Octopamine is a neurotransmitter, neuromodulator, and neurohormone widely distributed in the insect nervous system. Released by octopaminergic neurons, it binds to octopaminergic receptors coupled with G proteins to modulate the concentration of second messengers inside cells (Farooqui 2012). Through these mechanisms, octopamine modulates foregut activity, walking, flying and ventilation of insects. The levels of this neurotransmitter increases after exposure to various stresses, such as heat and cold, mechanical or chemical stress (Armstrong et al. 2006).

This biogenic amine modulates behaviour and physiology of insects, with effects similar to norepinephrine in vertebrates. It is secreted in response to heat stress to defend the nervous system from this detrimental factor. Additionally, a Drosophila Inactive strain with defective thermotolerance is characterized by decreased locomotor activity and lower octopamine levels (Armstrong and Robertson 2006). Octopamine was also shown to directly modulate thermoregulatory fanning response in Apis mellifera (Cook et al., 2017). Therefore, octopamine appear to play an important role in the thermoregulatory processes in insects.

The first step initiating a thermoregulation is temperature sensing via thermoreceptor neurons in the peripheral nervous system. These neurons are linked to the central nervous system and influence physiological and behavioural reactions to temperature, such as temperature preference and heat avoidance (Gonzalez-Tokman et al., 2020). In insects four fundamental types of thermoreceptors have been identified: warm (respond to moderate warming), cold (respond to moderate cooling) and two classes of nociceptors (respond to noxious heat and noxious cold) (Barbagallo and Garrity, 2015). Perception of thermal stimuli is mediated by receptors belonging to Transient Receptor Potential (TRP) family, known as thermo-TRP, which are also involved in temperature sensation in insects (Dillon et al., 2009).

After temperature sensing with thermoreceptors, the information is sent to neurosecretory cells in central parts of the insect nervous system involved in thermoregulation. This affects the expression and function of neurotransmitters, exerting an effect on internal physiology and behaviour. While studies on thermosensation have primarily focused mainly on primary sensory neurons, the mechanisms involved in further processing of thermal information in the central nervous system are largely unstudied (Gonzalez-Tokman et al., 2020). An open question remains regarding the involvement of neurotransmitters in insects’ response to temperature. We hypothesize that changes in cockroaches’ thermal behaviour that occur after thermo-TRP receptors activation are octopamine-dependent.

We decided to evaluate the involvement of octopamine in TRP-ligand-induced changes in Periplaneta americana. Thermo-TRPs have been proven to be involved in thermosensation, and their activation induces changes in thermoregulatory behaviour. To induce TRP activation, we used capsaicin, a noxious heat receptor activator. Capsaicin induces changes in cockroaches thermal behaviour (Maliszewska et al., 2018a). We evaluated whether, after TRP activation by capsaicin, octopamine is released in response to chemical and thermal stress. Octopamine’s involvement in the capsaicin response was determined through:

behavioural methods – thermal preferences and grooming, as it has been shown that octopamine elicits intensive leg grooming (Carrazoni et al. 2016),

physiological methods – heart rate, as octopamine regulates the heartbeat in the American cockroach (Enan 2001),

biochemical methods – octopamine level using liquid chromatography-mass spectrometry (LC-MS) technique and protein kinase A (PKA) activity, as this kinase is activated in the octopamine receptor cellular pathway (Bischof and Enan, 2004).