Dynamic state-dependent modelling predicts optimal usage patterns of responsive defences

Chemical defences against predation often involve responses to specific predation events where the prey expels fluids, such as haemolymph or gut contents, which are aversive to the predator. The common link is that each predation attempt that is averted results in an energetic cost and a reduction in the chemical defences of the prey, which might leave the prey vulnerable if the next predation attempt occurs soon afterwards. Since prey appear to be able to control the magnitude of their responses, we should expect them to trade-off the need to repel the current threat against the need to preserve defences against future threats and conserve energy for other essential activities. Here we use dynamic state-dependent models to predict optimal strategies of defence deployment in the juvenile stage of an animal that has to survive to maturation. We explore the importance of resource level, predator density, and the costs of making defences on the magnitude of the responses and optimal age and size at maturation. We predict the patterns of investment and the magnitude of the deployment of defences to potentially multiple attacks over the juvenile period, and show that responses should be smaller when the costs of defences and/or predation risk are higher. The model enables us to predict that animals in which defences benefit the adult stage will employ different strategies than those that do not use the same defences as adults, and thereby experience a smaller reduction in body size as a result of repeated attacks. We also explore the effect of the importance of adult size, and find that the sex and mating system of the prey should also affect defensive strategies. Our work provides the first predictive theory of the adaptive use of responsive defences across taxa.     

Characterisation of calcitonin gene-related peptide-immunoreactive neurons in the myenteric plexus of rat colon

A mechanical or chemical stimulus applied to the intestinal mucosa induces motility reflexes in the rat colon. Enteric neurons containing calcitonin gene-related peptide (CGRP) have been suggested as intrinsic primary afferent neurons responsible for mediating such reflexes. In the present study, immunohistochemistry was performed on whole-mount stretch preparations to investigate chemical profiles, morphological characteristics and projections of CGRP-containing neurons in the myenteric plexus of the rat colon. CGRP-positive neuronal cell bodies were detected in preparations incubated with colchicine-containing medium, whereas CGRP-positive nerve fibres were found in colchicine-untreated preparations. These neurons had large oval or round cell bodies that were also immunoreactive for the calcium-binding protein calretinin and neurofilament 200. Myenteric neurons positive for both calretinin and neurofilament 200 had several long processes that emerged from the cell body, consistent with Dogiel type II morphology. Application of the neural tracer DiI to the intestinal mucosa revealed that DiI-labelled myenteric neurons each had an oval or round cell body immunoreactive for calretinin. Thus, CGRP-containing myenteric neurons are Dogiel type II neurons and are immunoreactive for calretinin and neurofilament 200 in the rat colon. These neurons probably project to the intestinal mucosa. This study was supported by a Waseda University Grant for Special Research Projects (2008A-889).     

Reflex gastric motor inhibition caused by intraperitoneal bradykinin: Antagonism by Hoe 140, a bradykinin antagonist

Bradykinin (BK) has been reported to have mixed excitatory/inhibitory effects on gastrointestinal motility. The present study examined the mechanism responsible for the inhibition of gastric motor activity caused by intraperitoneal administration of BK. Gastric motor activity was measured by recording the intragastric pressure (IGP) of phenobarbital-anesthetized rats via a transesophageal catheter. To facilitate the study of inhibitory influences, gastric motility was stimulated by neurokinin A (NKA), which on intravenous injection evoked reproducible gastric contractions as measured by a rise of IGP. Intraperitoneal injection of BK (0.1–10 nmol) inhibited the NKA-induced increase in IGP in a dose-dependent manner, and the effect of epigastric administration of BK was not significantly different from that of intraperitoneal administration. The inhibitory effect of intraperitoneal BK on gastric motility was due to an effect on BK₂ receptors because it was blocked by prior intraperitoneal injection of the BK₂ antagonist Hoe 140. The specificity of this BK antagonist was demonstrated by its inability to antagonize the effect of intraperitoneal hydrochloric acid (HCl), which, like BK, inhibited the NKA-induced gastric contractions. Because the BK- and HCl-induced inhibition of the NKA-induced rise of IGP was abolished by acute removal of the celiac-superior mesenteric ganglion complex, but left unaltered by acute bilateral subdiaphragmatic vagotomy, it is inferred that intraperitoneal BK inhibits gastric motor activity via activation of an autonomic reflex that involves prevertebral ganglia.     

Prepulse inhibition of the startle reaction in the locust Locusta migratoria (Insecta: Orthoptera: Acridoidea)

A fast startle reaction of unrestrained sitting locusts (Locusta migratoria) can be elicited by sound pulses of steep rise time above 80 dB. The reaction consists of a fast jerky movement of legs and body with a mean latency of 35 ms and graded amplitude. The fast startle reaction did not result in any positional change; this was in contrast to acoustically induced escape reactions of flying Orthoptera. The startle reaction could be inhibited by pure tone stimuli of much lower intensity (60 dB) presented 160 ms before the startle-eliciting noise. This type of reflex modification is a striking convergence to the well-known prepulse inhibition of the mammalian startle response where it has been used to assess sensory thresholds. In the locust, prepulses between 3 and 20 kHz suppressed the startle reaction completely, with thresholds in the locust's hearing range as known from tympanal nerve recordings. No inhibition could be observed at prepulse frequencies of 40 kHz, although this frequency lies within the locust's hearing range. The presence of prepulse inhibition in an invertebrate preparation shows that it is not restricted to vertebrates.