An economic approach to animal models of alcoholism

Researchers have long sought an animal model for human alcohol consumption. This article describes an economic-based approach to a model of alcohol preference in rats. The procedures are based on an analogy between clinical accounts of human drinking and the economic analysis of consumption. Both clinical and economic investigators typically define consumption patterns in terms of the influence of negative consequences. For example, the clinical account emphasizes the persistence of heavy drinking despite mounting alcohol-related aversive consequences, and in economic analyses, the term "inelastic demand" is used to refer to the persistence of consumption despite large increases in prices. In the experimental procedure described here, rats worked for alcohol and food. Presses on one lever earned a drink of 10 percent alcohol plus saccharin, and presses on a second lever earned isocaloric drinks of a starch solution. After behavior stabilized, the response requirements (which are analogous to prices) for one or both drinks were increased. The rats maintained baseline alcohol consumption levels despite large increases in the "price" of alcohol. In contrast, the same price increases markedly reduced starch intake. That is, food consumption was sensitive to price hikes, but alcohol consumption was not. The results demonstrate that a common economic framework can be used to describe human and animal behavior and, hence, the possibility of an animal model of human alcohol consumption. The article also points out that economic concepts provide a framework for understanding a wide range of human drinking patterns, including controlled social drinking and excessive alcoholic drinking.

In vitro analog of operant conditioning in aplysia. I. Contingent reinforcement modifies the functional dynamics of an identified neuron

Previously, an analog of operant conditioning in Aplysia was developed using the rhythmic motor activity in the isolated buccal ganglia. This analog expressed a key feature of operant conditioning, namely a selective enhancement in the occurrence of a designated motor pattern by contingent reinforcement. Different motor patterns generated by the buccal central pattern generator were induced by monotonic stimulation of a peripheral nerve (i.e., n.2,3). Phasic stimulation of the esophageal nerve (E n.) was used as an analog of reinforcement. The present study investigated the neuronal mechanisms associated with the genesis of different motor patterns and their modifications by contingent reinforcement. The genesis of different motor patterns was related to changes in the functional states of the pre-motor neuron B51. During rhythmic activity, B51 dynamically switched between inactive and active states. Bursting activity in B51 was associated with, and predicted, characteristic features of a specific motor pattern (i.e., pattern I). Contingent reinforcement of pattern I modified the dynamical properties of B51 by decreasing its resting conductance and threshold for eliciting plateau potentials and thus increased the occurrences of pattern I-related activity in B51. These modifications were not observed in preparations that received either noncontingent reinforcement (i.e., yoke control) or no reinforcement (i.e., control). These results suggest that a contingent reinforcement paradigm can regulate the dynamics of neuronal activity that is centrally programmed by the intrinsic cellular properties of neurons.

In vitro analog of operant conditioning in aplysia. II. Modifications of the functional dynamics of an identified neuron contribute to motor pattern selection

Previously, an analog of operant conditioning was developed using the buccal ganglia of Aplysia, the probabilistic occurrences of a specific motor pattern (i.e., pattern I), a contingent reinforcement (i.e., stimulation of the esophageal nerve), and monotonic stimulation of a peripheral nerve (i.e., n.2,3). This analog expressed a key feature of operant conditioning (i.e., selective enhancement of the probability of occurrence of a designated motor pattern by contingent reinforcement). In addition, the training induced changes in the dynamical properties of neuron B51, an element of the buccal central pattern generator. To gain insights into the neuronal mechanisms that mediate features of operant conditioning, the present study identified a neuronal element that was critically involved in the selective enhancement of pattern I. We found that bursting activity in cell B51 contributed significantly to the expression of pattern I and that changes in the dynamical properties of this cell were associated with the selective enhancement of pattern I. These changes could be induced by an explicit association of reinforcement with random depolarization of B51. No stimulation of n.2,3 was required. These results indicate that the selection of a designated motor pattern by contingent reinforcement and the underlying neuronal plasticity resulted from the association of reinforcement with a component of central neuronal activity that contributes to a specific motor pattern. The sensory stimulus that allows for occurrences of different motor acts may not be critical for induction of plasticity that mediates the selection of a motor output by contingent reinforcement in operant conditioning.

Neural changes after operant conditioning of the aerial respiratory behavior in Lymnaea stagnalis

In this study, we demonstrate neural changes that occurred during operant conditioning of the aerial respiratory behavior of Lymnaea stagnalis. Aerial respiration in Lymnaea occurs at the water interface and is achieved by opening and closing movements of its respiratory orifice, the pneumostome. This behavior is controlled by a central pattern generator (CPG), the neurons of which, as well as the motoneurons innervating the pneumostome, have previously been identified and their synaptic connections well characterized. The respiratory behavior was operantly conditioned by applying a mechanical stimulus to the open pneumostome whenever the animal attempted to breathe. This negative reinforcement to the open pneumostome resulted in its immediate closure and a significant reduction in the overall respiratory activity. Electrophysiological recordings from the isolated CNSs after operant conditioning showed that the spontaneous patterned respiratory activity of the CPG neurons was significantly reduced. This included reduced spontaneous activity of the CPG interneuron involved in pneumostome opening (input 3 interneuron) and a reduced frequency of spontaneous tonic activity of the CPG interneuron [right pedal dorsal 1 (RPeD1)]. The ability to trigger the patterned respiratory activity by electrical stimulation of RPeD1 was also significantly reduced after operant conditioning. This study therefore demonstrates significant changes within a CPG that are associated with changes in a rhythmic homeostatic behavior after operant conditioning.

Animal models of craving: a roundtable discussion

Five experts respected for their work in the development of animal models of alcohol craving offer their perspectives in a roundtable discussion format. The panel members discuss the various definitions and theories of alcohol craving and the benefits and limitations of using animal models to study alcohol craving in humans. Animal models have helped further the understanding of craving by providing information about behavior associated with craving. Animal models do have limitations, however. The fact that animals cannot "talk" about their feelings poses difficulties for researchers seeking to map an animal analog of craving onto the human experience.

Contingent-dependent enhancement of rhythmic motor patterns: an in vitro analog of operant conditioning

Operant conditioning is characterized by the contingent reinforcement of a designated behavior. Previously, feeding behavior in Aplysia has been demonstrated to be modified by operant conditioning, and a neural pathway (esophageal nerve; E n.) that mediates some aspects of reinforcement has been identified. As a first step toward a cellular analysis of operant conditioning, we developed an in vitro buccal ganglia preparation that expressed the essential features of operant conditioning. Motor patterns that represented at least two different aspects of fictive feeding (i.e., ingestion-like and rejection-like motor patterns) were elicited by tonic stimulation of a peripheral buccal nerve (n.2,3). Three groups of preparations were examined. In a contingent-reinforcement group, stimulation of E n. was contingent on the expression of a specific type of motor pattern (i.e., either ingestion-like or rejection-like). In a yoke-control group, stimulation of E n. was not contingent on any specific pattern. In a control group, E n. was not stimulated. The frequency of the reinforced pattern increased significantly only in the contingent-reinforcement group. No changes were observed in nonreinforced patterns or in the motor patterns of the control and yoke-control groups. Contingent reinforcement of the ingestion-like pattern was associated with an enhancement of activity in motor neuron B8, and this enhancement was specific to the reinforced pattern. These results suggest that the isolated buccal ganglia expressed an essential feature of operant conditioning (i.e., contingent reinforcement modified a designated operant) and that this analog of operant conditioning is accessible to cellular analysis.

Amygdalar lesions block discriminative avoidance learning and cingulothalamic training-induced neuronal plasticity in rabbits

Learning to fear dangerous situations requires the participation of neurons of the amygdala. Here it is shown that amygdalar neurons are also involved in learning to avoid dangerous situations. Amygdalar lesions severely impaired the acquisition of acoustically cued, discriminative instrumental avoidance behavior of rabbits. In addition, the development of anterior cingulate cortical and medial dorsal thalamic training-induced neuronal plasticity in the early stages of behavioral acquisition was blocked in rabbits with lesions. The development of training-induced neuronal plasticity in the medial dorsal and anterior thalamic nuclei in late stages of behavioral acquisition was also blocked in rabbits with lesions. These results indicate that the integrity of the amygdala is essential for the establishment of both early and late training-induced cingulothalamic neuronal plasticity. It is hypothesized that amygdalar training-induced neuronal plasticity in the initial trials of conditioning represents a substrate of learned fear, essential for the early and late cingulothalamic plasticity that is involved in mediation of acquisition of the instrumental avoidance response.