Septohippocampal cholinergic pathway and penile erections induced by dopaminergic and cholinergic stimulants

Investigating the prevalence of erectile dysfunction among men exposed to organophosphate insecticides

INTRODUCTION

Erectile dysfunction (ED) poses a significant disease morbidity and contributor to male infertility, where an estimated 20-40% of men are affected annually. While several risk factors have been identified in the etiology of ED (e.g., aging, heart disease, diabetes, and obesity), the complete pathogenesis remains to be elucidated. Over the last few decades, the contribution of environmental exposures to the pathogenesis of ED has gained some attention, though population studies are limited and results are mixed. Among environmental contaminants, organophosphate (OP) insecticides represent one of the largest chemical classes, and chlorpyrifos is the most commonly used OP in the U.S. OP exposure has been implicated in driving biological processes, including inflammation, reactive oxygen species production, and endocrine and metabolism disruption, which have been demonstrated to adversely affect the hypothalamus and testes and may contribute to ED. Currently, studies evaluating the association between OPs and ED within the U.S. general population are sparse.

METHODS

Data were leveraged from the National Health and Nutrition Examination Survey (NHANES), which is an annually conducted, population-based cross-sectional study. Urinary levels of 3,5,6-trichloro-2-pyridinol (TCPy), a specific metabolite of the most pervasive OP insecticide chlorpyrifos, were quantified as measures of OP exposure. ED was defined by responses to questionnaire data, where individuals who replied "sometimes able" or "never able" to achieve an erection were classified as ED. Chi-square, analysis of variance (ANOVA), and multivariable, weighted linear and logistic regression analyses were used to compare sociodemographic variables between quartiles of TCPy exposure, identify risk factors for TCPy exposure and ED, and to analyze the relationship between TCPy and ED.

RESULTS

A total of 671 adult men were included in final analyses, representing 28,949,379 adults after survey weighting. Approximately 37% of our cohort had ED. Smoking, diabetes, aging, Mexican-American self-identification, and physical inactivity were associated with higher ED prevalence. Analysis of TCPy modeled as a continuous variable revealed nonsignificant associations with ED (OR = 1.02 95% CI [0.95, 1.09]). Stratification of total TCPy into quartiles revealed increased odds of ED among adults in the second and fourth quartiles, using the first quartile as the reference (OR = 2.04 95% CI [1.11, 3.72], OR = 1.51 95% CI [0.58, 3.93], OR = 2.62 95% CI [1.18, 5.79], for quartiles 2, 3, and 4, respectively).

CONCLUSIONS

The results of our study suggest a potential role for chlorpyrifos and other OPs the pathogenesis of ED. Future studies are warranted to validate these findings, determine clinical significance, and to investigate potential mechanisms underlying these associations.

Role of acetylcholine in control of sexual behavior of male and female mammals

The results of studies using systemic or central applications of cholinergic drugs suggest that acetylcholine makes important contributions to the neurochemical control of male- and female-typical reproductive behaviors. In males, cholinergic control seems largely specific to some elements or aspects of copulatory behavior that can vary significantly across species. Synapses in or near the medial preoptic area represent part of this mechanism, but the entire system appears to extend more widely, perhaps especially to one or more structures flanking some part of the lateral ventricle. In females, the lordosis response that essentially defines sexual receptivity is clearly responsive to cholinergic drugs. The same seems likely to be true of other elements of female sexual behavior, but additional studies will be needed to confirm this. Changes in cholinergic activity may help to mediate estrogenic effects on female sexual behavior. However, estrogen exposure can increase or decrease cholinergic effects, suggesting a relationship that is complex and requires further analysis. Also presently unclear is the localization of the cholinergic effects on female sexual responses. Though periventricular sites again have been implicated, their identity is presently unknown. This review discusses these and other aspects of the central cholinergic systems affecting male and female sexual behaviors.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Sexual behavior in male rodents

The hormonal factors and neural circuitry that control copulation are similar across rodent species, although there are differences in specific behavior patterns. Both estradiol (E) and dihydrotestosterone (DHT) contribute to the activation of mating, although E is more important for copulation and DHT for genital reflexes. Hormonal activation of the medial preoptic area (MPOA) is most effective, although implants in the medial amygdala (MeA) can also stimulate mounting in castrates. Chemosensory inputs from the main and accessory olfactory systems are the most important stimuli for mating in rodents, especially in hamsters, although genitosensory input also contributes. Dopamine agonists facilitate sexual behavior, and serotonin (5-HT) is generally inhibitory, though certain 5-HT receptor subtypes facilitate erection or ejaculation. Norepinephrine agonists and opiates have dose-dependent effects, with low doses facilitating and high doses inhibiting behavior.

How REM sleep deprivation and amantadine affects male rat sexual behavior

There are conflicting findings about the sexual effects of REM sleep deprivation (REMd). Otherwise, several studies show a dopaminergic hypersensitivity after REMd. The effect of REMd and amantadine (AMA) was studied for standard measures and temporal patterning in the first experiment, in four groups: normal with vehicle, normal with AMA (5.0 and 10 mg/kg), REMd with vehicle and REMd with AMA (5.0 and 10.0 mg/kg). REMd reduced mount latency (ML), intromission latency (IL) and mount number (MN) and increased copulatory efficiency (CE) and hit rate factor. REMd also reduced the mount bout number (MBN) and increased the sexual interaction (mount bout time, MBT) among male and female during copula. AMA stimulates initiation and hit rate factors and accelerates the temporal patterning of sexual behavior, evoking fewer and quicker mount bouts. In the experiments with combination of REMd and AMA administration, AMA did not increase behavior effects evoked by REM deprivation, probably due to a top or a bottom effect, depending on the measures considered. A second experiment studied the effects of AMA (1.25 to 5.0 mg/kg) and REMd on the sexual reflexes of nonimmobilized male rats. REMd enhanced the AMA effects upon the seminal emission reflex, but inhibited the penile erection reflex elicited by 1.25 mg/kg of AMA. Curiously, our results showed that REMd, like AMA, a dopaminergic agonist, causes similar effects of sexual behavior in the male rat, particularly those related to arousal mechanism and hit rate factor. The results are discussed and the effects of REMd probably involve dopaminergic hypersensitivity and increased sexual motivational response.

Neural pathways and central sites involved in penile erection: neuroanatomy and clinical implications

Penile erection occurs in response to tactile, visual, and imaginative stimuli in humans. In animals olfactory and auditory cues are particularly important. The participation of multiple sites with the brain and spinal cord, and coordination of somatic and autonomic pathways make sexual behavior in general, and erection in particular, vulnerable to neurologic injury and disease. Sites within the brain and spinal cord act in concert to process, coordinate, then distribute the neural inputs necessary for sexual behavior including erection. Activation of neurons in some of these regions either pharmacologically or by electrical stimulation has been associated with penile tumescence. This review will provide a geographic framework for understanding the neuroanatomical basis of penile erection based primarily on animal data. Following discussion of the anatomical substrates, a clinical correlation is then provided to confirm and reinforce these experimental observations.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel potential antidementia drug, restores the regional cerebral blood flow response abolished by scopolamine but not by HA-966: a positron emission tomography study with unanesthetized rhesus monkeys

The interactions of FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel potential antidementia drug, with cholinergic and glutamatergic neuronal systems were evaluated with respect to its effects on the regional cerebral blood flow (rCBF) response to vibrotactile stimulation in unanesthetized rhesus monkeys with [15O]H2O and high resolution positron emission tomography (PET). Under a saline condition, the vibrotactile stimulation given on the right forepaw induced a significant increase in the rCBF response in the contralateral somatosensory cortex of the monkey brain. Systemic administration of scopolamine (50 microg/kg, i.v.), a muscarinic cholinergic receptor antagonist, completely abolished the rCBF response to the stimulation, and the abolishment lasted, at least, up to 4 h after scopolamine injection. The scopolamine-induced abolishment of rCBF response was restored by the administration of FK960 at relatively wide dosing range from 1 to 1000 microg/kg (i.v. ), and the recovery by FK960 on the rCBF response lasted for 1 h following the administration of FK960 at doses of 100 and 1000 microg/kg. On the other hand, the rCBF response abolished by 1000 microg/kg of (+)-3-amino-1-hydroxy-2-pyrrolidone (HA-966), an antagonist of the glycine modulatory site on the N-methyl-d-aspartate (NMDA) receptors, was not restored by FK960 (1000 microg/kg, i.v.). These findings suggest that FK960 reverses the abolished rCBF response to somatosensory stimulation via enhancement of cholinergic neurotransmission but not via the glutamatergic one.

The neuropharmacology of yawning

Yawning is a phylogenetically old, stereotyped event that occurs alone or associated with stretching and/or penile erection in humans and in animals from reptiles to birds and mammals under different conditions. Although its physiological function is still unknown, yawning is under the control of several neurotransmitters and neuropeptides at the central level as this short overview of the literature on the neurochemistry of yawning shows. Among these substances, the best known are dopamine, excitatory amino acids, acetylcholine, serotonin, nitric oxide, adrenocorticotropic hormone-related peptides and oxytocin, that facilitate yawning and opioid peptides that inhibit this behavioral response. Some of the above compounds interact in the paraventricular nucleus of the hypothalamus to control yawning. This hypothalamic nucleus contains the cell bodies of oxytocinergic neurons projecting to extra-hypothalamic brain areas that play a key role in the expression of this behavioral event. When activated by dopamine, excitatory amino acids and oxytocin itself, these neurons facilitate yawning by releasing oxytocin at sites distant form the paraventricular nucleus, i.e. the hippocampus, the pons and/or the medulla oblongata. Conversely, activation of these neurons by dopamine, oxytocin or excitatory amino acids, is antagonized by opioid peptides, that, in turn, prevent the yawning response. The activation and inhibition, respectively of these oxytocinergic neurons is related to a concomitant increase and decrease, respectively, of paraventricular nitric oxide synthase activity. However, other neuronal systems in addition to the central paraventricular oxytocinergic neurons are involved in the control of yawning, since they do not seem to be involved in the expression of yawning induced by the stimulation of acetylcholine or serotoninergic receptors, nor by adrenocorticotropic hormone (ACTH) and related peptides. Nitric oxide is also involved in the induction of yawning by the latter compounds and neuronal links, for instance between dopamine and acetylcholine and dopamine and serotonin, seem to be involved in the yawning response. Finally, other neurotransmitters, i.e. gamma-aminobutyric acid (GABA) and noradrenaline, and neuropeptides, i.e. neurotensin and luteinizing hormone-releasing hormone (LH-RH), influence this behavioral response. In conclusion, in spite of some recent progress, little is known of, and more has to be done to identify, the neurochemical mechanisms underlying yawning at the central level.

Monoaminergic influences on temporal patterning of sexual behavior in male rats

We evaluated the effects of the serotonin (5-HT) presynaptic uptake blocker fluoxetine (FLX) and the dopamine (DA)/noradrenaline (NE) releaser amantadine (AMA), separately and in combination, on the temporal patterning of male rat sexual behavior. FLX alone increased intermount-bout intervals, time-outs, grooming time, ejaculation latency, number of mounts per mount bout, and number of mount bouts per ejaculation. AMA alone had the opposite effect on these measures. Additionally, AMA, when given in combination with FLX, completely reversed the FLX-induced deficits in copulatory behavior. We interpret our results as suggesting an interaction between 5-HT and catecholamines in the temporal patterning of male rat copulatory behavior.

Amantadine stimulates sexual behavior in male rats

The effects of amantadine on sexual behavior, penile erection, and seminal emission of male rats was studied. Amantadine significantly decreased latency of mounts in all doses (1.25 to 50 mg/kg), and decreased the number of mounts and intromission latency at the highest doses used. The lowest dose of amantadine significantly increased ejaculation latency and intromission frequency, while higher doses significantly reduced it, which indicates a biphasic response of the drug. Additionally, seminal emission, erections, and genital grooming were significantly induced by amantadine. Amantadine-induced seminal emissions were impaired by spinal cord transection, which suggests the involvement of supraspinal structures in the drug action. Haloperidol and atropine sulphate significantly reduced seminal emissions and penile erections induced by amantadine. These results demonstrate that amantadine stimulates sexual behavior and genital reflexes in male rats and suggest a facilitatory effect of the drug that probably involves different mechanisms of action.

Involvement of raphe-hippocampal serotonergic and septo-hippocampal cholinergic mechanisms in the penile erection induced by FR121196, a putative cognitive enhancer

FR121196 (N-[4-acetyl-1-piperazinyl]-4-fluorobenzenesulfonamide), a putative cognitive enhancer, induced penile erection in naive rats; the dose-response curve was bell-shaped with the maximum response obtained at the dose of 3.2 mg/kg. The response to FR121196 was abolished in rats treated with intra-raphe injections of 5,7-dihydroxytryptamine or systemic injections of p-chlorphenylalanine (150 mg/kg, i.p. for three consecutive days) as well as in rats with electrolytic medial-septum lesion or surgical fimbria-fornix lesion. In addition, the penile erection induced by FR121196 (3.2 mg/kg) was dose-dependently attenuated by pindolol (0.1-3.2 mg/kg), a serotonin (5-HT)1 antagonist with beta-antagonistic activity, but not by metoprolol, a selective beta=antagonist. The inhibitory activity was shared by ICS205-930, a 5-HT3 antagonist, but not by ketanserin, a 5-HT2 antagonist, or sulpiride, a dopamine D2 antagonist. Scopolamine (0.032-1 mg/kg), but not methyl-scopolamine (0.032-1 mg/kg), also attenuated the penile erection induced by FR121196. Neurochemical analysis revealed that intraperitoneal injection of FR121196 significantly elevated the levels of 5-HT and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus and that raphe-lesion significantly reduced both 5-HT and 5-HIAA levels without affecting choline-acetyltransferase activity in all cortical and subcortical regions examined. It is thus postulated that FR121196 facilitates the raphe-hippocampal serotonergic pathway resulting in an activation of the septo-hippocampal cholinergic pathway and finally induces the penile erectile response.

Role of the dopaminergic, serotonergic and cholinergic link in the expression of penile erection in rats

The neural mechanisms underlying the penile erection induced by serotonergic, cholinergic and dopaminergic stimulants were comparatively investigated. Fenfluramine (0.1-10 mg/kg, i.p.), pilocarpine (0.032-3.2 mg/kg) and apomorphine (0.01-1 mg/kg) induced penile erection in rats with bell-shaped dose-response curves. The penile erection induced by fenfluramine (1 mg/kg) was dose-dependently antagonized by pindolol (0.1-3.2 mg/kg), a 5-HT1 antagonist, or scopolamine (0.032-1 mg/kg), a muscarinic antagonist, but not by sulpiride (1-32 mg/kg), a dopaminergic antagonist. The penile erection induced by pilocarpine (0.32 mg/kg) was countered by pindolol or scopolamine but not by sulpiride, while that induced by apomorphine (0.032 mg/kg) was countered by all three antagonists. Septo-hippocampal cholinergic deafferentations by medial septum lesioning or fimbria-fornix transection also significantly attenuated the penile erection induced by fenfluramine or apomorphine, but scarcely affected that induced by pilocarpine. Raphe lesion by injections of 5,7-dihydroxytryptamine, a serotonergic neurotoxin, into the median- and dorsal-raphe nuclei significantly attenuated the penile erections induced by fenfluramine and apomorphine but not that by pilocarpine. These results suggest that a neuronal link between the dopaminergic, serotonergic and cholinergic systems plays a crucial role in the expression of penile erection; dopaminergic stimulation causes an activation of the raphe serotonergic neurons which in turn enhances the septo-hippocampal cholinergic pathway and results in expression of penile erection.

Possible involvement of the septo-hippocampal cholinergic and raphe-hippocampal serotonergic activations in the penile erection induced by fenfluramine in rats

Fenfluramine (0.1-10 mg/kg, i.p.) induced penile erection in naive rats with a bell-shaped dose response curve. The response to fenfluramine (1 mg/kg) was antagonized by scopolamine (0.032-1 mg/kg) but not by methyl-scopolamine (0.032-1 mg/kg) pre-treatment. The septo-hippocampal cholinergic deafferentations by medial septum lesion or fimbria-fornix transection also attenuated the penile erection induced by fenfluramine, whereas dopaminergic blockade by sulpiride (3.2-100 mg/kg) had hardly any effect. Pindolol (0.1-3.2 mg/kg), a 5-HT1 antagonist, and ICS205-930, a 5-HT3 antagonist, but not ketanserin, a 5-HT2 antagonist, inhibited the penile erection induced by fenfluramine. Depletion of 5-HT not only by systemic injections of p-chlorophenylalanine (150 mg/kg i.p. at 72, 48 and 24 h before the test) but also by the injections of 5,7-dihydroxytryptamine (5,7-DHT), a serotonergic neurotoxin, into the median- and dorsal-raphe nuclei significantly attenuated the fenfluramine-induced penile erections. Neurochemical analyses revealed that the raphe-lesion significantly reduced the contents of serotonin (5-HT) and its major metabolite, 5-hydroxyindole acetic acid (5-HIAA) but not choline acetyltransferase (ChAT) activities in all the cortical and subcortical regions examined. The results suggest that fenfluramine facilitates the expression of penile erection in rats through an activation of the septo-hippocampal cholinergic pathway as a consequence of excitation of the raphe-hippocampal serotonergic pathway in which 5-HT1 and/or 5-HT3 receptors appear to play a regulatory role.

Effects of apomorphine, physostigmine and vasoactive intestinal peptide on penile erection and yawning in diabetic rats

The present report describes for the first time the effects of systemic administration of apomorphine and of physostigmine, as well as the effects of central and systemic administration of vasoactive intestinal peptide (VIP), on penile erection and yawning in rats with streptozotocin-induced diabetes. Systemic administration of apomorphine induced both penile erection and yawning in non-diabetic rats but not in diabetic rats, while that of physostigmine induced only yawning in non-diabetic rats, and neither yawning nor penile erection in diabetic rats. Intracerebroventricular administration of VIP induced both penile erection and yawning in non-diabetic rats, but neither was induced in diabetic rats. Application of VIP as an ointment to the surface of the glans penis induced penile erection but not yawning in both non-diabetic and diabetic rats. Thus, penile erection and yawning are less easily induced in diabetic rats than in non-diabetic rats. Grooming occurred whenever penile erection was induced, but was not associated with yawning.

A screening concept based on a hypothesis led to the development of a putative cognitive enhancer that stimulates penile erection

Starting from the hypothesis that drugs which specifically activate the hippocampal cholinergic nerve activity may ameliorate memory impairments, we carried out a series of evaluations for a novel cognitive enhancer using enhancement of penile erection as a sign of cholinergic activation, and found FR64822. The compound facilitated penile erection in naive rats, and it ameliorated scopolamine-induced amnesia of rats in passive avoidance tasks with bell-shaped dose-response curves, while it dose-dependently reduced body weight gain in Zucker fatty rats. Pretreatment with sulpiride (32 mg/kg, p.o.) hardly affected the former two activities, but significantly reduced the anorectic activity in Zucker rats. Further evaluation of FR64822 derivatives characterized a second compound, FR121196, which induces penile erection and memory enhancement, but not body weight reduction. Memory enhancing and erection stimulating activities of FR121196 were abolished in rats treated with either cysteamine (200 mg/kg, s.c.), a somatostatin depletor, or lesioning of the serotonergic raphe nuclei. Thus, classic whole animal studies based on a hypothesis proved to be efficient for reaching our objective, the discovery of a new drug. They also gave us insight into the common somatostatinergic and serotonergic mechanisms underlying penile erection and memory improvement.

Hippocampal oxytocin mediates apomorphine-induced penile erection and yawning

Repeated episodes of penile erection and yawning can be induced in male rats either by low doses of the dopaminergic agonist apomorphine or by oxytocin given systematically or into a lateral ventricle (ICV), respectively, or after microinjection of the two substances directly in the paraventricular nucleus (PVN) of the hypothalamus. These behavioral responses are prevented in a dose-dependent manner by the ICV administration of the potent oxytocin antagonist d(CH2)5Tyr(Me)-Orn8-vasotocin. In contrast, the PVN injection of d(CH2)5Tyr(Me)-Orn8-vasotocin (1-30 ng), while effective in preventing oxytocin effect, was unable to prevent apomorphine response. On the other hand, apomorphine-, but not oxytocin-induced penile erection and yawning was prevented by electrolytic lesion of the medial septum (MS). Such a lesion decreased oxytocin content by about 45% in the hippocampus. The above results suggest that the hypothalamic-hippocampal oxytocinergic pathway mediates apomorphine-induced penile erection and yawning and that oxytocin is involved at different levels in the CNS for the control of these behavioral responses.

Differential effects of physostigmine and pilocarpine on the spatial memory deficits produced by two septo-hippocampal deafferentations in rats

Rats that had received two kinds of septo-hippocampal deafferentations, medial septum (MS) lesion and fimbria-fornix (FF) transection, were assayed for brain cholineacetyltransferase (ChAT) activity and spatial memory in an 8-arm radial maze task. Both lesions produced profound and long-lasting spatial memory impairments, which were characterized by a reduction in the numbers of correct arm choices and first correct choices, a reduction in the percent of correct choices and an increase in the number of errors. The degree of memory impairment was severer in FF- than in MS-lesioned rats, and paralleled that of decreases in ChAT activity in the hippocampus. MS lesion reduced ChAT activity in the hippocampus by approximately 45%, while FF lesion almost completely depleted the activity. An intraperitoneal injection of physostigmine (0.0032-0.32 mg/kg), an acetylcholinesterase (AChE) inhibitor, significantly ameliorated the spatial memory deficit induced by MS lesion, but hardly affected that by FF lesion. In contrast, intraperitoneal doses (0.032-3.2 mg/kg) of pilocarpine, a muscarinic agonist, showed a significant improvement of both types of memory deficit with bell shaped dose-response curves. The drug was more potent in the FF- than in the MS-lesioned rats. These results suggest that the septo-hippocampal cholinergic system plays a crucial role in the maintenance of spatial memory, and that the degree of septo-hippocampal deafferentation affects the efficacy of cholinergic drugs.