GABAergic mediation of indirect transsynaptic control over basal and spatial memory testing-induced activation of septo-hippocampal cholinergic activity in mice

Medial septum-diagonal band of Broca (MSDB) GABAergic regulation of hippocampal acetylcholine efflux is dependent on cognitive demands

The septohippocampal pathway contains cholinergic, GABAergic, and glutamatergic projections and has an established role in learning, memory, and hippocampal theta rhythm. Both GABAergic and cholinergic neurons in the medial septum-diagonal band of Broca (MSDB) have been associated with spatial memory, but the relationship between the two neuronal populations is not fully understood. The present study investigated the effect of selective GABAergic MSDB lesions on hippocampal acetylcholine (ACh) efflux and spatial memory during tasks that varied in memory demand. Male Sprague Dawley rats were given GABAergic lesions of the MSDB using GAT1-saporin (GAT1-SAP) and examined on spontaneous exploration (Experiment 1) and non-matching to position without (NMTP; Experiment 2) and with a delay (DNMTP; Experiment 3), while concurrently using in vivo microdialysis to measure hippocampal ACh efflux. Intraseptal GAT1-SAP treatment did not alter baseline or behaviorally stimulated hippocampal ACh efflux or maze exploration (Experiment 1). Moreover, GAT1-SAP did not alter evoked hippocampal ACh efflux related to NMTP nor did it impair working memory in NMTP (Experiment 2). In contrast, both ACh efflux and performance in DNMTP were impaired by intraseptal GAT1-SAP. Thus, GABAergic MSDB neurons are important for spatial working memory and modulate hippocampal ACh efflux under conditions of high memory load. The relationship between the septohippocampal cholinergic and GABAergic systems and working memory will be discussed.

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.

Possible interaction of cholinergic and GABAergic systems between MS and CA1 upon memory acquisition in rats

The present study explored the possibility that cholinergic and GABAergic systems of medial septum (MS) might influence acquisition of memory by regulation of acetylcholine (Ach) and γ-aminobutyric acid (GABA) receptors function in hippocampus and vice versa. The step-through passive avoidance (PA) task was used. The results showed that pre-training intra-MS/CA1 administration of nonselective muscarinic Ach antagonist, scopolamine (0.5, 1 and 2 μg/rat) and GABA(A) receptor agonist, muscimol (0.01 and 0.02 μg/rat) impaired, while acetylcholinesterase inhibitor, physostigmine (0.5 and 1 μg/rat) and GABA(A) receptor antagonist, bicuculline (0.25 μg/rat) improved memory acquisition. Moreover, intra-CA1/MS administration of a subthreshold dose of muscimol or bicuculline increased and reversed the impairment induced by scopolamine in MS/CA1 respectively (cross injection). Also, the result revealed that, intra-CA1/MS administration subthreshold dose of muscimol reduced improvement of memory induced by physostigmine in the MS/CA1, respectively (cross injection). On the other hand, subthreshold dose of bicuculline in CA1/MS did not alter memory improvement induced by physostigmine in the other site (MS/CA1). In conclusion, both cholinergic and GABAergic systems not only seem to play a role in the modulation of memory in the MS and CA1 but also to have a complex interaction.

The memory-impairing effects of septal GABA receptor activation involve GABAergic septo-hippocampal projection neurons

Septal infusions of the gamma-aminobutyric acid (GABA)(A) agonist muscimol impair memory, and the effect likely involves the hippocampus. GABA(A) receptors are present on the perikarya of cholinergic and GABAergic septo-hippocampal (SH) projections. The current experiments determined whether GABAergic SH projections are involved in the memory-impairing effects of septal GABA(A) receptor activation. Experiment 1 tested whether combining septal co-infusions of subeffective doses of muscimol with scopolamine, a drug that selectively influences GABA SH projections, would produce memory deficits. Experiment 2 tested whether hippocampal infusions of a GABA(A) receptor antagonist would block the effects of septal muscimol infusions. Fifteen minutes prior to assessing spontaneous alternation (SA) or training in a multiple trial inhibitory avoidance (CMIA) task, male Sprague-Dawley rats were given septal infusions of vehicle, muscimol, scopolamine, or co-infusions of muscimol with scopolamine, or septal infusions of vehicle or muscimol combined with hippocampal infusions of vehicle or bicuculline. Septal co-infusions of muscimol with scopolamine significantly impaired SA and CMIA. Hippocampal bicuculline infusions blocked deficits produced by septal muscimol infusions in SA and attenuated deficits produced in CMIA. Combined, these findings suggest that GABAergic SH projections are involved in the memory-impairing effects of septal GABA receptor activation.

Microdialysis measures of functional increases in ACh release in the hippocampus with and without inclusion of acetylcholinesterase inhibitors in the perfusate

Because brain extracellular acetylcholine (ACh) levels are near detection limits in microdialysis samples, an acetylcholinesterase (AChE) inhibitor such as neostigmine is often added to microdialysis perfusates to increase ACh levels in the dialysate, a practice that raises concerns that the inhibitor might alter the results. Two experiments compared functional differences in ACh release with and without neostigmine. In the first experiment, 30-60% increases in extracellular ACh concentrations in the hippocampus were evident during food-rewarded T-maze training with 20-500 nm neostigmine in the perfusate but no increases were seen without neostigmine. In the second experiment, 78% increases in ACh release in the hippocampus were seen after injections of the GABA(A) receptor antagonist, bicuculline, into medial septum only if neostigmine (50 nm) was included in the perfusate. These findings suggest that, in the hippocampus, endogenous brain AChEs are very efficient at removing extracellular ACh, obscuring differences in ACh release in these experiments. Therefore, inclusion of AChE inhibitors in the microdialysis perfusate may be necessary under some conditions for observations of functional changes in release of ACh in the hippocampus.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Glucose increases hippocampal extracellular acetylcholine levels upon activation of septal GABA receptors

Activation of septal GABA receptors impairs learning and memory and this effect likely involves an influence on the hippocampus. We found previously that the memory-impairing effects of septal infusions of the GABA agonist muscimol are reversed by hippocampal infusions of glucose and suggested that glucose reverses these deficits by increasing hippocampal acetylcholine (ACh). In this study, we report that septal infusions of muscimol produce dose-dependent decreases in ACh levels in hippocampal dialysates. Importantly, increasing glucose levels in the hippocampus elevates hippocampal extracellular ACh levels in rats given septal infusions of muscimol, but not in rats given vehicle. Thus, glucose increases hippocampal extracellular ACh levels when the ACh system is inhibited, an effect that likely contributes to the effects of glucose on memory.

Septal GABAergic and hippocampal cholinergic systems interact in the modulation of anxiety

According to Gray [(1982) The neuropsychology of anxiety: an enquiry into the function of the septo-hippocampal system. Oxford: Oxford University Press; (1991) Neural systems, emotion and personality. In: Neurobiology of learning, emotion, and affect (Madden J, ed), pp 273-306. New York: Raven Press; Gray JA, McNaughton N (2000) The neuropsychology of anxiety. Oxford: Oxford University Press], the septum and the hippocampus act in concert to control anxiety. In the present study we examined a possible interaction between septal GABAergic and hippocampal cholinergic systems in the shock-probe burying test, an animal model of anxiety. In experiment 1, we found that a 10-ng infusion of muscimol in the medial septum produced a significant suppression of burying behavior, whereas lower doses (2.5 and 5.0 ng) did not. In experiment 2, we found a significant suppression of burying behavior after a 20-microg infusion of physostigmine into the dorsal hippocampus, but not after lower-dose infusions (5 and 10 microg). In experiment 3, we infused combined sub-effective doses of physostigmine and muscimol in the hippocampus and medial septum respectively. The combination of sub-effective doses of physostigmine (5 microg) and muscimol (2.5 ng) significantly reduced burying of the shock probe. The results indicate that the hippocampal cholinergic and septal GABAergic systems act synergistically in the modulation of anxiety.

Septal infusions of glucose or pyruvate, but not fructose, produce avoidance deficits when co-infused with the GABA agonist muscimol

Although glucose typically enhances memory or reverses memory deficits, glucose can also produce memory deficits when co-infused with the gamma-aminobutyric acid (GABA) agonist muscimol into the medial septum (Parent & Gold, 1997; Parent, Laurey, Wilkniss, & Gold, 1997). To date the mechanisms underlying the memory-impairing interaction between GABA and glucose remain unknown. Here we investigate whether this effect is the result of hyperosmolar conditions or may involve glucose metabolism. Male Sprague-Dawley rats were given one-trial inhibitory avoidance training after receiving septal infusions of vehicle (phosphate-buffered saline, 0.5 microl), the GABA(A) agonist muscimol (3 nmol), glucose (16.5, 33, or 66 nmol), fructose (33 nmol), pyruvate (33 nmol), or a solution containing muscimol combined with glucose, fructose, or pyruvate. Retention performance was tested 48 h later. Infusions of glucose, pyruvate, fructose, or muscimol alone did not affect retention performance. However, co-infusions of all doses of glucose (16.5, 33, or 66 nmol) or the glycolytic end product pyruvate with muscimol impaired retention performance. Co-infusions of fructose with muscimol did not affect retention performance. These results suggest that the memory-impairing interaction between glucose and muscimol does not result from hyperosmolar conditions, because equiosmolar concentrations of fructose do not mimic the effects of glucose and the memory deficits do not vary as a function of glucose concentration. The finding that pyruvate mimicked the effects of glucose and impaired memory when combined with muscimol suggests that glucose metabolism may be involved in the memory-impairing interaction between glucose and GABA(A) receptors in the medial septum.

Insulin-like growth factor-I inhibits endogenous acetylcholine release from the rat hippocampal formation: possible involvement of GABA in mediating the effects

Evidence suggests that insulin-like growth factor-I (IGF-I) plays an important role during brain development and in the maintenance of normal as well as activity-dependent functioning of the adult brain. Apart from its trophic effects, IGF-I has also been implicated in the regulation of brain neurotransmitter release thus indicating a neuromodulatory role for this growth factor in the central nervous system. Using in vitro slice preparations, we have earlier reported that IGF-I potently inhibits K(+)-evoked endogenous acetylcholine (ACh) release from the adult rat hippocampus and cortex but not from the striatum. The effects of IGF-I on hippocampal ACh release was sensitive to the Na(+) channel blocker tetrodotoxin, suggesting that IGF-I might act indirectly via the release of other transmitters/modulators. In the present study, we have characterized the possible involvement of GABA in IGF-I-mediated inhibition of ACh release and measured the effects of this growth factor on choline acetyltransferase (ChAT) activity and high-affinity choline uptake in the hippocampus of the adult rat brain. Prototypical agonists of GABA(A) and GABA(B) receptors (i.e. 10 microM muscimol and 10 microM baclofen) inhibited, whereas the antagonists of the respective receptors (i.e. 10 microM bicuculline and 10 microM phaclofen) potentiated K(+)-evoked ACh release from rat hippocampal slices. IGF-I (10 nM) inhibited K(+)- as well as veratridine-evoked ACh release from rat hippocampal slices and the effect is possibly mediated via the activation of a typical IGF-I receptor and the subsequent phosphorylation of the insulin receptor substrate-1 (IRS-1). The inhibitory effects of IGF-I on hippocampal ACh release were not additive to those of either muscimol or baclofen, but were attenuated by GABA antagonists, bicuculline and phaclofen. Additionally, in contrast to ACh release, IGF-I did not alter either the activity of the enzyme ChAT or the uptake of choline in the hippocampus. These results, taken together, indicate that IGF-I, under acute conditions, can decrease hippocampal ACh release by acting on the typical IGF-I/IRS receptor complex while having no direct effect on ChAT activity or the uptake of choline. Furthermore, the evidence that effects of IGF-I could be modulated, at least in part, by GABA antagonists suggest that the release of GABA and the activation of its receptors may possibly be involved in mediating the inhibitory effects of IGF-I on hippocampal ACh release.

Infusions of physostigmine into the hippocampus or the entorhinal cortex attenuate avoidance retention deficits produced by intra-septal infusions of the GABA agonist muscimol

Septal gamma-aminobutyric acid (GABA) receptor activation is known to disrupt memory formation, although the mechanisms underlying this impairment remain unclear. The present study explored the possibility that high levels of septal GABA receptor activity might impair memory by down-regulating acetylcholine (ACh) function in archicortex and entorhinal cortex. To test this possibility, rats were trained on an avoidance task 15 min after receiving intra-septal infusions of vehicle or muscimol (5 nmol/0.5 microl) combined with unilateral intra-hippocampal (10 microl/1 microl) or intra-entorhinal cortex (1.875 microg/0.25 microl) infusions of vehicle or the acetylcholinesterase inhibitor physostigmine. We demonstrate that these infusions do not alter acquisition performance on a continuous multiple trial inhibitory avoidance task. However, intra-septal infusions of muscimol dramatically impair retention performance 48 h later. More importantly, infusions of physostigmine into the hippocampus or the entorhinal cortex, at doses that do not influence acquisition or retention performance when infused alone, attenuate the impairing effects of the muscimol infusions on retention. We suggest that high levels of septal GABA receptor activity might impair memory by down-regulating ACh levels in the hippocampal region, and that such memory impairments can be ameliorated by increasing ACh levels in the hippocampus or entorhinal cortex.

Septal GABAergic and hippocampal cholinergic systems modulate anxiety in the plus-maze and shock-probe tests

According to Gray [The neuropsychology of anxiety: an inquiry into the function of the septo-hippocampal system. Oxford: Oxford Univ. Press, 1982; Neural systems, emotion and personality. In: Madden VJ, editor. Neurobiology of learning, emotion, and affect. New York: Raven Press, 1991. p. 273-306.] the septum and hippocampus act in concert to control anxiety. In the present study we examined the roles of these structures in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. We found that microinfusions (20 ng/0.4 microl) of the GABA(A) agonist muscimol into either the lateral or the medial septum increased rats' open-arm exploration in the plus-maze test, and decreased their burying behavior in the shock-probe test. We also found that infusions of the acetylcholinesterase inhibitor physostigmine (10 microg/microl) into the dorsal hippocampus, like intraseptal muscimol (20 ng/0.4 microl), increased open-arm exploration in the plus-maze test, and decreased burying behavior in the shock-probe test. Although combined infusions of intraseptal muscimol and intrahippocampal physostigmine did not increase the magnitude of anxiolysis, this may have been due to "ceiling" effects. Overall, the results confirm that septal GABAergic and hippocampal cholinergic systems are both involved in the modulation of anxiety.

Increasing acetylcholine levels in the hippocampus or entorhinal cortex reverses the impairing effects of septal GABA receptor activation on spontaneous alternation

Intra-septal infusions of the gamma-aminobutyric acid (GABA) agonist muscimol impair learning and memory in a variety of tasks. This experiment determined whether hippocampal or entorhinal infusions of the acetylcholinesterase inhibitor physostigmine would reverse such impairing effects on spontaneous alternation performance, a measure of spatial working memory. Male Sprague-Dawley rats were given intra-septal infusions of vehicle or muscimol (1 nmole/0.5 microL) combined with unilateral intra-hippocampal or intra-entorhinal infusions of vehicle or physostigmine (10 microg/microL for the hippocampus; 7.5 microg/microL or 1.875 microg/0.25 microL for the entorhinal cortex). Fifteen minutes later, spontaneous alternation performance was assessed. The results indicated that intra-septal infusions of muscimol significantly decreased percentage-of-alternation scores, whereas intra-hippocampal or intra-entorhinal infusions of physostigmine had no effect. More importantly, intra-hippocampal or intra-entorhinal infusions of physostigmine, at doses that did not influence performance when administered alone, completely reversed the impairing effects of the muscimol infusions. These findings indicate that increasing cholinergic levels in the hippocampus or entorhinal cortex is sufficient to reverse the impairing effects of septal GABA receptor activation and support the hypothesis that the impairing effects of septal GABAergic activity involve cholinergic processes in the hippocampus and the entorhinal cortex.

GABA receptors and benzodiazepine binding sites modulate hippocampal acetylcholine release in vivo

In the present study, the regulation of acetylcholine release from the ventral hippocampus by gamma-aminobutyric acid (GABA) was investigated in vivo. GABA receptor agonists and antagonists were administered locally in the medial septum and the adjacent vertical limb of the diagonal band of Broca, or in the hippocampus by retrograde dialysis. Acetylcholine release was measured in the ventral hippocampus. In addition, the modulation of acetylcholine release via septal benzodiazepine binding sites was assessed by intraseptal administration of an agonists and an antagonist at the benzodiazepine binding site. Intraseptal administration of the GABA(A) receptor agonist muscimol and the GABA(B) receptor agonist baclofen, but not the agonist of the benzodiazepine binding site midazolam, decreased acetylcholine release in the hippocampus. The GABA(A) receptor antagonist bicuculline and the antagonist of the benzodiazepine binding site flumazenil, but not the GABA(B) receptor antagonist 3-N-(3,4,-dichlorobenzyl) aminopropyl-P-diethoxymethyphosphinic acid (CGP 52432) increased acetylcholine release in the hippocampus upon intraseptal administration. The same GABA receptor ligands were administered in the ventral hippocampus. CGP 52432 induced a small increase in acetylcholine release, whereas baclofen, muscimol and bicuculline did not affect local acetylcholine release. Thus, endogenous GABA causes tonic inhibition of acetylcholine release in the ventral hippocampus via septal GABA(A) receptors and, to a lesser extent, via GABA(B) receptors in the medial septum and hippocampus. The GABAergic inhibition in the medial septum is reduced by antagonists of the benzodiazepine binding site.

Involvement of medial septal glutamate and GABAA receptors in behaviour-induced acetylcholine release in the hippocampus: a dual probe microdialysis study

In the present study, the role of medial septal receptors in behaviour-induced increase in acetylcholine (ACh) release in hippocampus was investigated using dual-probe microdialysis in combination with a simple behavioural procedure. gamma-Aminobutyric acid (GABA) and glutamate receptor agonists and antagonists were administered by retrograde dialysis into the medial septal area, while ACh was measured in the ventral hippocampus. Rats were behaviourally activated by immobilization or handling, but only handling was performed during drug-treatment. The GABAA receptor agonist muscimol did not affect ACh release, but blocked the handling-induced increase in ACh release completely. In addition, muscimol administration induced behavioural activity. Administration of the GABAA receptor antagonist bicuculline increased ACh release more than 2-fold. However, handling-induced increase in ACh release, expressed as percentage of drug-induced release, was similar to that of controls. Administration of the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the septal area increased hippocampal ACh levels, but reduced the handling-induced increase in ACh release. The NMDA antagonist, 3-((R)-2-carboxypiperazin-4-yl) (CPP) increased ACh levels moderately, and reduced handling-induced increase in ACh release. However, similarly to muscimol, CPP administration induced behavioural activity. The AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) did not affect behaviour or basal ACh levels, but attenuated the handling-evoked ACh release. We conclude that the handling-induced increase in hippocampal ACh levels is mediated via stimulation of septal non-NMDA, and possibly NMDA receptors, whereas GABAA receptors are probably not involved. The feasibility of the experimental design is further discussed.

Intraseptal infusions of muscimol impair spontaneous alternation performance: infusions of glucose into the hippocampus, but not the medial septum, reverse the deficit

As observed with intraseptal injections of opioid receptor agonists, direct infusions of GABAergic receptor agonists into the medial septum impair performance on several tasks that involve spatial or working memory processes in rats. Because the effects of opioid-induced impairments can be reliably reversed by concomitant intraseptal infusions of glucose, the experiments reported here determined whether impairments produced by GABAergic agonists would similarly be reversed by glucose. The findings of Experiment 1 showed, in male Sprague-Dawley rats, that intraseptal infusions of the GABA agonist muscimol (1 or 3 nmol/0.5 microliter) impaired spontaneous alternation performance. The results of Experiment 2 indicated that intraseptal infusions of glucose (8, 17, or 33 nmol) or glutamate (15 or 30 nmol) did not attenuate the muscimol-induced deficit on spontaneous alternation performance, whereas infusions of the GABAergic antagonist bicuculline methiodide (0.1 nmol) did. However, the findings of Experiment 3 indicated that glucose injections (50 nmol/0.5 microliter) into the hippocampus did reverse the impairing effect of the intraseptal muscimol infusions. Combined, these findings suggest that the neurochemical regulation of learning and memory may involve hierarchical interactions between particular neurotransmitter and neuroanatomical systems. Specifically, medial septal GABAergic effects on spontaneous alternation prevail over those of glucose or glutamate in the medial septum, but are overridden by the effects of glucose in the hippocampus.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Trans-synaptic stimulation of cortical acetylcholine and enhancement of attentional functions: a rational approach for the development of cognition enhancers

Activation and restoration of cholinergic function remain major foci in the development of pharmacological approaches toward the treatment of cognitive dysfunctions associated with aging and dementia. Our research has been guided by the hypothesis that (re)activation of cortical cholinergic inputs is achieved as a result of trans-synaptic disinhibition of basal forebrain cholinergic neurons. This approach depends on the ability of benzodiazepine receptor (BZR) inverse agonists to reduce the potency of GABA to block neuronal excitation. BZR inverse agonists were found to augment cortical ACh efflux through interaction with cognition-associated activation of this system. Cortical cholinergic inputs have been implicated in the processing of behaviorally significant stimuli, i.e., attentional functions. Using a recently developed and validated task for the measurement of sustained attention, or vigilance, administration of BZR inverse agonists were found to selectively increase the number of false alarms in intact animals. However, in animals with a 50-70%, but not > 90%, loss of the cortical cholinergic inputs, treatment with BZR inverse agonists alleviated the lesion-induced impairment in sustained attention and enhanced activated cortical ACh efflux. A rational development of cognitive enhancers will benefit from experiments in which cognitive and neuropharmacological variables are assessed simultaneously, thus allowing the analysis of interactions between cognition-associated neuronal activity and the neuronal and cognitive effects of putative cognition enhancers.

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.

Behavioral correlates of single units in the medial septal area: the effect of ethanol

Experimental manipulations that compromise the medial septal area consistently and selectively impair working memory. The electrophysiological and pharmacological properties of medial septal neurons have been studied extensively, but the relation between medial septal neuronal activity and ongoing behavior has not been systematically analysed. Working memory was assessed in a continuous conditional discrimination task, and behavioral performance was correlated with medial septal single unit activity. Operant performance and the activity of rhythmically active neurons were continuously monitored during a 90 min test session, and peri-event time histograms of unit activity were constructed around relevant task events. Rats received intraperitoneal injections of either saline or ethanol (0.75 g/kg) 5 min before testing. Of the 52 medial septal neurons recorded under saline conditions, approximately 80% had significant behavioral correlates. Thirty-five per cent of these neurons were selectively activated at the time of the response and 65% at the time of the reward. Response-related activity was not selective for responses to the right or left lever, or to a particular type of trial, but in 61% of the cases was correlated with the accuracy of the response. In ethanol-treated rats, working memory was impaired, single unit activity was disrupted, and the behavioral correlates were less frequent and robust, especially the response-related correlates that were accuracy-sensitive. The results suggest that the medial septal area is involved in guiding accurate responses and processing rewards, and may contribute to the ethanol-induced impairments in working memory.

Consequences of selective blockade of septal noradrenergic afferents on anxiety and spatial working memory performance in mice

This experiment was designed to investigate the role of septal noradrenergic (NA) afferents in the control of anxiety and spatial working memory. To this end, C57Bl/6 mice were infused bilaterally into the lateral septal nuclei with 500 ng/0.2 microliter of BE 2254, a selective alpha 1 postsynaptic adrenoceptor antagonist. The consequences of this reversible treatment were evaluated 20 min later on the anxiety level measured in an elevated plus-maze and on spatial working memory, evaluated under four different conditions via the learning of a delayed nonmatching to place (DNMTP) rule achieved in an eight-arm radial maze. In these conditions, the BE 2254, as well as the saline-injected control group, showed an elevation of the anxiety level that may be the indirect expression of a nonspecific septal dysfunction induced by the vehicle injection rather than the normal behavioral response produced by the decrease of septal NA activity. This septal dysfunction also impaired spatial working memory but only when mnesic difficulty of the task is increased, suggesting that this impairment expresses a general memory deficit rather than a working memory deficit per se. A lack of spatial working memory deficits in BE 2254 or saline-injected animals was also observed in two other conditions of the behavioral protocol. However, when treatments were applied before the first exposure of animals to the radial maze (exploration session), only the group which received BE 2254 was impaired during the acquisition session for the rule performed 24 h later. This delayed perturbation seems to be linked, at this stage of the learning procedure, to the lack of NA-dependent processes taking place during the exploration session. Taken together, these data suggest that septal NA mechanisms are more essential at initial stage of this learning, when animals process new features of the situation, than during the expression of spatial working memory per se.

Spatial working memory over long retention intervals: dependence on sustained cholinergic activation in the septohippocampal or nucleus basalis magnocellularis-cortical pathways?

Previous direct neurochemical studies of the temporal dynamics of cholinergic activation in the septohippocampal and nucleus basalis magnocellularis-cortical pathways at various stages during repeated testing of mice with selective spatial reference or working memory protocols [Durkin and Toumane (1992), Behav. Brain Res. 50, 43-52] showed that the post-test durations of cholinergic activation in each pathway varied as a function of the type of memory tested and the level of task mastery. Since (i) the hippocampal formation is considered to constitute a critical component of a temporary memory buffer, and (ii) working memory items are not thought to be submitted to consolidation and permanent storage, we postulated that the duration of testing-induced cholinergic activation in the septohippocampal pathway may govern the maintenance of the working memory trace over the retention interval. In order to test directly this hypothesis C57 B1/6 mice were extensively trained (one trial/day, 25-30 days) on an identical selective working memory task to attain high levels of retention (> 80% correct), but using either 5 min (Group 1), or 60 min (Group 2) retention intervals. At various times (30 s-75 min) following the initial acquisition phase of the test, cholinergic activity in the hippocampus and frontal cortex was quantified using measures of high-affinity choline uptake. Whereas cholinergic activation was observed in both pathways at 30 s post-acquisition and throughout the 5 min retention interval in Group 1, the situation in Group 2 is different, activation of the septohippocampal pathway being maintained for only 15 min, while activation in the nucleus basalis magnocellularis-cortical pathway is maintained for the totality of the 1 h retention interval. The nucleus basalis magnocellularis-cortical cholinergic pathway, in addition to its role in long-term reference memory storage processes may, thus, via an intervention in the temporal encoding of information, also subsume a complementary intermediate-term buffer storage role in working memory situations requiring retention intervals in excess of 15 min in mice. This secondary, "backup", function of the nucleus basalis magnocellularis-cortical pathway would thus liberate the septohippocampal complex from its primary active role in the temporary maintenance and/or accessibility of the working memory trace in these particular cases requiring long retention intervals.

Cognitive functions of cortical ACh: lessons from studies on trans-synaptic modulation of activated efflux

Trans-synaptic modulation of cortical ACh efflux is a useful approach for determining the functions of cortical ACh. Bilateral modulation of basal forebrain GABAergic transmission by benzodiazepine-receptor agonists and inverse agonists decreases and increases, respectively, activated cortical ACh efflux. The determination of behavioral functions which are mediated via activated cortical ACh efflux, and therefore subject to the effects of basal forebrain GABA-cholinergic manipulations, should promote analyses of the functions of cortical ACh. Trans-synaptic approaches to enhance activated cortical ACh efflux offer some potential for the treatment of cognitive dysfunctions associated with impaired cortical cholinergic transmission.

Time gradient for post-test vulnerability to scopolamine-induced amnesia following the initial acquisition session of a spatial reference memory task in mice

The time course for vulnerability to the amnestic effects of the cholinergic antagonist, scopolamine, during the postacquisition period has been investigated. We have examined the effects of post-test injections of scopolamine (1 mg/kg ip) given at different times from 30 s for up to 6 h following the end of the first acquisition session of a concurrent spatial discrimination (reference memory) protocol in an 8-arm radial maze on subsequent long-term (24 h) retention performance in C57BL/6 mice. Results show that the immediate (30 s) post-test injection of scopolamine-HCl on Day 1 produces marked perturbation (amnesia) of long-term retention as attested to by significant deficits in various indices of spatial discrimination performance gain on Day 2 as compared to control subjects injected either with scopolamine-MBr or saline. The severity of this scopolamine-induced amnesia declines only slightly as a function of the treatment period 30 s-3 h post-test. However, no evidence for amnesia is observed if scopolamine-HCl injections are delayed for 6 h postsession. This important latter observation attests to the absence of any significant proactive effects of scopolamine on the ability of mice to perform the retention test via possible long-term effects on attention, motivation, or locomotor performance. These results thus constitute evidence for the existence of a limited (30 s-3 h) time gradient for vulnerability of the early memory trace to disruption by scopolamine. The present results are discussed in relation to our previous direct neurochemical observations describing the differential time courses of intervention of the ascending septohippocampal and nBM-cortical cholinergic pathways in the postlearning period. In particular, the presently observed time window concerning post-test vulnerability to scopolamine-induced amnesia corresponds more closely to the time course of the acute activation of the nBM-cortical cholinergic pathway, induced by testing with the same spatial memory protocol as used in the present study in mice.