Dopamine and adenosine mediate substance P-induced depression of evoked IPSCs in the rat nucleus accumbens in vitro

Zebrafish (Danio rerio) as a model to assess the effects of cocaine as a drug of abuse and its environmental implications

Cocaine (COC) use concerns are on the increase for both authorities and civil society. Despite this, it is important to investigate COC effects or those of its main metabolite, belzoylecgonine (BE), in consolidated aquatic model organisms, such as the zebrafish (Danio rerio). This (mini) review consists in an assessment regarding toxicological studies carried out employing zebrafish (embryos, larvae or adults) exposed to COC and/or BE indexed at the SCOPUS and Web of Science databases. Ten different endpoints were analyzed in both embryos and larvae, whereas only four were analyzed in adults. Of the 23 studies, only five investigated COC and/or BE effects following an environmental approach when exposing zebrafish, while most (18 studies) analyzed COC effects under a drug of abuse approach. Cocaine exposure was noted as altering the expression of several genes, such as those linked to COC transport proteins, dopamine receptors, SP substance production, the tachykinin system, and the tyrosine hydroxylase enzyme. BE exposure resulted in more oxidative and proteomic effects than COC in embryos. Cocaine abstinence resulted in hyperactivity associated with stereotypy in adult fish, in addition to reduced responses to visual stimuli to red light and neuronal development pattern alterations. Cocaine was noted as accumulating in zebrafish eyes, possibly due to melanin binding, and causing dose-response cardiac effects in both embryos and adults. Despite the different effects addressed by our survey, we emphasize the lack of COC and BE exposure assessments in zebrafish employing an environmental point of view.

Perseveration by NK1R-/- ('knockout') mice is blunted by doses of methylphenidate that affect neither other aspects of their cognitive performance nor the behaviour of wild-type mice in the 5-Choice Continuous Performance Test

The underlying cause(s) of abnormalities expressed by patients with attention deficit hyperactivity disorder (ADHD) have yet to be delineated. One factor that has been associated with increased vulnerability to ADHD is polymorphism(s) of TACR1, which is the human equivalent of the rodent NK1 (substance P-preferring) receptor gene (Nk1r). We have reported previously that genetically altered mice, lacking functional NK1R (NK1R-/-), express locomotor hyperactivity, which was blunted by the first-line treatment for ADHD, methylphenidate. Here, we compared the effects of this psychostimulant (3, 10 and 30 mg/kg, intraperitoneally) on the behaviour of NK1R-/- mice and their wild types in the 5-Choice Continuous Performance Test, which emulates procedures used to study attention and response control in ADHD patients. Methylphenidate increased total trials (a measure of 'productivity') completed by wild types, but not by NK1R-/- mice. Conversely, this drug reduced perseveration by NK1R-/- mice, but not by wild types. Other drug-induced changes in key behaviours were not genotype dependent, especially at the highest dose: for example, % omissions (an index of inattentiveness) was increased, whereas % false alarms and % premature responses (measures of impulsivity) declined in both genotypes, indicating reduced overall response. These findings are discussed in the context of the efficacy of methylphenidate in the treatment of ADHD. Moreover, they lead to several testable proposals. First, methylphenidate does not improve attention in a subgroup of ADHD patients with a functional deficit of TACR1. Second, these patients do not express excessive false alarms when compared with other groups of subjects, but they do express excessive perseveration, which would be ameliorated by methylphenidate.

Psychostimulants, antidepressants and neurokinin-1 receptor antagonists ('motor disinhibitors') have overlapping, but distinct, effects on monoamine transmission: the involvement of L-type Ca2+ channels and implications for the treatment of ADHD

Both psychostimulants and antidepressants target monoamine transporters and, as a consequence, augment monoamine transmission. These two groups of drugs also increase motor activity in preclinical behavioural screens for antidepressants. Substance P-preferring receptor (NK1R) antagonists similarly increase both motor activity in these tests and monoamine transmission in the brain. In this article, the neurochemical and behavioural responses to these three groups of drugs are compared. It becomes evident that NK1R antagonists represent a distinct class of compounds ('motor disinhibitors') that differ substantially from both psychostimulants and antidepressants, especially during states of heightened arousal or stress. Also, all three groups of drugs influence the activation of voltage-gated Ca(v)1.2 and Ca(v)1.3 L-type channels (LTCCs) in the brain, albeit in different ways. This article discusses evidence that points to disruption of these functional interactions between NK1R and LTCCs as a contributing factor in the cognitive and behavioural abnormalities that are prominent features of Attention Deficit Hyperactivity Disorder (ADHD). Arising from this is the interesting possibility that the hyperactivity and impulsivity (as in ADHD) and psychomotor retardation (as in depression) reflect opposite poles of a behavioural continuum. A better understanding of this pharmacological network could help explain why psychostimulants augment motor behaviour during stress (e.g., in preclinical screens for antidepressants) and yet reduce locomotor activity and impulsivity in ADHD. This article is part of the Special Issue entitled 'CNS Stimulants'.

Substance P mRNA expression during zebrafish development: influence of mu opioid receptor and cocaine

Zebrafish has emerged as an important vertebrate animal model for the study of human diseases and for developmental studies in mammals. Since there are few studies of the tachykinin 1 gene (TAC1), precursor of substance P (SP), in relation to embryonic development, we aimed to study the expression of SP transcript (mRNA) and determine the influence of cocaine and opioid receptors on the expression of this neuropeptide. In order to analyse the spatial and temporal SP mRNA expression in zebrafish, we cloned - based on human TAC1 sequence - the sequence that originates SP. Phylogenetic analyses of the precursor of SP, revealed an alignment in the fish cluster, with a clear distinction from other species (amphibians, birds and mammals). Real time PCR (qPCR) results showed that SP mRNA was expressed in several stages of embryonic development, where it increased progressively from gastrula-8hpf (hour post-fertilisation) to the end of the embryogenesis-72hpf. SP mRNA was expressed mainly in the spinal cord in embryos at 20-30hpf, whereas at 36, 42 and 48hpf embryos SP mRNA was expressed mainly in the CNS telencephalon, diencephalon, hypothalamus, rhombomeres, epiphysis and in peripheral areas (heart and somites). Exposure of embryos to 1.5μM cocaine altered the SP mRNA expression at 24 (increasing) and 48hpf (decreasing). We also report that knockdown of μ-opioid receptor induced an increase of SP mRNA expression while the knockdown of the two delta opioid receptors did not produce changes in SP mRNA expression. In conclusion, SP mRNA in zebrafish is expressed during embryonic development in the CNS and peripherally, suggesting that SP would play a critical role during embryogenesis. Furthermore, cocaine exposure and the knockdown of μ-opioid receptor affect the SP mRNA expression. These observations can be important in the pain and addiction field where SP is involved.

Substance P inhibits GABAB receptor signalling in the ventral tegmental area

Substance P (SP) and its receptors are involved in anxiety-related behaviours and regulate the intake of drugs of abuse and alcohol. Within the midbrain ventral tegmental area (VTA), a region that is clearly involved in the control of these behaviours, SP is released by stress and has been shown to trigger relapse. SP activates neurokinin (NK) receptors, which excites midbrain dopamine (DA) neurons and leads to increased DA in target regions. In this study, we have investigated the mechanisms underlying SP actions in the VTA, specifically investigating interactions between SP and GABA(B) receptors. We show that in VTA neurons, NK receptor activation closes an inwardly rectifying potassium channel, and moreover inhibits GABA(B) receptor-mediated transmission through an interaction that depends upon phospholipase C (PLC), intracellular calcium and protein kinase C (PKC).

Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway

Substance P is thought to play an essential role in several forms of supraspinally mediated analgesia. The actions of substance P on synaptic transmission within descending analgesic pathways, however, are largely unknown. Here, we used whole-cell recordings from rat midbrain slices to examine the effects of substance P on GABAergic and glutamatergic transmission within the periaqueductal gray (PAG), a key component of a descending analgesic pathway that projects via the rostral ventromedial medulla (RVM) to the spinal cord dorsal horn. We found that substance P reversibly decreased the amplitude and increased the paired-pulse ratio of evoked IPSCs recorded from identified PAG-RVM projection neurons and from unidentified PAG neurons. Substance P had no effect on miniature IPSCs, implying an indirect mode of action. The effects of substance P were abolished by metabotropic glutamate type 5 and cannabinoid CB1 receptor antagonists, but unaltered by NMDA, GABA(B), mu,delta-opioid, adenosine A(1), and 5HT(1A) receptor antagonists. Consistent with a role for endogenous glutamate in this process, substance P increased the frequency of action potential-dependent spontaneous EPSCs. Moreover, the effect of substance P on evoked IPSCs was mimicked and occluded by a glutamate transport inhibitor. Finally, these effects were dependent on postsynaptic G-protein activation and diacylglycerol lipase activity, suggesting the requirement for retrograde signaling by the endocannabinoid 2-arachidonoylglycerol. Thus, substance P may facilitate descending analgesia in part by enhancing glutamate-mediated excitation and endocannabinoid-mediated disinhibition of PAG-RVM projection neurons.

Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK(1)) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK(1) receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK(1) immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK(1) receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK(1) immunogold distributions. The NK(1) receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK(1) receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK(1) gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK(1) receptors. These results provide ultrastructural evidence that NK(1) receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.

Role of protein kinase C in substance P-induced synaptic depression in the nucleus accumbens in vitro

OBJECTIVES

This study set out to determine the roles of protein kinase A (PKA) and protein kinase C (PKC) signalling cascades in substance P- (SP-) mediated synaptic depression in the nucleus accumbens.

MATERIALS AND METHODS

We used whole-cell patch recording in rat forebrain slices to study the effects of excitatory and inhibitory modulators of PKA and PKC to determine their effects on SP-induced synaptic depression.

RESULTS

We showed that cAMP and PKC, but not PKA, are involved in SP-induced synaptic depression. Bath application of SP (1 microM) depressed evoked excitatory postsynaptic currents (EPSCs) by -27.50 +/- 5.6% (n = 8). Pretreatment of slices with 10 microM forskolin or rolipram prevented SP (1 microM) from depressing evoked EPSCs (-0.8 +/- 6.7%, n = 6; p > 0.05 and 1.6 +/- 5.6%, n = 8; p > 0.05, respectively). Furthermore, 8-bromo cAMP (1 mM) also blocked the effect of SP (-0.5 +/- 14.8, n = 4, p > 0.05). However, H-89 (1 microM) did not block the SP-induced synaptic depression (-32.3 +/- 4.0%, n = 4, p < 0.05). By contrast, PKC inhibitors bisindolylmaleimide (1 microM; 4.0 +/- 5.1%, n = 6; p > 0.05) and calphostin C (400 nM; -6.7 +/- 6.5%, n = 4, p > 0.05) both blocked SP-induced synaptic depression. Phorbol dibutyrate caused a synaptic depression of -33.0. +/- 5.0% and abolished the effect of SP (1 microM, -5.9 +/- 8.6%, n = 4, p > 0.05).

CONCLUSION

Our findings demonstrate that PKC and cAMP are involved in SP-induced synaptic depression while PKA is apparently not involved. Involvement of multiple signalling pathways may reflect the fact that SP uses several intermediates to depress EPSCs.

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.

A possible mechanism for the effect of modifiable lateral inhibition in the striatum on the selection of conditioned reflex motor responses

A mechanism is proposed for the effects of striatal dopamine-modifiable lateral inhibition on the selection of conditioned reflex motor responses. According to this mechanism, activation of dopamine D1 (D2) receptors on strionigral (striopallidal) neurons facilitates long-term depression (potentiation) of the inhibitory inputs simultaneously with potentiation (depression) of the excitatory inputs, of sufficient strength to open NMDA channels. For " weak" excitation, insufficient to open NMDA channels, the modification rules were of the opposite sign. Activation of presynaptic D2 (D1) receptors leads to decreases (increases) in GABA release from strionigral (striopallidal) axon terminals innervating strionigral (striopallidal) cells. As a result, dopamine-modifiable lateral inhibition simultaneously increases both the potentiation (depression) of the excitatory inputs to "strongly" activated strionigral (striopallidal) neurons, increasing (decreasing) their activity, and increases the depression (potentiation) of the excitatory inputs to the "weakly" activated strionigral (striopallidal) neurons, decreasing (increasing) their activity. Subsequent reorganization of neuron activity in the cortex-basal ganglia-thalamus-cortex circuit facilitates selection of conditioned reflex motor responses by further increasing (decreasing) the activity of those motor cortex neurons which were "strongly" ("weakly") excited by the striatum in conditions of dopamine release in response to the conditioned stimulus.

Colocalization of CART with substance P but not enkephalin in the rat nucleus accumbens

CART peptide is a novel neurotransmitter that, due to its distribution in the brain and its modulation of dopamine systems, may be involved in aspects of reward and drug abuse. In the nucleus accumbens (NAcc), CART peptide immunoreactivity (IR) is colocalized with substance P-IR in neurons. Approximately 86% of CART-IR cells colocalize with substance P, while only 19% of substance P-IR neurons contain CART. CART peptide does not colocalize with enkephalin-IR in this region. The substance P-CART colocalization exists in a rostro-caudal gradient with more colocalization in rostral regions. The presence of CART in substance P NAcc neurons suggests that CART neurons may be a subset of the basal ganglia direct pathway or that CART neurons are involved in limbic projections of the NAcc, such as to the ventral pallidum.

Cholecystokinin inhibits evoked inhibitory postsynaptic currents in the rat nucleus accumbens indirectly through gamma-aminobutyric acid and gamma-aminobutyric acid type B receptors

We recently reported that cholecystokinin (CCK) excited nucleus accumbens (NAc) cells and depressed excitatory synaptic transmission indirectly through gamma-aminobutyric acid (GABA), acting on presynaptic GABAB receptors (Kombian et al. [2004] J. Physiol. 555:71-84). The present study tested the hypothesis that CCK modulates inhibitory synaptic transmission in the NAc. Using in vitro forebrain slices containing the NAc and whole-cell patch recording, we examined the effects of CCK on evoked inhibitory postsynaptic currents (IPSCs) recorded at a holding potential of -80 mV throughout CCK-8S caused a reversible inward current accompanied by a concentration-dependent decrease in evoked IPSC amplitude. Maximum IPSC depression was approximately 25% at 10 microM, with an estimated EC50 of 0.1 microM. At 1 microM, CCK-8S induced an inward current of 28.3 +/- 4.8 pA (n=6) accompanied by an IPSC depression of -18.8% +/- 1.6% (n=6). This CCK-induced IPSC depression was blocked by pretreatment with proglumide (100 microM; -3.7% +/- 6.9%; n=4) and by LY225910 (100 nM), a selective CCKB receptor antagonist (4.4% +/- 2.6%; n=4). It was not blocked by SCH23390 (10 microM; -23.5% +/- 1.3%; P < 0.05; n=7) or sulpiride (10 microM; -21.8% +/- 5.1%; P <0.05; n=4), dopamine receptor antagonists. By contrast, it was blocked by CGP55845 (1 microM; -0.4% +/- 3.4%; n=5) a potent GABAB receptor antagonist, and by forskolin (50 microM; 9.9% +/- 5.2%; n=4), an adenylyl cyclase activator, and H-89 (1 microM; 6.9% +/- 3.9%; n=4), a protein kinase A (PKA) inhibitor. These results indicate that CCK acts on CCKB receptors to increase extracellular levels of GABA, which then acts on GABAB receptors to decrease IPSC amplitude.

Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats

The nucleus tractus solitarii (NTS) is essential for coordinating arterial baroreflex control of blood pressure. The primary baroreceptor afferent fibres make their first excitatory synaptic contact at second-order NTS neurones with glutamate as the major neurotransmitter. Glutamate regulates its own release by activating presynaptic metabotropic glutamate autoreceptors (mGluRs) on the baroreceptor central terminals to suppress its further release in frequency-dependent manner. Gamma-aminobutyric acid (GABA) interneurones provide the major inhibitory synaptic input. It is the integration of excitatory and inhibitory inputs that shapes the NTS output of baroreceptor signals. We hypothesized that glutamate released from the primary central afferent terminals can spill over to presynaptic mGluRs on GABA interneurones to suppress GABA release at the second-order baroreceptor neurones. We assessed GABA transmission in second-order baroreceptor neurones identified by attached aortic depressor nerve (ADN) boutons. The medial NTS was stimulated to evoke GABA inhibitory postsynaptic currents (eIPSCs). Glutamate spillover, generated by brief 2 s, 25 Hz trains of stimuli applied to the tractus solitarius (TS), induced a small (10%) but significant reduction in the eIPSC amplitudes. The depression was enhanced to a 25% decrease by increasing glutamate in the cleft with a glutamate-uptake inhibitor (M-trans-pyrrolidine-2,4-dicarboxylic acid, 1 mum), blocked by a Group II mGluR antagonist (LY341495, 200 nm) and mimicked by a Group II agonist ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl; L-CCG-I). A presynaptic mGluR locus was established by the mGluR agonist-mediated increase in the paired-pulse ratio of two consecutive eIPSCs in conjunction with the decrease in the first eIPSC, and a decrease in the frequency (39-46% reduction at EC(50) concentration), but not amplitude, of spontaneous and miniature GABA IPSCs. The data indicate that endogenous glutamate activation of Group II presynaptic mGluRs can decrease GABA release at the first central synapses, suggesting a heterosynaptic role for the Group II mGluRs in shaping baroreceptor signal transmission.

NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Substance P (SP) is a well-established pain messenger in the spinal cord, although its role in substantia gelatinosa (lamina II) still remains elusive. We carried out patch-clamp recordings on lamina II neurons from transverse mouse spinal cord slices (P8-12), using the selective NK1 receptor agonist [Sar9,Met(O2)11]-SP (SM-SP, 3-5 microM) in the presence of NBQX. Activation of NK1 receptors was confirmed after pre-incubation with selective NK1 antagonist L732,138 (4 microM) that consistently blocked the effects of SM-SP (nine neurons). After SM-SP challenge and spontaneous inhibitory post-synaptic current (sIPSC) analysis, 50% of recorded neurons (15 out of 30) were found to display a transient increase in frequency; in five neurons this was also associated with increase of peak amplitude. Five out of eight neurons displayed pure GABAA microM) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After miniature IPSC analysis, a significant increase in frequency was observed in three out of 14 SM-SP responsive neurons. At least four different morphological types were apparent among NK1-responsive neurons after filling with Lucifer Yellow/biocytin: fusiform with dorso-ventral dendritic arbors (i); round-to-oval with dendritic arborization mainly directed to lamina I (ii) or III (iii), and round-to-oval with dendrites sparsely distributed all around the cell body (iv). Thus, there was no correlation between morphology and electrophysiological properties of responsive neurons. Our observations provide new insights on the processing of sensory neurotransmission in spinal cord, and indicate that activation of NK1 receptors is involved in the maintenance of the inhibitory tone of substantia gelatinosa interneurons.

Cholecystokinin activates CCKB receptors to excite cells and depress EPSCs in the rat rostral nucleus accumbens in vitro

The peptide cholecystokinin (CCK) is abundant in the rat nucleus accumbens (NAc). Although it is colocalized with dopamine (DA) in afferent terminals in this region, neurochemical and behavioural reports are equally divided as to whether CCK enhances or diminishes DA's actions in this nucleus. To better understand the role of this peptide in the physiology of the NAc, we examined the effects of CCK on excitatory synaptic transmission and tested whether these are dependent on DA and/or other neuromodulators. Using whole-cell recording in rat forebrain slices containing the NAc, we show that sulphated CCK octapeptide (CCK-8S), the endogenously active neuropeptide, consistently depolarized cells and depressed evoked excitatory postsynaptic currents (EPSCs) in the rostral NAc. It caused a reversible, dose-dependent decrease in evoked EPSC amplitude that was accompanied by an increase in the decay constant of the EPSC but with no apparent change in paired pulse ratio. It was mimicked by unsulphated CCK-8 (CCK-8US), a CCK(B) receptor-selective agonist, and blocked by LY225910, a CCK(B) receptor-selective antagonist. Both CCK-8S and CCK-8US induced an inward current with a reversal potential around -90 mV that was accompanied by an increase in input resistance and action potential firing. The CCK-8S-induced EPSC depression was slightly reduced in the presence of SCH23390 but not in the presence of sulpiride or 8-cyclopentyltheophylline. By contrast, it was completely blocked by CGP55845, a potent GABA(B) receptor-selective antagonist. These results indicate that CCK excites NAc cells directly while depressing evoked EPSCs indirectly, mainly through the release of GABA.