Effects of atipamezole and tetrahydroaminoacridine on nucleus basalis lesion-induced EEG changes

Pharmacological properties, central nervous system effects, and potential therapeutic applications of atipamezole, a selective alpha2-adrenoceptor antagonist

Atipamezole is an alpha2-adrenoceptor antagonist with an imidazole structure. Receptor binding studies indicate that its affinity for alpha2-adrenoceptors and its alpha2/alpha1 selectivity ratio are considerably higher than those of yohimbine, the prototype alpha2-adrenoceptor antagonist. Atipamezole is not selective for subtypes of alpha2-adrenoceptors. Unlike many other alpha2-adrenoceptor antagonists, it has negligible affinity for 5-HT1A and I2 binding sites. Atipamezole is rapidly absorbed and distributed from the periphery to the central nervous system. In humans, atipamezole at doses up to 30 mg/subject produced no cardiovascular or subjective side effects, while at a high dose (100 mg/subject) it produced subjective symptoms, such as motor restlessness, and an increase in blood pressure. Atipamezole rapidly reverses sedation/anesthesia induced by alpha2-adrenoceptor agonists. Due to this property, atipamezole is commonly used by veterinarians to awaken animals from sedation/anesthesia induced by alpha2-adrenoceptor agonists alone or in combination with various anesthetics. Atipamezole increased sexual activity in rats and monkeys. In animals with sustained nociception, atipamezole increased pain-related responses by blocking the noradrenergic feedback inhibition of pain. In tests assessing cognitive functions, atipamezole at low doses has beneficial effects on alertness, selective attention, planning, learning, and recall in experimental animals, but not necessarily on short-term working memory. At higher doses atipamezole impaired performance in tests of cognitive functions, probably due to noradrenergic overactivity. Recent experimental animal studies suggest that atipamezole might have beneficial effects in the recovery from brain damage and might potentiate the anti-Parkinsonian effects of dopaminergic drugs. In phase I studies atipamezole has been well tolerated by human subjects.

Combined cholinergic and 5-HT2 receptor activation suppresses thalamocortical oscillations in aged rats

The present study investigated whether combined stimulation of the cholinergic system and 5-hydroxytryptamine (5-HT) subtype 2 receptors can suppress neocortical high-voltage spindles (HVSs) reflecting thalamocortical oscillations in aged rats. Cholinesterase inhibitors-tetrahydro-aminoacridine (THA: 1.0 and 3.0 mg/kg i.p.) and physostigmine (0.36 mg/kg i.p.)- and a 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 and 1.0 mg/kg SC)-suppressed HVSs in aged rats. A combination of subthreshold doses of THA (0.3 mg/kg i.p.) and DOI (0.1 mg/kg s.c.) suppressed HVSs more effectively than either drug alone. Furthermore, a 5-HT2 receptor antagonist, ketanserin (5.0 and 20.0 mg/kg s.c.), reduced the efficacy of THA (1.0 and 3.0 mg/kg i.p.) and physostigmine (0.12 and 0.36 mg/kg i.p.) in decreasing HVSs. THA and ketanserin slightly decreased, physostigmine tended to increase, and DOI significantly increased behavioral activity of the rats, demonstrating that the effects of the drugs on behavioral activity may be separated from their effects on generation of thalamocortical oscillations. The results suggest that activation of the cholinergic system and 5-HT2 receptors has additive effects in the suppression of thalamocortical oscillations in aged rats.

The effects of cholinergic drugs on rat neocortical high-voltage spindles in ketanserin-treated rats

To investigate the roles of the cholinergic system and 5-HT2 receptors in the modulation of thalamocortical oscillations, we studied the effects of systemic (s.c.) administration of anticholinesterases (physostigmine, tetrahydroaminoacridine) and muscarinic acetylcholine receptor agonists (pilocarpine, oxotremorine) on spontaneous thalamically generated rhythmic neocortical high-voltage spindles in adult rats pretreated with either saline or ketanserin, a 5-HT2 receptor antagonist. Ketanserin at 20.0 mg/kg increased the number of high-voltage spindles. In saline-treated rats, tetrahydroaminoacridine 3.0 and 9.0 mg/kg was able to decrease high-voltage spindles, whereas in ketanserin 20.0 mg/kg-treated rats only the highest dose of tetrahydroaminoacridine (9.0 mg/kg) decreased high-voltage spindles. Both doses of physostigmine, 0.12 and 0.36 mg/kg, decreased high-voltage spindles in both saline and ketanserin 20.0 mg/kg-treated rats. Lower doses of tetrahydroaminoacridine (1.0 mg/kg) and physostigmine (0.06 mg/kg) were ineffective in both saline- and ketanserin 20.0 mg/kg-treated rats. Pilocarpine 3.0 mg/kg and oxotremorine 0.1 and 0.9 mg/kg decreased high-voltage spindles in saline-treated rats. However, in rats treated with ketanserin 20.0 mg/kg, only the lower doses of pilocarpine (0.3 and 1.0 mg/kg) and oxotremorine (0.03 mg/kg) were able to decrease the high-voltage spindles. The results suggest that activation of the cholinergic system and activation of 5-HT2 receptors have additive effects in the suppression of thalamocortical oscillations and related neocortical high-voltage spindles in rats, thus maintaining effective information processing in thalamocortical networks.

Suppression of neocortical high-voltage spindles by nicotinic acetylcholine and 5-HT2 receptor stimulation

To investigate the roles of the nicotinic acetylcholine receptor and the serotonin (5-hydroxytryptamine; 5-HT) subtype 2 receptor in the modulation of rat thalamocortical oscillations, the effects of systemic (s.c.) administration of nicotine, a nicotinic acetylcholine receptor agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 receptor agonist, on neocortical high-voltage spindle activity occurring during quiet waking-immobility behavior in aged (28 months of age) and adult (7 months of age) rats were studied. Nicotine 0.1 and 0.3 mg/kg alleviated the age-related increase of neocortical high-voltage spindles, whereas in adult rats only nicotine 0.3 mg/kg was effective. DOI 0.3, 1.0 and 2.0 mg/kg suppressed high-voltage spindles in both aged and adult rats. In aged rats, a combination of subthreshold doses of nicotine (0.03 mg/kg) and DOI (0.1 mg/kg) decreased neocortical high-voltage spindles, whereas in adult rats two different subthreshold dose combinations (nicotine 0.03 or 0.1 mg/kg+DOI 0.1 mg/kg) had no effect. p-Chlorophenylalanine (400 mg/kg/day i.p. for 3 consecutive days) treatment decreased brain serotonin concentration (> 80% reduction), but did not affect high-voltage spindles. However, in both aged and adult rats, p-chlorophenylalanine treatment blocked the decrease in high-voltage spindle activity produced by DOI 0.3 mg/kg, though not the decrease produced by higher doses of DOI (1.0 and 2.0 mg/kg). It is important that, in adult rats, p-chlorophenylalanine treatment was able to abolish the decrease in high-voltage spindle activity seen after a relatively high dose of nicotine (0.3 mg/kg). The results suggest that nicotinic acetylcholine and 5-HT2 receptors may act in concert to suppress neocortical high-voltage spindling in rats, and that intact brain serotonergic systems may be important for some of the therapeutic effects of nicotine.

Modulation of rat neocortical high-voltage spindle activity by 5-HT1/5-HT2 receptor subtype specific drugs

To investigate the role of serotonin (5-hydroxytryptamine; 5-HT) receptors in the modulation of rat thalamocortical oscillations, we studied the effects of 5-HT1/5-HT2 receptor subtype specific drugs on neocortical high-voltage spindle activity in adult male rats. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.03, 0.1, 0.3 and 1.0 mg/kg s.c.), had no effect on neocortical high-voltage spindle activity. Furthermore, a mixed 5-HT1/5-HT2 receptor antagonist, methysergide (1.0, 5.0 and 15.0 mg/kg i.p.), had no effect, whereas a non-specific mixed 5-HT1/5-HT2 receptor antagonist, methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.), significantly increased neocortical high-voltage spindles. Of the 5-HT2 receptor antagonists, ritanserin (0.1, 1.0 and 5.0 mg/kg s.c.) had no effect, whereas ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) increased neocortical high-voltage spindles, but only at the highest dose used. A 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5, 1.0 and 2.0 mg/kg s.c.), at the two highest doses significantly decreased neocortical high-voltage spindle activity, and this effect was blocked by the 5-HT2 receptor antagonists, ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) and ritanserin (1.0 and 5.0 mg/kg s.c.), as well as by methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.) and methysergide (1.0, 5.0 and 15.0 mg/kg i.p.). Furthermore, unilateral intrathalamic infusions, but not intrahippocampal control infusions, of DOI (10 and 50 micrograms/1.0 microliters/rat) decreased neocortical high-voltage spindle activity and systemic administration of ketanserin (20.0 mg/kg s.c.) completely blocked this effect. The present results suggest that (1) the serotonergic system modulates rat thalamocortical oscillations as measured by neocortical high-voltage spindle activity, (2) activation of 5-HT2 receptors, possibly located in the thalamus, with a specific 5-HT2 receptor agonist, DOI, causes a reduction in rat neocortical high-voltage spindle activity.

Tacrine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in Alzheimer's disease

Tacrine is a centrally acting cholinesterase inhibitor with additional pharmacological activity on monoamine levels and ion channels. It has been postulated that some or all of these additional properties may also be relevant to the mode of action of the drug. There are wide interindividual variations in pharmacological and clinical response to tacrine, possibly related to interindividual variation in bioavailability. Tacrine appears to improve cognitive function and behavioural deficits in a proportion of patients with Alzheimer's disease, at dosages of 80 to 160 mg/day. In the best designed trials, 30 to 51% of evaluable patients showed an improvement of at least 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale, versus 16 to 25% of placebo recipients. A similar proportion of tacrine recipients were judged to have improved when global assessment scales were used. There was a significant dose-response relationship up to 160 mg/day. However, large numbers of patients were withdrawn during the trials, many because of tacrine-associated increases in transaminase levels. Elevated liver enzyme levels occurred in about 50% of tacrine recipients (reaching clinical significance in about 25%). Cholinergic symptoms also occurred more often in tacrine recipients than in those receiving placebo. A gradual increase in tacrine dosage, at 6-week intervals, is recommended when initiating therapy, and weekly serum transaminase monitoring is required for 6 weeks after each dosage increase. Despite the limitations implied by the low proportion of responders and high incidence of hepatic adverse effects associated with therapy, tacrine appears to make a measurable difference in both cognitive and behavioural function in a proportion of patients with Alzheimer's disease--a welcome advance in an area previously devoid of acceptable treatment options.

Effects of some cholinergic agonists on neocortical slow wave activity in rats with basal forebrain lesions

Chronic rats, prepared with unilateral injections of kainic acid in the left basal forebrain, displayed prominent large amplitude slow wave activity in the neocortex ipsilateral to the injection. Oxotremorine and pilocarpine, given systemically following pretreatment with methyl scopolamine to block peripheral muscarinic effects, restored low voltage fast activity (LVFA) in a dose-related manner. Oxotremorine was more potent than pilocarpine. Arecoline was not consistently effective. Tetrahydroaminoacridine abolished abnormal 4-6 Hz rhythmical slow waves in the left neocortex but had little effect on large amplitude irregular slow waves. Direct-acting cholinergic agonists can restore near-normal neocortical activity after extensive cholinergic deafferentation of the neocortex.