Effects of several 5-HT1A agonists on hippocampal rhythmical slow activity in unanesthetized rats

Bidirectional modulation of hippocampal long-term potentiation under stress and no-stress conditions in basolateral amygdala-lesioned and intact rats

Hippocampal long-term potentiation (LTP) is widely considered as a cellular model for learning and memory formation. We have shown previously that protein synthesis-independent, early dentate gyrus (DG) LTP, lasting approximately 4-5 h, can be transformed into a late-LTP with a duration of > or = 24 h by a brief acute swim stress experience (high-stress condition). This reinforcement requires the activation of mineralocorticoid receptors and protein synthesis. The basolateral amygdala (BLA) is known to modulate glucocorticoid effects on the consolidation of spatial/contextual memory via a beta-adrenergic mechanism. Interestingly, hippocampal DG-LTP can also be indirectly modulated by beta-adrenergic and cholinergic/muscarinergic processes. Here, we show that the reinforcement of early-DG-LTP under high-stress conditions depends on the processing of novel spatial/contextual information. Furthermore, this reinforcement was blocked in BLA-lesioned animals compared with sham-operated and intact controls; however, it was not dependent on beta-adrenergic or cholinergic/muscarinergic receptor activation. In contrast, under low-stress conditions, the induction of late-LTP in BLA-lesioned animals is facilitated, and this facilitation, again, was dependent on beta-adrenergic activation. The data suggest that DG-LTP maintenance can be influenced by the BLA through different mechanisms: a short-lasting corticosterone-dependent and beta-adrenergic-independent mechanism and a long-lasting mechanism that facilitated hippocampal beta-adrenergic mechanisms.

5-HT1A agonists induce hippocampal theta activity in freely moving cats: role of presynaptic 5-HT1A receptors

Electrical activity in the dorsal hippocampus was recorded in freely moving cats in response to intravenous administration of 5-HT1A agonist and antagonist drugs. Administration of low doses of the selective 5-HT1A agonists 8-OH-DPAT (5-20 micrograms/kg) and ipsapirone (20-100 micrograms/kg) produced rhythmic slow activity (theta) in the hippocampal EEG within 30 s. Similar effects were observed with BMY 7378 (20 and 100 micrograms/kg), which acts as an agonist at presynaptic (somatodendritic) 5-HT1A receptors and as an antagonist at postsynaptic 5-HT1A receptors. Power spectral analyses showed that all three compounds produced a dose-dependent increase in the EEG power occurring in the theta frequency band (3.5-8.0 Hz) as a proportion of total power from 0.25 to 30.0 Hz (relative theta power). The increase in relative theta power produced by 8-OH-DPAT (20 micrograms/kg) was greatly attenuated by spiperone (1 mg/kg), a highly effective 5-HT1A autoreceptor antagonist. Administration of spiperone alone had no significant effect on relative theta power. These results are discussed in relationship to the effects of these drugs on serotonergic neuronal activity. Our results suggest that preferential activation of presynaptic 5-HT1A receptors, and subsequent inhibition of serotonin neurotransmission, facilitates the appearance of hippocampal theta activity in awake cats.

5-Hydroxytryptamine (5-HT) agonists: effects on neocortical slow wave activity after combined muscarinic and serotonergic blockade

In freely-moving rats treated with a combination of reserpine (10 mg/kg, i.p.) and scopolamine (5 mg/kg, i.p.), neocortical low voltage fast activity (LVFA) associated with continuous multiunit activity (MUA) was abolished and replaced by 2-6 Hz large irregular slow activity (LISA) above 1.5 mV associated with a burst-suppression pattern of MUA. Administration of the monoamine oxidase inhibitor pargyline (50-100 mg/kg, i.p.) completely suppressed 2-6 Hz LISA and restored normal-appearing LVFA and continuous MUA. The 5-hydroxytryptamine (5-HT) receptor agonists quipazine (0.5-20 mg/kg, i.p.), (+/-)-DOI (0.1-5 mg/kg, s.c.), and buspirone (0.1-10 mg/kg, i.p.), but not 8-hydroxy-2-(di-n-propylamine) tetraline (8-OH-DPAT, 0.05-0.8 mg/kg, s.c.) and RU 24969 (1-30 mg/kg, i.p.), produced a partial suppression of 2-6 Hz LISA and restored some lower voltage activity (< 1 mV) above 6 Hz associated with continuous MUA. However, as opposed to pargyline, no receptor agonist tested restored continuous, normal-appearing LVFA. Even though agonists at 5-HT receptors can produce some activation of neocortical slow wave activity after combined cholinergic and serotonergic blockade, this effect is not equivalent to that observed after restoration of endogenous 5-HT transmission.