Central effects of muscarinic agonists and antagonists on hippocampal theta rhythm and blood pressure in the anaesthetised rat

Symptomatic treatment of Alzheimer’s disease: identification of biomarkers to aid translation from bench to bedside

In the absence of robust pharmacodynamic markers, the potential success of novel therapeutic agents for the symptomatic relief of Alzheimer’s disease is largely unknown until the drugs enter relatively large studies, assessing clinical outcome over a 6-month period. In order to increase the efficiency of future clinical development there is, therefore, a need to identify pharmacodynamic markers of drug response, pharmacodynamic models that allow early prediction of efficacy and markers to aid the stratification of the patient population. Using literature available from cholinesterase inhibitors, memantine and Ginkgo biloba, this review focuses on the identification of potential pharmacodynamic markers/models and highlights the utility of these end points throughout the drug discovery process, from preclinical to clinical development.

Antioxidant response analysis in the brain after pilocarpine treatments

Cholinergic and gabaergic systems play an important role generating electroencephalographic activity and regulating vigilance states. Pilocarpine is a cholinergic agonist commonly used to induce seizures and an epilepticus-like state in rodents. A relationship between status epilepticus and reactive oxygen species has been also suggested which could result in seizure-induced neurodegeneration. The aim of this study was to evaluate the existence of oxidative damage as well as the antioxidant enzyme response in cortex and hippocampus after the administration of an intraperitoneal (350 mg/kg) and an intracerebroventricular (360 microg, 1 microl) pilocarpine injection in rats. The GABA agonist muscimol (1 mg/kg, i.p.), with described neuroprotective properties, was used as a negative control. Only systemic pilocarpine induced oxidative damage. Malondialdehyde levels, as a marker of lipid peroxidation (LP), increased in both regions (55-56%). Catalase (52-80%) and superoxide dismutase (53-60%) activities also rose in both regions but glutathione peroxidase activity only increased in cortex (45%). Glutathione reductase and caspase-3 activity did not change. In conclusion, systemic pilocarpine produced oxidative brain damage, whereas local pilocarpine brain injection had no effects. Moreover, the enzymatic determinations performed in this study are a good tool to study brain injury in pharmacological manipulations such as the ones used in short recording EEG studies.

Genetic dissection of theta rhythm heterogeneity in mice

Rhythmic oscillatory activities at the theta frequency (4-12 Hz) in the hippocampus have long-attracted attention because they have been implicated in diverse brain functions, including spatial cognition. Although studies based on pharmacology and lesion experiments suggested heterogeneity of these rhythms and their behavioral correlates, controversies are abundant on these issues. Here we show that mice harboring a phospholipase C (PLC)-beta1(-/-) mutation (PLC-beta1(-/-) mice) lack one subset of theta rhythms normally observed during urethane anesthesia, alert immobility, and passive whole-body rotation. In contrast, the other subset of theta rhythms observed during walking or running was intact in these mutant mice. PLC-beta1(-/-) mice also have somewhat disrupted theta activity during paradoxical sleep but do have an atropine-resistant component of theta rhythm. In addition, carbachol-induced oscillations were obliterated in hippocampal slices of PLC-beta1(-/-) mice. Interestingly, PLC-beta1(-/-) mice showed deficits in a hidden platform version of the Morris water maze yet performed well in motor coordination tests and a visual platform version of the Morris water maze. The results genetically define the existence of at least two subtypes of theta rhythms and reveal their association with different behaviors.

Cholinergic involvement in the cortical and hippocampal Fos expression induced in the rat by placement in a novel environment

Placing rats into a series of novel environments induced vigorous c-fos expression in the infralimbic, anterior cingulate and retrosplenial cortices, and in the hippocampus. Pretreatment with the antimuscarinic drugs scopolamine and atropine was able to greatly suppress novelty-induced Fos expression at these sites. Placement into the novel environments also induced Fos expression in the habenula and the paraventricular thalamic nucleus, but the response at these sites did not appear to be sensitive to cholinergic blockade. These findings suggest that cholinergic mechanisms play an important role in ability of behavioral events to influence cortical and hippocampal immediate-early gene expression and are consistent with the possibility that some of the effects of anticholinergic drugs on placticity and learning may be mediated through alterations in the expression of these genes.

The therapeutic potential of neuronal KCNQ channel modulators

Neuronal KCNQ (Kv7) channels (KCNQ2-5 or Kv7.2-7.5, disclosed to date) were discovered by virtue of their homology with a known cardiac channel involved in long QT syndrome (KvLQT or KCNQ1, Kv7.1) and first disclosed in 1998. The involvement of KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) in a benign idiopathic neonatal epilepsy, KCNQ4 (Kv7.4) in a form of congenital deafness, and the discovery that neuronal KCNQ heteromultimers were among the molecular substrates of M-channels, resulted in a high level of interest for potential drug development strategies. A number of small-molecule modulators were quickly identified, including openers or activators such as the antiepileptic drug candidate retigabine and the structurally-related analgesic drug flupirtine (Katadolon trade mark Asta Medica), and a group of KCNQ channel inhibitors/blockers originally developed for cognition enhancement. All of these data have suggested a rich target profile for modulators of neuronal KCNQ channels, including a variety of neuronal hyperexcitability disorders and conditions for openers, such as the epilepsies, acute pain, neuropathic pain, migraine pain and some neurodegenerative and psychiatric disorders. KCNQ blockers could likewise have utility in disorders characterised by neuronal hypoactivity, including cognition enhancement and perhaps disorders of mood. Emerging patent literature suggests significant interest in neuronal KCNQ modulation in the pharmaceutical industry and significant chemical diversity concerning KCNQ modulation.

Effects of metrifonate on the hippocampal theta rhythm of freely moving intact and MS-lesioned mice

Changes in hippocampal electroencephalogram (EEG) have been suggested to be closely associated with spatial learning ability. Spatial learning can be improved in medial septal (MS)-lesioned mice by metrifonate, a cholinesterase inhibitor. We designed this study to investigate the effects of metrifonate on the hippocampal theta oscillation of intact and MS-lesioned mice. Intact and MS-lesioned C57BL mice were treated with acute injections of metrifonate (doses: 15, 50 and 100 mg/kg ip). These included a dose that considerably improved spatial memory of MS-lesioned mice in our earlier study. In addition, subtype selective muscarinic agents, BIBN-99, AF267B and AF150(S) were used. Recordings of hippocampal theta during movement and awake immobility revealed a dramatic reduction of theta in the lesioned animals. Metrifonate induced prominent changes in the EEG of intact mice, but not of MS-lesioned mice. The effect of metrifonate was not mimicked by two selective M(1)-agonists and was augmented by a combined injection of a selective M(2)-antagonist. These data suggest that improved spatial learning by the cholinesterase inhibitor metrifonate is unrelated to its effects on the hippocampal EEG. These two effects may be mediated through different muscarinic receptor subtypes.

Effects of cholinergic drugs on neocortical EEG and flash-visual evoked potentials in the mouse

The effects of single intraperitoneal injection of two cholinesterase inhibitors, physostigmine (PHY; 0.01, 0.025, 0.05, 0. 1, 0.2 mg/kg) and heptylphysostigmine (HEP; 0.5, 2, 6 mg/kg) on electroencephalographic (EEG) activity and flash visual evoked potentials (f-VEP) in the occipital cortex were compared in DBA/2 mice. EEG spectral analysis of awake periods showed that PHY at all doses and HEP at 2 mg/kg induced an increase of power in the 4.25- to 7-Hz frequency band. Furthermore, PHY at the higher doses and HEP at all doses induced a decrease of power in the 7.25- to 12-Hz frequency band, while the lower doses of PHY (0.01, 0.025 mg/kg) produced an increase of this band. EEG effects elicited by the two drugs were similar, when doses displaying analogous biochemical effects (acetylcholinesterase inhibition) were used (i.e. 0.01 and 0. 025 mg/kg of PHY versus 0.5 and 2 mg/kg of HEP). PHY and HEP induced similar changes in f-VEPs. Amplitudes of early and late components (P1N1, N1P2, P4N4 and particularly N1P3) were enhanced, while amplitudes of middle components were depressed after all doses. The peak latency measures were generally delayed, even though, after the lower doses, a trend to a latency reduction was evident in late components. This finding might indicate a possible effect on stimulus speed diffusion by 'low therapeutic' doses, analogous to the ones used in men. Our data show that both drugs are effective in modifying EEG and f-VEP parameters connected with brain cholinergic function, although in a very narrow dose range.

The profile of sabcomeline (SB-202026), a functionally selective M1 receptor partial agonist, in the marmoset

1. Sabcomeline (SB-202026, 0.03 mg kg(-1), p.o.), a potent and functionally selective M1 receptor partial agonist, caused a statistically significant improvement in the performance of a visual object discrimination task by marmosets. No such improvement was seen after RS86 (0.1 mg kg(-1), p.o.). 2. Initial learning, which only required an association of object with reward and an appropriate response to be made, was not significantly affected. Reversal learning, which required both the extinction of the previously learned response and the acquisition of a new response strategy, was significantly improved after administration of sabcomeline (0.03 mg kg(-1), p.o.). 3. Sabcomeline (0.03 and 0.1 mg kg(-1), p.o.) had no significant effect on mean blood pressure measured for 2 h after administration in the conscious marmoset. 4. Sabcomeline (0.03 mg kg(-1), p.o.) caused none of the overt effects such as emesis or behaviours often seen after the administration of muscarinic agonists, e.g. face rubbing and licking. 5. This is the first study to demonstrate cognitive enhancement by a functionally selective M1 receptor partial agonist in a normal (i.e. non-cognitively impaired) non-human primate and this effect was seen at a dose which did not cause side effects. 6. Perseverative behaviour and deficient acquisition of new information are seen in patients with Alzheimer's disease (AD). Therefore the data suggest that sabcomeline might be of therapeutic benefit in the treatment of AD.

Alteration of cholinergic pressor and antinociceptive responses in rats pretreated with the cholinergic toxin AF64A

1. In the present study, the pressor and antinociceptive effects of physostigmine and oxotremorine were investigated in rats injected with AF64A intracerebroventricularly. 2. Physostigmine (50-100 micrograms/kg, i.v.)-induced pressor responses were significantly lower in AF64A-injected rats compared with saline-injected animals, whereas oxotremorine (20-80 micrograms/kg, i.v.)-induced responses were found to be similar to those seen in the saline group. 3. The physostigmine (100 micrograms/kg, s.c.)-induced antinociceptive effect was totally abolished by AF64A treatment, but that of oxotremorine (30 micrograms/kg, s.c.) remained unchanged at the tail-flick test. 4. The results of this study present functional evidence for AF64A-produced substantial loss of cholinergic neurons involved in the regulation of blood pressure and nociception but not in postsynaptic muscarinic receptors.

Design of [R-(Z)]-(+)-alpha-(methoxyimino)-1-azabicyclo[2.2.2]octane-3-acetonitri le (SB 202026), a functionally selective azabicyclic muscarinic M1 agonist incorporating the N-methoxy imidoyl nitrile group as a novel ester bioisostere

Loss of cholinergic function is believed to be implicated in the cognitive decline associated with senile dementia of the Alzheimer type (SDAT). The disease is characterized by progressive loss of muscarinic receptors located on nerve terminals while postsynaptic muscarinic M1 receptors appear to remain largely intact. Muscarinic agonists acting directly on postsynaptic receptors offer the prospect of countering the cholinergic deficit in SDAT. This study describes a novel series of azabicyclic muscarinic agonists, which incorporate an oxime ether or modified oxime ether group as an ester bioisostere. Modification of the oxime ether function by the introduction of electron withdrawing groups led to the finding that the (Z)-N-methoxy imidoyl nitrile group serves as a stable methyl ester bioisostere. This culminated in the discovery of the quinuclidinyl N-methoxy imidoyl nitrile R-(+)-(Z)-5g which is a functionally selective muscarinic M1 partial agonist currently in phase III clinical trials for the treatment of SDAT. The selective profile of R-(+)-(Z)-5g can be rationalized in terms of the relative affinity of the compound at muscarinic receptor subtypes, the degree of agonist efficacy, and brain penetrancy.

Carbachol-induced pressor responses and muscarinic M1 receptors in the central nucleus of amygdala in conscious rats

The type of muscarinic receptor in the central nucleus of the amygdala that mediates the carbachol-evoked pressor responses was investigated in conscious unrestraint Sprague-Dawley rats. Carbachol (100 ng) injected into the lateral cerebral ventricle caused a significant rise in blood pressure of 31.8+/-4.5 mmHg and a decrease in heart rate of 80.0+/-12.2 beats/min. Pirenzepine (10-75 nmol) injected into the central nucleus of the amygdala inhibited carbachol-induced pressor responses dose-dependently. The bradycardic response to carbachol was also inhibited by pirenzepine, but no dose-dependency was observed. Injection of pirenzepine into the basolateral amygdala at a dose (50 nmol) that inhibited carbachol-induced changes in mean arterial pressure and heart rate when injected into the central nucleus of the amygdala failed to exert any inhibition. Methoctramine at a dose of 50 nmol injected into both the central nucleus of the amygdala and the basolateral amygdala did not cause any significant alteration in the responses. These results indicate that muscarinic M1 receptors in the central nucleus of the amygdala are involved in cardiovascular regulation mediated by central cholinergic pathways.

Modulation of the pressor response elicited by carbachol and electrical stimulation of the amygdala by muscarinic antagonists in conscious rats

1. The nature of the muscarinic receptor involved in mediating cardiovascular changes caused by unilateral microinjection of carbachol (5 nmol) into, and electrical stimulation (200-300 microA) of, the amygdaloid complex was investigated in conscious, unrestrained female Sprague-Dawley rats. 2. Unilateral microinjection of carbachol (5 nmol; n = 6) and electrical stimulation (200-300 microA, 80 Hz, 30 s; n = 4) caused a significant rise in blood pressure of 21 +/- 4 mmHg and 25 +/- 5 mmHg, respectively. These changes were associated with no overall effect on heart rate. The effects of electrical stimulation were found to be repeatable. 3. Pretreatment i.c.v. with pirenzepine (5-20 mmol; n = 6-7 for each dose), dose-dependently inhibited the rise in blood pressure induced by carbachol, whereas AF-DX 116 (100 nmol; n = 6) failed to have any effect on the carbachol-induced pressure response. Neither antagonist alone had any effect on resting baseline variables. 4. Unilateral microinjections of atropine sulphate (1-100 nmol; n = 4-6 for each dose), pirenzepine (0.03-10 nmol; n = 4 for each dose) or AF-DX 116 (10-60 nmol; n = 4-5 for each dose), into the amygdala, dose-dependently inhibited the rise in blood pressure caused by electrical stimulation (200-300 microA). The ID50 values were 1.05, 0.23 and 39.5 nmol, respectively. Although pirenzepine seemed to be more potent than atropine, this difference was not significant. 5. It is concluded that the rise in blood pressure elicited by unilateral microinjection of carbachol into, or electrical stimulation of, the amygdaloid complex is mediated by M1-muscarinic receptors.

Acute and chronic arecoline: effects on a scopolamine-induced deficit in complex maze learning

These studies tested the effect of arecoline, a nonselective muscarinic agonist, administered either acutely or by chronic peripheral infusion via osmotic minipumps, on a scopolamine-induced deficit in a Stone (14 unit) T-maze task in rats. Scopolamine alone (0.125-1.0 mg/kg, IP) dose-dependently impaired maze acquisition, increasing maze run-times and to a lesser extent, the number of errors committed. Neither acute administration of arecoline (5.0 and 10.0 mg/kg, IP), when tested against a deficit induced by scopolamine (0.25 mg/kg, IP), nor chronic arecoline administration (30 and 50 mg/kg per 24 h), when tested against a deficit induced by scopolamine (0.5 mg/kg), were able to ameliorate the decrements in maze performance. In fact, the higher dose of arecoline (50 mg/kg per 24 h) infused over 10 days potentiated the scopolamine-induced deficit, with respect to latency. These data indicate that dose selection is of great importance when employing arecoline in tests of learning and memory and that the influence of the method of administration of arecoline on the behavioural outcome warrants further study.

Involvement of M2 and non-M2 muscarinic receptors in hippocampal theta rhythm induced by carbachol infusion into the septum of the rat

Binding and autoradiographic studies have shown the presence of a rather high density of M2 muscarinic subtype receptors and the apparent absence or low density of the M1 subtype in the septum. We tested the hypothesis that, in the urethane-anesthetized rat, septal M2 receptors are involved in the generation of the hippocampal theta (theta) rhythm induced by intraseptal administration of carbachol, a potent cholinomimetic agent. Carbachol-induced theta was blocked by local infusion of the unspecific muscarinic antagonist agent, atropine (20 micrograms (29.55 nM)), given 10 min prior to carbachol. The intraseptal administration of low to high doses of gallamine (range: 20-180 micrograms (22.43-201.90 nM)), a specific M2 antagonist which displays high affinity for the septal region, resulted in significant changes in the electrophysiological characteristics of carbachol-induced theta but failed to abolish this rhythm. It is suggested that the latter may have resulted from a combined activation of both M2 and non-M2 receptors at septal level.

The design of novel muscarinic partial agonists that have functional selectivity in pharmacological preparations in vitro and reduced side-effect profile in vivo

Antagonist/agonist binding ratios (NMS/Oxo-M ratio) were used as an index of the efficacy of novel compounds acting at muscarinic receptors. These binding ratios have been used with a range of functional pharmacological assays to investigate the effects of varying the efficacy of muscarinic agonists. This strategy has been used as a means of obtaining functional receptor selectivity by exploiting differences in effective receptor reserves. The oxadiazole and pyrazine muscarinic agonists L-670,548 (NMS/Oxo-M ratio 1100) and L-680,648 (NMS/Oxo-M ratio 690) are amongst some of the most potent and efficacious agonists known. Decreasing the efficacy of compounds from these series, resulted in compounds with functional selectivity. The chloropyrazine L-689,660 (NMS/Oxo-M ratio 28) was an agonist on the rat superior cervical ganglion (M1), a partial agonist on the guinea-pig ileum (M3), but was an antagonist in the guinea-pig atria (M2). Synthesis of compounds with even lower predicted efficacy, such as the cyclopropyloxadiazole L-687,306 (NMS/Oxo-M ratio 15), maintained agonist activity in the ganglion, but showed antagonist activity in the M3 ileal, as well as the M2 atrial preparations. When tested in vivo these compounds did not produce many of the side effects associated with more efficacious agonists, particularly those associated with the cardiovascular system. However, they were active in reversing scopolamine-induced deficits in a variety of behavioural paradigms. This approach shows how functional selectivity for muscarinic receptor subtypes can be achieved in vitro, that in vivo reduces the dose-limiting side effects normally associated with muscarinic agonists.