Motivational Systems in Muroid Rodents

The similarity of motivating stimuli of defense among various species of muroid rodents has not usually been recognized, because there are important aspects of learning and ontogenetic experience which determine which motivating stimuli are effective in each individual animal [Adams, 1979b]. Moving visual stimuli are not effective motivating stimuli for defense in laboratory animals which have been raised in small cages without a chance to escape, but they are effective if such animals are raised with opportunity to escape into a "safe chamber" [Clark and Galef, 1977]. Similarly, handling, which involves dorsal tactile stimulation and/or restraint produces defense in wild muroid rodents but may be habituated in laboratory animals or if wild rodents are captured and handled when young [Stone, 1932; Boice, 1971; Galef, 1970b].

Pain is a powerful motivating stimulus of defense. It facilitates practically every motor pattern of defense. In a large number of studies on laboratory strains of muroid rodents, it has been used routinely to facilitate flight locomotion (escape testing procedures), freezing (unconditioned-stimulus for the so-called "conditioned emotional response"), and fighting (shock-induced upright posture, boxing, biting). Pain may also produce tail-rattling in M musculus [St. John, 1973], foot-thumping in Ne albigula and floridans [Logan and Boice, 1969] and Mer unguiculatus [Routtenberg and Dramis, 1967], hissing and squealing in R norvegicus [Berg and Baeninger, 1973], secretion of defense pheromones in R norvegicus (Valenta and Rigby, 1968] and M musculus [Carr et al, 1970], and secretion of corticosteroids by the adrenal cortex in R norvegicus [Freidman et al, 1967] and M musculus [Hennessy et al, 1977]. Previously neutral stimuli may be conditioned by pairing with pain, in which case they become conditional stimuli which can elicit defense when presented in isolation. This is routinely used in psychology experiments in which the paradigms of conditioned escape or avoidance and conditioned emotional reaction produce the motor patterns of flight or freezing. Upright posture and boxing has also been conditioned to previously neutral auditory stimuli [Creer et al, 1966; Vernon and Ulrich, 1966].

Handling of muroid rodents activates defense. Galef [1970b] compared responses of wild and laboratory R norvegicus to handling and obtained motor patterns of escape, squealing, biting, urination, and defecation, especially in the wild rats. Similar results were obtained from wild and laboratory M musculus by Connor [1975], including motor patterns of jumping, squealing, and biting. In various species of Ne, handling may produce flight, squealing, biting, urination, and defecation [Rainey, 1956; Linsdale and Tevis, 1951, p 233]. Handling is the classical method for producing tonic immobility in many kinds of vertebrates, including species of Perornyscus [Vestal, 1975]. It also facilitates the pituitary-adrenal secretion of ACTHand corticosteroid hormones [Friedman et al, 1967].

Handling involves many different types of sensory stimuli, including dorsal tactile stimulation, restraint, and vestibular stimulation. The precise contribution of each type of stimulation to the elicitation of defense has not been determined. We have shown, from an analysis of behavioral sequences during intermale social behavior in R norvegicus, that dorsal tactile stimulation by itself is a motivating stimulus for defense. It elicits the responses of defensive sideways posture, defensive upright posture, crouch, and full submissive posture with the intensity of the response being proportional to the intensity of the stimulus [Lehman and Adams, 1977]. The sensory analyzers of dorsal tactile stimulation are so sensitive that even air movement over the back may elicit squealing, jumping, and defensive upright posture in rats with forebrain lesions [Maire and Patton, 1954; Turner, 1970], while air puffs delivered to the side of the animal are ineffective [Turner, 1970].

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