Decreased locomotor and investigatory exploration after denervation of catecholamine terminal fields in the forebrain of rats.

Exploratory behaviors were examined after bilateral microinjections of 6-hydroxydopamine into two hypothalamic sites that produced different patterns of denervation of forebrain catecholamine terminal fields. After anterolateral injections rats locomoted and reared less in a novel open field, responded abnormally to changes in the degree of novelty of the open field, and investigated a novel object less. These are deficits in exploratory behavior because they were not secondary to the inhibition of open-field behavior by hyperemotionality, by general motor disability, or by the failure to detect novel spaces or objects. Such anterolateral injections produced loss of catecholamine fibers, determined histochemically, in neocortical, hippocampal, anterolateral hypothalamic, mesolimbic, mesocortical, and anteromedioventral striatal terminal fields and loss of dopaminergic perikarya in the A10 and anteromedial A9 cell groups. No deficits in exploratory behaviors occurred, however, after bilateral anteromedial 6-hydroxydopamine injections that denervated neocortical, hippocampal, and anteromedial hypothalamic catecholamine terminal fields. A critical forebrain catecholaminergic innervation for exploratory responses to novel stimuli may be within areas that were denervated by anterolateral but not by anteromedial hypothalamic 6-nydroxydopamine injections. These areas are mesolimbic, mesocortical, anteromedioventral, and anterolateral hypothalamic terminal fields.     

Neostriatal dopamine and sensory inattention

Damage to the mesotelencephalic dopamine-containing projection of rats results in a sensory inattention, characterized by impairments in orientation toward somatosensory, visual, and olfactory stimuli. The present experiments were performed to establish which branch of this dopaminergic system is responsible for these sensorimotor deficits. Two approaches were used. In the first, individual dopamine-innervated forebrain sites were damaged by localized 6-hydroxydopamine injection into, or by electrolytic lesions of, these regions. In the second, rats were given tegmental 6-hydroxydopamine injections that damaged the entire mesotelencephalic projection and subsequently received intracerebral injections of the dopamine agonist apomorphine into specific forebrain sites in an attempt to reinstate orientation. The results demonstrate that dopaminergic terminals in the neostriatum are critical for orientation. Unilateral neostriatal 6-hydroxydopamine injections or electrolytic lesions reduced orientation to contralateral touch, whereas similar damage to other dopamine-innervated forebrain structures did not. Further, the results suggest that dopaminergic terminals in the anterior neostriatum are especially important for orientation to touch of the rostral body surface while those in the posterior neostriatum are most critical for orientation to caudal touch. After damage to all branches of the mesotelencephalic dopaminergic system, orientation to touch was reinstated by injection of apomorphine into the neostriatum but not by injection into the other dopamine-innervated forebrain regions tested.     

Escape performance after inescapable shock in selectively bred lines of mice: response maintenance and catecholamine activity.

The effects of inescapable shock on subsequent escape performance and shock-elicited activity were examined in six lines of mice selectively bred for differences in general locomotor activity. The line differences in locomotor activity were found to be unrelated to the differences observed on shock-elicited activity. However, escape performance following exposure to inescapable shock was predictable from the levels of shock-elicited activity. Those lines that displayed the greatest decline in motor activity during shock likewise displayed the most pronounced escape deficits. The line differences in escape performance induced by inescapable shock could be mimicked by treatment with a tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine. As predicted, the lines that displayed the least interference after tyrosine hydroxylase inhibition exhibited the smallest reduction in levels of catecholamines. The effects on escape performance following inescapable shock are interpreted in terms of the role of response maintenance deficits produced by catecholamine depletion.     

The behavioral effects of depletions of brain serotonin induced by 5,7-dihydroxytryptamine vary with time after administration

Ether-anesthetized Sprague-Dawley rats were depleted of brain serotonin (5HT) by intraventricular injections of 50 micrograms 5,7-dihydroxtryptamine (57DHT). Oral pretreatment with 25 mg/kg desmethylimipramine was used to protect brain noradrenergic neurons from 57DHT. Liquid chromatographic assays revealed that this treatment did not significantly alter catecholamine levels but depleted hippocampal 5HT by 92% and striatal 5HT by 45%. Three or eleven days after lesioning, locomotor and exploratory behavior was characterized in separate groups of animals with a behavioral pattern monitor (BPM). On Days 4 and 12, the animals were retested following saline or 1.0 mg/kg amphetamine. Three days after depletion, lesioned rats exhibited a decrease rate of habituation of locomotor activity relative to controls. When challenged with amphetamine (1.0 mg/kg), 5HT-depleted rats exhibited increased corner and decreased center activity, as well as stereotyped patterns of locomotion. Eleven days following lesion, 5HT-depleted rats exhibited habituation rates greater than controls; amphetamine challenge yielded patterns of activity similar to those of control animals. These results show that central serotonergic pathways play an important role in modulating both spontaneous and amphetamine-elicited activity in rats, and that compensatory mechanisms operate over time to alter the behavioral effects of 57DHT-induced depletions of brain 5HT.     

The NK3 receptor agonist senktide ameliorates scopolamine-induced deficits in memory for object, place and temporal order

Senktide, a potent neurokinin-3 receptor (NK₃-R) agonist, increases acetylcholine (ACh) release in the striatum, the prefrontal cortex (Schäble et al., 2011), the amygdala and hippocampus, presumably via postsynaptic mechanisms. A promnestic action of NK₃-R agonists has been described in a variety of learning/memory tasks. The memory-enhancing effects of NK₃-R agonists and their activating influence on ACh suggest a possible role of the NK₃-R in learning and memory via cholinergic modulation. Deterioration of the cholinergic system in the basal forebrain has been associated with learning and memory deficits and cholinergic agents have promnestic effects in a variety of learning paradigms. The anticholinergic drug, scopolamine, a muscarinic ACh receptor antagonist, incurs deficits in a variety of learning tasks and provides a useful tool to investigate the role of the cholinergic systems in mechanisms underlying learning and memory. The aim of this study was to ascertain the effect of the NK₃-R agonist, senktide, in the scopolamine-induced deficit model. We hypothesized that senktide treatment would attenuate scopolamine-induced (subcutaneous – s.c. 0.75 mg/kg) memory impairment in three novelty preference paradigms based on spontaneous object exploration: namely object recognition, object–place recognition and object recognition for temporal order. Administration of senktide reversed the scopolamine-induced memory deficits by re-establishing object recognition (s.c. 0.2 mg/kg), object–place recognition (0.2 and 0.4 mg/kg), as well as object recognition for temporal order (0.4 mg/kg) in adult Wistar rats. These results indicate memory enhancing effects of senktide in animals subjected to scopolamine-induced memory impairments and indicate that the promnestic action of NK₃-R agonists is mediated by muscarinic cholinergic mechanisms.     

Disturbances in exploratory behavior and functional recovery in the Y and radial mazes following dopamine depletion of the lateral septum

The effects of 6-hydroxydopamine (6-OHDA) injected into the lateral septum in rats were investigated for spontaneous alternation behavior in a Y maze and for spatially oriented behavior in an 8-arm radial maze. The performance of the animals in these tests was assessed under two physiological states, food-satiated and food-deprived. The selective depletion of septal dopaminergic concentrations leads to behavioral disturbances in both the Y and the radial mazes. These deficits disappeared when the animals with 6-OHDA lesions were food-deprived. These results confirm other studies from our laboratory and support two conclusions. First, lesions of dopaminergic neurons lead to behavioral impairments which resemble those found after the total lesion of the structure they innervate. Second, these behavioral impairments are responsive to therapeutic treatments, manipulations of the internal or external environment, or the level of arousal, since under certain conditions a recovery of function can occur in the absence of dopaminergic neurons. These two points provide additional support for a nonspecific role for the dopaminergic neurons originating in the ventral tegmental area. These neurons could have a permissive role in the functioning of the forebrain structures they innervate.     

Reduction in paradoxical sleep after L-dopa administration in rats

The EEG and EMG activity of two groups of rats (N = 8 each) was recorded during one experimental and 5 control days. On the experimental day, one group received 125.0 mg/kg L-dopa preceded by 50.0 mg/kg benserazide hydrochloride. The other group received 1.0 mg/kg haloperidol, immediately before the recording session. After L-dopa plus benserazide administration the total time and the number of episodes of paradoxical sleep and slow wave sleep decreased significantly compared to control sessions.     

Glucose effects on mecamylamine-induced memory deficits and decreases in locomotor activity in mice.

Peripheral glucose administration attenuates the effects of muscarinic cholinergic antagonists on several measures, including spontaneous alternation, inhibitory avoidance, and locomotor activity. The present study examined glucose interactions with mecamylamine, a nicotinic cholinergic antagonist, on these measures. Mecamylamine (5 mg/kg, sc) significantly impaired spontaneous alternation performance. Glucose (100 mg/kg, ip) administered with mecamylamine attenuated the impairment. Treatment with hexamethonium (5 and 10 mg/kg, sc), a peripheral nicotinic blocker, did not impair performance. Pretraining treatment with mecamylamine, but not hexamethonium, significantly reduced later retention latencies on inhibitory avoidance tests. Glucose, administered with mecamylamine prior to training, significantly attenuated the impaired test performance. Mecamylamine, but not hexamethonium, significantly decreased locomotor activity. In contrast to the attenuating effects of glucose on the other measures above, glucose administered with mecamylamine potentiated the decreased locomotor activity. These findings demonstrate that glucose influences the behavioral effects of a nicotinic cholinergic antagonist in a manner generally similar to that of muscarinic cholinergic antagonists, and supports previous evidence that circulating glucose interacts with central cholinergic functions.     

SLV330, a cannabinoid CB1 receptor antagonist,ameliorates deficits in the T-maze,object recognition and Social Recognition Tasks in rodents

Cannabinoid CB₁ receptor (CB₁R) signaling has been suggested to play an important role in the regulation of memory and cognition. In the present study, our aim was to investigate whether the CB₁R antagonist SLV330 (doses ranging from 0.3 to 10 mg/kg, given orally, p.o.) could ameliorate impairments in distinct aspects of cognition using different disruption models in both mice and rats. Effects of SLV330 were tested on working memory deficits in the T-maze Continuous Alternation Task (T-CAT) in mice; episodic memory deficits in the Object Recognition Task (ORT) and Social Recognition Task (SRT) in rats. The acetylcholinesterase inhibitor (AChEI) donepezil (Aricept®, approved for symptomatic treatment of Alzheimer’s disease) and nicotine were used as reference compounds.SLV330 markedly improved aging and scopolamine-induced memory deficits in the T-CAT in mice with a lowest effective dose (LED) of 1 mg/kg p.o., while reversing the cognitive dysfunction induced by the N-methyl-d-aspartate (NMDA) antagonist dizocilpine (MK-801) only at the middle dose of 3 mg/kg. In the ORT, we have found that combined administration of subthreshold doses of SLV330 (1 mg/kg, p.o.) and the AChEI donepezil (0.1 mg/kg, p.o.), that had no discernable effects on performance when given alone, enhanced memory performance in Wistar rats with deficits induced by the muscarinic antagonist scopolamine, suggestive of additive synergistic effects of SLV330 and donepezil on cognitive impairment. Finally, SLV330 was found to have cognition enhancing properties in a time delay paradigm in the SRT at a LED dose of 3 mg/kg (p.o.).In conclusion, the CB₁R antagonist SLV330 was found to clearly improve memory in several preclinical models for cognitive impairment.     

Activation-induced restoration of sensorimotor functions in rats with dopamine-depleting brain lesions.

Bilateral electrolytic lesions of the lateral hypothalamus or intraventricular 6-hydroxydopamine injections produced substantial depletions of striatal dopamine in rates. All animals with brain damage showed marked sensorimotor impairments. However, they began to move and respond appropriately to environmental stimuli when placed in a sink of water, in a shallow ice bath, or among a colony of cats or rats. A reversal of the sensorimotor dysfunctions was still apparent shortly after the animals were removed from each activating situation. However, the terapeutic effects dissipated rapidly, and by 4 hr after an exposure the rats responded as poorly as they had prior to activation. These findings are strikingly similar to the "paradoxical kinesia" seen in parkinsonism, a clinical disorder attributed to degeneration of central dopamine-containing neurons. Collectively, they suggest the importance of activation in maintaining responsiveness to senory stimuli in rats following dopamine-depleting brain lesions.     

Environmental events that modify the catecholamine fluorescence of the A2 cell bodies in nucleus tractus solitarii

The fluorescence intensity of the catecholamine cell bodies of the A2 group--as determined by microfluorimetry--increased significantly due to exposure of the rats to the following environmental events: placement of a rat from the colony cage into a cold (4 degree C) room for a period of 10 min, isolation of a rat from the colony for 6--7 days, and satiation of gouped rats by allowing them access to sweetened milk for 15 min. The following events failed to affect the intensity of the cells of A2: reduction of the group colony size from eight to three rats per colony for 6--7 days and presentation of water instead of milk after the rats had experienced 6--7 days of milk satiation. These results indicate that aversive as well as rewarding environmental events activate the catecholamine cells of A2.     

Differential effects on body weight of central 6-hydroxydopamine lesions in obese (ob/ob) and diabetes (db/db) mice

Obese (ob/ob) and diabetes (db/db) mice are genetic mutants that have been shown to have altered levels of central catecholamines as well as syndromes of obesity, hyperphagia, and hyperglycemia. Because of catecholamines, and particularly norepinephrine (NE), are implicated in the control of feeding, levels of central catecholamines were experimentally reduced in ob/ob and db/db mice to investigate the role of the catecholamines in these cases of spontaneously occurring obesity. Lesions produced by 6-hydroxydopamine (6-OHDA) were used to produce large depletions of NE and dopamine (DA) in both ob/ob and db/db mice and in lean control mice of the same background strains. In the db/db but not the ob/ob, central catecholamine depletion was accompanied by a significant and persistent weight loss and by a reduction in plasma glucose levels when compared with vehicle-infused controls. Treatment with the NE uptake blocker desmethylimipramine (DMI) prior to 6-OHDA infusions attenuated NE but not DA depletion. Diabetes mice that received DMI pretreatment showed a weight loss and decrease in plasma glucose proportional to the amount of NE depletion. Lean mice that received the 6-OHDA treatments showed only a transient weight loss and no significant change in blood glucose. It is concluded that abnormalities in central noradrenergic systems may account for part of the obesity syndrome observed in the diabetes mouse.     

Modulation by 6-hydroxydopamine of expression of the phenylethanolamine N-methyltransferase (PNMT) gene in the rat heart during immobilization stress

Phenylethanolamine N-methyltransferase (PNMT) is the final enzyme in the catecholamine synthesizing cascade that converts noradrenaline (NA) to adrenaline (Adr). Both of these catecholamines are physiologically important hormones and neurotransmitters in mammals with profound influence on the activity of the cardiovascular system. Although PNMT activity and gene expression have been reported in the neonatal and also adult rat heart, little is known about the identity of the cells expressing PNMT mRNA. In this study, we have shown that besides PNMT in neuronal and intrinsic cardiac cells, this enzyme is expressed also in rat cardiomyocytes, as shown by immunofluorescence in isolated cardiomyocytes. To determine which cells in the heart more sensitively show stress-induced changes in PNMT mRNA expression, we performed chemical sympathectomy by administration of 6-hydroxydopamine (6-OHDA), which destroys catecholaminergic terminals. We determined PNMT mRNA levels in the left atria and ventricles of control and stressed rats. In the rats treated with 6-OHDA, PNMT mRNA levels were not changed under normal, physiological conditions compared to vehicle treated rats. Similar results were observed on isolated cardiomyocytes from control and 6-OHDA treated rats. However, 6-OHDA treatment prevented immobilization-induced increase in PNMT mRNA expression. The results allow us to propose that in the heart, the immobilization-induced increase in PNMT gene expression is probably not in cardiomyocytes, but in neuronal cells.     

Deficient passive and one-way active avoidance acquisition following medial forebrain bundle lesions in rats.

Cathodal electrolytic lesions of the medial forebrain bundle (MFB) at posterior hypothalamic levels in male hooded rats produced a mild, transient hypodipsia and lowered jump thresholds to footshock. The lesions produced marked deficits in passive avoidance performance in a paradigm that paired discrete, linearly incrementing footshock intensities with contact of a water spout following 48 hr of water deprivation. Intraperitoneal injections of DL-5-hydroxytryptophan (75 mg/kg), the immediate metabolic precursor of serotonin, had no effect on the passive avoidance performance of either experimental or operated control subjects. Lesions of the MFB also resulted in deficient acquisition in a one-trial step-through passive avoidance paradigm not using motivation to drink and caused a severe acquisition deficit in a one-way active avoidance task. Lesions of the septal nuclei produced lowered jump thresholds but did not affect acquisition in the first passive avoidance task. The results are interpreted as indicating a lesion-induced deficiency in fear learning, independent of the serotonergic functions of the MFB.     

Cognitive aspects of congenital learned helplessness and its reversal by the monoamine oxidase (MAO)-B inhibitor deprenyl

Cognitive processes are assumed to change with learned helplessness, an animal model of depression, but little is known about such deficits. Here we investigated the role of cognitive and related functions in selectively bred helpless (cLH, n = 10), non-helpless (cNLH, n = 12) and wild type (WT, n = 8) Sprague Dawley rats. The animals were exposed to an open field for 10 min on each of two test days. On the third day, an object exploration paradigm was carried out. The animals were later tested for helplessness. Both cLH and cNLH rats were more active than WTs on the first day in the open field. Over trials, cNLH and WT rats lowered their activity less than cLH rats. This resistance-to-habituation co-varied with a resistance to develop helplessness. In cLH rats, higher ‘anxiety’ or less time spent in the center of the open field co-varied with severe helplessness. In WTs, a greater reactivity to novel objects and to a spatially relocated object predicted lower levels of helplessness. In cLH rats (n = 4–5 per group), chronic treatment with a high dose of the monoamine oxidase (MAO)-B inhibitor deprenyl (10 mg/kg; i.p.), an anti-Parkinson, nootropic and antidepressant drug, attenuated helplessness. Remarkably, helplessness reversal required the experience of repeated test trials, reminiscent of a learning process. Chronic deprenyl (10 mg/kg; i.p.) did not alter locomotion/exploration or ‘anxiety’ in the open field. In conclusion, helplessness may be related to altered mechanisms of reinforcement learning and working memory, and to abnormalities in MAO-A and/or MAO-B functioning.     

Chronic oral methylphenidate induces post-treatment impairment in recognition and spatial memory in adult rats

Recent pre-clinical research has indicated that chronic treatment with methylphenidate (Ritalin^®) in young animals can result in lasting and potentially detrimental alterations in brain function that can persist into adulthood. Chronic methylphenidate-induced neuronal alterations may result in behavior and cognitive deficits that include increases in behavioral responses and impairment in recognition memory. This study compared the cognitive consequences following chronic treatment with two doses (5 and 10 mg/kg) of methylphenidate on recognition and spatial memory in adult male Long-Evans rats using an established oral dosing procedure. The animals were then tested in the Object Recognition test at 14 days post treatment and the Object Placement test at 21 days post treatment. The results indicate that repeated exposure to oral methylphenidate impaired the performance of rats in these tests. The current findings add to recent research demonstrating negative consequences in rats pre-treated with methylphenidate, and extend previous findings to include deficits in spatial recognition memory.     

Effects of 6-hydroxydopamine lesions of the nucleus accumbens septi and olfactory tubercle on feeding, locomotor activity, and amphetamine anorexia in the rat.

Bilateral 6-hydroxydopamine lesions of the nucleus accumbens septi (NAS) and olfactory tubercle (OT) caused enhanced intake of wet mash in 23-hr-food-deprived rats tested in photocell activity cages during restricted 30-min sessions. This mild hyperphagia was accompanied by a significant hypoactivity in the group with NAS/OT lesions. No hyperphagia was observed during a prolonged 120-min test session or in free-feeding tests conducted in the home cage. Anorexia induced by d-amphetamine (.5 and 1.5 mg/kg) was unaltered by the lesion, although the locomotor stimulant action of the drug was attenuated. A second experiment showed that the NAS/OT lesion also enhanced food intake in the photocell cages during 30-min sessions with dry food pellets but that food-associated drinking was concomitantly reduced. The results are consistent with the hypothesis that the behavioral changes caused by mesolimbic neuron destruction result in part from an inability to switch from one behavioral activity to another.     

Conditioning of the microcirculation in the rat

The serotonin-catecholamine balancing mechanism maintains normal vascular tonus and permeability of the microcirculation. Serotonin provokes local vasodilatation and increases capillary permeability in the skin of rats. Released or injected catecholamines inhibit the serotonin-induced peripheral hemodynamic alterations. Following eleotroconvulsive treatment (ECT) (US), there is a catecholamine release and the microcirculatory response to serotonin does not occur. When the administration of a visual stimulus (in six consecutive occasions) proceedingly coincided with the administration of the ECT, it was noted that the light became an effective stimulus for this system. Light alone is an indifferent stimulus to catecholamine release. Thus, the inhibition of the serotonin-induced vasodilatation by ECT (via catecholamine release) became conditioned to the visual-conditional stimulus. Since the net effect of extrinsic serotonin depends on free catecholamines at the vascular bed, the intensity of the serotonin-induced vascular response reflects alterations in the chemical balancing mechanism. This mechanism can be conditioned with the present technique. The result indicates that there is central nervous system mediation.     

Blood glucose and brain function: interactions with CNS cholinergic systems

We recently found that glucose injections attenuate amnesia and hyperactivity produced by scopolamine, a muscarinic antagonist. The present study examined whether glucose would augment behavioral effects produced by a muscarinic agonist, physostigmine. In experiment I, doses were first determined for which neither glucose (10 mg/kg) nor physostigmine (0.05 mg/kg) altered scopolamine-induced hyperactivity. However, combined glucose-physostigmine injections significantly reduced scopolamine hyperactivity. Experiment II evaluated the effects of glucose on physostigmine-induced tremors. Glucose (10, 100, and 250 mg/kg) or saline injections were given 20 min before physostigmine injections (0.4 or 0.05 mg/kg). Observations of glucose effects on the severity of physostigmine-induced tremors were then obtained at 5-min intervals for 25 min after physostigmine injections. Glucose (100 mg/kg) significantly facilitated the onset of tremors when injected before either dose of physostigmine, and augmented (at 100 and 250 mg/kg) tremor severity when injected before the lower dose of physostigmine. These findings indicate that glucose can facilitate the actions of a cholinergic agonist on two behaviors, locomotor activity and tremors, adding support to the view that circulating glucose levels can modulate central cholinergic function. More generally, the results provide additional evidence that circulating glucose levels can influence brain function.     

Chronic sildenafil (Viagra) administration reduces anxiety in intact and castrated male rats

Epidemiological research indicates that sildenafil (Viagra) abuse is associated with increased risk behaviors. The present study employs the open field, a standard animal model used in the field of anxiety research, to examine whether chronic exposure to sildenafil affects anxiety and risk-taking behaviors in gonadally intact and castrated male Wistar rats. Sildenafil (10 mg/kg) or saline were administered three times a week for three weeks. Animals were tested once a week in the open field during and after drug treatment. Sildenafil treatment increased the number of center entries and time spent in the center in intact and castrated animals during and after treatment, suggesting that repeated drug use decreases anxiety. Sildenafil also restored the deficits in exploration and locomotion produced by castration, indicating that sildenafil effects on open field behaviors are independent of endogenous androgens. We caution against generalizing from this study to human behaviors, but propose that the behavioral effects produced by a chronic high dose of sildenafil warrant further studies into its abuse potential.