Behavior during hippocampal microinfusions: anticholinesterase-induced locomotor activation

Spatial learning impairment in prepubertal guinea pigs prenatally exposed to the organophosphorus pesticide chlorpyrifos: Toxicological implications

Exposure of the developing brain to chlorpyrifos (CPF), an organophosphorus (OP) pesticide used extensively in agriculture worldwide, has been associated with increased prevalence of cognitive deficits in children, particularly boys. The present study was designed to test the hypothesis that cognitive deficits induced by prenatal exposure to sub-acute doses of CPF can be reproduced in precocial small species. To address this hypothesis, pregnant guinea pigs were injected daily with CPF (25mg/kg,s.c.) or vehicle (peanut oil) for 10days starting on presumed gestation day (GD) 53-55. Offspring were born around GD 65, weaned on postnatal day (PND) 20, and subjected to behavioral tests starting around PND 30. On the day of birth, butyrylcholinesterase (BuChE), an OP bioscavenger used as a biomarker of OP exposures, and acetylcholinesterase (AChE), a major molecular target of OP compounds, were significantly inhibited in the blood of CPF-exposed offspring. In their brains, BuChE, but not AChE, was significantly inhibited. Prenatal CPF exposure had no significant effect on locomotor activity or on locomotor habituation, a form of non-associative memory assessed in open fields. Spatial navigation in the Morris water maze (MWM) was found to be sexually dimorphic among guinea pigs, with males outperforming females. Prenatal CPF exposure impaired spatial learning more significantly among male than female guinea pigs and, consequently, reduced the sexual dimorphism of the task. The results presented here, which strongly support the test hypothesis, reveal that the guinea pig is a valuable animal model for preclinical assessment of the developmental neurotoxicity of OP pesticides. These findings are far reaching as they lay the groundwork for future studies aimed at identifying therapeutic interventions to treat and/or prevent the neurotoxic effects of CPF in the developing brain.

Rearing on hind legs, environmental novelty, and the hippocampal formation

Many mammals spontaneously rear on their hind legs in response to novelty. The current paper is the first review of rearing behaviour, and is intended to collate findings from different perspectives that are not usually brought together. We suggest that rearing is a useful marker of environmental novelty, that the hippocampal formation is a crucial component of the system controlling rearing in novel environments, and that rearing is one of several ethological measures that can profitably be used to assess hippocampal learning and memory. Consideration is given to the following topics: the possible functions of rearing in information-gathering and escape behaviour; the modulation of rearing by various factors such as anxiety/ fear emotionality; comparative perspectives on rearing; neuroanatomical circuits involved in rearing with particular reference to the hippocampal formation and its afferents and efferents; and the role of the hippocampal formation in uncharted and mismatch environmental novelty. The review concludes with testable predictions about rearing, environmental novelty and the hippocampus.

Microinjection of neostigmine into the pontine reticular formation of the mouse: further evaluation of a proposed REM sleep enhancement technique

Microinjections of cholinergic agonists into the pontine reticular formation (PRF) powerfully induce rapid eye movement sleep (REMS) in cats but have comparatively weaker effects in rats. Recently, the cholinomimetic neostigmine has been reported to strongly enhance REMS following microinjection into the PRF of the mouse. That study used behavioral assessments of locomotion in lieu of electrophysiological measures of muscle tone to identify REMS. We sought to confirm that the behavioral state induced in mice by PRF injections of neostigmine meets standard electroencephalogram (EEG) and electromyogram (EMG) criteria for defining REMS. Cortical EEG, nuchal muscle EMG, and PGO waves were recorded from male C57BL/6N mice with chronic indwelling cannulae for the delivery of neostigmine to the PRF. Recordings were made during midday following injections of neostigmine (8.8 mM, 50 nl), 2 h after lights on (LD 12:12). Neostigmine induced a behavioral state characterized by low amplitude, highly desynchronized cortical EEG with little theta, no PGO waves, and a sustained high muscle tone. Behavioral states meeting standard criteria for slow-wave sleep (SWS) and REMS were significantly suppressed compared to baseline recordings, and REMS onset was delayed by 3 h. Consistent with earlier reports, neostigmine did strongly suppress locomotor activity in open field tests and in the home cage. Due to the failure to meet criteria for defining REMS, we conclude that neostigmine microinjection into the PRF of the mouse induces an abnormal waking state rather than REMS.

The ventral subiculum modulation of prepulse inhibition is not mediated via dopamine D2 or nucleus accumbens non-NMDA glutamate receptor activity

Prepulse inhibition of the acoustic startle reflex is an operational measure of sensorimotor gating. The neural substrates of prepulse inhibition may be relevant to the pathophysiology of neuropsychiatric disorders that are characterized by sensorimotor gating deficits, including schizophrenia. Studies have demonstrated abnormalities within the hippocampal formation of schizophrenia patients, and animal studies have revealed that the hippocampus, and specifically the ventral subiculum, regulates prepulse inhibition. The ventral subiculum sends a dense glutamatergic projection to the nucleus accumbens, and the nucleus accumbens is known to potently regulate prepulse inhibition via dopaminergic and non-N-methyl-D-aspartate (non-NMDA) glutamatergic mechanisms. In the present study, we examined whether the hippocampal regulation of prepulse inhibition is mediated through subiculo-accumbens glutamatergic efferents. Intra-ventral subiculum infusion of NMDA dose dependently reduced prepulse inhibition, and this effect of NMDA was reversed by co-infusion of the NMDA receptor antagonist D,L-amino-5-phosphonovaleric acid (AP5). The prepulse inhibition-disruptive effect of intra-ventral subiculum NMDA infusion was not prevented by infusion of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) into the nucleus accumbens core or shell subregions. Pretreatment with the D2 receptor antagonist haloperidol also failed to block the prepulse inhibition-disruptive effects of intra-ventral subiculum NMDA infusion. Thus, the present findings suggest that while prepulse inhibition is regulated by NMDA activity in the ventral subiculum, this effect does not appear to be mediated via nucleus accumbens dopamine D2 receptors or via nucleus accumbens non-NMDA glutamatergic substrates.

Carbachol infusion into the dentate gyrus disrupts sensorimotor gating of startle in the rat

Prepulse inhibition (PPI) is the decrease in a startle response that occurs when the startling stimulus is preceded by a weaker stimulus or "prepulse". Schizophrenic patients exhibit abnormally low levels of PPI when the prepulse precedes the startle stimulus by less than 500 ms. A similar deficit in sensorimotor gating can be demonstrated in rats after stimulation of D2 dopamine (DA) receptors by systemic administration of DA agonists or by infusion of DA directly into the nucleus accumbens. We now demonstrate that carbachol infusion into the dentate gyrus of the hippocampal formation disrupts PPI in the rat. This disruption of sensorimotor gating occurs when the startling stimulus is either acoustic or tactile. Carbachol infusion into the neocortex has no effect on PPI. While pretreatment with the D2 DA receptor antagonist spiperone reverses the disruption of PPI caused by systemic administration of apomorphine, this pretreatment fails to reverse the disruption of PPI induced by carbachol infusion into the hippocampus. These results demonstrate that pharmacologic stimulation of the hippocampus disrupts sensorimotor gating in the rat by a mechanism distinct from that of DA agonists. Prepulse inhibition of the startle reflex is an animal model in which pharmacologic stimulation of the hippocampus mimics the deficits in sensorimotor gating observed in schizophrenic patients.

Increases in diversive exploration in rats during hippocampal microinfusions of isoproterenol but not methoxamine

To test the hypothesis that the increase in locomotor activity and the diversification of stimulus sampling observed during intrahippocampal microinfusions of norepinephrine are mediated by beta adrenergic receptors, rats were infused with either the beta agonist isoproterenol or the alpha-1 agonist methoxamine while being tested in a Behavioral Pattern Monitor. The behavioral profile induced by isoproterenol was qualitatively similar to that observed previously with norepinephrine. Methoxamine had no significant effects. The ability of isoproterenol to mimic the norepinephrine-induced blockade of the hyperactivity associated with intrahippocampal microinfusions of the cholinergic agonist carbachol was also tested. Carbachol produced hyperactivity which was attenuated by the coinfusion of isoproterenol. These results are consistent with the hypothesis that increases in diversive exploration and stimulus sampling induced by intrahippocampal norepinephrine are due to the activation of beta adrenergic receptors within the dentate gyrus.

Brain NADH and jumping behavior in the rat

The effects of either the reduced Nicotinamide Adenine Dinucleotide (NADH) microinjected into the brain or amphetamine injected peritoneally on jumping behavior were observed in 68 rats. The enhanced jumps in the group with amphetamine are the greatest among the three experimental groups. The enhanced effect of NADH microinjected into the caudate nucleus is stronger than those of NADH in the frontal cortex. The effects of extra NADH in the frontal cortex on the jumps are dose-dependent. NADH concentrations in the brain of rats with amphetamine increased immediately after behavioral procedure. The increased concentrations in the brain from both extra and intra sources are related to the enhanced jumps of rats.

Neuropharmacological evidence to suggest that the nucleus accumbens and subpallidal region contribute to exploratory locomotion

Rats showed a substantial increase in locomotor activity when novel objects were introduced into an open-field apparatus. This exploratory locomotion was reduced significantly when glutamic acid diethyl ester HCl, a glutamate antagonist, was injected bilaterally into the nucleus accumbens or when gamma-aminobutyric acid was injected bilaterally into the subpallidal region. These observations provide preliminary support for a contribution of the nucleus accumbens and subpallidal region to exploratory locomotion.

A study of the contribution of hippocampal-accumbens-subpallidal projections to locomotor activity

Locomotor activity recorded in an automated open-field apparatus was increased substantially by unilateral injections of carbachol, a cholinergic agonist, into the dentate gyrus of the hippocampus. Hyperactivity elicited in this way was reduced significantly when glutamate antagonists were injected into the ipsilateral nucleus accumbens. Injecting gamma-aminobutyric acid into the ipsilateral subpallidal region also reduced the hyperactivity from injections of carbachol into the dentate gyrus. When these compounds were injected into the contralateral accumbens and subpallidal region, respectively, there was little or no reduction in the carbachol-elicited locomotor activity. These observations suggest that neural pathways fom hippocampus to accumbens to subpallidal region may contribute to locomotor activity.

Effects of ganglioside treatments on lesion-induced behavioral impairments and sprouting in the CNS

Recent findings suggest that exogenous gangliosides improve recovery of a learned behavior (alternation in a T-maze) which is thought to be related to sprouting after lesions of the entorhinal cortex. In the present investigation, we studied an unlearned behavior (the open-field hyperactivity resulting from bilateral entorhinal lesions) to evaluate whether ganglioside treatments reduce the severity of initial postlesion impairments or improve recovery. We also examined whether the treatments enhance the sprouting of septodenate fibers which parallels the recovery of open-field activity. The typical behavioral changes induced by bilateral entorhinal lesions include hyperactivity, reduced habituation of activity, and a gradual time-dependent return toward control levels. We found that rats treated with total brain gangliosides (30 mg/kg) showed a smaller lesion-induced increase, consistently lower levels, and greater within-session habituation of activity than did saline-treated counterparts. Control rats treated with gangliosides did not exhibit a reduction in activity, suggesting that the effect was on lesion-induced hyperactivity rather than on activity, per se. Ganglioside-treated rats showed a slight, but consistently smaller lesion-induced sprouting response by the septodentate pathway than did untreated counterparts at all postlesion intervals examined (3, 5, 7, and 10 days). The present findings indicate that ganglioside treatments reduce the severity of the initial behavioral effects after entorhinal lesions without enhancing the sprouting by septodentate fibers.