Motivationally neutral stimulation of the nucleus basalis induces specific behavioral memory

Abstract

The cholinergic system has been implicated in learning and memory. The nucleus basalis (NB) provides acetylcholine (ACh) to the cerebral cortex. Pairing a tone with NB stimulation (NBstm) to alter cortical state induces both associative specific tuning plasticity in the primary auditory cortex (A1) and associative specific auditory behavioral memory. NB-induced memory has major features of natural memory that is induced by pairing a tone with motivational reinforcers, e.g., food or shock, suggesting that the cholinergic system may be a “final common pathway” whose activation promotes memory storage. Alternatively, NB stimulation might itself be motivationally significant, either rewarding or punishing. To investigate these alternatives, adult male rats (n = 7) first formed a specific NB-induced memory (CS = 8.0 kHz, 2.0 s paired with NBstm, ISI = 1.8 s, 200 trials), validated by post-training (24 h) frequency generalization gradients (1–15 kHz) of respiration interruption that were specific to the CS frequency. Thereafter, they received the same level of NBstm that had induced memory, while confined to one quadrant of an arena, and later tested for place-preference, i.e., avoidance or seeking of the quadrant of NBstm. This NBstm group exhibited neither preference for nor against the stimulated quadrant, compared to sham-operated subjects (n = 7). The findings indicate that specific associative memory can be induced by direct activation of the NB without detectable motivational effects of NB stimulation. These results are concordant with a memory-promoting role for the nucleus basalis that places it “downstream” of motivational systems, which activate it to initiate the storage of the current state of its cholinergic targets.

Introduction

The cholinergic system has long been implicated in learning and memory. For example, pharmacological blockade of the cholinergic system impairs many forms of memory (Deutsch, 1971, Flood et al., 1981, Power et al., 2003, Rudy, 1996). Cholinergic agonists and cholinesterase antagonists can facilitate memory (Introini-Collison and McGaugh, 1988, Stratton and Petrinovich, 1963), promote recovery of memory from brain damage (Russell, Escobar, Booth, & Bermudez-Rattoni, 1994) and achieve rescue from memory deficits in transgenic mice (Fisher, Brandeis, Chapman, Pittel, & Michaelson, 1998). Also, several non-cholinergic treatments that facilitate memory, such as adrenergic agents and stress hormones, affect memory via actions on the cholinergic system (Salinas, Introini-Collison, Dalmaz, & McGaugh, 1997).

The nucleus basalis of the basal forebrain (NB) is the major source of extrinsic acetylcholine (ACh) to the cerebral cortex (Bigl et al., 1982, Johnston et al., 1979, Luiten et al., 1987, Mesulam et al., 1983, Rye et al., 1984). Activation of the NB to release ACh in the cerebral cortex has a profound effect on cortical state. It shifts the spectrum of the electroencephalogram (EEG) from higher voltage slower waves, indicative of drowsiness or sleep, to lower voltage fast waves (cortical “desynchronization” or “activation”) that are characteristic of an alert waking state and also rapid-eye-movement (REM) sleep. Both states are thought to promote information processing in the cerebral cortex. Cortical activation depends upon the release of ACh (Casamenti et al., 1986, Celesia and Jasper, 1966, Detari et al., 1983, Detari et al., 1984, Detari et al., 1987, Jimenez-Capdeville et al., 1997, Juhasz et al., 1985, Kukorelli et al., 1986, Rasmusson et al., 1992, Rasmusson et al., 1994, Rasmusson et al., 1996, Szymusiak and McGinty, 1986) that engages cortical muscarinic receptors (Phillis and York, 1968, Szerb, 1964).

In addition to its role in activating the cortex, the NB can induce cortical plasticity. For example, stimulation of the nucleus basalis (NBstm) paired with sensory stimulation produces enduring facilitation of responses in the somatosensory cortex (Tremblay, Warren, & Dykes, 1990), atropine-sensitive persistent modification of evoked responses (Metherate & Ashe, 1992, Metherate and Ashe, 1993) and facilitation of responses to tone (Edeline et al., 1994, Edeline et al., 1994) in the primary auditory cortex (A1). Pairing a tone with NBstm produces associative, frequency-specific shifts of neuronal tuning (Bakin and Weinberger, 1996, Bjordahl et al., 1998, Dimyan and Weinberger, 1999, Ma and Suga, 2003) and enlargement of auditory cortical representation of the paired tone frequency (Kilgard and Merzenich, 1998a, Kilgard and Merzenich, 1998b, Kilgard et al., 2002). NB-induced associative receptive field tuning shifts are dependent upon the engagement of muscarinic receptors in the auditory cortex (Miasnikov, McLin, & Weinberger, 2001).

The nucleus basalis not only induces specific cortical plasticity, but it also induces behavioral memory. (We use the term “behavioral memory” to distinguish actual memory from learning-related neural plasticity, which unfortunately is often called “memory”.) Thus, pairing a tone with NBstm produces conditioned stimulus (CS)-specific behavioral memory as indexed by training (i.e., pairing) with one frequency but later testing with many frequencies to obtain behavioral generalization gradients. (Preferential responses to the CS frequency would indicate that subjects learned about the specific frequency, while responses to most frequencies would indicate that learning was not specific to the CS frequency but rather learned about tones in general (Mackintosh, 1974, Mostofsky, 1965, Pavlov, 1927).)

We found previously that pairing a tone with stimulation of the nucleus basalis induces memory that is both associative and contains detail about the absolute frequency of the conditioned stimulus. Rats that received extensive pairing of a single tone with NBstm (3000 trials over 15 days) later exhibited behavioral frequency response profiles (for both the interruption of ongoing respiration and changes in heart rate) in the absence of NBstm that were maximal at the CS frequency. In contrast, rats receiving unpaired stimulation failed to develop such behavioral CS-specificity (McLin et al., 2002a, McLin et al., 2003). Thereafter, we found that such extensive training is not necessary. Rather, specific associative memory can be induced rapidly, with a single training session of 200 trials (Miasnikov, Chen, & Weinberger, 2006). Additionally, the NB can control the amount of detail that is learned. Thus, pairing an 8.0 kHz tone with weak (∼45 μA) stimulation of the NB that produces minimal EEG activation induces associative memory that is equal across the frequency spectrum (i.e., a “flat” generalization gradient). In contrast, moderate stimulation (∼65 μA), that causes stronger EEG activation, induces associative auditory memory that is specific to the frequency band of the conditioned stimulus (Weinberger, Miasnikov, & Chen, 2006).

Despite the extensive documentation of the involvement of the cholinergic system in learning and memory, and of the role of the nucleus basalis in modulating cortical state, mediating specific cortical plasticity and inducing behavioral memory, the actual role of the nucleus basalis is unknown. Two major functional explanations are readily apparent. First engagement of the NB/ACh system could itself serve as a positive or negative reinforcer, i.e., be rewarding or punishing. Second, the NB/ACh system could be “downstream” of motivational systems but be engaged by them (both positive and negative in valence) to promote the storage of the information currently being processed, perhaps throughout the cortex and at any other cholinergic targets. To investigate these two alternatives, we used a place-preference test in an arena divided into four quadrants (Bardo et al., 1995, Hasenohrl et al., 1989; see also Panos et al., 1999, Sahraei et al., 2004). We first induced specific auditory memory in rats and later confined them to a quadrant of an arena while they received the same NBstm that had been used to induce memory. Subsequently they were allowed to move freely at which time we determined whether or not they exhibited a preference either for or against the stimulated location.