Introduction
The striatum is the central part of the basal ganglia that integrates and processes information from the cortex, the hippocampus, the substantia nigra, and the thalamus (Lanciego et al. , 2012; Haber, 2016). While functions of the ventral striatum are associated with the reward system, reinforcement, and emotions, the dorsal striatum (neostriatum) composed of the caudate nucleus and the putamen mainly participates in motor control and operational learning (Lanciegoet al. , 2012; Haber, 2016). Communication of the striatum with other brain regions is mediated by spiny projection neurons (SPNs), GABAergic striatofugal cells, which are traditionally divided into the two populations (Kreitzer, 2009). In the dorsal striatum, activation of SPNs induces direct (striatonigral) pathway of the basal ganglia that stimulates the motor cortex and promotes initiation of movements (Bateupet al. , 2010; Wall et al. , 2013). Cells of the second population initiate indirect (striatopallidal) pathway suppressing the activity of the motor cortex and locomotion (Bateup et al. , 2010; Wall et al. , 2013).
Proper activity of the striatal SPNs, in turn, is maintained by dopamine provided by neurons of the substantia nigra pars compacta (SNc) (Gantzet al. , 2018). According to the classical concept, bursts of nigrostriatal dopamine differentially stimulate SPNs of direct pathway and inhibit SPNs of indirect one facilitating activation of the direct circuits and suppressing indirect ones (Kreitzer, 2009; Lanciegoet al. , 2012). Low dopamine signaling, in opposite, results in predominance of indirect pathway (Duty & Jenner, 2011). Thus, dopamine coordinates the work of stimulating and suppressing circuits and provides optimal level of striatal activation necessary for maintaining locomotor activity according to the currents needs. Plenty of data indicate that physical activity is promoted by enhanced metabolism of dopamine and its massive release in synapses between dopaminergic neurons of SNc and striatal SPNs (Korchounov et al. , 2010; Gepshtein et al. , 2014). At the same time insufficient dopamine signaling in the dorsal striatum can be associated with motor disorders, including akinesia, bradykinesia, tremor, changed muscle tone, and postural abnormalities (inability to maintain equilibrium under dynamic or static conditions), observed in patients with Parkinson’s disease and in corresponding animal models (Duty & Jenner, 2011; Palakurthi & Burugupally, 2019).
It is well-known that microgravity, the main negative factor of spaceflights, strongly impairs locomotor function provoking such disorders as impaired gait, postural instability, and changes in muscle tone (Reschke et al. , 1998; Lacquaniti et al. , 2017; Tayset al. , 2021). Similar alterations were observed in ground-based conditions with volunteers which experienced antiorthostatic hypokinesia (head-down bed rest) (Parry & Puthucheary, 2015) and in experimental animals exposed to hindlimb unloading (HU) (Canu et al. , 2007). The hindlimb unloading (HU) model is a widely used approach to simulate microgravity in ground-based animal studies. It effectively reproduces such negative effects of spaceflights as removal of the support loading from the hindlimbs, cerebrospinal fluid shift, and degeneration in bones and muscles (Morey-Holton & Globus, 2002; Globus & Morey-Holton, 2016).
Furthermore, investigation of experimental animals revealed that real microgravity significantly affects dopaminergic system of the brain (Popova et al. , 2015, 2020; Tsybko et al. , 2015). Thus, 30-day spaceflight induced a considerable reduction in mRNA of proteins responsible for dopamine synthesis (tyrosine hydroxylase, TH), degradation (catechol-O-methyltransferase, monoamine oxidases), and postsynaptic effects (dopamine receptors) in the nigrostriatal system (Popova et al. , 2015). In addition, studies revealed reduced nigrostriatal expression of neurotrophins GDNF (glial cell-line derived nervous factor) and CDNF (cerebral dopamine neurotrophic factor) which are necessary for optimal functioning of dopaminergic neurons (Tsybkoet al. , 2015). However, available data concern only long-term effect of microgravity, while the main processes of adaptation are expected to occur during a shorter period of exposure. In addition, these data are associated mainly with analysis of mRNA which does not always fully correlate with protein amount and activity (Popova et al. , 2020). Thus, further investigations are needed to disclose detailed mechanisms of altered dopaminergic regulation in the brain under pathological and extremal conditions associated with restricted motility, loss of sensorimotor stimuli, and muscle disuse.
Ground-based animal studies could provide more information about detailed mechanisms of dysregulation in the dopaminergic system of the brain. However, the studies showed that in rodents the effect of 30-day HU on the dopaminergic system considerably differed from those of actual spaceflight (Kulikova et al. , 2017). Expression of some crucial markers affected by the real microgravity (TH, GDNF, CDNF) showed no changes in response to HU, while others, such as dopamine receptors D1 (D1R), demonstrated the opposite alterations. These data confirmed that HU model could not be considered as universal approach to study the physiological effects of microgravity (Kulikova et al. , 2017). Nevertheless, it is still can be perspective to investigate various alterations induced by ground-based pathological conditions such as inactivity, bed rest, and immobilization (Globus & Morey-Holton, 2016).
Social isolation (SI) is another factor that can produce a number of negative physiological outcomes including chronic stress, motor decline, and cognitive disorders (Arzate-Mejía et al. , 2020; Vitale & Smith, 2022). For example, it was shown that in elderly people, loneliness was associated with more rapid rate of motor decline manifesting as decrease in muscle strength, walking speed, and physical performance (Buchman et al. , 2010; Philip et al. , 2020). Animal models of SI also revealed dysregulation of dopamine signaling, however, main changes were observed in the ventral striatum (Benderskyet al. , 2021; Zhang et al. , 2021; McWain et al. , 2022), while alterations the nigrostriatal dopaminergic system still remain unclear.
On the other hand, SI is the important factor that should be mentioned both in spaceflight and ground-based experiments. Experimental data indicated that isolation contributed to negative effects of gravitational unloading including violations in the musculoskeletal apparatus and locomotor activity (Morey-Holton et al. , 2000; Tsvirkun et al. , 2012; Tahimic et al. , 2019). Thus, studying combined effects of SI and HU on the brain systems responsible for locomotor control is also of considerable interest.
The aim of the present work was to analyze the effect of short-term (3-day) HU and SI on the dopaminergic regulation of the dorsal striatum in mice. Obtained results showed that HU negatively affected dopamine biosynthesis and dopamine-mediated signaling in the nigrostriatal system indicating a decrease in dopaminergic neurotransmission. Contribution of SI into effects of HU was also revealed.