Involvement of D2 dopaminergic receptors in the emotional and motivational responses induced by injection of CCK-8 in the posterior part of the rat nucleus accumbens

When CCK-8 was injected in the rat posterior nucleus accumbens, where it is in part co-localized with dopamine, a decrease in exploration of the four hole box and the elevated plus maze was observed. In this study, a selective destruction of the dopaminergic mesoaccumbens pathway induced by local injection of 6-hydroxydopamine (6-OHDA) in the nucleus accumbens was found to suppress the CCK-8-evoked behavioral effects. Moreover, an ex vivo measurement of the dopaminergic metabolism has been performed after injection of CCK-8 in the posterior nucleus accumbens by electrochemical detection of dopamine and its metabolites extracted from punches of brain tissue. The results showed that CCK-8 decreased the turnover of dopamine in the posterior part but not in the anterior part of the nucleus accumbens or in the ventral tegmental area. Furthermore, sulpiride, a selective antagonist for D2 dopamine receptors, but not SCH 23390, a selective antagonist for D1 dopamine receptors, prevented CCK-8-induced behavioral responses. Taken together, these results suggest that CCK-8 could be involved in behavioral adaptation to situations producing change in emotional and/or motivational states through modulation of presynaptic D2 receptor functioning.

Developmental effect of testosterone on estrogen sensitivity of the rat preoptic neurons with axons to the ventral tegmental area

The neonatal effect of testosterone on neuronal sensitivity to estrogen was examined in the medial preoptic area of the rat when adult. Electrical stimulation of the ventral tegmental area induced antidromic action potentials in 683 neurons in 106 urethane-anesthetized animals which consisted of 38 males castrated on the day of birth (day 1), 30 females androgenized by testosterone injection on day 5, and 38 normal females. All females were used after ovariectomy when adult. Histological localization of the antidromically activated neurons was similar in all the animal groups. The latency jump in 40% of the potentials indicated that activated preoptic axons terminate in the stimulation site. The latency for activation was in the range 1.6-43.7 ms and the threshold was as low as 60 microA. The absolute refractory period did not exceed 1700 microseconds. Estrogen increased the threshold among 278 cells in the normal females and 181 cells in the neonatally castrated males. Preoptic neurons in the androgenized females (n = 224) differ from those in others in that their threshold, regardless of estrogen treatment, was at the ceiling values seen in the other groups in the presence of estrogen. Estrogen also prolonged the absolute refractory period in the females but not in the androgenized females. The reduced excitability and the lack of estrogen sensitivity may be responsible for the insensitivity of the androgenized females to feminine actions of estrogen, that may presumably underlie a sex difference in certain behavioral functions.

Role of the lateral tegmental field in the central sympatho-inhibitory effect of 8-hydroxy-2-(di-n-propylamino)tetralin in the cat

Cardiovascular neurones with sympathetic nerve and cardiac related activity have been detected in the lateral tegmental field and have been classified depending on their response to baroreceptor activation. Baroreflex activation decreased and increased the firing rate of sympatho-excitatory neurones and sympatho-inhibitory neurones respectively. Intravenous (i.v.) administration of 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) (1 to 30 micrograms/kg) produced a dose-dependent decrease in the firing rate of sympatho-excitatory neurones in conjunction with a decrease in renal sympathetic nerve activity. 8-OH-DPAT had a variable effect (excitation or inhibition) on the firing rate of sympatho-inhibitory lateral tegmental field neurones. Both the excitatory and the inhibitory effects of 8-OH-DPAT were reversed by spiperone (1 mg/kg i.v.). Microinjection of 8-OH-DPAT (2 nmol/site) into the lateral tegmental field decreased blood pressure and renal sympathetic nerve activity. Compared with the effects elicited by microinjection of 8-OH-DPAT into the rostral ventrolateral medulla, the renal sympatho-inhibitory response elicited by 8-OH-DPAT injected into the lateral tegmental field appeared smaller. Chemical lesions of the lateral tegmental field neurones with kainic acid produced hypertension and sympatho-excitation. The correlation between renal sympathetic nerve activity and the R wave of the electrocardiogram disappeared after treatment with kainic acid. Subsequent i.v. 8-OH-DPAT (1 to 100 micrograms/kg) failed to alter blood pressure and renal sympathetic activity. In contrast clonidine (1 to 10 micrograms/kg) still decreased blood pressure, heart rate and renal sympathetic nerve activity in kainic acid-lesioned animals and these effects were reversed by the alpha 2-adrenoceptor antagonist, methoxy-idazoxan (10 micrograms/kg i.v.). The data described indicate that the lateral tegmental field plays a role in the central cardiovascular effects of 8-OH-DPAT and that the integrity of this area is necessary for the expression of the sympatho-inhibitory effects of the drug.

[Magnetic resonance imaging in chronic toluene abuse, and volitional hyperkinesia]

We described the central nervous system impairments and findings of magnetic resonance imaging (MRI) of the brain in three chronic toluene abusers. Patient 1 and 2 had a history of chronic toluene inhalation for 1 or 2 years and patient 3 has a history of it for 16 years. The patients were evaluated after an abstinence period of at least 4 weeks, to avoid neurologic effects of acute intoxication. Neurologic signs included cerebellar, cognitive and pyramidal dysfunctions in all 3 patients and hyperkinésie volitionnelle (HV; volitional hyperkinesia) in patient 3 who had a long history of toluene inhalation. This HV was found to be tremulous by surface electromyography. Appearance of HV in chronic toluene abuse seems to be related to a period or quantity of inhalation of toluene. T2-weighted MRI in the 3 patients revealed the following abnormalities: (1) increased signal intensity of middle cerebellar peduncle and cerebellar white matter, deep cerebral white matter, and posterior limb of internal capsule; and (2) decreased signal intensity of thalamus and basal ganglia. Proton-weighted MRI in patient 3 with HV showed decreased signal intensity of lateral portion of the thalamus and tegmentum of the brainstem. These MRI findings correlate remarkably well with the neurologic signs seen in the 3 patients. Cerebellar, cognitive, and pyramidal dysfunctions appear to be related to involvement of middle cerebellar peduncle (cerebellar white matter), deep cerebral white matter, and internal capsule, respectively, and HV seems to correlate with involvement of the thalamus or tegmentum of the brainstem, especially the latter.

Suppressive effect of L-dopa on dopamine cells remaining in the ventral tegmental area of rats previously exposed to the neurotoxin 6-hydroxydopamine

Ever since the introduction of levo-3,4-dihydroxyphenylalanine (L-dopa) for the treatment of Parkinson's disease, there has been concern that it might accelerate the degeneration of dopamine neurones. Using rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB), we have studied the effect of chronic L-dopa treatment on the survival of dopamine cells which remain in the ventral tegmental area (VTA) ipsilateral to a 6-OHDA lesion. Following lesion surgery, rats were treated with L-dopa and carbidopa administered in the drinking water for 27 weeks. At the end of the treatment period, the number of dopamine cells remaining in each of the lesioned and intact substantia nigra (SN) and VTA were assessed, using tyrosine hydroxylase immunohistochemistry. Chronic L-dopa treatment resulted in an apparent reduction in the number of dopamine neurones remaining in the VTA ipsilateral to the lesion, whereas it had no effect on the number of dopamine cells remaining in the intact SN and VTA. This finding suggests a possible suppressive effect in vivo of L-dopa on dopamine cells in the midbrain of adult animals that have been previously exposed to 6-OHDA.

Ventral tegmental microinjections of quinpirole decrease ethanol and sucrose-reinforced responding

The effects of microinjections in the ventral tegmental area (VTA) of the dopamine D2/D3 agonist quinpirole on ethanol- and sucrose-reinforced responding were tested. Two groups of Long-Evans rats were trained to lever press on a fixed-ratio 4 schedule with 10% ethanol (v/v) (n = 8) and 75% sucrose (w/v) (n = 10) presented as the reinforcer. Weekly bilateral injections of quinpirole were tested in the ethanol group (0.0, 0.001, 0.01, 0.1, and 1.0 micrograms/microliters) and sucrose group (0.0, 0.01, 0.1, 1.0, and 10.0 micrograms/microliters). Quinpirole dose dependently decreased ethanol- and sucrose-reinforced responding, but the dose-effect curve for sucrose was shifted two orders of magnitude to the right of the ethanol curve. Temporal response patterns during control sessions for both reinforcers were characterized by initial high rates that terminated after approximately 10 min. VTA injections of quinpirole dose dependently delayed the onset of the first response and the initial high rate period, and resulted in an early termination of responding following onset. These data support the hypothesis that dopamine activity in the VTA is involved in the regulation of ethanol-reinforced responding in a manner similar to that of other reinforcers.

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Serotonergic influence from nucleus raphe obscurus on neurones in the periaqueductal grey matter in the rat

In rats anaesthetised with urethane, iontophoretic application of 5-hydroxytryptamine (5-HT, 5-70 nA) produced changes in ongoing activity of 41/44 neurones in the periaqueductal grey matter (PAG). The majority (85%) of responsive cells were inhibited and 15% were excited. The inhibitions were mimicked in 5/7 cells by iontophoresis of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 10-30 nA) whilst excitation was produced in 3/5 cells by iontophoresis of the 5-HT2 agonist alpha-methyl-5-HT (10-30 nA). Selective activation of neuronal perikarya in nucleus raphe obscurus (NRO) by microinjection of 50-100 nl D,L-homocysteic acid (DLH) inhibited ongoing activity of 25/31 neurones tested in the PAG for periods of 30-580 s, mean 183.5 s. The duration of the inhibition was potentiated by between 36 and 300% during iontophoresis of the 5-HT re-uptake blocker paroxetine (1-25 nA, 6/6 cells). The results indicate that there is an extensive inhibitory serotonergic input to the PAG which originates, at least in part, from NRO.

Prazosin modulates the firing pattern of dopamine neurons in rat ventral tegmental area

Previous studies have indicated a noradrenergic modulation of midbrain dopamine cell activity. The effects of systemic administration of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-antagonist idazoxan on midbrain dopamine cell firing were now studied with extracellular recording from single dopamine neurons in the ventral tegmental area of chloral hydrate-anaesthetized male rats. Prazosin (0.15-0.6 mg/kg i.v.) dose dependently decreased burst firing and regularized the firing pattern of dopamine neurons, while the firing rate was unaffected. The prazosin-induced effects were abolished by pretreatment with reserpine. Idazoxan (0.5-2.0 mg/kg i.v.) increased firing rate and burst firing and made the firing pattern less regular, probably by increasing adrenergic transmission via blockade of presynaptic alpha 2-adrenoceptors. The effects of idazoxan were blocked by prazosin. The present results indicate that noradrenergic neurons modulate the dopamine cell firing pattern via excitatory postsynaptic alpha 1-adrenoceptors. This mechanism might be involved in the pathogenesis and pharmacological treatment of schizophrenia.

Amphetamine injected into the ventral tegmental area sensitizes the nucleus accumbens dopaminergic response to systemic amphetamine: an in vivo microdialysis study in the rat

Different groups of rats received three injections of either D-amphetamine (2.5 micrograms/0.5 microliters/side) or saline into the ventral tegmental area (VTA), one injection every third day. Two weeks following the last injection, dopamine (DA) neurotransmission in the nucleus accumbens (N. Acc.) was assessed with in vivo microdialysis before and after a challenge with systemic D-amphetamine (1.0 mg/kg, i.p.). Prior to challenge, basal extracellular concentrations of DA in the N.Acc. did not differ in VTA amphetamine- and saline-preexposed animals. Following challenge, however, both groups showed an increase in N.Acc. DA but this was significantly greater (2-fold) in VTA amphetamine-preexposed animals. These latter animals also showed significantly higher DA metabolite levels in comparison to saline-preexposed animals prior to (DOPAC) as well as after challenge (HVA). These findings extend those of behavioral experiments showing that intra-VTA amphetamine produces sensitized locomotor responding to drug challenge (J. Pharmacol. Exp. Ther., 245 (1988) 1095-1102; Brain Res., 516 (1990) 99-106; Ann. NY Acad. Sci., 654 (1992) 444-447) and demonstrate that amphetamine applied to the somatodendritic region of mesolimbic DA neurons sensitizes these neurons as evidenced by their enhanced N.Acc. DA response to a systemic amphetamine challenge.

Comparison of the effects of the cholecystokinin-B receptor antagonist, PD 134308, and the cholecystokinin-A receptor antagonist, L-364,718, on dopamine neuronal activity in the substantia nigra and ventral tegmental area

Extracellular single-unit recording techniques were used to study the effects of the cholecystokinin-A (CCK-A) antagonist, L-364,718, and the CCK-B antagonist, PD 134308, on DA neuronal activity in chloral hydrate anesthetized rats. Neither L-364,718 (0.1-1.6 mg/kg i.v.) nor PD 134308 (0.1-6.4 mg/kg) altered the basal firing rate of substantia nigra or ventral tegmental area DA neurons. The ability of PD 134308 and L-364,718 to alter the apomorphine-induced inhibition of substantia nigra DA neurons was assessed. Pretreatment with L-364,718 (0.6 or 4.16 mg/kg i.v.) did not shift the apomorphine dose-response curve (0.5-32 micrograms/kg i.v.). In contrast, PD 134308 (0.6 or 6.4 mg/kg i.v.) produced dose-related, significant shifts to the right of the apomorphine dose-response curves. However, these effects were small in comparison to the haloperidol (0.1 mg/kg i.p.)-induced shift of the apomorphine curve. These data suggest that in the substantia nigra there may be a tonic level of CCK release that, through actions on CCK-B receptors, may modulate DA agonist-induced inhibition of DA neuronal activity.

Effects of dizocilpine (MK-801) on rat midbrain dopamine cell activity: differential actions on firing pattern related to anatomical localization

The effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine ((+)-MK-801) on the firing pattern of midbrain dopamine neurons were studied with single cell recording techniques in male albino rats anaesthetized with chloral hydrate. The extracellularly recorded electrical activity of single, identified dopamine neurons was studied with respect to firing rate, burst firing and regularity of firing. MK-801 (0.01-1.0 mg/kg IV) induced different effects in different subgroups of midbrain dopamine neurons. In the substantia nigra, firing rate was increased while the pattern was regularized and burst firing slightly increased. In the ventral tegmental area, firing rate and regularity of firing was also increased while effects on burst firing were bidirectional. Histological inspections revealed that neurons which responded with an increase in burst firing were mainly located in the nucleus paranigralis subdivision of the ventral tegmental area, while cells responding with a decrease were predominantly found in the nucleus parabrachialis pigmentosus subdivision. The effects of MK-801 were similar to previously described effects of phencyclidine, another non-competitive NMDA antagonist. The present effects of MK-801 might shed some light on the mechanisms involved in psychotic symptoms induced by phencyclidine and other non-competitive NMDA antagonists.

MK-801, phencyclidine (PCP), and PCP-like drugs increase burst firing in rat A10 dopamine neurons: comparison to competitive NMDA antagonists

Extracellular single-unit recordings were used to assess the effects of PCP and PCP-like drugs (MK-801 and TCP) on the burst firing of ventral tegmental A10 dopamine neurons in the rat. The effects of these noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists were compared to the potent and competitive NMDA antagonists CGS 19755 and (+/-)CPP, and to BTCP, a PCP-derivative possessing little affinity for the PCP binding site within the ion channel gated by NMDA. PCP, MK-801, and TCP produced dose-dependent increases in the firing rate, which were accompanied by increases in the amount of burst activity, the number of action potentials within a burst, and the conversion of nonbursty cells to bursty. However, the coefficient of variation, a measure of the regularity of firing, was not significantly altered. These predominately excitatory effects contrast with the inhibition of firing, decrease in bursting, and regularization of pattern produced by BTCP. CGS 197555 and (+/-)CPP failed to alter any of the measured parameters. Thus, the increase in firing rate and amount of burst activity of dopamine neurons produced by PCP and PCP-like drugs, and the resultant hyperdopaminergia within the mesolimbic-mesocortical regions, could underlie the psychotomimetic properties of these compounds. Moreover, this effect would not appear to be related to a loss of activity at the NMDA recognition site, as evidenced by the lack of effect of the competitive NMDA antagonists.

Differential development of autoreceptor subsensitivity and enhanced dopamine release during amphetamine sensitization

Various changes in the function of dopamine neurons have been proposed to underly the development of behavioral sensitization to the locomotor stimulant effects of d-amphetamine. The present study examined the relative importance of two such mechanisms after both short (3-4 days off) and longer (10-14 days off) withdrawals from repeated amphetamine or saline injection (1 mg/kg/day, days 1-5 and 8-12). First, single-unit recording was used to examine the sensitivity of impulse-regulating somatodendritic autoreceptors located on mesoaccumbens dopamine neurons in the rat ventral tegmental area. Second, in vivo microdialysis was used to examine the ability of amphetamine challenge to increase extracellular dopamine levels in the rat nucleus accumbens. Amphetamine-treated rats exhibited robust behavioral sensitization at both time points as compared to saline-treated rats. At 3 to 4 days off, autoreceptor subsensitivity was observed in the ventral tegmental area of amphetamine-treated rats, but there was no significant change in the ability of amphetamine to increase extracellular dopamine levels in nucleus accumbens. However, after 10 to 14 days off, autoreceptor subsensitivity was no longer observed, but amphetamine challenge resulted in a significantly greater increase in extracellular dopamine levels in amphetamine-treated as compared to saline-treated rats. These findings suggest that autoreceptor subsensitivity is a transient effect which may be related to the development of sensitization, whereas enhancement of amphetamine-stimulated dopamine release does not accompany early stages of behavioral sensitization, but may be involved in the persistence of the phenomenon after longer withdrawal periods.

Effect of inhibiting enkephalin catabolism in the VTA on motor activity and extracellular dopamine

The mixed inhibitor of enkephalin catabolism, kelatorphan, was microinjected into the ventral tegmental area (VTA) of rats to determine if endogenous enkephalins can modulate dopamine transmission in the mesoaccumbens projection. The concentration of extracellular dopamine content in the nucleus accumbens was monitored using in vivo microdialysis simultaneously with measuring motor behavior. Kelatorphan microinjection into the VTA produced a dose-related increase in motor activity and extracellular dopamine in the nucleus accumbens. While the change in extracellular dopamine was modest as compared to exogenous stimulation by a mu agonist such as DAMGO, there was a marked increase in the extracellular content of dopamine and serotonin metabolites. This suggests that mesoaccumbens dopamine transmission is under tonic control of endogenous enkephalins at the ventral tegmental area level.

Modulation of desynchronized sleep through microinjection of alpha 1-adrenergic agonists and antagonists in the dorsal pontine tegmentum of the cat

Noradrenaline is involved in the regulation of the sleep/waking cycle by acting through various receptor types. In previous studies we investigated the role of beta- and alpha 2-adrenergic receptors through local microinjections of various drugs into the dorsal pontine tegmentum (DPT) of the cat. This region is known to be crucially involved in desynchronized sleep execution. In this study we examined the role of alpha 1-adrenergic receptors. The alpha 1-agonist methoxamine and the alpha 1-antagonist prazosin were injected into the DPT of freely moving, unanaesthetized cats. We found that methoxamine notably reduced desynchronized sleep, and that this effect was both dose-dependent and site-specific. These effects were prevented by the subsequent injection of prazosin. On the other hand, the injection into the DPT of prazosin alone produced scarce or inconsistent effects on the sleep/waking cycle.

Acute depolarization block of A10 dopamine neurons: interactions of morphine with dopamine antagonists

Extracellular single-unit recording techniques were used to determine the effects of morphine, administered either systematically (intravenous) or locally (microiontophoresis), on ventral tegmental area (A10) dopamine (DA) neuronal activity in animals pretreated with D1 (SCH 23390) or D2 (pimozide) DA receptor antagonists. In rats pretreated with the D2 antagonist pimozide, A10 DA neurons readily entered a state of apparent depolarization block in response to either i.v. or iontophoretically applied morphine. Whether the inactivation of DA neurons was induced by systemic or local morphine, it was reversed in 22 of 27 cases by iontophoretic administration of the inhibitory amino acid transmitter gamma-aminobutyric acid (GABA), suggesting depolarization block as the underlying mechanism. Pretreatment of rats with the D1 antagonist SCH 23390 did not significantly alter the tendency of A10 DA neurons to enter apparent depolarization block in response to morphine. These data support recent behavioral evidence suggesting that the combination of systemic pimozide and ventral tegmental area morphine can result in depolarization inactivation of the mesoaccumbens DA reward system.

Regulatory behaviour, exploration and locomotion following NMDA or 6-OHDA lesions in the rat nucleus accumbens

The effects of bilateral, N-methyl-D-aspartate (NMDA)-induced lesions of the nucleus accumbens (N.Acc.) on regulatory and behavioural responding were studied in rats and compared with the effects of bilateral 6-hydroxydopamine (6-OHDA) lesions. After postoperative body weight, food and water intake had been monitored for a period of 4 weeks, rats were tested in an exploration-choice box. Spontaneous locomotion and the locomotor and stereotypy responses to different doses of dopaminergic agonists were measured subsequently. Detailed assessment of NMDA-induced lesion volumes showed that on average 81.53% of total N.Acc. area was damaged, depending on excitotoxin dose. Tyrosine hydroxylase immunohistochemistry was used to confirm loss of mesolimbic dopamine neurones following 6-OHDA. Analysis of the behavioural data showed that NMDA N.Acc. lesions significantly enhanced exploratory behaviour, spontaneous locomotor activity and the locomotor response to a low dose of D-amphetamine. By comparison, 6-OHDA lesions did not affect exploration and spontaneous locomotion but significantly attenuated the locomotor response to a low dose of D-amphetamine. Regulatory responses were unaffected 28 days after surgery, although NMDA-lesioned rats took longer to recover from postoperative hypodipsia. The results suggest that NMDA N.Acc. lesions induce a deficit in the control of general locomotor output and are consistent with the hypothesis that the N.Acc. functions as an interface between sensory input and locomotor output and that it is needed to channel activity levels appropriately.

The stimulation of central kappa opioid receptors decreases male sexual behavior and locomotor activity

Systemic injections of the kappa (kappa) opioid receptor agonist U-50,488H decreased male sexual behavior, locomotor activity, body temperature and bodily grooming, and induced body flattening. The U-50,488H-induced inhibitions of male sexual behavior were prevented by systemic injections of naloxone and by intra-cranial injections of the kappa opioid antagonist nor-binaltorphimine (NBNI). Injections of NBNI to either the ventral tegmental area (VTA) or the nucleus accumbens septi (NAS) increased female-directed behavior, and prevented the U-50,488H-induced decreases in female-directed behavior. Intra-VTA NBNI prevented U-50,488H-induced decreases in the mean number of ejaculations, intra-NAS NBNI prevented U-50,488H-induced increases in copulation latencies. Intra-medial preoptic area (mPOA) injections of NBNI increased female-directed behavior, and attenuated U-50,488H-induced decreases in female-directed behavior as well as U-50,488H-induced increases in both copulation and ejaculation latencies. Injections of NBNI dorsal to the mPOA were ineffective. Two of 26 days following the central injection of NBNI, systemic injections of U-50,488H remained behaviorally ineffective, leaving both sexual behavior and locomotor activity undiminished. These results suggest that the stimulation of central kappa opioid receptors inhibits sexual behavior in the male rat; perhaps endogenous kappa opioid agonists induce sexual refractory periods.

Cocaine's colocalized effects on synaptic serotonin and dopamine in ventral tegmentum in a reinforcement paradigm

The effect of subcutaneous (SC) cocaine (20 mg/kg) on synaptic concentrations of the biogenic amines, dopamine (DA), and serotonin (5-HT) in Ventral Tegmental Area, (VTA-[A10]) was studied in freely moving and behaving rats (rattus norvegicus) with in vivo voltammetry (in vivo electrochemistry). The actual detection of the biogenic amines was on-line and within a temporal resolution of seconds. Simultaneously, the psychostimulant behavior induced by cocaine was studied by infrared photocell beam detection. The results show that cocaine concurrently and significantly increased synaptic concentrations of DA (p less than 0.0001) and 5-HT (p less than 0.004) in VTA. Serotonin changes were accompanied by a notable oscillatory pattern. Importantly, DA and 5-HT changes in VTA were significantly and positively correlated (p less than 0.01). Moreover, psychostimulant behaviors induced by cocaine were significantly increased over control values (p less than 0.0001). Psychostimulant behaviors were significantly correlated with concurrently changing synaptic concentrations of DA (p less than 0.01) and also with 5-HT to a lesser degree. Additionally, behavioral data indicate that cocaine may exhibit an anxiolytic effect during acute administration because agoraphobic behavior, as shown by increased central ambulatory behavior, was dramatically reduced by cocaine. Summarily, the present findings show that cocaine increased synaptic concentrations of DA in VTA, an action that is correlated with cocaine-induced psychostimulant behavior. The DA-ergic effect appears to be tonically maintained. Furthermore, new findings demonstrate a colocalized, cocaine induced 5-HT-ergic effect in VTA, which keeps pace with cocaine-induced alterations in DA-ergic neurotransmission. Thus, 5-HT may be a relay or a gating mechanism for a DA reward signalling pathway for cocaine.

A comparison of the effects of mesencephalic injections of neurotensin(1-13) and neuromedin N on brain electrical self-stimulation

Neuromedin N (NM-N), a hexapeptide that shares a four amino acid C-terminal homology with the tridecapeptide, neurotensin (NT), has been suggested as a potential neurotransmitter or neuromodulator that could interact with the NT-sensitive receptors. In this experiment, we compared the effects of an equimolar concentration of NM-N and NT(1-13) injected in the ventral tegmental area (VTA) on brain electrical self-stimulation (SS), a behavior previously shown to be potentiated by VTA injections of NT(1-13). Rats implanted with a stimulating electrode in the mesencephalic central gray and a guide cannula in the VTA were trained to lever press to obtain rewarding electrical stimulations. Functions relating the rate of lever pressing to the stimulation frequency were determined, on separate daily tests, before and after the injection of 3 nmol of NM-N, NT(1-13), or an equal volume of saline vehicle. At this concentration, both NM-N and NT(1-13) produced a significant facilitation of SS when compared to saline vehicle, an effect that was not seen when the peptides were injected outside the VTA. The facilitation of SS by NM-N, however, was much weaker and of a shorter duration than the one produced by NT(1-13). The shorter time course and the weaker behavioral effect of NM-N compared to NT(1-13) are consistent with its lower potency at the NT receptor and its faster rate of enzymatic degradation in the VTA, and suggest that NM-N potentiated the reward-relevant neural signal by acting on mesencephalic NT receptors.

Lack of tolerance to ethanol-induced stimulation of mesolimbic dopamine system

Rats maintained for 10 days on a 10% ethanol solution as the sole source of fluid developed marked tolerance to ethanol-induced loss of righting reflex, but no tolerance to the stimulatory effects on mesolimbic dopaminergic system. An ethanol challenge stimulated both the electrical activity of A10 dopaminergic cells and dopamine output in the ventral striatum of behaviourally tolerant animals and of controls to the same extent. These results are compatible with the hypothesis that an increased dopamine neurotransmission in the limbic system participates in the reinforcing effect of ethanol.

Regulation of somatodendritic dopamine release in the ventral tegmental area by opioids and GABA: an in vivo microdialysis study

Microdialysis of the ventral tegmental area in conscious rats was used to evaluate the influence of opioids and GABA agonists on extracellular levels of GABA and somatodendritically released dopamine. The administration of morphine through the dialysis probe elicited significant, dose-dependent increases in the levels of extracellular dopamine and significantly reduced the extracellular concentration of GABA. In contrast, a dose-dependent decrease in somatodendritic extracellular dopamine was produced following the administration of the GABAB agonist baclofen. The increase in dopamine levels elicited by morphine (100 microM) was completely blocked by either baclofen (100 microM) coadministration or peripheral injection of naloxone (2 mg/kg, i.p.). Application of the GABAA agonist muscimol produced a significant increase in both extracellular levels of dopamine and locomotor activity. The present results, together with other electrophysiological, neurochemical, and behavioral data, support a hypothesis that stimulation of mu-opioid or GABAA receptors inhibits the activity of GABAergic afferents to dopamine neurons, thereby removing tonic inhibitory regulation, whereas stimulation of GABAB receptors directly inhibits dopamine neurons.