Classical conditioning of activities of salivary neurones in the cockroach

Abstract

SUMMARY Secretion of saliva to aid swallowing and digestion is a basic physiological function found in many vertebrates and invertebrates. For mammals, classical conditioning of salivation in dogs was reported by Pavlov a century ago. However, conditioning of salivation or of related neural activities in non-mammalian species has not been reported. In many species of insects, salivation is regulated by salivary neurones. In this study, we found that salivary neurones of the cockroach Periplaneta americanaexhibited a strong response to sucrose solution applied to the mouth and a weak response to odours applied to an antenna, and we studied the effect of conditioning on the activities of salivary neurones. After three sets of differential conditioning trials in which an odour was presented just before the presentation of sucrose solution and the other odour was presented alone,the response of salivary neurones to sucrose-associated odour significantly increased but that to the odour presented alone was unchanged. Backward pairing trials in which an odour was presented after the presentation of sucrose solution were not effective in achieving conditioning. Our study of the change in the level of saliva secretion in response to electrical stimulation of salivary neurones suggested that the magnitude of increase in odour response of salivary neurones by conditioning is sufficient to lead to an increased level of salivation. This study suggests classical conditioning of salivation in an insect.

Participation of NO signaling in formation of long-term memory in salivary conditioning of the cockroach

“…Drinking water and sugar-free yeast extract were provided ad libitum. The preparation for extracellular recordings from salivary neurons was described previously [10,27]. In short, individual animals were immobilized ventral side up on a paraffin plate.…”

“…The number of spikes was counted using LabVIEW software (National Instruments, Tokyo). For the study of effects of drugs, animals were injected with 3 l of ５ cockroach saline [27] containing drugs into the hemolymph of the head. All drugs used in this study were purchased from Sigma.…”

“…Salivary neurons exhibit spontaneous spike activities [10,[27][28][29][30]. Responses of salivary neurons to peppermint or vanilla odor (R) were measured as relative increase of the spike frequency from the spontaneous level, i.e., (Rs-Ro)/Ro, where Rs or Ro is the summed spike frequency of two salivary neurons during a 2-s period of odor stimulation or that during a 2-s period before the onset of odor stimulation, respectively [10].…”

“…We have reported that cockroaches, as do dogs [24] and humans [4], exhibit salivary conditioning, namely, they exhibit an increased level of salivation in response to an odor paired with sucrose reward [27,28]. This conditioning can be monitored by changes in ４ activities of salivary neurons of the subesophageal ganglion, the axons of which run along the salivary duct and innervate salivary glands [27,29].…”

“…This conditioning can be monitored by changes in ４ activities of salivary neurons of the subesophageal ganglion, the axons of which run along the salivary duct and innervate salivary glands [27,29]. The activity of salivary neurons can be monitored in rigidly immobilized animals and thus this conditioning system is suitable for the study of brain mechanisms of learning and memory.…”