Autoregulation of dopamine synthesis in subregions of the rat nucleus accumbens

Postnatal manganese exposure does not alter dopamine autoreceptor sensitivity in adult and adolescent male rats

Administering manganese chloride (Mn) to rats on postnatal day (PD) 1-21 causes long-term reductions in dopamine transporter levels in the dorsal striatum, as well as a persistent increase in D1 and D2 receptor concentrations. Whether dopamine autoreceptors change in number or sensitivity is uncertain, although D2S receptors, which may be presynaptic in origin, are elevated in Mn-exposed rats. The purpose of this study was to determine if early Mn exposure causes long-term changes in dopamine autoreceptor sensitivity that persist into adolescence and adulthood. To this end, male rats were exposed to Mn on PD 1-21 and autoreceptor functioning was tested 7 or 70 days later by measuring (a) dopamine synthesis (i.e., DOPA accumulation) in the dorsal striatum after quinpirole or haloperidol treatment and (b) behavioral responsiveness after low-dose apomorphine treatment. Results showed that low doses (i.e., "autoreceptor" doses) of apomorphine (0.06 and 0.12 mg/kg) decreased the locomotor activity of adolescent and adult rats, while higher doses increased locomotion. The dopamine synthesis experiment also produced classic autoreceptor effects, because quinpirole decreased dorsal striatal DOPA accumulation; whereas, haloperidol increased DOPA levels in control rats, but not in rats given the nerve impulse inhibitor γ-butyrolactone. Importantly, early Mn exposure did not alter autoreceptor sensitivity when assessed in early adolescence or adulthood. The lack of Mn-induced effects was evident in both the dopamine synthesis and behavioral experiments. When considered together with past studies, it is clear that early Mn exposure alters the functioning of various dopaminergic presynaptic mechanisms, while dopamine autoreceptors remain unimpaired.

Effects of repeated and acute aripiprazole or haloperidol treatment on dopamine synthesis in the dorsal striatum of young rats: comparison to adult rats

The purpose of the present study was to determine whether repeated treatment with the D2 partial agonist aripiprazole or the D2 antagonist haloperidol alters dopamine (DA) synthesis characteristics in the dorsal striatum of young rats. To this end, rats received a daily pretreatment regimen of aripiprazole or haloperidol on postnatal days (PD) 10-20 and were tested 24 or 72 h later after an acute injection of vehicle, aripiprazole, haloperidol, or quinpirole (a D2 agonist). For comparison purposes, adult rats were pretreated with an 11-day regimen of saline or haloperidol on PD 70-80 and DA synthesis was measured after acute drug treatment on PD 83. Dorsal striatal DA synthesis was determined by measuring L-dihydroxyphenylalanine accumulation after NSD-1015 treatment. In a separate experiment, the ability of repeated drug treatment to up-regulate dorsal striatal D2 receptors was assessed in young and adult rats 72 h after drug discontinuation. The major findings of this study were that: (a) acute treatment with haloperidol and aripiprazole increased DA synthesis while quinpirole reduced it; (b) pretreatment with haloperidol and aripiprazole blunted the synthesis-modulating effects of acutely administered dopaminergic drugs; and (c) DA synthesis of young and adult rats was affected in a qualitatively similar manner by DA agonist, antagonist, and partial agonist drugs. In conclusion, results from the present study suggest that synthesis-modulating autoreceptors in the dorsal striatum are functionally mature by the end of the preweanling period and DA synthesis declines to near basal levels during the course of repeated aripiprazole treatment.

Electrophysiological evidence of mediolateral functional dichotomy in the rat accumbens during cocaine self-administration: tonic firing patterns

Given the increasing research emphasis on putative accumbal functional compartmentation, we sought to determine whether neurons that demonstrate changes in tonic firing rate during cocaine self-administration are differentially distributed across subregions of the NAcc. Rats were implanted with jugular catheters and microwire arrays targeting NAcc subregions (core, dorsal shell, ventromedial shell, ventrolateral shell and rostral pole shell). Recordings were obtained after acquisition of stable cocaine self-administration (0.77 mg/kg/0.2mL infusion; fixed-ratio 1 schedule of reinforcement; 6-h daily sessions). During the self-administration phase of the experiment, neurons demonstrated either: (i) tonic suppression (or decrease); (ii) tonic activation (or increase); or (iii) no tonic change in firing rate with respect to rates of firing during pre- and post-drug phases. Consistent with earlier observations, tonic decrease was the predominant firing pattern observed. Differences in the prevalence of tonic increase firing were observed between the core and the dorsal shell and dorsal shell-core border regions, with the latter two areas exhibiting a virtual absence of tonic increases. Tonic suppression was exhibited to a greater extent by the dorsal shell-core border region relative to the core. These differences could reflect distinct subregional afferent processing and/or differential sensitivity of subpopulations of NAcc neurons to cocaine. Ventrolateral shell firing topographies resembled those of core neurons. Taken together, these observations are consistent with an emerging body of literature that differentiates the accumbens mediolaterally and further advances the likelihood that distinct functions are subserved by NAcc subregions in appetitive processing.

Subnanomolar dopamine D3 receptor antagonism coupled to moderate D2 affinity results in favourable antipsychotic-like activity in rodent models: I. neurochemical characterisation of RG-15

RG-15 (trans-N-[4-[2-[4-(3-cyano-5-trifluoromethyl-phenyl)-piperazine-1-yl]-ethyl]-cyclohexyl]-3-pyridinesulfonic amide dihydrochloride) displayed subnanomolar affinity to human and rat dopamine D3 receptors (pKi 10.49 and 9.42, respectively) and nanomolar affinity to human and rat D2 receptors (pKi 8.23 and 7.62, respectively). No apparent interactions were found with the other 44 receptors and four channel sites tested in this study. RG-15 inhibited dopamine-stimulated [35S]GTPgammaS binding in membranes from rat striatum, in murine A9 cells expressing human D2L receptors and in CHO cells expressing human D3 receptors (IC50 values were 21.2, 36.7 and 7.2 nM, respectively). In these tests RG-15 showed the highest affinity toward D3 receptors when compared to amisulpride, haloperidol and SB-277011. RG-15, similar to haloperidol and amisulpride, dose-dependently inhibited in vivo [3H]raclopride binding in mouse striatum, enhanced dopamine turnover and synthesis rate in mouse and rat striatum and olfactory tubercle. SB-277011 did not change [3H]raclopride binding in mouse striatum nor biosynthesis or turnover rates in either region in mice or rats. RG-15 and haloperidol, but not SB-277011, antagonised dopamine synthesis inhibition induced by the D3/D2 full agonist 7-OH-DPAT in GBL-treated mice. RG-15, but not SB-277011, elevated plasma prolactin levels. In vitro receptor binding and functional experiments demonstrated that RG-15 had an antagonist profile on both D3 and D2 receptors. with high selectivity for dopamine D3 receptors over D2 receptors. However, in vivo, its neurochemical actions were similar to those of D2 receptor antagonists. Neurochemical comparison of RG-15 with antagonists having a different affinity and selectivity toward D3 and D2 receptors indicate that D3 receptors have little, if any, role in the control of presynaptic dopamine biosynthesis/release in dopaminergic terminal regions.

Asymmetric modulation of a catecholamine-regulated protein in the rat brain, following quinpirole administration

We previously reported a brain-specific 40 kDa catecholamine-regulated protein (CRP40) that binds dopamine (DA) and related catecholamines. CPR40 shares significant sequence homology with human heat shock protein (Hsp70), GRP78/BIP, and human #BQ24193 protein. Recent studies with the DA D(2) receptor antagonist, haloperidol, demonstrated a significant increase in expression of CRP40 in the striatum (STR). The objective of the present study was to investigate CRP40 expression in various brain regions following treatments with the DA D(2)/D(3) receptor agonist quinpirole (QNP) in rats and examine possible relationships between neurochemical parameters and locomotor activity. Rats received injections of either QNP (0.5 mg/kg, for 27 days every third day) or saline (SAL) and their locomotor activities were measured for 90 min after each injection. At injection 9, QNP-treated rats showed locomotor activity that was significantly greater than SAL controls (F(2,28) = 3.88, P < 0.05, Duncan's multiple range test, P < 0.05). Neurochemically, acute QNP-treated rats demonstrated significant differential expression of CRP40 in the left/right prefrontal cortex (PFC) relative to SAL-treated rats (-17.76 +/- 2.10%, -10.35 +/- 1.23%, P < 0.001). Chronic QNP significantly decreased CRP40 expression in the STR, ventral tegmental area (VTA), and left/right PFC (-24.85+/- 2.10%, -18.15 +/- 5.64%, -49.13 +/- 7.05%, -25 +/- 3.63%, P < 0.001). Finally, chronic QNP treatment resulted in a significant increase in CRP40 levels in the nucleus accumbens (NA) (+39.32 +/- 7.00%, P < 0.001). Heat shock protein (i.e., Hsp70 or Hsc70) expression remained unaltered following QNP treatment. Since QNP is a DA D(2)/D(3) agonist, alterations in CRP40 expression following QNP treatment suggest the protein's function in dopaminergic neurotransmission.

Nucleus accumbens dopamine and learned fear revisited: a review and some new findings

A role for the nucleus accumbens (NAcc) and its dopamine (DA) innervation in fear and fear learning is supported by a large body of evidence, which has challenged the view that the NAcc is solely involved in mediating appetitive processes. Unfortunately, due to conflicting findings in the aversive conditioning literature the role of the NAcc in aversive conditioning remains unclear. This review focuses on the results of recent in vivo microdialysis studies that have examined the release of NAcc DA during Pavlovian aversive conditioning. In addition, we present additional new findings, which re-examine the involvement of NAcc DA in aversive conditioning. DA release was measured in the NAcc core using in vivo microdialysis during discrete cue Pavlovian aversive conditioning in four experiments. In all cases no change in DA levels was observed either during training or in response to the CS presentations despite robust behavioural evidence of discrete cue Pavlovian aversive conditioning. These findings contrast with some previous studies that show that primary and conditioned aversive stimuli increase DA release in the NAcc. We suggest that the inconsistencies in the literature might be due to procedural differences in the measurement of aversive conditioning, and the precise location of the probe in the NAcc region. Hence, rather than discount an involvement of NAcc DA in affective processes, we propose that functionally dissociable sub-regions of the NAcc may contribute to different aspects of Pavlovian aversive learning.

Acute administration of cocaine regulates the phosphorylation of serine-19, -31 and -40 in tyrosine hydroxylase

Acute cocaine can inhibit catecholamine biosynthesis by regulating the enzymatic activity of tyrosine hydroxylase via alterations in the phosphorylation state of the enzyme. The mechanisms underlying acute cocaine-dependent regulation of tyrosine hydroxylase phosphorylation have not been determined. In this study, 0, 15 or 30 mg/kg cocaine was administered intraperitoneally to rats and the phosphorylation state of tyrosine hydroxylase in the brain was examined using antibodies specific for the phosphorylated forms of serine-19, -31 and -40 in tyrosine hydroxylase. In the caudate and nucleus accumbens, cocaine dose-dependently decreased the levels of phosphorylated serine-19, -31 and -40. In the ventral tegmental area, the levels of phosphorylated serine-19, but not serine-31 and -40, were decreased by 15 and 30 mg/kg cocaine. In the amygdala, the levels of phosphorylated serine-19, but not serine-31 or -40, were decreased. The functional effects of these alterations in phosphorylation state were assessed by measuring tyrosine hydroxylase activity in vivo (accumulation of DOPA after administration of the decarboxylase inhibitor NSD-1015). Acute administration of 30 mg/kg cocaine significantly decreased l-DOPA production in caudate and accumbens but not in amygdala. These data suggest that the phosphorylation of serine-31 or -40, but not serine-19, is involved in the regulation of tyrosine hydroxylase activity by acute cocaine.

Preclinical evaluation of GBR12909 decanoate as a long-acting medication for methamphetamine dependence

Methamphetamine (METH) abuse is a growing health problem, and no treatments for METH dependence have been identified. The powerful addictive properties of METH are mediated by release of dopamine (DA) from nerve terminals in mesolimbic reward pathways. METH stimulates DA release by acting as a substrate for DA transporter (DAT) proteins, thereby triggering efflux of DA from cells into the synapse. We have shown that blocking DAT activity with high-affinity DA uptake inhibitors, like GBR12909, can substantially reduce METH-evoked DA release in vitro, suggesting GBR12909 may have potential as a pharmacotherapy for METH dependence. The purpose of the present study was to examine the neurobiological effects of a long-acting oil-soluble preparation of GBR12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-hydroxy-3-phenylpropyl) piperazinyl decanoate, or GBR-decanoate). Male rats received GBR-decanoate (480 mg/kg, i.m.) or its oil vehicle, and were tested using a variety of methods one and two weeks later. Ex vivo autoradiography showed that GBR-decanoate decreases DAT binding in DA-rich brain regions. In vivo microdialysis in the nucleus accumbens revealed that GBR-decanoate elevates baseline levels of extracellular DA and antagonizes the ability of METH to evoke DA release. The dopaminergic effects of GBR-decanoate were sustained, lasting for at least two weeks. Rats pretreated with GBR-decanoate displayed enhanced locomotor responses to novelty at one week, but not two weeks, postinjection. Administration of the D(2)/D(3) receptor agonist quinpirole (10 and 100 microg/kg, s.c.) decreased locomotor activity and suppressed plasma prolactin levels; quinpirole-induced responses were not altered by GBR-decanoate. Thus, GBR-decanoate is able to elevate basal synaptic DA levels and block METH-evoked DA release in a persistent manner, without significant perturbation of DA receptor function. The findings suggest that GBR-decanoate, or similar long-acting agents, should be evaluated further as potential treatment adjuncts in the management of METH addiction in humans.