inoculation. Control medium caused slight cellular reaction, mostly polymorphonuclear leucocytes (PMNs), but no significant increase in skinfold thickness. SK/SD provoked no reaction. PHA stimulated a marked increase in skinfold thickness; the cellular reaction was predominantly PMNs, with some macrophages occurring after several days. IFA and CFA promoted a long-lasting increase in skinfold thickness, and a mixed histological picture of PMNs and macrophages; later in the response, especially to CFA, giant cells and some lymphocytes occurred. PPD stimulated a small increase in skinfold thickness with a timing (2-3 days) consistent with delayed hypersensitivity (DTH); there was, however, no erythema or palpable oedema or induration. The histology was an initial multifocal reaction of PMNs with a later phase of lymphocytes and macrophages with some granuloma formation. Other cell types (eosinophils, basophils) were seen in varying proportions in all reaction sites. M. bovis was isolated from four badgers; the cellular reaction to PPD was stronger than in uninfected animals, but other aspects of the skin response were unaffected. This study shows the capacity of badgers for strong inflammatory responses, and is the first report of a DTH response to PPD in this species.     

Plasma levels of dihydrotestosterone in the cycling rat: implications for the regulation of lordosis behavior

Exogenously administered dihydrotestosterone (DHT) is a potent inhibitor of sexual behavior in ovariectomized, estrogen-primed rats and endogenous DHT has been implicated as an inhibitor of sexual behavior in cycling rats. To determine the temporal relationship between DHT levels and the expression of sexual behavior, females were tested at midnight of each day of the 4-day estrous cycle for lordosis response to male mounts and subsequently bled. Plasma DHT levels were highest (177 +/- 10 pg/ml) at proestrus (P), when lordosis was fully expressed, fell to significantly lower levels (136 +/- 8, p less than 0.025) at estrus (E), and rose at diestrus I and diestrus II (DII) (152 +/- 8 and 154 +/- 10 respectively). Other female rats were bled from a jugular cannula at 4-hr intervals between midnight of DII and midnight of E. Plasma DHT was elevated from 1200 of P to 0400 of E and fell rapidly through 0800 of E. This elevation of endogenous DHT coincides with the expression of lordosis behavior, and the magnitude of this peak is many times lower than the concentrations reached by the minimal exogenous dose that inhibits lordosis behavior. This suggests that cyclic changes of DHT in the peripheral circulation do not inhibit lordosis behavior during the estrous cycle.