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.

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.

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.

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.

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.

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.

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.