Presynaptic muscarinic receptor mechanisms and submandibular responses to stimulation of the parasympathetic innervation in bursts in rats

Submandibular secretory responses to electrical stimulation of the parasympathetic innervation at variable frequencies were measured in anaesthetized rats. Selective blockade by pirenzepine and by methoctramine occurred at doses (50 and of 300 nmol kg (-1), i.v., respectively) that did not inhibit the responses to exogenous acetylcholine. In the presence of methoctramine, the nerve-evoked fluid responses were increased by 100% at 1 Hz independently of the total number of impulses (10-300), suggesting that M2 receptor activation inhibits transmitter release. The magnitude of the increase was inversely related to frequency of stimulation. The protein concentrations in the fluid responses were not significantly affected by methoctramine. Pirenzepine had an inhibitory effect on the fluid secretory responses, which was dependent of frequency, as well as of number of impulses, suggesting that M1 receptor activation facilitates transmitter release. At 10 Hz given intermittently (for 1 s at 10-s intervals), pirenzepine reduced the fluid response by 25%. The protein release was substantially and significantly reduced by pirenzepine independent of frequency but only during long periods of stimulation (300 impulses). It is concluded that muscarinic M1 receptor activation normally has a facilitatory effect on transmitter release, and that the facilitation occurs during short, intense stimulation. Muscarinic M1 receptors are, however, likely to regulate protein secretion by other mechanisms. Muscarinic M2 receptors, on the other hand, normally inhibit cholinergic transmission at low frequencies. Similar to findings in the alimentary tract of several species, stimulation in bursts at high frequencies is a more efficient stimulation pattern than continuous low frequency stimulation. This pattern of stimulation thus takes advantage of transient facilitation and avoids the inhibition at less intense neuronal activity.     

Long-term adaptation of crayfish neurons depends on the frequency and number of impulses

Increasing the impulse activity of crustacean neurons for a few days causes long-lasting changes in transmitter release, which are termed 'long-term adaptation' (LTA) in previous studies. Both the amount of transmitter released at the beginning of a stimulus train, and synaptic fatigue during repetitive stimulation, are reduced. The present study examines the dependence of these synaptic changes on the frequency and number of impulses used to elicit LTA. Fatigue resistance develops consistently when crayfish phasic motor neurons are stimulated for 3 days with as few as 9,000 impulses per day, and occurs in response either to low frequency stimulation (0.2 or 0.5 Hz), or to stimulation in short bursts at a moderate average frequency (2.5 Hz). In contrast, the reduction in initial transmitter release does not appear consistently when the frequency and number of impulses are both low (9,000 impulses per day delivered at 0.2 Hz), but does occur at the moderate stimulus frequency (2.5 Hz) and when a larger number of impulses (18,000) are delivered at a low frequency (0.5 Hz). The data suggest that the two changes in synaptic transmission that comprise LTA have different stimulus requirements.     

Effects of dopamine depletion on striatal neurotensin: biochemical and immunohistochemical studies

Interactions between striatal dopamine (DA) and neurotensin (NT) have been suggested by anatomical, behavioral, and biochemical studies. Nigrostriatal DA neurons, in contrast to mesocorticolimbic DA neurons, do not appear to contain NT. Thus, distinct neuronal elements subserve interactions between DA and NT within the striatum. We have previously demonstrated that reserpine-induced depletion of striatal DA is accompanied by a dose- and time-dependent increase in striatal NT concentrations. In order to further characterize the effects of reserpine and to define the mechanism by which reserpine acts to increase striatal NT concentrations, we have used immunohistochemical and biochemical approaches. Immunohistochemical examination of rats pretreated with reserpine revealed marked increases in the density of NT-like immunoreactive (NT-li) perikarya and fibers, and the development of NT-li patches. Pretreatment with reserpine had no apparent effect on NT synthesis, as assessed by examination of cycloheximide-induced inhibition of protein synthesis. However, reserpine administration resulted in a significant decrease in the release of both DA and NT into the striatal extracellular fluid, as measured by in vivo microdialysis. These data suggest that the increase in striatal NT concentrations observed after reserpine treatment results from decreased release, rather than increased synthesis of the peptide.     

Local infusion of the serotonin antagonists ritanserin or ICS 205,930 increases in vivo dopamine release in the rat medial prefrontal cortex

Previous research has indicated that atypical antipsychotic drugs like clozapine preferentially increase dopamine (DA) release from the mesocortical, relative to the nigrostriatal, system. While these drugs generally have weak affinity for the D2 receptor subtype, they are potent antagonists of the 5-hydroxytryptamine₂ (5-HT2) receptor. Research into neurotransmitter interactions indicates that 5-HT modulates DA release, but the nature of this interaction may depend upon the specific 5-HT receptor subtype and the neuronal location of that subtype. The present research tested the hypothesis that 5-HT2 receptors localized near mesocortical DA nerve terminals regulate DA release. This was accomplished by infusing the specific 5-HT2 antagonist ritanserin directly into the medial prefrontal cortex through reverse dialysis in vivo in the rat. Cortical extracellular fluid was then extracted by microdialysis and DA was subsequently assayed by HPLC with electrochemical detection. These results were compared to the systemic administration of ritanserin (1.0–5.0 mg/kg i.p.) and the local application of ICS 205,930, an antagonist at the 5-HT3/4 receptor subtypes. Both 5-HT antagonists increased cortical DA levels when infused locally at concentrations of 100 μM (12 nmoles/60 min), and these results were concentration-dependent. Systemically administered ritanserin also dose-dependently increased cortical DA efflux. These results indicate that atypical antipsychotic drugs may increase mesocortical DA release by antagonizing 5-HT receptors located in the prefrontal cortex. Furthermore, 5-HT may normally inhibit cortical DA release by actions at the 5-HT2 receptor subtype. © 1996 Wiley-Liss, Inc.     

Interactions between the duration of stimulation and noradrenaline on cholinergic transmission in the myenteric plexus-smooth muscle preparation

Output of acetylcholine (ACh), electromyogram (EMG) recordings and contractions of myenteric plexus-longitudinal muscle strip preparations from the guinea-pig ileum were studied during stimulation by single impulses or by trains (30 Hz; 2 to 128 impulses) under control conditions and in the presence of noradrenaline (NA). During supramaximal stimulation NA (2.5 microM) inhibited both contractions of the smooth muscle and the release of ACh evoked by single impulses more effectively than those evoked by train stimulation so that in a train of 4 impulses the output of ACh per impulse after the 2nd to 4th impulses was 69 to 104% higher than the output after the 1st impulse. During submaximal stimulation, contractions and ACh release evoked by single impulses were almost completely inhibited by NA. The neurogenic EMG, a direct consequence of the localized action of released transmitter (ACh), was recorded in the longitudinal muscle 4 and 10 mm aborally from the focal stimulation site. The incidence of the neurogenic response was much higher at the proximal (4 mm) than at the distal (10 mm) site and was proportional to the number of impulses in a train (100 Hz). NA inhibited propagation of the neurogenic response evoked by single impulses whereas its effect during train stimulation was less. It is concluded that in the course of train stimulation, sites of transmission more distant from the stimulation focus was recruited, and consequently the secretion of ACh in succeeding impulses was enhanced. NA could interfere with this process; it might inhibit the invasion by action potentials of cholinergic nerve terminal varicosities, thereby reducing the release of ACh.     

电刺激多巴胺能神经纤维或末梢对纹状体多巴胺释放的影响

采用快速周期伏安法,以碳纤维微电极在体研究了电刺激黑质－纹状体多巴胺（ＤＡ）能神经通路前脑内侧束（ＭＦＢ）或刺激纹状体内的神经纤维末梢对纹状体ＤＡ释放量的影响。结果如下：电刺激ＭＦＢ与刺激纹状体诱发纹状体的ＤＡ释放量明显不同。二者的ＤＡ释放量取决于所用的刺激强度、脉冲和频率;当给予２．０ｍＡ,２００次脉冲,１２５Ｈｚ电刺激ＤＡ能神经纤维ＭＦＢ时,可诱发纹状体（２１．５６±５．７７）μｍｏｌ／Ｌ的最大ＤＡ释放量,而在ｎｏｍｉｆｅｎｓｉｎｅ存在的前提下,给予０．１０ｍＡ,３０次脉冲,２０Ｈｚ电刺激纹状体ＤＡ能神经末梢时,仅诱发（９．０２±２．７２）μｍｏｌ／Ｌ的最大ＤＡ释放量。以上结果表明,尽管刺激强度、脉冲、频率均构成影响ＤＡ释放的相关因素,但刺激ＤＡ能神经纤维和末梢诱发的ＤＡ释放量及所用刺激参数均不相同,提示在刺激不同部位的实验研究中,应重视适宜刺激参数的选择和应用,以便获得更好的实验结果。     

Effect of cholecystokinin-8 microinjection into ventral tegmental area on dopamine release in nucleus accumbens of rats: an in vivo voltammetric study

1. Effects of microinjection of cholecystokinin-8 (CCK-8) into the rat ventral tegmental area were studied on dopamine (DA) release from nucleus accumbens (Acb), using fast cyclic voltammetry and carbon fibre microelectrode. 2. DA release was evoked by electrical stimulation of the medial forebrain bundle (MFB). DA release from Acb was increased with increasing intensity or frequency of electrical stimulation in a dose-dependent manner and was inhibited by microinjection of CCK-8 (0.5 microg/kg) into the ventral tegmental area. 3. The release of DA was clearly reduced at all the intensities (0.25, 0.5, 0.75 and 1.0 mA) tested following CCK injection into the VTA, which was statistically significant (P<0.05). But the reduced percentage of DA release did not show significant changes between the data obtained with stimuli of different intensities (P>0.05). 4. While no change could be found with the stimuli of 10Hz, the DA release was significantly suppressed by injection of CCK-8 at the other three frequencies tested (50 Hz, 100 Hz and 250 Hz). Furthermore, although the differences in the reduced amounts of DA release obtained at 50 Hz, 100 Hz and 250 Hz were statistically significant, the reduced percentage seemed to be not closely related to the frequency applied (P>0.05). 5. These results indicate that CCK-8 is involved in the regulation of midbrain DA neurotransmission, and thereby implicated in disorders, such as Parkinson's disease, that involve malfunctions of the basal ganglion DA neuronal systems.     

Effects of acute repetitive transcranial magnetic stimulation on dopamine release in the rat dorsolateral striatum

Animal studies have shown that descending pathways from the frontal cortex modulate dopamine (DA) release in the striatum. This modulation is thought to be relevant to the pathophysiology of Parkinson's disease. In human, repetitive transcranial magnetic stimulation (rTMS) can result in functional changes in the cortex. The present study intended to clarify the effects of acute rTMS treatment using various stimulation intensities on the extracellular DA concentrations in the rat dorsolateral striatum. The frontal brain of each rat received acute rTMS treatment, which consisted of 500 stimuli from 20 trains in a day. Each train was applied at 25 Hz for 1 s with 1-min intervals between trains. The neurochemical effects of acute rTMS treatment were investigated by determining the extracellular concentrations of DA in the rat dorsolateral striatum using in vivo microdialysis. Acute rTMS treatment of the frontal brain using the stimulation intensity of almost 110% motor threshold (MT) markedly and continuously increased the extracellular DA concentrations in the rat dorsolateral striatum. The present study demonstrates that acute rTMS treatment of the frontal brain affects the DAergic neuronal system in the rat dorsolateral striatum, and may have therapeutic implications for Parkinson's disease.     

Regional differences in the regulation of dopamine and noradrenaline release in medial frontal cortex, nucleus accumbens and caudate-putamen: a microdialysis study in the rat.

Dopamine (DA) and noradrenaline (NA) extracellular levels have been measured by microdialysis in the medial frontal cortex (MFC), nucleus accumbens (NAc) and caudate-putamen (CP) under baseline conditions in awake and halothane-anaesthetized rats, and after application of three types of stimuli which are likely to activate the brainstem catecholaminergic systems: mild stressors (handling and tail pinch), rewarded behavior (eating palatable food without prior food deprivation) and electrical stimulation of the lateral habenular nucleus. Changes were studied with and without uptake blockade (10 microM nomifensine in the perfusion fluid). The influence of calcium concentration (1.2 or 2.3 mM in the perfusion fluid) on DA and NA overflow was tested in some cases. Handling and tail pinch stimulated both DA and NA overflow in MFC, and enhanced NA overflow in NAc. By contrast, these mildly stressful stimuli had only marginal effects on DA overflow in NAc and no effects on either DA or NA overflow in CP. Eating behavior was accompanied by increased DA and NA overflow in MFC but had no effect in NAc. These regional differences were similar also when the manipulations were applied under uptake blockade, which indicates that the more pronounced changes seen in MFC did not simply reflect a more sparse innervation (i.e. lower density of uptake sites) in the MFC compared to the more densely innervated NAc and CP areas. Stimulation of the lateral habenula induced a 2-3-fold increase in NA overflow in both MFC, NAc and CP but had no consistent effect on DA overflow in any region. The effect on NA release was abolished by a transection of the ipsilateral fasciculus retroflexus (which carries the efferent output of the lateral habenula). The results show that the forebrain DA and NA projections to cortical and striatal targets are differentially regulated during ongoing behavior, that the mesocortical and mesostriatal DA systems respond quite differently to stressful and rewarding stimuli; and that the NA projection to MFC (like the dopaminergic one) is more responsive to stressful and rewarding stimuli than the ones innervating the striatum (NAc and CP). The results support the view that environmental stimuli evoking emotional arousal (whether aversive or non-aversive) are accompanied by increased DA and NA release above all in the MFC and only to a minor extent in limbic and striatal areas.     

Dopamine D1-like receptor modulates layer- and frequency-specific short-term synaptic plasticity in rat prefrontal cortical neurons

The mesocortical dopamine (DA) input to the prefrontal cortex (PFC) is crucial for processing short-term working memory (STWM) to guide forthcoming behavior. Short-term plasticity-like post-tetanic potentiation (PTP, < 3 min) and short-term potentiation (STP, < 10 min) may underlie STWM. Using whole-cell voltage-clamp recordings, mixed glutamatergic excitatory postsynaptic currents (EPSCs) evoked by layer III or layer V stimulation (0.5 or 0.067 Hz) were recorded from layer V pyramidal neurons. With 0.5 Hz basal stimulation of layer III, brief tetani (2 x 50 Hz) induced a homosynaptic PTP (decayed: approximately 1 min). The D1-like antagonist SCH23390 (1 microm) increased the PTP amplitude and decay time without inducing changes to the tetanic response. The tetani may evoke endogenous DA release, which activates a presynaptic D1-like receptor to inhibit glutamate release to modulate PTP. With a slower (0.067 Hz) basal stimulation, the same tetani induced STP (lasting approximately 4 min, but only at 2x intensity only) that was insignificantly suppressed by SCH23390. With stimulation of layer-V-->V inputs at 0.5 Hz, layer V tetani yielded inconsisitent responses. However, at 0.067 Hz, tetani at double the intensity resulted in an STP (lasting approximately 6 min), but a long-term depression after SCH23390 application. Endogenous DA released by tetanic stimulation can interact with a D1-like receptor to induce STP in layer V-->V synapses that receive slower (0.067 Hz) frequency inputs, but suppresses PTP at layer III-->V synapses that receive higher (0.5 Hz) frequency inputs. This D1-like modulation of layer- and frequency-specific synaptic responses in the PFC may contribute to STWM processing.     

Mechanical stimulation of cervical vertebrae modulates the discharge activity of ventral tegmental area neurons and dopamine release in the nucleus accumbens

BackgroundGrowing evidence suggests that mechanical stimulation modulates substrates in the supraspinal central nervous system (CNS) outside the canonical somatosensory circuits.Objective/Methods: We evaluate mechanical stimulation applied to the cervical spine at the C7-T1 level (termed “MStim”) on neurons and neurotransmitter release in the mesolimbic dopamine (DA) system, an area implicated in reward and motivation, utilizing electrophysiological, pharmacological, neurochemical and immunohistochemical techniques in Wistar rats.ResultsLow frequency (45–80 Hz), but not higher frequency (115 Hz), MStim inhibited the firing rate of ventral tegmental area (VTA) GABA neurons (52.8% baseline; 450 s) while increasing the firing rate of VTA DA neurons (248% baseline; 500 s). Inactivation of the nucleus accumbens (NAc), or systemic or in situ antagonism of delta opioid receptors (DORs), blocked MStim inhibition of VTA GABA neuron firing rate. MStim enhanced both basal (178.4% peak increase at 60 min) and evoked DA release in NAc (135.0% peak increase at 40 min), which was blocked by antagonism of DORs or acetylcholine release in the NAc. MStim enhanced c-FOS expression in the NAc, but inhibited total expression in the VTA, and induced translocation of DORs to neuronal membranes in the NAc.ConclusionThese findings demonstrate that MStim modulates neuron firing and DA release in the mesolimbic DA system through endogenous opioids and acetylcholine in the NAc. These findings demonstrate the need to explore more broadly the extra-somatosensory effects of peripheral mechanoreceptor activation and the specific role for mechanoreceptor-based therapies in the treatment of substance abuse.     

Release and elimination of dopamine in vivo in mice lacking the dopamine transporter: functional consequences

In mice lacking the dopamine transporter (DAT), the amplitude of dopamine (DA) release and the kinetics of dopamine elimination were measured in vivo using carbon fibre electrodes combined with amperometry. DA release was evoked by electrical stimulation of the medial forebrain bundle. The amplitude of DA release per pulse was lower (7% in striatum and 21% in nucleus accumbens) than in wild-type mice. Inhibition of monoamine oxidases (MAOs) by pargyline, but not of catechol-O-methyltransferase (COMT) by tolcapone, slowed down DA elimination in knockout mice. As DA half-life was two orders of magnitude higher in these mice, the DA diffusion distance was 10-times higher than in wild-types (100 and 10 microm, respectively). In knockout mice, alpha-methyl-p-tyrosine induced a much faster decline of DA release and haloperidol was less effective in potentiating DA release. Therefore, DA release was more dependent on DA synthesis than in normal animals but was less influenced by D2 autoregulation. Dopaminergic neurons exhibit two kinds of discharge activity, i.e. single spikes and bursts of 2-6 action potentials. In wild-type mice, stimuli mimicking bursts evoked significant increases in extracellular DA over its basal level sustained by tonic activity. However, in mice lacking the DAT, low frequency firing resulted in consistently high extracellular DA levels that could not be distinguished from DA levels achieved by high frequency firing. Therefore, the burst firing activity cannot be specifically translated into phasic changes in extracellular DA. This deficit might contribute to the difficulties of these mice in spatial cognitive function.     

偏侧猴PD模型STN慢性高频电刺激对脑内多巴胺系统的影响

目的探讨丘脑底核(STN)慢性高频电刺激对猴偏侧帕金森病(PD)模型脑内多巴胺(DA)系统的影响。方法采用单侧颈内动脉注入1-甲基-4-苯基-1,2,3,6四氢吡啶(MPTP)建立猴偏侧PD模型2只,体内植入脑深部电刺激(DBS)系统,行右侧STN慢性高频电刺激。在电刺激前后不同时间点采用微透析技术检测纹状体区细胞外液的多巴胺(DA)及其代谢产物高香草酸(HVA)含量,腰穿取脑脊液标本测量脑脊液中DA、HVA含量,单光子放射计算机断层扫描(SPECT)检测脑内纹状体区巴胺转运体(DAT)及多巴胺D2受体(D2R)的变化。结果猴偏侧PD模型在给予单侧STN慢性高频电刺激后纹状体区DA、HVA明显增高。SPECT显示在有效刺激后纹状体区DAT特异性摄取率增高,D2R特异性摄取率下降。脑脊液中多巴胺及其代谢产物的含量与术前相比无明显差异。结论通过微透析检测提示在给予STN有效慢性高频电刺激后提高了纹状体区DA及其代谢产物的升高,DAT特异性摄取率增高,D2R特异性摄取率下降提示纹状体区的代谢活性有明显升高,这可能是STN-DBS治疗帕金森病的重要机制之一。     

Mesocortical dopamine and HPA axis regulation: role of laterality and early environment

The infralimbic (IL) cortex is importantly involved in regulating behavioral and physiological responses to stress, including those of the hypothalamic-pituitary-adrenal (HPA) axis. The mesocortical dopamine (DA) system is an important afferent modulator of this region, is highly stress sensitive and frequently shows functional hemispheric asymmetry. Postnatal handling stimulation facilitates development of cortical asymmetry and is also associated with optimal HPA axis regulation. The present study examines the poorly understood role of the mesocortical DA system in regulating HPA axis function in adult rats which were handled (H) or nonhandled (NH) postnatally. In the first experiment, unilateral intra-IL cortex injection of the DA (D1/D2) antagonist alpha-flupenthixol into either hemisphere significantly exaggerated the restraint stress-induced increases in plasma adrenocorticotrophic hormone and corticosterone in NH rats. In H rats, the same effect was lateralized to the right IL cortex. In a second experiment, post mortem neurochemical analysis of DAergic measures in the IL cortex was conducted in H and NH animals following either acute or repeated (5 times) restraint stress. DAergic measures in the right IL cortex were significantly correlated with reduced stress hormone activation in both H and NH rats, especially in repeatedly restrained rats. However, while H rats showed a significant rightward shift in DA metabolism with repeated stress experience, NH rats shifted DA metabolism to the left. It is suggested that, during stress, mesocortical DA release normally acts in an adaptive, negative feedback capacity preventing excessive HPA activation and, with repeated stress, the right IL cortex is particularly important in this capacity. As well, the selective enhancement of DA metabolism in the right IL cortex of H rats may underlie, in part, their typically superior ability to adapt to stress and constrain HPA activity.     

d-Amphetamine-induced increase in neurotensin and neuropeptide Y outflow in the ventral striatum is mediated via stimulation of dopamine D1 and D2/3 receptors

The neuroanatomical and functional relationships between dopamine (DA) and neurotensin (NT) and DA and neuropeptide Y (NPY) suggest a role for these neuropeptides in DA-related neuropsychiatric disorders. By employing a microdialysis technique in conjunction with radioimmunoassay (RIA), the effects of d-amphetamine per se or after pretreatment with DA receptor antagonists on NT and NPY outflow were determined in the ventral striatum (VSTR) of the rat. One hour after a subcutaneous (s.c.) injection of saline, the DA-D(1) receptor antagonist SCH 23390 (0.3 mg/kg), or the DA-D(2/3) receptor antagonist raclopride (1.0 mg/kg), animals were injected s.c. with either saline or d-amphetamine (1.5 mg/kg). d-Amphetamine significantly increased extracellular NT- and NPY-like immunoreactivity (LI) concentrations compared with control animals. Administration of SCH 23390 or raclopride did not significantly affect NT-LI or NPY-LI concentrations. However, pretreatment with either SCH 23390 or raclopride abolished the stimulatory effect of d-amphetamine on NT-LI and NPY-LI. These findings demonstrate that d-amphetamine increases extracellular concentrations of NT-LI and NPY-LI in the VSTR through a mechanism that initially involves stimulation of either DA-D(1) or DA-D(2/3) receptors but appears to require both. In conclusion, changes in dopaminergic neurotransmission via DA-D(1) and DA-D(2/3) receptors affect the outflow of both NT and NPY in the VSTR.     

High frequency stimulation of the subthalamic nucleus increases the extracellular contents of striatal dopamine in normal and partially dopaminergic denervated rats

The subthalamic nucleus (STN) has come under focus in Parkinson disease (PD) because of recent advances in the understanding of the functional organization of the basal ganglia in normal and pathological conditions. Manipulations of the STN have been described to compensate for some imbalance in motor output of the basal ganglia in animal models of PD and have been proposed as a potential therapeutic target in humans. Indeed, high frequency stimulation (HFS) (130 Hz) of the STN has beneficial effects in severe parkinsonian patients but the precise mechanisms underlying these clinical results remain to be elucidated. To date, very little is known concerning the effect of HFS-STN on striatal dopaminergic transmission. Since it has been reported that dopaminergic medication may be reduced in PD patients under HFS-STN, our goal was to study the effect of HFS-STN on striatal dopamine (DA) transmission by using intracerebral microdialysis in normal and partially DA denervated rats. Our results show that HFS STN induces a significant increase of extracellular DA in the striatum of normal and partially DA lesioned rats while striatal extracellular levels of DOPAC were not affected. We conclude that HFS-STN acts directly and/or indirectly on striatal DA levels in control or partially DA lesioned rats.     

Visceral afferent stimulation-evoked changes in the release of peptides into the parabrachial nucleus in vivo

Previous investigations have demonstrated that the peptides substance P (SP), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neurotensin (NT) and somatostatin (SOM) significantly modulate the glutamate-mediated transmission of visceral information through the parabrachial nucleus (PBN) to the ventrobasal thalamus. In addition, we have shown that the staining intensity of SOM, CCK and NT in the PBN decreases significantly following 2 h of vagal stimulation as visualized using immunohistochemistry. As well, the staining intensity of both SP and CGRP in the PBN were shown to increase under similar conditions. The present investigation was done to determine whether the altered peptide staining intensity of these peptides observed following 2 h of vagal stimulation was the result of an altered peptide release from terminals within the PBN. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbitol and instrumented to record blood pressure and heart rate and for the stimulation of the cervical vagus nerve. A push–pull perfusion cannula was lowered into the region of the PBN for the continuous sampling of extracellular fluid. Radioenzymatic quantification of the perfusates for peptide content revealed that the extracellular fluid concentration of CGRP and SP increased significantly during the 2 h of vagal stimulation. When the vagal stimulation was terminated, the release of both CGRP and SP decreased significantly below prestimulated values for approximately 30 min before returning to prestimulated levels shortly thereafter. In contrast, there was a significant decrease in the release of CCK, SOM and NT into the PBN during the period of vagal stimulation. Extracellular perfusate levels of these peptides returned to normal upon termination of stimulation. These results demonstrate that terminal release of CGRP and SP is significantly increased and terminal release of CCK, SOM and NT is significantly decreased in the PBN during 2 h of vagal stimulation. These results are consistent with our previous finding that the immunohistochemical staining intensity of CGRP and SP is increased while that of CCK, SOM and NT is decreased following vagal stimulation.     

Neurotensin increases extracellular striatal dopamine levels in vivo.

This study employed intracranial microdialysis to assess the effects of neurotensin (NT) infusion on extracellular dopamine (DA) and DA metabolite concentrations in the rat striatum and nucleus accumbens, and the effects of NT on alterations in extracellular DA levels induced by cocaine and the DA D-2 receptor agonist, quinpirole. Direct NT infusion (.10, 1.0, 10.0 microM) did not significantly affect extracellular DA in the nucleus accumbens, but did produce a significant increase in the DA metabolite homovanillic acid (HVA). In contrast, direct NT infusion produced an increase in striatal DA levels, without altering DA metabolites. Neurotensin infusion (.10 microM) into the striatum significantly attenuated the peak DA increase induced by an intraperitoneal (IP) injection of a low dose (10.0 mg/kg) but not a high dose (30.0 mg/kg) of cocaine. Neurotensin infusion (.10 microM) did not affect the decrease in DA and its metabolites induced by an IP injection of a low dose of quinpirole (.03 mg/kg), but did alter the decrease in HVA induced by a high dose of quinpirole (.10 mg/kg). These results suggest that NT differentially affects in vivo DA release in the striatum and nucleus accumbens, and further strengthens the assertion that NT is an important modulator of dopaminergic function.     

Evidence that the stimulatory effect of neurotensin on dopamine release in rat nucleus accumbens slices is independent of dopamine D2-receptor activation

Neurotensin (NT, 10(-9) M and 10(-7) M) increased the electrically stimulated release of [3H]dopamine (DA) in rat nucleus accumbens slices. This effect was not altered by activation of DA D2-receptors with high or low concentrations of quinpirole (10(-6) M and 10(-8) M) or blockade of DA D2-receptors with near maximal concentrations of sulpiride (10(-6) M and 10(-5) M). The sulpiride-mediated increase in [3H]DA release and the release induced by NT were additive. These results suggest that NT functions independently of DA D2-receptor activation to modulate DA release in the nucleus accumbens. NT did not modulate the simultaneous release of [14C]acetylcholine.     

Parametric analysis of the effects of cocaine and cocaine pretreatment on dopamine release in the nucleus accumbens measured by fast cyclic voltammetry

Fast cyclic voltammetry was used to measure dopamine (DA) release in the nucleus accumbens of anaesthetized rats, in response to electrical sine-wave stimulation of the ventral tegmental area. Voltammetric signals followed increases in either frequency (50–100 Hz), intensity (50–100 μA) or duration (0.5–5.0 s) of the stimulus. Cocaine administration (10 mg/kg) preferentially increased DA release by weak electrical stimuli. Cocaine pretreatment (3×10 mg/kg, two weeks earlier) preferentially increased DA release by stronger stimuli, and the effects of acute cocaine were potentiated in these animals. The effects of increasing stimulus duration conformed to first order kinetics. Cocaine pretreatment selectively increased the kinetic parameter representing maximal release, while acute cocaine administration preferentially decreased the parameter representing the stimulus duration eliciting half maximal release. The lack of statistical interaction between these two effects suggests that sensitization of the response to acute cocaine by cocaine pretreatment may simply reflect an increase in the size of the releasable pool of DA.