Muscarinic receptors on dopamine terminals in the cat caudate nucleus: Neuromodulation of [H]dopamine release in vitro by endogenous acetylcholine

The directly acting muscarinic receptor agonist oxotremorine (1.8–10 μM) produced an increase in electrically evoked [³H]dopamine release from slices of the cat caudate. The maximal increase caused by oxotremorine was 40%, and was antagonized by the muscarinic receptor blocking agent atropine (0.1 μM). Exposure to the acetylcholinesterase (AChE) inhibitor physostigmine (1 μM) resulted in a 50% increase in electrically evoked [³H]dopamine release. The increase caused by physostigmine was also antagonized by atropine (0.1 μM).Atropine did not, however, alter the modulations in [³H]dopamine release mediated by the dopamine autoreceptor: the increase in electrically evoked [³H]dopamine release caused by the dopamine receptor antagonist S-sulpiride (0.1 μM) and the decrease caused by the dopamine receptor agonist pergolide (30 nM) were unaffected by atropine (0.1 μM). These results indicate that the muscarinic receptor-mediated and dopamine autoreceptor-mediated presynaptic effects on [³H]dopamine release are independent.The present results suggest that in the electrically depolarized caudate slice in vitro, released endogenous acetylcholine may interact with muscarinic receptors faciliting depolarization-evoked [³H]dopamine release,ifAChE is inhibited. These muscarinic receptors may be located on dopamine nerve terminals. In the context of present neuroanatomical knowledge, the action of released endogenous acetylcholine on dopamine terminals may be a non-synaptic neuromodulation.     

Rapid desensitization of presynaptic dopamine autoreceptors during exposure to exogenous dopamine

When nomifensine is employed to inhibit neuronal uptake, exposure to dopamine (DA) (0.1–0.3 μM) or apomorphine (0.01–0.1 μM) inhibited, in a concentration-dependent manner, the electrically evoked release of [³H]dopamine from slices of the rabbit caudate nucleus. Apomorphine inhibited transmitter release independently of the time of exposure to the drug (6–32 min). On the other hand, the inhibitory effect of exogenous dopamine occurred only if a short period (4–12 min) of exposure was employed. In studies on the electrically evoked release of [³H]acetylcholine in slices of the rabbit caudate nucleus there was no evidence for desensitization to apomorphine or exogenous dopamine at the level of the dopamine receptors that inhibit [³H]acetylcholine release. These results indicate that the dopamine autoreceptors modulating [³H]dopamine release in the caudate nucleus become subsensitive after a few minutes of exposure to exogenous dopamine. This effect does not occur at the level of the dopamine receptors which inhibit the release of [³H]acetylcholine.     

Neurotransmitter synthesis, storage and release by aggregating cell cultures of rat brain

Rotation-mediated aggregating cell cultures of mechanically dissociated fetal (15–16 days gestation) rat brains between 25 and 35 days in vitro were examined for their ability to synthesize neurotransmitters and putative neurotransmitters from radioactively labeled precursors added to the culture medium. Cultures derived from whole brain synthesized [³H]acetylcholine from [³H]choline, [³H]γ-aminobutyric acid from l-[³H]glutamic acid, [³H]dopamine from l-[³H]tyrosine, [³H]dopamine and [³H]norepinephrine from l-[³H]dihydroxyphenylalanine, and [³H]serotonin from l-[³H]tryptophan. Veratridine increased and tetrodotoxin decreased the rate of [³H]-dopamine synthesized by aggregates derived from midbrain plus hindbrain. In chase experiments in which aggregates were incubated for 4 h with radioactively labeled precursors and then for 4 h with non-radioactively labeled precursors, addition of veratridine (50 μM) during the second 4 h incubation significantly decreased the amounts of radioactively labeled acetylcholine, l-glutamic acid, dopamine and serotonin recovered from aggregates. Tetrodotoxin (5 μM) present during the chase significantly increased the amounts of [³H]acetylcholine and [³H]dopamine recovered from the aggregates. In addition, reserpine (4 μM) markedly depleted [³H]dopamine from aggregates in these experiments. These results indicate that these cultured cells synthesized neurotransmitters and in addition suggest that some of these compounds are stored by and released from electrically active cells within the aggregates.     

Presynaptic effect of 7-OH-DPAT on evoked [H]-acetylcholine release in rat striatal synaptosomes

The objective of the present experiments was to study the presynaptic effect of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT, a D₂-like dopamine receptor agonist) on [³H]-acetylcholine ([³H]-ACh) release induced by potassium (15 mM, 25 mM and 60 mM), potassium channel-blockers (4-aminopyridine, 4-AP; tetraethylammonium, TEA and quinine) and veratridine to gain insight into the mechanisms involved in the activation of the D₂ dopamine-receptor subtype located at striatal cholinergic nerve terminals. 7-OH-DPAT (1 μM) inhibited the evoked [³H]-ACh release induced by K⁺ 15 mM in a similar percentage than that obtained during basal conditions (30% and 27%, respectively). Nevertheless, in the presence of 25 mM and 60 mM of K⁺ the inhibitory effect of 7-OH-DPAT was completely abolished. 4-AP (1–100 μM) and TEA (1 and 5 mM) significantly enhanced [³H]-ACh release, showing 69.32%±7.60% (P<0.001) and 52.27%±5.64% (P<0.001), respectively, at the highest concentrations tested. In these conditions, 7-OH-DPAT (1 μM) inhibited the release induced by potassium channel-blockers 25–27%. Quinine (0.1–1 μM) did not alter [³H]-ACh release either in the presence or absence of 7-OH-DPAT. Veratridine 10 μM evoked [³H]-ACh release in the presence of a low-calcium medium, but in such conditions 7-OH-DPAT (1 μM) did not modify the neurotransmitter release in the absence or presence of veratridine. Present data indicate that activation of the presynaptic D₂ dopamine receptor inhibits the [³H]-ACh release by increasing K⁺ conductance, as high K⁺ concentrations abolished the inhibitory control of 7-OH-DPAT on [³H]-ACh release. This effect could be mediated by potassium channels different from those sensitive to 4-AP, TEA and quinine. In addition, the presynaptic D₂ dopamine-receptor activation seems to not involve changes in intracellular Ca²⁺.     

NMDA regulation of dopamine release from proximal and distal dendrites in the cat substantia nigra

The NMDA regulation of the dendritic release of [³H]dopamine ([³H]DA) synthesized from [³H]tyrosine was investigated in vitro using a microsuperfusion procedure in the pars compacta (SNC) and the pars reticulata (SNR) of the cat substantia nigra. The spontaneous release of [³H]DA was threefold higher in the SNC than in the SNR and amphetamine (1 μM) enhanced similarly [³H]DA release in both nigral areas. In the absence of magnesium, NMDA (50 μM) stimulated markedly the release of [³H]DA in the SNC and SNR, these effects being completely prevented by MK 801 (1 μM), the NMDA receptor antagonist. The DA uptake inhibitor, nomifensine (5 μM), increased the amount of [³H]DA recovered in SNC (×2) and SNR (×3) superfusates but did not significantly modify the NMDA-evoked responses. The effects of NMDA seen in the absence or presence of nomifensine persisted when the two nigral areas were continuously superfused with tetrodotoxin (1 μM). These results are in favor of the presence of NMDA receptors on dopaminergic dendritic arborizations and indicate that the stimulation of these receptors facilitates in a similar way the release of DA from proximal and distal dendrites.     

Somatostatin and cholecystokinin octapeptide differentially modulate the release of [H]acetylcholine from caudate nucleus but not cerebral cortex: Role of dopamine receptor activation

The effects of somatostatin (SOM) and cholecystokinin octapeptide (CCK-8) on basal and potassium-induced release of acetylcholine (ACh) were investigated in slices of rat caudate nucleus (CN) and, for comparison, cerebral cortex (CX). Potassium (5–55 mM) produced a concentration-dependent increase in the release of [³H]ACh in the presence of extracellular Ca²⁺. SOM (1 μ), CCK-8 (1 μM) and the dopamine (DA) receptor agonist, apomorphine (APO, 30 μM) inhibited the K⁺-induced (35 mM) release of [³H]ACh by 26–32% from CN, but did not affect ACh release from CX. Other peptides (1 μM), such as Met-enkephalin, vasoactive intestinal peptide, thyrotropin-releasing hormone and substance P, had no effect on release of [³H]ACh in CN or CX. Sulpiride (SULP), a dopamine receptor antagonist, prevented the effects of APO and SOM, but not CCK-8, to inhibit [³H]ACh release. The results indicate that: (1) SOM and CCK-8 inhibit the release of [³H]ACh in CN, but not CX; and (2) the inhibitory effect of SOM, but not CCK-8, on [³H]ACh release is mediated by dopaminergic mechanisms.     

Release of [C]adenosine derivatives from superfused synaptosome preparations. Effects of depolarizing agents and metabolic inhibitors

Adenine 5′-mononucleotides of synaptosome beds from guinea pig neocortex were labelled by incubation with [8-¹⁴C]adenosine, and excess adenosine was then removed with precursor-free medium. During continuous superfusion, labelled adenosine derivatives were released at a rate of about 0.5% of the synaptosome¹⁴C/min and this rate was increased 2.5-fold by depolarization with 33 mM K⁺. Some depolarizing agents and metabolic inhibitors were examined for action on the release of [¹⁴C]-adenosine derivatives from the synaptosome preparations, and also on the rate of lactate production by these preparations, both before and during K⁺ depolarization. The synaptosome content of adenine 5′-mononucleotides following exposure of the synaptosome beds to these compounds was also estimated.Ouabain, 0.1 mM, brought about an increase in¹⁴C-labelled compounds output, but it caused little alteration in lactate output and some decrease in the adenylate energy charge of the synaptosomes. Veratridine, 80 μM, increased markedly both the output of radioactivity and the lactate production. Tetrodotoxin, 1 μM, when added together with veratridine, completely abolished the effect of veratridine on the efflux of [¹⁴C]adenosine derivatives, but this was not associated with a complete blockade of the output of lactate.Sodium cyanide, 2.5 mM, FCCP, 6 μM together with iodoacetate, 2.5 mM, caused an increase in the output of¹⁴C-labelled compounds, and a decrease in the adenylate energy charge. The production of lactate was also increased by sodium cyanide and FCCP, but it was completely inhibited by iodoactate. Oligomycin, 4.65 μM, and amytal, 1 mM, added in the incubation medium before labelling the synaptosome beds with [8-¹⁴C]adenosine, did not affect very much the output of [¹⁴C]adenosine derivatives, while the output of lactate increased independently of the depolarization with 33 mM K⁺.The release of adenosine derivatives from superfused synaptosome beds induced by depolarizing agents or metabolic inhibitors did not seem to be due either to an effect of membrane permeability changes that follow a decrease of ATP supply, or to an increased metabolic rate occurring during nerve ending stimulation. It is concluded that this release of adenosine derivatives appeared to be a process triggered primarily by the influx of Na⁺ and the increased intrasynaptosomal calcium, and closely related to the process of neurotransmission.     

On the role of calcium ions in the presynaptic alpha-receptor mediated inhibition of [H]noradrenaline release from rat brain cortex synaptosomes

Rat brain cortex synaptosomes, previously labeled by incubation with [³H]noradrenaline ([³H]NA) were continuously superfused with Krebs-Ringer media. Release of [³H]NA was induced by superfusion with medium containing either 15 mM K⁺, 20 μM veratrine or 1 μM of the calcium-ionophore A 23187 and was strongly dependent on the concentration of Ca²⁺ in the medium. Noradrenaline (1μM, in the presence of the uptake inhibitor desipramine) inhibited K⁺-induced [³H]NA release by activation of presynaptic alpha-receptors. When the Ca²⁺-concentration in the medium was reduced, or the Mg²⁺-concentration increased, [³H]NA release appeared to be more susceptible to alpha-receptor mediated inhibition.Noradrenaline (1 μM) inhibited [³H]NA release induced by 15 mM K⁺, in the presence of 0.075 Ca²⁺ and 10 mM Mg²⁺, by 86%. Veratrine-induced release was also inhibited by alpha-receptor activation. However, [³H]NA release induced by the calcium-ionophore was not affected by alpha-receptor agonists. These results strongly support the view that alpha-receptor activation results in a decrease of the availability of Ca²⁺ for stimulus-secretion coupling processes. Presumably this is effected by an inhibition of voltage-sensitive calcium channels in the neuronal membrane associated with neurotransmitter release.     

Differential effects of dopamine agonists on evoked dopamine release from slices of striatum and nucleus accumbens in rats

The effects of dopamine receptor agonists on electrically evoked dopamine release from slices of nucleus accumbens were compared with the effects on release from striatal slices in rats. Apomorphine, which has equal potency at the dopamine D₂ and D₃ receptors, reduced the evoked dopamine release from both regions to the same extent (ED₅₀, 0.42 μM for nucleus accumbens; ED₅₀, 0.46 μM for striatum). Quinpirole of 7-[³H]hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT), which are much more potent at the D₃ receptor than at the D₂ receptor, reduced the evoked dopamine release from the nucleus accumbens (ED₅₀, 0.12 μM for quinpirole; 0.02 μM for 7-OHDPAT) much more than the release from the striatum (ED₅₀, 1.6 μM for quinpirole; 0.55 μM for 7-OHDPAT). These results suggest that the contribution of D₃ receptors in nucleus accumbens to regulate dopamine release from dopamine nerve terminals is much greater than that in striatum.     

Failure of carbachol to influence the release of somatostatin from slices of rat cerebral cortex

A[K⁺]-related, Ca²⁺-dependent efflux of immunoreactive somatostatin (IRS) from superfused slices of rat cerebral cortex has been observed; this release paralleled the release of both [¹⁴C]noradrenaline and [¹⁴C]GABA. However IRS release in this preparation was not stimulated by the muscarinic agonist carbachol at low (10 μM) or high (500 μM) concentrations. Furthermore, 100 or 500 μM carbachol did not affect the IRS efflux from rat cortex slices incubated in the presence of 12, 25 or 53 mM K⁺.     

GABA and benzodiazepine-induced modification of [C]l-glutamic acid release from rat spinal cord slices

The spontaneous and potassium-evoked release of [¹⁴C]-label from rat spinal cord slices preloaded with [¹⁴C]l-glutamic acid and its modification by GABA and related drugs, such as flurazepam, was studied as a possible indirect measure of presynaptic inhibition and of the ability of benzodiazepines to augment it. GABA (100 μM) reduced the spontaneous release of [¹⁴C]-label (glutamate) provided that GABA metabolism was blocked by amino-oxyacetic acid (AOAA), but failed to reduce the potassium-evoked release of glutamate, although muscimol (10 μM) had some effect. In contrast, flurazepam (1–100 μM) did not affect spontaneous release but produced some inhibition of the evoked release (through a system insensitive to 10 μM bicuculline). This inhibition became more marked in the presence of both GABA and AOAA, and was then overcome by bicuculline. It is concluded that either some benzodiazeophine receptors must be occupied for GABA to produce an effect on evoked release and/or, that the benzodiazepines can only augment GABA function once a certain amount has been released. Studies of the rapid distribution of [¹⁴C]-label from glutamate, to GABA, glutamine and other amino acids, using high voltage electrophoresis, showed the importance of blocking metabolic pathways in studies of this kind.     

Modulation of noradrenaline release from hippocampal slices by hexachlorocyclohexane isomers. Effects of GABAergic compounds

The effects of hexachlorocyclohexane (HCH) isomers and some GABAergic compounds on [³H]noradrenaline (NA) release from rat hippocampal slices prelabelled with 80 nM [³H]NA were determined. The convulsant γ-HCH isomer facilitated (EC₅₀ = 21 μM) and the depressant δ-HCH isomer reduced (EC₅₀ = 48 μM) the Ca²⁺-dependent K⁺-evoked release of [³H]NA, whereas α- and β-HCH isomers did not show any effect. Moreover, α- and δ-HCH isomers antagonized the facilitation of evoked [³H]NA release induced by the γ-HCH isomer. The GABAergic convulsant drugs, bicuculline, picrotoxin and pentylenetetrazol, did not cause any modification of the evoked [³H]NA release even at high concentrations. Neither bicuculline nor picrotoxin blocked the effects of HCH isomers on K⁺-evoked release of [³H]NA. Exposure of slices to diazepam reduced the K⁺-evoked release of [³H]NA (EC₅₀ = 33 μM) in a manner similar to that of the δ-HCH isomer. In addition, diazepam (50 μM) blocked the γ-HCH effect and caused an additive inhibitory response with the δ-HCH isomer. On the other hand, diazepam and δ-HCH induced a time-dependent Ca²⁺-independent enhancement of basal [³H]NA release. The results suggest that modulation of [³H]NA release in the hippocampus by HCH isomers may be involved in the central actions of these compounds, and that sites other than the classic GABA_A receptor may underlie their presynaptic mechanisms of action.     

Effect of chronic desipramine treatment on adrenoceptor modulation of [H]dopamine release from rat nucleus accumbens slices

The effects of α₂- and β-adrenoceptor agonists on the 25 mM K⁺-induced release of [³H]dopamine ([³H]DA from nucleus accumbens slices of chronic desipramine (DMI)- and saline-treated rats were investigated using a superfusion technique. The K⁺-induced release of [³H]DA from nucleus accumbens slices was shown to be Ca²⁺-dependent by ascorbic acid. In experiments with isoproterenol, ascorbic acid was added to the superfusion media in order to prevent the otherwise rapid oxidation of the drug. The K⁺-induced release of [³H]DA from nucleus accumbens slices of saline-treated rats was significantly decreased by the α₂-adrenoceptor agonist, clonidine(10 μM; 89 ± 2.4%of control values; P < 0.002), and significantly enhanced by the β-adrenoceptor agonist, isoproterenol(1 and 10 μM; 122 ± 4.3and171 ± 2.9%of control values, P < 0.002andP < 0.001, respectively). The basal release of [³H]DA was strongly enhanced by 10 μM but not 1 μM isoproterenol. Chronic DMI pretreatment (10 mg/kg i.p. for 28 days) did not significantly alter the K⁺-induced release of [³H]DA. Chronic DMI treatment attenuated the α-adrenoceptor-mediated inhibition of [³H]DA release, while the β-adrenoceptor-mediated stimulation remained unchanged. The net effect of chronic DMI treatment therefore would appear to be a facilitation of dopaminergic neurotransmission in the mesolimbic system. This is consistent with behavioural evidence which suggests that the function of the mesolimbic dopaminergic reward system is facilitated by chronic treatment with antidepressant drugs.     

Bidirectional movement of γ-aminobutyric acid in cerebral cortex slices incubated in small volumes of medium

Rat cerebral cortex slices were incubated in small volumes of medium in order to study the bidirectional movement of γ-aminobutyric acid (GABA) between tissue and medium, and to measure the extent of release of endogenous amino acids of interest as potential neurotransmitters. The apparent uptake, real uptake, and net change of GABA were calculated from the changes in [¹⁴C]GABA and endogenous GABA in the medium.The extent of release of endogenous amino acids was dependent upon the amino acid, the duration of incubation, and the volume of incubating medium. Substantial amounts of endogenous amino acids may accumulate in the medium under conditions commonly employed in amino acid uptake studies. The accumulation of endogenous GABA may affect the calculation of GABA uptake when this is based on the initial specific activity of exogenous [¹⁴C]GABA. The difference between apparent uptake and real uptake was particularly prominent in the presence of the depolarizing agents, potassium or veratridine, which caused a marked increase in GABA release. Exchange or net release of GABA was found when slices were incubated briefly in media containing 1.3 μM or 12.5 μM exogenous GABA; net uptake accounted for 50% of observed uptake when slices were incubated in media containing 25 μM GABA.     

Effect of 6R- -erythro-5,6,7,8-tetrahydrobiopterin and infusion of -tyrosine on the in vivo l-[β-C]DOPA disposition in the monkey brain

The effect of 6R- -erythro-5,6,7,8-tetrahydrobiopterin (6R-BH₄) and -tyrosine infusion on [¹¹C]dopamine synthesis was analyzed in the striatum of Rhesus monkey using positron emission tomography (PET). The rate for decarboxylation from -[β-¹¹C]DOPA to [¹¹C]dopamine was calculated using a graphical method with cerebellum as a reference region. Although the peripheral administration of 6R-BH₄ at low dose (2 mg/kg) did not provide a significant increase in the rate of dopamine biosynthesis, a high dose of 6R-BH₄ (20 mg/kg) induced an elevation of the rate. This 6R-BH₄-induced elevation of the dopamine synthesis rate was further dose-dependently enhanced by the continuous infusion of -tyrosine (0.2 and 1.0 μmol/min/kg). -Tyrosine infusion with a rate of 1.0 μmol/min/kg caused an enhancement of the rate even during low dose administration of 6R-BH₄ (2 mg/kg). -Tyrosine infusion alone did not induce any elevation of the dopamine biosynthesis rate. The analysis of plasma indicated that the metabolic ratios of -[β-¹¹C]DOPA to each metabolite were not affected by 6R-BH₄ and/or -tyrosine infusion. The results suggest that the low dose loading of tyrosine facilitates the activity of 6R-BH₄ on the presynaptic dopamine biosynthesis, and also that the combined effects can be monitored by PET using -[β-¹¹C]DOPA as a biochemical probe.     

β-adrenoceptor stimulation-induced increase in the number of α2-adrenoceptors of cerebral cortical membranes in rats

Effects of pre-treatment of synaptic membranes with β-adrenoceptor agonists and cholera toxin on [³H]clonidine and [³H]yohimbine binding were examined in rat cerebral cortex. Pre-incubation of cerebral cortical membranes with isoproterenol (10 or 200 μM) or dobutamine (1, 10 or 200 μM) at 37 °C for 40 min caused a significant elevation of specific [³H]clonidine binding but treatment with salbutamol (10 or 200 μM) did not. Scatchard analysis showed that 200 μM isoproterenol treatment resulted in a significant elevation of high affinity component of [³H]clonidine binding which was significantly decreased by the addition of 10μM GTP. A significant elevation in high affinity [³H]clonidine binding was observed by treatment with 100 μg/ml cholera toxin, while a significant decrease in low affinity one was by the treatment. Specific [³H]yohimbine binding was also elevated by 10 or 200 μM isoproterenol treatment. It is suggested that stimulation of β-receptors, presumably β₁-subtype could elevate the number of agonist and antagonist binding sites in α₂-receptors in synaptic membranes by partially mediated by stimulatory and/or inhibitory GTP binding regulatory proteins.     

Downregulation of muscarinic- and 5-HT1B-mediated modulation of [H]acetylcholine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts of septal origin

Adult Long-Evans female rats sustained electrolytic fimbria-fornix lesions and, two weeks later, received intrahippocampal suspension grafts of fetal septal tissue. Sham-operated and lesion-only rats served as controls. Between 6.5 and 8 months after grafting, both the [³H]choline accumulation and the electrically evoked [³H]acetylcholine ([³H]ACh) release were assessed in hippocampal slices. The release of [³H]ACh was measured in presence of atropine (muscarinic antagonist, 1 μM), physostigmine (acetylcholinesterase inhibitor, 0.1 μM), oxotremorine (muscarinic agonist, 0.01 μM–10 μM), mecamylamine (nicotinic antagonist, 10 μM), methiothepin (mixed 5-HT₁/5-HT₂ antagonist, 10 μM), 8-OH-DPAT (5-HT_1A agonist, 1 μM), 2-methyl-serotonin (5-HT₃ agonist, 1 μM) and CP 93129 (5-HT_1B agonist, 0.1 μM–100 μM), or without any drug application as a control. In lesion-only rats, the specific accumulation of [³H]choline was reduced to 46% of normal and the release of [³H]ACh to 32% (nCi) and 43% (% of tissue tritium content). In the grafted rats, these parameters were significantly increased to 63%, 98% and 116% of control, respectively. Physostigmine reduced the evoked [³H]ACh release and was significantly more effective in grafted (−70%) than in sham-operated (−56%) or lesion-only (−54%) rats. When physostigmine was superfused throughout, mecamylamine had no effect. Conversely, atropine induced a significant increase of [³H]ACh release in all groups, but this increase was significantly larger in sham-operated rats (+209%) than in the other groups (lesioned: +80%; grafted: +117%). Oxotremorine dose-dependently decreased the ([³H]ACh) release, but in lesion-only rats, this effect was significantly lower than in sham-operated rats. Whatever group was considered, 8-OH-DPAT, methiothepin and 2-methyl-serotonin failed to induce any significant effect on [³H]ACh release. In contrast, CP 93129 dose-dependently decreased [³H]ACh release. This effect was significantly weaker in grafted rats than in the rats of the two other groups. Our data confirm that cholinergic terminals in the intact hippocampus possess inhibitory muscarinic autoreceptors and serotonin heteroreceptors of the 5-HT_1B subtype. They also show that both types of receptors are still operative in the cholinergic terminals which survived the lesions and in the grafted cholinergic neurons. However, the muscarinic receptors in both lesioned and grafted rats, as well as the 5-HT_1B receptors in grafted rats show a sensitivity which seems to be downregulated in comparison to that found in sham-operated rats. In the grafted rats, both types of downregulations might contribute to (or reflect) an increased cholinergic function that results from a reduction of the inhibitory tonus which ACh and serotonin exert at the level of the cholinergic terminal.     

Biochemical characterization of the intramembrane interaction between neurotensin and dopamine D2 receptors in the rat brain

In order to better understand the neuroleptic-like effects of neurotensin in vivo, the effects of neurotensin in vitro on dopamine D₂ and D₁ agonist and antagonist binding sites were characterized in membranes from the neostriatum and the subcortical limbic area. Neurotensin increased theK_D bu not theB_max value ofS(−)[N-propyl-³H(N)]propylnorapomorphine ([³H]NPA) binding sites with a maximal increase of 20–40% at 3–10 nM of neurotensin in both areas. TheK_D increase was preferentially due to an increase in the dissociation rate. The maximal reduction of [³H]NPA binding (35%) was obtained within 5 min from the addition of neurotensin. Neurotensin increased theK_H of dopamine vs [³H]raclopride binding and, in the presence of GTP, alsoK_L. Neurotensin did not affect the percentage of binding sites in the high vs low affinity states or the binding characteristics of [³H]spiperone, [³H]SKF 38393, and [³H]SCH 23390. Serotonin (10 nM), neuropeptide Y (10 nM), Substance P (10 nM), dynorphin A (10 nM), morphine (10 nM), nicotine (100 nM),γ-amino-n-butyric acid (1 μM), orN-methyl-d-aspartate (1 μM) did not affect [³H]NPA binding. These results indicate that neurotensin in vitro selectively reduces D₂ agonist affinity by an enhancement of the dissociation rate. This antagonistic intramembrane interaction may underlie the neuroleptic-like effects of neurotensin at low concentrations in vivo on D₂ agonist binding, dopamine release, and on D₂-mediated behaviours.     

Enhanced stimulus-secretion coupling from brains of aged mice

Fractional calcium-dependent release of accumulated [³H]dopamine and γ-[¹⁴C]aminobutyric acid (GABA), but not [³H]norepinephrine or [¹⁴C]choline-acteylcholine, from isolated mouse forebrain synaptosomes was enhanced in 12- as compared to 2-month-old mice. In the case of [³H]-dopamine, differences in the storage of accumulated dopamine may have been responsible for these differences. No differences were observed in GABA accumulation or storage. The enhanced release of accumulated GABA in synaptosomes from 12- vs. 2-month-old mice is discussed in terms of age-related changes in secretion processes.     

Dopamine released from dendrites in the substantia nigra controls the nigral and striatal release of serotonin

Using push-pull cannulae, the release of endogenously synthesized [³H]serotonin was estimated in both substantia nigra and caudate nuclei of ‘encéphale isolé’ cats. The unilateral nigral application of dopamine (10⁻⁷ M) reduced [³H]serotonin release in ipsilateral structures whereas α-methylparatyrosine (10⁻⁴ M) induced opposite effects. Both treatments decreased [³H]serotonin release in the contralateral caudate nucleus but not in the contralateral substantia nigra. As a working hypothesis it is suggested that the effects of observed are related to changes in the activity of nigroraphe neurons regulated by dopamine released from dendrites of the nigrostriatal dopaminergic neurons. However it cannot vet be excluded that the local changes in [³H]serotonin release induced by the nigral application of dopamine or α-methylparatyrosine result from presynaptic modulation.