Research reportAMPA receptor mediated behavioral plasticity in the isolated rat spinal cord
Highlights
▸ We further examine the role of the glutamatergic system on instrumental learning. ▸ The role of the AMPA Receptor on instrumental learning is highlighted. ▸ Extends our hypothesis that the plasticity of the spinal cord can be saturated.
Introduction
Neurons within the spinal cord are plastic and can support a range of behavioral phenomena [1]. Using traditional learning tasks, the isolated spinal cord has been found to support single stimulus learning [2], Pavlovian conditioning [3], [4], and instrumental learning [5]. Our laboratory has focused on the last form of learning, in part because instrumental learning contributes to the recovery of function after spinal cord injury (SCI) [6], [7].
Evidence that spinal neurons are sensitive to instrumental (response–outcome) relations has been obtained using animal subjects that have undergone a complete transection of the thoracic spinal cord, which blocks all communication between the brain and neurons below the injury. In a typical experiment [5], [8], a response–outcome relationship is instituted by administering shock to the tibialis anterior muscle of one hind leg whenever that leg is extended (controllable shock). Over time, subjects in this condition exhibit a progressive increase in flexion duration that minimizes net shock exposure. Other subjects receive shock at the same time and for the same duration, but independent of leg position (uncontrollable shock). Uncontrollably shocked subjects do not exhibit an increase in flexion duration and later fail to learn when given controllable shock to either the pretreated (ipsilateral) or opposite (contralateral) leg. Further work has shown that just 6 min of intermittent, uncontrollable, shock to the leg or tail impairs learning for up to 48 h [9].
Both the acquisition of spinal instrumental learning and the induction of the learning deficit depend on a glutamatergic signal [10], [11]. The N-methyl-d-aspartic acid receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) are part of the same family of ionotropic glutamate receptors [12]. Engaging the AMPAR, through the binding of glutamate, results in rapid depolarization of the cell and delayed activation of the NMDAR [13]. Activation of the NMDAR allows Ca2+ ions to flow freely into the cell [14], [15]. A strong Ca2+ influx initiates intracellular mechanisms that modify synaptic communication, altering components thought to contribute to learning and memory [16], [17], including the open probability of NMDARs, activating AMPARs and AMPAR trafficking at the synaptic cleft [12], [18]. Prolonged high frequency stimulation has been linked to an NMDAR-dependent enhancement of synaptic function (long-term potentiation [LTP]) mediated by an up-regulation of AMPARs [12], [16]. Conversely, stimulation parameters that lead to an overall reduction in synaptic efficacy (long-term depression [LTD]) produce a reduction of AMPAR function [12].
Enumerable studies have examined the role of the NMDAR in behavioral plasticity, both within the brain and spinal cord [15], [16], [19]. In contrast, while much is known about the AMPAR at the cellular level [20], [21], [42], relatively few studies have examined how pharmacological manipulations that target this receptor impact behavior (excluding studies designed to examine AMPA-mediated neurotoxic effects). At the level of the spinal cord, the limited work that has been done indicates that manipulations that impact AMPAR function affect nociceptive reactivity [22], [23]. Also as found within the brain, chronic spinal administration of AMPAR agonists can induce a lasting effect (excitotoxicity) that results in tissue damage and a loss of plasticity [46].
In prior studies, we have assumed that alterations in AMPAR function play a pivotal role in both instrumental learning and the induction of the learning deficit [8], [24], [25], but this has not been tested. Instead, we have assumed that evidence that the NMDAR plays an essential role implicates, through association, the AMPAR. From this perspective, instrumental learning reflects an NMDAR/AMPAR-dependent alteration in a spinal circuit that increases flexion duration. The learning deficit was then explained by positing uncontrollable stimulation saturates glutamate-dependent plasticity [24], [26]. While this reasoning seems sound given the standard view, derived from studies of hippocampal LTP, NMDAR's within the spinal cord may act in a distinct manner. For example, it is generally assumed that NMDAR antagonists affect the induction, but not the maintenance, of amygdala-dependent plasticity [27]. However, administration of an NMDAR antagonist disrupts both the induction and maintenance of instrumental learning and C-fiber dependent windup [25], [28]. The latter suggests that both effects may depend on slow NMDAR-mediated synaptic potentials and the sensitization of the NMDAR [28]. The presumption AMPA-signaling plays a pivotal role is also called into question by studies linking spinal LTP to C-fiber activity and the neurokinin (substance P) receptor [29], [30]. A final complexity stems from the observation that approximately a third of the NMDARs within the spinal cord are presynaptic, which suggests that the receptor may also regulate transmitter release [31].
Thus, while our theorizing has assumed that the AMPAR plays a central role in spinal plasticity, we lack any direct evidence that the AMPAR is involved and current data implicate other neural signals. We address this issue by testing the impact of the AMPAR antagonist CNQX on instrumental learning (Experiments 1 and 2) and the induction of the learning deficit (Experiment 3). We also examine whether pretreatment with the agonist AMPA has an acute (Experiment 4), and/or long-term (Experiment 5), effect on instrumental learning. We hypothesize that the AMPAR signal is a necessary component of spinal instrumental learning.
Section snippets
Animals
All subjects, male Sprague-Dawley rats (100–120 days old; 300–450 g), were obtained from Harlan Laboratories (Houston, TX). Subjects were individually housed with water and food ad libitum, and maintained on a 12-h light–dark cycle. Behavioral testing and surgeries were performed during the light portion of the cycle. All experiments were carried out in accordance with the NIH standards for the care and use of laboratory animals (NIH publications No. 80-23), and were approved by the University
Experiment 1: CNQX disrupts acquisition of instrumental learning
Experiment 1 examined whether 40–80 nmol of CNQX affects instrumental learning. If AMPAR activation is necessary for instrumental learning, then blocking the action of the AMPAR with an antagonist (CNQX) should inhibit instrumental learning.
Spinally transected rats (N = 18, 6 per group) were placed in the instrumental learning apparatus and the exterior portion of the catheter was threaded through a hole in the tube to administer the drug. Subjects received CNQX (40 nmol or 80 nmol) or its vehicle
General discussion
The present experiments confirmed that the AMPAR is involved in spinal learning. The AMPAR antagonist CNQX produced a dose-dependent disruption in instrumental learning (Experiment 1). CNQX also disrupted the maintenance of the instrumental response (Experiment 2) and the induction of the learning deficit (Experiment 3). Administration of the agonist AMPA impaired learning when subjects were tested soon after drug treatment (Experiment 4), but did not affect learning when subjects were tested 24
Conclusions
The present experiments demonstrate that manipulations that impact AMPAR-mediated communication within the spinal cord can affect both instrumental learning and the long-term consequences of uncontrollable stimulation, which may be characterized as adaptive and maladaptive plasticity, respectively. The demonstrated role in learning suggests glutamatergic systems play a pivotal role in the abstraction of instrumental relations and/or the maintenance of instrumental behavior (memory). The fact
Acknowledgements
This study was supported by NS041548 & HD058412. A portion of the data from this study has been previously presented in abstract form.
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