Mediation of passive avoidance learning by nicotinic hippocampo-entorhinal components in young rats

Nicotinic ACh receptors in the hippocampal circuit; functional expression and role in synaptic plasticity

Acetylcholine (ACh) can regulate neuronal excitability in the hippocampus, an important area in the brain for learning and memory, by acting on both nicotinic (nAChRs) and muscarinic ACh receptors. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca (MS-DBB), and we investigated how their activation regulated hippocampal synaptic plasticity. We found that activation of these endogenous cholinergic inputs can directly induce different forms of hippocampal synaptic plasticity with a timing precision in the millisecond range. Furthermore, we observed a prolonged enhancement of excitability both pre- and postsynaptically. Lastly we found that the presence of the α7 nAChR subtype to both pre- and postsynaptic sites appeared to be required to induce this plasticity. We propose that α7 nAChRs coordinate pre- and postsynaptic activities to induce glutamatergic synaptic plasticity, and thus provide a novel mechanism underlying physiological neuronal communication that could lead to timing-dependent synaptic plasticity in the hippocampus.

Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability throughout the nervous system by acting on both the cys-loop ligand-gated nicotinic ACh receptor channels (nAChRs) and the G protein-coupled muscarinic ACh receptors (mAChRs). The hippocampus is an important area in the brain for learning and memory, where both nAChRs and mAChRs are expressed. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca, the activation of which can activate both nAChRs and mAChRs in the hippocampus and regulate synaptic communication and induce oscillations that are thought to be important for cognitive function. Dysfunction in the hippocampal cholinergic system has been linked with cognitive deficits and a variety of neurological disorders and diseases, including Alzheimer's disease and schizophrenia. My lab has focused on the role of the nAChRs in regulating hippocampal function, from understanding the expression and functional properties of the various subtypes of nAChRs, and what role these receptors may be playing in regulating synaptic plasticity. Here, I will briefly review this work, and where we are going in our attempts to further understand the role of these receptors in learning and memory, as well as in disease and neuroprotection.

Nicotinic ACh receptors in the hippocampus: role in excitability and plasticity

INTRODUCTION

The nicotinic ACh receptors (nAChRs) are in the cys-loop family of ligand-gated ion channels. They are widely expressed throughout the brain, including in the hippocampus where they are thought to be involved in regulating excitability, plasticity, and cognitive function. In addition, dysfunction in hippocampal nAChRs has been linked to a variety of neurological disorders and diseases, including Alzheimer's disease, schizophrenia, and epilepsy. In order to understand how to treat nAChR-related disorders and diseases, it is critical to understand how these receptors participate in normal brain function; this entails not only understanding the biophysical properties of ion channel function and their pattern of expression but also how these receptors are regulating excitability and circuit behavior.

DISCUSSION

The primary cholinergic input to the hippocampus comes from the medial septum and diagonal band of Broca; however, the mechanistic details are unknown of how activation of cholinergic receptors, either through exogenous nAChR ligands or the activation of endogenous acetylcholine release, regulates hippocampal network activity. This entails direct study of the excitatory and inhibitory neuronal networks, as well as the role of nonneuronal cells, in regulating hippocampal function.

CONCLUSIONS

Here, I will review the latest work from my laboratory in which we have attempted to do just that, with the overall goal of learning more about the role of the hippocampal nAChR in synaptic plasticity.

Characterization of a nicotine-sensitive neuronal population in rat entorhinal cortex

The entorhinal cortex (EC) is a part of the hippocampal complex that is essential to learning and memory, and nicotine affects memory by activating nicotinic acetylcholine receptors (nAChRs) in the hippocampal complex. However, it is not clear what types of neurons in the EC are sensitive to nicotine and whether they play a role in nicotine-induced memory functions. Here, we have used voltage-sensitive dye imaging methods to locate the neuronal populations responsive to nicotine in entorhino-hippocampal slices and to clarify which nAChR subtypes are involved. In combination with patch-clamp methods, we found that a concentration of nicotine comparable to exposure during smoking depolarized neurons in layer VI of the EC (ECVI) by acting through the non-alpha7 subtype of nAChRs. Neurons in the subiculum (Sb; close to the deep EC layers) also contain nicotine-sensitive neurons, and it is known that Sb neurons project to the ECVI. When we recorded evoked EPSCs (eEPSCs) from ECVI neurons while stimulating the Sb near the CA1 region, a low dose of nicotine not only enhanced synaptic transmission (by increasing eEPSC amplitude) but also enhanced plasticity by converting tetanus stimulation-induced short-term potentiation to long-term potentiation; nicotine enhanced synaptic transmission and plasticity of ECVI synapses by acting on both the alpha7 and non-alpha7 subtypes of nAChRs. Our data suggest that ECVI neurons are important regulators of hippocampal function and plasticity during smoking.

Role of ventral hippocampus in acquisition, consolidation and retrieval of rat's passive avoidance response memory trace

By means of local administration of tetrodotoxin (TTX) a fully reversible functional inactivation of rat's ventral hippocampus (VH) was obtained in order to characterize the role of this structure in the memorization of a conditioned passive avoidance response (PAR). In Experiment 1, on permanently cannulated animals, TTX (10 ng in 1.0 microl saline) or saline (1.0 microl) was injected uni- or bilaterally in the VH, respectively, 1 h before PAR acquisition, immediately after PAR acquisition, and 1 h before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that both pre-acquisition and pre-retrieval VH uni- or bilateral blockades were followed by significant PAR retention impairment, while in post-acquisition only the bilateral blockade determined PAR retention impairment. In Experiment 2, on three different groups of rats, TTX (10 ng in 1 microl saline) was bilaterally administered, under general ketamine anesthesia (100 mg/kg b.w.), into the VH at different post-acquisition delays (0.25, 1.5, 6 h). Retrieval testing, 48 h after treatment, showed that post-acquisition bilateral VH blockade caused PAR impairment only when performed 0.25 h after acquisition. The results clearly indicate a role of VH during acquisition, consolidation and retrieval of PAR engram. The experimental evidence is discussed in comparison to previous results concerning TTX dorsal hippocampus blockade effects on rat's PAR and in relation to hippocampal connectivity with the medial septal area and the amygdala.

Age-dependent effects of hippocampal muscarinic receptor blockade on memory-based learning in the developing rat

The effects of ventral intrahippocampal injections of atropine sulfate on patterned single alternation (PSA), a discrimination task that requires intact short-to-intermediate-term memory, were examined in the developing rat at 16-17 and 28-32 days of age. Atropine treatment disrupted simple acquisition in some 16- to 17-day-old pups by interfering with approach to the goal, but did not eliminate PSA at either 8- or 15-s intertrial intervals when approach was normal. In the older rats, atropine treatment delayed the onset and reduced the magnitude of PSA, indicating a reduced memory-based discrimination. These results provide additional support for an increasing role of muscarinic receptors in learning and memory as this system matures in the developing rat, and suggest different mechanisms for PSA at the two ages.

Hippocampal nicotinic cholinergic mechanisms mediate spontaneous alternation and fear during ontogenesis but not later in the rat

Spontaneous alternation was examined in young rats following microinjections of antinicotinic agents into one of the 4 hippocampal sites: anterodorsal, or posteroventral dentate gyrus, hippocampal gyrus, or entorhinal cortex. In control and saline-injected animals, the alternation rate was shown to grow suddenly from 40 to 80% between days 15 and 17 (the adult level being 85-90%), to regress partly (to 55%) between days 20 and 30, and return to a near-adult level (75%) by day 40. Meanwhile fear responses to environment (defecation and vocalization) emerged between days 20 and 25, increased to a maximum until day 30, and returned to the typically low adult level by day 40. Injections of mecamylamine (5, 20 micrograms) or hexamethonium (5, 20 micrograms) into any of the 4 sites significantly reduced the rate of alternation from as early as day 10 on, but were no longer effective from day 30 on; on the other hand, they did not alter the level of defecation, but had a tendency to lower the level of vocalization on day 30 only. These results indicate that hippocampal nicotinic cholinergic mechanisms play a role in spontaneous alternation and appear to be involved in the control of one fear reaction (vocalization) until day 30.

Dopamine D-2 receptor mediation of response suppression learning of young rats

In three experiments, the effects of augmenting or blocking dopamine (DA) D-2 receptor activity on the ontogeny of response suppression learning of preweanling rat pups were determined. In the initial experiment, rat pups were trained to traverse a straight alley for nipple attachment to an anesthetized dam. When footshock (0.2 mA, 0.5 sec) was made contingent on responding, younger (11- and 13-day-olds) rat pups were deficient to older (17- and 19-day-olds) pups at withholding punished responding. In the subsequent experiments, response suppression learning was assessed after injecting 11- and 17-day-old rat pups with the specific DA D-2 agonist, LY 171555 (0.005-, 0.01-, and 0.1-mg/kg, i.p.), or the specific DA D-2 antagonist, sulpiride (5.0-, 15.0-, and 50.0-mg/kg, i.p.). LY 171555 enhanced the punished responding of both the 11- and 17-day-old rat pups; whereas, sulpiride increased the punished responding of the 17-, but not the 11-day-olds. In four additional experiments, the effects of LY 171555 and sulpiride on the locomotor activity, nociception, and reinforcement processes of 17-day-old rat pups was assessed. Rat pups given LY 171555 (0.01 mg/kg, i.p.) exhibited enhanced locomotor activity and a trend towards hyperanalgesia using a hot plate task. Sulpiride (15.0 mg/kg, i.p.) completely antagonized LY 171555's activity enhancing effects and had hyperalgesic properties. In two experiments, sulpiride did not affect the nonpunished appetitive responding of the 17-day-olds; whereas, haloperidol-treated pups responded on fewer reinforced trials than did saline-treated pups. Therefore, these results indicate that the response suppression learning of 17-day-old rat pups is mediated, at least partially, by a DAD-2 receptor system, and that D-2 receptors are also involved in the locomotor activity and nociceptive responses of young rat pups.

Enhancement of passive avoidance learning through small doses of intra-amygdaloid physostigmine in the young rat. Its relation to the development of acetylcholinesterase

Passive avoidance learning was studied in young rats 7-20 days old, in control conditions and after bilateral injections of physostigmine into the lateral amygdaloid nucleus. Acquisition in controls was possible from postnatal Day 8 on, progressed markedly after Day 11, and nearly reached maturity by Day 20. Physostigmine differentially altered acquisition depending on the dose: facilitation with low doses, no effect with moderate doses, and impairment with high doses. Enhanced learning through small doses of physostigmine was observed at all ages from Day 8 on, and was greater with 0.2 microgram than with 0.1 microgram. Maturation of the cholinergic innervation of the amygdaloid region was also studied between Days 9-20 using acetylcholine-esterase histochemistry. The results suggest that passive avoidance learning is dependent on amygdaloid cholinergic mechanisms early in life. In addition, very immature cholinergic systems, which are known to be uninfluenced by anticholinergic agents, react to anticholinesterases.

Development of amygdaloid cholinergic mediation of passive avoidance learning in the rat. II. Nicotinic mechanisms

Young rats 10-30 days of age received bilateral injections of antinicotinic and/or nicotinic agents into the lateral (L), the basolateral (BL), or the cortical (CO) amygdaloid nucleus, and were trained to learn a cool-draft stimulus passive-avoidance task, 17 min later. Mecamylamine produced age- and dose-dependent acquisition deficits; these deficits appeared on day 11, increased similarly after injections into any of the three nuclei until day 16, and decreased thereafter, more rapidly after administration into CO nucleus than after injections into L and BL nuclei. In the latter nucleus, the deficits had nearly disappeared on day 30. Nicotine injected alone induced slight facilitatory effects, significant at 20 days but not reliable at earlier stages. However, nicotine could hinder the antagonistic effects of mecamylamine, when given in combination, as early as the 11th day of age on. The results suggest the existence of nicotinic synaptic elements in the basal lateral part of the rat amygdala; however, these seem to play an important role in passive avoidance learning only during the early stages of ontogenesis.

Development of amygdaloid cholinergic mediation of passive avoidance learning in the rat. I. Muscarinic mechanisms

Passive avoidance learning was studied in young rats 13-30 days of age following bilateral injections of saline or antimuscarinic and/or muscarinic agents into three amygdaloid nuclei--lateral (L), basolateral (BL), and cortical (CO). While acquisition was not influenced by saline injections into various other cerebral structures, it was significantly altered by similar injections into these amygdaloid nuclei, especially by those into the BL nucleus, suggesting that this nucleus is particularly involved in passive avoidance learning. Atropine induced significant deficits from as early as 13 days on. These deficits increased and were of similar strength after injections into any of the three studied nuclei until day 16; after that age, they diminished slightly following CO and L nuclei administration, while remaining substantial after BL nucleus injections at all ages, even at 30 days. No facilitatory effects could be elicited by arecoline injected alone, while arecoline could antagonize the disturbing effect of atropine, when given in combination, from day 13 on. These results suggest a muscarinic cholinergic mediation of passive avoidance learning through the synaptic elements located in the basal lateral part of the amygdala in the young rat.

Hippocampal muscarinic cholinergic mediation of spontaneous alternation and fear in the developing rat

Spontaneous alternation was examined in young rats following microinjections of anticholinergic agents into 4 hippocampal sites: anterodorsal or posteroventral dentate gyrus, hippocampus or entorhinal complex. The rate of alternation remained around 40% at 5, 10, and 15 days, increased suddenly to 80% on day 17, did not vary until day 20, regressed partly and temporarily until day 30, and returned to a near-adult level on day 40. Concomitantly with the transient regression of alternation between days 20 and 40, fear responses to environment were seen to emerge (boluses and squeaks), to reach a maximum on day 30, and to return to a low level by day 40. Injections of atropine (4, 8 micrograms) or scopolamine (4, 10 micrograms) into any of the 4 sites significantly reduced the rate of alternation from day 17 on. Only the highest doses were active at 10 and 15 days. These results demonstrate that spontaneous alternation and hippocampal muscarinic cholinergic mechanisms develop simultaneously and progress suddenly on postnatal day 17. Atropine and scopolamine also affected fear responses, abolishing or potentiating them according to the site of injection, showing that hippocampal cholinergic mechanisms exert complex influences on fear-induced emotional reactions.