Effects of reserpine on dopamine metabolite in the nucleus accumbens and locomotor activity in freely moving rats

The effect of reserpine (2 mg/kg i.p.) on both locomotor activity and the turnover of dopamine metabolite in the rat nucleus accumbens was estimated by using an activity monitor (Animex) and by in vivo brain microdialysis. Three to five hours after reserpine administration locomotor activity was reduced and there was a concomitant increase in the level of the dopamine metabolite, homovamillic acid. These findings suggest that depletion of dopamine from the nucleus accumbens may result in decreased locomotor activity. The data support the notion that dopamine in this tissue contributes to the control of locomotion.     

Effect of direct and indirect dopamine agonists on brain extracellular ascorbate levels in the striatum and nucleus accumbens of awake rats

Systemic administration of direct and indirect dopamine agonists resulted in increased extracellular ascorbic acid levels in the striatum and, to a lesser degree, in the nucleus accumbens as measured by in vivo voltammetry. Intraperitoneal d-amphetamine sulfate (5mg/kg) increased ascorbate concentrations in striatal extracellular fluid. Amphetamine also increased extracellular ascorbate levels in the nucleus accumbens although more gradually and to a lesser extent. Intraperitoneal phenethylamine hydrochloride (20 mg/kg) following pargyline hydrochloride pretreatment (20 mg/kg) increased extracellular ascorbate levels in the striatum significantly above the small increase seen in the nucleus accumbens. The direct acting dopamine agonists Ly-141865 and Ly-163502 when given i.p. at 1 mg/kg, resulted in increased extracellular ascorbate concentrations in both brain areas, again with a significantly greater effect in the striatum. These results indicate that brain extracellular ascorbate levels can be modulated by dopaminergic neuro-transmission and that this modulation is quantitatively different in different dopamine-containing brain structures.     

Measurement of extracellular 5-HT and 5-HIAA in hippocampus of freely moving rats using microdialysis: long-term applications

Extracellular levels of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in the ventral hippocampus of the awake rat using microdialysis. The basal level of 5-HT in hippocampal dialysates was very close to the detection limit of our assay. However, addition of a 5-HT re-uptake blocker, citalopram, to the perfusion medium resulted in a 3-fold rise of 5-HT levels in dialysates and provided a stable baseline. Under these conditions, extracellular levels of 5-HT and 5-HIAA could be measured up to 11 days after dialysis probe implantation. 5-HT measured in dialysate was Ca²⁺-dependent both 24 h and 7 days after surgery. In comparison K⁺-induced depolarization caused a 9-fold increase in 5-HT output 24 h after probe implantation and this effect had disappeared 10 days later, although behavioral activation following K⁺ application was similar 24 h and 11 days after surgery. Systematic administration of -trytophan had no significant effect on 5-HT but increased 5-HIAA levels by 90%. These results suggest that in the presence of citalopram, 5-HT in hippocampal dialysates is derived from serotonergic neurones. The microdialysis method appears most useful for measurement of extracellular 5-HT 24–72 h after probe implantation. Finally, our data indicate that extracellular 5-HIAA mainly reflects intraneuronal metabolism of 5-HT prior to release.     

Cocaine treatment increases extracellular cholecystokinin (CCK) in the nucleus accumbens shell of awake,freely moving rats,an effect that is enhanced in rats that are behaviorally sensitized to cocaine

Cholecystokinin (CCK) is co-localized with dopamine, is known to modulate dopamine neurotransmission and is involved in behavioral sensitization to psychostimulants. To better understand its role, CCK was measured by microdialysis in the nucleus accumbens (NAC) shell in response to cocaine in drug-naive rats and in rats that are behaviorally sensitized to cocaine. Basal extracellular levels of CCK in drug-naive rats were 0.17 pg/20 min fraction, while in cocaine-sensitized rats, they were significantly higher (0.56 pg). Treating drug-naive rats with cocaine caused a significant increase in CCK to 0.58 pg. Cocaine treatment of cocaine-sensitized rats increased CCK to 0.98. When analyzed as a function of time after cocaine treatment, these increases were sustained and were significantly different from CCK levels of saline-treated rats. In cocaine-sensitized rats, CCK levels following cocaine treatment were also significantly higher than levels in drug-naive animals receiving a single injection of cocaine. These results provide evidence for an activation of the mesolimbic and/or cerebral cortical CCK system in response to repeated cocaine administration. These results provide a neurochemical basis for an important role of CCK (via modulation of dopamine neurotransmission) in expression of cocaine sensitization.     

Cannabidiol increases Fos expression in the nucleus accumbens but not in the dorsal striatum

Preclinical and clinical studies suggest that cannabidiol (CBD), a major component of Cannabis sativa, could produce antipsychotic effects without causing extra-pyramidal side-effects. In the present paper we employed the detection of Fos protein to investigate neuronal activation in the dorsal striatum and nucleus accumbens of male Wistar rats after systemic administration of CBD (120 mg/kg), haloperidol (1 mg/kg) or clozapine (20 mg/kg). Only haloperidol was able to increase the number of Fos immunoreactive neurons (FIr) in the dorsal striatum (vehicle: 0.07 +/- 0.07/0.1 mm(2), haloperidol: 28.3 +/- 8.9/0.1 mm(2), p < 0.01). In contrast, both haloperidol and CBD significantly increased FIr in the nucleus accumbens (Vehicle: 0 +/- 0/0.1 mm(2), haloperidol: 7.2 +/- 2.7/0.1 mm(2), CBD: 4.0 +/- 1.9/0.1 mm(2), p < 0.05). Clozapine also produced a barely significant increase in FIr (3.0 +/- 1.7/0.1 mm(2), p = 0.062). These results show that CBD is able to induce FIr in a limbic- but not in a motor-related area.     

A study of the effects of buspirone, BMY 13805, and 1-PP on dopaminergic metabolism in the nucleus accumbens using in vivo voltammetry in freely moving rats

The effects of buspirone, a buspirone analogue (BMY 13805) and a buspirone metabolite (1-PP) on dopaminergic metabolism in the nucleus accumbens were investigated using in vivo voltammetry. Differential pulse voltammetry coupled with electrochemically pretreated carbon fiber electrodes was used to provide a continuous and selective measure of the 3,4-dihydroxyphenylacetic acid (DOPAC). An implanted micromanipulator enabled the use of freely moving animals. Buspirone injections induced a marked and rapid increase of the DOPAC peak in the nucleus accumbens. Buspirone was 10 times more potent when injected subcutaneously than intraperitoneally. BMY 13805 and 1-PP were without effect on dopaminergic metabolism in the nucleus accumbens. In conclusion, the anxiolytic properties of these drugs and their effects on dopaminergic metabolism do not appear related.     

Systemic cocaine challenge after chronic cocaine treatment reveals sensitization of extracellular dopamine levels in nucleus accumbens but direct cocaine perfusion into nucleus accumbens does not: implications for the neural locus of cocaine sensitization

Rats were treated chronically with 20 mg/kg/day cocaine (by intraperitoneal injection) for 16 days, followed by 7 days of cocaine wash-out. On the next day, rats were challenged with an acute dose of cocaine administered by one of two routes (systemic or intracranial), and extracellular dopamine (DA) in the nucleus accumbens (Acb) was measured by in vivo microdialysis. Rats acutely challenged systemically with 20 mg/kg cocaine showed enhanced Acb extracellular DA levels (compared to control rats that had not previously received chronic cocaine). However, rats acutely challenged with intracranial cocaine by perfusion of 10⁻⁵ M cocaine directly into the Acb did not. It is suggested that both the development and triggering of cocaine sensitization, as manifested by enhanced Acb DA content to subsequent acute cocaine challenge, may involve more than just neural mechanisms occurring locally within the Acb.     

Alterations in amphetamine-induced locomotor activity and stereotypy after electrical stimulation of the nucleus accumbens and neostriatum

The exacerbation of the locomotor and stereotypic effects of amphetamine after repeated drug administration is well documented. To elaborate on the involvement of the nigrostriatal and mesolimbic dopamine (DA) systems in modulating behavioral sensitization, locomotor activity and the time spent engaged in repetitive stereotyped behaviors following systemic amphetamine injection were assessed after electrical stimulation of the nucleus accumbens and neostriatum. It was found that exposure to repeated sessions of high frequency, low current stimulation of the anteromedial neostriatum and nucleus accumbens significantly enhanced the locomotor excitation induced by administration of 3.0 mg/kg of amphetamine. Stereotypic behaviors were also modified as a function of electrical stimulation of these brain regions, with the development of a significant decrease in the duration of focused head and body movements corresponding to the facilitated locomotor effects of the drug. Taken together, these data provide additional evidence demonstrating the interdependent relationship between amphetamine-elicited locomotor activity and stereotypy, and were discussed in terms of a functional interaction between mesolimbic and nigrostriatal systems in determining the behavioral profile of amphetamine administration.     

Behavioral sensitization to amphetamine is not accompanied by changes in glutamate receptor surface expression in the rat nucleus accumbens

We examined whether behavioral sensitization to amphetamine is associated with redistribution of glutamate receptors (GluR) in the rat nucleus accumbens (NAc) or dorsolateral striatum (DLSTR). Following repeated amphetamine treatment and 21 days of withdrawal, surface and intracellular levels of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) or NMDA receptor subunits were determined using a protein cross-linking assay. In contrast to our previous results in cocaine-sensitized rats, we did not observe redistribution of GluR1 or GluR2 to the cell surface in the NAc after amphetamine withdrawal, although a small increase in total GluR1 was found in the shell subregion. Nor did we observe activation of signaling pathways associated with cocaine-induced AMPA receptor trafficking or changes in NMDA receptor subunits. No significant changes were observed in the DLSTR. We also investigated the effect of administering a challenge injection of amphetamine to amphetamine-sensitized rats 24 h prior to biochemical analysis based on prior studies showing that cocaine challenge decreases AMPA receptor surface expression in the NAc of cocaine-sensitized rats. GluR1 and GluR2 were not significantly altered in either NAc or DLSTR, although a modest effect on GluR3 cannot be ruled out. Our results suggest that glutamate transmission in the NAc is dramatically different in rats sensitized to amphetamine versus cocaine.     

Oscillatory entrainment of striatal neurons in freely moving rats

Oscillations and synchrony in basal ganglia circuits may play a key role in the organization of voluntary actions and habits. We recorded single units and local field potentials from multiple striatal and cortical locations simultaneously, over a range of behavioral states. We observed opposite gradients of oscillatory entrainment, with dorsal/lateral striatal neurons entrained to high-voltage spindle oscillations ("spike wave discharges") and ventral/medial striatal neurons entrained to the hippocampal theta rhythm. While the majority of units were likely medium-spiny projection neurons, a second neuronal population showed characteristic features of fast-spiking GABAergic interneurons, including tonic activity, brief waveforms, and high-frequency bursts. These fired at an earlier spindle phase than the main neuronal population, and their density within striatum corresponded closely to the intensity of spindle oscillations. The orchestration of oscillatory activity by networks of striatal interneurons may be an important mechanism in the pathophysiology of neurological disorders such as Parkinson's disease.     

Respiratory sinus arrhythmia in freely moving and anesthetized rats.

Heart rate increases during inspiration and slows during postinspiration; this respiratory sinus arrhythmia helps match pulmonary blood flow to lung inflation and maintain an appropriate diffusion gradient of oxygen in the lungs. This cardiorespiratory pattern is found in neonatal and adult humans, baboons, dogs, rabbits, and seals. Respiratory sinus arrhythmia occurs mainly due to inhibition of cardioinhibitory parasympathetic cardiac vagal neurons during inspiration. Surprisingly, however, a recent study in anesthetized rats paradoxically found an enhancement of cardiac vagal activity during inspiration, suggesting that rats have an inverted respiratory sinus arrhythmia (Rentero N, Cividjian A, Trevaks D, Pequignot JM, Quintin L, and McAllen RM. Am J Physiol Regul Integr Comp Physiol 283: R1327-R1334, 2002). To address this controversy, this study examined respiratory sinus arrhythmia in conscious freely moving rats and tested whether the commonly used experimental anesthetics urethane, pentobarbital sodium, or ketamine-xylazine alter respiratory sinus arrhythmia. Heart rate significantly increased 21 beats/min during inspiration in conscious rats, a pattern similar to the respiratory sinus arrhythmia that occurs in other species. However, anesthetics altered normal respiratory sinus arrhythmia. Ketamine-xylazine (87 mg/kg and 13 mg/kg) depressed and pentobarbital sodium (60 mg/kg) abolished normal respiratory sinus arrhythmia. Urethane (1 g/kg) inverted the cardiorespiratory pattern so that heart rate significantly decreased during inspiration. Our study demonstrates that heart rate normally increases during inspiration in conscious, freely moving rats, similar to the respiratory sinus arrhythmia pattern that occurs in other species but that this pattern is disrupted in the presence of general anesthetics, including inversion in the case of urethane. The presence and consequences of anesthetics need to be considered in studying the parasympathetic control of heart rate.     

Chronic amphetamine alters D-2 but not D-1 agonist-induced behavioral responses in rats

In two experiments, using different drug doses and periods of drug administration, rats were given amphetamine (AMPH) either continuously (via slow-release pellets), or intermittently (via injections). In both experiments, only the rats pretreated with intermittent AMPH subsequently showed heightened responsivity to the selective D-2 dopamine agonist LY171555 but not to SKF38393 (a D-1 agonist). This altered response to LY171555 was still present 30 days after the AMPH withdrawal, implying that D-2 dopamine receptors at least partially mediate AMPH inverse tolerance effects. The behavioral response to the D-2 agonist was clearly different in animals receiving high versus low doses of AMPH, suggesting that different drug-state learning may have occurred during pretreatment. In a third experiment, in which rats were given repeated daily injections of either the D-1 or the D-2 agonist, only rats pretreated with the D-2 agonist and subsequently injected with the D-2 agonist clearly showed heightened responsivity. These data imply an important role of D-2 receptors in the AMPH inverse tolerance effect.     

The determination of the nociceptive thresholds in freely moving rats

A simple method for determination of nociceptive thresholds in free-moving rats is put forward. The method is based on the registration of vocalization reaction caused by the feed through the lungs connected with the stimulator by thin wires electrodes of gradually increasing impulses of constant electric current. The examples of effect of morphin and naloxone on the nociceptive thresholds as well the alteration of thresholds during predatory aggression are given.     

Circadian rhythms of urinary electrolyte excretion in freely moving rats

In 8 freely moving rats the circadian variation in the eletrolyte excretion was studied. Food was available during either the dark or the light period. The lights were on from 0800–2000 hr. Potassium, phosphate and magnesium showed peak excretion values during the dark period under both feeding conditions, although the maxima occurred 2.5 hr earlier when the rats were fed during the light period; minimum excretion was recorded just prior to feeding. Sodium excretion followed a different pattern; for animals fed during the night, maximum excretion occurred almost at the end of the dark period and minimum excretion at the start of the feeding period. For day-fed animals these values were recorded 5 and 4 hours earlier, respectively. Calcium excretion reached a maximum after the feeding period and a minimum shortly after the onset of feeding. From this study it can be concluded that the peak excretions of potassium, phosphate and magnesium are only slightly influenced by the feeding regimen, indicating that they depend mainly on an endogenous rhythm. In contrast, the minimum excretion of these ions is determined by feeding. For calcium maximum as well as minimum excretion is correlated with the feeding regimen. The excretion pattern of sodium differs from that of calcium, as well as potassium, phosphate and magnesium, indicating that it is controlled by a different mechanism.     

Dietary protein content affects the profiles of extracellular amino acids in the medial preoptic area of freely moving rats

This study examined the effect of protein consumption on extracellular amino acid concentrations in the medial preoptic area (MPOA) of rats. Rats were given free access to diets containing 0, 25 or 50 % protein for 3-h duration, starting from the onset of dark cycle (1800 h). The microdialysis probe was implanted into the MPOA at 1500 h. Dialysates were collected every 20 min from 1700 h to 2100 h. Amino acid concentrations in dialysate samples were determined by reverse phase-HPLC. Extracellular amino acid concentrations in the MPOA were elevated by protein consumption within 20 to 40 min following the start of the meal. The 50 % protein diet resulted in increased (p<0.05) alanine, glutamine, isoleucine, leucine, methionine, tyrosine and valine concentrations, when compared with both baseline and the 0% protein diet. When the 25 % protein diet was fed, amino acid concentrations in the MPOA were between those after the 0 and 50% protein diets. The ratio of tryptophan to the total branched-chain amino acids in extracellular fluid was highest after the 0% protein diet and increased with time. We conclude that extracellular amino acid profiles in the MPOA are affected by dietary protein content.     

An intragastric amino acid mixture influences extracellular amino acid profiles in the lateral hypothalamic area of freely moving rats

We tested the effect of equicaloric loads of glucose (0.89 g) or a balanced amino acid mixture (0.85 g) on extracellular amino acid concentrations in the brains of freely moving rats. At 15:30 hours, the microdialysis probe was inserted into the lateral hypothalamic area of ambulatory rats, and food and water were removed. Dialysates were collected every 20 min from 1 h prior to gavage (18:00 hours) and until 3 h after the gavage. Amino acid concentrations in the dialysate were determined by reverse-phase HPLC. Following the amino acid gavage, extracellular amino acid concentrations significantly increased from baseline for alanine, isoleucine, leucine, methionine, threonine, tyrosine, and valine. Those elevations occurred within 20-40 min following the amino acid load, and lasted up to 100 min. After the glucose and water treatments, amino acid concentrations were either not affected or gradually diminished from baseline. We conclude that extracellular amino acid concentration in the lateral hypothalamus is influenced by the composition of food consumed.     

Prenatal amphetamine reduces alpha but not beta adrenergic receptor binding in brain of adult rats

Adults offspring of rats treated with daily injection of d,l-amphetamine sulfate (0.5 mg/kg s.c.) during pregnancy showed a significant decrease in brain alpha but not beta adrenergic receptor binding, without apparent changes in receptor affinity. This change may be a consequence of long lasting alterations in the metabolism of brain catecholamines produced by amphetamine administration at fetal age, and may account for the behavioral alterations described in these animals.     

Firing properties of substantia nigra dopaminergic neurons in freely moving rats

Single unit recordings were obtained from putative dopaminergic neurons in the substantia nigra of awake, freely moving rats. The cells exhibited waveforms, range of firing rates and types of firing patterns identical to those of identified DA neurons of anesthetized or paralyzed rats. Two firing patterns were observed: single spike activity and a bursting mode with spikes of progressively diminished amplitude and increased duration within each burst. The degree of burst firing varied considerably among the cells and individual cells sometimes switched from one pattern of firing (e.g. predominantly single spike) to another (e.g. bursting), although the determinants of these transitions are, at this time, unclear. Putative DA neurons were inhibited by i.v. apomorphine and excited by i.v. haloperidol. Haloperidol also reversed the apomorphine-induced inhibition of firing. Inhibitions and excitations were associated with a reduction and elevation, respectively, in burst firing. The effects of the two drugs were identical to their effects in immobilized rats. In several cases, a putative DA neuron was observed to fire all of its spikes in near coincidence with at least one other cell with identical electrophysiological characteristics. This form of interaction (i.e. presumed electrical coupling) between DA cells is only rarely observed in anesthetized or paralyzed rats and may play a significant role in the normal functioning of the nigrostriatal DA system.