Effects of serotonergic activity in nucleus accumbens septi on drug-induced circling

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Pre- and postsynaptic sites for serotonin modulation of GABA-containing neurons in the shell region of the rat nucleus accumbens

The shell of the nucleus accumbens received a dense serotonergic innervation and contains abundant gamma-aminobutyric acid (GABA)-immunoreactive neurons. Moreover, serotonin (5-hydroxytryptamine: 5-HT) and GABA have been implicated in a variety of common motivational and motor-related functions partially ascribed in this brain area. We used immunoelectron microscopy of antisera directed against 5-HT and GABA in the same section of tissue to examine whether there were cellular substrates that might indicate more specific sites for functional interactions involving these transmitters in the shell region of the rat nucleus accumbens. Immunogold-silver labeling for GABA was localized to perikarya, dendrites, axons and axon terminals, whereas immunoperoxidase labeling for 5-HT was restricted to axons and axon terminals. Approximately half (187/366) of the 5-HT-immunoreactive axon terminals apposed or formed synaptic junctions with postsynaptic neurons. These junctions were mainly of the symmetric-type (83/187) characteristic of inhibitory transmitters, and were equally prevalent on dendrites with and without detectable gold-silver labeling for GABA. Of the 187 5-HT-labeled axon terminals with recognized synaptic contacts, 36% also showed convergence on a common dendrite with a GABA-labeled axon terminal. In addition, 5-HT- and GABA-immunoreactive axon terminals were commonly (83/366) identified in direct apposition to one another. Within a single plane of section, 41% of the apposed GABA-immunoreactive axon terminals formed symmetric-type junctions with dendrites or somata, whereas, the apposed 5-HT-labeled axon terminals rarely showed postsynaptic contacts. These results indicate that 5-HT-containing axon terminals may postsynaptically inhibit GABAergic neurons and their targets within the shell of the rat nucleus accumbens. Additionally, our results strongly suggest that, in this brain region, appositions between 5-HT and GABA axons and axon terminals may facilitate presynaptic interactions between these transmitter systems.

Injection of 5-HT into the nucleus accumbens reduces the effects of d-amphetamine on responding for conditioned reward

Injection of d-amphetamine into the nucleus accumbens potentiates responding for stimuli paired with a primary reward. A previous study showed that this potentiating effect of d-amphetamine on responding for conditioned reward (CR) was attenuated by peripherally injected d-fenfluramine, a 5-hydroxytryptamine (5-HT) releaser and re-uptake inhibitor. The present experiments further examined the effects of manipulating 5-HT function within the nucleus accumbens on responding for CR, and on the potentiation of CR responding following intra-accumbens injection of d-amphetamine. Water deprived rats were trained to associate a compound stimulus with water delivery during a conditioning phase. During a test phase water was not delivered, but the compound stimulus was delivered according to a random ratio 2 schedule following a response on one of two levers. Rats responded at a higher rate on the lever delivering this CR. d-Amphetamine (10 micrograms) injected into the nucleus accumbens enhanced responding on the CR lever. Co-injections of 5-HT (5 and 10 micrograms) into the nucleus accumbens abolished the response-potentiating effect of d-amphetamine but were without effect on the base-line level of responding for CR. This reduction by 5-HT of the response potentiating effect of d-amphetamine was prevented by prior treatment with the 5-HT receptor antagonist metergoline (1 mg/kg). Responding for water was not altered by 5-HT and so the effects of 5-HT on responding for CR cannot be due to a change in the motivation to seek the primary reward. Thus, elevating 5-HT activity within the nucleus accumbens antagonises the effect of d-amphetamine on responding for CR within the nucleus accumbens. These results suggest that 5-HT within the nucleus accumbens may play an important role in mediating incentive motivation by modulating dopaminergic neurotransmission.

Antidepressant treatment and limbic serotonergic mechanisms regulating rat locomotor activity

The effects of chronic administration of desipramine, citalopram, and electroconvulsive shocks (ECS) on changes in rat motility after intraaccumbens (NAS) injections of selective serotonergic drugs were studied in intact and 5.7-DHT lesioned animals. It was shown that local injections of 8-OHDPAT and DOI-HCl depressed rat locomotor activity. Their effects appeared to be mediated postsynaptically, and could be antagonized by NAN-190 and ritanserin, respectively. Chronic but not acute pretreatment of rats with antidepressants (21 days long; the experiment was performed 24 h after the last dose) as well as repeated ECS (shocks were applied five times every second day), antagonized behavioral depression after 8-OHDPAT and DOI-HCl. The influence of antidepressant treatment was prevented by serotonergic lesions. Chronic administration of antidepressants and ECS did not equivocally affect the levels or metabolism of 5-HT, dopamine, and noradrenaline in the rat limbic forebrain. It is concluded that the present data indicate diminished activity of 5-HT systems related to the 5-HT1A and 5-HT2 receptors in the limbic nucleus, after chronic antidepressant treatment. This effect of drugs and ECS concerns nervous processes linked with the function of postsynaptically localized 5-HT receptor subtypes, and it probably depends on intact presynaptic 5-HT innervation.

Ultrastructure of serotonin-immunoreactive terminals in the core and shell of the rat nucleus accumbens: cellular substrates for interactions with catecholamine afferents

The nucleus accumbens is composed of a core region involved in motor functions and a shell region implicated in emotional and motivational processes. Both of these regions receive serotonin- and dopamine-containing afferents. We examined whether the serotonin innervation or relation to catecholamine (mainly dopamine) axons in the nucleus accumbens shows common features or specializations corresponding to the noted functional differences in core and shell subregions. To address this question, we examined the ultrastructure of serotonin-containing axons and their relation to catecholamine-containing afferents in either the core or shell of the nucleus accumbens. Single coronal sections through the rat forebrain were processed for immunoperoxidase labeling of serotonin and immunogold silver labeling of tyrosine hydroxylase, the catecholamine-synthesizing enzyme. Varicose processes showing peroxidase product for serotonin by light microscopy were confirmed to be axons and terminals by electron microscopy. In a quantitative analysis of serotonin-immunoreactive terminals forming one or more contacts in single sections, some common features were observed. For the core (n = 120) and the shell (n = 82), 41% formed synaptic junctions with unlabeled dendrites, 75% were in apposition with unlabeled terminals, which often formed asymmetric junctions, and 20% were in apposition with axons or terminals containing tyrosine hydroxylase. Thus, in both the core and shell of the nucleus accumbens, serotonin terminals synapse on postsynaptic neurons and are likely to modulate or be modulated by presynaptic interactions with excitatory axons forming asymmetric junctions and by catecholaminergic afferents. Marked differences in the morphology of serotonin axons were also seen in the core versus shell of the nucleus accumbens. By light microscopy, serotonin-immunoreactive axons were thicker and more varicose than those found in the core. Ultrastructural analysis confirmed that, in contrast to the core, serotonin-immunoreactive axons and terminals in the shell were larger in cross-sectional diameter size (0.7 micron vs. 0.3 micron). Additionally, serotonin axon terminals in the shell contained more numerous immunoreactive large dense core vesicles and more frequently formed symmetric as opposed to asymmetric contacts with dendrites. The larger size and more numerous dense core vesicles in serotonin-immunoreactive terminals in the shell support the concept that serotonin or co-existing neurotransmitter may be more tonically released in the shell versus core of the nucleus accumbens.

Tianeptine raises dopamine and blocks stress-induced noradrenaline release in the rat frontal cortex

The effects of various doses of tianeptine on extracellular concentrations of dopamine were studied in the frontal cortex and nucleus accumbens of the rat. At 5 and 10 mg/kg intraperitoneally, tianeptine raised extracellular dopamine in the nucleus accumbens but only the higher dose caused a significant increase in the frontal cortex. At 10 mg/kg tianeptine significantly raised the concentrations of dihydroxyphenylacetic acid and homovanillic acid in both brain regions. In another experiment, 10 and 20 mg/kg tianeptine did not modify the extracellular concentrations of noradrenaline in the frontal cortex but dose dependently blocked the increase in extracellular noradrenaline caused by restraint stress.

Reduction in cortical 5-HT3 binding sites following a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle in rats

Serotonergic mechanisms have been implicated in pathophysiology of Parkinson's disease, an illness where the dopamine deficiency represents the prime biochemical deficit. Present interest centres on the possible involvement of serotonergic receptors in modulating dopamine transmission. In this paper the binding of the selective 5-HT3 antagonist [3H]GR 65630 was studied in rats with a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle. The maximal density of specific [3H]GR 65630 binding was reduced in homogenates of entorhinal (17.1%, P < 0.05) and prefrontal cortex (27.5%, P < 0.05) on the lesioned side of the rat brain compared to the control tissues. An increase in affinity for [3H]GR 65630 binding was also found in homogenates of prefrontal cortex (33.8%, P < 0.05). No changes in the characteristics of [3H]GR 65630 binding to homogenates from the amygdala and hippocampus were observed. These data suggest that altered dopamine function may affect serotonergic mechanisms in the cortex in Parkinson's disease.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

Serotonergic mechanisms in the nucleus accumbens affected by chronic desipramine treatment

The effects of repeated treatment of rats with desipramine on 5-HT mechanisms within the nucleus accumbens (NAS) have been studied in a functional model. Local microinjections of 5-HT, quipazine as well as 5-HT1A receptor agonist buspirone, 8-OH-DPAT and NDO-008, inhibited rat locomotor activity in the open-field test. The effect of 5-HT and buspirone was blocked by serotonergic receptor antagonists methysergide and cyanopindolol, respectively. Chronic, but not acute treatment of rats with desipramine (10 mg/kg, PO, twice a day for 21 days, tests were performed 24 h after the last dose) significantly attenuated behavioral depression after 5-HT and quipazine microinjections, while the effect of buspirone was left unchanged. On the basis of present data, it may be concluded that whereas both accumbens 5-HT1A and 5-HT2 receptors appear to be important to regulation of animals' motility, only 5-HT2 receptors seem to be the most likely targets of antidepressive treatment. These data, along with previously reported changes in limbic noradrenergic and dopaminergic activity after antidepressive treatment, may explain the energizing influence of drugs and electroconvulsive shocks on psychomotor retardation, a part of endogenous depression.

The effect of intraventricular administration of the 5-HT3 receptor agonist 2-methylserotonin on the release of dopamine in the nucleus accumbens: an in vivo chronocoulometric study

In the present study we have examined the effects of the serotonin3 (5-HT3) agonist 2-methylserotonin (2-Me-5HT) on the dopamine (DA) release in the nucleus accumbens (NAc) of rats using in vivo chronocoulometric recording. The intraventricular (i.c.v.) administration of 2-Me-5HT dose-dependently increased the DA release in the NAc. This effect was blocked by the selective 5-HT3 antagonist BRL-43694 (granisetron), but not by the 5-HT1/5-HT2 antagonist metergoline. The i.c.v. injection of 8-hydroxydipropylaminotetraline (8-OHDPAT, a selective 5-HT1a agonist) or (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, a 5-HT2/5-HT1c agonist) failed to alter the DA release in the NAc. The increase in the DA release produced by 2-Me-5HT was abolished in animals that had received acute bilateral injections of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. Our results suggest that the 2-Me-5HT-induced DA release in the NAc is mediated by 5-HT3 receptors. In addition, 2-Me-5HT induced effect is dependent upon the impulse flow of DA cells.

1-(3-Trifluoromethylphenyl) piperazine (TFMPP) in the ventral tegmental area reduces the effect of desipramine in the forced swimming test in rats: possible role of serotonin receptors

1-(3-Trifluoromethylphenyl)piperazine (TFMPP), a serotonin1 (5-HT1) receptor agonist, injected i.p. in doses of 0.1 and 0.6 mg/kg, did not modify the immobility time of rats in the forced swimming test but significantly antagonized the effect of a 7 days treatment with 10 mg/kg per day desipramine (DMI). A similar effect was found on infusing 1 and 5 micrograms/microliters TFMPP bilaterally into the ventral tegmental area (VTA). Infusion of 5 micrograms/microliters TFMPP into the nucleus accumbens or into the globus pallidus did not modify the effect of DMI. The effect of 5 micrograms TFMPP infused into the VTA was prevented by the i.p. administration of 5 mg/kg metergoline, a non-selective serotonin receptor antagonist. Infusion of 5 micrograms/microliters 8-hydroxy-2-(di-n-propylamino)tetralin, a specific 5-HT1A receptor agonist, into the VTA did not modify the effect of DMI. Besides acting as a 5-HT1B receptor agonist, TFMPP may also act on other 5-HT receptor types, but available evidence suggests that its former action is more important. It thus appears that 5-HT1 receptors in the VTA, presumably of the 5-HT1B type, act by preventing the anti-immobility effect of DMI. The role of VTA dopamine and non-dopamine cells in the effect of TFMPP is discussed.

5-Hydroxytryptamine acts at 5-HT2 receptors to decrease potassium conductance in rat nucleus accumbens neurones

1. Intracellular recordings were made from neurones in the nucleus accumbens in slices from the rat brain maintained in vitro. 2. 5-Hydroxytryptamine (5-HT.1-100 microM) depolarized 170 of 203 (84%) neurones and caused them to discharge action potentials. The depolarization was associated with an increase in the input resistance, and was reversed in polarity by conditioning hyperpolarization; this reversal potential was linearly related to the logarithm of the extracellular potassium concentration. 3. Application of 5-HT to neurones voltage-clamped near their resting potential (typically about -80 mV) caused an inward current and a decrease in the slope conductance. The current caused by 5-HT reversed polarity at the potassium equilibrium potential. Analysis with an equivalent circuit model of the neurone at steady state indicated that 5-HT selectively reduced the inward rectifier potassium conductance. 4. The depolarization caused by 5-HT persisted in tetrodotoxin (1 microM). It was reduced but not abolished by a solution that contained lower levels of calcium (0.24 instead of 2.4 mM), higher levels of magnesium (5 instead of 1.2 mM), and cobalt (2 mM). 5. The depolarization caused by 5-HT was competitively antagonized by the 5-HT2 antagonists ketanserin and mianserin with dissociation equilibrium constants of 3 and 45 nM respectively: spiperone (300 nM) also blocked the action of 5-HT. The depolarization was not mimicked or blocked by a number of other agonists and antagonists selective for the 5-HT1 and 5-HT3 receptor types.

Drug-induced place preference in rats with 5,7-dihydroxytryptamine lesions of the nucleus accumbens

The conditioned place preference (CPP) paradigm was used to determine a role for serotonin in the nucleus accumbens in the mediation of the rewarding properties of D-amphetamine morphine and diazepam. The effect of these drugs on CPP was examined in controls and in animals with 5,7-dihydroxytryptamine lesions of the nucleus accumbans. The results from control animals confirmed that D-amphetamine (1.5 mg/kg, i.p.), morphine (2.0 mg/kg, i.p.) and diazepam (1.0 mg/kg, i.p.) produced place preference for a distinctive environment that had previously been paired with injections of the drug. In animals with 80% reduction of 5-hydroxytryptamine content of the nucleus accumbens, D-amphetamine CPP was unchanged and morphine CPP was attenuated compared with controls. Diazepam CPP was not apparent in animals with the lesion. In separate experiments, characteristic behavioural effects of the drugs under study were examined in control and in animals with lesion. The results showed a tendency for increased amphetamine hyperlocomotion, enhanced morphine activity and analgesia and decreased diazepam anti-anxiety effect in animals with lesions. Thus, the 5,7-dihydroxytryptamine lesions of the nucleus accumbens differently influenced the CPP induced by the drugs studied and, with the exception of diazepam, the various behavioural effects elicited by each drug. The findings suggest that serotonin-containing neurones of the nucleus accumbens are a component of the neural circuitry that mediates the rewarding properties of morphine, probably of diazepam, but not of D-amphetamine.

Effects of K+-stimulation and precursor loading on the in vivo release of dopamine, serotonin and their metabolites in the nucleus accumbens of the rat

The in vivo efflux of endogenous 3,4-dihydroxyphenylethylamine (DA 5-hydroxytryptamine (5-HT), 3,4-dihydroxyphenyl-acetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-indoleacetic acid (5-HIAA) in the nucleus accumbens of the anesthetized rat was studied using a push-pull cannula. Local perfusion for 10 minutes with 35 mM K+ significantly (P less than 0.01) increased the release of DA and 5-HT, but not their metabolites, from their respective control levels of 0.95 and 0.04 pmol/15 min to 2.5 and 0.23 pmol/15 min. Exposure to 35 mM K+ a second and third time resulted in a decrement in the amount of stimulated release for both DA and 5-HT. This decrease was prevented by local perfusion for 10 minutes with 50 uM L-tyrosine and -tryptophan starting 30 minutes before each episode of depolarization. The baseline amounts of DOPAC, HVA and 5-HIAA observed in the perfusates were several fold higher than the basal levels found for 5-HT and DA. In the absence of precursors, the efflux of DOPAC, HVA and 5-HIAA decreased approximately 60, 40 and 25%, respectively, from the first to the last baseline fraction collected. Addition of precursors prevented the decrease for DOPAC and 5-HIAA but not for HVA. The data indicated that (a) the in vivo release of DA and 5-HT, along with their metabolites, could be simultaneously measured with the present procedure, and (b) when using the push-pull cannula, local perfusion with precursors may be necessary following periods of sustained and/or repeated stimulation in order to replenish the monoamine transmitter pools.

Effects of 5,7-dihydroxytryptamine lesions of the nucleus accumbens on rat intravenous morphine self-administration

The role of serotonergic innervations of the nucleus accumbens in the processes maintaining intravenous morphine self-administration were assessed. Pairs of male rat littermates were implanted with intravenous jugular catheters and bilateral injection guide cannulae into the central medial nucleus accumbens, made physically dependent on morphine and then allowed to intravenously self-administer with continuous access. When stable baselines of drug intake were obtained (2-3 weeks), one of each pair received bilateral microinjections of vehicle and the other 5,7-dihydroxytryptamine (5,7-DHT) into the nucleus accumbens. Response independent infusions of morphine were delivered for 24 hours at the previous rate of self-injection and the animals were again allowed to self-administer while drug intake was monitored for thirteen days. The littermate pairs were then sacrificed by immersion in liquid nitrogen, the brains removed at -20 degrees C and frozen sections of the cannulae tract taken for histological assessment. The nucleus accumbens, anterior caudate nucleus and pyriform cortex were removed at -20 degrees C and biogenic monoamine content determined. The 5,7-DHT lesions resulted in a significant increase in drug intake and significantly decreased the content of serotonin (5-HT) and 5-hydroxyindoleacetic acid in the nucleus accumbens (-49% and -30%, respectively) and 5-HT in the anterior caudate nucleus (-14%) and pyriform cortex (-17%). Dose-effect relationships were assessed in four additional animals before and after similar bilateral 5,7-DHT lesions.(ABSTRACT TRUNCATED AT 250 WORDS)