Diminished regulation of mesolimbic dopaminergic activity in rat after chronic inorganic lead exposure

Previous studies in this laboratory have indicated that chronic lead (Pb) exposure during development induces a neurotoxicity in dopamine (DA) neurons that is primarily presynaptic in nature and at least partially related to altered regulation of DA synthesis. A primary form of DA synthesis regulation is the inhibition exerted on synaptic tyrosine hydroxylase activity via dopaminergic autoreceptors. This study assessed the functional status of this mechanism in Pb-exposed rats employing a pharmacological model. At parturition dams received 0.2% Pb acetate (1090 ppm) in the drinking water while control dams received distilled water. Offspring were weaned to and maintained on the same solution given their dams until termination at 125 days. Rats were given saline or 6,7-dihydroxy-2-dimethylaminotetralin (TL-99, 2.5-20 mg/kg ip) 40 min before termination followed 10 min later by 750 mg/kg ip of gamma-butyrolactone (GBL) or saline. The ability of TL-99 to prevent the GBL-induced increase in DA content was significantly diminished in nucleus accumbens (NAc) of exposed rats compared to controls, indicating that chronic Pb impairs receptor-mediated regulation of DA synthesis in mesolimbic neurons. No effect of Pb was observed in caudate-putamen. In animals receiving only saline injections concentrations of the DA metabolites, homovanillic acid and dihydroxyphenylacetic acid, were significantly decreased by Pb in the range of 17-31% and 12-24%, respectively. DA content was also significantly diminished by Pb in ventral tegmental area of these latter groups. These findings suggest that chronic Pb has multiple actions on central nervous system dopaminergic neurons consisting of an impaired regulation of DA synthesis that is apparently independent of a decrease in DA release.

Racial differences in cardiovascular reactivity to mental arithmetic

One hypothesized mechanism for the higher rates and greater severity of essential hypertension among blacks is that this group is particularly susceptible to stress-induced beta-adrenergically mediated cardiovascular hyperreactivity. In this study, we compared the cardiovascular responses to mental arithmetic in 20 white and 17 black, young, normotensive males. Contrary to expectations, the black subjects exhibited significantly smaller changes in heart rate, and systolic and diastolic blood pressure. These findings suggest that cardiovascular reactivity to a mental stressor known to produce beta-adrenergically mediated responses may be lower in some normotensive blacks compared to their white counterparts.

Racial differences in blood pressure and forearm vascular responses to the cold face stimulus

The mechanisms responsible for the higher incidence of essential hypertension in blacks than in whites are the object of much research attention. One hypothesis is that the development of hypertension in blacks is associated with exaggerated blood pressure reactivity, particularly those responses mediated by vasoconstriction. Racial differences in blood pressure responses to cold stimulation of the forehead, a known alpha-adrenergic vasoconstrictive stimulus, were examined in health, college-age males. Compared to white subjects, black subjects exhibited significantly greater increases in systolic and diastolic blood pressure, as well as increases in forearm vascular resistance, in response to cold stimulation. This preliminary evidence of increased peripheral vascular reactivity in blacks suggests that known racial differences in hypertension prevalence might derive in part from physiological differences in sympathetic nervous system reactivity.

Type A behavior pattern and blood glucose control in diabetic children

We studied the relationship between presence of Type A behavior pattern and glycemic response to stress in children with insulin dependent diabetes mellitus (IDDM). Twelve male (six Type A and six Type B) and nine female (four Type A and five Type B) insulin-dependent diabetic patients between the ages of 8 and 16 years received a standard meal and blood glucose values were assessed two hours later. All subjects then played a competitive videogame for 10 minutes following which blood glucose was assessed again. Preprandial and postprandial blood glucose values did not differ between the groups. However, only Type A subjects showed a hyperglycemic response to the videogame stress. Type A subjects also demonstrated significantly higher glycohemoglobin values. In order to assure that this effect was due to a differential response to stress and not simply a difference in metabolic response to a meal, a second study was conducted in which blood glucose values were assessed at one, two and three hours following a standard meal. No significant differences in postprandial blood glucose values were observed between Type A and Type B subjects. These data support previous research which has suggested that some but not all patients with IDDM show a hyperglycemic response to stress.

Cerebral cortical GABA and benzodiazepine binding sites in genetically seizure prone rats

Adult male and female genetically seizure-prone rats were assessed for sound-induced seizures. Heterozygous control groups were compared with mild seizure (designated GEPR 3) and severe seizure animals (GEPR 9). Groups of animals were killed and crude synaptosome fractions (P2) prepared from freshly dissected cerebral cortices. Binding sites for gamma-aminobutyric acid (GABA) were assessed by [3H]-muscimol in the absence or presence of excess GABA and/or pentobarbital. Binding sites for benzodiazepines were assessed by [3H]-flunitrazepam in the presence or absence of clonazepam. Compared to controls, GEPR 3 animals had a modest increase and GEPR 9 animals a larger increase in Bmax for both high and low affinity GABA sites, with no change in Kd. Chloride-dependent, barbiturate-enhanced GABA binding (increased Bmax) was observed in all conditions and groups. Likewise benzodiazepine binding (Bmax) increased slightly in GEPR 9 animals. There were no observed changes in binding sites for a survey of biogenic amines. Seizure-prone animals appear to have compensatory denervation-like supersensitivity for their most prominent inhibitory receptor, which may or may not be linked to the seizure event.

Type A behavior, family history of hypertension, and cardiovascular responsivity among Black women

The purpose of this study was to determine the effects of Type A behavior and family history of hypertension on cardiovascular reactivity to mental stress in a group of employed black women. Measures of heart rate and of systolic blood pressure (SBP) and diastolic blood pressure (DBP) were taken at rest, during a mental arithmetic task, and during the Type A Structured Interview (SI). Results indicated that the Type A behavior pattern was associated with SBP and DBP hyperresponsivity during the SI but not during mental arithmetic. Additionally, certain speech components of the Type A pattern, as well as features of the potential-for-hostility component, were also related to cardiovascular responses during the SI. Family history of hypertension did not influence the cardiovascular parameters either alone or in combination with Type A behavior. The results suggest that many of the cardiovascular response characteristics of the Type A pattern that have been observed in predominantly white samples also hold true for blacks. Replication of these findings with other subgroups of blacks, such as young females and middle-aged males, will help document the generality of these findings within the black population.

Effects of 6-OHDA lesions of the central medial nucleus accumbens on rat intravenous morphine self-administration

The function of dopaminergic innervations of the central medial nucleus accumbens in the processes maintaining intravenous morphine self-administration was assessed by lesioning with 6-OHDA and comparing drug intake with sham-vehicle treated littermates. Localized bilateral lesions of this structure resulted in significant increases in morphine intake shifting the dose-effect relationship to the right with twice the dose necessary to maintain prelesion rates of self-administration. Content of dopamine and dihydroxyphenylacetic acid was decreased in the nucleus accumbens after the lesion, but unchanged in the adjacent pyriform cortex and anterior caudate nucleus-putamen, while serotonin was significantly decreased in the pyriform cortex. High affinity uptake measurements also suggested nucleus accumbens dopaminergic and pyriform cortex serotonergic innervations to be affected by the lesion. The shift to the right in the dose effect relationship after the lesion suggests these neuronal systems to be excitatory to the processes mediating self-administration.

The inhibited power motive, type A behavior, and patterns of cardiovascular response during the structured interview and Thematic Apperception Test

The Type A behavior pattern and the inhibited power motive have been implicated in the development of coronary heart disease (CHD). Since it is widely believed that enhanced cardiovascular responsivity may be one mechanism by which individuals develop CHD, the present study examined the relationship of Type A behavior and the inhibited power motive to different patterns of cardiovascular response during two behavioral tasks. Forty-one (24 Type A's, 17 Type B's) male undergraduates underwent the Type A structured interview (SI) and the Thematic Apperception Test (TAT) while a broad range of cardiovascular functions were simultaneously recorded. Different patterns of cardiovascular response were observed during the SI and TAT, and Type A's showed a greater tendency than Type B's to exhibit increased heart rate (HR), systolic blood pressure (SBP), and forearm blood flow (FBF) during the SI and the preparatory phase (but not the story-telling phase) of the TAT. The inhibited power motive was not related to enhanced cardiovascular responsivity during the SI or TAT. The implications of these findings for the development of CHD are discussed.

Withdrawal from morphine generalizes to a pentylenetetrazol stimulus

Rats trained to discriminate pentylenetetrazol (PTZ) from saline in a two-lever food-reinforced operant task were given a three-day course of morphine, 15 to 45 mg/kg tid, ip. On the third day naloxone produced dose-dependent generalization to the PTZ stimulus, with 66% of subjects selecting the PTZ lever after the highest dose (0.32 mg/kg). Following termination of morphine injections, generalization of spontaneous withdrawal was tested. Approximately 50% of subjects selected the PTZ lever at 24 and 48 hrs after the last morphine, and by 96 hrs the percentage of subjects selecting the PTZ lever had dropped to 11%. Rats that chose the PTZ lever at 48 hrs were given diazepam, 5.0 mg/kg, which blocked the PTZ-like stimulus. These data demonstrate that morphine withdrawal produces a stimulus with PTZ-like characteristics which can be blocked by an anxiolytic, and they suggest that the PTZ discrimination may have general utility for investigating drug dependence and withdrawal in animals.

Self-administration of methionine enkephalin into the nucleus accumbens

Microinfusions of the endogenous opiate neurohumor, methionine enkephalin, into the nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation of this reinforcing stimulus since naloxone effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside the ventral tegmental area, questioning the current dopamine hypothesis for the initiation of these reinforcement processes.

Limbic acetylcholine turnover rates correlated with rat morphine-seeking behaviors

Acetylcholine (ACh) turnover rates were measured in fourteen brain regions of rats intravenously self-administering morphine and in yoked-morphine and yoked-vehicle infused littermates to identify cholinergic neuronal pathways potentially involved in opiate reinforcement processes. Rats receiving chronic passive administration of morphine had increased ACh turnover rates in the frontal cortex and diagonal band and decreased rates in the medial septum. The significant changes in animals self-administering the drug were prominent in limbic regions with increases in the frontal cortex and decreases in the pyriform cortex, nucleus accumbens, amygdala and ventral tegmental area. Some components of opiate reinforcement may be mediated by increases in the activity of cholinergic ventral pallidal and diagonal band fibers innervating the frontal cortex and by decreases in activity of cholinergic fibers innervating the ventral tegmental area. These data and turnover rates for dopamine, norepinephrine, serotonin, aspartate, glutamate and gamma-aminobutyric acid previously determined in similarly treated animals are consistent with two neuronal circuits that may be involved in opiate seeking behaviors and opiate reinforcement processes.

Limbic muscarinic cholinergic and benzodiazepine receptor changes with chronic intravenous morphine and self-administration

Muscarinic cholinergic and benzodiazepine receptor affinities and densities were evaluated in membranes from seven brain regions of rats intravenously self-administering morphine and in littermates receiving yoked-morphine or yoked vehicle infusions to identify neuronal systems potentially involved in mediating opiate reinforcement processes. Passive morphine infusion resulted in increases in muscarinic cholinergic receptor densities in the pyriform cortex and in decreases in the cingulate cortex while benzodiazepine receptor densities were decreased in both the hippocampal formation and entorhinal-subicular cortex compared to littermates receiving passive infusions of vehicle. Morphine self-administration resulted in decreased muscarinic cholinergic receptor densities in the frontal and entorhinal-subicular cortices and increases in the amygdaloid complex compared to littermates receiving yoked passive drug. These data are in agreement with acetylcholine turnover rate measurements in these animals and support the proposed role of cholinergic innervations of the frontal and entorhinal-subicular cortices and amygdaloid complex in opiate reinforcement processes.

Reinforcer interactions under concurrent schedules of food, water, and intravenous morphine

Responding by six rats was maintained under a concurrent chained fixed-ratio 1, fixed-ratio 9 schedule (conc chain FR1 FR9 ) of food, water, and morphine presentations. The subjects had continuous access to the schedule contingencies on a reversed 12-h light-dark cycle. Local rates and temporal patterns were very similar for responding maintained by the three reinforcers with food and water intake occurring predominantly during the dark cycle, while morphine infusions were evenly distributed. Food and water extinction (24-h duration) decreased the number of ratios completed on both the food and water levers. Moreover, food extinction resulted in a large increase in I.V. morphine self-administration. Morphine extinction increased responding on the morphine lever while almost eliminating responding on the water lever. Changes in the dose of morphine (2.5-40 mg/kg/injection) did not significantly affect food and water intake, but were inversely related to responding on the morphine lever. Saline substitutions resulted in effects similar to those observed during morphine extinction. The schedule used in this study provides a method for examining the specificity of a number of pharmacological and neurochemical manipulations.

Caffeine and cardiovascular responses to stress

Caffeine and psychologic stress have similar physiologic effects. Moderate doses of caffeine were found to elevate blood pressure in healthy, young males during periods of rest and stress. Blood pressure during stress was also significantly higher after caffeine had been consumed. The elevation of blood pressure due to caffeine appears to add to that elicited by stress. The implications of these results for prevention and treatment of cardiovascular disease are discussed.

Turnover rates of amino acid neurotransmitters in regions of rat cerebellum

The turnover rates of aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, alanine, serine, and glycine were measured in five regions of rat cerebellum. Turnover rates of the putative neurotransmitters (aspartate, glutamate, and GABA) were 2-20-fold higher than those of alanine and serine, and generally consistent with the proposed neurotransmitter functions for these amino acids. However, glutamate turnover was high and similar in magnitude in the deep nuclei and granule layer, suggesting possible release, not only from parallel fibers, but from mossy fibers as well. The differential distribution of turnover rates for GABA supports its neuronal release by Purkinje, stellate, basket, and Golgi cells, whereas aspartate may be released by both climbing and mossy fibers. The distribution of glycine turnover rates is consistent with release from Golgi cells, whereas alanine may be released from granule cell parallel fibers. Turnover rates measured in two other motor areas, the striatum and motor cortex, indicated that utilization of these amino acid neurotransmitters is differentially distributed in brain motor regions. The data indicate that turnover rate measurements may be useful in identifying neurotransmitter function where content measurements alone are insufficient.