Selective deficits in attentional performance on the 5-choice serial reaction time task following pedunculopontine tegmental nucleus lesions.

Sustained attention requires the integrity of basal forebrain cholinergic systems. The pedunculopontine tegmental nucleus (PPTg) has direct and indirect connections (via the thalamus) with the basal forebrain, suggesting that the PPTg may also play an important role in attentional processes. We examined this hypothesis by testing the effects of PPTg lesions in rats on performance in the 5-choice serial reaction time test. Bilateral lesions reduced accuracy, increased errors of omission, and increased the latency to correct responses. The deficits were more severe when neuronal damage was bilateral and concentrated in the posterior PPTg. Attentional demands of the task were increased by decreasing the stimulus duration, the stimulus brightness, or the inter-trial interval, and by introducing random bursts of white noise. These challenges impaired performance of all animals, but the magnitude of deficit was increased in the lesioned group. Conversely, lesion-induced deficits were partially alleviated when the attentional demands of the task were reduced. This pattern of results suggests that PPTg lesions produce a global deficit in attention, rather than a specific impairment in one process. The PPTg may control attentional processes through its direct projections to the forebrain cholinergic system or, indirectly, through activation of thalamocortical projections.

Pedunculopontine tegmental nucleus lesions impair stimulus--reward learning in autoshaping and conditioned reinforcement paradigms.

The role of the pedunculopontine tegmental nucleus (PPTg) in stimulus-reward learning was assessed by testing the effects of PPTg lesions on performance in visual autoshaping and conditioned reinforcement (CRf) paradigms. Rats with PPTg lesions were unable to learn an association between a conditioned stimulus (CS) and a primary reward in either paradigm. In the autoshaping experiment, PPTg-lesioned rats approached the CS+ and CS- with equal frequency, and the latencies to respond to the two stimuli did not differ. PPTg lesions also disrupted discriminated approaches to an appetitive CS in the CRf paradigm and completely abolished the acquisition of responding with CRf. These data are discussed in the context of a possible cognitive function of the PPTg, particularly in terms of lesion-induced disruptions of attentional processes that are mediated by the thalamus.

Lesions of the pedunculopontine tegmental nucleus increase sucrose consumption but do not affect discrimination or contrast effects.

Pedunculopontine tegmental nucleus (PPTg) lesions block place preferences to drugs or food only when animals are nondeprived. PPTg lesions also disrupt operant responding, but lesioned rats cannot discriminate active from inactive levers. It is not clear, therefore, whether PPTg lesions block reward or disrupt the ability to differentiate changes in reward magnitude. These hypotheses were tested by measuring sucrose consumption, choice, and contrast effects after PPTg lesions. Both sham and lesioned rats consumed greater amounts of a sucrose solution as the concentration and level of deprivation were increased. Given a choice between 2 solutions, all rats consumed more of the higher concentration. Both groups exhibited contrast effects when the concentration was shifted from 32% to 4% within a session. Somewhat surprisingly, lesions increased sucrose intake when rats were food-restricted. These results suggest that PPTg lesions do not disrupt primary motivation or the ability to evaluate and respond to changes in reward strength.

Discriminable excitotoxic effects of ibotenic acid, AMPA, NMDA and quinolinic acid in the rat laterodorsal tegmental nucleus.

Excitotoxins are valuable tools in neuroscience research as they can help us to discover the extent to which certain neurones are necessary for different types of behaviour. They have distinctive neurotoxic effects depending on where they are infused, and this study was conducted to delineate the neurotoxic profiles of excitotoxins in the laterodorsal tegmental nucleus (LDTg). Two 0.1 microl infusions of 0.1 M ibotenate, 0.1 M quinolinate, 0.04-0.1 M NMDA, or 0.05-0.015 M AMPA, were made unilaterally into the LDTg under either pentobarbitone or Avertin anaesthesia. The injection needle was oriented at an angle of 24 degrees from vertical in the mediolateral plane. After 23-27 days, sections through the mesopontine tegmentum were processed using standard histological procedures for NADPH-diaphorase histochemistry, tyrosine hydroxylase or 5-hydroxytryptamine immunohistochemistry, and Cresyl violet. Lesions were assessed in terms of the size of the damaged area (identified by reactive gliosis), the extent of cholinergic cell loss in the mesopontine tegmentum (by counting NADPH-diaphorase-positive neurones), and neuronal loss induced in the locus coeruleus and dorsal raphe nucleus. Ibotenate induced compact lesions in the LDTg (more than 80% cholinergic loss) and did little damage to the locus coeruleus and dorsal raphe nucleus. Quinolinate and low doses of AMPA and NMDA made very small lesions with less than 35% cholinergic loss, while at higher doses, AMPA and NMDA induced large areas of reactive gliosis but killed only a proportion of the cholinergic neurones. AMPA appeared to have a particular affinity for noradrenergic neurones in the locus coeruleus, with the 0.015 M dose injected into the LDTg typically destroying the majority of these neurones. The results are discussed in the context of what is known about the mechanisms of excitotoxins and the glutamate receptor profile of mesopontine neurones.

On the relationships between the striatum and the pedunculopontine tegmental nucleus.

In this essay we consider the role of the pedunculopontine tegmental nucleus as a striatal output station. We review the relevant anatomical, electrophysiological, behavioral, and pathological studies and conclude that the pedunculopontine tegmental nucleus occupies an important position in striatal outflow, receiving motor output from the dorsal striatum and information from the ventral striatum relating to limbic processes of motivation and reinforcement. The hypothesis we present is that the pedunculopontine tegmental nucleus is at the very least an integral component of the limbic-motor interface, although in discussing this concept we also assess the likelihood that the limbic-motor interface is in fact a distributed system-that is, that limbic-motor interfacing is not all done by a single structure in the central nervous system but that different aspects of it are served by different systems. We present the hypothesis that the pedunculopontine tegmental nucleus is one critical site through which limbic information concerned with motivation, reinforcement, and the construction of novel associations can gain access to a stream of motor outflow coming from the caudate-putamen and directed toward pontomedullary systems without reference back to the cerebral cortex. This hypothesis is important because it highlights striatal outflow, which is not processed through the cortical re-entry systems, and also emphasizes the importance of pontine systems in cognitive processing.

Colocalization of ionotropic glutamate receptor subunits with NADPH-diaphorase-containing neurons in the rat mesopontine tegmentum.

Tegmental cholinergic neurons vary their discharge patterns across the sleep-wake cycle, and glutamate is suggested to play an important role in determining these firing patterns. Cholinergic and noncholinergic neurons in the mesopontine tegmentum have different susceptibilities to various excitotoxins, presumably because of heterogeneity in the expression of glutamate receptor subtypes in this area. By using a double-labeling procedure that combines nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) histochemistry and avidin-biotin-peroxidase immunocytochemistry with diaminobenzidine as the chromogen, we compared the colocalization of AMPA receptor subunits GluR1, GluR2/3, and GluR4, kainate receptor subunits GluR5/6/7, and an NMDA receptor subunit NMDAR1 on NADPH-diaphorase-positive (cholinergic) neurons in the mesopontine tegmentum. Throughout the brainstem, neurons immunoreactive for GluR2/3 and NMDAR1 were most numerous, whereas neurons labeled for GluR1, GluR4, and GluR5/6/7 were less common. Specifically within the mesopontine tegmentum, the proportion of double-labeled neurons in the diaphorase-containing cell population was highest with GluR1 (43%) and lowest with GluR5/6/7 (12%). Regardless of the receptor subunit type, the greatest numbers of double-labeled neurons were observed in the pedunculopontine tegmental nucleus pars compacta and the fewest in the dorsal aspect of the laterodorsal tegmental nucleus. In addition, there were regional differences in the relative expression of receptor subunits and diaphorase-positive neurons across the subdivisions of the tegmental cholinergic column. Because each ionotropic subunit confers distinctive properties to a receptor channel, the present results suggest that mesopontine cholinergic neurons have nonuniform responses to glutamate and are also discriminable from basal forebrain cholinergic neurons in terms of glutamate receptor configuration.

Is the cuneiform nucleus a critical component of the mesencephalic locomotor region? An examination of the effects of excitotoxic lesions of the cuneiform nucleus on spontaneous and nucleus accumbens induced locomotion.

The cuneiform nucleus and the pedunculopontine tegmental nucleus have both been suggested as possible sites for the mesencephalic locomotor region (MLR), an area from which controlled stepping on a treadmill can be elicited following electrical or chemical stimulation in a decerebrate animal. It has been shown that excitotoxic lesions of the pedunculopontine tegmental nucleus impair neither spontaneous locomotion nor locomotion induced by stimulation of the nucleus accumbens. Excitotoxic lesions of the cuneiform nucleus have not previously been investigated. Rats received either bilateral ibotenate or sham lesions of the cuneiform nucleus combined with bilateral implantation of guide cannulae aimed at the nucleus accumbens. On recovery from surgery spontaneous locomotion was tested, followed by accumbens-stimulated locomotion. For nucleus accumbens stimulation, each rat received bilateral microinjection of each of three doses of d-amphetamine (10.0, 20.0 and 30.0 micrograms) and a vehicle only injection. Locomotor activity was recorded following the injection. In comparison to the sham-lesioned group, the ibotenate-lesioned group showed no differences in either spontaneous or amphetamine-induced locomotor activity. These results suggest that, like the pedunculopontine tegmental nucleus, the cuneiform nucleus is not involved in the direct mediation of spontaneous or accumbens-induced locomotion, and thus is very unlikely to be the anatomical substrate of the MLR. The role of the cuneiform nucleus in other types of behavioural control is discussed.

Modulation of dopamine efflux in the nucleus accumbens after cholinergic stimulation of the ventral tegmental area in intact, pedunculopontine tegmental nucleus-lesioned, and laterodorsal tegmental nucleus-lesioned rats.

Microinjections of the cholinergic receptor agonist nicotine and the cholinesterase inhibitor neostigmine were made into the ventral tegmental area (VTA) of urethane-anesthetized rats, and dopamine (DA) efflux in the nucleus accumbens was measured using in vivo chronoamperometry. Dose-dependent increases in the chronoamperometric signals corresponding to increased DA efflux were observed in the nucleus accumbens of normal intact rats after cholinergic stimulation of the VTA. The source of the cholinergic input to the VTA was investigated by making excitotoxic lesions in either the laterodorsal tegmental nucleus (LDTg) or the pedunculopontine tegmental nucleus (PPTg). Compared with sham-operated control animals, which showed the same response as intact, nonlesioned rats, ibotenate lesions of the LDTg attenuated the stimulatory effects of intra-VTA neostigmine on DA efflux in the nucleus accumbens. In contrast, rats with ibotenate lesions of the PPTg showed normal nucleus accumbens DA eflux after intra-VTA injections of neostigmine. Such lesions in the PPTg attenuate DA efflux in the caudate-putamen stimulated by injections of neostigmine into the substantia nigra pars compacta (SNc). The present data show that cholinergic neurons in the LDTg, but not the PPTg, regulate the activity of DA-containing neurons in the VTA, which complements previous data showing that cholinergic neurons in the PPTg regulate DA-containing neurons in the SNc.

The pedunculopontine tegmental nucleus: where the striatum meets the reticular formation.

The pedunculopontine tegmental nucleus (PPTg) contains a population of cholinergic neurons (the Ch5 group) and non-cholinergic neurons. There appears to be functional interdigitation between these two groups, which both have extensive projections. The principal ascending connections are with thalamic nuclei and structures associated with the striatum, including the substantial nigra pars compacta. The descending connections are with a variety of nuclei in the pons, medulla and spinal cord, concerned with autonomic and motor functions. In the past, emphasis has been laid on the role of the PPTg in locomotion and behavioural state control. In this review, we emphasise the role of the PPTg in processing outputs from the striatum. The non-cholinergic neurons receive outflow from both dorsal and vental striatum, and lesions of the PPTg disrupt behaviour associated with each of these. Our review indicates that the PPTg is less concerned with the induction of locomotion and more concerned with relating reinforcement (information about which comes from the ventral striatum) with motor output from the dorsal striatum. The conclusions we draw are: (1) the PPTg is an outflow system for the striatum, but also forms a 'subsidiary circuit', returning information to striatal circuitry; in this, the PPTg has an anatomical organisation that resembles that of the substantia nigra. (2) As well as a role in the mediation of REM sleep, cholinergic PPTg neurons have an important role in the waking state, providing feedback into the thalamus and striatum. (3) The precise function of the computations performed on striatal outflow by the PPTg is uncertain. We discuss whether this function is complementary (parallel to other routes of striatal outflow), integrative (modifying other forms of striatal outflow) or both.

Outflow from the nucleus accumbens to the pedunculopontine tegmental nucleus: a dissociation between locomotor activity and the acquisition of responding for conditioned reinforcement stimulated by d-amphetamine.

Output of neuronal information from the nucleus accumbens to the ventral pallidum is known to be a critical pathway in the expression of locomotion and incentive-related behaviour. Some signals from this structure are relayed forward through the dorsomedial nucleus of the thalamus to the medial prefrontal cortex, but the other major pathway from this site is a descending innervation to the pedunculopontine tegmental nucleus. Information carried by these descending neurons has been linked with both the output of locomotor activity and incentive-related information. Previous studies carried out in this laboratory have shown no changes in locomotor activity--either spontaneous or in response to systemic administration of d-amphetamine or apomorphine--in rats with excitotoxic lesions of the pedunculopontine tegmental nucleus. The present experiments compare the effects of ibotenate lesions of this nucleus in tests of locomotor activity or the acquisition of responding with conditioned reinforcement, following injections of d-amphetamine directly into the nucleus accumbens. In general agreement with previous results, ibotenate lesions of the pedunculopontine tegmental nucleus did not alter locomotion stimulated directly from the nucleus accumbens. However, comparable lesions in a group of trained rats produced an array of deficits in the conditioned reinforcement paradigm. Most notably, these rats directed their attention almost entirely towards pressing the levers (practically ignoring the food-hopper panel), but did not appear to be able to discriminate between them, while controls focused almost all their efforts on pressing the reinforcing lever (virtually ignoring the non-reinforcing lever) and the food-hopper panel. These results indicate that pedunculopontine tegmental nucleus lesions disrupt an element of reward-related responding, but do not affect the production of locomotor activity. This highlights the unlikely existence of specific "locomotion-inducing" centres in the mesencephalon and implicates the pedunculopontine tegmental nucleus in the formation of stimulus-reward associations. These data are discussed with respect to a role for the pedunculopontine tegmental nucleus in response selection.

An investigation into the role of the pedunculopontine tegmental nucleus in the mediation of locomotion and orofacial stereotypy induced by d-amphetamine and apomorphine in the rat.

As the pedunculopontine tegmental nucleus has an important anatomical position as an output station for the striatum, its role in the mediation of behaviour stimulated by d-amphetamine and apomorphine was investigated. Bilateral ibotenate lesions were made in either the pedunculopontine tegmental nucleus or, as a control, in the adjacent deep mesencephalic nucleus; sham lesions were made using phosphate buffer. Over the 14 days after surgery there were no significant differences in the rats' body weight or food intake. Deep mesencephalic lesioned rats spilled more food and drank more water (never more than 5 ml more) than controls or pedunculopontine tegmental lesioned rats. Spontaneous locomotion and that elicited by d-amphetamine or apomorphine were not affected by ibotenate lesions of either the pedunculopontine tegmental nucleus or deep mesencephalic nucleus. At higher doses of d-amphetamine and apomorphine, however, excessive biting and licking were observed in the pedunculopontine tegmental nucleus, but not deep mesencephalic nucleus, lesioned rats. Such orofacial stereotypies are never observed in normal rats after systemic injection of d-amphetamine. Post mortem analysis showed that ibotenate lesions of the pedunculopontine tegmental nucleus had destroyed cholinergic and non-cholinergic neurons there but had left the deep mesencephalic nucleus intact; ibotenate lesions of the deep mesencephalic nucleus destroyed neurons in that structure but not the pedunculopontine tegmental nucleus. These data demonstrate that lesions in the pedunculopontine tegmental nucleus and deep mesencephalic nucleus have different effects, measured histologically and behaviourally; that neither spontaneous locomotion nor that stimulated by d-amphetamine or apomorphine is dependent on the integrity of the pedunculopontine tegmental nucleus; and that the pedunculopontine tegmental nucleus plays an important role in mediating orofacial activity stimulated by these drugs. The data are discussed in terms of their implications for understanding outflow from the caudate-putamen and nucleus accumbens.

Barbiturate anaesthesia reduces the neurotoxic effects of quinolinate but not ibotenate in the rat pedunculopontine tegmental nucleus.

We have previously demonstrated that ibotenate (IBO) injected into the pedunculopontine tegmental nucleus (PPTg) damages all neurones there while quinolinate (QUIN) makes relatively selective lesions of cholinergic neurones. We now compare the effects of two anaesthetics, sodium pentobarbitone and Avertin (tribromoethanol/tert-amylalcohol dissolved in ethanol, saline and phosphate buffer) on three doses of IBO and QUIN in the PPTg. Diaphorase-positive cell loss after QUIN was attenuated under barbiturate, the relative selectivity of QUIN for diaphorase-positive neurones was lost and lesion volumes were uniformly small compared with lesions made under Avertin anaesthesia. IBO toxicity was unaffected by anaesthesia. These data are discussed with reference to the actions of excitotoxins at glutamate receptor subtypes and interactions of barbiturates with the GABAA receptor.

A comparison of behaviour following stimulation of the anterior substantia nigra by direct cholinergic agonists and anticholinesterases.

Microinjections of carbachol, a muscarinic cholinergic receptor agonist, into the anterior substantia nigra increase feeding, drinking and sexual behaviour if there is a pre-existing tendency to respond and a low baseline rate of behaviour. The present experiment was undertaken to compare the effects of carbachol with other cholinergic stimulants. Groups of 6-12 satiated rats received 0.5 microliter microinjections into the anterior substantia nigra of 0.1-5.0 micrograms carbachol, 0.1-5.0 micrograms nicotine, 2.5-10.0 micrograms eserine, and 1.25-5.0 micrograms or 0.1-1.0 microgram neostigmine (each dissolved in sterile saline) and the effects on feeding, drinking, locomotion, grooming, rearing and sniffing were examined. Carbachol, nicotine and low doses of neostigmine stimulated eating in a dose-dependent manner. The increased feeding following neostigmine was over a shorter time-period than following carbachol or nicotine. Neither carbachol nor nicotine had any significant effect on behaviour other than eating. The higher doses of neostigmine increased the frequency of sniffing and rearing, but not eating, and no dose of eserine had a clear effect on behaviour. These data are discussed in terms of their relationship to the cholinergic input to substantia nigra which excites pars compacta dopamine-containing neurones.