Developmental Phenotypic and Transcriptomic Effects of Exposure to Nanomolar Levels of 4-Nonylphenol, Triclosan, and Triclocarban in Zebrafish (Danio rerio)

Triclosan, triclocarban and 4-nonylphenol are all chemicals of emerging concern found in a wide variety of consumer products that have exhibited a wide range of endocrine-disrupting effects and are present in increasing amounts in groundwater worldwide. Results of the present study indicate that exposure to these chemicals at critical developmental periods, whether long-term or short-term in duration, leads to significant mortality, morphologic, behavioral and transcriptomic effects in zebrafish (Danio rerio). These effects range from total mortality with either long- or short-term exposure at 100 and 1000 nM of triclosan, to abnormalities in uninflated swim bladder seen with long-term exposure to triclocarban and short-term exposure to 4-nonylphenol, and cardiac edema seen with short-term 4-nonylphenol exposure. Additionally, a significant number of genes involved in neurological and cardiovascular development were differentially expressed after the exposures, as well as lipid metabolism genes and metabolic pathways after exposure to each chemical. Such changes in behavior, gene expression, and pathway abnormalities caused by these three known endocrine disruptors have the potential to impact not only the local ecosystem, but human health as well.

Developmental phenotypic and transcriptomic effects of exposure to nanomolar levels of metformin in zebrafish

Metformin is found in the majority of lakes and streams in the United States, leading to widespread environmental exposure. Results of the present study indicate that extended duration metformin exposure at critical developmental periods leads to decreased survival rates in zebrafish (danio rerio), an NIH approved human model. Significant abnormalities are seen with extended duration metformin exposure from 4 h post fertilization up to 5 days post fertilization, although short term metformin exposure for 24 h at 4-5 days post fertilization did not lead to any significant abnormalities. Both extended and short term duration did however have an impact on locomotor activity of zebrafish, and several genes involved in neurological and cardiovascular development were differentially expressed after exposure to metformin. The changes seen in behavior, gene expression and morphological abnormalities caused by metformin exposure should be examined further in future studies in order to assess their potential human health implications as metformin prescriptions continue to increase worldwide.

Detection of endocrine disrupting chemicals in Danio rerio and Daphnia pulex: Step-one, behavioral screen

Anthropogenic surface and ground water contamination by chemicals is a global problem, and there is an urgent need to develop tools to identify and elucidate biological effects. Contaminants of emerging concern (CECs) are not typically monitored or regulated and those with known or suspected endocrine disrupting potential have been termed endocrine disrupting chemicals (EDCs). Many CECs are known to be neurotoxic (e.g., insecticides) and many are incompletely characterized. Behavioral responses can identify chemicals with neuroactive properties, which can be relevant to EDC mechanisms (e.g., neuroendocrine disturbances). Two freshwater species, Daphnia pulex and Danio rerio, were evaluated for swimming behavior alterations resulting from 24-hr exposure to 9 CECs: triclosan, triclocarban, chlorpyrifos, dieldrin, 4-nonylphenol, bisphenol-A, atrazine, metformin, and estrone. This is the first step in the development of a bioassay for detecting estrogenic and/or anti-androgenic activity with the goal to evaluate complex mixtures of uncharacterized contaminants in water samples. The second step, described in a subsequent report, examines transcriptome alterations following chemical exposure. Significant differences in the swimming behavior response and sensitivity were found across chemicals within a species and across species for a given chemical in this unique optical bioassay system. In the concentration ranges studied, significant behavioral alterations were detected for 6 of 9 CECs for D. pulex and 4 of 9 CECs for D. rerio. These results underscore the utility of this bioassay to identify behavioral effects of sublethal concentrations of CECs before exploration of transcriptomic alterations for EDC detection.

The phenotypic and transcriptomic effects of developmental exposure to nanomolar levels of estrone and bisphenol A in zebrafish

Estrone and BPA are two endocrine disrupting chemicals (EDCs) that are predicted to be less potent than estrogens such as 17β-estradiol and 17α-ethinylestradiol. Human exposure concentrations to estrone and BPA can be as low as nanomolar levels. However, very few toxicological studies have focused on the nanomolar-dose effects. Low level of EDCs can potentially cause non-monotonic responses. In addition, exposures at different developmental stages can lead to different health outcomes. To identify the nanomolar-dose effects of estrone and BPA, we used zebrafish modeling to study the phenotypic and transcriptomic responses after extended duration exposure from 0 to 5 days post-fertilization (dpf) and short-term exposure at days 4-5 post fertilization. We found that non-monotonic transcriptomic responses occurred after extended duration exposures at 1 nM of estrone or BPA. At this level, estrone also caused hypoactivity locomotive behavior in zebrafish. After both extended duration and short-term exposures, BPA led to more apparent phenotypic responses, i.e. skeletal abnormalities and locomotion changes, and more significant transcriptomic responses than estrone exposure. After short-term exposure, BPA at concentrations equal or above 100 nM affected locomotive behavior and changed the expression of both estrogenic and non-estrogenic genes that are linked to neurological diseases. These data provide gaps of mechanisms between neurological genes expression and associated phenotypic response due to estrone or BPA exposures. This study also provides insights for assessing the acceptable concentration of BPA and estrone in aquatic environments.

Toxic effects of combined stressors on Daphnia pulex: Interactions between diazinon, 4-nonylphenol, and wastewater effluent

Contaminant exposure in aqueous systems typically involves complex chemical mixtures. Given the large number of compounds present in the environment, it is critical to identify hazardous chemical interactions rapidly. The present study utilized a prototype for a novel high-throughput assay to quantify behavioral changes over time to identify chemical interactions that affect toxicity. The independent and combined effects of 2 chemicals, diazinon (an insecticide) and 4-nonylphenol (a detergent metabolite), on the swimming behavior of the freshwater crustacean Daphnia pulex were examined. Cumulative distance and change in direction were measured repeatedly via optical tracking over 90 min. Exposure to low concentrations of diazinon (0.125-2 µM) or 4-nonylphenol (0.25-4 µM) elicited significant concentration- and time-dependent effects on swimming behavior. Exposure to 0.5 µM 4-nonylphenol alone did not significantly alter mean cumulative distance but did elicit a small, significant increase in mean angle, the measure of change in direction. When 0.5 µM 4-nonylphenol was used in combination with diazinon (0.125-0.5 µM), it augmented the adverse impact of diazinon on the swimming behavior of Daphnia. Additionally, enhanced sensitivity to diazinon was observed in animals exposed to treated wastewater effluent for 24 h prior to a diazinon challenge. The present experiments demonstrate that exposure to 4-nonylphenol and complex chemical mixtures (e.g., treated wastewater) can enhance the toxicity of exposure to the insecticide diazinon.

Optical bioassay for measuring sublethal toxicity of insecticides in Daphnia pulex

Many emerging contaminants tend to be biologically active at very low concentrations, occur in water as part of complex mixtures, and impact biota in ways that are not detected using traditional toxicity tests (e.g., median lethal concentration). To evaluate emerging contaminants, the authors developed a method for detecting sublethal behavioral effects by quantifying the swimming behavior of Daphnia pulex, a model organism for studying aquatic toxicity. This optical tracking technique is capable of measuring many swimming parameters, 2 of which-cumulative distance and angular change-are presented. To validate this technique, 2 prototypical compounds that exhibit different modes of action as well as corresponding insecticides that are commonly found in surface waters were investigated. The acetylcholinesterase (AChE) inhibitor physostigmine was used as the prototypical compound for the large number of AChE inhibitor insecticides (e.g., chlorpyrifos). Nicotine was used as the prototypical compound for neonicotinoid insecticides (e.g., imidacloprid). Results demonstrate that this assay is capable of detecting sublethal behavioral effects that are concentration-dependent and that insecticides with the same mode of action yield similar results. The method can easily be scaled up to serve as a high-throughput screening tool to detect sublethal toxic effects of a variety of chemicals. This method is likely to aid in enhancing the current understanding of emerging contaminants and to serve as a novel water-quality screening tool.

Postnatal Inorganic Lead Exposure Reduces Midbrain Dopaminergic Impulse Flow and Decreases Dopamine D1 Receptor Sensitivity in Nucleus Accumbens Neurons

Lead treatment via drinking water for 3 to 6 weeks at 250 ppm was found to significantly decrease the number of spontaneously active dopamine (DA) neurons in both the substantia nigra and ventral tegmental area that were recorded using standard extracellular electrophysiological recording techniques. Lead exposure did not affect the discharge rate or discharge pattern of these DA neurons. No significant decrease in the number of tyrosine hydroxylase immunopositive cells was detected in lead-treated animals relative to controls even though the length of lead exposure was extended beyond that of the electrophysiological studies. The significant lead-induced decrease in spontaneously active cells observed in the electrophysiological studies was, therefore, not due to cell death. An acute drug challenge with the DA receptor agonist apomorphine at a dose known to hyperpolarize midbrain DA neurons (50 mug/kg i.v.) was used to determine whether hyperpolarization would normalize the number of spontaneously active DA neurons. The results suggest that depolarization inactivation was most likely not the cause for this lead effect. The D(1) receptor agonist SKF-38393 [1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol] was iontophoretically applied to type I nucleus accumbens (Nacb) neurons. The results demonstrated that type I Nacb neurons have a significantly lower basal discharge rate in lead-treated animals relative to controls and that the Nacb DA D(1) receptors were significantly less sensitive to SKF-38393 in the lead-treated animals. Therefore, lead exposure decreases DA neuron impulse flow presynaptically and decreases DA D(1) receptor sensitivity postsynaptically in the nucleus accumbens.

Inorganic lead exposure in the rat activates striatal cFOS expression at lower blood levels and inhibits amphetamine-induced cFOS expression at higher blood levels

The impact of inorganic lead exposure on dopamine (DA) neurotransmission in the basal ganglia was examined. Amphetamine (AMPH)-induced cFOS immunoreactivity (cFOS-IR) in the striatum was determined after a 3-week exposure to lead acetate (0, 50, or 250 ppm). On the 21st day of lead exposure, rats were challenged with AMPH (4 mg/kg i.p.) or saline vehicle (Veh) and were assayed for presence of cFOS-IR. In the untreated control (Con) group, AMPH challenge (Con/AMPH) increased cFOS-IR expression by approximately 35-fold over Veh challenge (Con/Veh) (P < 0.01). In the Pb50/Veh group, cFOS-IR expression was approximately 7-fold greater than in the Con/Veh group (P < 0.05). Given that there was negligible cFOS-IR expression in the Con/Veh group, this indicates that the Pb50 exposure induced cFOS expression. The increase in cFOS-IR in the Pb50/AMPH was also significant (P < 0.01), but it was not different from the Con/AMPH (P > 0.20). Neither the Pb250/Veh group nor the Pb250/AMPH group had a significant increase in cFOS-IR relative to Con/Veh (P > 0.20). These results indicate that chronic 50 ppm lead exposure induced a low but statistically significantly level of cFOS gene activation and that it did not affect the AMPH-induced cFOS activation. However, chronic 250 ppm lead exposure inhibited AMPH-induced activation of cFOS in the striatum by about 89%. Therefore, lead is capable of both activating cFOS expression at low levels of exposure (mean blood lead level 21.6 +/- 1.9 microg/dl) and inhibiting AMPH-induced cFOS expression at higher levels of exposure (mean blood lead level 47.4 +/- 2.6 microg/dl).

Protein kinase C does not mediate the inhibitory action of lead on vitamin D3-dependent production of osteocalcin in osteoblastic bone cells

The level of osteocalcin in serum is lower in lead-intoxicated children than in their normal counterparts. To explain this clinical observation, we investigated the mechanism of action of lead on vitamin D3-dependent osteocalcin production. Lead (5-20 microM) blocked the stimulating effects of vitamin D3 on osteocalcin production in cultured rat osteosarcoma cells (ROS 17/2.8). It is often suggested that activation of protein kinase C (PKC) is a critical mediator of the toxic actions of lead. Treatment of ROS cells with Gö6976, an inhibitor of PKC alpha and beta isozymes, produced similar effects as lead on vitamin D3-dependent osteocalcin production, while activation of PKC by phorbol-12-myristate-13-acetate (TPA) did not reverse or mimic this effect of lead. Thus activation of PKC is not consistent with the actions of lead on vitamin D3-dependent osteocalcin production. Measurement of PKC enzyme activity showed that 10 microM lead treatment does not activate or inhibit the activity of PKC in ROS cells. Western blot analysis indicated that lead treatment does not translocate PKC alpha, beta, or zeta from cytosol to membrane. Therefore, we concluded that PKC does not mediate the cellular toxicity of lead on vitamin D3-dependent osteocalcin production.

Postnatal inorganic lead exposure decreases the number of spontaneously active midbrain dopamine neurons in the rat

This study examined the effect of lead (Pb) exposure during postnatal development on the electrophysiological activity of midbrain dopamine (DA)-containing neurons. Single-cell electrophysiological recordings were made in the substantia nigra (SN) and ventral tegmental area (VTA) of chloral hydrate anesthetized rats. In this post-weaning exposure protocol 22-day-old male Sprague-Dawley rats were exposed to Pb- (100, 250, and 500 ppm) or Na-acetate in the drinking water for a period ranging from 3 to 6 weeks. Animals were exposed up to the day of electrophysiological recording. One Pb- and one Na-treated animal were recorded each experimental day. The post-weaning exposure protocol used in this study resulted in a significant Pb-dependent decrease in the number of spontaneously active DA neurons at the time of electrophysiological recording. Analysis of covariance, using duration of exposure as the covariate (i.e. 3, 4, 5, or 6 weeks), did not indicate that there was a consistent relationship between exposure duration and the number of spontaneously active DA neurons. However, the effect of Pb was dependent on the level of Pb exposure through the drinking water. At the 250 and 500 ppm level of exposure, Pb produced a significant decrease in the number of spontaneously active DA neurons in both anatomical regions. The number of active DA neurons was not significantly affected by the 100 ppm Pb treatment over the 3-6 weeks exposure period. The average discharge rate, and the percentage of spontaneously active DA neurons classified as having discharge patterns with bursts (i.e. 'bursting DA neurons'), was not changed at any of the three levels of Pb exposure. Based on results obtained from electrophysiological studies, the effect of selected Pb exposure levels, 250 and 500 ppm, were examined during the postnatal period using tyrosine hydroxylase (TH) immuno-histochemistry to determine if Pb affects the survival of dopamine neurons within SN and VTA. TH immuno-histochemical studies revealed that the reduction in the number of spontaneously active DA neurons in animals treated with 250 and 500 ppm Pb was probably not related to the physical loss of cells (e.g. necrosis or apoptosis).

Effects of chronic low-level oral lead exposure on prepulse inhibition of acoustic startle in the rat

Previous work has suggested that the behavioral effects of chronic low-level lead exposure on fixed interval (FI) operant behavior result from enhanced dopaminergic neurotransmission in the nucleus accumbens (Cory-Slechta et al., J Pharmacol Exp Ther 286: 794-805, 1998). The present studies were designed to further characterize the effects of chronic low-level oral lead exposure on another behavior that is modulated by dopaminergic neurotransmission in the nucleus accumbens. In these studies acoustic startle and the prepulse inhibition (PPI) of startle were studied in rats following chronic low-level oral lead exposure. Weanling male rats were treated for 5-6 weeks with lead via drinking water (250 ppm lead acetate; controls drank 250 ppm sodium acetate). Acoustic startle reactivity (95, 105, and 115 dB noise bursts) and PPI (prepulses of 1-8 dB over the 70-dB background) of startle were tested following lead exposure. Lead exposure did not affect body weight. Lead exposure also did not significantly affect baseline [i.e., no prepulse inhibition (NO-PPI)] acoustic startle as measured by 1) startle amplitude on the first startle trial (105 dB), 2) the average startle amplitude for the first ten trials (105 dB), or 3) the average startle amplitude for the NO-PPI trials during PPI testing (95, 105, and 115 dB). Lead exposure also did not affect the latency to onset for the startle response. In contrast, for both the 105 dB and 115 dB acoustic startle stimuli, chronic low-level oral lead exposure significantly attenuated the capacity of an acoustic prepulse to reduce the startle response. This effect was present whether the data were presented and analyzed as raw change from baseline or as the percentage of baseline startle. Given the strong link between the modulation of PPI and dopaminergic neurotransmission in the nucleus accumbens, the present data support the hypothesis that chronic low-level oral lead exposure facilitates dopamine neurotransmission in the nucleus accumbens.

An electrophysiological evaluation of serotonergic dorsal raphe neurons in Maudsley rats

Extracellular electrophysiological recording techniques were used to study serotonergic dorsal raphe (DRN) neurons in Maudsley Reactive (MR), Maudsley Non-Reactive (MNRA) and Sprague Dawley (SD; reference control strain) rats. No significant differences were observed in the average discharge rates of DRN neurons from SD, MR AND MNRA rats. The sensitivity of DRN neuron somatodendritic 5-HT1A autoreceptors to the inhibitory effects of i.v. 8-OH-DPAT or alpha 1-adrenoceptors to the excitatory effects of iontophoretic phenylephrine did not differ significantly among strains. These findings are discussed in light of the previously reported strain-dependent differences in anxiety-like behavior and noradrenergic locus coeruleus neurons.

Prenatal haloperidol reduces the number of active midbrain dopamine neurons in rat offspring

The dopamine (DA) receptor antagonist, haloperidol (HAL, 1.25 or 5 mg/kg), or vehicle, dimethyl sulfoxide (DMSO), was administered (SC) daily to pregnant Sprague-Dawley dams from gestational day (GD) 8 to GD 20. The average body weight of 2-week-old male offspring was significantly lower in all of the HAL-treated groups relative to controls. In extracellular electrophysiological studies, the male 2-week-old offspring from all HAL treatment groups were found to have significantly reduced average numbers of spontaneously active midbrain dopamine (DA)-containing neurons in both the substantia nigra (A9) and the ventral tegmental area (A10) relative to controls. In DA neurons classified as bursting neurons, HAL exposure (5 mg/kg) caused a significantly increased level of burst activity in A10 but not A9 DA neurons relative to controls. For both the A9 and A10 regions, the proportion of DA neurons classified as bursting or nonbursting was unaffected by HAL treatment. These results suggest that prenatal HAL exposure influences the development of midbrain DA neurons.

Repeated stimulation of dopamine D2-like receptors: reduced responsiveness of nigrostriatal and mesoaccumbens dopamine neurons to quinpirole

Extracellular recording techniques were used to study antidromically activated nigrostriatal (NSDA) and mesoaccumbens (MADA) dopamine neurons in chloral hydrate-anesthetized rats. Repeated 14-day i.p. treatment with the dopamine D2-like receptor agonists, quinpirole (2 mg/kg/day) or EMD 23448 (2.6 mg/kg/day), resulted in a significant decrease in the average potency and efficacy of i.v. quinpirole (cumulative doses administered on day 15) to inhibit the spontaneous activity of NSDA neurons relative to vehicle controls. Repeated 14-day quinpirole treatment caused a significantly greater decrease in the sensitivity of MADA neurons to i.v. quinpirole challenges than NSDA neurons. When the effects on NSDA neurons were examined after a shorter treatment period, the decrease in the average potency and efficacy of i.v. quinpirole appeared to occur after only 2 days of i.p. quinpirole treatment (2 mg/kg/day). Iontophoretic studies, however, indicated that the average dopamine sensitivity of somatodendritic dopamine autoreceptors on MADA neurons, but not NSDA neurons, was significantly lower relative to controls after 14-day quinpirole treatment (2 mg/kg/day). These results suggest that this quinpirole treatment regimen can differentially affect the average sensitivity of somatodendritic dopamine autoreceptors on MADA and NSDA neurons. The somatodendritic autoreceptors on MADA neurons appear to be more sensitive to the effects of repeated 14-day quinpirole treatment than those on NSDA neurons.

Ontogeny of nigrostriatal dopamine neuron autoreceptors: iontophoretic studies

This study characterized somatodendritic dopamine (DA) autoreceptors on nigral DA-containing neurons during postnatal developmental in chloral hydrate-anesthetized rats. Antidromically activated nigrostriatal DA (NSDA) neurons from 2-week-old animals were found to be less sensitive to the inhibitory effects of cumulative i.v. doses (1-32 micrograms/kg) of the DA agonists apomorphine (D2/D3/D1) and quinpirole (D2/D3) than those from adults. The age-dependent difference in DA agonist sensitivity was found to be of significantly greater magnitude for apomorphine than for quinpirole. When a single i.p. dose (64 micrograms/kg) of apomorphine that elicits a moderate level of inhibition was administered, however, no significant differences between the sensitivity of 2-week-old and adult NSDA neurons were found. In iontophoretic studies, no age-dependent (1, 2 and 4 week-olds and adults) differences in nigral DA neuron sensitivity to the inhibitory effects of apomorphine, quinpirole and the D3/D2 agonist, 7-hydroxy-dipropylaminotetralin HBr were found. Iontophoretic studies with the DA antagonists, eticlopride (D2/D3) and 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (D1), and i.v. studies with the DA agonists 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (D1) and N-allyl-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (D1) indicate that somatodendritic DA autoreceptors on 2-week-old NSDA neurons appear to be of the D2/D3 subtype. These results suggest that functional adult-like somatodendritic DA autoreceptors are present on nigral DA neurons during early postnatal development. Given the conflict between the iontophoretic and i.v. results, however, the nature of any potential age-dependent differences in somatodendritic autoreceptor sensitivity to DA agonists will need to be examined further in vitro.

Electrophysiological characteristics of locus coeruleus neurons in the Maudsley reactive (MR) and non-reactive (MNRA) rat strains

Extracellular single-unit recording techniques were used to evaluate the physiological and pharmacological characteristics of noradrenergic locus coeruleus (LC) neurons in urethane-anesthetized Maudsley reactive (MR) and non-reactive (MNRA) rat strains, a presumed genetic model for differences in 'anxiety'. LC neurons from MNRA rats were found to have a significantly higher basal discharge rate than LC neurons from either the MR or Sprague-Dawley (SD) rats. The discharge pattern of MNRA LC neurons also differed significantly from that of LC neurons from SD and MR rats, with LC neurons from MNRA rats exhibiting a burst-like pattern of discharge. Finally, MNRA LC neurons were significantly less sensitive to the inhibitory effects of i.v. clonidine on spontaneous activity.

Postnatal development of mesoaccumbens dopamine neurons in the rat: electrophysiological studies

The postnatal development of antidromically identified mesoaccumbens dopamine (MADA) neurons were examined with single-unit electrophysiological techniques. Rats were anesthetized with chloral hydrate. The physiological characteristics of 1-, 2-, 4- and 5-week-old rat pups were compared to adults (7-9-weeks-old). The basal discharge rate, conduction velocity, antidromic latency and discharge patterns of MADA neurons were not significantly different among the 4- and 5-week-old and adult MADA neurons. MADA neurons from 1- and 2-week-old pups, however, had significantly lower mean basal discharge rates and significantly lower mean conduction velocities than MADA neurons from the older animals (i.e., 4-weeks old, 5-weeks old and adults). 1- and 2-week-old MADA neurons were also found to have significantly longer mean antidromic latencies than MADA neurons from older animals. Significantly fewer 1- and 2-week-old MADA neurons were found to discharge in a bursting pattern when compared to MADA neurons from older animals. These results indicate that during early postnatal development MADA neurons are spontaneously active, but still physiologically immature. The results of the present study are discussed in the context of previous developmental electrophysiological studies of nigrostriatal dopamine neurons.

Repeated amphetamine administration: role of forebrain in reduced responsiveness of nigrostriatal dopamine neurons to dopamine agonists

The effects of repeated amphetamine treatment on single antidromically identified nigrostriatal dopamine-containing (NSDA) neurons were evaluated in rats. The inhibitory potency and efficacy of dopamine (DA) agonists on NSDA neuron spontaneous discharge rate were examined after amphetamine treatment. Repeated amphetamine treatment (14 days, 1 or 6 mg/kg/day i.p.) dose-dependently decreased the sensitivity of NSDA neurons to the inhibitory effects of the i.v. administered quinpirole. The amphetamine-induced alteration in sensitivity to apomorphine and quinpirole was abolished by acute hemitransection of the forebrain/midbrain connections. No change in the responsiveness or sensitivity of NSDA neurons to the inhibitory effects of iontophoretically applied DA was detected after amphetamine treatment for 14 days (4 or 6 mg/kg/day) or 28 days (6 mg/kg/day). These results suggest that these amphetamine regimens alter the sensitivity/responsiveness of forebrain DA receptors but not NSDA cell somatodendritic DA autoreceptors in a dose-dependent manner.

Repeated SKF 38393 and nigrostriatal system neuronal responsiveness: functional down-regulation is followed by up-regulation after withdrawal

The effects of chronic administration of the D1 dopamine (DA) agonist SKF 38393 on the basal activity and electrophysiological and pharmacological responsiveness of nigrostriatal DA neurons were examined by means of extracellular, single-unit recording techniques. Chronic D1 stimulation failed to alter either the basal activity of DA neurons or the potency of quinpirole to induce inhibition of these cells. However, 28-day SKF 38393 treatment (but not 14-day treatment) eliminated the ability of subsequent (24 h later) acute SKF 38393 to alter the rate-dependent nature of quinpirole-induced inhibition. In contrast, one week after a 28-day SKF 38393 treatment we found that quinpirole-induced inhibition by itself was no longer rate-dependent, an effect which was reversed by acute pretreatment with the D1 antagonist SCH 23390. This latter finding is suggestive of enhanced endogenous D1 tone. Similarly, 28-day SKF 38393 treatment eliminated the effect of subsequent acute SKF 38393 on sciatic nerve stimulation-induced inhibition of nigrostriatal DA neurons, whereas one week after the chronic D1 regimen these cells were highly sensitive to acute D1 enhancement of the response to sciatic nerve stimulation. In order to address the postsynaptic effects of chronic D1 stimulation, the influence of iontophoretically administered SKF 38393 was examined on type I caudate neurons. Again, 28-day SKF 38393 treatment resulted in reduced sensitivity of caudate neurons tested 24 h later, and an enhanced sensitivity was observed one week after the completion of chronic SKF 38393 administration. Thus, chronic SKF 38393 induced functional desensitization of D1 receptors, but one-week withdrawal was followed by sensitization.

Estimation of fetal weight by means of ultrasound: a comparison of methods

An accurate prediction of birth weight during gestation can provide useful information for assessing fetal and newborn health status, enabling the clinician to better predict infant morbidity and mortality. Two previously reported standard methods for birth-weight estimation used data collected in utero to derive formulas by least-squares linear regression. The rationale for the inclusion of particular variables in these equations, however, has not been clearly defined. This study was undertaken to examine the efficacy of some previously used variables as well as some new variables in estimating fetal birth weight. The authors used measurements of femur length, biparietal diameter, and abdominal circumference from 107 fetuses (2500 to 4000 g) as variables to compare the two standard methods of birth-weight estimation. A new formula is presented that is derived from a simple model based on the known relationship between volume and weight. The head is represented as a sphere and the body as a cylinder. This study presents a more systematic approach to formula development in which statistical biases are minimized by examining the underlying distributions of the variables used to predict birth weight.

Dopamine neuron ontogeny: electrophysiological studies

The ontogeny of nigrostriatal dopamine (NSDA) neurons was examined with single-unit extracellular electrophysiological methods. The physiological and pharmacological characteristics of 2-, 4-, and 5-week-old rat pup NSDA neurons were compared with those of adults (8-10 weeks old). Although the basal discharge rate, conduction velocity, and firing pattern of NSDA neurons from 4- and 5-week-old rats were similar to adults, the 2-week-old-rats differed significantly in all three of these physiological characteristics. The conduction velocity and basal discharge rate were found to be significantly lower in the 2-week-old pups relative to adults. In addition, there were significantly fewer bursting NSDA neurons in 2-week-olds than there were in adults. Two and 4-week-olds exhibited significantly lower sensitivity to cumulative intravenous doses of apomorphine. In contrast, the sensitivity to cumulative intravenous doses of quinpirole was found to be similar across all age groups. It is evident that the pharmacological and physiological properties of NSDA neurons are in a dynamic state of flux during postnatal development. These electrophysiological findings are discussed in the context of the perinatal development of midbrain DA systems.

Statistical analysis of dose-response curves in extracellular electrophysiological studies of single neurons

The application of polynomial regression and the analysis of covariance (ANCOVA) to dose-response (DR) data derived from extracellular electrophysiological studies of midbrain dopamine neurons and noradrenergic locus coeruleus neurons in vivo is demonstrated and discussed. Third-order polynomial regression was found to be a better method for estimating ED50 values than probit analysis of linear regression. ANCOVA provides a more powerful statistical method than ANOVA for detecting significant differences in ED50 values or DR curves when a confounding variable such as basal discharge rate is present. The methods of analysis presented herein should be useful in the analysis of other types of neurons in electrophysiological studies.

Autonomic actions of cocaine

The development of our knowledge of the physiological, pharmacological, and biochemical actions of cocaine has in essence occurred in parallel with the development of our knowledge about the function of the autonomic nervous system. Cocaine is a sympathomimetic compound with potent local anesthetic properties. The principal hypothesis accepted to date to explain the sympathomimetic effects of cocaine is that this drug inhibits neuronal monoamine neurotransmitter reuptake by binding to a transporter or uptake site thereby increasing the effective concentration of neurotransmitter at adrenergic receptor sites. Much of the available evidence for this hypothesis has come from studies utilizing in vitro or in situ techniques. There have been relatively fewer studies examining the impact of cocaine on the autonomic nervous system in the intact animal. In addition, few studies have examined the effects of cocaine on central autonomic function. Past studies concerning the mechanism of action of cocaine are reviewed and recent data addressing the cardiovascular, respiratory, and central autonomic effects of cocaine are discussed.

Repeated amphetamine: reduced dopamine neuronal responsiveness to apomorphine but not quinpirole

Extracellular recordings from single nigrostriatal dopamine (DA) neurons in rats revealed significantly reduced neuronal sensitivity to the inhibitory effects of i.v. apomorphine following repeated amphetamine (4 mg/kg per day i.p., 14 days). This effect was reversed by acute SCH 23390. Quinpirole sensitivity was reduced in amphetamine-treated rats only following acute SKF 38393 pretreatment. These results suggest that, in amphetamine-treated animals, D-1 receptor activation is important for the expression of reduced nigrostriatal DA neuron sensitivity to apomorphine.

Chronic cocaine reduces alpha 2-adrenoceptor elicited mydriasis and inhibition of locus coeruleus neurons

The effects of chronic cocaine (50 mg/kg per day for two weeks) administration on two alpha 2-adrenoceptor-mediated responses were studied in rats. Chronic administration of cocaine significantly (compared to sham controls) attenuated the alpha 2-adrenoceptor-mediated inhibition of noradrenergic locus coeruleus (LC) neurons as well as alpha 2-adrenoceptor elicited mydriasis. Noradrenergic LC neurons from the cocaine treated and sham sham groups differed significantly in their responsiveness to the inhibitory effects of clonidine (ED50 values micrograms/kg: sham 7.35 +/- 1.13 and cocaine-treated 17.17 +/- 4.40, P less than 0.05). The ED50 values for the mydriatic response were sham 5.71 +/- 0.49 and cocaine-treated 16.42 +/- 0.69 micrograms/kg, respectively, P less than 0.001. No differences in cardiovascular responses to systemically injected clonidine between the chronic cocaine- and sham-treated groups were observed. Chronic cocaine treatment attenuates the two alpha 2-adrenoceptor-mediated responses most likely via an interaction with central catecholaminergic neurotransmission.

Cocaine-elicited mydriasis in the rat: pharmacological comparison to clonidine, D-amphetamine and desipramine

Agents known to influence adrenergic function were examined for their mydriatic effects in urethane-anesthetized rats. Both the direct acting adrenergic agonist, clonidine, and the "indirect" acting agonists, cocaine, desipramine and amphetamine elicited mydriatic responses. The polar alpha-2 adrenoceptor agonist, 4-hydroxyclonidine, did not elicit a mydriatic response when administered systemically; however, it did produce a pronounced mydriatic response when administered i.c.v. Inasmuch as the selective lipophilic alpha-2 adrenoceptor antagonist, yohimbine, but not the polar alpha adrenoceptor antagonist, phentolamine, reversed the mydriatic effects of clonidine and cocaine, these data suggest that centrally located alpha-2 adrenoceptors elicit the above mydriatic response. The direct acting alpha-2 adrenoceptor agonists, clonidine (i.v.) and 4-hydroxyclonidine (i.c.v.), were the most efficacious of the agents studied in eliciting the mydriatic response. The indirect acting agents, amphetamine, desipramine and cocaine, were less efficacious. The rank order potency (ED-50) of these drugs was as follows: clonidine greater than desipramine greater than cocaine = amphetamine. The mydriatic effects of cocaine were attenuated by yohimbine and reserpine pretreatments. In addition, the local anesthetic, procaine, and the polar cocaine analog, cocaine methiodide, were significantly less efficacious than cocaine. These results suggest that cocaine elicits mydriasis by indirectly acting at central and postsynaptically located alpha-2 adrenoceptors.

Cocaine and central monoaminergic neurotransmission: a review of electrophysiological studies and comparison to amphetamine and antidepressants

Psychomotor stimulants (e.g. cocaine and amphetamine) and many antidepressants are believed to elicit their psychotropic actions by interacting primarily with central monoaminergic neurons. The acute central neuronal effects of amphetamine and antidepressants have been extensively investigated in rats utilizing extracellular single unit electrophysiological and microiontophoretic techniques in vivo. In recent years the chronic effects of these compounds on the above neuronal systems have also been reported. Such investigations have proliferated because of the realization that the mechanisms underlying the psychotomimetic effects (e.g. amphetamine and cocaine) and mood elevation (i.e. antidepressants) observed with the administration of these drugs are more accurately reflected in chronic studies. For many years it has been assumed that cocaine and amphetamine produce very similar if not identical psychotropic effects through their actions on central monoaminergic neurotransmission. In terms of effects on single monoaminergic neurons, this assumption had gone by untested until two years ago, when the first report of the electrophysiological effects of cocaine on central monoaminergic (locus ceruleus) neurons appeared in the literature (61). This review discusses recent electrophysiological studies with cocaine at the level of single identified monoaminergic neurons and compares such data with that previously reported for amphetamine and antidepressants. In addition to identifying some of the similarities and differences between these compounds, this review also highlights some of the gaps in our knowledge regarding the effects of these drugs on central monoaminergic neurotransmission.

Reciprocal pre- and postsynaptic actions of cocaine at a central noradrenergic synapse

Single-unit microelectrode studies were conducted to test the effects of systemic cocaine HCl on spontaneously firing single noradrenergic locus ceruleus (presynaptic) and cerebellar Purkinje (postsynaptic) neurons in rats in vivo. The spontaneous neuronal activity of all locus ceruleus neurons was inhibited by cocaine in a dose-dependent manner (0.5 to 2 mg/kg). These doses of cocaine elicited a predominant activation of postsynaptic Purkinje neurons. No effect of cocaine on neuronal action potential amplitude or slope was observed. Similar doses of the local anesthetic agent, procaine, did not affect action potential amplitudes or slopes of either locus ceruleus or Purkinje neurons. In addition, although cocaine elicited a significantly greater absolute change in the discharge rate of locus ceruleus neurons than of Purkinje neurons, the effects of procaine on those neurons were not significantly different from each other. The inhibition of locus ceruleus neurons by cocaine was significantly attenuated by pretreatment either with the alpha 2-adrenoceptor antagonist, yohimbine, or with reserpine. The activation of Purkinje neurons by cocaine was also significantly attenuated by reserpine pretreatment. Systemic cocaine administration (1 mg/kg, i.v.) did not potentiate the inhibitory effects of either locus ceruleus stimulation or local iontophoretic application of norepinephrine on Purkinje neuron discharge rate. We conclude that cocaine potently inhibits locus ceruleus neurons and this effect probably elicits Purkinje cell activation through disinhibition.

Cardiovascular effects of cocaine in anesthetized and conscious rats

This study examined the cardiovascular and respiratory effects of cocaine and procaine in anesthetized and conscious rats. Intravenous cocaine (0.16-5 mg/Kg) elicited a rapid, dose dependent increase in mean arterial pressure of relatively short duration. In pentobarbital anesthetized (65 mg/Kg, i.p.) animals, the pressor phase was generally followed by a more prolonged depressor phase. These effects on arterial pressure were generally accompanied by a significant tachypnea and at larger doses (2.5 and 5 mg/Kg, i.v.), bradycardia. Procaine (0.31 and 1.25 mg/Kg, i.v.) produced similar cardiovascular and respiratory effects (depressor phase, tachypnea) in pentobarbital anesthetized animals. In conscious-restrained animals, both cocaine and procaine (1.25 mg/kg, i.v.) produced pressor responses. The subsequent depressor response was, however, absent in both cases. The cardiovascular effects of cocaine (0.25-1 mg/Kg, i.v.) in urethane anesthetized (1.25 g/Kg, i.p.) animals were essentially similar to those observed in conscious animals. Procaine (1mg/Kg) did not produce any significant cardiovascular effects in urethane anesthetized animals, but did elicit tachypnea. Reserpine pretreatment (10 mg/Kg, i.p.) did not significantly attenuate the pressor response in urethane anesthetized animals. Phentolamine pretreatment (3 mg/Kg, i.v.) did significantly antagonize the pressor effect in urethane anesthetized animals. These results suggest that: the depressor phase is likely due to a interaction between local anesthetic activity (cocaine and procaine) and barbiturate anesthesia, the cardiovascular effects of cocaine in conscious animals are more similar to those observed in urethane anesthetized rats than in pentobarbital anesthetized rats and the pressor effect in urethane anesthetized rats is apparently due to a reserpine resistant catecholaminergic mechanism.

Cocaine modulation of central monoaminergic neurotransmission

Extracellular microelectrode studies were conducted to test the effects of cocaine HCl on the activity of spontaneously firing single serotonergic dorsal raphe (DRN), noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental (VTA) and zona compacta (ZC) neurons, and cerebellar Purkinje neurons (PC) in urethane anesthetized rats in vivo. Cocaine (0.0625-4 mg/kg) predominantly inhibited all of the central monoaminergic neurons and predominantly activated cerebellar Purkinje neurons. Cocaine (1 mg/kg, IV) failed to potentiate the inhibitory effects of LC stimulation on PC neurons. The temporal effects of intravenous cocaine on arterial pressure (i.e., pressor response) were not directly correlated with the effects on neurons. Cocaine did not decrease the amplitude or slope of neuron action potentials, and the effects of cocaine on firing rate were not shared by similar doses of procaine. Reserpine pretreatment (10 mg/kg, IP) attenuated the effects of cocaine (1 mg/kg, IV) on DRN, LC, and PC neurons. Specific adrenoceptor antagonists antagonized the inhibitory effects of cocaine on LC (piperoxane, yohimbine) and VTA (haloperidol) neurons. These results suggest that the central effects of cocaine on presynaptic monoaminergic neurons may in part be mediated by augmented monoamine neurotransmission at autoreceptors and that the effects of cocaine on postsynaptic target cells (PC) may be more complex, requiring the analysis of both pre- and postsynaptic elements.

Electrophysiological actions of cocaine on noradrenergic neurons in rat locus ceruleus

Extracellular microelectrode experiments were conducted to study the effects of cocaine HCl on the activity of spontaneously firing single locus ceruleus (LC) noradrenergic neurons in vivo. The responses of single identified noradrenergic LC neurons to the systemic (i.v.) administration of cocaine were observed over a wide range of doses (0.0625-2.0 mg/kg). The spontaneous activity of all LC neurons receiving doses greater than the threshold dose (0.0625 mg/kg) was inhibited in a dose-dependent manner. The local anesthetics, procaine and mepivacaine, did not affect LC neuronal activity, action potential amplitude or slope. The inhibitory effects of cocaine on spontaneous LC neuron activity was reversed by the subsequent i.v. administration of the specific alpha-2 adrenoceptor antagonist, piperoxone, but not the opiate receptor antagonist, naloxone. Pretreatment with another alpha-2 adrenoceptor antagonist, yohimbine (5 mg/kg i.p.), attenuated significantly the inhibition of spontaneous LC activity elicited by i.v. cocaine. Intravenous cocaine produced a brief increase in mean arterial pressure which did not appear to be correlated with the more sustained inhibition of LC neurons. Reserpine pretreatment (10 mg/kg i.p.) attenuated significantly the inhibitory effects of cocaine on LC activity. These results suggest that the inhibition of spontaneous LC neuron activity by i.v. cocaine is most likely mediated by an augmented action of catecholamines at central alpha-2 adrenoceptors and not by the local anesthetic effects of cocaine.

Electrophysiological effects of cocaine on central monoaminergic neurons

The electrical activity of three different single, identified, spontaneously firing central neurons was monitored by extracellular microelectrodes. Intravenous cocaine administration (0.25-2 mg/kg) elicited an activation of cerebellar Purkinje neurons (PN) and an inhibition of serotonergic dorsal raphe (DRN) and noradrenergic locus coeruleus (LC) neurons. These effects were not temporally correlated with the increase in mean arterial pressure elicited by the intravenous administration of cocaine. In addition, the administration of procaine, a structurally related local anesthetic, did not significantly affect any of the three central neurons studied. Our results suggest that cocaine has potent effects on the activity of DRN, LC and PN neurons, which are not directly related to its cardiovascular or local anesthetic actions.

Cardiovascular effects of perfusion of the rostral rat hypothalamus with clonidine: differential interactions with prazosin and yohimbine

The present studies examined the role of alpha 1- versus alpha 2-adrenoceptors in the cardiovascular effects of clonidine administered into the anterior hypothalamic/pre-optic (AH/PO) region of the forebrain by the push-pull perfusion technique. Push-pull cannulas were placed bilaterally into the AH/PO region of anesthetized, paralyzed and ventilated rats. Perfusion of this area with artificial CSF (0.015 ml/min), yohimbine (5 or 50 microM) for 30 min did not affect mean arterial blood pressure or heart rate. Perfusion of the AH/PO region with clonidine (0.55-5.50 mM) resulted in a concentration-dependent reduction of mean arterial pressure and heart rate. The hypotensive effects of clonidine were found to be greater than the bradycardic effects, when expressed as a percent of pre-infusion baseline values. Co-perfusion with yohimbine (5, 50 microM) significantly attenuated the hypotensive, but not the bradycardic, effects of a single concentration (1.75 mM) of clonidine; this selective antagonism of the hypotensive effect of clonidine by yohimbine was concentration dependent. In contrast to yohimbine, co-perfusion with 5 microM prazosin did not significantly affect either the clonidine-induced hypotension or bradycardia. Co-perfusion with the higher concentration (50 microM) of prazosin significantly reversed the bradycardic, but not the hypotensive, effects of 1.75 mM clonidine. These results suggest that AH/PO clonidine perfusion depresses both mean arterial pressure and heart rate, and that the clonidine-induced hypotension is due to alpha 2-adrenoceptor activation, while the clonidine-induced bradycardia is due to alpha 1-adrenoceptor activation.

Effects of cocaine on the electrical activity of single noradrenergic neurons from locus coeruleus

The effects of intravenous (i.v.) cocaine HCl on single identified spontaneously firing noradrenergic neurons in the nucleus locus coeruleus (LC) were studied in rats in vivo. Cocaine (0.25-1 mg/kg) produced inhibition of spontaneously firing LC neurons, which was reversed by the administration of the selective alpha 2-adrenoceptor antagonist, piperoxane (250 micrograms/kg, i.v.). Procaine, a local anesthetic that is structurally related to cocaine, did not inhibit LC neurons in doses up to 4 mg/kg, i.v. These results suggest that cocaine in low doses has significant central sympathomimetic effects at the single noradrenergic neuron level and that the inhibition of spontaneous activity may be mediated by alpha 2-adrenoceptors. Our results also indicate that cocaine in pharmacologically relevant doses, can significantly affect central alpha 2-adrenoceptor regulatory processes.

Cardiovascular effects of centrally perfused clonidine

Earlier work has indicated that the systemic cardiovascular actions of clonidine might be mediated by caudal brainstem centers, especially the nucleus of the solitary tract (NTS). This study sought to define the mode of clonidine action on the NTS more explicitly using the technique of push-pull perfusion on urethane-anesthetized rats. The NTS of stereotaxically mounted subjects was unilaterally perfused with an artificial cerebrospinal fluid at 25 microliter/min. Clonidine was added to the medium at concentrations of 5 to 500 microM, without interruption of flow, for test periods of 10 min. Systemic drug actions were expressed in terms of mean arterial pressure (MAP) and heart rate (HR), both of which were recorded continuously throughout the experiment. Decreases occurred in both MAP and HR following clonidine perfusion at all concentrations. However, the dose-effect relationship for the blood pressure response was dependent to some extent on control pressure. When this was considered as a variable, the drug-induced pressure effects were significantly dose-dependent. Control HR values were more stable than pressure and dose-related decreases following clonidine administration were highly significant. The clonidine concentrations investigated here were considerably lower than those previously studied by microinjection. The observed dose-related depression of MAP and HR under basal conditions may be related to specific alpha 2-adrenergic receptor activation of the NTS.