Although the salivary glands have a low rate of cell turnover, they are relatively radiosensitive. To study the possible mechanism behind this inherent radiosensitivity, a rat model was developed in which saliva can be collected after local irradiation of the parotid gland without the use of anesthetics or stressful handling. Saliva secretion was induced by the partial muscarinic receptor agonist pilocarpine (0.03-3 mg/kg) with or without pretreatment with the beta-adrenoceptor antagonist propranolol (2.5 mg/kg), or the full muscarinic receptor agonist methacholine (0.16-16 mg/min), and measured during 5 min per drug dose before and 1, 3, 6 and 10 days after irradiation. The maximal secretory response induced by pilocarpine plus propranolol was increased compared to that with pilocarpine alone but did not reach the level of methacholine-induced secretion, which was about five times higher. One day after irradiation a decrease in maximal pilocarpine-induced secretion was observed (-22%) using the same dose of pilocarpine that induces 50% of the maximal response (ED(50)), in both the absence and presence of propranolol, indicating that the receptor-drug interaction was not affected by the radiation at this time. The secretory response to methacholine 1 day after irradiation, however, was normal. At day 3 after irradiation, the maximal methacholine-induced secretion was also affected, whereas pilocarpine (+/-propranolol)-induced maximal secretion decreased further. At day 6 after irradiation, maximal secretory responses had declined to approximately 50% regardless of the agonist used, whereas ED(50) values were still unaffected. No net acinar cell loss was observed within the first 10 days after irradiation, and this therefore could not account for the loss in function. The results indicate that radiation does not affect cell number or receptor-drug interaction, but rather signal transduction, which eventually leads to the impaired response. We hypothesize that the early radiation effect, within 3 days, may be membrane damage affecting the receptor-G-protein signaltransfer. Later critical damage, however, is probably of a different nature and may be located in the second-messenger signal transduction pathway downstream from the G protein, not necessarily involving cellular membranes.

1 In the portal vein of permanently cannulated, freely moving, unanaesthetized rats, methacholine (MCh) is able to inhibit the electrically-evoked endogenous noradrenaline (NA) overflow. This inhibition is mediated by presynaptic inhibitory muscarinic heteroreceptors. 2 By use of pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) and AF-DX 116 as M1-, M3-, and M2-selective antagonists respectively, the MCh (0.1 uM)-induced inhibition of the electrically-evoked NA overflow could be reversed to the control stimulation value dose-dependently.3 The potency order of the antagonists was: 4-DAMP > AF-DX 116 > pirenzepine, pIC50 values being 8.50, 7.96 and 7.01, respectively. 4 From these results it was concluded that the inhibitory presynaptic heteroreceptors in the portal vein of concious unrestrained rats are of the cardiac M2-subtype.

1 In the portal vein of the freely moving unanaesthetized rat, the existence of presynaptically located inhibitory muscarinic receptors was investigated by use of the muscarinic agonist methacholine (MCh) 2 Infusion of MCh (0.3pgmin-1) did not significantly inhibit the endogenous noradrenaline (NA) overflow in portal plasma. However, after inducing high intra-synaptic concentrations of NA by blocking the presynaptic a2-adrenoceptors with yohimbine (lmgkg-1), MCh (0.3jpgmin-1) was able to reduce the yohimbine-induced enhanced NA overflow by 38%. 3 The MCh-induced inhibition was almost completely abolished after blockade of the presynaptic muscarinic receptors with atropine (0.6 mg kg~-). 4 During electrical stimulation of the portal vein nervous plexus the evoked NA overflow was strongly inhibited (95%) during MCh-infusion (0.3jugmin'). Again atropine (0.6 mgkg 1) was able to reverse this inhibition. 5 These results show the existence of presynaptic muscarinic receptors inhibiting endogenous NA overflow from the portal vein nervous plexus under conditions of enhanced sympathetic activity in the freely moving rat.

This Roadmap paper covers the field of precision preclinical radiation studies in animal models. It is mostly focused on models for cancer and normal tissue response to radiation, but also discusses other disease models. The recent technological evolutions in imaging, irradiation, dosimetry and monitoring that have empowered these kinds of studies is discussed, and many developments in the near future are outlined. Finally, clinical translation and inverse translation are discussed.