The effects of graded forelimb afferent volleys on acetylcholine release from cat sensorimotor cortex

Substitution of natural sensory input by artificial neurostimulation of an amputated trigeminal nerve does not prevent the degeneration of basal forebrain cholinergic circuits projecting to the somatosensory cortex

Peripheral deafferentation downregulates acetylcholine (ACh) synthesis in sensory cortices. However, the responsible neural circuits and processes are not known. We irreversibly transected the rat infraorbital nerve and implanted neuroprosthetic microdevices for proximal stump stimulation, and assessed cytochrome-oxidase and choline- acetyl-transferase (ChAT) in somatosensory, auditory and visual cortices; estimated the number and density of ACh-neurons in the magnocellular basal nucleus (MBN); and localized down-regulated ACh-neurons in basal forebrain using retrograde labeling from deafferented cortices. Here we show that nerve transection, causes down regulation of MBN cholinergic neurons. Stimulation of the cut nerve reverses the metabolic decline but does not affect the decrease in cholinergic fibers in cortex or cholinergic neurons in basal forebrain. Artifical stimulation of the nerve also has no affect of ACh-innervation of other cortices. Cortical ChAT depletion is due to loss of corticopetal MBN ChAT-expressing neurons. MBN ChAT downregulation is not due to a decrease of afferent activity or to a failure of trophic support. Basalocortical ACh circuits are sensory specific, ACh is provided to each sensory cortex "on demand" by dedicated circuits. Our data support the existence of a modality-specific cortex-MBN-cortex circuit for cognitive information processing.

Reticulo-cortical activity and behavior: A critique of the arousal theory and a new synthesis

It is traditionally believed that cerebral activation (the presence of low voltage fast electrical activity in the neocortex and rhythmical slow activity in the hippocampus) is correlated with arousal, while deactivation (the presence of large amplitude irregular slow waves or spindles in both the neocortex and the hippocampus) is correlated with sleep or coma. However, since there are many exceptions, these generalizations have only limited validity. Activated patterns occur in normal sleep (active or paradoxical sleep) and during states of anesthesia and coma. Deactivated patterns occur, at times, during normal waking, or during behavior in awake animals treated with atropinic drugs. Also, the fact that patterns characteristic of sleep, arousal, and waking behavior continue in decorticate animals indicates that reticulo-cortical mechanisms are not essential for these aspects of behavior.

These puzzles have been largely resolved by recent research indicating that there are two different kinds of input from the reticular activating system to the hippocampus and neocortex. One input is probably cholinergic; it may play a role in stimulus control of behavior. The second input is noncholinergic and appears to be related to motor activity; movement-related input to the neocortex may be dependent on a trace amine.

Reticulo-cortical systems are not related to arousal in the traditional sense, but may play a role in the control of adaptive behavior by influencing the activity of the cerebral cortex, which in turn exerts control over subcortical circuits that co-ordinate muscle activity to produce behavior.

Inactivation of prefrontal cortex abolishes cortical acetylcholine release evoked by sensory or sensory pathway stimulation in the rat

Sensory stimulation and electrical stimulation of sensory pathways evoke an increase in acetylcholine release from the corresponding cortical areas. The pathways by which such sensory information reaches the cholinergic neurons of the basal forebrain that are responsible for this release are unclear, but have been hypothesized to pass through the prefrontal cortex (PFC). This hypothesis was tested in urethane-anesthetized rats using microdialysis to collect acetylcholine from somatosensory, visual, or auditory cortex, before and after the PFC was inactivated by local microdialysis delivery of the GABA-A receptor agonist muscimol (0.2% for 10 min at 2 microl/min). Before PFC inactivation, peripheral sensory stimulation and ventral posterolateral thalamic stimulation evoked 60 and 105% increases, respectively, in acetylcholine release from somatosensory cortex. Stimulation of the lateral geniculate nucleus evoked a 57% increase in acetylcholine release from visual cortex and stimulation of the medial geniculate nucleus evoked a 72% increase from auditory cortex. Muscimol delivery to the PFC completely abolished each of these evoked increases (overall mean change from baseline = -7%). In addition, the spontaneous level of acetylcholine release in somatosensory, visual, and auditory cortices was reduced by 15-59% following PFC inactivation, suggesting that PFC activity has a tonic facilitatory influence on the basal forebrain cholinergic neurons. These experiments demonstrate that the PFC is necessary for sensory pathway evoked cortical ACh release and strongly support the proposed sensory cortex-to-PFC-to-basal forebrain circuit for each of these modalities.

The role of acetylcholine in cortical synaptic plasticity

This review examines the role of acetylcholine in synaptic plasticity in archi-, paleo- and neocortex. Studies using microiontophoretic application of acetylcholine in vivo and in vitro and electrical stimulation of the basal forebrain have demonstrated that ACh can produce long-lasting increases in neural responsiveness. This evidence comes mainly from models of heterosynaptic facilitation in which acetylcholine produces a strengthening of a second, noncholinergic synaptic input onto the same neuron. The argument that the basal forebrain cholinergic system is essential in some models of plasticity is supported by studies that have selectively lesioned the cholinergic basal forebrain. This review will examine the mechanisms whereby acetylcholine might induce synaptic plasticity. It will also consider the neural circuitry implicated in these studies, namely the pathways that are susceptible to cholinergic plasticity and the neural regulation of the cholinergic system.

The role of basal forebrain neurons in tonic and phasic activation of the cerebral cortex

The basal forebrain and in particular its cholinergic projections to the cerebral cortex have long been implicated in the maintenance of cortical activation. This review summarizes evidence supporting a close link between basal forebrain neuronal activity and the cortical electroencephalogram (EEG). The anatomy of basal forebrain projections and effects of acetylcholine on cortical and thalamic neurons are discussed along with the modulatory inputs to basal forebrain neurons. As both cholinergic and GABAergic basal forebrain neurons project to the cortex, identification of the transmitter specificity of basal forebrain neurons is critical for correlating their activity with the activity of cortical neurons and the EEG. Characteristics of the different basal forebrain neurons from in vitro and in vivo studies are summarized which might make it possible to identify different neuronal types. Recent evidence suggests that basal forebrain neurons activate the cortex not only tonically, as previously shown, but also phasically. Data on basal forebrain neuronal activity are presented, clearly showing that there are strong tonic and phasic correlations between the firing of individual basal forebrain cells and the cortical activity. Close analysis of temporal correlation indicates that changes in basal forebrain neuronal activity precede those in the cortex. While correlational, these data, together with the anatomical and pharmacological findings, suggest that the basal forebrain has an important role in regulating both the tonic and the phasic functioning of the cortex.

Spread of dendritic excitation in layer 2/3 pyramidal neurons in rat barrel cortex in vivo

In layer 2/3 pyramidal neurons of barrel cortex in vivo, calcium ion concentration ([Ca2+]) transients in apical dendrites evoked by sodium action potentials are limited to regions close to the soma. To study the mechanisms underlying this restricted pattern of calcium influx, we combined two-photon imaging of dendritic [Ca2+] dynamics with dendritic membrane potential measurements. We found that sodium action potentials attenuated and broadened rapidly with distance from the soma. However, dendrites of layer 2/3 cells were electrically excitable, and direct current injections could evoke large [Ca2+] transients. The restricted pattern of dendritic [Ca2+] transients is therefore due to a failure of sodium action-potential propagation into dendrites. Also, stimulating subcortical activating systems by tail pinch can enhance dendritic [Ca2+] influx induced by a sensory stimulus by increasing cellular excitability, consistent with the importance of these systems in plasticity and learning.

Responses of cortical EEG-related basal forebrain neurons to brainstem and sensory stimulation in urethane-anaesthetized rats

The basal forebrain can be considered to be a rostral extension of the ascending reticular activating system. A large number of neurons in the basal forebrain have been shown to display higher firing rates when low-voltage fast activity is present in the cortical EEG as opposed to states characterized by large slow waves in both unanaesthetized and anaesthetized animals. However, a smaller number of cells with increased discharge rate during slow waves was also observed in most of these studies. While it is likely that these two types of neurons have opposite roles in the regulation of cortical activation, it is not known how they respond to inputs from the brainstem or the periphery. In the present study, extracellular recordings were made in the basal forebrain of urethane-anaesthetized rats. A total of 52 neurons were studied in which the firing rate was significantly higher during fast cortical EEG waves (F-cells), and 14 neurons in which activity was significantly greater during slow waves (S-cells). The two cell types responded differently to stimulation of the pedunculopontine tegmental nucleus (PPT) and dorsal raphe nucleus (DRN) with short (0.5-1 s) trains of pulses and to noxious sensory stimuli (tail pinch). These stimulations excited most F-cells (80-96%) and inhibited the majority of S-cells (55-67%). In the few F-cells that were inhibited by stimulation, the response varied with the background firing rate of the cell: the higher the firing rate at the time of stimulation, the higher the probability of observing an inhibitory response. In contrast, single electrical pulses delivered to the PPT and DRN excited the majority (72%) of both F- and S-cells. Previous in vitro studies have shown that the application of acetylcholine or serotonin has predominantly inhibitory effects on basal forebrain cholinergic neurons. The predominantly excitatory effect of noxious, PPT and DRN stimulation on F-cells therefore suggests that glutamatergic or other excitatory afferents play a more dominant role in regulating basal forebrain neurons. We have previously shown that F-cells are more prevalent than S-cells. In combination, these findings suggest that basal forebrain neurons, and F-cells in particular, are important in mediating the ascending excitatory drive from the brainstem to the cerebral cortex.

Basal forebrain grafts in the rat neocortex restore in vivo acetylcholine release and respond to behavioural activation

Acetylcholine release in the frontal cortex of awake rats after acute or chronic lesions of the nucleus basalis magnocellularis and grafting of cholinergic-rich basal forebrain tissue was studied by in vivo microdialysis. Three to four weeks and five months after a unilateral quisqualic acid lesion of the nucleus basalis, and five months after lesion and cortical implantation of a basal forebrain cell suspension, acetylcholine release was characterized during a range of pharmacological and behavioural manipulations. Neostigmine (5 microM) was added to the perfusion fluid in order to inhibit the degradation of acetylcholine. The extracellular levels of acetylcholine in normal animals increased three- to four-fold when KCl (100 mM) was added to the perfusion medium and was reduced by 80% after addition of tetrodotoxin (1 microM). The nucleus basalis lesion resulted in a 60% reduction in baseline acetylcholine levels compared to normal and the response to KCl-evoked depolarization was significantly reduced. There were no differences between the acute and chronic lesion groups during any of the manipulations performed. Rats with grafts showed baseline levels of acetylcholine about 70% higher than normal, and responded to both KCl (two-fold increased acetylcholine release) and tetrodotoxin (85% reduced levels). All groups showed lower acetylcholine levels during halothane anaesthesia (on average 70-85% reduction). Sensory stimulation by handling resulted in a two-fold increase in acetylcholine release in normal animals, whereas the absolute responses in the lesioned controls were significantly weaker. Rats with grafts increased their acetylcholine release after handling to an extent not different to normal or lesioned controls. Immobilization stress induced an almost two-fold increase in cortical acetylcholine levels in normal rats, whereas the effect in the lesion-only groups was very weak. The grafts responded to the immobilization with an enhanced acetylcholine overflow that was significantly higher than in lesioned controls. The results showed that the reduction in frontocortical acetylcholine release induced by excitotoxic lesions of the nucleus basalis did not recover spontaneously over several months. Intracortical cholinergic-rich grafts obtained from the fetal basal forebrain provided a source of acetylcholine release with firing-dependent properties which could be modulated by behaviourally stressful stimuli. The ability of the grafts to respond to behavioural manipulation strongly suggests that the host brain can functionally influence graft neuronal activity during ongoing behaviour. Host control of graft activity may play a role in the recovery of the lesion-induced deficits seen with these types of grafts.

Effect of adenosine receptor agonists on spontaneous and K(+)-evoked acetylcholine release from the in vivo rat cerebral cortex

Repeated applications of elevated K+ (100 mM) in artificial cerebrospinal fluid (CSF) were used to evoke an efflux of acetylcholine (ACh) from the in vivo rat cerebral cortex using a cortical cup technique. Elevated K+ reproducibly increased the levels of ACh in cup superfusates by a factor of 3-5-fold above basal levels (27.2 +/- 9.7 nM). The adenosine A1 receptor agonist N6-cyclopentyl adenosine (CPA), at a concentration of 10(-8) M, depressed basal, but not K(+)-evoked ACh efflux. 10(-6) M CPA increased basal, but did not alter K(+)-evoked, ACh efflux. The A2 selective agonist CGS 21680 did not alter either basal, or K(+)-evoked, ACh efflux. The inhibitory effects of 10(-8) M CPA on ACh efflux would be consistent with the presence of adenosine A1 receptors on cholinergic nerve terminals in the cerebral cortex. At a higher concentration (10(-6) M) CPA elevated basal release, possibly by activating low affinity A2 receptors. The failure of CGS 21680 (10(-8) M) to alter basal ACh release suggests an absence of high affinity A2 receptors in these terminals. Whereas elevated K+ in cup superfusates consistently enhanced ACh efflux from the cerebral cortex, this increase was not affected by either CPA or CGS 21680. High K(+)-evoked release of cerebral cortical ACh may be an inappropriate model for the study of adenosine's actions on neurotransmitter release.

Acetylcholine release in the rat hippocampus as studied by microdialysis is dependent on axonal impulse flow and increases during behavioural activation

Changes in extracellular levels of acetylcholine and choline in the hippocampal formation were measured using intracerebral microdialysis coupled to high performance liquid chromatography with post-column enzyme reaction and electrochemical detection. Various pharmacological and physiological manipulations were applied to awake unrestrained normal rats and rats subjected to a cholinergic denervation of the hippocampus by a complete fimbria-fornix lesion (1-2 weeks previously). Low baseline levels of acetylcholine (about 0.3 pmol/15 min sample) could be detected in the absence of acetylcholinesterase inhibition in all animals. However, in order to obtain stable and more readily detectable levels, the acetylcholinesterase inhibitor neostigmine was added to the perfusion medium at a concentration of 5 or 10 microM and was used during all subsequent manipulations. Addition of neostigmine increased acetylcholine levels approximately 10-fold (to 3.7 pmol 15 min) in the normal rats, which was about 4-fold higher than the levels recovered from the denervated hippocampi. Depolarization by adding KCl (100 mM) to the perfusion fluid produced a 3-fold increase in the extracellular acetylcholine levels, and the muscarinic antagonist atropine (3 microM) resulted in a 4-fold increase in the normal rats, whereas these drugs induced only small responses in the denervated rats. Neuronal impulse blockade by tetrodotoxin (1 microM) resulted, in normal rats, in a 70% reduction in extracellular acetylcholine levels. Sensory stimulation by handling increased acetylcholine levels by 94% in the normal rats, whereas this response was almost totally abolished in the denervated hippocampi. Behavioural activation by electrical stimulation of the lateral habenula resulted in a 4-fold increase in acetylcholine release in normal animals, and this response was totally blocked by a transection of the lateral habenular efferents running in the fasciculus retroflexus. The levels obtained by lateral habenula stimulation were reduced by about 95% in the rats with fimbria-fornix lesions. Following an acute knife transection of the fimbria-fornix performed during ongoing dialysis, acetylcholine levels dropped instantaneously by 70%, indicating that the extracellular acetylcholine levels in the hippocampus are maintained by a tonic impulse flow in the septohippocampal pathway. The extracellular levels of choline were reduced by about 30% after the addition of neostigmine in the normal rats, and increased by about 50% in both normal and denervated rats after addition of KCl to the perfusion fluid. No changes could be detected after atropine, handling, lateral habenula stimulation, or acute fimbria-fornix or fasciculus retroflexus transection.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of identified septo-hippocampal neurons by noxious peripheral stimulation

Septo-hippocampal neurons (SHNs) were recorded from the medial septum-diagonal band area of rats anaesthetized with either urethane or fluothane. They were identified by their antidromic response to the electrical stimulation of the fimbria. Their responses to peripheral somatic noxious and non-noxious stimulation were studied. Non-noxious natural stimulations were relatively ineffective. In contrast, 68% of the SHNs were driven by noxious stimulation. The SHNs could be driven either by mechanical or thermal stimulation. Intraperitoneal injection of bradykinin excited about half of the SHNs. Some neurons were able to encode stimulus intensity (strength of the mechanical stimulation and/or temperature of the thermal stimulation). The receptive fields of the SHNs were large, usually involving half of the body or the whole body surface. These results suggest that SHNs, which are at the origin of the cholinergic septo-hippocampal pathway, might be involved in cerebral mechanisms related to nociception.

Effects of diazepam on adenosine and acetylcholine release from rat cerebral cortex: further evidence for a purinergic mechanism in action of diazepam

1 Diazepam administered intraperitoneally (0.25 mg/kg) enhanced the rate of efflux of [3H]-adenosine and its metabolites from rat cerebral cortex. At a lower dose (0.05 mg/kg), this effect could be detected in only one of four rats. 2 Diazepam (0.05 and 0.25 mg/kg i.p.) depressed acetylcholine release from the rat cerebral cortex. Its effect was reversed by theophylline. 3 Theophylline (15 and 30 mg/kg) enhanced acetylcholine release from the rat cerebral cortex. Diazepam (0.25 mg/kg) administered after theophylline failed to cause a reduction in the rate of release, rather there appeared to be a further enhancement of release. 4 Pentobarbitone sodium (5, 10 and 15 mg/kg i.p.) did not elicit any increase in adenosine release. 5 These results support the proposal that benzodiazepines may exert their pharmacological actions by preventing adenosine uptake, thus enhancing the levels of extracellular adenosine.

Catecholamines released from cerebral cortex in the cat; decrease during sensory stimulation

In an attempt to determine the functional role of catecholamine (CA) nerve terminals in cerebral cortex the release of endogenous norepinephrine (NE) and dopamine (DA) into superfusates from visual and somatosensory cortex of the cat have been measured by a sensitive radiometric enzymatic assay based on the methylation of CA by catechol-O-methyltransferase (COMT) in the presence of a [3H]-methyl donor and followed by resolution of 3H derivatives through a series of organic extractions. In the flaxedilized animal maintained under local anaesthesia with artificial respiration the concentration of CA measured in 30-min superfusates was fairly constant in a given experiment under basal conditions without sensory stimulation, but varied widely from one experiment to another. Variations in NE were often independent of those for DA. For visual cortex the average basal release of NE in experiments was 20.09 +/- 3.64 pg/min/sq.cm while the average for DA was 34.01 +/- 7.62 pg/min/sq.cm. In all experiments intermittent visual stimulation (15/sec) produced a significant reduction in release rate averaging about 42% for NE and 64% for DA in visual cortex. The reduction was relatively non-specific since visual or somatic sensory stimulation produced a decrease in release from both visual and somatic sensory cortical areas. Since it has been shown that there is a relatively non-specific increase in acetylcholine (ACh) release from sensory cortex during stimulation, it is proposed that ACh may regulate CA release at presynaptic CA terminals in the cortex as it does in the periphery. A marked increase in CA release observed on perfusing with nicotine or atropine is consistent with this hypothesis.