Glucoregulatory effects and prolonged duration of action of davalintide: a novel amylinomimetic peptide

AIMS

Davalintide is a second-generation amylinomimetic peptide possessing enhanced pharmacological properties over rat amylin to reduce food intake in preclinical models. The current experiments in rats describe additional glucoregulatory actions of davalintide consistent with amylin agonism, and explore the duration of action of these effects.

METHODS

Subcutaneous (SC) injection of davalintide slowed gastric emptying with equal potency to amylin (ED₅₀'s = 2.3 and 4.1 µg/kg). This effect was maintained for 8 h with davalintide, but not amylin. Intraperitoneal injection of davalintide also reduced food intake with a potency similar to amylin (ED₅₀'s = 5.0 and 11.3 µg/kg). Consistent with amylin agonism, davalintide (10 µg/kg, SC) suppressed the plasma glucagon response over 90 min following an intravenous arginine bolus in anaesthetized rats. The elimination t(½) of davalintide (200 µg/kg, SC) was 26 min, similar to the t(½) of amylin, suggesting that pharmacokinetic-independent mechanisms contribute to davalintide's enhanced duration of action. Binding kinetic studies using ¹²⁵I davalintide revealed no appreciable dissociation from the amylin nucleus accumbens receptor after 7 h while ¹²⁵I rat amylin did dissociate from this receptor (K(off) = 0.013/min). Sustained SC infusion of davalintide (275 µg/kg/day) or amylin (300) decreased plasma glucose after an oral glucose challenge at 2 weeks (by 27 and 31%) and suppressed gastric emptying at 3 weeks (by 29 and 47%), demonstrating durable glucoregulatory actions of both peptides.

CONCLUSIONS

These data show glucoregulatory properties of davalintide consistent with amylin agonism and suggest that slowed receptor dissociation plays a role in davalintide's prolonged pharmacodynamic actions.

Enhanced amylin-mediated body weight loss in estradiol-deficient diet-induced obese rats

In rodents, ovariectomy (OVX) elicits weight gain and diminished responsiveness to homeostatic signals. Here we characterized the response of obese OVX rats to peripheral amylin. Rats received sham surgery (SHAM), OVX, or OVX with hormonal replacement (17β-estradiol, 2 μg per 4 d; OVX+E) and were infused with vehicle or amylin (50 μg/kg · d) for 28 d. Amylin reduced body weight (5.1 ± 1.1%) and food intake (10.9 ± 3.4%) in SHAM rats but was twice as efficacious in OVX rats in reducing weight (11.2 ± 1.9%) and food intake (23.0 ± 2.0%). There were no differences between amylin-treated SHAM and OVX+E rats. OVX decreased metabolic rate (∼24%) and increased respiratory exchange ratio relative to SHAM. Amylin partially normalized metabolic rate (13% increase) in OVX rats and decreased respiratory exchange ratio in OVX and SHAM rats. Regarding central mechanisms, amylin infusion corrected the OVX-induced decrease in hippocampal neurogenesis and increased immobility in the forced swim test. Additionally, amylin increased neurogenesis (∼2-fold) within the area postrema of OVX rats. To assess the contribution of endogenous leptin to amylin-mediated weight loss in OVX rats, amylin was administered to SHAM or OVX Zucker diabetic fatty rats. In SHAM rats, amylin infusion reduced food intake but not body weight, whereas in OVX Zucker diabetic fatty rats, food intake, body weight, and insulin were reduced. Overall, amylin induced greater body weight loss in the absence of estradiol via central and peripheral actions that did not require leptin. These findings support the clinical investigation of amylin in low estradiol (e.g. postmenopausal) states.

Davalintide (AC2307), a novel amylin-mimetic peptide: enhanced pharmacological properties over native amylin to reduce food intake and body weight

OBJECTIVE

The current set of studies describe the in vivo metabolic actions of the novel amylin-mimetic peptide davalintide (AC2307) in rodents and compares these effects with those of the native peptide.

RESEARCH DESIGN AND METHODS

The anti-obesity effects of davalintide were examined after intraperitoneal injection or sustained peripheral infusion through subcutaneously implanted osmotic pumps. The effect of davalintide on food intake after lesioning of the area postrema (AP) and neuronal activation as measured by c-Fos, were also investigated.

RESULTS

Similar to amylin, davalintide bound with high affinity to amylin, calcitonin and calcitonin gene-related peptide receptors. Acutely, davalintide displayed greater suppression of dark-cycle feeding and an extended duration of action compared with amylin (23 versus 6 h). Davalintide had no effect on locomotor activity or kaolin consumption at doses that decreased food intake. Davalintide-induced weight loss through infusion was dose dependent, durable up to 8 weeks, fat-specific and lean-sparing, and was associated with a shift in food preference away from high-fat (palatable) chow. Metabolic rate was maintained during active weight loss. Both davalintide and amylin failed to suppress food intake after lesioning of the AP and activated similar brain nuclei, with davalintide displaying an extended duration of c-Fos expression compared with amylin (8 versus 2 h).

CONCLUSION

Davalintide displayed enhanced in vivo metabolic activity over amylin while retaining the beneficial properties possessed by the native molecule. In vitro receptor binding, c-Fos expression and AP lesion studies suggest that the metabolic actions of davalintide and amylin occur through activation of similar neuronal pathways.

Combination therapy with amylin and peptide YY[3-36] in obese rodents: anorexigenic synergy and weight loss additivity

Circulating levels of the pancreatic beta-cell peptide hormone amylin and the gut peptide PYY[3-36] increase after nutrient ingestion. Both have been implicated as short-term signals of meal termination with anorexigenic and weight-reducing effects. However, their combined effects are unknown. We report that the combination of amylin and PYY[3-36] elicited greater anorexigenic and weight-reducing effects than either peptide alone. In high-fat-fed rats, a single ip injection of amylin (10 microg/kg) plus PYY[3-36] (1000 microg/kg) reduced food intake for 24 h (P < 0.05 vs. vehicle), whereas the anorexigenic effects of either PYY[3-36] or amylin alone began to diminish 6 h after injection. These anorexigenic effects were dissociable from changes in locomotor activity. Subcutaneous infusion of amylin plus PYY[3-36] for 14 d suppressed food intake and body weight to a greater extent than either agent alone in both rat and mouse diet-induced obesity (DIO) models (P < 0.05). In DIO-prone rats, 24-h metabolic rate was maintained despite weight loss, and amylin plus PYY[3-36] (but not monotherapy) increased 24-h fat oxidation (P < 0.05 vs. vehicle). Finally, a 4 x 3 factorial design was used to formally describe the interaction between amylin and PYY[3-36]. DIO-prone rats were treated with amylin (0, 4, 20, and 100 microg/kg.d) and PYY[3-36] (0, 200, 400 microg/kg.d) alone and in combination for 14 d. Statistical analyses revealed that food intake suppression with amylin plus PYY[3-36] treatment was synergistic, whereas body weight reduction was additive. Collectively, these observations highlight the importance of studying peptide hormones in combination and suggest that integrated neurohormonal approaches may hold promise as treatments for obesity.

Antiobesity action of peripheral exenatide (exendin-4) in rodents: effects on food intake, body weight, metabolic status and side-effect measures

BACKGROUND

Exenatide (exendin-4) is an incretin mimetic currently marketed as an antidiabetic agent for patients with type 2 diabetes. In preclinical models, a reduction in body weight has also been shown in low-fat-fed, leptin receptor-deficient rodents.

OBJECTIVE

To more closely model the polygenic and environmental state of human obesity, we characterized the effect of exenatide on food intake and body weight in high-fat-fed, normal (those with an intact leptin signaling system) rodents. As glucagon-like peptide-1 receptor agonism has been found to elicit behaviors associated with visceral illness in rodents, we also examined the effect of peripheral exenatide on kaolin consumption and locomotor activity.

METHODS AND RESULTS

High-fat-fed C57BL/6 mice and Sprague-Dawley rats were treated with exenatide (3, 10 and 30 microg/kg/day) for 4 weeks via subcutaneously implanted osmotic pumps. Food intake and body weight were assessed weekly. At 4 weeks, body composition and plasma metabolic profiles were measured. Kaolin consumption and locomotor activity were measured in fasted Sprague-Dawley rats following a single intraperitoneal injection of exenatide (0.1-10 microg/kg). Exenatide treatment in mice and rats dose-dependently decreased food intake and body weight; significant reductions in body weight gain were observed throughout treatment at 10 and 30 microg/kg/day (P<0.05). Decreased body weight gain was associated with a significant decrease in fat mass (P<0.05) with sparing of lean tissue. Plasma cholesterol, triglycerides and insulin were also significantly reduced (P<0.05). Exenatide at 10 microg/kg significantly reduced food intake (P<0.05) but failed to induce kaolin intake. In general, locomotor activity was reduced at doses of exenatide that decreased food intake, although a slightly higher dose was required to produce significant changes in activity.

CONCLUSION

Systemic exenatide reduces body weight gain in normal, high-fat-fed rodents, a model that parallels human genetic variation and food consumption patterns, and may play a role in metabolic pathways mediating food intake.

PYY[3-36] administration decreases the respiratory quotient and reduces adiposity in diet-induced obese mice

In rodents, weight reduction after peptide YY[3-36] (PYY[3-36]) administration may be due largely to decreased food consumption. Effects on other processes affecting energy balance (energy expenditure, fuel partitioning, gut nutrient uptake) remain poorly understood. We examined whether s.c. infusion of 1 mg/(kg x d) PYY[3-36] (for up to 7 d) increased metabolic rate, fat combustion, and/or fecal energy loss in obese mice fed a high-fat diet. PYY[3-36] transiently reduced food intake (e.g., 25-43% lower at d 2 relative to pretreatment baseline) and decreased body weight (e.g., 9-10% reduction at d 2 vs. baseline) in 3 separate studies. Mass-specific metabolic rate in kJ/(kg x h) in PYY[3-36]-treated mice did not differ from controls. The dark cycle respiratory quotient (RQ) was transiently decreased. On d 2, it was 0.747 +/- 0.008 compared with 0.786 +/- 0.004 for controls (P < 0.001); light cycle RQ was reduced throughout the study in PYY[3-36]-treated mice (0.730 +/- 0.006) compared with controls (0.750 +/- 0.009; P < 0.001). Epididymal fat pad weight in PYY[3-36]-treated mice was approximately 50% lower than in controls (P < 0.01). Fat pad lipolysis ex vivo was not stimulated by PYY[3-36]. PYY[3-36] decreased basal gallbladder emptying in nonobese mice. Fecal energy loss was negligible ( approximately 2% of ingested energy) and did not differ between PYY[3-36]-treated mice and controls. Thus, negative energy balance after PYY[3-36] administration in diet-induced obese mice results from reduced food intake with a relative maintenance of mass-specific energy expenditure. Fat loss and reduced RQ highlight the potential for PYY[3-36] to drive increased mobilization of fat stores to help meet energy requirements in this model.

Structure–activity relationship studies on 2-heteroaryl-4-arylimidazoles NPY5 receptor antagonists

A series of 2-heteroaryl-4-arylimidazoles with potent in vitro activity at the NPY5 receptor was developed. Introduction of electron-withdrawing groups on the 4-aryl ring led to a significant improvement of in vitro potency. Several analogues from this series had anorectic activity in rodent feeding models, but were also found to have undesired behavioral effects in spontaneous locomotor activity.

Structure-activity relationships in a series of NPY Y5 antagonists: 3-amido-9-ethylcarbazoles, core-modified analogues and amide isosteres

Beginning with carbazole 1a, the amide and alkyl substituents were optimized to maintain potency while adding solubilizing groups. Efforts to replace the 3-amino-9-ethylcarbazole core, a known carcinogen, used the SAR generated in the carbazole series for guidance and led to the synthesis of a number of core-modified analogues. In addition, an isosteric series, in which the amide was replaced with an imidazole, was prepared. Two potent new series lacking the putative toxicophore were identified from these endeavors.

In vitro and in vivo characterization of 3-[2-[6-(2-tert-butoxyethoxy)pyridin-3-yl]-1H-imidazol-4-yl]benzonitrile hydrochloride salt, a potent and selective NPY5 receptor antagonist

To investigate the anorectic potential of NPY5 receptor antagonists, we have profiled the in vitro and in vivo properties of 3-[2-[6-(2-tert-butoxyethoxy)pyridin-3-yl]-1H-imidazol-4-yl]benzonitrile hydrochloride salt (1). This compound was found to have excellent NPY5 receptor affinity and selectivity, potent functional antagonism, and good peripheral and central nervous system exposure in rats. This compound attenuated bovine pancreatic polypeptide induced food intake in rats but failed to demonstrate anorectic activity in rodent natural feeding models.

Diurnal variation in thermoregulatory response to chlorpyrifos and carbaryl in the rat

Time of day of exposure is rarely considered in the study of insecticide toxicology. It would be expected that the circadian temperature rhythm (CTR) as well as the circadian rhythms of other physiological processes would affect the efficacy of anticholinesterase (antiChE) insecticides. The ability of antiChE insecticides to alter core temperature (T(c)) could be affected by time of exposure in relation to the CTR. To this end, we assessed time of exposure on the efficacy of the antiChE insecticides chlorpyrifos (CHP) and carbaryl (CAR) to alter T(c) in the rat. T(c) and motor activity (MA) were monitored by radiotelemetry. Rats were dosed orally with 0, 30, and 50 mg/kg CHP or 0, 25 and 75 mg/kg CAR at 09:00 and 15:00 h. Both insecticides caused an acute decrease followed by a delayed increase in T(c) by 24-48 h post-exposure. The temperature index (TI) (area under curve of DeltaT(c) with time) was significantly greater when CHP was given at 15:00 h as compared with 09:00 h. The maximum decrease in T(c) was similar for morning and afternoon CHP. The TI following CAR was similar for morning and afternoon exposure. CHP suppressed the 24 h MA equally when given in the morning and afternoon. CAR was more effective in reducing MA when given in the morning as compared with the afternoon. The T(c) increase measured 24 h after dosing was greater when CHP was given in the morning. Overall, time of day affected the thermoregulatory toxicity of CHP but not CAR. Another experiment showed that the hypothermic efficacy of oxotremorine, a muscarinic agonist, was greater when injected at 09:00 h as compared with 15:00 h. Hence, cholinergic stimulation is probably not the only mechanism to explain the effects of the chronotoxicogical effects of some antiChE insecticides.

Ovarian hormones can organize the rat corpus callosum in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. Previous data suggest that CC sensitivity to testosterone ends by postnatal day 8 (P8). In contrast, responsivity to ovarian hormones extends as late as P25. The current series of experiments investigates whether ovarian hormone effects on the callosum are permanent and whether CC sensitivity to ovarian hormones extends beyond P25. We found that P70 ovariectomy (Ovx) did not affect callosal size, suggesting that ovarian hormone exposure sometime prior to P70 is sufficient to feminize the CC, and that once the callosum is feminized, the effects can not be reversed. We also found that P25 ovariectomy enlarged, or defeminized, adult female CC, whereas ovary transfer starting on P55 or P70 counteracted this enlarging effect, resulting in feminized adult CC. Thus, although a previously feminized callosum is not affected by P70 ovarian hormone removal, a not-yet feminized callosum can still be feminized after P70. These findings indicate that there is flexibility in the developmental window within which the female brain is responsive to the active feminization process initiated by ovarian hormones.

Field potential recordings in dentate gyrus of anesthetized rats: stability of baseline

Urethane is a standard anesthetic utilized for in vivo recordings in the hippocampus. In studies of long-term potentiation (LTP), the measure of interest is the response amplitude minutes to hours following train delivery. In the absence of experimental treatment, we have consistently observed upward drift in the amplitude of the population spike (PS) and EPSP slope of the dentate gyrus (DG) evoked field response in acute surgical preparations performed in the urethanized rat. The present study systematically monitored PS amplitude and EPSP slope in the DG every 30 minutes for 6 hours following optimal positioning of Teflon-coated bipolar stainless steel electrodes under urethane anesthesia. At maximal stimulus intensities, large time-dependent increases in PS amplitude (70-80%) were observed over the first 2-4 hours, an effect that was exaggerated at lower stimulus intensities. Increases in the EPSP slope were smaller in magnitude (20-30%) and stabilized within a shorter period of time (1-2 hours). Animals were warmed on a heating pad and body and brain temperature remained constant over the recording session. Reducing stimulating electrode size and recording with glass micropipettes did not alleviate the upward drift in response amplitude. Similar increases were also seen under pentobarbital anesthesia. To dissociate anesthetic from surgical effects, recordings were obtained from animals previously prepared with indwelling electrodes and injected with urethane. Although slight declines (10-15%) in EPSP slope occurred over time, no significant alterations in PS amplitude were seen in the chronic preparation at high stimulus intensities. Low stimulus intensities yielded a more variable response pattern and, in direct contrast to the acute preparation, time-dependent declines, not increases, were noted in both parameters. These data suggest that generalized surgical trauma contributes to the upward drift in response amplitude and indicate that long stabilization periods are required in acute surgical preparations for accurate field potential recordings.

Chronic developmental lead exposure and hippocampal long-term potentiation: biphasic dose-response relationship

Developmental exposure to lead (Pb) has long been associated with reductions in intellectual function in children and behavioral impairments in animal models of learning and memory. We have used long-term potentiation (LTP) in the dentate gyrus of the Pb-exposed rats to determine the potential of a reduced capacity for synaptic plasticity to contribute to Pb-induced cognitive dysfunction. Previous work demonstrated that developmental exposure resulting in moderate blood concentrations of Pb increase the threshold for induction of long-term potentiation (LTP) in the dentate gyrus in vivo. These findings were also suggestive of reductions in LTP magnitude (Gilbert et al., 1996). The present study was designed to further examine the effects of Pb on LTP magnitude and to determine if lower blood Pb levels commonly encountered in children are also effective in impairing synaptic plasticity in this rodent model. Pregnant dams were exposed to control tap water or 0.1, 0.2, 0.5 or 1.0% Pb-acetate in the drinking water beginning just prior to parturition (gestational day 16, GD16). Male offspring were weaned at 21 days of age (PN21) to the same solution given their dams and continued on this regimen until testing. As adults, animals were anesthetized with urethane and stimulating and recording electrodes placed in the perforant path and dentate gyrus, respectively. Post-train I/O functions taken 1 hour after delivery of a series of six high frequency (400Hz) trains revealed a reduced capacity for LTP of the PS amplitude and EPSP slope in Pb-exposed animals in all but the 1.0% group, indicative of a biphasic dose-effect relationship. The 1.0% Pb exposure was clearly less effective than the lower exposure levels in reducing LTP magnitude, and did not differ significantly from control values. The mechanisms underlying the reduced efficacy of higher exposure levels of Pb to impair LTP are not clear. Blood (26-117 microg/dl) and brain (220-1812 ng/g tissue) concentrations of Pb were elevated as a function of increasing exposure (0.1%-1.0%) and cannot readily account for the lack of an effect in the 1.0% group on LTP. We have observed a similar profile in hippocampal glutamate release employing a similar range of exposure levels, i.e., reduction of glutamate release that is absent at higher concentrations of Pb in the drinking water (Lasley et al., 1998). These and previously reported data suggest that the ability of Pb to diminish presynaptic transmitter release contributes to a reduced capacity for LTP at lower exposure levels. The reversal of the effect of Pb on glutamate release that accompanies higher exposure levels may serve to compensate for the mechanism underlying the LTP impairment and form the basis for the biphasic dose-response pattern seen with chronic developmental exposure.

The influence of developmental period of lead exposure on long-term potentiation in the adult rat dentate gyrus in vivo

Previous work has demonstrated an increase in the threshold for induction of long-term potentiation (LTP) in the dentate gyrus of animals chronically exposed to lead (Pb) from birth (Gilbert et al., 1996). The present study sought to extend these findings by evaluating the developmental periods critical for Pb-induced impairment of LTP. Rats were exposed to Pb through maternal milk and/or the drinking water over different developmental intervals: 1) beginning just prior to birth and continuing throughout life (PL); 2) beginning just prior to birth and terminating at weaning (PW); or 3) continuously from the early post-weaning period throughout life (WL). Pregnant dams received 0.2% Pb-acetate in the drinking water on gestational day (GD)16, with male offspring switched to the same solution (PL group) or tap water (PW group) at weaning on postnatal day (PND)21. Postweaning exposure began on PND30 and continued throughout life. As adults (PND130-210), field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia, and an ascending series of stimulus trains was administered to induce LTP and to determine its threshold. The magnitude of population spike (PS) LTP was reduced relative to controls in animals exposed throughout life (PL) and in animals exposed after weaning (WL). No impairment in PS LTP was evident in animals removed from Pb at weaning and tested as adults (PW). Similarly, thresholds for induction of PS LTP were elevated relative to controls in the PL and WL groups, but were not affected by Pb exposure limited to the lactational period (PW). Reductions in the magnitude of LTP of the EPSP slope were evident in posttrain I/O functions in all Pb-exposed groups, including the PW group. An elevated LTP threshold was evident in the EPSP slope measure in the continuously exposed group (PL) only. Thus Pb exposure restricted to the lactational period appeared less disruptive to adult LTP in the dentate gyrus than continuous exposure beginning around birth or weaning. However, EPSP slope LTP was impaired in animals exposed to Pb for as little as 30 days in the early postnatal period. An attenuated ability to support neuroplastic change in synaptic function may contribute to cognitive deficits associated with Pb-induced toxicity.

Chronic lead exposure accelerates decay of long-term potentiation in rat dentate gyrus in vivo

Long-term potentiation (LTP) is a model of synaptic plasticity believed to encompass the underlying neurobiological mechanisms that support memory function. Chronic developmental lead (Pb) exposure is known to be associated with cognitive dysfunction in children and animals. Disruption of the induction of long-term potentiation (LTP) has been reported in the hippocampus following chronic exposure to environmentally relevant levels of Pb in rats. Under urethane anesthesia, we have previously observed Pb-induced increases in the threshold for LTP induction. With higher train intensities, LTP was induced and no declines in the amplitude of responses within a 60-min posttrain period were evident. The present study was designed to assess the effects of Pb on the more enduring forms of LTP in the dentate gyrus of the conscious rat. Beginning in the late gestational period, rats were chronically exposed to 0.2% Pb(2+)-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, electrodes were permanently implanted in male offspring and field potentials evoked by perforant path stimulation were recorded from the dentate gyrus over several weeks. LTP was induced by delivering theta-burst patterned stimulation at a maximal stimulus intensity through the perforant path electrode, and input/output (I/O) functions were monitored for 1 month. Population spike (PS) amplitude was increased maximally 1 h after train delivery. The time constant of decay (tau) calculated from pooled data for each group yielded declines in PS amplitude by 63% in 17.4 days in controls and 13.4 days in Pb-exposed animals. Quantitative estimates of decay in individual animals were achieved in two ways: (1) by calculating difference scores in I/O functions from the maximal LTP at 1 h, and (2) by interpolating day to decay by 63% from declines from maximal LTP. The interpolated values were used to compare the incidence of animals showing decay of 63% within 1 week posttrain. Both analyses revealed a more accelerated rate of decay of LTP in animals developmentally exposed to Pb relative to controls. Endurance of potentiated responses for days to weeks is believed to be supported by structural modifications and synaptic growth. The reported effects of Pb on growth-related processes may thus contribute to a reduced persistence of LTP and the resulting cognitive deficits engendered by developmental Pb exposure.

Chronic developmental lead exposure increases the threshold for long-term potentiation in rat dentate gyrus in vivo

Chronic developmental lead (Pb) exposure has been long associated with cognitive dysfunction in children and animals. In an attempt to more directly relate the behavioral observations of impaired cognitive ability to Pb-induced effects on neuronal activity, we utilized the long-term potentiation (LTP) model of neural plasticity to assess synaptic function. Male rats were chronically exposed to 0.2% Pb(2+)-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. LTP threshold was determined by applying a series of stimulus trains of increasing intensities. Baseline testing of dentate gyrus field potentials indicated that input/output functions, maximal response amplitudes, and threshold currents required to evoke a population spike (PS) did not differ for control and Pb-exposed animals. Despite similarities in baseline synaptic transmission, Pb-exposed animals required a higher train intensity to evoke LTP than controls. With maximal train stimulation, however, control and Pb animals exhibited comparable levels of potentiation. These findings suggest that the mechanisms of LTP induction are preferentially impaired by Pb exposure. Although baseline synaptic transmission was not altered in Pb-exposed animals, decreases in glutamate release following high K+ perfusion and reductions in paired pulse facilitation have been reported in the intact animal. Pb-induced reductions in calcium influx through voltage-sensitive or N-methyl-D-aspartate (NMDA) receptor-dependent channels may mediate increases in LTP threshold. It is possible that the threshold changes in the induction of LTP reported here contribute to cognitive impairments associated with Pb exposure.

Absence of postnatal testosterone fails to demasculinize the male rat's corpus callosum

We had previously shown that elimination of testosterone from embryonic day 17 through adulthood reduced the midsagittal area of the male rat corpus callosum (CC). However, day 1 castration, performed after the 2-h post-birth testosterone surge, was without effect. To elucidate the contribution of this surge on the CC, male rats were delivered by cesarean section and castrated within 20 min. This procedure eliminated the 2-h postnatal rise in testosterone levels. The prenatal surge in testosterone, which occurs on embryonic day 18, remained intact. In adulthood, callosal area was examined in castrate males, sham males, and intact females. Castrate males and sham males had significantly larger CCs as compared to females. The two male groups did not differ from each other. Body weight was significantly higher in sham versus castrate males, establishing the effectiveness of the castration. These results show that hormonal organization of the CC in the male is the result of the independent action of prenatal testicular androgens, and suggest that the end of this period marks the end of callosal sensitivity to testicular hormone influence. In addition, this report documents sexual dimorphism of the CC in a third rat strain.

Lack of activational influence of ovarian hormones on the size of the female rat's corpus callosum

The sex difference in the midsagittal area of the adult rat corpus callosum (CC) has been shown to be mediated, in part, by gonadal steroids in early development, with the sensitive period of hormone action in the female extending at least up to postnatal day 25. Given this prolonged sensitivity, the current study attempted to delineate organizational vs. activational influences of gonadal hormones on the female rat CC. In Experiment 1, callosal size was examined across the estrous cycle at 52 and 90 days of age. In Experiment 2, females were ovariectomized at 78 days and CC parameters assessed at 110 days. Last, in Experiment 3, females were ovariectomized at 78 days and sacrificed at 110 days; in addition, sham females were sacrificed during proestrus or estrus. Neither stage of estrous cycle nor adult ovariectomy affected midsagittal CC size. These results provide evidence for organizational effects of ovarian steroids on the female callosum, with the sensitive period of hormone action ending sometime between days 25 and 78.

Sex differences in the distribution of axon types within the genu of the rat corpus callosum

Neuroanatomical sex differences have been documented in the rat neocortex, including dimorphism of its predominant commissure, the corpus callosum (CC). In particular, CC sex differences have been reported in the ultrastructure of the posterior callosal region, the splenium. Since the CC is a heterogeneous fiber tract with its axons arising from distinct cortical areas and passing through restricted regions along its length, it became of interest to ascertain whether cellular sexual dimorphisms may also be present in another division of the CC. To test this hypothesis, electron microscopy was used to examine axon composition in adult male and female rats in the anterior portion, the genu. The number and size of axons, the thickness of the myelin sheath, and the area within the genu occupied by these constituents, were quantified. Results showed a significant sex difference in the ratio of unmyelinated to myelinated axons, with females having a larger proportion of unmyelinated fibers. This effect was present for both (1) the number of axons, and (2) the area taken up by axonal fibers. No differences were found in the size of either axon type, or for myelin thickness. Comparison of these results with those from the splenium and possible mechanisms underlying this dimorphism are discussed.

Seizure thresholds in kindled animals are reduced by the pesticides lindane and endosulfan

Lindane and endosulfan are chlorinated hydrocarbon pesticides that bind to the gamma-aminobutyric-acidA (GABAA) receptor ionophore complex. We have previously demonstrated development of a persistent predisposition to the seizure-inducing properties of these pesticides following repeated administration of initially subconvulsive doses. A subsequent facilitation of electrical kindling development indicated that a chemical kindling mechanism may contribute to this enhanced sensitivity. We tested the bidirectionality of kindling transfer induced by chemical and electrical means by comparing the convulsive thresholds of lindane and endosulfan in electrically kindled and nonkindled animals. Amygdala kindled, implanted/nonstimulated, and nonimplanted controls were administered 0, 5, 10, or 20 mg/kg of lindane or endosulfan in corn oil by gavage, and each animal's behavior was scored for the presence of myoclonic jerks (MCJ) and clonic seizures. Dose-related increases in the number of animals displaying convulsions in response to both of these pesticides were observed. The dose of lindane (4.5 mg/kg) and endosulfan (4.3 mg/kg) producing MCJs in 50% of the animals (ED50) was reduced by more than 60% in kindled animals relative to controls. A second experiment evaluated the generalized seizure threshold in previously kindled animals treated with low doses of endosulfan (0, 2.5, and 5 mg/kg, po by gavage). Endosulfan reduced the intensity of electrical stimulation required to evoke seizures in amygdala kindled animals. These data indicate that these chlorinated hydrocarbon insecticides reduce seizure thresholds in amygdala-kindled animals with acute administration, and together with previous findings (see ref. 20) demonstrate bidirectional transfer between chemical and electrical kindling.(ABSTRACT TRUNCATED AT 250 WORDS)

Ovarian estrogen acts to feminize the female rat's corpus callosum

The rat corpus callosum (CC) is sexually dimorphic, with the male CC being larger. Ovariectomy (Ovx) on day 12 has been shown to eliminate this sex difference, with callosal values of Ovx females approaching those of male controls. This suggested that postnatal ovarian estrogen affects the size of the female CC. In the present experiment, one group of female rats received Ovx on day 12, and a second group received Ovx followed by chronic implantation of a silastic tube containing beta-estradiol on day 25. Unmanipulated males and sham females served as controls. Examination of the CC at 110 days confirmed our prior findings that males have larger callosa than females and that the Ovx group had increased CC's compared to sham controls. Our new finding was that estrogen treatment was capable of reversing the effects of Ovx. Ovx+estrogen-treated females had decreased CC size as compared to Ovx alone. Indeed, they also had smaller CC values than control females. These findings indicate that ovarian estrogen plays a role in determining CC morphology and that estrogen in the female acts to inhibit overall callosal growth as measured by changes in gross callosal size.

An examination of the anticonvulsant properties of voltage-sensitive calcium channel inhibitors in amygdala kindled seizures

Voltage-sensitive calcium (VSC) channels may contribute to epileptogenesis. A systematic examination of the anticonvulsant efficacy of different classes of VSC channel inhibitors, however, is lacking in chronic seizure models. The present study evaluated representatives from three different classes of VSC channel inhibitors for their protection against amygdala kindled seizures. Adult male rats (n = 12) were kindled to stage 5 seizures (GS), and a threshold intensity required to evoke a GS was determined. The Ca(++)-channel inhibitors (verapamil 0, 10, 20, 40 mg/kg; nimodipine 0, 5, 25, 50 mg/kg; nitrendipine 0, 25, 50, 100 mg/kg and flunarizine 0, 20, 40, 80 mg/kg) were administered 60-90 min prior to amygdala stimulation at the established threshold. None of the drugs altered threshold for inducing a seizure. Verapamil, a phenylalkylamine, and the dihydropyridines nimodipine and nitrendipine were without effect on kindled seizures. The diphenylalkylamine, flunarizine, was found to be the most efficacious, reducing AD duration and duration of clonic seizure activity by more than 60% in most animals. Flunarizine also decreased the severity of behavioral seizures, with 40% of the animals displaying Stage 1-2 seizures only. It is concluded that some VSC Ca(++)-channel inhibitors do possess anticonvulsant potential. Thus influx of extracellular calcium through VSC channels may contribute to the expression of kindled seizures.

The NMDA antagonist, MK-801, suppresses long-term potentiation, kindling, and kindling-induced potentiation in the perforant path of the unanesthetized rat

Antagonism of NMDA-mediated transmission by MK-801 has been shown to block long-term potentiation (LTP) in vitro and delay electrical kindling of the amygdala. The present experiment sought to examine the relationship between synaptic potentiation of the perforant path-granule cell synapse and development of perforant path kindling. MK-801 (0.1 and 1.0 mg/kg) blocked induction of LTP of the perforant path in the unanesthetized animal measured 24 h after train delivery. The 1.0 mg/kg dosage also increased afterdischarge (AD) thresholds, delayed kindling development from daily stimulation of the perforant path (means = 8.82 +/- 1.19 and 22.9 +/- 3.66 sessions to the first stage 5 seizure), and increased AD durations. Kindling produced a significant potentiation of the EPSP (47%) and population spike (49%) after the first evoked AD in control animals. No significant enhancement of either component of the field potential was observed in MK-801-treated animals. Animals treated with this dosage of MK-801, did, however, kindle in the absence of potentiation at this synapse. It was concluded that although NMDA-mediated potentiation may facilitate kindling, synaptic potentiation does not appear to be a critical requirement for kindling to develop. These findings support the notion that development of the burst response and not synaptic enhancement may be the critical physiological alteration that underlies the kindling phenomenon.

Pyrethroids and enhanced inhibition in the hippocampus of the rat

The pyrethroid insecticides have been divided into two classes on the basis of their biochemical actions and behavioral indices of toxicity. Both types of pyrethroids have effects on sodium conductance, and Type II pyrethroids have been reported to antagonize gamma-aminobutyric acid (GABA) by interacting with the t-butyl-bicyclophosphorothionate (TBPS)/picrotoxinin binding site. The dentate gyrus of the hippocampus is equipped with GABAergic recurrent inhibitory circuits. The present experiment was designed to demonstrate dissociation in the biochemistry of pyrethroids by activating the perforant path with pairs of stimulus pulses and monitoring the recurrent inhibition in this circuit. Antagonism of GABA leads to a reduction in inhibition, measured as an increase in the size of the population spike in response to the second pulse of the pair. The GABAergic properties of the pyrethroids were assessed by examining paired pulse inhibition before and after oral treatment with 20 mg/kg of cismethrin (Type I), 20 mg/kg of fenvalerate, or 10 mg/kg of deltamethrin (Type IIs). Input/output (I/O) functions revealed a reduction in excitatory postsynaptic potential (EPSP) following cismethrin and deltamethrin. Population spike height was unaffected. Fenvalerate had no effect on I/O functions. In contrast to the prediction of reduced inhibition following treatment with Type II pyrethroids, deltamethrin and fenvalerate increased inhibition up to 500 and 150 ms interpulse intervals, respectively. Cismethrin was without effect on paired pulse inhibition. These findings fail to provide evidence of GABA antagonistic properties of Type II pyrethroids and may be best explained by a differential effect of these three pyrethroids on sodium channel kinetics.

Enhanced susceptibility to kindling by chlordimeform may be mediated by a local anesthetic action

The formamidine pesticides amitraz and chlordimeform have recently been shown to be potent proconvulsants (Gilbert 1988). Two main neuroactive properties have been identified as mediators of formamidine neurotoxicity, alpha-2 adrenergic agonism and local anesthetic actions. These two proposed mechanisms of formamidine action were contrasted using electrical kindling of the amygdala. Male rats were administered 0, 10 and 40 mg/kg of the local anesthetic lidocaine, 0, 0.01 and 0.10 mg/kg of the alpha-2 adrenergic agonist clonidine or 0, 10 and 30 mg/kg chlordimeform, IP, once per day. After each injection, kindling stimulation was delivered through chronically-implanted electrodes. The high dosage of chlordimeform and both dosages of lidocaine enhanced the rate of kindling development (mean sessions to stage 5 seizures = 8.6 +/- 1.16, 10.15 +/- 1.04 and 8.5 +/- 0.95, respectively) relative to controls (mean = 14.59 +/- 1.36). Afterdischarge (AD) durations were increased over the first seven sessions by both treatments, but the total cumulative AD did not differ from controls. Clonidine, by contrast, delayed kindling development (mean 27.57 +/- 1.97) and shortened the mean AD duration over the first seven sessions. These data provide support for a local anesthetic action of chlordimeform and stand in contrast to several recent demonstrations of alpha-2 activity of formamidines as a primary contributor to formamidine toxicity.

Central Ca++-channel blockade reverses ethanol-induced poikilothermia in the rat

Two series of experiments were performed to determine the possible involvement of Ca++ channels in the thermolytic action of ethanol administered at a room temperature of 22 degrees C. In one group of 11 adult female Sprague-Dawley rats, stainless steel guide cannulae were implanted stereotaxically above the lateral cerebral ventricle. Prior to an experiment, a thermistor probe was inserted into the colon so that core temperature could be monitored continuously for up to six hours or until the temperature had returned to a previous baseline level. When the animal's body temperature had stabilized, a dose of 4.0 g/kg in a v/v solution of 20% ethanol was given by intragastric gavage. After the body temperature had declined by about 2.0 degrees C, ordinarily 30 min after ethanol administration, either control CSF or the vehicle plus one of four doses of verapamil (8.3, 25, 50 and 100 micrograms) was infused intracerebroventricularly (ICV) in a volume of 10 microliter. In a second group of 7 unoperated rats, either 4.0 g/kg ethanol or a physiological saline control solution was administered isovolumetrically by intragastric gavage; then, 30 min later, either 3.0 or 10.0 mg/kg verapamil was injected intraperitoneally. At an ambient temperature of 22 degrees C, ethanol gavage produced a significant decline in colonic temperature which was unaffected by physiological saline given by the same route. Although the CSF control vehicle was without effect, verapamil administered ICV attenuated the thermolytic action of ethanol in all doses tested; however, the lowest dose exerted its antagonist effect but with a longer latency. Conversely, when verapamil was given systemically, the hypothermic action of ethanol was significantly potentiated in a dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Day care services: an alternative to institutional care

The day care program at St. Camillus Nursing Home in Syracuse, New York is described. The growth rate of the program,, the observed benefits to the participants, and acceptance by their family members indicate its value to the community. The admission criteria are well defined and non-restrictive, and there is a wide scope of services. However, a greater number of participants and expansion of the services would be feasible if Medicare patients could be included in the program besides the present Medicaid and private physician referrals. Such reimbursement should be provided for the sake of participants and their families, as well as for the economics of the total health structure. This would open the doors for day care programs in many other communities across the nation.