Introduction
Tactile stimulation (TS) is defined as a positive sensory experience resulting from physical contact (touch) between individuals or via artificial devices (Bolognesi et al., 2019; Soares et al., 2011). TS is involved in various relationships between animals, such as grooming in primates (Dunbar, 2010), cooperative interactions between fish (Bshary and W��rth, 2001), therapeutic massage in humans (Field et al., 2005), and in the context of animal husbandry (Gauy et al., 2022). In these contexts, TS plays a crucial role in promoting animal welfare and strengthening social bonds. To the moment, the association between DA and TS has only been observed in cleaner-client context, i.e. always the presence of a fish as a "reward" (Messias et al., 2016a, 2016b). Here, we aimed to test if the TS per se can have the rewarding role, not given by other living organisms but provided by an artificial apparatus. Thus, we investigated the role of DA in modulating the response to tactile stimuli and the choice of TS in an important commercial species of teleost fish, the gilthead seabream (Sparus aurata). We predicted that if TS is processed as a reward (or positive valence) and modulated by exogenous changes in available DA levels, blocking endogenous levels would function as an omission of reward, resulting in behavioural compensation characterised by the seeking the physical stimulation. Conversely, increased DA levels would induce the opposite effect.
Materials and methods
Fish were first exposed to an artificial apparatus consisting of a rectangular PVC frame fixed in the centre of the aquarium, with vertically attached sticks lined with silicone bristles providing TS as fish swam through them. Groups of four individuals received seven days of TS and were then randomly assigned to one of three exogenous treatments: D1 agonist, D1 antagonist, or saline (12 groups per treatment). Fish then returned to an aquarium containing an apparatus with sticks half with/half without silicone bristles for analysis of post-treatment preference (60 min of video recording).
Secondly, to test the efficiency of our DA exogenous manipulations, we needed to understand the effects that intramuscular injections would have on the DA brain level of Sparus aurata individuals, which areas would be affected, and how long the effects would last. Four timepoints were defined at which the fish would be euthanised by deepening anaesthesia, following an intramuscular injection of one of three treatments: 15 minutes, 30 minutes, 45 minutes and 60 minutes. Six individuals were euthanised at each time point for each compound, totalling n = 24 in each treatment, for subsequent brain HPLC analysis of DA and DOPAC.
Results and discussion
We found that all treatments sought the TS at a similar frequency but those under DA blockage showed a greater relative preference for the TS area, having a higher relative frequency of crossing the bristles than those receiving other treatments. In the second experiment, fish treated with D1 blocker showed a significant reduction in DA concentration in the forebrain after 60 min, confirming the antagonist's effectiveness. These results suggest that TS is associated with dopaminergic reward pathways in fish, similarly to mammals. Although TS in the natural environment of S. aurata is not described in the literature as in other species (see Soares et al., 2011), it seems however able to generate positive responses associated with reward. It is thus possible that TS occurs in a more subtle way for S. aurata, such as through touch between conspecifics and elements present in their natural environment, or even from water movements that produce tactile perception between individuals. Our results suggest that body tactile stimulation enrichment may have a great potential for aquaculture application, particularly in this important commercial species.
Acknowledgments
This study was supported by the S��o Paulo Research Foundation (FAPESP) [Grants to EGF # 2023/02306-1; scholarship for BCS #2023/04059-1, post-doc scholarship for ACSG #2023/09880-5]. MCS is supported by the Foundation for Science and Technology (FCT), Portugal (CEEC Individual-2021.01458.CEECIND/CP1668/CT0003).
References
Bolognesi, M. C., Gauy, A. C. D. S., Gon��alves-De-Freitas, E., 2019. Tactile stimulation reduces aggressiveness but does not lower stress in a territorial fish. Sci. Rep. 9(1), 40. https://doi.org/10.1038/s41598-018-36876-1
Bshary, R., W��rth, M., 2001. Cleaner fish Labroides dimidiatus manipulate client reef fish by providing tactile stimulation. Proc. R. Soc. B. 268(1475), 1495–1501. https://doi.org/10.1098/rspb.2001.1495
Dunbar, R. 2008. The social role of touch in humans and primates: Behavioural function and neurobiological mechanisms. Neurosci. Biobehav. Rev. 34(2), 260–268. https://doi.org/10.1016/j.neubiorev.2008.07.001
Field, T., Hernandez-Reif, M., Diego, M., Schanberg, S., Kuhn, C., 2005. Cortisol decreases and serotonin and dopamine increase following massage therapy. Int. J. Neurosci. 115(10), 1397–1413. https://doi.org/10.1080/00207450590956459
Gauy, A. C. D. S., Bolognesi, M. C., Gon��alves-De-Freitas, E. 2022. Long-term body tactile stimulation reduces aggression and improves productive performance in Nile tilapia groups. Sci. Rep. 12(1), 20239. https://doi.org/10.1038/s41598-022-24696-3
Messias, J. P. M., Paula, J. R., Grutter, A. S., Bshary, R., Soares, M. C., 2016a. Dopamine disruption increases negotiation for cooperative interactions in a fish. Sci. Rep. 6(1), 20817. https://doi.org/10.1038/srep20817
Messias, J. P. M., Santos, T. P., Pinto, M., Soares, M. C., 2016b. Stimulation of dopamine D1receptor improves learning capacity in cooperating cleaner fish. Proc. R. Soc. B. 283(1823), 20152272. https://doi.org/10.1098/rspb.2015.2272
Soares, M. C., Oliveira, R. F., Ros, A. F., Grutter, A. S., Bshary, R., 2011. Tactile stimulation lowers stress in fish. Nat. Comm. 2(1), 534. https://doi.org/10.1038/ncomms1547