Neurons in the guinea-pig lateral septum generate rhythmic bursting activities, in vitro, following application of carbachol

Somato-dendritic nicotinic receptor responses recorded in vitro from the medial septal diagonal band complex of the rodent

The medial septal diagonal band area (MS/DB), made up of GABAergic and cholinergic neurones, plays an essential role in the generation and modulation of the hippocampal theta rhythm. To understand the part that the cholinergic neurones might play in this activity, we sought to determine whether postsynaptic nicotinic receptor responses can be detected in slices of the rodent MS/DB by puffing on acetylcholine (ACh). Neurones were characterized electrophysiologically into GABAergic and cholinergic neurones according to previous criteria. Responses of the MS/SB neurones to ACh were various combinations of fast depolarizations (1.5-2.5 s), fast hyperpolarizations (3-4 s) and slow depolarizations (20-30 s), the latter two being blocked by atropine. The fast depolarizations were partially or not blocked with cadmium and low calcium, tetrodotoxin, and antagonists of other ionotropic receptors, and were antagonized with 25 microm mecamylamine. Pharmacological investigation of the responses showed that the alpha 7* nicotinic receptor type is associated with cholinergic neurones and 10% of the GABAergic neurones, and that non alpha 7* nicotinic receptor subtypes are associated with 50% of the GABAergic neurones. Pharmacological dissection of evoked and spontaneous postsynaptic responses, however, did not provide evidence for synaptic nicotinic receptor transmission in the MS/DB. It was concluded that nicotinic receptors, although prevalent on the somatic and/or dendritic membrane compartments of neurones in the MS/DB, are on extrasynaptic sites where they presumably play a neuromodulatory role. The presence of alpha 7* nicotinic receptors on cholinergic neurones may also render these cells specifically vulnerable to degeneration in Alzheimer's disease.

Noradrenergic responses of neurones in the mediolateral part of the lateral septum: alpha1-adrenergic depolarization and rhythmic bursting activities, and alpha2-adrenergic hyperpolarization from guinea pig brain slices

Bath application of noradrenaline on neurones of class A, B and C within the mediolateral part of the lateral septum from guinea pig brain slices elicited depolarization (through alpha1-adrenoceptors) or hyperpolarization (through alpha2-adrenoceptors) and rhythmic bursting activities (through alpha1-adrenoceptors). A co-existence of these two types of adrenoceptors (alpha1 and alpha2) mediating opposite effects on membrane excitability was sometimes observed on the same neurone. Three types (I, II and III) of neurones were identified on the basis of their bursting properties during noradrenaline application. With the bursting activities persisting under tetrodotoxin and their frequency being sensitive to changes in membrane potential, these neurones could be considered as conditional bursters. An afterdepolarizing potential could promote burst initiation in the three types of neurones. Neuronal input resistance decreased at the afterdepolarizing potential peak. Application of low-Na+ medium blocked the generation of bursts and afterdepolarizing potentials induced by noradrenaline in the three types of neurones. Furthermore, the bursts were dependent on the presence of Ca(2+) in the medium in a subpopulation of neurones. We conclude that afterdepolarizing potentials and bursts induced by noradrenaline are generated by a cationic conductance largely permeable to Na+ in neurones of the mediolateral part of the lateral septum.

Characterization of carbachol-induced rhythmic bursting discharges in neurons from guinea pig lateral septum slices

A brain slice preparation from guinea pigs and intracellular recording techniques were used to examine the effects of carbachol application on the three classes (A, B, and C) of neurons (n = 68, 40 of class A, 12 of class B, 16 of class C) within the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited a sustained depolarization associated with an increase in membrane input resistance, action-potential firing and triggered rhythmic bursting discharges in 59% of recorded neurons. According to the configuration of these bursts, LSml neurons were classified into type I, II, and III neurons with reference to their response to carbachol. The frequency of spontaneous bursts was increased by depolarization caused by applied DC current in the three types of neurons. Bursts in type II and III neurons were voltage and dose dependent. These dependences were responsible for a continuum of variation in carbachol responses in these two types of neurons. As the neuron depolarized in the presence of carbachol, spontaneous action potentials increased in frequency and slow afterdepolarizing potentials (sADPs) appeared and preceded the occurrence of the first burst. These sADPs from adjacent action potentials appeared to progressively increase to initiate a burst. In the presence of carbachol, sADPs and bursts were also observable after action potentials evoked by depolarizing current pulses at the resting membrane potential (RMP) in LSml neurons. Evoked sADPs and bursts were associated with an apparent increase in input conductance. Application of low Na+ medium blocked both the sADP and bursts. Application of zero Ca2+ medium either 1) blocked completely the generation of sADPs and bursts (n = 16), or 2) did not block bursts (n = 14). Evoked sADPs and bursts were blocked by tetraethylammonium but were resistant to external Cs+. The results indicate that the activation of cholinergic receptors does not differentially affect the three classes of LSml neurons. The responses to carbachol in type II and III neurons form a continuum of variation, whereas these of type I neurons constitute a discrete entity. The selective cholinergic induction of a sADP, and the progressive activation of these sADPs in LSml neurons are thought to be responsible for the onset of the three types of rhythmic bursting discharges. We propose that sADPs and bursts induced by carbachol are generated by a cationic conductance largely permeable to Na+. In a subpopulation of LSml neurons (n = 16), the bursts are dependent on the presence of Ca2+ in the medium.