Electrolytic lesions of the substantia innominata and lateral preoptic area attenuate the 'supersensitive' locomotor response to apomorphine resulting from denervation of the nucleus accumbens

Orexin-1 Receptor Signaling in Ventral Pallidum Regulates Motivation for the Opioid Remifentanil

Signaling at the orexin-1 receptor (OxR1) is important for motivated drug taking. Using a within-session behavioral economics (BE) procedure, we previously found that pharmacologic blockade of the OxR1 decreased motivation (increased demand elasticity) for the potent and short-acting opioid remifentanil and reduced low-effort remifentanil consumption. However, the mechanism through which orexin regulates remifentanil demand is currently unknown. Previous work implicated OxR1 signaling within ventral pallidum (VP) as a potential target. VP is densely innervated by orexin fibers and is known to regulate opioid reward. Accordingly, this study sought to determine the role of VP OxR1 signaling in remifentanil demand and cue-induced reinstatement of remifentanil seeking in male rats. Intra-VP microinjections of the OxR1 antagonist SB-334867 (SB) decreased motivation (increased demand elasticity; α) for remifentanil without affecting remifentanil consumption at low effort. Baseline α values predicted the degree of cue-induced remifentanil seeking, and microinjection of SB into VP attenuated this behavior without affecting extinction responding. Baseline α values also predicted SB efficacy, such that SB was most effective in attenuating reinstatement behavior in highly motivated rats. Together, these findings support a selective role for VP OxR1 signaling in motivation for the opioid remifentanil. Our findings also highlight the utility of BE in predicting relapse propensity and efficacy of treatment with OxR1 antagonists.SIGNIFICANCE STATEMENT Abuse of opioids has risen rapidly and continues to be a major health crisis. Thus, there is an urgent need to better understand the neurobiological and behavioral mechanisms underlying opioid addiction. Here, we investigate the role of orexin-1 receptor signaling (OxR1) within ventral pallidum (VP) in remifentanil demand and cue-induced reinstatement of remifentanil seeking. Using a within-session behavioral economics procedure, we show that intra-VP microinjections of the OxR1 antagonist SB-334867 decreased motivation (increased demand elasticity) without affecting remifentanil consumption at low effort. We also found that SB microinjected intra-VP attenuated cue-induced reinstatement of remifentanil seeking. Together, our results support a role for VP OxR1 signaling in opioid reward.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Estradiol-sensitive projection neurons in the female rat preoptic area

Electrical stimulation of the preoptic area (POA) interrupts the lordosis reflex, a combined contraction of back muscles, in response to male mounts and the major receptive component of sexual behavior in female rat in estrus, without interfering with the proceptive component of this behavior or solicitation. Axon-sparing POA lesions with an excitotoxin, on the other hand, enhance lordosis and diminish proceptivity. The POA effect on the reflex is mediated by its estrogen-sensitive projection to the ventral tegmental area (VTA) as shown by the behavioral effect of VTA stimulation as well as by the demonstration of an increased threshold for antidromic activation of POA neurons from the VTA in ovariectomized females treated with estradiol benzoate (EB). EB administration increases the antidromic activation threshold in ovariectomized females and neonatally castrated males, but not in neonatally androgenized females; the EB effect is limited to those that show lordosis in the presence of EB. EB causes behavioral disinhibition of lordosis through an inhibition of POA neurons with axons to the VTA, which eventually innervate medullospinal neurons innervating spinal motoneurons of the back muscle. The EB-induced change in the threshold or the axonal excitability may be a result of EB-dependent induction of BK channels. Recordings from freely moving female rats engaging in sexual interactions revealed separate subpopulations of POA neurons for the receptive and proceptive behaviors. Those POA neurons engaging in the control of proceptivity are EB-sensitive and project to the midbrain locomotor region (MLR). EB thus enhances lordosis by reducing excitatory neural impulses from the POA to the VTA. An augmentation of the POA effect to the MLR may culminate in an increased locomotion that embodies behavioral estrus in the female rat.

Regional and cell-type-specific effects of DAMGO on striatal D1 and D2 dopamine receptor-expressing medium-sized spiny neurons

The striatum can be divided into the DLS (dorsolateral striatum) and the VMS (ventromedial striatum), which includes NAcC (nucleus accumbens core) and NAcS (nucleus accumbens shell). Here, we examined differences in electrophysiological properties of MSSNs (medium-sized spiny neurons) based on their location, expression of DA (dopamine) D1/D2 receptors and responses to the μ-opioid receptor agonist, DAMGO {[D-Ala²-MePhe⁴-Gly(ol)⁵]enkephalin}. The main differences in morphological and biophysical membrane properties occurred among striatal sub-regions. MSSNs in the DLS were larger, had higher membrane capacitances and lower Rin (input resistances) compared with cells in the VMS. RMPs (resting membrane potentials) were similar among regions except for D2 cells in the NAcC, which displayed a significantly more depolarized RMP. In contrast, differences in frequency of spontaneous excitatory synaptic inputs were more prominent between cell types, with D2 cells receiving significantly more excitatory inputs than D1 cells, particularly in the VMS. Inhibitory inputs were not different between D1 and D2 cells. However, MSSNs in the VMS received more inhibitory inputs than those in the DLS. Acute application of DAMGO reduced the frequency of spontaneous excitatory and inhibitory postsynaptic currents, but the effect was greater in the VMS, in particular in the NAcS, where excitatory currents from D2 cells and inhibitory currents from D1 cells were inhibited by the largest amount. DAMGO also increased cellular excitability in the VMS, as shown by reduced threshold for evoking APs (action potentials). Together the present findings help elucidate the regional and cell-type-specific substrate of opioid actions in the striatum and point to the VMS as a critical mediator of DAMGO effects.

Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex

Anatomical and functional refinements of the meso-limbic dopamine system of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas the anteromedial VTA has few if any projections to the ventral striatum, and the lateral VTA largely projects to the ventrolateral striatum (accumbens core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into the anterior VTA. A review of the literature suggests that (1) the midbrain has corresponding zones for the accumbens core and medial shell; (2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and (3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed learning than the lateral projection system, as described in the variation-selection hypothesis of striatal functional organization.

P2 receptor-mediated effects on the open field behaviour of rats in comparison with behavioural responses induced by the stimulation of dopamine D2-like and by the blockade of ionotrophic glutamate receptors

The effects of the P2 receptor ligands 2-methylthio ATP (2-MeSATP; 10 pmol)--as a non-specific agonist--and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10 pmol)--as a non-selective antagonist--after bilateral intra-accumbens injection on the locomotor response were investigated in an open field situation. The P2 receptor-mediated effects on the pattern of locomotor activity were compared with the effects caused by the dopamine D2-like receptor agonist quinpirole (10 pmol) and by the combination of the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 10 pmol) with the alpha-amino-3-hydro-5-methyl-4-isoxazolpropionic acid (AMPA) and kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 30 pmol). The intra-accumbens injection of all tested compounds elicited an increase in the locomotor activity over a test period of 20 min when compared with the controls. No statistically significant differences could be evaluated between the different drug-treated groups. However, a more detailed analysis--using further behavioural parameters such as the number of movement direction changes, the effective running time and the running speed--revealed two basically different patterns of locomotor activity. The locomotor response induced by the injection of 2-MeSATP or quinpirole was characterised by a continuous and consistent locomotion, whereas the enhanced locomotor activity elicited by PPADS or CPP/CNQX was determined by an increased running speed accompanied by more disruptions and more changes of movement direction. The coadministration of 2-MeSATP and quinpirole led to an enhancement of locomotor activity in a limited post-treatment interval. The effects of both compounds could be abolished by the pre-treatment with the D2/D3 receptor antagonist sulpiride (100 pmol). Coadministration of PPADS and CPP/CNQX caused additive effects suggesting that the pathway mediated by P2 and ionotrophic glutamate receptors is different. The stimulation of P2 receptors in the nucleus accumbens (NAc) modulates the locomotion in the direction to be to be longer lasting, more consistent and more goal directed.

Distinct pattern of c-fos mRNA expression after systemic and intra-accumbens amphetamine and MK-801

Pharmacological manipulation of both dopamine and glutamate systems affects motor responses in laboratory animals. The two systems, however, seem to act in opposite ways, since direct or indirect activation of dopamine receptors induces similar stimulatory effects to those seen following blockade of N-methyl-D-aspartate receptors. In the present study we compared the pattern of c-fos activation induced by systemic and intra-accumbens administration of the non-competitive N-methyl-D-aspartate antagonist MK-801 and the indirect dopamine agonist amphetamine. Systemic MK-801 induced c-fos mRNA expression in the motor cortex and preferentially in the motor thalamus, i.e. ventrolateral nucleus. Systemic amphetamine, on the other hand, enhanced c-fos mRNA expression in the shell of the accumbens and in limbic thalamic nuclei such as the anteroventral and anterodorsal nuclei. The main effect observed after intra-accumbens administrations of either drug was enhanced c-fos expression in the thalamus, somewhat similar to what seen following systemic administration. In fact also in this case there was a preferential activation of the limbic thalamus by amphetamine and the motor thalamus by MK-801. The present results confirm that different neural substrates underlie behavioral effects induced by systemic administrations of N-methyl-D-aspartate receptor antagonists and dopamine agonists. Further they suggest that intra-accumbens manipulation of the two neural systems could affect different efferent pathways from this structure activating different thalamic targets.

Locomotor activity induced by the non-competitive N-methyl-D-aspartate antagonist, MK-801: role of nucleus accumbens efferent pathways

We have recently shown that focal administration of dizocilpine hydrogen maleate (MK-801, a non-competitive N-methyl-D-aspartate antagonist) within the nucleus accumbens increases locomotor activity in a dopamine-independent manner. The purpose of this study was to investigate the neural network underlying locomotor stimulation induced by N-methyl-D-aspartate receptor blockade in the accumbens. In the first experiment, we examined the effect of different doses (1, 5 and 25 nmol) of the active and inactive enantiomers of the N-methyl-D-aspartate antagonist, (+)- and (-)-MK-801, respectively, focally administered in the nucleus accumbens. Only the active enantiomer induced a significant increase in locomotor activity; furthermore, the effect induced by the two highest doses of (+)-MK-801 was significantly different from that induced by (-)-MK-801. In the second part of the study, we performed ibotenic acid lesions to the major output nuclei of the accumbens, the ventral pallidum, mediodorsal thalamus, ventrolateral/ventromedial thalamus and pedunculopontine tegmental nucleus, to observe their effect on locomotor activity induced by focal (+)-MK-801 (25 nmol) administration into the accumbens. None of the lesions had any effect on spontaneous locomotor activity. Hyperactivity induced by accumbens MK-801 administrations was unaffected by ibotenic acid lesions of the pedunculopontine tegmental nucleus, while lesions of the mediodorsal thalamus induced only a partial inhibition. In contrast, ibotenic acid lesions of the ventral pallidum and ventrolateral/ventromedial thalamus completely blocked the motor response induced by accumbens MK-801. These data indicate that the intact mediodorsal thalamus, which has been proposed as a part of the loop that relays accumbens information to the prefrontal cortex, does not seem to be a structure of primary importance in MK-801 locomotor activity. On the contrary, the motor nuclei of the thalamus appear to play a more relevant role, suggesting that different neural substrates may mediate dopamine and glutamate functional output from the nucleus accumbens.

Effect of intracerebral administration of NMDA and AMPA on dopamine and glutamate release in the ventral pallidum and on motor behavior

The present study investigates the modulation of the ventral tegmental area (VTA)-ventral pallidum (VP) dopaminergic system by glutamate agonists in rats. The glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were infused via reversed microdialysis into the VTA, and dopamine (DA), glutamate, and aspartate levels in the VTA and ipsilateral VP were monitored together with motor behavior screened in an open field. NMDA (750 microM) infusion, as well as AMPA (50 microM) infusion, induced an increase of DA and glutamate levels in the VTA, followed by an increase of DA levels in the ipsilateral VP and by enhanced locomotor activity. The increase of DA in the VP was similar after administration of these two glutamate agonists, although motor activity was more pronounced and showed an earlier onset after NMDA infusion. Glutamate levels in the VP were not increased by the stimulation of DA release. It is concluded that DA is released from mesencephalic DA neurons projecting to the VP and that these neurons are controlled by glutamatergic systems, via NMDA and AMPA receptors. Thus, DA in the VP has to be considered as a substantial modulator. Dysregulation of the mesopallidal DA neurons, as well as their glutamatergic control, may play an additional or distinct role in disorders like schizophrenia and drug addiction.

Neuronal activity in female rat preoptic area associated with sexually motivated behavior

Single unit activities were recorded from 31 neurons in the preoptic area (POA) of female rats engaging in sexual interactions. Concurrent videotape recordings were used to establish a relationship between neuronal activity and particular behavioral events. In 14 of the 31 neurons, the firing rate changed in association with bouts of sexual activity. The remaining 17 fired with more variability regardless of episodes of sexual interactions. Peri-event histograms identified four types of neurons: type 1 (n=4) increased their firing rate when the female rats initiated proceptive behavior; type 2 (n=4) showed a brief activation when the male mounted; type 3 (n=4) fired in response to intromission, and type 4 (n=2) were inhibited prior to and throughout the display of lordosis reflex. Type 1 neurons fired at significantly higher rates during the solicitatory period, from the initiation of solicitatory locomotion to the male mounts. Their activity was suppressed when the males mounted successfully with intromission. Types 1-3 neurons were recorded from the transitional region between the medial and lateral POAs. Type 4 neurons were located more medially in the medial POA. Systemic injection of pimozide, a dopamine receptor blocker, diminished firing in type 1 neurons and abolished proceptivity. The firing pattern in type 1 neurons appeared to embody the motivational state of the animal with an implication for a consummatory value of penile intromission. Visceral or somatosensory inputs may be responsible for short bursts in types 2 and 3 neurons. Type 4 neurons behaved exactly as if they inhibit the execution of the lordosis reflex. The results showed separate sets of POA neurons each specifically associated with proceptive and receptive components of female rat sexual behavior.

The ventral pallidum mediates disruption of prepulse inhibition of the acoustic startle response induced by dopamine agonists, but not by NMDA antagonists

Prepulse inhibition (PPI) of the acoustic startle response is observed when the startling noise pulse is preceded by a weak, non-startling stimulus. PPI has been considered as a measure for sensorimotor gating mechanisms. Disruption of PPI can be found in schizophrenic patients as well as after blockade of NMDA receptors or stimulation of dopamine receptors in rats. The neuronal circuitry which regulates PPI consists of cortico-limbic brain structures where the nucleus accumbens (NAC) plays a key role. The NAC exerts its modulating effects on PPI by way of a projection from the ventral pallidum (VP) to the pedunculopontine tegmental nucleus (PPTg). We recently postulated that the reduction of PPI by intra-NAC infusion of glycine-site NMDA antagonists is not mediated by the VP. We tested here this hypothesis in rats with excitotoxic lesions of the VP which were systemically treated with apomorphine or MK-801 or received intraNAC infusions of dopamine or the glycine-site NMDA antagonist 7-chlorokynurenic acid. Lesioned rats showed a marked deficit in PPI after MK-801 and 7-chlorokynurenate treatment but not after apomorphine or dopamine injection, in contrast to sham-lesioned controls showing deficits in PPI under all conditions. These data provide behavioral evidence for the existence of a pathway which does not include the VP for the mediation of sensorimotor gating deficits. We propose that a direct connection between the NAC and PPTg may be responsible for the effects of NMDA/glycine receptor blockade, whereas the VP is an indispensable relay for the disruptive effects on PPI exerted by the NAC dopamine system.

Different neural mechanisms underlie dizocilpine maleate- and dopamine agonist-induced locomotor activity

This study evaluated and compared the role of mesoaccumbens dopamine and the ventral pallidal region in the locomotor stimulatory action of the non-competitive N-methyl-D-aspartate antagonist dizocilpine maleate and dopamine agonists. Intra-accumbens injections of both amphetamine (1, 5 and 25 nmol) and dizocilpine maleate (1, 5, 25 and 50 nmol) induced a dose-dependent increase in locomotor activity. The N-methyl-D-aspartate antagonist was somewhat less effective than amphetamine. 6-Hydroxydopamine dopamine-depleting lesions of the nucleus accumbens completely blocked locomotor stimulation induced by focal administrations of amphetamine (5 nmol), but were ineffective in altering the actions of dizocilpine maleate (50 nmol). Ibotenic acid lesions of the ventral pallidal region and muscimol injections into this area also prevented the stimulatory effects of systemic amphetamine (1 mg/kg), while having no effect on the locomotor-activating actions of systemic dizocilpine maleate (0.3 mg/kg). Microdialysis studies revealed that systemically administered apomorphine (2 mg/kg) significantly decreased extracellular GABA in the pallidum, which was accompanied by substantial increases in locomotor output. Systemically administered dizocilpine maleate (0.3 mg/kg), on the other hand, also increased locomotor activity without having any effect on pallidal GABA. These data, taken together, indicate that while the locomotor effects of dopamine agonists are dependent upon intact mesoaccumbens dopamine and involve GABAergic efferents from the nucleus accumbens to the ventral pallidum, dizocilpine maleate's stimulatory actions are independent of such mechanisms.

Dissociation of locomotor and conditioned place preference responses following manipulation of GABA-A and AMPA receptors in ventral pallidum

1. This study examined the roles of GABAergic and glutamatergic neurotransmission in ventral pallidum (VP) in conditioned place preference and locomotor activity. 2. Picrotoxin (0.1 microgram), a GABA antagonist, and (+/-)alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 0.14 microgram), a non-NMDA glutamatergic agonist, were injected bilaterally into VP through implanted cannulae. 3. Both drugs produced a robust increase in locomotion, but neither produced conditioned place preference. 4. These results suggest a dissociation of locomotor activity and reward at the level of ventral pallidum. In addition, it was argued that the GABAergic projection from nucleus accumbens to ventral pallidum may not be involved in the processing of reward initiated from dopaminergic activation in nucleus accumbens.

6-Hydroxydopamine lesion of ventral pallidum blocks acquisition of place preference conditioning to cocaine

In parallel with nucleus accumbens (NAS), ventral pallidum (VP) also receives a dopaminergic projection from the ventral tegmental area (VTA). The present study examined the involvement of this mesopallidal dopaminergic system in the action of cocaine. In the first experiment, the effect of cocaine injections on VP dopamine was examined by microdialysis. Intraperitoneal (i.p.) injections of cocaine 5-20 mg/kg dose-dependently increased the extracellular dopamine level in VP 2.5-4.5-fold. In addition, intra-VP perfusion of 20 microM cocaine induced a 12-fold increase of dopamine locally. The second experiment examined the role of VP dopamine in cocaine-induced conditioned place preference (CPP) and locomotor activation. Rats received bilateral intra-VP injections of 3-4 microg 6-OHDA or ascorbic acid vehicle in 0.5 microl volume. Tissue assays indicated that the 6-OHDA-lesioned rats had significantly lowered dopamine concentration in VP, but not in NAS or striatum. As a group, 6-OHDA lesions blocked the development of CPP to 5 mg/kg cocaine but not to 10 mg/kg cocaine. However, rats with more than 60% depletion in VP dopamine did not develop CPP to cocaine at either dose. Preference for the cocaine-paired side correlated significantly with dopamine concentration in VP, but not in NAS or striatum. It was concluded that VP dopamine may play a critical role in the initial rewarding effect of cocaine. 6-OHDA lesions also blocked locomotor activation induced by 5 mg/kg cocaine but had no effect on 10 mg/kg cocaine-induced locomotion. Dopamine concentration in VP did not correlate with the locomotor activation response to cocaine at either dose. These findings further establish the involvement of the mesopallidal dopaminergic system in the action of cocaine.

Contribution of the nucleus accumbens to cocaine-induced responses of ventral pallidal neurons

The present study characterized the responses of ventral pallidal (VP) neurons to intravenously (iv) administered cocaine (0.003, 0.01, 0.03, 0.1, 0.3, and 1.0 mg/kg) in chloral hydrate-anesthetized rats. Eighty-four percent (16/19) of the tested neurons displayed rate changes following cocaine administration. Fifty-three percent responded by increasing firing rate, with an EMAX of 217 +/- 26% of basal activity and an ED50 of 0.07 +/- 0.03 mg/kg. Neurons that responded with a rate decrease (26%) had an EMAX of 14.3 +/- 9.0% of basal control and an ED50 of 0.04 +/- 0.02 mg/kg. One neuron (5%) displayed a biphasic response pattern. Haloperidol (0.2 mg/kg) attenuated cocaine-induced effects in 90% of the tested neurons. Given the responsiveness of VP neurons to cocaine, the extensive innervation of the VP by the nucleus accumbens (NAC), and the importance of the NAC in regulating cocaine-induced effects, it is likely that NAC activity may affect VP responses to cocaine. To test this possibility, the influence of NAC on cocaine-induced VP activity was evaluated. Unilateral inactivation of the NAC with microinjections of procaine (40 mu g/2 mu l/2 min) did not alter the proportion of VP neurons responsive to subsequent systemic administration of cocaine (0.1, 1.0 mg/kg iv) or the EMAX for those neurons showing a rate decrease. However, for the population of neurons showing a cocaine-induced rate increase, intra-NAC procaine significantly enhanced EMAX to 392 +/- 74% of control. These data suggest that the ability of VP neurons to respond to iv cocaine is independent of the NAC. However, the magnitude of the cocaine-induced effect appears to be dependent on NAC influences.

Conditioned place preference and locomotor activation produced by injection of psychostimulants into ventral pallidum

The ventral pallidum (VP) is often viewed as an output structure of the nucleus accumbens septi (NAS). However, VP, like NAS, receives a dopaminergic input from the ventral tegmental area. These experiments investigated some behavioral effects of microinjection into VP of drugs which enhance dopaminergic transmission. Injection of 25 micrograms dopamine or 5-10 micrograms amphetamine into VP produced hypermotility. In contrast, injection of 12.5-50 micrograms cocaine initially suppressed, then increased, activity. Injection of 100 micrograms cocaine only produced hypomotility in the 1-h period examined. The hypomotility following cocaine seemed to be a local anesthetic effect, because it was mimicked by 50-200 micrograms procaine. Procaine did not, however, produce subsequent hypermotility. Conditioned place preference (CPP) was produced by 10 micrograms amphetamine and 50 micrograms cocaine but not 100 micrograms procaine. We conclude that injection of cocaine into VP unlike similar injections into NAS, produces CPP. These results support the idea of an involvement of dopamine in VP in reward and locomotor activation, independent of dopamine in NAS. The use of intracerebral injections of cocaine is complicated, however, by an apparent local anesthetic effect of the drug.

Diametrically opposite effects of estrogen on the excitability of female rat medial and lateral preoptic neurons with axons to the midbrain locomotor region

Electrical stimulation of the midbrain locomotor region (MLR) in 76 ovariectomized, urethan-anesthetized female rats elicited antidromic action potentials in 252 preoptic neurons. Thresholds and refractory periods for the activation ranged from 60 to 1550 microA and 1.3 to 5.0 ms, respectively. The probability distribution for the peak-to-peak amplitude (2-14 mV) or the overall duration (0.7-4.4 ms) was bell-shaped, whereas that for the latency (1.8-33.5 ms) was distinctively bimodal with a division at 12.0 ms. Two groups of preoptic neurons of a similar soma size therefore project to the MLR presumably via different routes. In 121 neurons with latencies < or = 12.0 ms, estrogen lowered the antidromic activation thresholds (nested analysis of variance, P < 0.02), but 131 neurons with latencies > 12.0 ms had their thresholds increased (P < 0.005) and refractory periods prolonged (P < 0.02) by estrogen. Even though both overlapped in part, many potentials with the shorter latencies were recorded from the medial part of the lateral preoptic area (mLPO), lateral to the recording sites of the longer-latency potentials in the medial preoptic area (MPO). The observed antagonistic effects of estrogen on the two groups of preoptic neurons with axons to the MLR may contribute to increased locomotor activity in female rats in estrus.

Cocaine abuse and dependence

Western countries experienced a widespread cocaine epidemic during the 1980s, and the number of frequent users has not declined in this decade. A key factor in the development of this epidemic has been the introduction of "crack," an affordable form of cocaine that appears to be more addicting than the powder. Epidemiologic studies indicate a high incidence of polysubstance abuse among cocaine abusers and probable gender differences in patterns of abuse and response to treatment. An abstinence syndrome has been documented in outpatients after the acute cessation of cocaine; the symptoms perhaps depend on the presence of cues to evoke craving of cocaine and thus are not detected in inpatient settings. Cocaine is a psychostimulant drug that possesses euphorigenic and reinforcing properties. The fact that various animal species self-administer cocaine through the intravenous route provides a reliable animal model for the study of the molecular mechanism of cocaine action and for the characterization of the anatomical substrates responsible for the rewarding properties of the drug. A multisynaptic, allocorticolimbic-accumbens-pallidal circuitry has been identified that seems to play an important role. This pathway may also be part of the neuronal substrates that mediate the reinforcing properties of other classes of abused drugs and, perhaps, motivated behavior in general. Because of this potent reinforcing nature of cocaine in humans, the problem of designing effective therapy for its addiction has not been simply solved. Clinical treatments, guided by animal studies and designed for specific attack of symptoms of the abstinence syndrome, craving and anhedonia, have been tested. To date, only a few agents have proved effective in controlled trials (amantadine, bromocriptine, carbamazepine, and desipramine) and these have limitations of side effects or delayed onset of action. Agents that interact with specific subcomponents of the dopamine system or its connections offer promise for the development of successful agents to treat cocaine abuse and craving in humans.

Excitotoxic lesions of the core and shell subregions of the nucleus accumbens differentially disrupt body weight regulation and motor activity in rat

The behavioral effects of bilateral N-methyl-D-aspartate (NMDA) lesions of the core and medial shell subregions of the nucleus accumbens were evaluated in rats. Body weight was monitored for 2 weeks following surgery. Locomotor activity and open field behavior were recorded 1 week after surgery. The core-lesion group had difficulty recovering from the lesion and had significantly lower weights throughout the experiment. The shell-lesion group had normal recovery and weighed significantly more than controls over the course of the experiment. In the activity cage test, the core-lesion group was hyperactive when compared to controls and to the shell-lesion group. Activity of the shell-lesion group was similar to that of their sham-controls. Three weeks postlesion, the core-lesion group was still significantly more active. In the open field test, peripheral locomotion scores were significantly higher in the core-lesion group when compared to their controls, whereas the scores of the shell-lesion group were similar to controls. In the other open field measures, both lesion groups were hyperactive; however, the scores of core-lesion group were significantly higher than those of the shell-lesion group on all measures. Histological analysis indicated small, discrete areas of damage within the core or medial shell accumbens regions. These preliminary results suggest that these two subregions can be behaviorally differentiated.

N-methyl-D-aspartic acid-induced lesions of the nucleus accumbens and/or ventral pallidum fail to attenuate lateral hypothalamic self-stimulation reward

The role of ventral striatum in the maintenance and transmission of a hypothalamic intracranial self-stimulation (ICSS) reward signal was investigated using the rate-frequency multiple-curve shift paradigm. The excitotoxin N-methyl-D-aspartic acid (NMDA) was bilaterally administered into the nucleus accumbens (15 micrograms per side), the ventral pallidum (15 micrograms per side) or the juncture between the two structures (20 micrograms per side) creating three lesion groups. Both the nucleus accumbens (NAC) lesion group and the ventral pallidum (VP) lesion group displayed substantial NMDA-induced damage which was generally restricted to the intended limbic structure. The NMDA lesions in the third group displayed extensive damage to both the NAC and VP, as intended, but also typically diffused into adjacent medial structures. NMDA-induced lesions in all groups caused a suppression in motor/performance activity at all currents tested. Contrary to motor effects, reward efficacy was relatively unaffected for the NAC and VP groups. The lack of reward effects may be due to plasticity of neuronal systems and redundancy of circuit connections. However, this explanation is questionable given the fact that NMDA lesions which encompassed both the NAC and VP had little effect on reward efficacy. The above data suggests that the nucleus accumbens and the ventral pallidum are not critical for ICSS rewards stimulation and that hypothalamic ICSS reward signals are processed downstream from these limbic structures.

Trends in place preference conditioning with a cross-indexed bibliography; 1957-1991

The purpose of this work is to present a perspective of the conditioned place preference (CPP) test by offering an overview of the empirical research from 1957-1991. The intent is not to extensively analyze the controversies inherent to any behavioral technique but rather to present a survey of research using a descriptive statistics approach to explore topical issues. The objectives of this work are three-fold: (a) to provide an exhaustive bibliography of the CPP literature including articles, journal abstracts, book chapters and critical reviews; (b) to provide a cross-index of identified key words/drugs tested; and (c) to give an overview of selected procedural issues underlying CPP testing.

Differential effects of quinpirole in the nucleus accumbens depending on the initial level of locomotor activity

Effects of dopamine D1 and D2 receptor agonists (SKF 38393 and quinpirole, respectively) on locomotion were studied in two behavioural situations characterized by low and high level of exploratory locomotor activity. Administration of quinpirole bilaterally into the nucleus accumbens increased locomotor activity at the low initial level of activity and decreased locomotor activity at the high activity level, while the administration of SKF 38393 increased locomotor activity in both behavioural situations. It was concluded that quinpirole has differential effects on locomotion, depending on the initial level of activity.

Functionally selective neurochemical afferents and efferents of the mesocorticolimbic and nigrostriatal dopamine system

In summary, evidence is presented that the mesocorticolimbic and nigrostriatal dopamine systems form functionally selective afferents to different parts of the basal ganglia and these inputs are paralleled by functionally selective outputs. The ventral striatal region of the nucleus accumbens and olfactory tubercle has a dopamine input that is critical for locomotor activation produced by psychomotor stimulant drugs and some non-drug states. These regions also appear critical for the reinforcing actions of psychomotor stimulants such as cocaine and amphetamine, and these regions may also be involved in the activation associated with non-drug rewards. Both psychomotor stimulant-induced locomotor activation and reinforcement may selectively involve dopamine D1 receptors. The functional efferents of this system appear to involve the region of the ventral pallidum and more specifically GABAergic mechanisms of the posterior medial (sublenticular) ventral pallidum. The relationship of this circuitry with the revised concept of the "extended amygdala" is an area of current work. The nigrostriatal dopamine system forms a functionally selective afferent system to the dorsal striatum and appears to be critical for the focused stereotyped behavior associated with high doses of psychomotor stimulants. This dopamine input also appears to be involved in non-drug-induced conditioned reaction time performance and may selectively involve dopamine D2 receptors. The functional efferents of this system appear to involve both direct and indirect GABAergic connections to the substantia nigra reticulata and dorsal pallidum, respectively. Activation of the GABAergic connection to the dorsal pallidum (indirect connection) appears to mimic the action of dopamine in the dorsal striatum, whereas activation of the GABAergic connection to the substantia nigra reticulata (direct connection) appears to modulate striatal dopamine function. These results show an important functional role for the globus pallidus in the output of the dorsal striatum and emphasize the parallel functional processing of both dorsal and ventral striatum.

Morphological differences between projection neurons of the core and shell in the nucleus accumbens of the rat

The somatodendritic morphology of projection neurons in the shell and core of the rat nucleus accumbens was studied. These cells were retrogradely labelled with Fast Blue from the ventral mesencephalon (substantia nigra/ventral tegmental area) and subsequently injected intracellularly with Lucifer Yellow and processed immunocytochemically. Digitized reconstructions revealed that the cell bodies of neurons located throughout the nucleus are small-to-medium in size. Neurons in the shell have significantly fewer dendritic arbours with fewer branch segments, fewer terminal segments, and lower spine densities than those in the core. Values for the same parameters are significantly greater for cells in lateral than in medial parts of the shell but the same for neurons located within and without enkephalin enriched parts of the core, with an exception of spine density being significantly greater in the enkephalin-rich compartment. Calculations based on these data reveal that neurons in the core have as much as 50% more surface area than those in the shell, which suggests that core neurons have a greater potential for collecting synaptic information than have shell cells. Furthermore, the differential distribution and action of various neurochemicals such as dopamine in the shell and core, supports the idea that different morphologies reflect the presence of distinct neuronal circuits in these two territories.

Dopaminergic involvement in locomotion elicited from the ventral pallidum/substantia innominata

Microinjection of the indirect GABAA antagonist, picrotoxin, or the mu opioid agonist, Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO), into the ventral pallidum and substantia innominata (VP/SI) increases locomotor activity in rats. The VP/SI has direct and indirect projections to the region of the ventral mesencephalon containing dopamine perikarya, and to certain dopamine terminal fields, including the nucleus accumbens. Thus, it is possible that modulation of the mesocorticolimbic dopamine system by pharmacological stimulation in the VP/SI may play a role in the locomotor stimulant response. It was shown that pretreatment with dopamine receptor antagonists, either peripherally or microinjected into the nucleus accumbens significantly attenuated the motor stimulant effect of DAGO or picrotoxin injection into the VP/SI. Injection of either picrotoxin or DAGO into the VP/SI increased the levels of dopamine metabolites in the nucleus accumbens and prefrontal cortex. Thus, the motor stimulant response following pharmacological stimulation of the VP/SI appears to be mediated by increased dopamine neurotransmission via feedback mechanisms to the mesocorticolimbic dopamine system.

Subregions of the caudate nucleus and their in- and output channels in oro-facial dyskinesia: a behavioural and retrograde tracing study in the cat

Previous studies have shown that the feline caudate nucleus contains DPI-sensitive (caput nuclei caudati, anterodorsal part; r-CRM) and DPI-insensitive (caput nuclei caudati, rostromedial part; CRM) regions. Stimulation of dopamine receptors within the r-CRM by dopamine or DPI are known to elicit oro-facial dyskinesia (OFD), i.e. a syndrome of tic-like contractions of the facial muscles in combination with tongue protrusions. OFD is also elicited from the sub-commissural part of the globus pallidus (scGP), a first order output station of the r-CRM, but not from the CRM. On the basis of these data it has been hypothesized that (1) OFD is a specific feature of the r-CRM, but not the CRM; (2) effects elicited from the r-CRM are funneled via the scGP, and that (3) r-CRM and CRM are differentially innervated. Cats were bilaterally equipped with cannulas directed at the CRM or r-CRM and scGP. Following recovery from the operation the cats received bilateral injections of DPI into CRM (5 micrograms/5 microliter) or r-CRM (5 and 10 micrograms/5 microliter), the latter in combination with muscimol (50 and 100 ng/1 microliter) into the scGP or its solvent. Subsequently, behaviour was analyzed. OFD, quantified in number of tongue protrusions, was only elicited from the r-CRM, but not from the CRM confirming previously reported data in this respect. Furthermore the effect varied according to the dose used. The OFD elicited from the r-CRM was found to be blocked at the level of the scGP by local injections of muscimol, a GABA agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

The contribution of basal forebrain to limbic-motor integration and the mediation of motivation to action

The contribution of hippocampal glutamatergic and VTA dopaminergic inputs to the nucleus accumbens and the role of accumbens--ventral and subpallidal GABAergic pathway in integrating the limbic signals into motor responses via pedunculopontine nucleus were examined with electrophysiological and behavioural techniques. Stimulation of hippocampal input to the accumbens activates GABAergic output to the subpallidal area which leads to suppression of spontaneous firing of subpallidal neurons, while activation of dopamine receptors in the accumbens suppresses GABAergic output to subpallidal area and thus increases the firing of picrotoxin-sensitive ventral pallidal neurons. However, both treatments induced hypermotility suggesting the functional heterogeneity of the ventral and subpallidal areas in "limbic-motor integration". Furthermore, both hippocampal output signals and dopaminergic input to the accumbens descend via ventral and subpallidal areas serially to the pedunculopontine nucleus, the region of the mesencephalic locomotor region. In addition, a parallel ascending pathway from the subpallidal area to the mediodorsal nucleus, and subsequently to the medial prefrontal cortex, probably mediates behaviour, e.g. food hoarding, that requires higher cognitive processing.

Effect of learned behavior upon conditioned place preference to cathinone

The purpose of this study was to examine whether first training rats to discriminate the stimulus cues produced by an indirect dopamine agonist, cathinone, would influence a subsequent test of preference. The conditioned place preference (CPP) paradigm was used to evaluate the reinforcing effects of l-cathinone in four differently treated groups of rats. Half of the animals were trained to discriminate the interoceptive cues produced by 0.8 mg/kg cathinone in a two-lever, food-motivated operant task. The other animals were equally divided between two groups, one receiving saline and noncontingent reinforcements on the same schedule as those trained to discriminate cathinone; the other group, the "yoked-control" rats, received the same cathinone and saline regimen of administration as the discrimination-trained animals. Results of CPP testing indicate that cathinone produced a statistically significant conditioned place preference only in the group trained to discriminate cathinone and not in the saline or yoked control groups. Furthermore, when half of the cathinone discrimination-trained rats were pretreated with the dopamine release inhibitor CGS 10746B, the conditioned place preference to cathinone was attenuated. The results would indicate that pairing cathinone with a nonpreferred environment tended to make the rat spend more time in that environment and the amount of time spent in the cathinone-associated environment can be increased by prior discrimination training and decreased by diminished dopamine function in the brain.

GABAergic projection from nucleus accumbens to ventral pallidum mediates dopamine-induced sensorimotor gating deficits of acoustic startle in rats

Previous studies have demonstrated that increased mesolimbic dopamine (DA) activity disrupts sensorimotor gating as measured by prepulse inhibition (PPI) of the acoustic startle response (ASR) in rats. Other behavioral changes following mesolimbic DA activation are mediated through GABAergic efferent projections from the nucleus accumbens (NAC) to the ventral pallidum (VP). In this experiment, we examined whether PPI deficits in rats following mesolimbic DA activation are mediated through these same GABAergic substrates. PPI was significantly disrupted in rats following infusion of DA (40 micrograms) into the NAC, and this effect was reversed by infusion of a low dose (10 ng) of the GABA agonist muscimol into the VP. In a second experiment, we tested the hypothesis and the loss of PPI following intra-NAC DA infusion results from a disruption of GABAergic activity within the VP. Consistent with this hypothesis, infusion of the GABA antagonist picrotoxin (0-0.2 micrograms) into the VP caused a significant loss of PPI. These findings strongly suggest that the accumbens-ventral pallidal GABAergic circuitry is a substrate for the decrease in sensorimotor gating induced by mesolimbic DA overactivity.

The ventral pallidum plays a role in mediating cocaine and heroin self-administration in the rat

The hypothesis that the ventral pallidum is an important site mediating psychomotor stimulant and opiate reinforcement was tested in rats trained to self-administer i.v. cocaine or heroin. Ibotenic acid lesions of the ventral pallidum produced significant decreases in cocaine and heroin self-administration behavior maintained on a fixed-ratio 5 schedule of reinforcement, suggesting an attenuation of the reinforcing value of cocaine and heroin. On a progressive-ratio schedule, ventral pallidal lesions produced significant decreases in the highest ratio obtained in rats self-administering cocaine. Similar results were observed with heroin in a progressive-ratio procedure modified to produce higher levels of responding; lesions of the ventral pallidum produced a significant decrease in the highest ratio obtained. Further, the i.v. co-administration of naloxone and heroin produced a decrease in progressive-ratio responding relative to heroin alone using the modified progressive-ratio schedule. These results suggest that the ventral pallidum is an important site mediating the reinforcing effects of cocaine and heroin and that the nucleus accumbens-ventral pallidum circuit may be a common pathway for both stimulant and opiate reinforcement.

Effect of acute and daily cocaine treatment on extracellular dopamine in the nucleus accumbens

The behavioral stimulant effect of peripheral cocaine injection into rats is augmented following daily administration. In vivo dialysis in the nucleus accumbens of conscious rats was used to determine if the increased behavioral response following daily cocaine administration is associated with an increase in extracellular dopamine concentration. Acute injection of cocaine (15 mg/kg, ip) produced an elevation in extracellular dopamine concentration in the nucleus accumbens. Following daily pretreatment with cocaine (15 mg/kg, ip X 4 days), a subsequent acute injection of cocaine (15 mg/kg, ip) significantly elevated the extracellular dopamine levels compared to that produced by a single acute injection. Although the levels of extracellular dopamine metabolites was significantly lowered by both acute cocaine and daily cocaine, no difference between these two groups of animals was measured. The increase in extracellular dopamine following a single acute injection of cocaine was not correlated to the motor stimulant response. However, after daily pretreatment with cocaine the motor stimulant response to acute cocaine was positively correlated with the increased extracellular concentration of dopamine in the nucleus accumbens. These data demonstrate that enhanced dopamine release into the nucleus accumbens may mediate the behavioral sensitization produced by daily injections of cocaine, but that other neural systems are influential in mediating the acute motor stimulant effect of cocaine.

Hyperactivity following posterior cortical injury is lateralized, sensitive to lesion size and independent of the nigrostriatal dopamine system

In a previous study, we reported that a specific size cortical suction lesion of the right posterior cortex in rats produced hyperactivity and increased concentrations of dopamine in the nigrostriatal pathway. The present study extended those findings by addressing whether this phenomenon is lateralized to the right posterior cortex and whether the increases in nigrostriatal dopamine are necessary for the behavioral changes produced by the lesion. Right posterior cortical lesions of 1.8 mm diameter produced spontaneous hyperactivity over a 30-day postoperative period while identically placed 1.3 mm or 2.4 mm diameter lesions did not. Left hemisphere lesions of 1.3 mm, 1.8 mm or 2.4 mm diameter also failed to produce hyperactivity. The hyperactivity response to 1.8 mm diameter lesions of the right posterior cortex was not blocked by 70% depletions in nigrostriatal dopamine produced by bilateral nigral injections of 6-hydroxydopamine two weeks prior to cortical lesions. These findings suggest that hyperactivity following right posterior cortical lesions is lateralized and is not dependent upon changes in the nigrostriatal dopaminergic pathway.

The use of place conditioning in studying the neuropharmacology of drug reinforcement

The place conditioning paradigm has proven successful in identifying the neural mechanisms of drug reinforcement. Two classes of drugs, opiates and psychomotor stimulants, have received the most study, and in each case an important role for DA neurons of the mesolimbic system has been established. Moreover, both receptor subtypes, D1 and D2, appear to be involved. Despite this progress, the substrates of drug reward are not completely understood. First, a role for DA has not been established for all stimulants: DA receptor blockade failed to affect conditioned place preferences produced by the stimulants methylphenidate, nomifensine, or bupropion. Second, preliminary evidence suggests that intact serotonergic transmission is important in morphine place conditioning, but a similar consistent finding has not been observed with amphetamine place conditioning. Further study may reveal an interesting dissociation of serotonin's role in the rewarding effects of psychomotor stimulants and opiates. Finally, the role of the opiate receptor subtype kappa is not known; also, the significance of the several anatomical sites that support opiate place conditioning remains unclear.

Involvement of the substantia innominata/ventral pallidum complex in transmitting forelimb muscular rigidity evoked from the nucleus accumbens in rats

In the present study it was investigated whether muscular rigidity elicited from the nucleus accumbens could be transmitted via the substantia innominata/ventral pallidum (SI/VP) complex. For this purpose rats were equipped with cannulae aimed at the nucleus accumbens and/or the SI/VP, and with electrodes in the triceps muscle of the forelimb. In the first set of experiments it was found that the GABA agonist muscimol (10-25 ng/0.5 microliters) dose-dependently increased tonic EMG activity upon injection into the SI/VP region. The GABA antagonist bicuculline (50 ng/0.5 microliters) antagonized the effects of muscimol (25 ng/0.5 microliters). In the second set of experiments it appeared that the increase in tonic EMG activity induced by haloperidol (1000 ng/0.5 microliters) injected into the nucleus accumbens was antagonized by subsequent injection of bicuculline (25 ng/0.5 microliters) into the SI/VP complex. These data suggest that the SI/VP complex may be one station through which the muscular rigidity elicited from the nucleus accumbens is transmitted to lower brain structures.

Ventral pallidal neuronal responses to dopamine receptor stimulation in the nucleus accumbens

The possible role of ventral pallidum (VP) in expressing dopaminergic actions in the nucleus accumbens was studied electrophysiologically using extracellular single unit recording and iontophoretic techniques in urethane-anaesthetized rats. Microinjections of dopamine (130 mM, 5-10 micrograms/0.2-0.4 microliters) into the nucleus accumbens resulted in a gradual, but prolonged, increase in the firing rate of VP neurones. Injections of the D1 agonist SKF38393 (34 mM, 2 micrograms/0.2 microliters), followed by the D2 agonist quinpirole (40 mM, 2 micrograms/0.2 microliters) into the accumbens, but not in the reverse order, resulted in a similar increase in the activity of VP neurones, mimicking the dopaminergic effect. Injections of either the D1 or the D2 agonist alone into the accumbens, however, produced no significant changes. Furthermore, iontophoretic application of picrotoxin, a gamma-aminobutyric acid (GABA) antagonist, or naloxone, an opiate (including enkephalin) antagonist on the same VP neurone which responded to accumbens dopamine injection also increase its spontaneous firing rate. Thus, pre-activation of D1 receptors in the accumbens was essential for the subsequent physiological expression of D2 receptors in inducing an increase in the firing rate of VP neurones. Dopamine in the accumbens may suppress the tonic inhibitory GABAergic and enkephalinergic outputs to the VP, resulting in an increase in firing rate of VP neurones. Since previous behavioural studies have shown that dopaminergic stimulation in the accumbens increases locomotor activity, the increased firing rate of ventral pallidal neurones may be expressing the postsynaptic actions of dopamine receptor stimulations in the accumbens as initiation of locomotor activity.

The functional output of the mesolimbic dopamine system

In summary, the nucleus accumbens, located at the interface of the limbic projections from the amygdala, hippocampus, and cingulate cortex, and receiving extrapyramidal fibers from midbrain DA-containing nuclei, is well situated to form neural circuitry that mediates the behaviorally activating properties of several stimulants. Efferent GABAergic fibers projecting from the nucleus accumbens to the ventral pallidum translate integrated limbic and extrapyramidal information to lower motor circuitry; some of this information appears to be carried by ventral pallidal efferent fibers projecting to the dorsomedial nucleus of the thalamus. It seems very possible that activation of this circuitry by positive reinforcing environmental stimuli, through the release of endogenous DA or opiate compounds, might contribute to motivated behavior. Indeed, environmentally generated locomotor activity can be blocked by disruption of this circuitry following destruction of N. Acc. DA terminals. It is also tempting to speculate that pathological changes in activity within this system might disrupt normal reinforcement contingencies, and contribute to the affective components of both psychiatric and neurologic disease states.

Lesions of the dorsomedial nucleus of the thalamus, medial prefrontal cortex and pedunculopontine nucleus: effects on locomotor activity mediated by nucleus accumbens-ventral pallidal circuitry

A GABAergic nucleus accumbens-ventral pallidum projection is believed to serve as the critical first-order accumbens efferent pathway underlying the behavioral expression of mesolimbic dopamine (DA) activity in the rat. In a series of experiments, we studied the effects of lesions of several ventral pallidal efferent terminal regions on the rat locomotor response to apomorphine following 6-hydroxydopamine denervation of the nucleus accumbens. Lesions of the dorsomedial nucleus of the thalamus (DMT), but not the medial prefrontal cortex or the predunculopontine nucleus, significantly depressed the 'supersensitive' locomotor response to apomorphine. Lesions of the DMT did not depress baseline locomotion, but did diminish the locomotor activation produced by intracerebral injection of the gamma-aminobutyric acid antagonist picrotoxin into the ventral pallidum. These results suggest that accumbens-pallidothalamic circuitry plays a crucial role in translating the effects of mesolimbic DA activity to lower motor circuitry responsible for locomotor behavior in the rat.

Locomotion elicited by lateral hypothalamic stimulation in the anesthetized rat does not require the dorsal midbrain

Locomotor stepping elicited by lateral hypothalamic stimulation in the anesthetized rat is blocked by lesions in the anterior ventromedial midbrain. This study determined in acute experiments whether the dorsal midbrain regions implicated in locomotion were also part of the necessary pathway. Rats were anesthetized with Nembutal and held in a stereotaxic apparatus so that stepping responses rotated a wheel. Stepping was elicited by stimulation of the lateral hypothalamus (up to 100 microA, 0.5 ms cathodal pulses, 50 Hz, 10-s train length). Nine rats received unilateral lesions ipsilateral to the locomotor electrode and 3 rats received bilateral lesions. None of the dorsal midbrain lesions reduced locomotion elicited by ipsilateral lateral hypothalamic stimulation. Therefore the following regions are unnecessary for this type of locomotion: the dorsal and lateral central gray, the tegmentum lateral to the central gray, and in particular the area cuneiformis and the dorsal aspect of the pedunculopontine region. The neural systems required for lateral hypothalamic locomotion are located ventral to the superior cerebellar peduncle.

Glutamate and picrotoxin injections into the preoptic basal forebrain initiate locomotion in the anesthetized rat

This study determined the locomotor effects of glutamate and picrotoxin injections and electrical stimulation in the preoptic basal forebrain. Male rats, anesthetized with Nembutal, were held in a stereotaxic apparatus such that stepping rotated a wheel. Cathodal stimulation (0.5-ms pulses, 50-Hz frequency, 10-s train, less than 100 microA) was applied through a 30-gauge stainless-steel, insulated cannula to find locomotor sites. Glutamate (20 mM or 2 M) or picrotoxin (100 or 200 ng) were injected in volumes of 0.2 or 0.1 microliter of saline at a rate of 1 microliter/5 min. Electrical stimulation elicited locomotion (principally hindlimbs) in 32 sites which included the lateral and medial preoptic areas and the bed nucleus of the stria terminalis (BST). Stimulation in 23 sites, most in the BST and septal area, failed to produce locomotion. Stepping was elicited by glutamate and electrical stimulation in 15 sites. Glutamate was ineffective at 21 sites, at 6 of these sites electrical stimulation was effective. Longer bouts of locomotion were produced by 20 nM glutamate. Picrotoxin produced more intense and prolonged locomotion than glutamate. It was effective in 15 sites, at 12 of which electrical stimulation was also effective. At some ventral sites, picrotoxin-elicited stepping was continuous, at others it appeared in bursts of 5-20 s duration. At dorsal sites, the locomotor bursts were punctuated by episodes of pelvic flexion. Picrotoxin was ineffective at 12 sites, 7 of which were effective with electrical stimulation. These results indicate that activity of neurons in the preoptic basal forebrain can initiate locomotion.

Modification of place preference conditioning in mice by systemically administered [Leu]enkephalin

[Leu]enkephalin (300 micrograms/kg, i.p.) induced either a positive or negative conditioned place preference response in mice depending on whether the animals were trained against or towards their initial preference. Induction of a positive preference (300 micrograms/kg) was partially blocked by simultaneous addition of methylnaloxonium (10 mg/kg, i.p.), an opioid antagonist that does not readily cross the blood-brain barrier; methylnaloxonium alone (3 or 10 mg/kg) had no effect on the place preference response. The results indicate that [Leu]enkephalin treatment reverses the initial preference of the animal regardless of training, and that some aspect of the [Leu]enkephalin effect on place preference conditioning is mediated by peripheral opioid receptors. These findings challenge the notion that place preference conditioning is a simple measure of opioid reward.

"Selective" D-1 and D-2 receptor antagonists fail to differentially alter supersensitive locomotor behavior in the rat

The dopamine receptor antagonists SCH 23390 and spiperone show highly selective in vitro affinity for D-1 and D-2 dopamine receptor subtypes, respectively. We studied the effects of these selective antagonists on the supersensitive locomotor response to apomorphine in rats following 6- hydroxydopamine (6OHDA) lesions of the nucleus accumbens (N. Acc.). Both D-1 and D-2 receptor antagonists produced dose-dependent blockade of the supersensitive locomotor response at doses that did not depress baseline locomotor activity. The behavioral properties of these D-1 and D-2 receptor antagonists were further examined using a simple step-down motor task. Both antagonists produced catalepsy as evidenced by dose-dependent increases in step- down latency. These results indicate that drugs with distinct in vitro dopamine binding affinities cannot be distinguished on the basis of their ability to inhibit supersensitive locomotor activity or simple motor tasks in rats in vivo.

The neural substrates for the motor-activating properties of psychostimulants: a review of recent findings

Several different classes of pharmacological agents produce syndromes of behavioral activation in humans and infrahumans. While many of these agents, including direct and indirect sympathomimetics, methylxanthines, opiates and several neuropeptides have very distinct neurochemical profiles, it is not clear whether their behavioral stimulant action results from their action on a common neural substrate, or instead from their action on parallel but separate activation "circuits.' Using photocell measurements of motor activity in rats, it has been possible to demonstrate that some agents with very distinct neurochemical identities act on common neural substrates to produce behavioral activation, while other agents act on completely distinct brain regions. Specifically, the locomotor-activating properties of direct and indirect sympathomimetics and opiates appear to result from their action within the basal ganglia, including the ventral striatum and globus pallidus, while the activating properties of caffeine and the neuropeptide, corticotropin releasing factor (CRF) appear to be independent of this circuitry. These findings suggest the presence of at least two separate neural systems capable of mediating behavioral activation.

Evidence for, and nature of, the tonic inhibitory influence of habenulointerpeduncular pathways upon cerebral dopaminergic transmission in the rat

The potential role of the habenula in the transsynaptic regulation of the activity of ascending dopaminergic systems has been investigated in the rat by studying the effect of an acute interruption of impulse traffic in the diencephalic conduction system (stria medullaris-habenula-fasciculus retroflexus) and of pharmacological manipulation of various neurotransmitter systems in the interpeduncular nucleus on dopamine metabolism in several dopaminergic projection fields. The bilateral infusion of tetrodotoxin into the fasciculus retroflexus (which conveys the habenulointerpeduncular tract) of conscious rats markedly increased homovanillic acid levels and dopamine synthesis and utilization in the medial prefrontal cortex, nucleus accumbens, olfactory tubercle and striatum. Similar changes in dopamine metabolism were observed in these areas after bilateral infusion of tetrodotoxin into the stria medullaris (which conveys most of the afferents to the habenula). Infusion of atropine (0.4-1 micrograms) into the interpeduncular nucleus increased homovanillic acid concentrations and dopamine utilization in the medial prefrontal cortex and nucleus accumbens but not in the olfactory tubercle and striatum. Moreover, intra-interpeduncular injection of oxotremorine (17 micrograms) antagonized the increase in dopamine utilization in the nucleus accumbens (but not in the olfactory tubercle) induced by an intrafasciculus retroflexus infusion of tetrodotoxin. Local infusion of naloxone (20 micrograms) into the interpeduncular nucleus increased homovanillic acid concentrations in the nucleus accumbens and olfactory tubercle but not in the medial prefrontal cortex and striatum. In contrast, intra-interpeduncular nucleus infusion of the substance P antagonist D-Arg1, D-Pro2, D-Trp7,9, Leu11-substance P or of substance P antiserum failed to alter homovanillic acid levels in the 4 dopamine-rich areas investigated. Finally, intraraphé medianus (but not intraraphé dorsalis) infusion of muscimol (25 ng) moderately increased dopamine synthesis in the nucleus accumbens and striatum. The present findings suggest that the habenulointerpeduncular pathways exert a tonic inhibitory influence on mesocortical, mesolimbic and mesostriatal dopaminergic neurons. Cholinergic and/or opioid peptidergic neurons coursing through the fasciculus retroflexus as well as ascending serotonergic neurons originating in the raphé medianus could take part in this inhibitory control of ascending dopaminergic neurons.

Changes in cortical acetylcholine output induced by modulation of the nucleus basalis

The modulatory inputs of the cholinergic neurons of the nucleus basalis have been investigated in midpontine transected and freely moving rats by measuring acetylcholine release from the cerebral cortex using the cortical cup technique. Acetylcholine release was found to be the same in both groups of rats indicating similar levels of activity of the cholinergic neurons ascending to the cortex. The electrical stimulation of the nucleus basalis was always followed by an increase in acetylcholine release. Conversely, in some experiments only the stimulation of the midbrain reticular formation enhanced acetylcholine output. The stimulation of the nucleus accumbens prevented the increase in acetylcholine release elicited by amphetamine. The dose-dependent increase in acetylcholine output following IP administration of amphetamine was also prevented by the 6-hydroxydopamine induced degeneration of the dopaminergic fibres. However injection of apomorphine in the nucleus basalis did not modify acetylcholine output. Direct injection of the GABAergic agonist muscimol resulted in a decrease in acetylcholine output which was prevented by picrotoxin. In conclusion, the cholinergic neurons ascending to the cortex can be inhibited by GABA receptors located in the nucleus basalis and stimulated indirectly by dopaminergic fibres.

Central dopamine hyperactivity in rats mimics abnormal acoustic startle response in schizophrenics

Schizophrenic patients show loss of the normal inhibition of the acoustic startle response (ASR) when the startle stimulus is preceded by a weak prepulse stimulus. In rats experimentally induced to have locally supersensitive dopamine receptors within forebrain dopamine terminal regions, we find a similar deficit in "prepulse inhibition" of the ASR. Our results suggest that there are intriguing parallels between the aberrant sensory gating of the ASR demonstrated by schizophrenics and that demonstrated by rats during stimulation of supersensitive brain dopamine receptors. Thus, prepulse inhibition of the ASR provides a useful paradigm for investigating the bridge between mesolimbic dopamine overactivity and the specific time-dependent information processing-sensory gating disturbances that have been identified in schizophrenic patients.