Alexander Jakubovic

Dopamine and preparatory behavior. II: A neurochemical analysis

Changes in the activity of dopamine-containing systems in relation to preparatory and consummatory feeding responses were investigated. In Experiment 1 rats were conditioned to associate food delivery with the presentation of a conditional stimulus (CS+). When sacrificed after exposure to the CS+ alone on a test trial, the ratio of the dopamine metabolite 3,4-dihydroxyphenylacetic acid to dopamine (DOPAC/DA ratio) was increased significantly in the nucleus accumbens. A similar trend in the ratio of homovanillic acid to dopamine (HVA/DA ratio) was also observed. Similar increases were observed in the striatum, but these were not statistically significant. In contrast, no increases were observed in the DOPAC/DA ratio or the HVA/DA ratio in either brain region when rats were permitted to consume an unsignaled meal for 7 min. These findings suggest that activation of dopamine terminals in the nucleus accumbens occurs during the anticipation of a meal, at which times the rat is engaged in preparatory feeding behaviors, but does not accompany the performance of short bouts of consummatory feeding behavior.

Anatomical analysis of the involvement of mesolimbocortical dopamine in the locomotor stimulant actions ofd-amphetamine and apomorphine

Lesion studies employing 6-hydroxydopamine (6-OHDA) suggest that locomotor hyperactivity induced by certain stimulant drugs is dependent on dopaminergic neurotransmission in the nucleus accumbens (NACC). However, studies to date have not adequately controlled for the reported effects of 6-OHDA on baseline (non-drug) activity and on DA levels in other terminal regions. Slow bilateral infusions of 6-OHDA into the NACC, but not into olfactory tubercle (OT) or medial prefrontal cortex (mPFCx), reducedd-amphetamine (0.5 mg/kg SC) hyperactivity and resulted in a “supersensitive” (hyperactive) response to a low dose of apomorphine (0.1 mg/kg SC) in photocell cages. Direct observation revealed no behavioral changes in OT lesioned rats challenged with apomorphine which might correspond to a “denervation supersensitivity” syndrome. Assays of DA and 5-hydroxytryptamine (5-HT) in mPFCx, OT, NACC, and caudate-putamen revealed that 6-OHDA infusion into NACC caused substantial DA loss in NACC, OT and mPFCx, whereas infusion at mPFCx or OT sites depleted DA locally (>85% loss) with little or no remote change. Concentrations of 5-HT were little altered by 6-OHDA, except for a local depletion in mPFCx. The present results confirm the importance of nucleus accumbens DA in the expression of locomotor stimulation induced by apomorphine andd-amphetamine, and suggest that the mPFCx and OT do not make an important contribution.

Increased dopamine metabolism in the nucleus accumbens and striatum following consumption of a nutritive meal but not a palatable non-nutritive saccharin solution

This study examined the concentrations of dopamine (DA) and its metabolites homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum and nucleus accumbens of rats that were either 20 hr food deprived or had been given 1 hr of access to food pellets, a liquid diet, or a palatable 0.4% saccharin solution. Significant increases were observed in the HVA/DA ratio in both structures following ingestion of either liquid diet or food pellets. Increases in the DOPAC/DA ratio were observed only after the ingestion of liquid diet. Ingestion of saccharin solution had no effect on any index of DA activity. These results indicate that the type of food ingested can influence dopaminergic responses to feeding, and argue against an exclusive role for motor or reward processes in determining DA activity.

The effects of chronic lithium on behavioral and biochemical indices of dopamine receptor supersensitivity in the rat

The effects of dietary lithium on several indices of dopamine receptor supersensitivity were examined in rats during withdrawal from chronic administration of haloperidol. Chronic haloperidol enhanced the locomotor stimulant action of d-amphetamine, and this effect was attenuated by lithium. In contrast, lithium did not affect the amphetamine response in animals that had not previously received haloperidol. Apomorphine-induced hypothermia was not influenced by the chronic haloperidol treatment. On the other hand, during withdrawal from chronic haloperidol, spontaneous locomotor activity (20 h) and apomorphine-induced stereotypy were increased, but neither of these effects was attenuated by lithium. In addition, lithium did not affect the chronic haloperidol-induced increase in 3H-spiperone binding sites in the striatum. Lithium alone had no effect on any of these measures except for causing a slight prolongation of the hypothermic effect of apomorphine. The results indicate that not all DA-receptor-mediated responses are enhanced by chronic administration of neuroleptics (e. g., apomorphine-induced hypothermia). In addition, while lithium reduces the effects of chronic haloperidol administration on d-amphetamine-induced locomotor activity, this is not because lithium prevents haloperidol-induced supersensitivity of postsynaptic DA receptors because more direct measures of this phenomenon (e.g., 3H-spiperone binding, apomorphine-induced stereotypy) are not affected by lithium.

Effects of cannabinoids on testosterone and protein synthesis in rat testis leydig cells in vitro

Various water-insoluble cannabinoids as well as SP-111A, the water-soluble derivative of delta 9-tetrahydrocannabinol (delta 9-THC), reduced hCG and dibutyryl-cAMP stimulated testosterone production by rat testicular Leydig cell preparations. With 0.15 microM (0.05 micrograms/ml) 8-beta-OH-delta 9-THC the inhibition was about 50% of stimulated testosterone synthesis. Dose-related inhibitions were apparent with other cannabinoids and their order of potency in inhibiting stimulated steroidogenesis by the interstitial cells in vitro was found to be: 8-beta-OH-delta 9-THC greater than or equal to 11-OH-delta 9-THC greater than CBN = CBD = CBG greater than or equal delta 9-THC = delta 8-THC. The non-stimulated, basal, steroidogenesis was not affected even with 15 microM cannabinoids. The incorporation of L-[U-14C]leucine into the protein of Leydig cells was markedly reduced by 15 microM cannabinoids under both basal and stimulated conditions. The inhibition of steroidogenesis as well as protein synthesis in rat testicular Leydig cell preparations by various cannabinoids cannot be correlated with their psychoactivity. The present data suggest that cannabinoids at very low concentrations may interfere directly in Leydig cells with both protein and testosterone synthesis, and thus with their function.

Intracarotid 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration: biochemical and behavioral observations in a primate model of hemiparkinsonism

Cynomolgus monkeys received intracarotid injections of the neurotoxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) to produce a chronic unilateral model of parkinsonism. Extensive dopamine (DA) depletion was observed in the caudate nucleus and putamen on the side ipsilateral to the injection and this was associated with contralateral tremor, rigidity, and bradykinesia. A dose of 1.25 mg of MPTP caused ipsilateral DA loss of 99.4% in the caudate nucleus, 99.8% in the putamen, and 74.2% in the nucleus accumbens. A dose of 2.5 mg caused ipsilateral DA depletion of 99.3% in the caudate nucleus, 99.5% in putamen, and 90.1% in the nucleus accumbens. The unilateral aspect of the lesion was dose sensitive, with the 2.5‐mg dose causing bilateral asymmetric DA depletion. Tissue concentrations of serotonin were not affected by the toxin. These findings confirm that intracarotid injection of MPTP may produce a useful primate model of hemiparkinsonism that can be associated with selective unilateral DA depletion when the appropriate dose of toxin is used.

Factors affecting the stability and separation of biogenic amines and their metabolites: Simultaneous measurement by HPLC with electrochemical detection

We describe a simple and sensitive method for the rapid and simultaneous quantification of dopamine, 3-methoxytyramine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, serotonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in the picogram range in small samples of brain tissue. After minimal sample preparation the amines were analyzed utilizing isocratic separation and reversed-phase high-performance liquid chromatography with amperometric detection. The effects of pH and methanol concentration in the solvent on the retention times of the amines on two different C-18 columns were investigated. Stabilities of the amines in solution were determined under various conditions. Light and air were found to be detrimental to the stability of indoles. In the absence of light, their stability was dependent on temperature and the presence of air; however, in the absence of air, light and/or temperature had little effect. The catechols were stable under most of these conditions. The assay has been applied to study the postmortem stability of dopamine, serotonin, and their metabolites in the striatum of rat brain. In the striatum 4 hr after death, the content of dopamine and 3,4-dihydroxyphenylacetic acid decreased by less than 20%, and 3-methoxytyramine increased by 158%, with no changes in serotonin, 5-hydroxyindoleacetic acid, and homovanillic acid.

Increased in vivo tyrosine hydroxylase activity in rat telencephalon produced by self-stimulation of the ventral tegmental area

Changes in the activity of dopaminergic neurons associated with intracranial self-stimulation of the ventral tegmentum were assessed by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of aromatic amino acid decarboxylase by NSD-1015. When compared to implanted unstimulated controls, DOPA concentrations were elevated significantly in the nucleus accumbens, striatum and olfactory tubercle in the hemisphere ipsilateral to the electrode, after a 30 min session of self-stimulation. The concentration of DOPA in the contralateral nucleus accumbens and striatum did not differ from control levels, although relative to control values it was significantly increased in the contralateral olfactory tubercle. A similar analysis of in vivo tyrosine hydroxylase activity in these brain regions following a 30 min session of lever pressing for food reward on a fixed-ratio (FR-8) schedule failed to reveal any significant changes relative to control subjects. These results are consistent with a role for dopamine in brain-stimulation reward obtained from electrical stimulation of the ventral tegmental area but do not provide evidence for dopaminergic mediation of the rewarding properties of food.

Excretion of THC and its metabolites in ewes' milk

Abstract After single iv injections of either 0.02 mg/kg or 1 mg/kg of [ 14 C ]Δ 9 - tetrahydrocannabinol , [14C]THC, to lactating ewes, radioactivity was detected in the milk at all subsequent time intervals tested (4–96 hr). Radioactivity was found in unchanged THC as well as in various unidentified metabolites. Only about 15% of the administered radioactivity was excreted by the ewes in the first 48 hr; most of this was in the urine and feces. Radioactivity appeared in the feces and urine of a lamb suckling milk from a ewe injected with [14C]THC, indicating transfer of THC and its metabolites via the milk. These results confirm previous literature reports indicating slow elimination of THC, and show that milk is an additional route of excretion.

Cerebral histamine levels are unaffected by MPTP administration in the mouse

The mechanism by which 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) depletes forebrain dopamine is not fully understood, but a necessary step involves the formation of neurotoxic MPP+ by monoamine oxidase type B (MAO-B). The histamine neurons in the brain contain MAO-B and are a possible site for the production of MPP+. Two weeks after MPTP injections (2 X 50 mg/kg i.p.) in C-57 mice, striatal dopamine was reduced by more than 70%. However, histamine levels in neocortex, hippocampus and hypothalamus were unaffected by this neurotoxic dose of MPTP. Thus, as in Parkinsons disease, central histaminergic systems appear to be spared in the MPTP model.