Conditioned circling in rats: bilateral involvement of the mesotelencephalic dopamine system demonstrated following unilateral 6-hydroxydopamine lesions.

High rates of conditioned circling have previously been associated with a bilateral augmentation of striatal dopamine metabolism. These results suggest that both striata subserve this response. The present experiment further assessed this possibility by determining the effects of unilateral 6-hydroxydopamine lesions of the mesotelencephalic dopamine system on conditioned circling. Rats were initially trained to circle in their preferred direction for water reinforcement. Upon establishment of this response, they received unilateral lesions at the level of the lateral hypothalamus either contralateral or ipsilateral to the reinforced direction of circling. Reinforced responding was virtually abolished in rats with contralateral lesions. In contrast, rats lesioned ipsilateral to the direction of reinforced circling exhibited only a 50% decrease in rate of reinforced responding. Non-reinforced responding was increased only in rats with contralaterally placed lesions. Following 5 postoperative test sessions, the experimental contingencies were reversed. 'Ipsilaterally lesioned' rats were now required to circle away from their lesion whereas 'contralaterally lesioned' rats had to turn towards their lesion. The 'Contralateral' group acquired the reversal, such that reinforced responding occurred more frequently than non-reinforced responding. However, reinforced rates of responding did not reach preoperative rates. Conversely, 'ipsilaterally lesioned' rats could not learn to turn contraversively and now made more non-reinforced than reinforced responses. These findings suggest that conditioned circling is mediated by a bilateral involvement of the mesotelencephalic dopaminergic systems. However, the specific role of each side appears to be dependent upon its relationship with the direction of circling emitted.

Effects of B-HT 920 on nigrostriatal and mesolimbic dopamine systems in normosensitive and supersensitive rats.

1. B-HT 920, a D2 dopamine receptor agonist, was tested for its ability to exert presynaptic actions in normosensitive rats, and for possible postsynaptic actions in rats made 'supersensitive' to apomorphine. 2. In normosensitive rats, B-HT 920 (0.01-0.3 mg kg-1, i.p.) increased dopamine concentrations and lowered metabolite levels to a similar extent in all four terminal regions examined (medial prefrontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen). Analogous effects were seen for 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid. 3. Rats which received bilateral 6-hydroxydopamine (6-OHDA) infusions into the caudate-putamen showed signs of postsynaptic dopamine receptor activation (stereotyped behaviour) in response to B-HT 920 (0.1 and 1.0 mg kg-1, i.p.) and to apomorphine (0.2 mg kg-1, s.c.). Similarly, B-HT 920 (0.1 mg kg-1) induced contralateral circling in rats that had received unilateral 6-OHDA infusions into the medial forebrain bundle; the rate of circling increased gradually over several weeks. 4. In contrast, bilateral 6-OHDA infusions into the nucleus accumbens resulted in a supersensitive (locomotor stimulant) response to a low dose of apomorphine (0.1 mg kg-1, s.c.), but not to B-HT 920 (0.01 and 0.1 mg kg-1). 5. In intact rats, withdrawal of chronic haloperidol treatment induced behavioural supersensitivity to apomorphine but not to B-HT 920.

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.

Influence of inherent directional biases on neurochemical consequences of conditioned circling.

High rates of conditioned circling have been associated with a bilateral increase in striatal (STR) dopamine (DA) metabolism in rats. Experiments 1 and 2 examined the extent to which inherent directional biases, which are critical in determining the magnitude and direction of drug-induced circling, would influence the acquisition and performance of conditioned circling. No behavioral effects were evident. However, a symmetrical bilateral enhancement in DA metabolism was observed in the STR irrespective of directional biases. These results provide further evidence for the bilateral involvement of the mesotelencephalic DA projection in conditioned circling. Although conditioned circling could be established and maintained by reinforcing the response with food (Experiment 3), food itself influenced DA metabolism and therefore precluded the detection of changes in DA metabolism specific to the circling response. Specifically, DA metabolism was augmented to a similar extent in animals given noncontingently presented food.

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.

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.

Anatomical analysis of the involvement of mesolimbocortical dopamine in the locomotor stimulant actions of d-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), reduced d-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 (greater than 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 and d-amphetamine, and suggest that the mPFCx and OT do not make an important contribution.

Fentanyl-induced conditional place preference: lack of associated conditional neurochemical events.

Three experiments were performed to determine if stimuli previously paired with the reinforcing effects of fentanyl elicit changes in the activity of dopaminergic neurons that are similar to the unconditional effects of the drug. Experiment 1 characterized the unconditional effects of fentanyl (0.04 mg/kg SC) on neurochemical indices of dopaminergic activity in rats. Both acute and repeated fentanyl injections (five injections administered at 48-h intervals) increased the concentrations of the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) within the striatum (STR), nucleus accumbens (NAS), and olfactory tubercle (OT). Acute injections elicited a greater increase in metabolite concentrations in the NAS than in the STR, suggesting that there are regional differences in the sensitivity of dopaminergic neurons to fentanyl. In experiments 2 and 3, fentanyl (five injections; 0.04 mg/kg SC) was paired with environmental stimuli using a place preference conditioning paradigm. The fentanyl-paired stimuli failed to elicit conditional changes in DOPAC or HVA concentrations within the STR, NAS, or OT even though rats exhibited a preference for the drug-paired compartment of the shuttle box. These results indicate that the secondary reinforcing effects of stimuli previously paired with fentanyl may not reflect the ability of these stimuli to elicit measurable changes in the activity of mesolimbic or nigrostriatal dopaminergic neurons.

Degradation of met-enkephalin by extracts of various regions of the human brain: effects of antipsychotics and narcotics in vitro.

Antischizophrenic drugs, reduced in a concentration-dependent fashion enkephalin degradation by the soluble and particulate fractions of the human cerebral cortex and cerebellum. The order of potency is as follows: thioridazine greater than chlorpromazine greater than fluphenazine greater than haloperidol greater than or equal to promazine with IC50 of 50, 80, 120, 200-250 micro M, respectively. Kinetic studies revealed non-competitive and competitive inhibition by thioridazine and chlorpromizine, respectively. Narcotics, were weak inhibitors of enkephalin degradation. For dl-, d-, l-methadone and l-alpha-acetylmethadol, IC50 was about 500 micro M, and 1000 micro M for heroin and morphine. It is suggested that inhibition of the degradation of endogenous morphinomimetic peptides in the CNS may be a crucial factor governing the pharmacology of some neuroleptics and other psychoactive drugs. Enkephalin-hydrolyzing activity was ubiquitous and exhibited considerable regional differences in the normal human and in Huntington's chorea brains. The rate of enkephalin degradation is generally higher in the subcortical nuclei than in the cortex and cerebellum. The highest hydrolytic activity was found in the substantia nigra, anterior thalamus, septal area, globus pallidus and caudate nucleus, in this decreasing order.

Dopaminergic and serotonergic correlates of stimulation-induced circling.

Rotation induced by electrical stimulation of the medial forebrain bundle at the level of the lateral hypothalamus was associated with increases in dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum ipsilateral to the site of stimulation (i.e. contralateral to direction of turning). The concentrations of DA, DOPAC and HVA within the nucleus accumbens (NAS) were not altered. In the olfactory tubercle (OT), concentrations of DA and both metabolites were, in general, elevated ipsilateral to the electrode. However, relative to non-stimulated controls, HVA concentrations were increased bilaterally in rats exhibiting circling. Stimulation-induced circling also resulted in a bilateral enhancement of striatal serotonin (5-HT) metabolism as indicated by elevated 5-hydroxyindoleacetic acid: 5-HT ratios. No changes in 5-HT metabolism were observed in the NAS. The utilization of 5-HT was elevated in the OT ipsilateral to the electrode in rats that exhibited stimulation-induced rotation. While most subjects that exhibited contraversive rotation in response to the stimulation demonstrated enhanced DA activity, the neurochemical changes were not observed in all subjects. As such, it is concluded that while stimulation of the mesotelencephalic DA system can be associated with stimulation-induced rotation it is not necessary for its elicitation.

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.

Protein and RNA synthesis in kainic acid-injected striata.

Injections of kainic acid into rat neostriata destroy neuronal soma in the injected area. Kainic acid-injected neostriata show a significant increase (as compared with control) in the incorporation in vitro of label from radioactive leucine into the TCA-insoluble protein fraction at all time periods studied (from 1 to 80 days after the kainic acid injection), with the greatest stimulation ((6-7-fold) occurring between the third and eighth day. At 1-5 days there was also increased incorporation in vitro of label from uridine into RNA. The increased protein synthesis is probably due to the astrocytes which invade the injected area and are still seen in electron microscopic studies 2-3 months after the injections.

D2 dopamine receptors in the rat prefrontal cortex: characterization and alteration by stress.

D2 dopamine (DA) receptors were characterized in the medial prefrontal cortex (MPFC) of the rat by employing radioligand binding techniques which greatly reduced [3H]spiperone binding to filters, S2 serotonin receptors and spirodecanone sites. Competition studies suggested that the MPFC contains a higher proportion of D2 [high] receptors than does the striatum. The IC50 values of DA receptor antagonists in assays of MPFC tissue were correlated with their antipsychotic potencies. Stress (footshock or tailshock) increased the density of MPFC D2 dopamine receptors and decreased their affinity for [3H]spiperone. These changes were delayed in onset, being present at 27 h but not at 3 h after exposure to stress. The binding assay detected the changes when it was performed at pH 7.9 but not when the pH was reduced to 6.2. D2 dopamine receptors in the striatum and nucleus accumbens were not affected by stress.

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 Parkinson's disease, central histaminergic systems appear to be spared in the MPTP model.

Tolerance to haloperidol-induced increases in dopamine metabolites: fact or artifact?

Haloperidol increased 3,4-dihydroxyphenylacetic acid and homovanillic acid concentrations in the striatum, nucleus accumbens and olfactory tubercle of both drug-naive rats and rats pretreated with haloperidol (10 injections). The increases in metabolite concentrations were greater in all brain regions of the naive rats, suggesting that haloperidol pretreatment resulted in a decreased responsiveness to the drug (tolerance). However, subchronic haloperidol injections also resulted in decreased basal metabolite concentrations in rats killed 48 h after the last injection. While the response of drug-experienced rats to haloperidol was attenuated relative to that of drug-naive rats, this difference could be accounted for entirely by the decreased basal metabolite concentrations that occur after repeated haloperidol injections.

Interaction of a water-soluble derivative of delta-9-tetrahydrocannabinol with [3H]diazepam and [3H]flunitrazepam binding to rat brain membranes.

We have tested the effect of a psychoactive water-soluble derivative of delta-9-tetrahydrocannabinol, SP-111A, on the binding of [3H]diazepam and [3H]flunitrazepam to rat brain membranes. It was found that SP-111A reduced the specific binding of [3H]diazepam and [3H]flunitrazepam. The inhibition by SP-111A was dependent not only on the concentration of the ligand but also on the protein content of membrane preparations. The inhibition of the specific binding of [3H]diazepam by SP-111A was found to be competitive with Ki value of 3.1 microM. In the presence of 7.5 microM SP111A the apparent Kd of [3H]diazepam binding increased from 4.3 nM to 12.5 nM, without affecting the Bmax. The inhibition of the specific binding of [3H]flunitrazepam by SP-111A was also competitive, however, the IC50 was higher than with [3H]diazepam. The inhibition by SP-111A appeared to be caused by its tight binding to the benzodiazepine binding sites of brain membranes.

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.