Preference and aversion for brain stimulations estimated by the conditioned place preference

Rats implanted with chronic electrode into the medial forebrain bundle at the level of the hypothalamus (which elicited self-stimulation behaviour) display conditioned place preference after repeated stimulations of this area; conversely rats implanted into the mesencephalic dorso-medial tegmentum (which elicited switch-off behaviour) present conditioned place aversion after such repeated stimulations. Furthermore rats implanted in these two area without exhibiting self-stimulation or switch-off behaviours also display preference or aversion for the location paired with the brain stimulations. It was thus hypothesized that the conditioned place preference procedure seemed to present a higher sensitivity than the bar pressing procedures to detect preference or aversion for brain stimulation. Moreover the activation of the medial forebrain bundle which was associated with positive affect is more effective for a long term retention of the preferred location than midbrain periventricular reticular activation which induces a negative affect.     

Opposite effects of cholecystokinin octapeptide (CCK-8) and tetrapeptide (CCK-4) after injection into the caudal part of the nucleus accumbens or into its rostral part and the cerebral ventricles

Neurons with colocalized cholecystokinin and dopamine are present predominantly in the ventral tegmental area and project mainly to the caudal part of the medial nucleus accumbens. The activity of this dopamine system can be evaluated by means of the intracranial self-stimulation behavior on male Wistar rats having chronic electrodes implanted into the medial forebrain bundle in the postero-lateral area of the hypothalamus. The direct injection of 150 pmol CCK-8 into the medio-caudal accumbens induced an increase of intracranial self stimulation while a similar administration into its rostral portion produced a slight decrease of intracranial self-stimulation. The administration of 300 pmol CCK-4 into the same medio-caudal part of the accumbens produced an inhibitory action on intracranial self stimulation lasting for 25 min. The injection of 70 to 1300 pmol CCK-4 into the cerebral ventricles produced no change on intracranial self-stimulation. The intracerebroventricular injection of 70 pmol CCK-8 induced a large decrease of intracranial self-stimulation lasting for 20 min. Sodium chloride 0.15 M or unsulphated CCK-8 injection were without effect in either case. These results support the ideas that intracerebroventricular CCK-8 injection inhibits accumbens dopaminergic activity but that CCK-8 injection into the medio-caudal part of the accumbens, where nerve terminals with colocalized CCK and DA are present, facilitates this dopaminergic activity. In addition at the level of medio-caudal accumbens, CCK-8 and CCK-4 have opposite effects.     

Effects of the afferent stimulation on neurons in the medial septal region slices and their modulation by biologically active compounds in hibernating and waking ground squirrels

Responses of the medial septal (MS-DB) neurons to electrical stimulation of the medial forebrain bundle (MFB) and their modulation by some neuropeptides and monoamines were investigated in brain slices taken from two groups of ground squirrels-hibernating (HGS) and waking (WGS). Electrical stimulation evoked mostly inhibitory effects of various duration. Besides, responses by phase reset of the background rhythmic bursts and short-latency single spike responses were observed. The neuropeptides identified in the brain of hibernators differentially and reversibly modulated responses even in those neurons where they did not influence the level and pattern of the background activity. Effects of the peptides were state-dependent. E.g., the peptide TSKYR increased the duration of inhibitory effects in the HGS but shortened them in the WGS, while TSKY which had low efficacy in the HGS, increased the duration of inhibition in the WGS. Dipeptide DY depressed inhibitory components and augmented excitatory components of responses in the HGS but was much less effective in the WGS. Effects of noradrenaline and serotonin had stronger correlation with their influence on spontaneous activity. It is suggested that endogenous substances provide for the increased latent excitability and reactivity of the MS-DB neurons during seasonal hibernation. Thus, the MS-DB may function as a "sentry post" participating in signal detection and urgent arousal during hibernation.     

The effect of tryptophan depletion on brain activation measured by functional magnetic resonance imaging during the Stroop test in healthy subjects

We investigated the role of serotonin in cognitive activation of the frontal cortex. The serotonergic system was affected by the administration of an amino acids mixture without tryptophan (tryptophan depletion). In a placebo-controlled double-blind cross-over study with 20 healthy volunteers, we tested the hypothesis that a tryptophan (serotonin) decrease affects the activation of prefrontal cortex by the Stroop test. Cognitive brain activation was evaluated by functional magnetic resonance imaging (fMRI). Tryptophan depletion decreased the plasma tryptophan level up to 90 % for five hours after the tryptophan-free drink had been consumed when compared with the same mixture with tryptophan (p?0.0001). Tryptophan depletion did not affect the Stroop test performance. We compared fMRI activation in both conditions (tryptophan depletion and placebo) with plasma tryptophan levels as the covariates. The tryptophan depletion increased the activation (fMRI signal) in the bilateral mediofrontal cortex, anterior cingulate and left dorsolateral prefrontal cortex. The present findings allow the postulate that serotonergic medial forebrain and cingulum bundle pathways play a role in the activity of cortical structures involved in Stroop test processing.     

Effects of the cannabinoid receptor agonist CP 55,940 and the cannabinoid receptor antagonist SR 141716 on intracranial self-stimulation in Lewis rats.

Lewis rats were trained to self-stimulate the medial forebrain bundle (MFB) using a rate-frequency paradigm. They were then tested for the effects of the cannabinoid receptor agonist CP 55,940, the selective cannabinoid receptor antagonist SR 141716 and the dopamine D1 receptor antagonist SCH 23390. CP 55,940 (0, 10, 25 and 50 microg/kg i.p.) had no effect on MFB self-stimulation behaviour as assessed by the M50, the stimulation frequency at which half-maximal response rates were obtained. With SR 141716, only a very high dose (20 mg/kg i.p.) caused a significant inhibition of the rewarding efficacy of the stimulation. This was seen as an increase in the M50. All other doses of SR 141716 (0, 1, 3, 10 mg/kg i.p.) were ineffective in modulating the M50. By comparison, a relatively low dose (0.06 mg/kg i.p.) of SCH 23390 caused a large increase in M50. These results indicate a relatively modest influence, if any at all, of exogenous or endogenous cannabinoids on reward-relevant neurotransmission.     

Acetaminophen inhibits liver trytophan-2,3-dioxygenase activity with a concomitant rise in brain serotonin levels and a reduction in urinary 5-hydroxyindole acetic acid

The effect of the analgesic agent, acetaminophen was determined on rat forebrain serotonin levels as well as hepatic tryptophan-2,3-dioxygenase (TDO) activity and urinary 5-hydroxyindole acetic acid (5-HIAA). The results show that acetaminophen administration (100mg/kg) over three hours does not affect the holoenzyme of tryptophan-2,3-dioxygenase but significantly inhibits the apoenzyme. This inhibition is accompanied by a concomitant rise in forebrain serotonin levels. This phenomenon is also accompanied by a reduction in urinary 5-HIAA levels. These results suggest that acetaminophen use is accompanied by changes in brain serotonin levels due to inhibition of hepatic tryptophan-2,3-dioxygenase activity. This in turn could explain the possible abuse potential of acetaminophen and its effects on mood at high doses.     

Intracranial self-stimulation (ICSS) in rodents to study the neurobiology of motivation

It has become increasingly important to assess mood states in laboratory animals. Tests that reflect reward, reduced ability to experience reward (anhedonia) and aversion (dysphoria) are in high demand because many psychiatric conditions that are currently intractable in humans (e.g., major depression, bipolar disorder, addiction) are characterized by dysregulated motivation. Intracranial self-stimulation (ICSS) can be utilized in rodents (rats, mice) to understand how pharmacological or molecular manipulations affect the function of brain reward systems. Although many different methodologies are possible, we will describe in this protocol the use of medial forebrain bundle (MFB) stimulation together with the 'curve-shift' variant of analysis. This combination is particularly powerful because it produces a highly reliable behavioral output that enables clear distinctions between the treatment effects on motivation and the treatment effects on the capability to perform the task.     

Peptides and self-stimulation of the medial prefrontal cortex in the rat: effects of intracerebral microinjections of substance P and cholecystokinin

The effects of intracerebral microinjections of substance P and cholecystokinin on self-stimulation of the medial prefrontal cortex of the rat were studied. Intracerebroventricular administration of substance P at doses of 2.5, 5, 10 and 20 micrograms produced a dose-related decrease in self-stimulation of the medial prefrontal cortex; spontaneous motor activity, measured as a control, was not affected. Unilateral microinjections into the medial prefrontal cortex of substance P at doses of 10 and 20 micrograms produced a decrease of self-stimulation of the ipsilateral side, but self-stimulation of the contralateral cortex, used as a control, was not affected. On the contrary, cholecystokinin in both intracerebroventricular administration at doses of 100, 200 and 400 ng, or intracortical microinjections into the medial prefrontal cortex at doses of 200, 400 and 800 ng, had no effect on self-stimulation of this cortical area. These results suggest that substance P, but not cholecystokinin, could be part of the neurochemical substrate underlying self-stimulation of the medial prefrontal cortex in the rat.     

Brain 3,4-dihydroxyphenylethyleneglycol levels are dependent on central noradrenergic neuron activity

The effect of manipulations aimed to alter brain noradrenergic neuron activity on the levels of free and conjugated DOPEG in discrete brain areas was studied in the rat. Electrical stimulation of the medial forebrain bundle for 10–20 min produced a frequency dependent elevation of free and conjugated DOPEG concentrations in the anterior cerebral cortex and in the hippocampus. In contrast, acute interruption of noradrenergic nerve impulse flow by application of tetrodotoxin (50–100 ng) into the medial forebrain bundle markedly diminished cortical and hippocampal DOPEG levels at 0.5–2 h post-injection. Cortical conjugated and free DOPEG levels were also reduced (by 80–96%) 2–3 weeks after bilateral electrolytic lesion of the locus coeruleus, 6-hydroxydopamine-induced lesion of the ascending noradrenergic pathways or noradrenergic denervation by the neurotoxic agent DSP4. Finally, the α-adrenoceptor blocking agents yohimbine (1–10 mg/kg, ip) and RX 781094 (3–10 mg/kg, ip) increased whereas the α-adrenergic agonist clonidine (0.01–1 mg/kg, sc) decreased DOPEG levels in the cerebral cortex, hypothalamus and septal areas. These data indicate that free and conjugated DOPEG formation is dependent on, and may serve as an index of, central noradrenergic neuron activity.     

Voltammetric and mathematical evidence for dual transport mediation of serotonin clearance in vivo

The neurotransmitter serotonin underlies many of the brain's functions. Understanding serotonin neurochemistry is important for improving treatments for neuropsychiatric disorders such as depression. Antidepressants commonly target serotonin clearance via serotonin transporters and have variable clinical effects. Adjunctive therapies, targeting other systems including serotonin autoreceptors, also vary clinically and carry adverse consequences. Fast scan cyclic voltammetry is particularly well suited for studying antidepressant effects on serotonin clearance and autoreceptors by providing real‐time chemical information on serotonin kinetics in vivo. However, the complex nature of in vivo serotonin responses makes it difficult to interpret experimental data with established kinetic models. Here, we electrically stimulated the mouse medial forebrain bundle to provoke and detect terminal serotonin in the substantia nigra reticulata. In response to medial forebrain bundle stimulation we found three dynamically distinct serotonin signals. To interpret these signals we developed a computational model that supports two independent serotonin reuptake mechanisms (high affinity, low efficiency reuptake mechanism, and low affinity, high efficiency reuptake system) and bolsters an important inhibitory role for the serotonin autoreceptors. Our data and analysis, afforded by the powerful combination of voltammetric and theoretical methods, gives new understanding of the chemical heterogeneity of serotonin dynamics in the brain. This diverse serotonergic matrix likely contributes to clinical variability of antidepressants.

     

Localization of serotonin in the hypothalamus and the mesencephalon of the guinea-pig

Summary The distribution of serotonin in the hypothalamus and the mesencephalon of guinea-pigs pretreated with both pargyline and L-tryptophan was investigated immunohistochemically using monoclonal antibodies to 5-HT. 5-HT-positive fibers and varicosities appeared distributed throughout the hypothalamus. Some areas showed a greater density of immunoreactivity: the suprachiasmatic nucleus, the region of the supraoptic crest, the area of the medial forebrain bundle, the ventral part of the nucleus ventromedialis, the median eminence and the ventral part of the mammillary bodies. 5-HT nerve fibers were also scattered in the posterior lobe of the pituitary. An extensive supraependymal plexus of immunoreactive axons was observed in most ventricular regions. No 5-HT positive cell bodies were present in the hypothalamus of the guinea-pig under our experimental conditions, whereas an intense serotonin immunoreactivity was detected in perikarya of the brain stem. 5-HT cell bodies were found predominantly in the raphe region including the nucleus raphe dorsalis and raphe medianus, nucleus interpeduncularis, reticular formation and dorsal area of the medial lemniscus.     

Alcohol deters the outgrowth of serotonergic neurons at midgestation

We have previously demonstrated that treatment of pregnant C57BL mice from gestation days 8 to 14 with alcohol with 20% ethanol-derived calories (EDC) reduced the number of serotonin (5-HT) neurons and retarded their migration in the fetal brains. In the present study, we obtained similar results with the use of 25% EDC and extended our previous findings by demonstrating that besides the alteration of the number of 5-HT neurons, prenatal alcohol exposure also affects their projecting fibers in their early development. Pregnant C57BL mice were divided into an alcohol-exposed (ALC) group given 25% EDC (4.49%, v/v), a pair-fed group to the ethanol-fed group (PF) and a chow-fed group (Chow). The PF and Chow groups served as controls. Our results showed that in the ALC group, when compared with the control groups, prenatal alcohol exposure with 25% EDC reduced the number of 5-HT-immunoreactive neurons in both the median and dorsal raphe, and the amount of 5-HT-immunoreactive fibers in the medial forebrain bundle (MFB). The diameter of the 5-HT-immunoreactive MFB was also reduced as a result of treatment. No significant differences of the above parameters were found between the PF and Chow groups. The previous and present work confirmed that alcohol reduces the normal formation and growth of 5-HT neurons in the midbrain. Furthermore, the projection of 5-HT fibers, in density as well as in distribution, is reduced in the major trajectory bundle. This may affect the amount of 5-HT fibers available to the forebrain. In light of the importance of the 5-HT system in brain development, alcohol may affect the growth of the forebrain through its effect on 5-HT signaling.     

Tripelennamine: enhancement of brain-stimulation reward

Thresholds for reinforcing electrical stimulation to the medial forebrain bundle were determined in rats by means of a rate-free psychophysical method. The acute administration of tripelennamine lowered the threshold for rewarding brain stimulation in a dose-dependent manner. These results suggest that the abuse liability of tripelennamine may be related to its ability to sensitize the central neural pathway that mediates reward.     

Septal nuclei in the mechanisms of lateral hypothalamic self stimulation in rabbits

Unilateral lesions of medial septal nucleus and lateral septal nuclei (dorsalis, intermedius, ventralis) decreased ipsilateral hypothalamic self-stimulation; the lesions of n. dorsalis, n. medialis and lateral septal nuclei (intermedius and ventralis) has the opposite effect on contralateral self-stimulation. The inhibition of ipsilateral self-stimulation was neither total nor permanent; 25-32% decrease in stimulation rate was seen, but behaviour returned to near-normal levels over a period of few days. In contrast, the augmentation of contralateral self-stimulation showed no significant change over the same period; in this case the 35-40% shift in stimulation rate was immediate and permanent. Bilateral lesions of septal nuclei had no effect if the initial level of self-stimulation rate was high and significantly increased self-stimulation.     

Central neural pathways for angiotensin-induced thirst.

Evidence is reviewed implicating the preoptic region in angiotensin-induced thirst. The most responsive area according to results obtained with behavioral, electrophysiological, and autoradiographic mapping techniques is at the caudal border of the medial preoptic region and rostral border of the anterior hypothalamus. The neural pathway from this preoptic site for angiotensin-induced thirst extends along the medial forebrain bundle through the midlateral hypothalamus to the paramedial midbrain tegmentum and to an area ventrolateral to the central gray. Lesions of this pathway in the midlateral hypothalamus and rostral midbrain significantly attenuated drinking induced by microinjections of angiotensin II into the preoptic area but did not disrupt water intake induced by microinjections of angiotensin II into the subfornical organ or cerebral ventricles. Although the efferent pathways from angiotensin-receptive sites in the subfornical organ and cerebral ventricles are unknown, it appears from these observations that the medial forebrain bundle is not involved. Lesions of the medial forebrain bundle-lateral hypothalamus also do not disrupt drinking induced by microinjections of hypertonic saline into the preoptic region although lesions placed 1 mm further lateral do. Since fat lateral hypothalamic lesions are without effect on drinking induced by centrally administered angiotensin II, this suggests that intracellular and extracellular thirst signals are subserved by separate neural pathways in the hypothalamus.     

Effects of Frequency and Pattern of Medial Forebrain Bundle Stimulation on Caudate Dialysate Dopamine and Serotonin

In vivo microdialysis was employed to detect changes in extracellular dopamine and serotonin in the rat caudate in response to electrical stimulation of the medial forebrain bundle. Extracellular dopamine concentrations increased linearly as a function of the frequency (4-33 Hz) of evenly spaced stimuli in both the presence and absence of cocaine added to the dialysate. Because dopamine neurons are known to fire in single-spike and burst patterns, stimulation pulses were also delivered in a bursting pattern. The response of extracellular dopamine was augmented in both the presence and absence of cocaine when the same number of stimuli were delivered in bursts as compared to an evenly spaced pattern. Serotonin, which was only assessed in the presence of cocaine, similarly increased linearly with frequency, but, in contrast to the dopamine response, levels of serotonin were not augmented by stimuli presented in bursts. These results suggest that microdialysis can be used to detect physiological changes in synaptic transmitter concentrations.     

Modulation of neuronal evoked responses to the medial septal area of hibernating ground squirrels by some neuropeptides after chronic disconnection between septum and preoptico-hypothalamic structures

Effects of some neuropeptides identified in the brain of hibernators (TSKYR, TSKY, DY) and of monoaminergic neurotransmitters (noradrenaline and serotonin) on responses of the medial septal neurons evoked by intraseptal electrical stimulation were analyzed in slices taken from the ground squirrels with chronic basal undercutting of the septum. Despite the elimination of direct contacts with the preoptic area and afferents ascending in the medial forebrain bundle, the neurons retained almost normal level of reactivity and distribution of the reaction types. The neuropeptides effectively modulated neuronal responses of various types, including oligosynaptic short-latency single-spike responses. The latter were strongly facilitated by the neuropeptides. As a rule, changes in the responses to electrical stimulation were independent of the spontaneous activity shifts (in 78% of the tests). It was suggested that the neuropeptides exert a double influence on the septal neurons: direct nonsynaptic effects on the pacemaker potential responsible for the background activity and modulation of synaptic processes. Our experiments showed that descending influences of the septo-hippocampal system are not crucial for the entrance into the hibernation state and its tonic maintenance. The influences of the thermoregulatory--circadian structures of the preoptico-hypothalamic area determine the paradoxically increased latent excitability of septal neurons that allows the septo-hippocampal system to gate external stimuli and organize arousal of the forebrain during hibernation in case of emergency.     

Effect of intraventricular oxytocin and vasopressin on self-stimulation in rats.

Oxytocin (500 mu u) and vasopressin (50 mu u) were injected into the lateral ventricle and its effect on hypothalamic self-stimulation has been studied. Oxytocin increased, while vasopressin decreased the self-stimulation rate tested 10-20 min following application. The hypothalamic and mesencephalic serotonin content decreased slightly while plasma corticosterone content did not change 20 min after oxytocin and vasopressin administration compared to the injected control animals. The data suggest that vasopressin and oxytocin have an opposite effect on self-stimulation and this action is not mediated through the brain serotoninergic or pituitary-adrenocortical axis.     

Presynaptic effects of biogenic amines modulating synaptic transmission between identified sensory neurons and giant interneurons in the first instar cockroach

Intracellular recording was used to investigate the modulatory effects of serotonin and octopamine on the identified synapses between filiform hair sensory afferents and giant interneurons in the first instar cockroach, Periplaneta americana. Serotonin at 10(-4) mol l(-1) to 10(-3) mol l(-1) reduced the amplitude of the lateral axon-to-ipsilateral giant interneuron 3 excitatory postsynaptic potentials. and octopamine at 10(-4) mol l(-1) increased their amplitude. Similar effects were seen on excitatory postsynaptic potentials in dorsal giant interneuron 6. Several lines of evidence suggest that both substances modulate the amplitude of excitatory postsynaptic potentials by acting presynaptically, rather than on the postsynaptic neuron. The fitting of simple binomial distributions to the postsynaptic potential amplitude histograms suggested that, for both serotonin and octopamine, the number of synaptic release sites was being modulated. Secondly, the amplitudes of miniature excitatory postsynaptic potentials recorded in the presence of tetrodotoxin were unaffected by either modulator. Finally, recordings from contralateral giant interneuron 3, which has two identifiable populations of synaptic inputs, showed that each modulator had a more pronounced effect on excitatory postsynaptic potentials evoked by the lateral axon than on those evoked by the medial axon. Immunocytochemistry confirmed that neuropilar processes containing serotonin are present in close proximity to these synapses.     

The state-dependent action of neuropeptides and monoamines on the background neuronal activity of the medial septal area in hibernating animals

Neuronal activity of the medial septal area (MS-DB) was recorded extracellularly in brain slices from two groups of Yakutian ground squirrels Citellus undulatus--hibernating (winter period) and actively waking (summer period). Effects of three neuropeptides identified in the brain of hibernators (TSKYR, TSKY, and DY) and of two monoamines (serotonin and noradrenaline) on spontaneous activity were analyzed. All neuropeptides reversibly changed the levels of the background activity, but in the hibernating ground squirrels (HGS) the level of reactivity (47-56%) was significantly higher than in the waking ground squirrels (WGS, 25-30%). Serotonin also showed some tendency to higher efficacy in the HGS. Only noradrenaline was equally effective and had absolutely dominating excitatory effect in both states, although the level of excitation in the HGS was higher. All other substances evoked excitatory and inhibitory effects in various proportions. Their distribution was state-dependent, the rate of development, intensity and duration of the effects were greater in the HGS. The experiments confirmed the data on higher excitability and reactivity of the septal neurons in the state of hibernation. It is suggested that the tested neuropeptides may participate in the control of hibernation.