Interhemispheric relationship between lateral hypothalamic self-stimulation and the region of the nucleus tegmenti pedunculo-pontinus

The central extended amygdala network as a proposed circuit underlying reward valuation

The phenomenon of medial forebrain bundle self-stimulation offers a powerful model of reward-based behavior. In particular, it appears to activate a neural system whose natural function is to compute the survival value or utility of present stimuli and to help orchestrate responses toward those inputs. Although the anatomical identity of this system is as yet unknown, recent descriptions of anatomical macrosystems within the basal forebrain lead to the proposal that it may be largely contained within the central extended amygdala network. This paper reviews decades' worth of behavioral and neurophysiological investigations of brain stimulation reward that support or are at least consistent with this idea. The proposed network circuitry underlying self-stimulation is also placed into the larger context of basal forebrain function, specifically, the role of the ventral striatopallidum in linking motivation to behavior, the role of the amygdala in detecting motivationally significant inputs, and the role of the magnocellular complex in communicating reward information to cortical and hippocampal targets.

Mesencephalic substrate of reward: possible role for lateral pontine tegmental cells

The present study was aimed at determining whether cells located in the lateral pontine tegmentum could constitute part of the neural circuitry that mediates the rewarding effect of mesencephalic electrical brain stimulation. Single action potentials were recorded from lateral pontine tegmental cells in urethane-anesthetized rats following antidromic activation from the ventral tegmental area and/or posterior mesencephalon. A total of 445 cells were recorded in 13 animals and of these, 44 were antidromically driven from the ventral tegmental area (n=13 ipsi-, n=5 contralateral), the posterior mesencephalon (n=8 ipsi-, n=5 midline), or from both sites (n=13). The occurrence of collision between ortho- and antidromic action potentials triggered by concurrent stimulation of both sites is consistent with psychophysical data obtained previously in behaving animals, and likewise suggests that the two sites are linked by uninterrupted axons. In five of the cells that were driven from both sites, the inter-electrode conduction time exceeded the difference in latencies, suggesting that stimulation of the ventral tegmental area and posterior mesencephalon triggered action potentials in different axonal branches of the same cell. Estimates of the end of the absolute refractory period ranged from 0.44 to 1.6 ms (ventral tegmental area) and from 0.3 to 2.0 ms (posterior mesencephalon), times that overlap with behaviorally derived estimates for mesencephalic reward-relevant neurons. These results suggest that cells originating in the lateral pontine tegmentum might constitute part of the directly-stimulated substrate responsible for the rewarding effect of mesencephalic electrical brain stimulation.

Impairment of two-way active avoidance after pedunculopontine tegmental nucleus lesions: effects of conditioned stimulus duration

It was investigated whether the disruptive effects of bilateral lesions of the pedunculopontine tegmental nucleus on two-way active avoidance might vary depending on variations of task demand. The animals were either subjected to bilateral electrolytic lesions of the pedunculopontine tegmental nucleus (Lesion groups) or were sham-operated (Control groups). All the rats were subjected to two 30-trial sessions of two-way active avoidance (separated by ten days), using either a 10-s conditioned stimulus (low task demand) or a 3-s conditioned stimulus (high task demand). The lesions induced a significant disruption of two-way active avoidance in the two conditions tested, but, in both lesioned and control rats, the number of avoidance responses was higher when the 10-s conditioned stimulus was used. In lesioned animals, the condition of high task demand was associated with a significant increase of escape failures. Lesions did not affect locomotor activity during the period of adaptation to the conditioning apparatus, but induced training-specific motor deficits (a decrease of intertrial crossings and an enhancement of escape latencies) regardless of the specific training conditions used. The results are discussed in terms of the influences of the pedunculopontine tegmental nucleus in thalamocortical and striatal systems.

Diazepam modifies the effect of pedunculopontine lesions on morphine but not on amphetamine conditioned place preference

We have previously shown that T-maze learning impairments caused by lesions to the pedunculopontine tegmental nucleus (PPTg) can be reversed by the anxiolytic diazepam. We now report that diazepam also reverses the effect of PPTg lesions on conditioned place preference (CPP) to morphine but not to amphetamine. Rats with bilateral sham or N-methyl-D-aspartate lesions (0.1 or 0.05 M) to the PPTg were trained in a unbiased CPP paradigm with 2 mg/kg morphine or 2 mg/kg D-amphetamine associated with one compartment of the apparatus and vehicle injections in the alternative compartment. After three drug/saline-compartment pairings, the preference of the animals was assessed by allowing them to explore the entire apparatus for 20 min. In contrast to sham-lesioned subjects, the rats with PPTg lesions did not show a preference for the compartment paired with morphine or amphetamine. In two experiments the expression of a morphine CPP was restored by injecting the lesioned animals with 1 mg/kg of diazepam 30 min before the test session. Diazepam pre-treatment did not restore the expression of amphetamine CPP.

Differential sensitivity of the caudal and rostral nucleus accumbens to the rewarding effects of a H1-histaminergic receptor blocker as measured with place-preference and self-stimulation behavior

A recent series of studies in rats has demonstrated positively reinforcing and memory enhancing effects following lesions of the nucleus tuberomammillaris, which is the only known source of neuronal histamine. The aim of the present experiments was to assess whether inhibition of histaminergic neurotransmission in the ventral striatum has positively reinforcing effects. In Experiment 1 rats with chronically-implanted cannulae were injected with the H1 receptor blocker d-( + )-chlorpheniramine at doses of 0.1, 1.0 and 10.0 microg into the rostral or caudal parts of the nucleus accumbens, a brain region known to be involved in reward-related processes. Immediately after the treatment the animals were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the drug-free test for conditioned place preference, when a choice among the four quadrants was provided, those rats injected with 10.0 microg chlorpheniramine in the caudal nucleus accumbens spent more time in the treatment corral, indicative of a positively rewarding drug action. In Experiment 2 the question was posed whether injection of chlorpheniramine into the nucleus accumbens influences electrical self-stimulation of the lateral hypothalamus. For this purpose rats were chronically implanted with two bipolar electrodes aimed at the lateral-hypothalami and with two additional guide cannulae aimed either at the rostral or caudal nucleus accumbens. After having established reliable self-stimulation behavior at one of the two electrode sites the animals were allowed to self-stimulate for one hour (baseline). Then they were unilaterally injected with 10.0 microg chlorpheniramine or vehicle and allowed to self-stimulate for another hour (test). On the next day the same procedure took place, except for the difference that the animals received an injection aimed at the hemisphere not treated so far. Animals treated with chlorpheniramine in the caudal and in the rostral nucleus accumbens displayed higher rates of ipsihemispheric self-stimulation behavior. Moreover, the animals treated with the H1 receptor blocker in the caudal nucleus accumbens displayed higher rates of ipsihemispheric self-stimulation than those having received an injection in the rostral pole. Upon completion of this part of the experiment all animals received an additional intraperitoneal treatment with chlorpheniramine (20 mg/kg) or vehicle, respectively, and were tested in the same way described above. This treatment also resulted in an amplification of intracranial self-stimulation behavior. These results support the hypothesis that histaminergic neurotransmission is involved in the inhibitory control of a central system subserving reward-related processes. The present data also further highlight the nucleus accumbens as functionally heterogenous along its rostrocaudal axis, with the caudal-shell subregion being more sensitive to antihistaminic induced reward than the rostral entity.

The H1- and H2-histamine blockers chlorpheniramine and ranitidine applied to the nucleus basalis magnocellularis region modulate anxiety and reinforcement related processes

This study examined the effects of the H1-antagonist chlorpheniramine and the H2-antagonist ranitidine on reinforcement and anxiety-parameters following unilateral injection into the vicinity of the nucleus basalis magnocellularis (NBM). In Experiment 1, rats with chronically implanted cannulae were injected with chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg) and were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the test for conditioned corral preference, when provided a choice between the four quadrants, only those rats injected with 10 or 20 microg chlorpheniramine spent more time in the treatment corral, indicative of a positively reinforcing action. None of the other doses of chlorpheniramine or of the H2-antagonist influenced rats' preference behavior. In Experiment 2, the elevated plus-maze (EPM) was used to gauge possible anxiolytic or anxiogenic effects of intra-basalis injection of chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg). A single injection of chlorpheniramine at 0.1 or 20 microg as well as ranitidine at 20 microg was found to exert anxiolytic-like effects in the EPM. Both compounds elevated the time spent on the open arms and increased scanning over the edge of an open arm. None of the other doses of the H1- and H2-antagonist influenced rats' behavior in the EPM. In sum, these findings show that H1- and H2-receptor antagonists differentially modulate reinforcement and fear-related processes in the NBM and thus, provide the first evidence for a behavioral relevance for the histaminergic innervation of this brain site.

Lesions of pontomesencephalic cholinergic nuclei do not substantially disrupt the reward value of medial forebrain bundle stimulation

This study examines the effects of lesioning the pedunculopontine tegmentum (PPTg) and laterodorsal tegmentum (LDTg) on the reward effectiveness of medial forebrain bundle (MFB) stimulation. Although the focus is on the effects of unilateral lesions made ipsilateral to stimulation sites in the hypothalamic and ventral tegmental MFB, the effects of contralateral lesions of both targets are also investigated. Reward effectiveness was assessed using the rate-frequency curve shift paradigm. In nine rats with unilateral PPTg lesions and five rats with unilateral LDTg lesions, the frequency required to maintain half-maximal response rats was generally not changed by more than 0.1 log units relative to prelesion baseline mean. In three rats with contralateral PPTg lesions and four rats with contralateral LDTg lesions, required frequency was also not substantially changed. The results are interpreted in terms of a previously proposed hypothesis regarding the role in MFB self-stimulation of ascending cholinergic input from the pontomesencephalon to ventral tegmental dopaminergic neurons.

Effects of pedunculopontine tegmental nucleus lesions on responding for intravenous heroin under different schedules of reinforcement

The pedunculopontine tegmental nucleus (PPTg) is believed to play important roles in reward and learning. We examined the effect of PPTg lesions (0.5 microl of 0.1 M NMDA injected bilaterally over 10 min) on the learning of an operant response for opiate reward. In 14 adult male Long-Evans rats, bilateral lesions of the PPTg disrupted the acquisition of responding for intravenous heroin (0.1 mg/kg infused at a rate of 0.25 ml/28 sec) on a fixed ratio-1 (FR-1) schedule of reinforcement. The 12 remaining lesioned animals increased their heroin intake over the acquisition sessions but did not reach the response levels of sham-lesioned animals on the 15th and final session. The sham- and PPTg-lesioned animals that learned the FR-1 task exhibited similar patterns of responding during extinction and reacquisition sessions. When tested on a progressive ratio (PR) schedule of reinforcement, however, PPTg-lesioned animals had lower break points than sham-lesioned animals. Asymmetric lesions, which destroyed the majority of the nucleus in one hemisphere only, did not produce any behavioral deficits. Rats that were lesioned after training also did not show deficits in responding under either FR or PR schedules. These findings suggest that PPTg lesions reduce the rewarding effect of opiates but do not disrupt the ability either to learn an operant response or the response requirements of a PR schedule.

Rewarding effects elicited by the microinjection of either AMPA or NMDA glutamatergic antagonists into the ventral tegmental area revealed by an intracranial self-administration paradigm in mice

In order to study the functional role of the trans-synaptic neuronal interaction between glutamatergic afferents and mesolimbic dopaminergic neurons in internal reward processes, BALB/c male mice were unilaterally implanted with a guide-cannula, the tip of which was positioned 1.5 mm above the ventral tegmental area (VTA). On each day of the following experimental period, a stainless steel injection cannula was inserted into the VTA in order to study the eventual self-administration behaviour of either the competitive N-methyl-D-aspartate antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (AP-7) or the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (3 ng/50 nL) using a spatial discrimination task in a Y maze. Mice rapidly discriminated between the arm enabling a microinjection of either of these glutamatergic antagonists and the neutral arm of the maze, and a robust self-administration of either of these compounds was observed from the first session of acquisition. These data provide strong evidence that the intra-VTA microinjection of either of these subclasses of glutamatergic antagonist produces an effect which is interpreted centrally by the experimental subjects as being highly rewarding. Once the self-administration response had been fully acquired by the experimental subjects, preinjection of the dopaminergic D2 antagonist, sulpiride (50 mg/kg i.p.), 30 min before the test, produced a rapid extinction of the self-administration response. This latter result demonstrates the dopaminergic D2 receptor dependence of this intra-VTA self-administration of both of these subclasses of glutamatergic antagonist. We conclude that the different glutamatergic afferent neuronal inputs to the VTA globally exert, in vivo, via the mediation of interposed endogenous GABAergic interneurons, a tonic trans-synaptic inhibitory regulation of neuronal activity in the mesolimbic dopaminergic pathway and that this complex neuronal interaction in the VTA plays a significant functional part in the modulation of internal reward processes.

Locomotion and stereotypy induced by scopolamine: contributions of muscarinic receptors near the pedunculopontine tegmental nucleus

In this study, we test whether blockade of muscarinic receptors near mesopontine cholinergic cell groups may contribute to locomotor activation induced by scopolamine. Unilateral or bilateral injections of scopolamine (10-150 micrograms) into the pedunculopontine tegmental nucleus (PPT) increased horizontal locomotion by 2-15 times in a dose-related way. Unilateral or bilateral injections of scopolamine into the PPT increased stereotypic behaviors (such as sniffing in one location or over large areas), self-biting and grooming. Carbachol (4 micrograms) injected into PPT reduced locomotion for 20 min, followed by 70 min of increased locomotion. When carbachol (4 micrograms) was injected into the PPT before scopolamine (3 mg/kg, i.p.), the activating effect of scopolamine was attenuated, but not when carbachol was injected after scopolamine. Therefore, carbachol appears to compete with scopolamine for muscarinic receptors near the PPT that mediate locomotor activating effects of systemic scopolamine. Haloperidol (0.1 mg/kg, i.p.) also attenuated the stereotypy and locomotion induced by scopolamine in the PPT. We hypothesize that scopolamine acts by blocking muscarinic receptors on mesopontine cholinergic neurons, thereby disinhibiting cholinergic neurons that can activate dopamine neurons.

The tuberomammillary nucleus projections in the control of learning, memory and reinforcement processes: evidence for an inhibitory role

The tuberomammillary nucleus (TM), a cluster of magnocellular cells in the posterior hypothalamus, is the main source of neuronal histamine in the brain. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. Our earlier work showed that the TM projection system may be involved in behavioral asymmetries and behavioral recovery after unilateral manipulations of the brain. Using horseradish peroxidase (HRP) labeling we found an increase in strength (structure and/or activity) in the crossed and uncrossed tuberomammillary-striatal projections in the course of recovery from behavioral asymmetries produced by unilateral removal of the rats' vibrissae, which were in the same direction as the asymmetries found in projections from the substantia nigra to the striatum. Experiments performed with unilateral lesions of the TM region provide evidence for an involvement of the TM system in reinforcement mechanisms. Unilateral destruction of the TM with direct current (DC) or ibotenic acid was found to increase the rate of lateral hypothalamic self-stimulation ipsilateral to the lesion site, suggesting that the TM (particularly the E2 subgroup in its rostral part) may function as a reinforcement inhibiting neural substrate. Experiments performed with bilateral DC or ibotenic acid lesions of the TM region suggest a role of the nucleus in learning and mnemonic processes. A bilateral electrolytic or neurotoxic lesion of the TM region was found to facilitate the performance of adult and behaviorally impaired aged rats in a variety of learning tasks, including a habituation paradigm, aversively motivated learning tasks and water mazes. Examination of the site of the neurotoxic lesion in the TM region with immunohistochemical techniques revealed a marked decline of histamine-staining neurons mainly in the rostral part of the TM nucleus, suggesting that the facilitatory effects on reinforcement and mnemonic processes might be related to the destruction of TM intrinsic histaminergic cells. In summary, the present results indicate that the TM nucleus is involved in neural plasticity and functional recovery following damage to the CNS and may function as an inhibitory neural substrate in the control of reinforcement and mnemonic processes.

N-methyl-D-aspartate lesions of the pedunculopontine nucleus block acquisition and impair maintenance of responding reinforced with brain stimulation

Excitotoxin lesions of the pedunculopontine tegmental nucleus have been found to block the acquisition of a conditioned place preference induced by morphine or amphetamine, and it has been suggested that such lesions may attenuate the primary reinforcing effects of these drugs and, possibly, other reinforcers. The present study examined the effects of pedunculopontine lesions on the reinforcing effects of brain stimulation. N-methyl-D-aspartate-induced lesions of the pedunculopontine nucleus prevented spontaneous acquisition of lever pressing for brain stimulation reinforcement during five daily 1 h sessions of training. The effective lesions damaged the retrorubral fields in addition to the pedunculopontine tegmental nucleus. N-methyl-D-aspartate (25 or 50 nmol) lesions of the retrorubral fields did not block acquisition of self-stimulation, however, controls reached their maximum rate of responding in the first session, responding of rats with retrorubral field lesions gradually increased over five days. When excitotoxin-induced lesions of the pedunculopontine nucleus were made after acquisition of self-stimulation, lesioned rats continued to respond to brain stimulation, but at a lower rate than controls. The results show that pedunculopontine lesions interfere with the learning and expression of a response reinforced by brain stimulation just as they block learning motivated by drugs and natural rewards. They also suggest that collateral damage to the retrorubral fields may contribute to the effects of pedunculopontine lesions on reinforced behaviour. These data support the view that the pedunculopontine tegmental nucleus is involved in the process by which reinforcers control purposive behaviour.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Self-stimulation of the MFB following parabrachial lesions

It has been reported previously that the parabrachial region supports robust self-stimulation. In the present study, we determined whether lesions of the parabrachial nucleus (PBN) influence the rewarding effect of medial forebrain bundle (MFB) stimulation. In 10 rats, stimulation electrodes were aimed at the lateral hypothalamus and/or ventral tegmental area and a lesioning electrode aimed at the PBN. Rate-frequency curves were collected at each of three stimulation currents at each electrode, before and after lesioning. Four rats showed virtually no change in the frequency required to sustain half-maximal performance following lesioning, and two showed some postlesion decreases. Only two rats showed substantial postlesion increases in required frequency; the lesions in these subjects damaged the region ventral to the superior cerebellar peduncle, just caudal to the decussation of the peduncle, but spared the PBN. Thus, the reward effectiveness of MFB stimulation does not appear to be altered substantially following PBN lesions but may decrease following damage to the neighboring pedunculopontine region.

Interhemispheric links in brain stimulation reward

The MFB substrate of self-stimulation (SS) has generally been viewed as a unilateral system. We re-examined this belief with pairs of moveable SS electrodes placed bilaterally in the MFB. Rats barpressed for trains of single or twin cathodal pulses of fixed intensity and width and of variable frequency. The first (C) and second (T) pulse of each pair was delivered through the left and right electrode or inversely. C-T intervals ranging from 0.2 to 5.0 ms were tested. The frequency of C pulses required for criterial bar-pressing was used to plot the stimulation efficacy (SE), as a function of the C-T interval and pulse presentation order. The electrodes were subsequently moved and the same procedure repeated for more ventral sites. With some pairs of contralateral hypothalamic (H) sites, the SE was independent of the C-T interval. However, with other pairs of contralateral H sites, the SE increased with C-T interval in a manner resembling a collision effect, with the important exception that no conduction time (CT) was apparent in the data. The absence of CT excludes the presence of a genuine collision effect. When one pulse was sent to the H and another to the contralateral ventral tegmentum (VT), the H-VT curve rose always earlier than the VT-H curve, thus resembling a transynaptic collision effect. However, the C-T interval at which the VT-H curve began rising (always 1.0 ms or less) fails to support the contention that the electrodes activated fibers separated by a synapse. Finally, a typical collision effect was noted with ipsilateral H-VT electrode placements, confirming the presence of direct linkage between ipsilateral MFB sites. Computer-generated data based on two parsimonious assumptions were found to match the empirical results. These assumptions were that each electrode activated a different branch of the same reward neuron and that conduction failure occurred at the branchpoint. The model, which posits that a large number of MFB reward neurons send branches to the other hemisphere, is testable and makes clear-cut predictions about the effects of lesions. In a preliminary test, we recorded the H and contralateral VT threshold frequencies before and after lesioning the H. The H threshold increased more when using small pulse current and remained constant throughout the 4-week testing period. The VT threshold was elevated more for intermediate pulse current and kept increasing with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between dopamine release in nucleus accumbens and place preference induced by substance P injected into the nucleus basalis magnocellularis region

The activity of the neurotransmitter dopamine in the nucleus accumbens is considered to be an important element in the central processing of reinforcement. Unilateral administration of the neurokinin substance P into the area of the nucleus basalis magnocellularis of rats was found to be reinforcing, as assessed by the conditioned place preference paradigm. Simultaneous in vivo microdialysis showed that administration of substance P into the area of the nucleus basalis magnocellularis could increase extracellular concentrations of dopamine in the contralateral nucleus accumbens. Only those animals in which the administration of substance P induced this increase in dopamine levels acquired place preference. Furthermore, the changes in extracellular dopamine levels after substance P administration had a bimodal time course with an acute increase (to about 160% of baseline) during the first hour after injection, with a low (to 120-130%) and enduring increase occurring thereafter. Interestingly, during this second increase there were indications for positive correlations with the degree of place preference induced by substance P. Further positive correlations with place preference were found in the levels of the serotonergic metabolite 5-hydroxyindoleacetic acid. In contrast to dopamine, these were observed ipsi- and contralateral to the side of substance P administration. By combining the methods of in vivo microdialysis and conditioned place preference it was shown that the reinforcing effect induced by unilateral substance P injection in the nucleus basalis magnocellularis is related to dopaminergic (and possibly serotonergic) mechanisms in the nucleus accumbens.

The role of neuropeptides in learning: focus on the neurokinin substance P

The neurokinin substance P (SP) can have neurotrophic as well as memory-promoting effects. The study of its mechanisms may provide new insights into processes underlying learning and neurodegenerative disorders. Our work shows that SP, when applied peripherally (i.p.), promotes memory and is reinforcing at the same dose of 37 nmol/kg. Most important, however, is the finding that these effects seemed to be encoded by different SP-sequences, since the N-terminal SP1-7 (185 nmol/kg) enhanced memory, whereas C-terminal hepta- and hexapeptide sequences of SP proved to be reinforcing in a dose equimolar to SP. These differential behavioral effects were paralleled by selective and site-specific changes in dopamine (DA) activity, as both SP and its C-, but not N-terminus, increased extracellular DA in the nucleus accumbens (NAc), but not in the neostriatum. The neurochemical changes lasted at least 2 h after injection. Direct application of SP (0.74 pmol) into the region of the nucleus basalis magnocellularis (NBM) was also memory-promoting and reinforcing, and again, these effects were differentially produced by the N-terminus and C-terminus, supporting the proposed structure-activity relationship for SP's effects on memory and reinforcement. In addition, it was found that a single injection of SP into the NBM led to an increase of extracellular DA in the contralateral NAc. This effect of SP was observed only in those animals where SP was reinforcing, providing evidence for a lateralized relationship between reinforcement induced by injection of SP into the NBM and DA activity in the NAc. Furthermore, the outcome of a series of experiments suggests, that SP may not only be considered to have memory-promoting effects in normal animals, but can also improve functional recovery after unilateral 6-OHDA lesion of the substantia nigra and after lesions of the hippocampus, and can counteract age-related performance deficits.

The relationship between self-stimulation and sniffing in rats: does a common brain system mediate these behaviors?

The relationship between brain self-stimulation and brain-stimulation induced sniffing behavior was examined at three brain sites (frontal cortex, hypothalamus and lower brain stem). In the first experiment, sniffing was elicited in the prefrontal cortex and pontine reticular formation (PRF) of anesthetized rats. These sites corresponded to reported self-stimulation sites. In non-anesthetized animals (Expt. 2), all self-stimulation sites in the medial prefrontal cortex (MPC) and lateral hypothalamic-medial forebrain bundle (LH-MFB) also supported sniffing. In the PRF, this was also the case except for one subject which exhibited self-stimulation and jaw movements without sniffing. After unilateral lesions either in the MPC or PRF, stimulation-induced sniffing from the ipsilateral LH-MFB was not influenced. While MPC lesions did not affect self-stimulation either, medial PRF lesions disrupted ipsilateral self-stimulation. In summary, stimulation-induced sniffing and self-stimulation behavior appear to share strikingly similar anatomical loci, but the PRF appears to be differentially involved in these behaviors. The results were discussed from an appetitive motivational hypothesis of self-stimulation.

Effects of pedunculopontine tegmental nucleus lesions on morphine-induced conditioned place preference and analgesia in the formalin test

The development of a conditioned place preference to morphine (2 mg/kg; three pairings) and the analgesic effect of morphine (0, 4 or 8 mg/kg) in the formalin test were studied in rats with sham or neurotoxin lesions of the pedunculopontine tegmental nucleus. Lesions were induced by bilateral infusions of N-methyl-D-aspartate (0.5 microliter of 0.1 M solution) or vehicle over 10 min. No anti-seizure medication was administered in the first experiment, whereas animals in the second experiment were injected with diazepam (1 mg/kg) immediately after surgery. In Experiment 1, behaviour in the conditioned place preference and formalin tests was assessed in separate groups of lesioned and control rats. In Experiment 2, the same animals received both sets of tests. In both experiments lesions of the pedunculopontine tegmental nucleus blocked the development of a conditioned place preference to morphine, but had no effect on the behavioural response to formalin, or on its inhibition by morphine. Examination of cholinergic-stained cells found no correlation between the magnitude of behavioural effects and the number of acetylcholine cells destroyed by the lesions. These results confirm that the pedunculopontine tegmental nucleus mediates the development of a morphine-induced conditioned place preference, but not the analgesic effect of morphine.

The tuberomammillary nucleus region as a reinforcement inhibiting substrate: facilitation of ipsihypothalamic self-stimulation by unilateral ibotenic acid lesions

The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, consists of five subgroups and is the only known source of brain histamine. Knowledge about the function of this nucleus is still scarce. In a previous study we found an increase in the rate of ipsihemispheric hypothalamic self-stimulation following a dc lesion in the rostroventral part of this nucleus, suggesting that this region has an inhibitory action on a neuronal reward system or on the brain's reinforcement mechanism. In the present study we examined whether this facilitating effect on reinforcement was due to the destruction of fibers passing through the lesion area or of intrinsic cells, by lesioning subgroups of the TM with ibotenic acid, an excitatory amino acid, that selectively destroys neural cell bodies, leaving fibers largely intact. Following such lesions in the rostroventral part of the TM the operant response rates increased over the six days of testing when the animals stimulated themselves in the lateral hypothalamus in the hemisphere located ipsilateral but not contralateral to the lesion. No significant changes in response rate occurred following the lesion in the caudal part of the ventral TM. The results indicate that the region influenced by the lesion exerts inhibitory control over lateral hypothalamic self-stimulation, and that it is possible that histamine-containing neurons are involved in this effect.

Lesions of the tegmental pedunculopontine nucleus: effects on the locomotor activity induced by morphine and amphetamine

One of the important questions in the neurobiology of motivation asks how the incentive impact of stimuli acting on the limbic system of the forebrain are ultimately translated into action and approach behavior. Bilateral ibotenic acid lesions of the tegmental pedunculopontine nucleus (TPP) (a brainstem output of the limbic system that receives neuronal input from limbic forebrain and midbrain sites identified as primary sites for psychoactive drug reward) have been shown previously to block the acquisition, but not the retention, of morphine and amphetamine conditioned place preferences in formerly drug-naive rats. These results suggest a deficit in the processing of the unconditioned rewarding effects of these drugs. The TPP projects to widespread parts of the brain and spinal cord involved in various somatomotor responses. Thus, we investigated the role of the TPP in morphine- and amphetamine-induced locomotion as assessed in an open field. We report that TPP lesions blocked the locomotor excitation, as well as the conditioned hyperactivity, produced by amphetamine. TPP lesions also blocked the conditioned increase in locomotion, but not the catalepsy, produced by morphine. TPP lesions were behaviorally specific in that the analgesic properties of morphine in a tail-flick test were not attenuated, nor did the lesions affect the locomotion induced by naloxone-precipitated withdrawal in morphine-dependent animals. We suggest that the neural circuits mediating the acute rewarding effects of drug stimuli acting at forebrain sites exit the limbic system in the TPP region of the brainstem, where motivation gains access to (or is isomorphic with) motor systems that initiate approach and exploration.

Schedule-controlled brain self-stimulation: has it utility for behavioral pharmacology?

We review evidence that schedule-controlled intracranial self-stimulation (ICSS) has properties in common with conventional reinforcements, such as food and water, but unlike the latter, animals will respond for ICSS for long periods of time at a near-constant rate. Schedule-controlled ICSS has proven to be more sensitive to drug-induced changes than has ICSS on a continuous reinforcement schedule, and it permits a more fine-grained analysis of the pattern of responding that results in the reinforcement. Evidence is accumulating that the schedule of ICSS itself leads to neurochemical changes in areas of the brain, such as the nucleus accumbens, in which reward processes occur. Results obtained from schedule-controlled ICSS would complement those obtained by drug self-administration studies which generally use intermittent reinforcement. A systematic examination of ICSS schedules at different brain sites would greatly facilitate our interpretation of drug effects and this would have utility for behavioral pharmacology.

Measuring threshold of reinforcing brain stimulation by the method of constant stimuli

A new method for measuring threshold of reinforcing electrical brain stimulation is described and results of a parametric study using this method are presented. Two groups of rats were trained under a concurrent FR-CRF (fixed ratio-continuous reinforcement) schedule of reinforcement provided by electrical stimulation of the lateral hypothalamus. The invariant intermittent FR schedule of reinforcement was used to maintain a baseline of behavior while a superimposed concurrent CRF schedule was used to measure reinforcement magnitude by varying the intensity of the CRF stimulus between zero and a maximum. Increasing and decreasing stimulus intensity on the CRF schedule leads to a gradual disappearance, respectively reappearance, of post-reinforcement pauses (PRPs) on the concurrent FR schedule, providing a criterion for changeover in schedule control, and thus, for threshold of reinforcement. To illustrate the measurement of threshold according to psychophysical requirements of the Method of Constant Stimuli, different CRF intensities were given in a randomized order. In one group of animals FR and CRF stimuli were given through the same electrode. Another group received FR and CRF stimuli through separate electrodes implanted in different hemispheres of the brain. For both groups the duration of the PRP was used as the dependent variable. The data of both groups showed a high negative correlation between the intensity of the CRF stimulus and the duration of the PRP, which is consistent with the results of experiments in which a Method of Limits procedure was used. On the basis of this relationship between CRF current intensity and PRP duration a threshold for reinforcing brain stimulation was calculated.