Scopolamine but not haloperidol disrupts training-induced neuronal activity in cingulate cortex and limbic thalamus during learning in rabbits

Memory performance and scopolamine: hypoactivity of the thalamus revealed by cytochrome oxidase histochemistry

Spatial memory learning is related to the functioning of a neuronal circuit composed of cortical, hippocampal and diencephalic brain regions. The Morris water maze (MWM) is frequently used to assess spatial memory in rats. In this study, the neuronal functional activity of some brain limbic system regions after a memory task in adult male Wistar rats injected with scopolamine (1.0mg/kg, i.p.) was assessed using cytochrome oxidase (COx) histochemistry. The rats were trained following a working memory schedule in the MWM. A trained group injected with saline and an untreated control group were examined to compare changes in COx activity in the dorsal hippocampus, anterior thalamus, mammillary nuclei, prefrontal cortex and ventral tegmental area. The scopolamine-treated group showed an impairment of spatial learning. Also, a decrease in COx activity was found in this group as compared to the saline group in the anteroventral and anteromedial thalamic nuclei. Overall, these findings suggest that memory deficits induced by scopolamine may be due to impairment of the cholinergic function in the anterior thalamic nuclei.

Metabotropic induction of persistent activity in layers II/III of anterior cingulate cortex

The anterior cingulate cortex (ACC), a limbic region associated with pain-related working memory and memory acquisition, receives a dense cholinergic innervation. To further understand the role of acetylcholine in ACC, we characterized the firing properties of pyramidal neurons following muscarinic receptor activation. Using whole-cell patch clamp recordings in acute brain slices, we report long-lasting nonsynaptic plateau potentials and persistent firing induced by carbachol (CCh) in pyramidal neurons in layers II/III of rat ACC. CCh responses were abolished by the muscarinic receptor antagonist atropine or by inhibitors of G proteins and phospholipase C. Inhibiting L-type calcium channels with nifedipine, removing extracellular calcium or chelating intracellular calcium with BAPTA also abolished plateau potentials and persistent firing. Blockade of nonselective cation channels with flufenamic acid, 2-aminoethyl diphenylborinate or SKF-96365 suppressed CCh responses and voltage-clamp recordings of CCh-sensitive currents revealed a transient receptor potential canonical-like cationic conductance. The group I metabotropic glutamate receptor (mGluR) agonist (S)-3,5-dihydroxyphenylglycine hydrate induced plateau potentials and persistent firing that were mediated by mGluR5. Our data demonstrate that receptor-operated channels drive calcium-dependent plateau potentials and persistent firing in layers II/III of ACC. Therefore, acetylcholine- and glutamate-evoked persistent activity in ACC may play a mnemonic role by allowing transient storage of information during pain processing.

Scopolamine impairs auditory delayed matching-to-sample performance in monkeys

Information concerning the major neurotransmitters critical for auditory memory is sparse. One possibility is the cholinergic system, important for performance in some tasks requiring visual short-term memory and attention [T.G. Aigner, M. Mishkin, The effects of physostigmine and scopolamine on recognition memory in monkeys, Behav. Neural. Biol. 45 (1986) 81-87; N. Hironaka, K. Ando, Effects of cholinergic drugs on scopolamine-induced memory impairment in rhesus monkeys, Jpn. J. Psychopharmacol. 16 (1996) 103-108; T.M. Myers, G. Galbicka, M.L. Sipos, S. Varadi, J.L. Oubre, M.G. Clark, Effects of anticholinergics on serial-probe recognition accuracy of rhesus macaques (Macaca mulatta), Pharmacol. Biochem. Behav. 73 (2002) 829-834; H. Ogura, T.G. Aigner, MK-801 Impairs recognition memory in rhesus monkeys: comparison with cholinergic drugs, J. Pharmacol. Exp. Ther. 266 (1993) 60-64; D.M. Penetar, J.H. McDonough Jr., Effects of cholinergic drugs on delayed match-to-sample performance of rhesus monkeys, Pharmacol. Biochem. Behav. 19 (1983) 963-967; M.A. Taffe, M.R. Weed, L.H. Gold, Scopolamine alters rhesus monkey performance on a novel neuropsychological test battery, Cogn. Brain Res. 8 (1999) 203-212]. Five rhesus monkeys were trained to perform an auditory go/no-go delayed matching-to-sample (DMTS) task wherein two acoustic stimuli (500ms), separated by variable memory delays (500ms, 2500ms, or 5000ms), were either identical sound presentations, i.e., match trials, or two different sound presentations, i.e., nonmatch trials. Sound stimuli were chosen semi-randomly from a large set sound set ( approximately 900). After reaching a criterion of 80% correct on the behavioral task, monkeys were injected with saline or doses of scopolamine hydrochloride mixed in saline (3 microg, 5 microg, and 10 microg per 1kg of weight), 30 min before training. Scopolamine impaired performance accuracy on match trials in a dose-dependent manner. Blocking muscarinic receptors with scopolamine did not significantly impair motor responses, food motivation, or responses to rewarded sound. These findings support the hypothesis that the cholinergic system is important for auditory short-term memory.

Effects of conditioning during amygdalar inactivation on training-induced neuronal plasticity in the medial geniculate nucleus and cingulate cortex in rabbits (Oryctolagus cuniculus)

This study addressed the amygdala's role in avoidance conditioning in rabbits (Oryctolagus cuniculus). Intra-amygdalar muscimol infusion before 60 or 120 conditioning trials blocked training-induced neuronal activity (TIA) in the medial geniculate (MG) nucleus. One hundred twenty trials with muscimol blocked TIA permanently, during conditioning with muscimol and then later without muscimol; 60 trials with muscimol blocked TIA only when muscimol was present. Cingulate cortical TIA was blocked only when muscimol was present. Behavioral learning did not occur with muscimol, but later learning was facilitated (i.e., savings occurred) in rabbits initially given muscimol plus training. These results define the time period wherein amygdalar processes initiate TIA in the MG nucleus and suggest that distinct forms of amygdalar processes induce TIA in the MG nucleus and cingulate cortex.

Ultrastructure of ascending cholinergic terminals in the anteroventral thalamic nucleus of the rat: a comparison with the mammillothalamic terminals

In this study, to identify the ultrastructure and distribution of ascending cholinergic afferent terminals in the anteroventral thalamic nucleus, we used an anti-vesicular acetylcholine transporter antibody as marker of cholinergic afferents, and characterized the immunoreactive terminals at the ultrastructural level. We then compared the distribution pattern of the cholinergic terminals and that of the mammillothalamic terminals identified by anterograde transport of a tracer injected into the mammillary body. The cholinergic terminals were small, and formed both symmetrical and asymmetrical synaptic contacts throughout the dendritic arborizations, particularly in the distal region. This distribution pattern differed from that of mammillothalamic terminals, that were of LR (large terminal containing round synaptic vesicles) type and were preferentially distributed in the proximal region of dendrites. We also found relatively numerous cholinergic terminals making contact directly with immunonegative excitatory terminals, both LR and SR (small terminal containing round vesicles) terminals, without clear postsynaptic specialization. A few cholinergic terminals even seemed to form a synaptic complex with the LR or SR terminals. These findings suggest that the ascending cholinergic afferents in the anteroventral thalamic nucleus can effectively modulate excitatory inputs from both the mammillothalamic and corticothalamic terminals, in close vicinity to a synaptic site.

The role of muscarinic cholinoceptors in the retrieval of an operant food-related conditioned reflex in cats

Experiments on cats showed that lesions affecting retrieval of an operant food-procuring reflex. occurring on a background of systemic administration of the centrally-acting muscarinic cholinoceptor blocker scopolamine (a non-selective M-cholinoceptor blocker) and trihexyphenidyl (a relatively selective M1-cholinoceptor blocker), might be associated with the central and peripheral side effects of these blockers, preventing performance of the conditioned reflex. It was established that when no side effects were present (low doses of trihexyphenidyl, 1 mg/kg), blockade of M1-cholinoceptors led to selective loss of the motor operant reflex while contextual behavior and other conditioned responses were retained or led to errors in performance of the reflex: this appears to be evidence that derangement of launching and performing the motor program is the most important component of the conditioned reflex. Systemic administration of trihexyphenidyl at a dose of 10 mg/kg, scopolamine at doses of 0.03 and 0.06 mg/kg, and the peripherally-acting non-selective blocker methylscopolamine at a dose of 0.03 mg/kg led to changes in the general functional state (disturbances in the emotional-motivational sphere), the extent of which depended on the individual sensitivity of the animal to the anticholinergic agents. The presence of side effects led to complete cessation of conditioned reflex activity, though this appeared not to be associated with memory impairment.

Medial septal microinfusion of scopolamine disrupts hippocampal activity and trace jaw movement conditioning

This study investigated the effects of microinfusion of scopolamine into the medial septum (MS Scp) on hippocampal neurophysiology and learning of the rabbit's classically conditioned jaw movement response. The percentage of hippocampal theta slow waves (2-8 Hz) decreased after drug infusion in the MS Scp group but did not change in control groups that received infusion of saline into the MS or scopolamine into the cortex. Unit recordings from the MS Scp group showed significantly smaller conditioning-related hippocampal neural responses than seen in controls, and during conditioning, rabbits in the MS Scp group took significantly longer to reach learning criterion than either control group. Thus, the neural and behavioral impairments previously reported for systemic muscarinic blockade were reproduced by microinfusions restricted to the medial septal nucleus.

The effects of scopolamine on changes in regional cerebral blood flow during classical conditioning of the human eyeblink response

We examined the effects of scopolamine on the functional anatomy of classical conditioning of the human eyeblink response. Ten healthy young normal female volunteers (mean age +/- SEM: 26.7 +/- 0.9 years) were administered 0.4 mg scopolamine intravenously 1 h before regional cerebral blood flow (rCBF) was measured with positron emission tomography (PET) and H215O. Scans occurred during three sequential phases: (1) explicitly unpaired presentations of the unconditioned stimulus (airpuff to the right eye) and conditioned stimulus (binaural tone), (2) paired presentations of the two stimuli (associative learning) and (3) explicitly unpaired presentation of the stimuli (extinction phase). Scopolamine impaired acquisition of the conditioned eyeblink response (54.7 +/- 4.9%) relative to 18 untreated subjects from two previous PET studies. Regions that showed significant relative increases in rCBF during conditioning included the right lateral occipital cortex, the right inferior occipital cortex, the right lateral temporo-occipital cortex, the left medial temporo-occipital cortex, the posterior cingulate, the right cerebellum/brain stem area and the medial cerebellum. Significant relative decreases in rCBF were measured in the thalamus, the left putamen/insula area, the right putamen and the left and middle cerebellar cortex. The data partially replicate previous findings in unmedicated young volunteers of conditioning-specific rCBF changes in the cingulate cortex, the cerebellar cortex, the insula and the lateral temporo-occipital cortex. Our finding of decreased rCBF in the thalamus and increased rCBF in the occipital cortex may be attributable to effects of scopolamine per se rather than conditioning. Our data lend further support to the notion that classical conditioning involves distributed changes in multiple systems within the central nervous system.

Training-stage related neuronal plasticity in limbic thalamus and cingulate cortex during learning: a possible key to mnemonic retrieval

This study is part of an ongoing project concerned with the analysis of the neural substrates of discriminative avoidance learning in rabbits. Multi-unit activity was recorded in 5 anterior and lateral thalamic nuclei and in 4 layers of 2 posterior cingulate cortical areas (29c/d and 29b) during learning. The rabbits learned to step in response to a warning tone to avoid a foot-shock, and to ignore a different tone not followed by shock. Excitatory training-induced unit activity (TIA, increased tone-elicited activity during training relative to a pretraining session with unpaired tone-shock presentations) and/or discriminative TIA (greater discharges to the warning than to the safe tone) developed during training in 11 of the 13 areas. Discriminative TIA in the thalamic nuclei increased monotonically as learning occurred. Anterodorsal (AD) thalamic excitatory TIA peaked in an early stage (the first session of training), laterodorsal thalamic and parvocellular anteroventral (AVp) excitatory TIA peaked in an intermediate stage (the session of the first behavioral discrimination), and magnocellular anteroventral (AVm) and anteromedial (AM) thalamic excitatory TIA peaked in a late stage (the session in which asymptotic behavioral discrimination first occurred). The excitatory TIA in these nuclei declined as training continued beyond the stage in which the peak occurred. Peaks of excitatory TIA developed in area 29c/d of posterior cingulate cortex in the early (layer IV), intermediate (layers I-III and V) and late (layer IV) training stages, as just defined. Only layer IV in area 29b of posterior cingulate cortex exhibited a peak of excitatory TIA, which occurred in the early and intermediate training stages. As in limbic thalamus, discriminative TIA increased monotonically over training stages in layers V and VI of areas 29c/d and in layer VI of area 29b. However, layers I-III and IV in area 29c exhibited peak discriminative TIA in the intermediate and late training stages, respectively. Lesion studies indicate that limbic thalamus and cingulate cortex are essential for learning. The peaks represent a unique topographic pattern of thalamic and cortical excitation elicited by the CS+. It is proposed that the peaks constitute a retrieval pattern, i.e. a unique topographic array of excitation. This pattern encodes the spatio-temporal context which defines the learning situation and is necessary for recall and output of the learned response.