The present studies evaluated the effects of aTHC exposure in rats and mice on increasingly complex aspects of synaptic physiology for two axonal systems in the young adult hippocampus. There was no effect of aTHC on input/output curves indicating that, for rodent S-C and LPP systems, the treatment did not disrupt basic mechanisms of synaptic transmission. There were, however, enduring aTHC effects on complex patterns of activity that normally occur during behavior and aTHC-associated impairments in two, mechanistically distinct forms of synaptic plasticity. Male rats were the least affected: aTHC did not alter frequency facilitation with trains of single pulse stimulation, theta burst responses, or LTP for either projection system (see Table 2 for summary). Male mice were anomalous: For this group alone, aTHC enhanced frequency facilitation and within theta train facilitation in field CA1 without effect on frequency facilitation in the LPP. Nevertheless, aTHC profoundly depressed LTP in the male mouse LPP. This dissociation of aTHC treatment effects on frequency facilitation and potentiation suggests that although the alterations in within-train responses may influence hippocampal circuit activity, most particularly during behaviors (retrieval, response to salient cues) that elicit beta and gamma frequency oscillations (Rangel et al., 2015; Alexander et al., 2018), these effects do not account for the marked influence of aTHC on LTP. Females of both species exhibited the most striking changes in synaptic plasticity with aTHC treatment: LTP was severely depressed in the two test pathways and this occurred without robust changes in other measures (Table 2). The results thus establish that the lasting effects of aTHC are markedly different between sexes and between rats and mice, and that daily exposure to a moderate dose of the phytocannabinoid (Huestis and Cone, 2004; Ruiz et al., 2020) during adolescence impairs mechanisms of enduring, memory-related synaptic plasticity within hippocampus without evident effects on basic synaptic transmission. This is to our knowledge the first report that aTHC impairs adult LTP (it reportedly does not affect potentiation in prefrontal cortex (Rubino et al., 2015).

Effects of aTHC on individual theta burst responses and theta train facilitation in mouse field CA1 suggest that early life THC exposure reduced GABAergic inhibition. In aTHC treated mice, the observed increase in response to later pulses in a theta burst is consistent with reduced GABA_AR-mediated inhibition in females (Arai et al., 1995) whereas the increase in theta train facilitation suggests reduced GABA_BR-mediated effects on release in males (Larson and Lynch, 1986; Davies et al., 1991). These results in mice accord with evidence for reduced GABAergic function, and markers of GABA transmission, in adult prefrontal cortex after aTHC treatment (Zamberletti et al., 2014; Renard et al., 2017). The absence of similar aTHC effects in rat, in the present studies, combined with reported increases in GABA release and receptor expression in rat hippocampus following adolescent CB₁R agonist treatment (Higuera-Matas et al., 2012) suggests there may be species differences in aTHC effects on inhibition in this structure. To further test this possibility, characterization of aTHC effects on GABAergic synapses in rat and mouse hippocampus is an important target for future studies.

In aTHC-treated male mice, LPP potentiation was profoundly reduced within the first 2 min after induction and then fell to baseline values. These findings suggest that aTHC affected signaling activities during or immediately after the high frequency stimulation used to induce potentiation. For the LPP in particular, buffering postsynaptic calcium or infusion of an NMDAR antagonist, but not mGluR5 or CB₁R antagonists, suppresses the earliest stages of potentiation in the manner seen with aTHC treatment (Wang et al., 2016b). We hypothesize that aTHC exposure altered developmental programs relating to activation of postsynaptic NMDARs in the LPP field and/or the elevation of intracellular calcium with high frequency stimulation. Why such changes would occur in LPP of male mice but not male rat or female mice is an open question, but substantial mouse vs. rat differences in the granule cell targets of the pathway have been described (Snyder et al., 2009). There is evidence that, with different treatment regimens, aTHC reduces NMDAR ligand (e.g., MK801) binding (Rubino et al., 2009; Higuera-Matas et al., 2012) but increases synaptic GluN2B levels (Zamberletti et al., 2016) in rat hippocampus but further work is needed to determine if changes in NMDAR function in the LPP field persist into adulthood and align with disturbances in STP.

The striking male-female differences in aTHC effects on potentiation of the S-C afferents to field CA1 were not entirely unexpected because potentiation in this system is sexually dimorphic, requiring local estrogen production and estrogen signaling via postsynaptic ERα in female but not male rodents (Vierk et al., 2012; Wang et al., 2018c; Gall et al., 2021). Activation of this signaling with estradiol infusion was shown here to be defective in CA1 of aTHC-treated females. This result suggests that a loss of synaptic estrogen action largely accounts for effects of aTHC exposure on S-C LTP in females and highlights the importance of identifying, in future studies, synaptic elements (receptors, signaling intermediaries) underlying impaired estrogen action. THC reportedly suppresses estrogen signaling via ERα in peripheral tissues by upregulating the competitive estrogen receptor β (ERβ) (Takeda et al., 2013). A mechanism of this kind, operating during development, could result in a lasting depression of estrogen actions required for female but not male LTP. Alternatively changes could be evident downstream from the estrogen receptors. In field CA1, estrogen infusion leads to transactivation of the neurotrophin receptor TrkB (Wang et al., 2016a) and ERα antagonism prevents synaptic TrkB activation with TBS in females only (Wang et al., 2016a). Thus impaired signaling downstream from ERα could disrupt the engagement of TrkB leading to a failure of LTP in females only.

Given the fundamental differences in mechanisms of LTP in the S-C and LPP systems it is quite likely that the molecular bases of aTHC effects also differ. The well-characterized field CA1 LTP is expressed as changes in AMPAR function in the postsynaptic element (Kauer et al., 1988; Muller and Lynch, 1988; Manabe and Nicoll, 1994; Lu et al., 1998; Granger and Nicoll, 2014). Although mechanisms and levels of field CA1 LTP are modulated by endocannabinoids (Carlson et al., 2002; Slanina et al., 2005; Monory et al., 2015) and contribute to metaplasticity in this system (Chevaleyre and Castillo, 2004), these agents and the CB₁R are not required for induction or expression of S-C potentiation (Chevaleyre and Castillo, 2004; Slanina et al., 2005; Lynch et al., 2013; Wang et al., 2016b; Nicoll, 2017). In contrast, LPP LTP is endocannabinoid-dependent in both rats and mice and is expressed presynaptically as an increase in neurotransmitter release. Specifically, 2-arachidonoyl glycerol (2-AG) serves as the retrograde messenger to presynaptic CB₁Rs that, in coordination with presynaptic integrins, recruit integrin-related kinases to effect an actin-dependent increase in neurotransmitter release (Wang et al., 2016a; Wang et al., 2018b). Unlike S-C potentiation, it is not known if LPP potentiation is sexually dimorphic or if, in females, it depends on estrogen function. Adolescent THC treatments do not have persistent effects on 2-AG levels (Rubino et al., 2015; Bara et al., 2021) and, although adolescent exposure causes transient and widespread reductions in the density and function of CB₁Rs in male and female rats (Rubino et al., 2008; Rubino et al., 2015; Silva et al., 2015), both measures and CB₁R gene expression reportedly return to normal in adult hippocampus and prefrontal cortex (Higuera-Matas et al., 2012; Rubino et al., 2015). Sustained reductions in CB₁R levels within the LPP field would provide a reasonable explanation for impairments in female LPP LTP. Alternatively, aTHC treatment in females could shift CB₁R signaling bias away from the integrin kinases (Wang et al., 2018b) and toward the more typical Munc18–1 target (Toonen, 2003; Schmitz et al., 2016), in essence bringing the system in line with CB₁R activities in other sites including CA1, and thereby removing a key factor that stabilizes potentiation in the LPP.

Previous studies using the Serial Odor task employed in the present studies showed that transient silencing of the LPP completely blocks acquisition of the ‘what’ (cue identity) element of episodic memory (Cox et al., 2019) and that LPP potentiation is required for olfactory encoding in this task (Wang et al., 2018a). Consistent with these observations, we found that the striking loss of LPP LTP with aTHC exposure was accompanied by defective ‘what’ learning: groups with aTHC-associated deficits in LPP-LTP (female rats and mice, male mice) performed at chance levels in the serial odor episodic task whereas aTHC-treated male rats, that exhibited normal LPP-LTP, had ‘what’ retention scores that were equal to those of vehicle controls. Similar associations were evident for the integrity of field CA1-LTP and spatial, OLM learning: aTHC-treated male rats and mice exhibited both robust CA1-LTP and normal, field CA1-dependent (Barrett et al., 2011; Babayan et al., 2012) OLM whereas aTHC-treated female rats and mice had impaired CA1-LTP and failed OLM. The latter result is in agreement with reported effects of aTHC on spatial learning using different aTHC treatment and behavioral paradigms (Zamberletti et al., 2014). Thus, regional defects in LTP described here were predictive of learning impairments and, clearly, the electrophysiological and behavioral consequences of aTHC exposure varied across telencephalic systems.

In rodents, the loss of episodic ‘what’ encoding by LPP silencing has been shown to disrupt acquisition of other episodic elements including acquisition of spatial and temporal information (Cox et al., 2019) whereas one can selectively block ‘where’ or ‘when’ acquisition by silencing other hippocampal subsystems (Cox et al., 2019). Studies with humans have described negative, later life effects on episodic memory following prolonged cannabis use in adolescence (Fletcher and Honey, 2006; Crane et al., 2013a; Crane et al., 2013b; Riba et al., 2015) but have not isolated effects on the three basic episodic elements evaluated in rodent work. However, reported deficits in the recall of personal narratives imply that, in human, problems with proper encoding of cue identities (what) are present. The possibility of pronounced sex differences in human episodic memory after adolescent cannabis usage has not been addressed. The long-standing issue of whether rats or mice are the better model for predicting human outcomes introduces a further complexity. A survey of the literature suggests that the rat is more translationally relevant with regard to hippocampal development and behavioral flexibility (Snyder et al., 2009). If this holds true for episodic memory, then results described here would predict that early life exposure to cannabis, even at moderate doses, will have larger effects in women than men with regard to encoding the continuous flow of everyday events into organized episodes. This possibility takes on added significance given evidence that episodic memory plays a critical role in diverse aspects of high cognitive function (Crane and Goddard, 2008).

In conclusion, we have shown that aTHC exposure has persistent effects on two, mechanistically distinct forms of LTP within hippocampus that are predictive of deficits in spatial and episodic memory encoding in adulthood. Results also demonstrate that these effects are more pronounced in females than males and suggest underlying mechanisms. Together these findings provide insights as to specific risks of daily cannabis use to the adolescent brain. Goals of future research will be to identify the components of estrogen signaling that are impaired by aTHC exposure and the consequences of persistent aTHC effects on plasticity for network functions underlying complex behaviors.