The variability in individual responsiveness to general anesthetics remains a critical issue in clinical practice. While pharmacokinetic factors often explain differences in dosing, growing evidence suggests that an individual’s environmental background and lifestyle factors significantly modulate neural sensitivity to anesthetics (pharmacodynamics) (Kil et al., 2012). Recently, environmental enrichment, a paradigm providing sensory, cognitive, and motor stimulation, has gained attention for its impact on the central nervous system and its potential interaction with anesthetic outcomes (Van Praag et al., 2000; Zhao et al., 2015). A comprehensive review highlighted that environmental enrichment exerts neuroprotective effects and modulates cognitive recovery following anesthesia, suggesting a profound interaction between environmental factors and anesthetic mechanisms (Chang and Tian, 2022). Furthermore, clinical studies have indicated that socioeconomic and educational backgrounds, factors reflecting aspects of human environmental enrichment, significantly influence patients’ preoperative anxiety and concerns regarding anesthesia (Al Modanat et al., 2024). This suggests a translational link whereby environmental history modulates internal emotional state relevant to anesthetic management. A key physiological effect of environmental enrichment is its robust anxiolytic and stress-reducing property. Previous behavioral and neurochemical studies have demonstrated that animals reared in an enriched environment (EE) exhibit reduced anxiety-like behaviors and altered stress responses compared with those in standard environments (Fox et al., 2006; Simpson and Kelly, 2011). These behavioral changes are often accompanied by functional modifications in the limbic system, particularly within the amygdala and hippocampus (Eckert et al., 2010; Ashokan et al., 2016). The hippocampus is essential for memory consolidation, and because amnesia is a cardinal endpoint of general anesthesia occurring at lower concentrations than immobility, this region is widely regarded as a primary neuroanatomical target for investigating anesthetic mechanisms (Hemmings et al., 2019; Platholi and Hemmings, 2022).

The relationship between emotional state and anesthetic sensitivity provides the rationale for the present study. We previously reported that artificial activation of the amygdala circuitry, mimicking a state of heightened anxiety or fear, significantly attenuates the inhibitory actions of general anesthetics on hippocampal synaptic transmission (Hirota et al., 2019). This finding aligns with the clinical observation that highly anxious patients often require larger doses of anesthetics (Maranets and Kain, 1999). Based on these findings, we hypothesized that EE rearing, which conversely reduces anxiety and amygdala excitability, would enhance the potency of general anesthetics in the limbic circuit. To test this hypothesis, we examined the effects of propofol and desflurane on synaptic transmission using a novel amygdala–hippocampal slice preparation that preserves the reciprocal connections between these regions. This study aims to elucidate how environmental history modulates the pharmacological profile of anesthetics at the circuit level, focusing on the GABAergic mechanisms within the amygdala–hippocampal network.

Electrophysiological studies were conducted of amygdala-hippocampal slices to determine how environmental enrichment modifies the actions of general anesthetics. Figures 3A,B shows representative recordings of PSs in the presence of either propofol or desflurane. In slices from both SE and EE rats, a standard test pulse (Protocol A) elicited a PS. When a pre-pulse was added to activate the amygdala circuitry (Protocol B), this PS was suppressed, demonstrating an inhibitory effect. The anesthetic-induced suppression of the PS was visibly more pronounced in slices from the EE group than in the SE group, suggesting enhanced sensitivity in EE rats.

To quantify these observations, concentration-response curves were generated using Protocol B. Propofol produced a concentration-dependent reduction in PS amplitude, with negligible effects on the EPSP slope (Figures 3C,D). The inhibitory action of propofol on PS amplitude was significantly greater in the EE group (main effect of group: P = 0.0025, two-way repeated measures ANOVA). The median IC50 for PS amplitude was 5.4 × 10⁻⁵ M [IQR: 5.1 × 10⁻⁵–5.6 × 10⁻⁵] in the EE group, which was significantly lower than 3.3 × 10⁻⁴ M [IQR: 2.7 × 10⁻⁴–3.5 × 10⁻⁴] in the SE group (P = 0.002, Mann–Whitney U test). The Hill coefficient did not differ significantly between groups (0.9 [IQR: 0.89–0.96] vs. 0.8 [IQR: 0.72–1.95], P = 0.39, Mann–Whitney U test). Similarly, a significant enhancement of inhibitory action was observed for desflurane in the EE group (P = 0.0021, two-way repeated measures ANOVA). Although the shift appeared less prominent than with propofol, the median IC50 was significantly lower in the EE group (6.5 vol% [IQR: 4.8–7.1]) compared to the SE group (10.4 vol% [IQR: 10.3–11.5], P = 0.002, Mann–Whitney U test). Notably, the median Hill coefficient was significantly higher in the EE group compared to the SE group (2.7 [IQR: 2.7–3.7] vs. 2.2 [IQR: 1.9–2.5], P = 0.015, Mann–Whitney U test).

Finally, to investigate the underlying mechanism for the enhanced anesthetic sensitivity in EE rats, Protocol C, which used a high-frequency tetanic pulse to induce a transient depletion of GABA stores (Markram and Tsodyks, 1996), resulting in disinhibition of the PS (Abbott and Regehr, 2004), was used (Figure 4A). After the tetanic pulse, the inhibitory action of the anesthetic on PS amplitude returned gradually. In the presence of propofol, this recovery was markedly faster in the EE group (main effect of group: P < 0.0057, two-way repeated-measures ANOVA), with a time constant (τ) of 36.6 s [IQR: 25.5–48.9], compared with 146.0 s [IQR: 110.6–642.8] in the SE group (P = 0.008, Mann–Whitney U test). This difference in recovery time was less pronounced in the presence of desflurane (τ = 237.8 s [IQR: 166.3–238.7] in EE vs. 450.3 s [IQR: 428.7–531.7] in SE, P = 0.056, Mann–Whitney U test).

This study demonstrated that an EE during rearing alters the effects of general anesthetics on synaptic transmission in the amygdala-hippocampal slice, yielding two primary findings. First, an EE potentiated the effects of the anesthetics: the inhibitory effects of propofol and desflurane on PS were significantly greater in rats raised in an EE compared with those raised in a SE. Second, this potentiation was more prominent with the intravenous anesthetic propofol than with the volatile anesthetic desflurane.

The GABAergic system is central to explaining our findings, both the overall enhancement of anesthetic effects by EE and the observed differences between the two anesthetics. First, this GABA-centric mechanism accounts for the general increase in anesthetic sensitivity under an EE. Previous studies have documented the profound impact of an EE on the central nervous system, showing that an EE enhances synaptic plasticity (Eckert et al., 2010; Ohline and Abraham, 2019) and induces structural and functional changes in the GABAergic system (Nakamura et al., 1999; Segovia et al., 2006). Thus, an EE likely amplifies the effects of general anesthetics by selectively strengthening GABAergic inhibitory synapses relative to excitatory ones.

Second, this GABA-centric view is supported by the distinct pharmacological actions of the two anesthetics. In the present study, though both agents reduced PS amplitude in a concentration-dependent manner, their effects on excitatory input were distinct: propofol spared the EPSP slope (Figure 3C), whereas desflurane suppressed it (Figure 3E). These results align with prior findings that propofol primarily exerts its inhibitory neuronal effect via GABA receptors, whereas desflurane also acts on other molecular targets (Alkire et al., 2008; Asahi et al., 2006; Wakasugi et al., 1999). The Hill coefficients further illuminate the specificity of an EE’s action. For propofol, a GABA-specific agent, the coefficient remained near 1.0 in both the SE and EE groups, indicating that an EE enhanced its potency (IC50) without altering its fundamental non-cooperative mechanism. In stark contrast, for desflurane, which has multiple targets, the Hill coefficient increased significantly from 1.9 to 2.9 in the EE group, indicating a change in its cooperative binding dynamics. The fact that the mechanism of action changed only for the non-specific anesthetic (desflurane), but not for the specific one (propofol), strongly suggests that an EE’s primary effect is selective enhancement of the GABAergic system’s efficiency, while its effects on other systems are more complex and alter the very nature of drug-receptor interactions. Therefore, the stronger potentiation of propofol, a highly GABA-specific anesthetic, by an EE provides compelling evidence that an EE enhances inhibitory networks, thereby augmenting the effects of anesthetics that primarily act through the GABAergic system.

Furthermore, the present findings suggest that the inhibitory synapses of EE rats recover their function more quickly, allowing the re-establishment of the anesthetic’s suppressive effect. This was demonstrated using Protocol C, with which the recovery from transient disinhibition was markedly faster in the EE group, an effect that was particularly prominent in the presence of propofol (Figure 4B). Although the precise mechanism was not determined in this study, one possibility is an enhanced capacity for GABA synthesis. For example, previous research has shown that an EE increases the expression of the GABA-synthesizing enzyme GAD (Sale et al., 2007). Such enhancement is thought to contribute to a more robust inhibitory neurotransmitter system, a possibility that supports the present functional evidence.

The implications of the present findings may extend to clinical anesthesia. The results, demonstrating that developmental environment alters anesthetic sensitivity in rats, suggest that an individual’s life history and background could be a critical, yet currently overlooked, factor in determining anesthetic dosage in humans. For instance, a patient from a highly EE might exhibit increased sensitivity to GABAergic intravenous anesthetics like propofol. For such individuals, a standard dose could lead to an overdose, increasing the risk of adverse events like prolonged recovery or excessive hemodynamic depression. Therefore, this study highlights the potential need for a more personalized approach to anesthesia, in which patient-specific environmental factors are considered to optimize dosing and enhance perioperative safety.

A key limitation of this study is that, although we demonstrated that EE enhances anesthetic sensitivity, the precise underlying mechanism remains to be elucidated. Potential factors, such as accelerated GABA reuptake, enhanced synthesis, or increased postsynaptic receptor sensitivity, were not directly investigated, and their respective contributions will require further research.

Second, regarding the experimental subjects, although the present study was conducted exclusively in male rats, previous studies have reported significant sex differences in the anesthetic actions of propofol (Hoymork and Raeder, 2005; Choong et al., 2013) and desflurane (Tercan et al., 2005; Wasilczuk et al., 2024). While our initial focus on males was based on conventional practice, recent studies indicate that male rodents can exhibit greater intra- and inter-individual variability than females, and that diurnal fluctuations in testosterone may influence brain measures more profoundly than estrous cycle fluctuations (Prendergast et al., 2014; Kaluve et al., 2022; Levy et al., 2023). Furthermore, it has been explicitly emphasized that the effects of housing conditions should be assessed for both males and females (Simpson and Kelly, 2011). Therefore, the exclusion of females limits the generalizability of our findings across sexes, and future studies are essential to examine potential sex differences in the interaction between EE and anesthetic sensitivity.

Third, with respect to the study design, in vivo behavioral assays (e.g., measurement of anesthetic induction time) were not performed. In this study, we focused exclusively on the slice preparation. This approach enabled a detailed mechanistic characterization of the amygdala–hippocampal circuit, leaving in vivo validation as an important subject for future investigation.

Finally, concerning the scope of the enrichment model, we acknowledge that our protocol differs from standard EE paradigms, which typically involve social housing. In our experimental design, we used individual housing to standardize the environment and to avoid the potential stress associated with the sequential removal of cage mates (Burman et al., 2008). The EE paradigm in this study—consisting of toys, shelters, and tunnels—should be interpreted as a model that focuses specifically on complex sensory, cognitive, and motor stimulation, representing a distinct subset of the broader definitions of environmental enrichment (Nithianantharajah and Hannan, 2006; Sztainberg and Chen, 2010; Simpson and Kelly, 2011). It should be noted that this individual housing inherently excludes social enrichment, is known to increase anxiety (Manouze et al., 2019), and may dampen enrichment effects (Hellemans et al., 2004). Importantly, however, our results demonstrate that these physical stimuli alone were sufficiently potent to induce a robust potentiation of anesthetic effects. This finding is consistent with previous reports indicating that the benefits of environmental complexity can be retained even in the absence of social interaction (Schrijver et al., 2002).

In conclusion, EE rearing enhanced the inhibitory actions of general anesthetics on hippocampal population spikes and accelerated recovery from synaptic disinhibition, when compared with SE rearing. This modification of anesthetic sensitivity was more prominent with intravenous anesthetics than with volatile agents.