Although the actions of dopamine throughout the brain are clearly linked to motivation and cognition, the specific role(s) of dopamine in the CA1 subfield of the ventral hippocampus (vH) is unresolved. Prior preclinical studies suggest that dopamine D receptors (DR) expressed on CA1 pyramidal cells exhibit a unique capacity to modulate mechanisms of long-term synaptic plasticity, but less is known about how interneurononal inputs modulate these cells. We hypothesized that inputs from μ opioid receptor (MOR)-expressing inhibitory interneurons selectively modulate the activity of postsynaptic DRs expressed on CA1 principal cells to shape neurotransmission in the rat vH. We used the whole-cell voltage clamp technique to test this hypothesis by measuring evoked inhibitory postsynaptic currents (eIPSCs) from CA1 principal cells in vH slices or GABA currents from acutely dissociated vH neurons. The eIPSC response recorded from CA1 neurons in vH slices was inhibited by either the MOR agonist DAMGO or the DR agonist PD128907, but pretreatment with DAMGO occluded any further inhibition by PD128907. GABAA currents measured in acutely dissociated vH CA1 neurons were inhibited by DR activation via PD128907, consistent with postsynaptic localization of D receptors. Kinetic alterations induced by the neuromodulatory agonists are consistent with selective targeting of postsynaptic DRs expressed on CA1 principal cells by MOR-expressing GABAergic inputs. Our findings suggest postsynaptic DR-mediated modulation of MOR-expressing GABAergic inputs is a site at which dopaminergic and opioidergic activity may contribute to disinhibition of vH excitatory neurotransmission, and, thus, influence critical physiological processes such as synaptic plasticity and network oscillations.

Gamma oscillations in brain activity (30-150 Hz) have been studied for over 80 years. Although in the past three decades significant progress has been made to try to understand their functional role, a definitive answer regarding their causal implication in perception, cognition, and behavior still lies ahead of us. Here, we first review the basic neural mechanisms that give rise to gamma oscillations and then focus on two main pillars of exploration. The first pillar examines the major theories regarding their functional role in information processing in the brain, also highlighting critical viewpoints. The second pillar reviews a novel research direction that proposes a therapeutic role for gamma oscillations, namely the gamma entrainment using sensory stimulation (GENUS). We extensively discuss both the positive findings and the issues regarding reproducibility of GENUS. Going beyond the functional and therapeutic role of gamma, we propose a third pillar of exploration, where gamma, generated endogenously by cortical circuits, is essential for maintenance of healthy circuit function. We propose that four classes of interneurons, namely those expressing parvalbumin (PV), vasointestinal peptide (VIP), somatostatin (SST), and nitric oxide synthase (NOS) take advantage of endogenous gamma to perform active vasomotor control that maintains homeostasis in the neuronal tissue. According to this hypothesis, which we call GAMER (GAmma MEdiated ciRcuit maintenance), gamma oscillations act as a 'servicing' rhythm that enables efficient translation of neural activity into vascular responses that are essential for optimal neurometabolic processes. GAMER is an extension of GENUS, where endogenous rather than entrained gamma plays a fundamental role. Finally, we propose several critical experiments to test the GAMER hypothesis.