Abstract
Background
With the recent legalization of recreational marijuana in Canada, increasing numbers of patients with gastrointestinal (GI) disorders are using cannabis to treat their pain, either alone or together with opioids. However, little is known about potential benefits of cannabinoids for treating visceral pain originating within the GI tract and whether the combined use of cannabinoids and opioids could enable the reduction or even discontinuation of opioids.
Aims
To investigate the effects of cannabinoids alone or in combination with opioids on colonic nociceptive nerves.
Methods
Extracellular afferent nerve recordings were obtained from ex vivo flat sheet preparations of male C57BL/6 mouse distal colons. Single unit analysis discriminated individual afferent neuron responses to mechanical probing of the colon with a 1g von Frey hair before and after superfusion of HU-210, a selective CB1 receptor agonist, HU-308, a selective CB2 receptor agonist, DAMGO, a selective mu-opioid receptor (MOR) agonist, or a combination. In parallel studies, perforated patch clamp techniques were employed to assess the rheobase as a measure of neuronal excitability in acutely dissociated dorsal root ganglia (DRG) neurons in the presence of one or more of these agonists. Data were analyzed using a one-way ANOVA with Bonferroni multiple comparisons test.
Results
Superfusion of HU-210 (1 μM), caused significant inhibition in afferent nerve mechanosensitivity compared to control (6.2±1.1 vs. 13.7±2.5 Hz, p=0.005, n=10); lower concentrations (10 nM and 100 nM) had no effect (p>0.99, n=11; p=0.600, n=10 respectively). Conversely, the CB2 agonist HU-308 (1 μM and 10 μM), did not alter mechanosensitivity (p>0.9, n=8 for both concentrations). Superfusion of HU-210 alone (100 nM) or DAMGO (1 nM) alone in the same recording had no effect, but when both agonists were superfused together, there was a significant reduction in mechanosensitivity (8.1±1.7 vs. 14.8±2.3 Hz, p<0.01, n=10). In patch clamp recordings of DRG neurons, HU-210 (1 μM) decreased excitability (i.e. increased rheobase, 94.4±9.4 vs. 62.7±6.4 pA; p=0.031, n=9), whereas a lower concentration (100 nM) had no effect. Similar to afferent nerve recordings, when applied alone, DAMGO (1 nM) and HU-210 (100 nM) did not affect rheobase (DAMGO: p>0.99, n=9; HU-210: p>0.99, n=10), whereas the combination of both agonists significantly decreased excitability (123.0±13.4 vs. 62.7±6.4 pA, p<0.01, n=10).
Conclusions
Activation of CB1 receptors inhibits mechanosensitivity of colonic afferent nerves while a CB2 agonist had no effect. Interestingly, combination of sub-threshold concentrations of CB1 and MOR agonists inhibited colonic afferent nerves and thus, may suggest that cannabinoids could enable opioid dose reduction or discontinuation.
Funding Agencies
None
Dopamine Alters the Fidelity of Working Memory Representations according to Attentional Demands
