The origin of tyrosine hydroxylase-immunoreactive fibers in the regions of the nucleus basalis magnocellularis of the rat

Disruption of positive- and negative-feature morphine interoceptive occasion setters by dopamine receptor agonism and antagonism in male and female rats

RATIONALE

Internally perceived stimuli evoked by morphine administration can form Pavlovian associations such that they can function as occasion setters (OSs) for externally perceived reward cues in rats, coming to modulate reward-seeking behaviour. Though much research has investigated mechanisms underlying opioid-related reinforcement and analgesia, neurotransmitter systems involved in the functioning of opioids as Pavlovian interoceptive discriminative stimuli remain to be disentangled despite documented differences in the development of tolerance to analgesic versus discriminative stimulus effects.

OBJECTIVES

Dopamine has been implicated in many opioid-related behaviours, so we aimed to investigate the role of this neurotransmitter in expression of morphine occasion setting.

METHODS

Male and female rats were assigned to positive- (FP) or negative-feature (FN) groups and received an injection of morphine or saline before each training session. A 15-s white noise conditioned stimulus (CS) was presented 8 times during every training session; offset of this stimulus was followed by 4-s access to liquid sucrose on morphine, but not saline, sessions for FP rats. FN rats learned the reverse contingency. Following stable discrimination, rats began generalization testing for expression of morphine-guided sucrose seeking after systemic pretreatment with different doses of the non-selective dopamine receptor antagonist, flupenthixol, and the non-selective dopamine receptor agonist, apomorphine, combined with training doses of morphine or saline in a Latin-square design.

RESULTS

The morphine discrimination was acquired under both FP and FN contingencies by males and females. Neither flupenthixol nor apomorphine at any dose substituted for morphine, but both apomorphine and flupenthixol disrupted expression of the morphine OS. This inhibition was specific to sucrose seeking during CS presentations rather than during the period before CS onset and, in the case of apomorphine more so than flupenthixol, to trials on which access to sucrose was anticipated.

CONCLUSIONS

Our findings lend support to a mechanism of occasion setting involving gating of CS-induced dopamine release rather than by direct dopaminergic modulation by the morphine stimulus.

Phase synchrony between prefrontal noradrenergic and cholinergic signals indexes inhibitory control

Inhibitory control is a critical executive function that allows animals to suppress their impulsive behavior in order to achieve certain goals or avoid punishment. We investigated norepinephrine (NE) and acetylcholine (ACh) dynamics and population neuronal activity in the prefrontal cortex during inhibitory control. Using fluorescent sensors to measure extracellular levels of NE and ACh, we simultaneously recorded the dynamics of prefrontal NE and ACh in mice performing an inhibitory control task. The prefrontal NE and ACh signals exhibited strong coherence at 0.4-0.8 Hz. Chemogenetic inhibition of locus coeruleus (LC) neurons that project to the basal forebrain region reduced inhibitory control performance to chance levels. However, this manipulation did not diminish the difference in NE/ACh signals between successful and failed trials; instead, it abolished the difference in NE-ACh phase synchrony between the successful and failed trials, indicating that NE-ACh phase synchrony is a task-relevant neuromodulatory feature. Chemogenetic inhibition of cholinergic neurons that project to the LC region did not impair the inhibitory control performance, nor did it abolish the difference in NE-ACh phase synchrony between successful or failed trials, further confirming the relevance of NE-ACh phase synchrony to inhibitory control. To understand the possible effect of NE-ACh synchrony on prefrontal population activity, we employed Neuropixels to record from the prefrontal cortex with and without inhibiting LC neurons that project to the basal forebrain during inhibitory control. The LC inhibition reduced the number of prefrontal neurons encoding inhibitory control. Demixed principal component analysis (dPCA) further revealed that population firing patterns representing inhibitory control were impaired by the LC inhibition. Disparities in NE-ACh phase synchrony relevant to inhibitory control occurred only in the prefrontal cortex, but not in the parietal cortex, somatosensory cortex, and the somatosensory thalamus. Taken together, these findings suggest that the LC modulates inhibitory control through its collective effect with cholinergic systems on population activity in the prefrontal cortex. Our results further revealed that NE-ACh phase synchrony is a critical neuromodulatory feature with important implications for cognitive control.

Neural Networks With Motivation

Animals rely on internal motivational states to make decisions. The role of motivational salience in decision making is in early stages of mathematical understanding. Here, we propose a reinforcement learning framework that relies on neural networks to learn optimal ongoing behavior for dynamically changing motivation values. First, we show that neural networks implementing Q-learning with motivational salience can navigate in environment with dynamic rewards without adjustments in synaptic strengths when the needs of an agent shift. In this setting, our networks may display elements of addictive behaviors. Second, we use a similar framework in hierarchical manager-agent system to implement a reinforcement learning algorithm with motivation that both infers motivational states and behaves. Finally, we show that, when trained in the Pavlovian conditioning setting, the responses of the neurons in our model resemble previously published neuronal recordings in the ventral pallidum, a basal ganglia structure involved in motivated behaviors. We conclude that motivation allows Q-learning networks to quickly adapt their behavior to conditions when expected reward is modulated by agent's dynamic needs. Our approach addresses the algorithmic rationale of motivation and makes a step toward better interpretability of behavioral data via inference of motivational dynamics in the brain.

Ventral Pallidum Neurons Encode Incentive Value and Promote Cue-Elicited Instrumental Actions

The ventral pallidum (VP) is posited to contribute to reward seeking by conveying upstream signals from the nucleus accumbens (NAc). Yet, very little is known about how VP neuron responses contribute to behavioral responses to incentive cues. Here, we recorded activity of VP neurons in a cue-driven reward-seeking task previously shown to require neural activity in the NAc. We find that VP neurons encode both learned cue value and subsequent reward seeking and that activity in VP neurons is required for robust cue-elicited reward seeking. Surprisingly, the onset of VP neuron responses occurs at a shorter latency than cue-elicited responses in NAc neurons. This suggests that this VP encoding is not a passive response to signals generated in the NAc and that VP neurons integrate sensory and motivation-related information received directly from other mesocorticolimbic inputs.

Neural differentiation of transplanted neural stem cells in a rat model of striatal lacunar infarction: light and electron microscopic observations

The increased risk and prevalence of lacunar stroke and Parkinson's disease (PD) makes the search for better experimental models an important requirement for translational research. In this study we assess ischemic damage of the nigrostriatal pathway in a model of lacunar stroke evoked by damaging the perforating arteries in the territory of the substantia nigra (SN) of the rat after stereotaxic administration of endothelin-1 (ET-1), a potent vasoconstrictor peptide. We hypothesized that transplantation of neural stem cells (NSCs) with the capacity of differentiating into diverse cell types such as neurons and glia, but with limited proliferation potential, would constitute an alternative and/or adjuvant therapy for lacunar stroke. These cells showed neuritogenic activity in vitro and a high potential for neural differentiation. Light and electron microscopy immunocytochemistry was used to characterize GFP-positive neurons derived from the transplants. 48 h after ET-1 injection, we characterized an area of selective degeneration of dopaminergic neurons within the nigrostriatal pathway characterized with tissue necrosis and glial scar formation, with subsequent behavioral signs of Parkinsonism. Light microscopy showed that grafted cells within the striatal infarction zone differentiated with a high yield into mature glial cells (GFAP-positive) and neuron types present in the normal striatum. Electron microscopy revealed that NSCs-derived neurons integrated into the host circuitry establishing synaptic contacts, mostly of the asymmetric type. Astrocytes were closely associated with normal small-sized blood vessels in the area of infarct, suggesting a possible role in the regulation of the blood brain barrier and angiogenesis. Our results encourage the use of NSCs as a cell-replacement therapy for the treatment of human vascular Parkinsonism.

Proteasomal abnormalities in cortical Lewy body disease and the impact of proteasomal inhibition within cortical and cholinergic systems

Dementia with Lewy bodies (DLB) accounts for 15-20% of the millions of people worldwide with dementia. In the current work we investigate the association between proteasome dysfunction and the development of cortical Lewy body pathology. Analysis of post-mortem cortical tissue indicated levels of the alpha-subunit of the 20S proteasome were significantly reduced in DLB cortex, but not Alzheimer's, in comparison to control and this reduction correlated with both the severity and duration of dementia. Application of proteasome inhibitors to rodent cortical primary neurones in vitro and by direct injection onto rodent cholinergic forebrain neurons in vivo gave rise to dose dependent neuronal death and in rodent cortex -- marked cholinergic deficits accompanied by the accumulation of inclusions that stained positive for alpha-synuclein and ubiquitin. These findings suggest that proteasomal abnormalities are present within cortical Lewy body disease and the experimental inhibition of proteasomal function mirrors the neuropathological changes seen within the disorder.

D1-like dopamine receptors selectively block P/Q-type calcium channels to reduce glutamate release onto cholinergic basal forebrain neurones of immature rats

Whole-cell patch-clamp recordings of non-NMDA glutamatergic EPSCs were made from identified cholinergic neurones in slices of basal forebrain (BF) of young rats (P13-P18), to investigate the subtypes of calcium channels involved in dopamine D(1)-like receptor-mediated presynaptic inhibition of the EPSCs. The BF cholinergic neurones were pre-labelled by intracerebroventricular injection of a fluorescent marker, Cy3-192IgG. A D(1)-like receptor agonist, SKF 81297 (30 microM) suppressed the EPSCs reversibly by about 30%, and this inhibition was reproducible. Calcium channel subtypes involved in the glutamatergic transmission were elucidated using selective Ca(2+) channel blockers. The N-type Ca(2+) channel blocker omega-conotoxin (omega-CgTX, 3 microM) suppressed the EPSCs by 57.5%, whereas the P/Q-type channel selective blocker omega-agatoxin-TK (omega-Aga-TK, 200 nM) suppressed the EPSCs by 68.9%. Simultaneous application of both blockers suppressed the EPSCs by 96.1%. The R-type Ca(2+) channel blocker SNX-482 (300 nM) suppressed the EPSCs by 18.4%, whereas nifedipine, the L-type Ca(2+) channel blocker (10 microM), had little effect. In the presence of omega-Aga-TK, SKF 81297, a dopamine D(1)-like receptor agonist, had no effect on the EPSCs. On the other hand, SKF 81297 could still inhibit the EPSCs in the presence of either omega-CgTX, SNX-482 or nifedipine. SKF 81297 had no further effect on the EPSCs when external Ca(2+) concentration was raised to 7.2 mM in the presence of omega-Aga-TK, but could still inhibit the EPSCs in high Ca(2+) solution after omega-CgTX application. Forskolin (FK, 10 microM), an activator of adenylyl cyclase pathway, suppressed the EPSCs, and the FK-induced effect was mostly blocked in the presence of omega-Aga-TK but not that of omega-CgTX. These results suggest that D(1)-like receptor activation selectively blocks P/Q-type calcium channels to reduce glutamate release onto BF cholinergic neurones.

Nitric oxide synthase and NADPH-diaphorase after acute hypobaric hypoxia in the rat caudate putamen

Changes in the production system of nitric oxide (NO), a multifunctional biological messenger known to participate in blood-flow regulation, neuromodulation, and neuroprotection or neurotoxicity, were investigated in the caudate putamen of adult rats submitted to hypobaric hypoxia. Employing immunohistochemistry, Western blotting, enzymatic assay, and NADPH-diaphorase staining, we demonstrate that neuronal nitric oxide synthase (nNOS) expression and constitutive nitric oxide synthase (cNOS) activity were transiently activated by 7 h of exposure to a simulated altitude of 8325 m (27,000 ft). In addition, endothelial nitric oxide synthase (eNOS) immunoreactivity and blood vessel NADPH-diaphorase staining peaked immediately after the hypoxic stimulus, whereas inducible nitric oxide synthase (iNOS) expression and activity remained unaltered. Nitrotyrosine formation, a marker of protein nitration, was evaluated by immunohistochemistry and Western blotting, and was found to increase parallel to nitric oxide synthesis. We conclude that the nitric oxide system undergoes significant transient alterations in the caudate putamen of adult rats submitted to acute hypobaric hypoxia.

Short-term, D2 receptor blockade induces synaptic degeneration, reduces levels of tyrosine hydroxylase and brain-derived neurotrophic factor, and enhances D2-mediated firing in the ventral pallidum

Repeated treatments with neuroleptics are associated with biochemical and morphological alterations in forebrain neurons as well as an upregulation of D2-mediated changes in neuronal function. The present study evaluated the histological and physiological effects of three once-daily treatments with two chemically divergent neuroleptics, haloperidol (1 mg/kg i.p./day) and eticlopride (3 mg/kg i.p./day), measured in rats 24 h after the last injection. It was determined that this short-term antagonism of D2-like receptors induced fiber and terminal degeneration and significantly decreased tyrosine hydroxylase (TH) and brain-derived neurotrophic factor (BDNF) immunoreactivity in the ventral pallidum (VP), as determined by optical density measurements. While other forebrain regions demonstrated changes in TH and BDNF, the neurodegeneration profile was unique to the VP. This was accompanied by an enhancement in the efficacy of the D2 agonist quinpirole to increase spiking rate of VP neurons recorded in chloral hydrate-anesthetized rats. These data indicate that short-term treatments with D2 antagonists are sufficient to induce changes in the biochemical and morphological profiles uniquely within the VP. Moreover, the functional ramifications of these changes appear to include profound alterations in the way dopamine regulates neuronal activity in this region.

Neuroprotective effect of rasagiline, a selective monoamine oxidase-B inhibitor, against closed head injury in the mouse

The potential neuroprotective effects of rasagiline, N-propargyl-1R-aminoindan, a selective monoamine oxidase-B inhibitor and its inactive enantiomer TVP 1022, N-propargyl-1S-aminoindan were assessed against the sequelae of closed head injury in the mouse. Injury was induced in the left hemisphere under ether anaesthesia. Rasagiline (0.2 and 1 mg/kg) or TVP1022 (1 and 2 mg/kg) injected 5 min after injury accelerated the recovery of motor function and spatial memory and reduced the cerebral oedema by about 40-50%, (P < 0.01). The neuroprotective effects on motor function and spatial memory, but not on cerebral oedema, were prevented by scopolamine (0.2 mg/kg). Daily injection of rasagiline (1 mg/kg) from day 3 after injury accelerated the recovery of spatial memory but not motor function.

CONCLUSIONS

Early administration of rasagiline or TVP1022 can reduce the immediate sequelae of brain injury. The mechanism of action does not appear to involve monoamine oxidase-B inhibition but could be mediated by the maintenance of cholinergic transmission in brain neurons.

Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

Distribution of catecholaminergic afferent fibres in the rat globus pallidus and their relations with cholinergic neurons

The topographical distribution of catecholaminergic nerve fibres and their anatomical relationship to cholinergic elements in the rat globus pallidus were studied. Peroxidase-antiperoxidase and two-colour immunoperoxidase staining procedures were used to demonstrate tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) immunoreactivities, combined with acetylcholinesterase (AChE) pharmacohistochemistry. TH immunoreactive nerve fibres were seen to enter the globus pallidus from the medial forebrain bundle. The greatest density of such fibres was found in the ventral region of the globus pallidus, which was also characterized by the greatest density of ChAT immunoreactive neurons. TH immunoreactive nerve fibres showed varicose arborizations and sparse boutons, which were occasionally seen in close opposition to cholinergic structures. In all regions of the globus pallidus, there were also larger, smooth TH immunoreactive nerve fibres of passage to the caudate putamen. A smaller number of DBH immunoreactive nerve fibres and terminal arborizations were found in the substantia innominata, internal capsule and in the globus pallidus bordering these structures. A few PNMT immunoreactive nerve fibres in the substantia innominata and internal capsule did not enter the globus pallidus. Electron microscopy revealed TH immunoreactive synaptic profiles in the ventromedial area of the globus pallidus corresponding to the nucleus basalis magnocellularis of Meynert (nBM). These made mainly symmetrical and only a few asymmetrical synaptic contacts with dendrites containing AChE reaction product. The results indicate that cholinergic structures in the nBM are innervated by dopaminergic fibres and terminals, with only a very small input from noradrenergic fibres.

Parvalbumin-containing neurons in the basal forebrain receive direct input from the substantia nigra-ventral tegmental area

By means of anterograde tracing of Phaseolus vulgaris-leucoagglutinin (PHA-L) it was determined if parvalbumin-immunoreactive neurons in the basal forebrain receive a direct synaptic input from the A9-A10 dopaminergic nuclei of the substantia nigra and ventral tegmental area. Forebrain sections were processed for immunocytochemical detection of PHA-L and parvalbumin (PV) at light and electron microscopic levels. At the ultrastructural level, PHA-L-labeled terminals were found to establish synaptic contacts with PV-immunoreactive neuronal somata in the ventromedial globus pallidus, the ventral pallidum, the internal capsule, and the substantia innominata. PV-containing neurons in pallidal and adjacent basal forebrain territories are thus directly targeted by presumably A9-A10 dopaminergic neurons and represent a novel aspect of midbrain dopaminergic control of basal forebrain neuronal activity.

Modulation of inhibitory transmission by dopamine in rat basal forebrain nuclei: activation of presynaptic D1-like dopaminergic receptors

The effects of dopamine (DA) on inhibitory transmission onto identified magnocellular neurons were examined in rat basal forebrain slices using whole-cell recording. IPSCs evoked by focal stimulation within basal forebrain nuclei were reversibly blocked by 10 microM bicuculline and had a decay time constant of 20.1 +/- 0.77 msec in the presence of 6-cyano-7-nitroquinoxalline-2,3-dione (5 mM). Bath application of DA reduced the amplitude of IPSCs up to 71.1 +/- 1.49% in a concentration-dependent manner between 0.003 and 1 mM (the IC50 value being 6.6 microM), without any effect on the holding current at -70 mV. DA (10 microM) reduced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of TTX (0.5 microM), without affecting their mean amplitude, rise time, and decay time constant. Furthermore, the DA-induced effect on mIPSCs remained unaffected by 100 microM cadmium, suggesting a presynaptic mechanism independent of calcium influx. SKF 81297, a D1-like agonist, mimicked DA-induced effect on evoked IPSCs (IC50, 10.9 microM), whereas R(-)-TNPA or (-)-quinpirole, D2-like agonists (30 microM), had little or no effect on the amplitude of evoked IPSCs. R(+)-SCH 23390, a D1-like antagonist, antagonized the DA-induced effect on IPSCs (K(B) 0.82 microM), whereas S(-)-eticlopride, a D2-like antagonist, showed slight antagonism (K(B) 7.8 microM). Forskolin (10 microM) reduced the amplitude of evoked IPSCs to approximately 58% of the control and occluded the inhibitory effect of DA. These findings indicate that DA reduces inhibitory transmission onto magnocellular basal forebrain neurons by activating presynaptic D1-like receptors.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. I. Dopamine-beta-hydroxylase- and tyrosine hydroxylase input to cholinergic neurons

Immunocytochemical double-labeling techniques were used at the light and electron microscopic levels to investigate whether dopamine-beta-hydroxylase and tyrosine hydroxylase-containing axons contact basal forebrain cholinergic neurons. Dopamine-beta-hydroxylase- and tyrosine hydroxylase-positive fibers and terminals were found in close proximity to cholinergic neurons throughout extensive basal forebrain areas, including the vertical and horizontal limb of the diagonal band nuclei, the sublenticular substantia innominata, bed nucleus of the stria terminalis, ventral pallidum, and ventrolateral globus pallidus. Cholinergic cells in some aspects of the globus pallidus appeared to be contacted by tyrosine hydroxylase-positive but not dopamine-beta-hydroxylase-positive fibers, suggesting dopaminergic input to cholinergic neurons in these regions. Direct evidence for the termination of dopamine-beta-hydroxylase and tyrosine hydroxylase-positive fibers on cholinergic neurons was obtained in electron microscopic double-immunolabeling studies. Using high magnification light microscopic screening, both qualitative and quantitative differences were noted in the catecholaminergic innervation of forebrain cholinergic neurons. For example, while many cholinergic neurons were in close proximity to single dopamine-beta-hydroxylase-positive varicosities, others, particularly those located in the substantia innominatabed nucleus of the stria terminalis continuum, were apparently contacted by labeled fibers in repetitive fashion. The findings of the present study, together with our preliminary biochemical experiments (Zaborszky et al. [1993] Prog. Brain Res. 98:31-49) suggest that catecholaminergic afferents can differentially modulate forebrain cholinergic neurons. Such interactions may be important in learning and memory processes, and their perturbations may contribute to the cognitive decline seen in aging and in disorders such as Alzheimer's and Parkinson's diseases.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. II. Substantia nigra-ventral tegmental area projections to cholinergic neurons

Previous observations indicate that the basal forebrain receives dopaminergic input from the ventral midbrain. The present study aimed at determining the topographic organization of these projections in the rat, and whether this input directly terminates on cholinergic neurons. Injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into discrete parts of the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNC) labeled axons and terminals in distinct parts of the basal forebrain, including medial and lateral septum, diagnoal band nuclei, ventral pallidum, globus pallidus, substantia innominata, globus pallidus, and internal capsule, where PHA-L-labeled terminals abutted cholinergic (choline acetyltransferase = ChAT-containing) profiles. Three-dimensional (3-D) computerized reconstruction of immunostained sections clearly revealed distinct, albeit overlapping, subpopulations of ChAT-immunoreactive neurons apposed by PHA-L-labeled input from medial VTA (mainly in vertical and horizontal diagonal band nuclei), lateral VTA and medial SNC (ventral pallidum and anterior half of substantia innominata), and lateral SNC (caudal half of the substantia innominata and globus pallidus). At the ultrastructural level, about 40% of the selected PHA-L-labeled presynaptic terminals in the ventral pallidum and substantia innominata were found to establish synaptic specializations with ChAT-containing profiles, most of which on the cell body and proximal dendritic shafts. Convergent synaptic input of unlabeled terminals that formed asymmetric synapses with the ChAT-immunoreactive profiles were often found in close proximity to the PHA-L-labeled terminals. These observations show that the cholinergic neurons in the basal forebrain are targets of presumably dopaminergic SNC/VTA neurons, and suggest a direct modulatory role of dopamine in acetylcholine release in the cerebral cortical mantle.

Dopamine D1-like receptor-mediated presynaptic inhibition of excitatory transmission onto rat magnocellular basal forebrain neurones

1. Excitatory postsynaptic currents (EPSCs) following focal afferent stimulation were recorded from patch-clamped magnocellular neurones in a thin-slice preparation of the rat basal forebrain. Evoked EPSCs had a mean decay time constant of 3.81 +/- 0.09 ms and were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM). 2. Bath-applied dopamine (DA) reduced evoked EPSC amplitude by up to 54.2 +/- 2.3% with an IC50 of 19.9 microM in normal Krebs solution (2.5 mM Ca2+, 1.2 mM Mg2+) without effect on postsynaptic holding current. 3. DA (30 microM) reduced the mean frequency of spontaneous miniature EPSCs recorded in 0.5 microM tetrodotoxin without affecting their mean amplitude, rise time or decay time constant. This effect was diminished by 100 microM Cd2+. 4. The effect of DA on evoked EPSCs was mimicked by the D1-like receptor agonist, SKF 81297 (IC50 25.6 microM), but not by the D2-like receptor agonist R(-)-TNPA (30 microM) or (-)-quinpirole (30 microM), and was antagonized by the D1-like receptor antagonist R(+)-SCH 23390 (estimated dissociation constant KB = 1.7 microM) but not by the D2-like receptor antagonist S(-)-eticlopride (10 microM). 5. Forskolin (10 microM) reduced evoked EPSCs to approximately 60% of the control amplitude, and occluded the effect of subsequent application of DA. 6. These results suggest that glutamatergic afferents to magnocellular basal forebrain neurones possess presynaptic D1-like DA receptors, and that activation of these receptors reduces excitatory glutamatergic transmission, probably via an adenylyl cyclase-dependent pathway.

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.

Effects of bilateral cholinotoxin infusions on the behavior and brain biochemistry of the rats

We examined behavioral and biochemical specificity and the general usefulness of the proposed rat model of Alzheimer's disease. Bilateral infusions of ethylcholine aziridinium (AF64A) into the basal magnocellular nuclei caused a deterioration of learning in passive and active avoidance tests, increased emotional reactivity, and decreased motoric activity. Choline acetyltransferase activity was decreased by 22% in the frontal cortex but increased by 8-10% in the hippocampus and hypothalamus. Noradrenaline and dopamine levels in the frontal cortex were decreased by 20%. In striatum, dopamine and its metabolites were strongly suppressed (by 50-60%). Also striatal noradrenaline (-48%) and 5-hydroxytryptamine (-34%) were significantly decreased. Hypothalamic 5-hydroxytryptamine was increased (+25%). Bilateral AF64A lesions decreased significantly (by 14-20%) activities of prolyl endopeptidase, dipeptidyl peptidase II and IV in hippocampal and frontal cortical brain homogenates. These results show that AF64A can be used to induce long-term learning deficits in the rat. However, striatal amine levels are also strongly suppressed, and are reflected as hypomotility and increased emotional reactivity. These changes may limit the usefulness of the rat model. Universally decreased peptidase activities offer interesting views regarding the role of peptidase inhibitors in amnestic disorders.

Contribution of the amygdala and nucleus accumbens to ventral pallidal responses to dopamine agonists

Neurons recorded from ventral pallidum/substantia innominata (VP) of the basal forebrain respond to dopaminergic agonists that activate either the D1 or D2 the receptor subtype. Major afferent systems to the VP originate within amygdaloid nuclei (AMN) and the nucleus accumbens (NA). Since both the AMN and the NA are dopaminoceptive, the present study sought to analyze the contribution of these afferent systems to VP responses to dopaminergic agonists. Single VP neurons were electrophysiologically recorded in vivo from chloral hydrate-anesthetized rats, and the following determinations were made. 1) Effects of pharmacologic inactivation of an afferent system were assessed by monitoring VP neurons during intracerebral microinjections of the local anesthetic procaine, administered directly into either the AMN or the NA. 2) With procaine-induced VP rate changes used to indicate an afferent influence on the recorded neuron, VP responses to apomorphine (an agonist that acts at D1 and D2 receptor subtypes), SKF38393 (a D1 agonist), or quinpirole (a D2 agonist) were determined and compared with responses in rats not receiving the procaine pretreatment. Following pharmacologic inactivation of either the AMN or the NA, approximately 80% of the VP neurons monitored demonstrated rate changes, illustrating that spontaneous neuronal firing in the Vp is dependent on tonically input systems. Following afferent cessation, responses to apomorphine and quinpirole remained intact, suggesting that the AMN or NA is not necessary for VP responding to the systemic administration of dopaminergic agonists that act at D2 receptors. In contrast, the number of neurons that responded to SKF38393 was diminished follow intra-AMN (but not intra-NA) procaine. This suggests that D1-induced VP responses are mediated, at least in part, via the AMN.

The development of neurons in the nuclei of the horizontal and vertical limb of the diagonal band of Broca of the rat: a qualitative and quantitative analysis of Golgi preparations

We have studied the morphological alterations of neurons in the nuclei of the horizontal (NHL) and vertical (NVL) limbs of the diagonal band of Broca of rats from late embryonic life to maturity using the Golgi-Stensaas and Golgi-Cox methods. During late embryonic life and in the first postnatal days, the two nuclei of the diagonal band of Broca were found to be located near the ventral surface of the brain. Shortly thereafter, neurons in the NHL and NVL gradually take up the positions which they normally occupy in adulthood. At this stage neurons were small with round or elongated somata and 1-3 primary dendrites that only occasionally bore spines and very seldom showed varicosities, features commonly shown by neurons at later postnatal ages. At birth, cells showing varying soma shapes and dendritic morphology were present, and by postnatal day 4 (P4) the three forms of neurons previously described in adult rats (Dinopoulos et al., J. Comp. Neurol., 272 (1988) 461-474) were readily distinguished. During the second postnatal week, the size of cell somata as well as the number, size and extent of dendritic branching underwent considerable increases in both nuclei and at P14 neurons showed features typical of their adult counterparts. In addition they showed a dramatic increase in the number of spines which was followed during the next 10 days by a substantial decrease. Overall, the dendritic geometry of neurons in the NHL and NVL did not change significantly after P14, although their cell bodies continued to increase in size until the middle of the fourth and fifth postnatal weeks respectively. These findings suggest that neurons in the nuclei of the diagonal band of Broca show continuous growth from embryonic life to the end of the second postnatal week when they acquire morphological features comparable to the adult. Thereafter they exhibit only minor morphological alterations with the exception of extensive spine elimination which is pronounced during the third postnatal week and continues until adulthood.

Chronic infusion of GABA and saline into the nucleus basalis magnocellularis of rats: II. Cognitive impairments

In order to assess sensorimotor and/or cognitive modifications following chronic inhibition of nucleus basalis magnocellularis (NBM) neurons, rats trained in two radial maze paradigms (the classical version of the test and a modified version introducing a one-hour delay between the fourth and the fifth choice) received chronic infusion of gamma-aminobutyric acid (GABA) into the NBM area. GABA (10 and 50 micrograms/microliters/h) was infused for 3 days into the NBM contralateral to their preferred turning direction in the radial maze. Simultaneously, saline (NaCl 0.9%; 1 microliter/h) was infused into the contralateral NBM. GABA and saline infusions were alternated for the subsequent 3-day period. One week later, we investigated the rats' ability to learn a multiple trial passive avoidance task. At the dose of 50 micrograms/microliters, GABA infusion produced (1) a turning bias ipsilateral to the side first infused with GABA, (2) transitory cognitive impairments in radial maze tasks and (3) a deficit in the acquisition of the passive avoidance task. At the dose of 10 micrograms/microliters, the same behavioral deficits were observed except that (1) the turning bias was reversed by the contralateral GABA infusion and (2) cognitive impairments in the radial maze were observed only when a delay was inserted between the fourth and the fifth choice. Histologically, we found a dose-dependent gliosis in the NBM area first infused with GABA. These data suggest a reactivity of the NBM to GABAergic manipulations and the intervention of this structure in both sensorimotor and cognitive processes involved in the radial maze paradigms.

Immunocytochemical localization of cholinergic terminals in the region of the nucleus basalis magnocellularis of the rat: a correlated light and electron microscopic study

The cholinergic circuitry in the nucleus basalis magnocellularis of the rat was investigated in a correlated light and electron microscopic study by using monoclonal antibodies against the acetylcholine-synthesizing enzyme, choline acetyltransferase, following the unlabelled antibody peroxidase-antiperoxidase immunocytochemical procedure. After the immunocytochemical approach, large cholinergic cells and a few immunoreactive fibres exhibiting a varicose appearance, were detected by light microscopy in portions of the nucleus basalis magnocellularis located within the anatomical limits of the globus pallidus, mostly in its ventromedial part. Cholinergic neurons and fibre-like structures were also found within the substantia innominata on the edge of globus pallidus. The same material studied by light microscopy was analysed with the electron microscope. At the ultrastructural level, the immunopositive neurons showed the same cytological characteristics and pattern of synaptic input as cholinergic basal forebrain cells. Additionally, scarce immunoreactive preterminal axons and terminal boutons were detected in the region. The immunoreactive terminals were scattered or formed occasional clusters and appeared as heavily immunostained vesicle-filled boutons making exclusively axodendritic synaptic contacts principally with immunonegative distal dendrites. Both symmetric and asymmetric synaptic contacts established between these structures were detected, although the symmetric contacts were the more numerous. The surface of postsynaptic immunonegative dendrites in asymmetric synaptic contact with immunoreactive terminals was generally covered by terminals that lacked detectable immunoreactivity. In contrast, those in symmetric synaptic contact with labelled terminals showed much sparser input from immunonegative terminals, suggesting that they may belong to interneurons. Very rarely, cholinergic terminals were detected in asymmetric synaptic contact with dendrites which also contained positive immunoreaction product. Asymmetric contacts were frequently characterized by the presence of subjunctional dense bodies. The detection of cholinergic terminals in the region of the nucleus basalis magnocellularis of the rat indicates that this region not only contains cholinergic projecting neurons, but receives a cholinergic input itself. Results of this study provide evidence of the existence of a cholinergic transmission in the basal forebrain of the rat, and also that acetylcholine might play a role in the regulation of the extrinsic cortical cholinergic innervation. The possible sources of this innervation are discussed.

Dopamine in the rat ventral pallidum/substantia innominata: biochemical and electrophysiological studies

Recent anatomical literature suggests that dopaminergic projections ascending from the midbrain terminate within the ventral pallidum/substantia innominata (VP/SI). The present investigation evaluated this possibility using standard biochemical and electrophysiologic approaches. Biochemical studies revealed that dopamine and its major metabolites are present within the rat VP/SI. Concentrations of these compounds were diminished greatly when dopaminergic neurons of the substantia nigra were destroyed. Electrophysiologic studies demonstrated that VP/SI neurons often respond to local applications of dopamine with a decrease in firing rate. These observations support the contention that dopamine regulates neuronal activity within the VP/SI and that cells of origin for at least a portion of this projection lie within the substantia nigra.

Afferent connections of the forebrain cholinergic projection neurons, with special reference to monoaminergic and peptidergic fibers

Earlier light microscopic data on afferent connections to the cholinergic forebrain neurons are reconsidered in the light of EM cross-identification of neurons and synapses by combinations of tracer and immunocytochemical techniques. Such studies suggest that brainstem monoaminergic afferents terminate on cholinergic forebrain neurons, and may modulate the activity of choline acetyltransferase levels in the postsynaptic neurons. A monosynaptic relationship between cholinergic forebrain neurons and neuropeptide Y and somatostatin containing axons is also supported by studies using double immunolabeling techniques at the EM level. These peptidergic afferents originate in part from locally arborizing neurons. Based upon the new data a circuit model for basal forebrain cholinergic neurons is proposed.

Afferents to the basal forebrain cholinergic cell area from pontomesencephalic--catecholamine, serotonin, and acetylcholine--neurons

The afferent input to the basal forebrain cholinergic neurons from the pontomesencephalic tegmentum was examined by retrograde transport of wheatgerm agglutinin-horseradish peroxidase in combination with immunohistochemistry. Multiple tyrosine hydroxylase-, dopamine-beta-hydroxylase-, serotonin- and choline acetyltransferase-immunoreactive fibres were observed in the vicinity of the choline acetyltransferase-immunoreactive cell bodies within the globus pallidus, substantia innominata and magnocellular preoptic nucleus. Micro-injections of horseradish peroxidase-conjugated wheatgerm agglutinin into this area of cholinergic perikarya led to retrograde labelling of a large population of neurons within the pontomesencephalic tegmentum, which included cells in the ventral tegmental area, substantia nigra, retrorubral field, raphe nuclei, reticular formation, pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus, parabrachial nuclei and locus coeruleus nucleus. Of the total population of retrogradely labelled neurons, a significant (approximately 25%) proportion were tyrosine hydroxylase-immunoreactive and found in the ventral tegmental area (A10), the substantia nigra (A9), the retrorubral field (A8), the raphe nuclei (dorsalis, linearis and interfascicularis) and the locus coeruleus nucleus (A6), Another important contingent (approximately 10%) was represented by serotonin neurons of the dorsal raphe nucleus (B7), the central superior nucleus (B8) and ventral tegmentum (B9). A small proportion (less than 1%) was represented by cholinergic neurons of the pedunculopontine (Ch5) and laterodorsal (Ch6) tegmental nuclei. These results demonstrate that pontomesencephalic monoamine neurons project in large numbers up to the basal forebrain cholinergic neurons and may represent a major component of the ventral tegmental pathway that forms the extra-thalamic relay from the brainstem through the basal forebrain to the cerebral cortex.